Smithells Metals Reference Book Part 17 pptx

Heat treatment at a lower temperature that will austenitize the high carbon case followed by quench- Table 29.7.. Illustratively, beams of kilowatt power may be transmitted through air a

Steel heat treatment 2P-9 293.2 Hardening

Rapid cooling from austenite The rate of cooling and the composition control the structures obtained In general steel specifications will give the recommended conditions Guidance can be obtained from two types of diagram This isothermal transformation diagrams (time/temperature/ trmsformation) show the degree of transformation through time for any temperature held from the austenite treatment These are helpful for austempering treatments but for normal hardening, continuous cooling transformation temperature diagrams (CCT) are more useful They give transformation conditions for particular compositions when cooled from austenite in air, oil or water Transformation starts and finishes for pearlite, bainite and martensite are shown M Atkins,

‘An Atlas of Continuous Cooling Transformation Diagrams’, British Steel Corporation, Rotherham,

UK should be consulted for this information If quench to martensite is to be followed by tempering,

temperature range 290-470 should be avoided since these can produce temper brittleness For equipment parmeters in induction surface hardening see Table 29.5

a gas at about 500°C For details ofcase depths and formulae in case hardening, see Table 29.6

(b) Core refining Heat treatment to refine the grain size of the core by austenitizing and quenching-see Table 29.7

(c) Hardening Heat treatment at a lower temperature that will austenitize the high carbon case followed by quench- Table 29.7

(d) Tempering A low tempering treatment to give stress relief and reduce brittleness, usually below 200°C

For economy, a single quench is sometimes used After carburizing, the temperature is allowed

to fall to betweed that of (a) and (b) followed by quench Steels for which this is acceptable are

noted in Table 29.7 Steels suitable for nitriding are listed in Table 29.8

Surface hardening by flame or induction methods for various steels are listed in Table 29.9 For suitable induction equipment see Table 29.5 Hardness‘profiles of case and core for case carburizing

steels of low, medium and high hardenability are given in Figure 29.3

29.3.4 Surface treatments

As well as carbon and nitrogen, surface diffusion treatments are available using aluminium, chromium, silicon boron and zinc These processes with their properties and applications are listed

in Table 29.10 Blueing of steel parts is included

29.3.5 High carbon steel and alloy steels

The same primary reactions apply to the heat treatment of high carbon steels as to the low carbon type, but the decarburizing reactions are of most importance A variety of controlled atmospheres

is available for industrial use and they are summarized below, the final choice being determined

by the quality of product required and whether the heat treating temperature is above or below the Ac, point

Above 680-700°C the atmospheres for treatment are:

1 High temperature endothermic gas, produced from rich hydrocarbon gas/air mixtures, plus

2 Completely burnt hydrocarbon gas, processed over hot charcoal, plus hydrocarbon if

3 Chemically purified burnt hydrocarbon gas, dry and free from carbon dioxide, plus

4 Nitrogen or nitrogen plus hydrocarbon

Table 29.6

(carbon diffusion) heat-resistant box surrounded by a

carburizing powder consisting of alkali carbonates, charcoal or coke tar, and molasses with a.barium carbonate energizer

1.25 mm ( h)

2.5 mm (20h)

Low capital cost Simple Labour intensive Heat

Liquid Parts suspended in molten salt Vary according to depth d = K P (h) Simple controls can be Poisonous salts and

bath containing sodium cyanide required Values of k automated Bath heat re vapoun Equipment

chloride and accelerators The salt

oil gas, or electricity (submerged

875°C = k= 0.018

925"C=k=0.025

carburizing, refining and

mass production Can be combined with quenching

High capital cost-not suited to jobbing work

A freshly made bath is therefore aged for a few hours at 700°C before use

Proportions of carbon and nitrogen may be varied Ammonia

is used to provide nitrogen

Low temperature cyanide bath, preaged to allow cyanate formation With low temperatures and long times the case is mostly nitride

Fully machined and heat-treated parts are nitrided in a mume in contact with ammonia gas

carbunzing

Lower than with gas carburizing Lower carburizing

temperatures increase nitrogen percentage

Hard wear resistant case

Longer equipment life

Hard and temper resistant case

Hard and wear-resistant

Improved fatigue properties Machined and hardened before casing

No distortion or grinding necessary

Similar to liquid nitriding:

used for crankshafts

camshafts, gear shift forks, etc

Poisonous salts

Similar to gas carburizing

Thin case Slow process Not suitable for heavy coarse work

Case brittle and can crack

or spall, if used with plain carbon steels, hence special steels necessary

2% nickel-Mo 665M17 34 0.17 0.5 2Ni0.25 Mo 850-880 (1) 760-780 (0) 700

Moderate shock resistance 2% nickel-Mo 665M33 35 0.23 0.5 1.7 Ni 0.25 Mo 850-880 (1) 760-780 (0) 850 15 30

(higher C)

‘20’ carbon 805M20 362 0.20 0.8 Ni 0.5 Cr 0.2 Mo 850-880 (1)* 780-820 (O)* 850 LOT shock mistance

alloy

Low nickel-Cr- - 325 0.22 0.5 1.7 Ni 0.5 0 0 2 5 Mo 850-880 (I )* 770-800 (O)* 850 15 40 Tough, medium strength

Mo

f nickel-Cr 635MlS 351 0.15 0.8 0.75 Ni 0.5 Cr 850-880 (I)* 780-820 (O)* 700 18 40 Good shock resistance

‘15’ carbon low 80511117 361 0.17 0.8 0.5 Ni 0.5 Cr 0.2 Mo 850-880 (1)* 780-820 (O)* 700 18 35 Moderate shock resistance alloy

1% nickel-Cr 63511117 352 0.17 0.8 1.0 Ni 0.75 Cr 850-880 (1)* 780-820 (O)* 850 15 27 Moderate shock resistance

HIGH HARDEN ABILITY

37 39B

2 Ni 1.5 Cr 0.2 Mo

Refine

“C 850-880 (1) 850-880 (1) 850-880 (1) 850-880 (2) 850-880 (2) 850-880 (a)*

850-880 (2)*

Harden

“C 760-780 (0)

760-780 (0) 750-780 (0) 760-780 (O)?

780-820 (0) 780-830 (O)*

780-820 (O)*

Tensile strengrh

MPa

Elongation

%

Impact roughness

As above Heavy duty gears etc

Section Yield Tensile Impact size strength strength toughness

Higher hardenability

Corrosion resistance

0.25-0.35% C gives core strength of 700-850 MPa

0.35-0.45% C gives core strength of 850-1 000

MPa

0.25 0.3 0.45 5.25 0.80 0.30 0.15 2.20

Table m.9

Free cutting

Alloy

~~ ~

29-16 Heat treatment

Table 29.10 DIFFUSION AND OTHER SURFACE TRFATMENTS OF STEEL

Aluminizing (calorizing) also for

nickel and cobalt alloys

Aluminium or ferro-aluminium

powder, volatile halide and

ceramic heated to 815-1 200°C

for 6-24 h

Chromizing Chromium powder,

halide and inert aggregate heated

800-1 300°C for about 20 h in

presence of hydrogen

Siliconking A pack of s i l i n

carbide or fmosilicon and

circulating gas at 900-1 0oO"C

Silicon chloride is carrier gas

Sheradizing Diffusion of zinc to

form a coating Zn dust and sand

packed around parts and heated

to 300400°C for 3-10 h Zn

vapour acts as carrier-see

BS 4291

Bor~nizing'~ Diffusion of boron

into plain carbon steel Treatment

at surface decreases to t 2 5 % Al

with deerease of thickness to

< 125 pm Coating is resistant to combustion and S gases

Surface > 12% Cr and ferritic

Coating about 8-9% Fe Can be

painted without pretreatment

High temperature oxidation resistance

Pump shafts, cylinder liners,

valves and valve guides Fittings

Small p s i n g s , forgings, castings,

nuts, bolts, washers Lengths of rod or tube

Hard surface applications

Nuts, bolts, etc

Below 680-700°C the atmospheres for treatment are:

1 Dry burnt hydrocarbon gas

2 Dry burnt ammonia

The nature and concentrations of the alloying elements deterrmn e the type of controlled atmosphere

which will ensure freedom from oxidation or decarburization during heat treatment

Austenitic and martensitic stainless steels, etc If the alloying elements form oxides of low

dissociation pressure, and are present in concentrations exceeding about 1%, the controlled

atmosphere must be quite free from oxygen-bearing gases, and the choice is limited to cracked

ammonia, dry hydrogen, or dry nitrogen/hydrogen mixtures

Straight nickel steels These can be bright heat treated successfully in atmospheres suitable for

their plain carbon steels equivalents, except that the use of a desulphurized atmosphere is advisable For this reason all alloy steels should be degreased before treatment if surface staining is to be avoided

Aluminium alloys 29-17

Martensitic types of stainless steel Such types are treated in cracked ammonia having a controlled addition of methane or propane to prevent decarburization, and quenched in the controlled atmosphere5 or oil

High speedsteels These are preferably treatment in salt bath furnaces, but where the nature and size of the work does not permit this a controlled atmosphere can be used6 The formation of an

oxide skin is not objectionable, since the tools are normally surface-ground before use, and such

a skin may in fact give added protection against decarburization

TOOL STEELS AND CAST STEELS

For heat treatments of tool steels see Table 22.50 For forged and rolled steels see Table 22.44

The conditions of heat treatment for these complex steels must be obtained from individual specifications

29.4 Cast iron treatments

29.4.1 Mnlleablizing

Whiteheart malleablizing-xiow little used-is produced by a simultaneous graphitizing and decarburizing treatment of white cast iron The decarburizing atmosphere can be produced by a suitable atmosphere or by reaction between the air in the furnace and the carbon in the coatings The resulting atmosphere, rich in carbon dioxide, is circulated and the carbon monoxide converted

to dioxide by addition of air of steam Batch or continuous furnaces operate at 1050°C For Blackheart malleablizing, decarburization is prevented by a completely neutral atmosphere

such as dry nitrogen or completely burned hydrocarbon gas stripped of carbon dioxide to 0.1% and water vapour to a dew point of -40°C Alternatively fully burned ammonia can be used-see Chapter 26-80 The carbide in the white iron changes to areas of temper carbon The process is

carried out at 1050°C (first stage) and 750450°C (second stage)

29d.2 Nodular cast iron or spheroidal graphite iron (SG)

The nodules of graphite are in a steel-like matrix that can be heat treated in ways similar to steel thus giving a great versatility of end products An important development is the amtempering treatment of nodular cast irons The nodular casting-(for production see 26.9)-is first austenitized and then quenched into a temperature of between 250 and 450°C where it is held so that transformation occuls from austenite to bainite It is then air cooled to ambient temperature Additions of copper, nickel and molybdenum can be used to facilitate and improve the

295 Aluminium alloys

29.5.1 A n d i n g

For softening aluminium alloys that have been hardened by cold work:

Alloys 1080A, 1050, 1200,5251, 5154A, 5454,5083-360°C for 20min

Alloys 3103,3105-lO0-425"C for 20 min

Heat-treatable alloys that have not been heat treated-360°C & 10°C for 1 h and cool in air Alloys that have been heat treated 400-425"C for 1 h and cool at 15"C/h to 300°C

For AI-Zn-Mg alloys of the 7000 series, after cooling in air, reheat to 225°C for 2-4 h

2951 stai)ilizing

To relieve internal stress Normally heat to 250°C followed by slow cooling is adequate

29-18 Heat treatment

295.3 Hardening

Conditions for solution treatment and ageing for both cast and wrought aluminium alloys are

given in Tables 29.11 and 29.12 For the alloy designation system and compositions see

Chapter 22 Temper designations are given in Table 29.13

Table 29.11 HEAT TREATMENT DATA FOR ALUMINIUM CASTING ALLOYS"

Solution treatment Material

hignation Alloy Temperature Time' Quench'

Precipitation treatment Temperature Time3

- 515-525 515-525 520-530 520-530 515-530 525-545 525-545

-

-

- 495-505

-

495-505

500-520

- 520-540 425-435 520-530 525-545 525-545 535-545

542 + 5

510k5 510k5

6-16

- 2-8

O at 160°C m4

Hot water Hot water Hot water Hot water Hot water Hot water Hot water

-

-

-

- Air blast Air blast

-

-

Boiling water

- Water at 30-100°C Oil at 160°C max4 Hot water Hot water Hot water Hot water Boiling water or oil at 80°C

Water (50-70°C) Water (50-70°C)

150-170 150-170 150-170

- 160-180 160-180 200-250 160-179

250 155-175 155-175 200-210

- 160-170 120-140 120-170 150-160

215 + 5

- 140+10

6-18

16

16

- 4-16? 4-16 4-16

- 8-10 2-4 8-12 8-12 7-9

4 12-16

Single figures am minimum tims at tempratwe for average castings and may have to be incnased for particular castings

Hot water means water at 70-8WC unless otherwise stated

The exact number of hours depends on the mechanical propetties required

The castings m a y be allowed to cool to 385495°C in the furnace before quenching The castim shall be allowed to stay in the

oil for not more than 1 h and m a y then be quenched in water or oooled in air

?The duration of the treatment shall he such as will produce the s@ed Brinell hardness io the castings

Aluminium alloys 29-19

Table 29.12 TYPICAL HEAT TREATMENT DATA FOR WROUGHT ALUMINIUM ALLOYSo

Times and tempemhues within the limits shown Some specifications give tighter Limits

Solution treatment

Ageing Timeat

50555

505 f 5

505 f 5 505f5

465 5

465f5 465+5

Water Water Water Water Water Water Water Water Water Water or oil Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water

or followed hy Water Water 85°C or oil Water or oil Water Water Water (6040°C)

or

followed by Water followed by Water 70°C followed by

-

OI

Room 155-165 Room Room 155-190 155-190 Room Room Room 155-205 Room 160-200 Room 165- 195 Room 160-180 160-180 Room 175-185 Room 170f 10 Room 170f 10 172+3*

120k3

17253 172f3*

135f5

135 f 5

135 f 5 135f5 110k5

12055 177f5

48

48

48

96 2-20 16-24 3-12 5-15 5-15 7-12

24 6-15 10-24

12

12

12

12 6-24 20-30

6-10

6-24 5-12 6-24 20-30 6-10

-

Heatiog to temperature at not more than ZO"C/h

$For temper designations see Table 29.13

Water below 40°C unkss otherwise stated

Table 29.13 ALUMINIUM ALLOY TEMPER DESIGNATIONS

Solution treated and naturally aged

Solution treated and artificially aged Solution treated, artificially aged and stabilized

29-20 Heattreamtent

Table 29.l3 ALXMNWM ALLOY lEMpER DESIGNATIONS - conaimwl

condition

Cooled from elevated temperature shaping process and naturally aged to stable condition

As T 1 but cold worked after cooling from elevated temperature

Solution treated, cold worked and naturally aged to stable condition

Solution treated and naturally aged to stable condition

Cooled from elevated temperature shaping process and artilicially aged

Solution treated and artificially aged Solution treated and stabilized (averaged)

Solution treated, cold worked and then artificially aged

Solution treated, artificially aged and then cold worked

Cooled from elevated temperature shaping process, aaificiaUy aged and thm cold worked

29.6.1 Environments for copper base heat treatments

Stream and carbon dioxide are virtually inactive to copper and both are used on an industrial

scale The forma has the disadvantage of giving rise to water staining, while the latter is less

economical Burnt hydrocarbon gas is used extensively for bright annealing copper, but the metal

is very susceptible to staining by hydrogen sulphide Organic sulphur and sulphur dioxide do not attack copper but may be converted to hydrogen sulphide so that complete desulphurization of the controlled atmosphere is advisable Sulphur compounds are much r e d u d if the gas is fully bumt

rather than partially burnt Cracked and burnt ammonia are suitable for copper annealing and are

sulphw free, but the former is little used for economic reasons

Tough pitch copper contains oxygen and hydrogen in the contmlled atmosphere must be less than

1% Burnt ammonia or burnt hydrocarbon gas can be used, however, embrittlement can OCCUT due

to reaction of steam with iron in the furnace producing hydrogen An open flame furnace in which

the preferably desulplnuized gas is fully burnt can be used in bright annealing

Copper-tin and copper-nickel alloys can be bright annealed as above but desulphurlzation must

be absolutely complete For alloys containing more readily oxidized elements, the range of suitable

atmospheres is restricted to ammonia or dry burnt ammonia

Copper-zinc alloys A successful industrial process11 has been developed for the full annealing

of brass with zinc content up to at least 37% CO, C02 and water vapour must be minimal and the atmosphere should contain some hydrogen Qpical composition is 25% H/N with max 0 2 1 vpm,

COz 20 vpm and de point -70°C Zinc has a considerable vapour pressure at annealing temperatures and this may affect the colour and surface finish Temperatures and soak times should be chosen with care Where finish is less critical, 'flash' annealing may be applied to light gauge material with

good results where rapid heating and cooling may be applied Commercial annealing is often carried out in the cheaper types of controlled atmospheres which produce a light oxide film sufficient to restrict excessive zinc loss

Gilding metal contains low zinc concentration and can be bright annealed readily in atmospheres

used for the annealing of copper, especially when in the form of coiled strip

29.6.2 Temperatures for annealing and stress relief

'Ihese are are set out in 'pdble 29.14 For more details on copper base alloys reference should be made to Copper Development Association data sheets

Magnesium alloys 2!4-21 Table 29.14 ANNEALING A N D STRESS RELIEVING OF COPPERS AND BRASSFST

IS0 alloy designation

400-650 425-650*

500-700 475-700

Stress reIhing range ("C)

150-200

175-225*

170-200 200-250 225-275*

175-225 225-275 225-275 225-275*

250-350 250-380

* Embrittlemeat will occur if heated in atmosphere containing ex- of hydrogen

For more details see Copper Development Association data TN 26 and 27

29.7.2 Environment

For temperatures over 4OO"C, surface oxidation takes place in air This can be suppressed by addition

of sufticient sulphur dioxide, carbon dioxide or other suitable oxidation inhibitor

In the case of castings to MEL ZE63A and related specifications, solution treatment should be carried out in an atmosphere of hydrogen and quenching of castings from solution treatment temperature of MEL QE22 is to be done in hot water

If microscopic examination reveals eutectic melting or high temperature oxidation, rectification cannot be achieved by reheat-treatment Quench from solution treatment should be rapid, either forced air or water quench From ageing treatment, air cool

29-22 Heat treatment

29.73 Conditiom for heat treatment of magaesiUm d o y castiagS

These are shown in Table 19.15 and for some wrought magnesium alloys in Table 19.16 Stress relief treatments are given in Table 19.17

Table 29.15 HEAT TREATMENT OF MAGNESIUM C A m G ALLOYS

Temperature Tim (h) Temperature Time @)

2112.2 ZrO.7 Th3.0

Zn5.5 ZrO.7 Th1.8

Ag1.5 ZrO.7 (30.07 Nd(RE)2.0 Ag2.5 ZrO.6 Nd(RE)2.5 Ag2.5 ZrO.6 Nd(RE)z.O

Mn0.3 Be0.0015 A17.519.5 Zn0.3/1.5 Mn0.15 A17.5/9.5 Zn0.3/1.5 Mn0.15

380-390 380-390 410-420 410-420

Nickel and cobalt alloys 29-23 Table 29.16 HEAT TRF$ATMENT OF MAGNESIUM WROUGHT ALLOYS

Specifications Composition Form Solution treatment

Notes: Ex-ext~sims, F-forgbw, S H 4 e e t hard rolled, SA-sheet annealed, *-cooled and a r t i l i i l y aged

29.8 Nickel and cobalt alloys

For bright annealing of nickel alloys, ammonia, burnt ammonia or hydrogen atmospheres can be

used but the dew point should be below -50°C All traces of sulphur gases should be excluded

from atmospheres in which nickel alloys are heat treated

The heat treatments of some nickel alloys are given in Table 29.18 and some cobalt alloys in Table 29.19 These alloys are normally used by name and therefore these tables do not give the alloy type The UNS number (see Chapter 1) is given when available

Table 29.18 HEAT TREATMENT OF NICKEL ALLOYS

Annealing Stress relief Sohttion treatment Ageins

Temp Time Temp Time Temp Time Temp Time

29-24 Heat treatment

Table 29.18 HEAT TREATMENT OF NICKEL AuOY%-continued

Annealing Stress relief Solution treatment Ageing

AMS 5668

Nora: FC=fumacc cool, AC=& cool, h=hours, hs=hours/iach of section, WQ=water quench, &=ageing during service,

few full &, SQ=quench Wow 540°C rapidly enough to prevent precipitation

Table 29.19 HEAT TRWTMENT OF COBALT ALLOYS

Annealing Stress relkf Temp Time Temp.(T)

Solution treatment Ageing

Notes: RAC=rapid air cool, AC=& cool, h=hours, hs=hours/inch of d o n , AS=ageing during Service, f=w full anneal,

@=quench balm W C rapidly enough to prevent precipitation

REFERENCES

1 I Jenkins, ‘Controlled Atmospheres for the Heat Treatment of Metals’, London, 1946

2 C E Peck, Metah and Alloys, 1944,19, 593; 1945,22,85

3 B Lustman, Mefal Progress, 1946,51, 850

4 L Fairbank and L G W Palethorpe, ‘Controlled Atmospheres for the Heat Treatment of Metals’, ISI, Special

5 0 Kubaschewski and C B Alcock, ‘Metallurgical Thermochemistry’, 5th edn, F’ergamon, Oxford, 1979 Report No 95, 1966, p 52

Refeences 29-25

6 ‘Cassel Manual of Heat Treatment and Case Hardening’, 7th edn, Sections 1.2.3 and 1.2.4, Cassel, London,

7 K J Irving, Low Alloy High Strength Steel Symposium, Nuremberg, 1970

8 R A Harding, ‘Ferrous Materials for Geam-A Review’, BCIRA Report No 1578,1984

9 P A Blackmore, ‘High Strength Nodular Irons’, BCIRA Report No 1581,1984

1966

10 “The Properties of Aluminium and Its Alloys’, 8th edn, Aluminium Federation, Birmingham, 1983

11 P H Elner and J B Carcol, Heat Treatment of Metals, 1978,4, 83

12 ‘Standard Practice for Heat Treatment of Magnesium Alloys’, ASTM 661-87

13 K H Habig, Materia& in Engineering, 1980,2,83

14 ASTM B637, ‘Heat Resisting Alloys’

30 Laser metalworking

30.1 Introduction

Laser radiation, in contrast to that from black body sources, can offer a high degree of spatial

coherence This means that it can transmit as a well-directed, often near-parallel beam which may then be focused to spot diameters comparable with the wavelength of the radiation Illustratively, beams of kilowatt power may be transmitted through air and then focused to intensities in excess

of lo6 W cm-2 which, when applied appropriately to a metal workpiece, permit the carrying out

of processes such as drilling, cutting and welding

Importantly, because these processes are carried out at intensities much greater than those of

conventional sources, heating is localized and there is usually a related reduction in phenomena

such as distortion, shrinkage and heat-affected-zones As a result the need for post-process machining may be eliminated so that components go into use directly; this can, of course, be an important factor in the overall economics Thus the unique combination of high focused power density,

coupled with the facility for beam delivery and flexible use at ambient pressure, has resulted in a

continuing growth in industrial applications in laser metal working

30.2 Lasers

30.2.1 Basic principles

The three essential components of a laser are the laser medium, the excitation source and the

optical resonator (Figure 30.1) The excitation source drives the atoms, ions or molecules of the

laser medium to a situation where there is an excess of those at high energy level over those at a low level This inversion of the normal thermodynamic population distribution leads to laser actioc:

Optical resonoto

t t t t t

Excito tion source

Figgre 30.1 Essentiak of a laser

3&2 h e r s

an excited member of the medium undergoing a transition from high to low energy will emit a photon, which in turn stimulates further emission, perfectly in phase, and at the same wavelength, from the other excited members of the medium The radiation is thus rapidly amplified; the role

of the optical resonator is to direct and control the radiation by allowing an appropriate fraction

to be bled off as a near-parallel beam while the remainder is circulated within the cavity to maintain

laser action The output is monochromatic, usually with high spatial and temporal coherence At present, metal working applications involve use of predominantly two laser types, carbon dioxide

(CO,) and neodymium YAG (Nd:YAG) The salient features of the two types are summarized in Table 30.1 and they are discussed in turn below

30.2.2 C 0 2 lasers

In these lasers, the laser action results from electric discharge excitation of a low pressure gas

mixture containing carbon dioxide The beam is invisible, having a wavelength 1 lying in a far intrared at L=10.6pm Industrial units are available with powers up to lOkW and above Whilst

early CO, lasers were almost exclusively of continuous wave (CW) operation, many low to medium power units now offer kilohertz pulsing capabilities Since the pulse power may be ten times the average power, such operation can give better coupling of the beam into reflective metal surfaces Additionally, pulsing may permit improved control of energy delivery to the work and this can be important, for example in the processing of thin sections Current trends in laser development have resulted in the use of radio frequency (RF) excitation of the discharge in some designs In general, research and development continues on the building of ever higher power CO, lasers Motivated

by the promise of increased single-pass weld penetration, work is underway on the design of improved 25 kW units

30.2.3 Nd:YAG lasers

Here the laser action results from the excitation of a solid rod of yttrium aluminium garnet, doped with neodymium, by intense white light from a lamp which may be pulsed or continuous As in the CO, laser, the beam is invisible but here it is in the near infrared at 1=1.06pm Currently,

most metal-working Nd:YAG lasers have average powers in the region of several hundred watts, and have an output which is pulsed A pulse may deliver tens of joules of optical energy in a millisecond, so that power a t the workpiece may be tens of kilowatts As a result, Nd:YAG lasers are well suited to drilling They are also used very successfully for cutting and, with less intense pulses, for welding and surface treatment Present trends in laser development have led to the availability of powers of 1 kW and above, and t o the increasing appearance of cw units Replacement

of rod geometry by that of slabs is claimed to ease cooling problems, reduce distortion and improve beam quality

Because high beam power is more readily achievable, and available, in CO, lasers than in Nd:YAG lasers, the latter have been mainly associated with fine-scale applications whereas CO, lasers tend to be used for larger-scale application However, with Nd:YAG units now offering beam powers of 1 kW and above, there is a greater overlap of capabilities in the cutting and welding of

metal thicknesses which represent a significant part of engineering production

30.2.4 Resonators

The beam-metal interaction process is strongly affected by the intensity distribution of the beam,

which in turn is influenced by the design of the optical resonator (or cavity) These fall into two categories: stable and unstable The terms refer to a mathematical description of the resonator which will not be attempted here Figure 30.2 compares practical realizations of a stable and unstable cavity

The stable cavity is particularly suited to long lasers of low aperture, and in a well-designed system only the lowest-order mode (TEM,,) will be sustained, yielding an output beam which has

a Gaussian radial intensity distribution

[=l,e-'/"

where I=intensity (as a function of radius), /,=on-axis intensity, r =radial position, w = 'spot radius' at which the intensity falls to l/e of axial value

This distribution has a strong central peak, steep sides and low wings; it is obtained also in the

beam spot after focusing and it appears to correspond to excellent metal penetration capabilities

Lasers 30-3

c+

Figure 30.2 (a) Stable and (b) unstable cavity resonators

On the other hand, a stable cavity can be arranged to sustain higher-order transverse modes so that instead of there being a lone spike of high intensity, additional rings appear around it (the

TEM,,* mode gives the first ring alone) The redistribution of power results in a broader, flatter

distribution in both the output beam and the focused spot Although such multimode beams do not appear to be particularly well suited to creation of deep, narrow cuts and welds, they may be preferable for surface treatment processes requiring an extended uniform distribution

If the laser gain medium tends towards a geometry of short length and high aperture, an unstable resonator must normally be used to ensure good mode control and focusability As seen in Figure

30.2(b), the output beam is annular This distribution (which should not be confused with the much less focusable TEM,,*) yields, after focusing, an Airey pattern distribution which is strongly peaked on axis but with some power in concentric rings For lasers with high gain where the central hoie in the annulus is small (high magnification cavity), the power after focusing is predominantly

in the central spike, giving generally very acceptable cutting and welding performance One special merit of the unstable resonator is that the out-of-focus annular distribution is well suited to metal surface treatment operations

30.2.5 Beam delivery

The beam emerging from the laser is only near-parallel; if it is transmitted a significant distance before use, the growth in diameter may need to be taken into account in choosing the diameter of beam lines and focus units Consider the case of a laser having TEM,, mode: the beam envelope

Figure 30.3 Laser beam focusing

typically consists of a waist region (diameter wl, left-hand side of Figure 30.3) and a region bounded

by straight lines of divergence a Then

F (right-hand side of Figure 30.3), the near field distribution at waist w , is transformed to the far field distribution at waist 0 2 Further lenses can effect further transformations, the product

speed cutting of thin sheet where depth of focus is less important, whilstf/lO tof/l5 focusing might

be chosen for high power welding of 15-nun thick steel where a broad depth offocus is desirable

As an alternative to beam transmission through air, optical fibres may be used for Nd:YAG lasers; quaxtz fibres offer good transmission at 1.06pm and are capable of operation with kilowatt power levels O n the other hand, suitable low-loss optical materials capable of transmitting high power 10.6pm radiation have not yet been developed Nevertheless, flexible beam delivery systems for CO, lasers d o exist based on line of sight transmission between suitably articulated mirrors

TABLE 30.1 CHARACTERISTICS OF NdYAG AND CO, LASERS

10W

Solid rod of N d Y A G White light excitation from flash tubes beside rod

Water cool rod

or AC up to R F (i) Operate discharge in glass tubes and water cool tube walls (ii) Blow gas through discharge and then cool in heat exchanger

$ 5 kW cw and RF, pulsable up to several kHz

1&20 kW mainly cw

6 10%

Stable or unstable Metal

e.g ZnSe, KCI

steel for example it is desirable to use very high intensity ( 2 lo7 W/cm') so that super-heating occurs (temperatures > 3000"C), leading to explosive ejection of liquid and solid material, thereby

yielding very efficient machining In deep penetration welding, where the creation of the welding capillary involves melting and carefully controlled vaporization, intensities of lo6 W/cmZ and a little above are normally employed Surface cladding requires good melting with negligible vaporization, Le t < 3000°C and intensities of 104-105 W/cm2 are used In transformation hardening, surface melting must not occur, Le temperature must be held below 1500°C for steel, and typical intensities are > lo3 w/cm2

Processing considerations 30-5

30.3.2 Beam coupling

Clean, smooth metal surfaces are good reflectors of infrared radiation although they are more absorbing at the Nd:YAG wavelength than that of the CO, laser Indeed, ‘perfect’ metal surfaces

may have reflectances in the range 60-90% However, reflectivity decreases with increasing

temperature, presence of oxides, onset of surface melting, and capillary formation The result is that in most practical applications even the CO, laser beam is efficiently coupled to the metal

In drilling, cutting and welding, the focus intensity is sufficiently high (particularly in the case

of pulsed beams) to initiate melting, surface disruption and capillary formation which yields efficient beam trapping In CO, laser welding, plasma formation associated with the capillary also contributes

by absorbing beam energy and redistributing it to the metal by conduction In cutting, the assist

gas jet keeps the cut slot (kerf) relatively clear so that the beam couples directly to the kerf leading edge and sides It is found that better coupling occurs in CO, laser cutting for a plane polarized beam having the electric vector lying along the cut (Brewster effect) The overall result is that the machining processes exhibit a very highly efficient utilization of the beam However, CO, lasers are not suited to the processing of gold, and they require good beam quality for processing of copper and aluminium

Surface melting, cladding and alloying can be carried out with high efficiency using Nd:YAG lasers, and with acceptable efficiency using C 0 2 Because there is no strong capillary feature in these processes, some CO, beam reflection does occur However, when powders are being fused into metal surfaces, their delivery into the interaction region can provide a surface with good absorptivity, and typical overall beam utilization efficiencies may be better than 30%

The implementation of efficient transformation hardening by CO, laser relies on the presence

of an absorbing coating This may be an oxide layer grown during processing, but it is more normal

to employ a pre-applied thin layer of colloidal graphite Such coatings can present greater than 80% absorptivity As in the case of cutting, the Brewster effect may be used to give better coupling

when the beam has to be inclined to the surface: the beam should be plane polarized with the electric vector lying in the plane of incidence-reflection

3033 Processing depths and rates

Cutting and welding

In these processes, prediction from first principles of the depth of penetration achievable with a

given laser beam is at present not a practical proposition This is a consequence of the complexity

of the beam interaction processes occurring in the kerf and keyhole Nevertheless, a simple energy balance may give a qualitative guide to the scaling of penetration with process speed Consider

full penetration taking place using beam power p in a plate of thickness d at speed u A parallel sided kerf and melt region occur in the case of cutting and welding respectively Let these have

widths w, and respectively A significant fraction of the applied power p is deposited in the metal, the remainder being lost by reflection and through transmission Of the power deposited, a fraction (perhaps of order a) corresponds to that required to heat and melt the volume of metal of interest, whilst the remainder is conducted into the work It should be noted that the weld region can

become a cut kerf with the application of an assist gas jet; use of a reactive gas has the effect of

increasing the power deposited in the kerf Therefore the approximation can be made that the

volume of melt produced per unit time is proportional to the beam power applied That is

P = k,vdw,

p = k,vdw,

whiere k,, k , are the constants of proportionality which contain contributions from reflectivity specific enthalpy for melting, etc Consider now two special cases:

In cutting, the width of the kerf may be comparable with the beam spot diameter and furthermore

may remain substantially constant over a (limited) speed range Thus from the above equation at

constant powe:

vd=constant, i.e vcclfd

A ?lot of speed against thickness at threshold of cutting penetration will therefore have the form

shown in Figure 30.4(a)

In welding, over part of the speed range, the aspect ratio of the melt, d / o , may tend to be maintained, i.e w,Kd and from the above equation, at constant power

ud2=constant, Le uccl/d2

(/inear scule, orb uniW

Figme 30.4 Characteristic relationships between process speed and workpiece thickness based on simple energy balance and ( a ) kerfwidth independent of speed, (b) kerf aspect ratio (thicknesslwidth) independent of speed

A plot of speed against thickness at threshold of weld penetration may have the form also shown

in Figure 30.4(b)

In practical application, processing may fall between these two special cases Nevertheless, the energy balance approach permits some understanding of the characteristic shapes of plots of speed and thickness which are observed, and it provides a guide to extrapolating performance

Su$ace transformation hardening

Here, a defocused beam is scanned over steel or cast iron, austenitizing a surface layer which then quenches by conduction into the bulk to give a martensitic layer It is possible to make relatively simple and accurate predictions regarding process depths and speeds because the laser energy is deposited on the metal surface and is conducted, according to classical heat flow, into the meal Because the depth heated is small compared with the extent of the heating spot, it is quite appropriate to use a one-dimensional heat flow model which considers a heating flux P deposited

for a time z in the surface of a semi-infinite body The dwell time z corresponds to a beam spot, moving over a workpiece at speed u, and having length 1 in the travel direction where ~ = l / u At the end of the heating pulse, the surface temperature T as a function of depth z is given by (Carslaw H.S and Jaeger J.C., 'Conduction of Heat in Solids', 2nd edn, Clarendon Press, Oxford, 1959, p75)

1 erfc x=l-erfx and ierfc x=-e-x2-xerfc x

f i

2F

K

and the surface temperature T, by

where K =thermal conductivity, k = thermal diffusivity = K/pc, p = workpiece density, c = workpiece specific heat

In laser transformation hardening, it is usual to take the surface close to (but not above) the melting point Thus in hardening steel, where T,= 1500"C, K = 0 3 5 W ~ m - ~ ~ " C - ' , and

k = 7 3 x 10-2cm2s-1, the last equation gives

F A < 1722 w crn-2s1/*

Cutting 3&7

if surface melting is to be avoided Typically absorbed intensities of 3 kW/cmz and 6 kW/cmz might

be used so that interaction times of g0.33 s and G0.083 s should be chosen respectively The resulting case depth can be estimated from the model by assuming that this is the same as the depth raised above austenitizing temperature The temperature as a function of depth can be rewritten in terms of a surface temperature T,

If hardening takes place down to a depth z' where T = 850"C, then since T, = 1500°C

z' ierfc - w0.3

2Jj;;

For the two cases considered above where T =0.33 s and0.083 s, z' x 1 mm and 0.5 mm respectively I[t is important to note from the foregoing that the process operates by the surface being heated very close to melting point Thus a high degree of uniformity is required in the beam spot, otherwise melt strips may occur Furthermore, a move towards greater case depth implies use of lower

intensities in order to provide sufficient heat diffusion time In practice, laser hardening is best

suited to depths < 1 mm; beyond that self-quenching may become problematic and distortion may become noticeable Also, in practice depth can be reduced by increasing processing speed-however there is a strong sensitivity in the interrelationship, and care is required

30.4 Catting

The cutting process is based on location of beam focus at the surface of the (moving) workpiece,

so that it melts the metal, and on provision of a jet of gas, usually coaxial with the laser beam,

which displaces the molten material This gas jet, if containing oxygen, may promote an exothermic reaction with the workpiece which enhances speed or penetration For a well-optimized process, the resulting kerf can be narrow and parallel sided, the cut faces smooth (exhibiting striations of limited amplitude), the rear edges of the cut substantially free of adherent dross, and the heat affected zone (HAZ) narrow Laser cutting currently occupies a niche which overlaps to some

extent (depending on laser and workpiece) with electro-discharge machining, plasma cutting and

abrasive water jet cutting It is now believed that the following factors contribute crucially to the achievement of good cut quality The radial intensity distribution of the beam should be narrow, steep sided and with little power in the wings; a Gaussian distribution appears suitable The gas jet must be accurately centred on the beam interaction point and it should deliver high pressure into the kerf to promote efficient removal of material; however, care is required in the use of very high pressures since adverse supersonic shock structures may be present (and, particularly for lower laser powder, high gas flows may cause a deleterious chilling) Furthermore, an excess of oxygen can lead to too much burning of the material with a concomitant decrease in edge quality For preference, the beam should be circularly polarized so that cutting performance is the same in all

directions The overall system (beam, jet, workpiece traverse) should be stable, smooth and free

of jitter However, the use of a pulsed laser beam may have advantages; it enables better control

of energy input for the cutting of thin sections and fine details, and indeed it is claimed that careful matching of pulse frequency to cutting speed permits achievement of very smooth cut edges There is a growing body of data on cutting performance from laser manufacturers and researchers

using a range of lasers and a wide variety of materials Some care should be exercised in the use

of such data because of the process dependences just described For present purposes it is convenient

to discuss performance by material type

Steels

The vast majority of laser metal cutting is applied to steel, much of which is mild steel Figure 30.5

provides a guide to cutting performance trends with oxygen assist; it shows illustrative data for both 1Vd:YAG and C O , lasers, bearing in mind that at present the former cover the range up to

1 kW and the latter up to 5 kW and above Generally speaking, edge quality improves for the cleaner, more highly alloyed compositions; thus tool steel gives a better edge finish than mild steel

The cutting of stainless steel is more difficult because it resists formation of slag so that the dross

is metallic rather than a friable oxide and it tends to adhere strongly to the rear edges Indeed it

is currently claimed that best edge quality on stainless steel is obtained with the use of N, assist gas at pressures up to 15 bar In broad terms, the speeds of Figure 30.5 should be reduced by approximately 50% for stainless steel cutting

Nickel alloys

Nickel alloys also exhibit a tendency to form a tenacious dross when laser cut Again the cutting

speeds of Figure 30.5 should be reduced by approximately 50%

Titanium

Titanium is highly reactive, and therefore can be cut with considerable exothermic enhancement

if oxygen is used However, care is required Not surprisingly, the cut edges are then highly oxidized

It is probably more relevant to utilize inert gas assist, e.g argon, in which case oxide-free edges can result although the speeds of Figure 30.5 are again reduced by approximately 50%

A wide range of proprietary cutting equipment is now available For very high precision, thin-section work this may consist of a Nd:YAG laser with le& focusing and a CNC X-Y table

Dri&q and engrauing 30-9

to move the work For thicker sections, where a COz laser is more appropriate, a similar arrangement may be used or alternatively moving optics may be employed so that the focus head moves in

X-Y over the fixed workpiece Movements over several metres are now commonplace For three dimensional cutting, the moving optics concept may be extended by the addition of a z axis together with one or two axes of rotation at the focus head Five, or even six, axes are employed because

it is desirable to cut with the beam normal to the workpiece surface As an alternative to moving optics based on the foregoing Cartesian system, robot arm systems are also available where the beam is conducted along the side of the arm, or even inside it One of the claimed advantages of Nd:YAG lasers is that their use with fibre optics considerably simplifies beam delivery

30.5 Drilling and engraving

The majority of drilling and engraving operations on metals are at present performed by pulsed Nd:YAG laser (although Nd:glass units may be used for drilling if lower repetition rates are acceptable) This is because the solid state lasers offer good coupling into metal surfaces, as well

as pulse power densities which are higher than those obtainable from most industrial COz units The high focal intensities lead to vigorous material expulsion in drilling and, in engraving, legible symbols may be generated by surface evaporation and the creation of a very thin, slightly disrupted melt layer

30.5.1 Drilling

The process arrangements for metal drilling are somewhat similar to those for cutting, i.e the beam

is focused on or close to the workpiece surface and a jet of assist gas is provided to aid expulsion

of material The laser is normally operated in free-running, pulsed-lamp mode (typical beam pulse length approximately 1 ms) and pulse energies up to 50 J may be employed Empirical process optimization can be used to choose between the regimes of high repetition rate/low pulse energy and the converse Two drilling techniques may be distinguished: percussion and trepanning In the former the beam is kept fixed with respect to the workpiece, and the hole diameter is determined largely by spot diameter, values ranging from approximately 0.1 mm to 1.5 mm Use of larger spots

leads to an unacceptable reduction of focal intensity and larger holes are therefore drilled by the trepanning technique The repetitively pulsing beam is then caused to describe on the work a circular trajectory, the diameter of which determines the diameter of the resultant hole In principle, trepanned holes of arbitrarily large size can be drilled but in practice the method is best suited to diameters up to approximately 3mm since thereafter mechanical drilling may offer a more cost-effective alternative Currently, laser drilling in metal can give hole depths in excess of 20mm Under particular circumstances, depth to diameter aspect ratios may approach 509 The trepanning technique results in excellent axial symmetry Holes are substantially parallel sided, although some degree of taper may be present, particularly in thicker workpieces However, holes can be drilled having minimal heat-affected zones and recast layers as well as absence of microcracking Furthermore, it is possible to drill holes which are inclined far off-normal to the workpiece surface Typical production applications include the drilling of metering and lubrication holes for the automotive industry and, most importantly, the large-scale drilling of cooling holes in nickel-alloy gas-turbine components for the aerospace industry

30.53 Engraving

The engraving of uncoated metal requires localized application of the focused laser beam to the workpiece surface A typical beam delivery system therefore incorporates a pair of galvanometer-type mirror units which permit steering of the beam over the work Alphanumeric characters can then

be created either via a dot-matrix array, or by tracing out the characters with a quasi-continuous

beam In the former, the laser is normally operated in free-running pulsed mode, synchronized with the line scanning system so that a dot may be produced at each co-ordinate oi the array; the beam may then be switched on or off to write the required characters In the latter, the laser is operated in Q-switched mode (tens of kilohertz) so that for typical mirror deflection speeds the written character is effectively continuous

The advantages of laser engraving of metals include the contactless nature of the marking, the minimized metallurgical and mechanical damage and the ability to engrave through glass A range

of commercially-available equipment is available Production applications include engraving of stock metal, engineering components and medical implants

30-10 Welding

30.6 Welding

Laser welding now spans a significant thickness range, from submillimetre to greater than 10mm For thin section work, Nd:YAG lasers are generally preferred because of better beam coupling and the good control associated with pulsed operation (although rapid quenching between pulses may induce cracking in sensitive materials) For sections of one or two millimetres and upwards,

CO, lasers are normally used but this range is becoming accessible to Nd:YAG with the advent

of kilowatt power levels

Although conduction-limited laser welding (carried out using beam intensities of around

5 x lo5 W/cm2) may be of interest in some applications, it is more usual to exploit the deep penetration capability of the beam by employing focused power densities of lo6 W/cmZ or greater The workpiece surface then undergoes melting and vaporization which is sufficiently intense to disrupt the melt to form a capillary or keyhole, thereby enabling the beam to penetrate relatively deeply into the material Using a pulsed or CW laser, and by suitably moving the work or beam, the keyhole translates along the join line, metal melting ahead and flowing round to solidify behind The process, like that of electron beam welding but without the vacuum requirement, thus produces welds which are energy efficient, of low shrinkage (because they are narrow) and of low distortion (because they are parallel-sided) Laser welding is particularly suited to autogenous welding, close fit-up of the parts being normally required, although filler material can be added if a gap exists; filler may be used to improve weld properties At higher CO, laser beam powers, plasma formation

at the workpiece becomes important since it can be responsible for the broadening of the weld beads, a reduction in workpiece penetration and (more desirably) a smoothing of both weld surfaces The plasma is frequently controlled by directing at it a jet of helium (which serves also to prevent

oxidation) As in the case of cutting, the best penetration is obtained with good-quality, tightly

focused beam spots However, it should be noted that, particularly with pulsed lasers, it is possible

to apply to the work power densities which are too high so that excessive evaporation and weld disruption occur Figure 30.6 provides a guide to weld penetration trends in the welding of alloys

of iron, nickel and titanium Care should be exercised in the use of the figure because of the sensitivity of values to conditions such as spot quality, plasma control, etc; in any case, it is often

Thickness mm

Figure 30.6 Illustrative performance in laser welding of steel Note: welding speeds slower than penetration-threshold values are required in order to create acceptable weld profiles; penetration at very slow welding speeds depends sensitively on pulsing and plasma effects

Figure 30.7 Laser weld configurations

necessary to reduce speed significantly below that at threshold of penetration, in order to achieve desirable weld geometry However, the capability of the laser to carry out keyhole welding at ambient pressure can permit a novel approach to component fabrication Indeed it is likely that the best exploitation of laser welding follows from a rethink of component design to match the process Figure 30.7 shows a selection of possible joint configurations

Laser welding is being applied to a wide range of materials This has involved exploration of a new regime of welding, for although it may be argued that the process is similar to that of the

electron beam, laser welding speeds are normally higher and cooling rates are faster so that different

weld properties may result Indeed there is no doubt much to be learned yet about process techniques such as the use of beam pulsing and spinning Similarly, there is scope for more work to better

understand the fine-scale scattered microporosity often seen in laser welding At higher power levels more attention is required to the overlapping and ramping out of circumferential welds Nevertheless,

laser welding is now exploited in production to achieve high-integrity joints in a wide range of

products There follow comments on the weldability of a number of materials types

Steels

Successful laser welding of mild steels requires choice of cleaner, low-carbon compositions which are not too rich in oxygen, sulphur or phosphorus otherwise weld disruption, porosity and

solidification cracking may occur If such effects are problematic, reduced welding speeds or use

of appropriate fillers may help Steels which have carbon equivalent greater than approximately

0.3% are difficult to weld without cracking because they have high hardenability and insufficient ductility to resist the shrinkage stresses encountered in many joint geometries Thus the welding

of planetary joints in gear assemblies may require shrink-fit assembly Interest in the use of

30-12 Transformation hardening

multikilowatt power levels for deep penetration welding has led to studies in the joining of

high-strength low alloy steels such as those used in pipelines and marine applications The indications here are that low-carbon casts are greatly to be preferred for the achievement of restricted hardness and acceptable impact properties, although the use of appropriate filler may help There is some evidence that flux-cored filler promotes in the weld an acicular ferrite microstructure have excellent impact properties

Most austenitic stainless steels weld extremely well by laser The welding of ferritic stainless, which is difficult by most processes because of grain growth and embrittlement, may benefit from the use of lasers because of reduced energy input

Non-ferrous alloys

Titanium and nickel, and many of their alloys, exhibit good weldability by laser On the other hand, the laser welding of aluminium and its alloys can require considerable care This is partly because of high reflectivity (in the case of the CO, laser) but more importantly because in those alloys containing Mg and Zn selective evaporation of these elements can lead to porosity and weld disruption The problem is greatly reduced when the alloying element is mainly copper

30.7 Transformation hardening

By use of a diffuse beam, the surface hardening of carbon steels and irons can be achieved by martensitic transformation The laser can be distinguished from alternative surface hardening techniques by one or more of the following attributes:

1 It characteristically operates as a rapidly moving source so that overall heat input (and therefore distortion) is minimized, and adequate quenching rates are obtained solely by conduction into the substrate

2 The beam can be manipulated and directed into bores and conventionally inaccessible regions without the hindrance of supply cables and pipes

3 Its heating patterns may be altered to suit the application

The power densities (usually 103-104W/cmz) and speeds (or beam dwell times) are chosen to

austenitize a relatively shallow case depth (usually < 1 mm) while avoiding surface melting The workpiece surface may be given a prior coating of colloidal graphite to ensure efficient COz laser

energy absorption at the surface As noted in Section 30.3.2., if access demands the use of an inclined CO, beam, it should be plane polarized to enable exploitation of the Brewster effect Illustratively, for a 0.5 mm case depth, coverage per kilowatt is approximately 65 mmz/s for cast

iron and 135 mm2/s for steel Preferred materials for laser hardening are those in which the carbon

is wiformly distributed, i.e steels in quenched and tempered conditions, and cast irons having pearlitic matrices In such materials, despite the brief thermal cycle, austenitization occurs quite quite uniformly with depth and a relatively flat hardness profile results Nevertheless, adequate hardness may be obtained in steels having a coarse normalized structure, or in ferritic irons This

is because the near-surface layer experiences temperatures close to melting and sufficient carbon diffusion may occur to yield a useful martensite Process conditions should be biased towards low intensities and long interaction times to aid diffusion However, such cases generally exhibit a strong decrease of hardness with distance from the surface

One of the key elements in laser hardening equipment is the means for achieving an extended,

uniform heating pattern A number of possible CO, laser beam techniques have been developed

to varying degrees These include: high-speed spot-rastering systems, beam scrambling based on faceted mirrors; and light pipes featuring multiple internal reflections In some applications multi-mode operation of the laser resonator may yield a beam distribution which is adequately flat-topped to avoid causing melt strips For treatment of components with special profiles, the use of tailored beam distributions may be required: for example the treatment of internal corners requires enhanced intensity there because of the increased heat-sinking; the converse is true of

external corners and knife edges The use of on-line surface temperature monitoring by optical

pyrometry and with associated feedback control of the process parameters has been demonstrated

as a useful adjunct to hardening equipment

The production implementation of laser hardening is at present much less established than say laser cutting Many of the reported applications relate to the automotive field and these include hardening of piston ring grooves and cylinder bores In the latter application, the use of discrete spiral tracks seems preferred to overall hardening

Surface cladding and 30-13

30.8 Surface cladding and alloying

These are surface treatment processes which involve substrate melting in conjunction with the addition of material to improve wear or corrosion resistance In cladding, the additive is fused and

then solidifies as a coating which is metallurgically bonded via the melting of only a very thin layer of the substrate; the composition of the final surface is close to that of the additive since it experiences little dilution by the substrate On the other hand, in alloying there is significant mixing

of molten substrate and additive and the resulting surface has composition and properties determined

by contributions from the two The attractions of using a laser for these processes concern localization of treatment, low cladding dilution, good geometrical control, efficient additive utilization and useful microstructures resulting from rapid cooling rates Although there is research and development activity on fine-scale treatments relevant to the electronics sector (including use

of Nd:YAG lasers) the current major effort is concerned with treatments for the automotive and power sectors performed mainly, but not exclusively, with CO, lasers The additive may be in two forms: solid or gas In the former it is preferable to use fhe material in powder form (cf rod or wire for example) because not only are the deposit width and thickness more readily varied, but the powder promotes better coupling of the beam for CO, lasers Normally the powder is supplied from a hopper via a delivery tube to the interaction point Transport through the tube may be under gravity or via an inert gas stream Alloying may also be carried out by arranging an appropriate gas atmosphere above the irradiated, molten surface; in this case it is usually adequate

to direct at the interaction point a laminar flow of the gas and to avoid entrainment of air The beam intensities used for cladding and alloying lie in the range 104-105 W/cm2

There is a natural tendency for cladding, rather than alloying, to occur when the additive has

a lower melting point than the substrate For example, a typically cobalt-based alloy (melting point

1340°C) can be clad on to steel (melting point 1SOOT) under conditions of minimal substrate melting Conversely alloying tends to occur when the additive has a higher melting point that the substrate; for example, in the alloying of silicon (melting point 1410°C) into aluminium (melting point 660°C) significant melting of the substrate and mixing with the silicon occur

It should be noted that all surface melting processes, including those carried out by laser, tend

to result in tensile stress in the as-treated layer This is because the recast surface, as it solidifies and cools, tries to contract but is prevented from doing so by a relatively massive cold substrate The effect is reduced by preheating the substrate and by heat-treatment after laser processing -Production-line exploitation of laser cladding and alloying is still relatively limited Most work appears to involve the cladding of cobalt-based hard-facing alloys on process plant components and on the interlock region of gas-turbine blading Research and development effort is being directed towards alloying processes for titanium and aluminium In the former, gas phase alloying using nitrogen results in a structure containing TiN and offering hardness up to 1OOOHV and more Alternatively, the introduction of carbides also leads to a surface offering high hardness In the case of aluminium, alloying with silicon can be carried out very controllably to yield strengthened microstructures having hardnesses double that of the parent alloy

However, in Class 1 operation, the installation is regarded as a laser system contained within a

protective enclosure from which personnel are excluded and which does not permit the escape of

radiation above the ‘Accessible Emission Limit’ (AEL) For example, in the case of CW Nd:YAG

or C O , lasers, it is prescribed that an operator must not encounter access to greater than 0.6 to 0.8mW of beam power Since the enclosed laser may have power of many kilowatts, it can be appreciated that much of the emphasis on safety provisions must centre on the engineering of the system so that there is a minimal risk that the beam can become errant and impinge on the enclosure

and that, if it does, it is not permitted to penetrate the enclosure The best current approaches tend

to embrace a range of fail-safe design features which may include the following:

Optical components in the beamline must be selected, mounted and maintained with care to

30-14 Bibliography

that they do not fail and allow the beam to become errant The workpiece must be presented in such a way that the beam couples to it and is not excessively reflected in a way to cause damage The enclosure must be capable of containing an errant beam: at lower laser powers, it may be adequate simply to select materials such as brick or copper; at high powers, such constructions may need to be augmented by, for example, heat-sensitive scanners which can register beam impingement on the enclosure and then shut down the laser before penetration occurs Preferably the processing should be continuously monitored: in some cases this may be done visually by a remote operators holding a dead-man’s handle switch which they release if they see something amiss; alternatively for example a simple photo-sensitive detector can register light from the normal beam-workpiece interaction and enable continued operations (loss of light would imply that the beam had become errant so that the laser would then be shut down) Where one laser can be used with several workstations, a carefully designed system of beam switching, beam isolation and door interlocks must be employed to permit safe operator access for setting-up in one workstation whilst processing takes place in another

It is important to note that, although the foregoing discussion has been concerned with safety issues to d o with the laser beam, in a materials processing installation all due care must also be taken with associated hazards including: high voltage power supplies; moving manipulators and robots; toxic fume from welding and cutting; fire due to hot spatter as well as impingment of the

transmitted or rdected beam; ultraviolet radiation from the welding plasma plume

30.10 Bibliography

Books

1 ‘An Introduction to Lasers and Masers’ A E Siegman, McGraw-Hill, 1971

2 ‘Lasers in Industry’ ed S S Charschan, Van-Nostrand-Reinhold, Princeton, New Jersey, 1972

3 ‘Industrial Applications of Lasers’ J F Ready, Academic Press, New York, 1978

4 ‘Laser and Electron Beam Processing of Materials’ ed C W White and P S Peercy, Academic Press, New

5 ‘Materials Processing, Theory and Practice, Vol 3 Laser Materials Processing’, ed M Bass, North-Holland,

6 ‘Laser Treatment of Materials’, ed B L Mordike, DGM Informationsgellschaft, 1987

York, 1980

1983

Industrial laser publications

7 Lasers and Optronics, published monthly, Gordon Elsevier Business Press (Annual Buying Guide also

8 Laser Foeus World, published monthly, Pennwell Publishing (Annual Buyers’ Guide also published)

9 ‘The Industrial Laser Annual Handbook‘, eds D Belforte and M Levitt, F’ennwell Books

published)

Laser beam principles

10 I J Spalding, ‘Characteristics of Laser Beams for Machining’, in ‘Physical Processes in Laser Materials

11 J T Luxon, ‘Optics for Materials Processing’, in the 1986 ‘Industrial Laser Annual Handbook’, eds D

12 L Marshall, ‘Applications a la Mode’, Laser Focus, April 1971, pp26-28

Interactions’, ed M Bertolotti, Plenum, 1983

Belforte and M Levitt, Pennwell Books, 1986, pp38-48

Cutting and drilling

13 D Schuocker, ‘Laser Cutting’, 1986 ‘Industrial Laser Annual Handbook’, eds D Belforte and M Levitt, Pennwell, pp87-107

14 J Fieret et a[., ‘Overview of Flow Dynamics in Gas-Assisted Laser Cutting’, Proc Conf High Power Lasers,

30 March-3 April, 1987, The Hague, SPIE Vol 801, pp243-250

15 G Brodh and H - 0 Ketting ‘Influence of the Purity of Cutting Oxygen in Laser Beam Flame Cutting’,

Schweissen and Schneiden, August 1989, DVS ppE124-126

16 J M Weick and W Bartel, ‘Laser Cutting without Oxygen and its Benefits for Cutting Stainless Steel’, Proc

Vi Int Conf Lasers in Manufacturing, 10-11 May 1989,ed W M Steen, IFS/Springer-Verlag, 1989,pp81-89

17 A Thompson, ‘CO, Laser Cutting of Highly Reflective Materials’, 1989 ‘Industrial Laser Annual Handbook‘,

eds D Belforte and M Levitt, Pennwell, 1989, pp149-153

18 A G Corfe, ‘Laser Drilling of Aero-Engine Components’, Roc 1st Int Conf Lasers in Manufactwing, 1-3 Nov 1982, Brighton, IFS/North-Holland, pp31-40

20 D T Swift-Hook and A E F Gick, ‘Penetration Welding with Lasers’, Welding Journal Research Supplement

21 P G Klemens, ‘Heat Balance and Flow Conditions for Electron Beam and Laser Welding’, J App Phys.,

52 (11) 492499s

May 1976,41 ( 9 , pp2165-2174

22 M Davis et al ‘Modelling the Fluid Flow in Laser Beam Welding’, Welding Journal, July 1986,167s-174s

23 A P Hoult, ‘Welding, Cutting and Drilling with the 1 kW Solid-state Oscillator-Amplifier Laser’, Ibid ref -

16, pp23-30

24 J Heyden et al., ‘Laser Welding of Zinc Coated Steel‘, Zbid ref 16, pp93-104

25 V Ram et al., ‘C8, Laser Beam Weldability of Zircalloy 2’, Welding Journal, July 1986, pp33-37

26 M N Watson, ‘Laser Welding of 6A1-4V Titanium Alloy’, The Welding Institute Research Bulletin, November

27 T Zacharia et al., ‘Weld Pool Development during GTA and Laser Beam Welding of Type 304 Stainless

28 J H P C Megaw et al., ‘Girth Welding of X-60 Pipeline with a lOkW Laser’, Proc SPIE/ANRT Conf

29 I J Stares et al., ‘Improved Microstructure and Impact Toughness of Laser Welds in a Pressure Vessel

30 M Sasaki et al., ‘CO, Laser Welding for Steel Strip Production Process, Proc 3rd Int Coll on Welding

1986, ~~381-385

Steel’, Parts I and II, Welding Journal Research Supplement, December 1989, ~ ~ 4 9 9 ~ 5 1 9 s

High Power Lasers and Their Industrial Applications, Innsbruck, April 15-18, 1986

Steel’, Metal Construction, March 1987 19 (3), pp123-126

and Melting by Electron and Laser Beams (CISFFEL) Lyon 5-9 September 1983 pp705-711

Surface treatment

31 W M Steen, ‘Surface Engineering with a Laser’, Metals and Materials, December 1985, pp730-736

32 D N H Trafford et al., Laser Treatment of Grey Iron’, Proc Heat Treatment ’79, The Metals Society,

33 A S Bransden et ~ l , ‘Laser Hardening of Ring Grooves in Medium Speed Diesel Engine Pistons’, Surface

34 J H P C Megaw et d., ‘Surface Cladding by Multikilowatt Laser’, Ibid ref 30, pp26S277

35 R M Macintyre, ‘Laser Hard-surfacing of Turbine Blade Shroud Interlocks’, in ‘Lasers in Mate&

1980, pp32-38

Engineering, 1986, 2 (2), pp107-113

Processing’ ed E A Metzbower, ASM 1983, pp230-239

Safety

36 ‘BS4803: 1983 Radiation Safety of Laser Products and Systems’, British Standards Institution 1983

37 ‘IEC825, Radiation Safety of Laser Products, Equipment Classification, Requirements and UserB Guide’, European Laser Safety Regulations (Note that this standard is being redrafted to include recommendations

on workstation enclosure design, and it is intended that in due course BS4803: 1983 will be aligned with this)

38 R D Ball et a!., ’The Assessment and Control of Hazardous By-products from Materials Processing with

C O , Lasers’, [bid ref 17, pp154-162

1 uide to corrosion control

31.1 Introduction

Metals may be chosen specifically for their resistance to a corrosive environment but in industry, where economic considerations affect the selection of materials, it may be less costly to choose a metal that has a comparatively short life, and carry out regular maintenance or replacement rather than a high initial capital investment in a resistant metal or alloy that will withstand the conditions of corrosion during the lifetime of the plant There are many examples where either of these two extremes has been the more economic choice, and therefore a wide choice of materials is required It may be that the most economical decision would be to use coatings (see Chapter 35),

cathodic protection or control of the environment (inhibitors, etc.) Resistant materials will be listed, where these are available, followed by the less resistant metals and alloys where shorter lifetimes may be tolerated

31.1.1 Types of corrosion

Corrosion damage to a metal or alloy can be (a) general or uniform corrosion, (b) Localized or pitting corrosion, and may be caused or enhanced by one or more of the following broad classifications:

bimetallic coupling (and dealloying),

crevice corrosion,

erosion corrosion,

stress corrosion cracking,

corrosion fatigue (and fretting corrosion),

Cydrogen embrittlement

31.1.2 Environments which cause corrosion

These may be broadly classified into three main groups:

- Sea water, river, potable, condensation

-Possible corrosion during storage, transit or erection

- Sulphuric hydrochloric, nitric, phosphoric

-Nitrogen compounds, ammonia, organic acids

-Concentration of river, well or sea water

31-2 Guide to corrosion control

Wood -Some vapours from wood are aggressive

Polymers - Some polymers, in contact with metals can cause corrosion

31.1.3 Accelerating factors

Corrosion rates may be drastically changed when temperature, flow rates, pressures and concentrations of chemicals are varied; corrosion of metals from these parameters can therefore be regarded as an ‘add on’ factor and under certain conditions of temperature, pressure and flow the corrosion rates may then become excessive There are many anomalies, e.g mild steel is not attacked by very high concentrations of H,SO, but rapidly at concentrations below 70% w/v Copper can withstand sea water but is attacked when the flow rate is excessive

31.1.4 Measurement of corrosion damage

Corrosion attack is not often uniform and it can be misleading to apply much of the published data which convert weight loss into penetration rate (mmyr-I) For instance, mild steel in sea water corrodes at approximately O.lmmyr-’, but pitting can occur up to 0.4mmyr-’ over a relatively small area of the total surface In the case of intergranular attack at the metal grain boundaries a relatively small rate of attack can cause deep penetration so that whole grains drop out Corrosion rates must also be accompanied by the type of attack and in the case of pitting by the probability of finding the deepest pit at a certain depth

In the case of high temperature oxidation and also for atmospheric corrosion, adherent corrosion products may be produced The measurement of weight gain is then recorded

Corrosion damage, although not excessive, can be very undesirable or even dangerous When metals are under tensile or cyclic stresses a small amount of pitting could give rise to stress concentrations that lead ultimately to failure by cracking Formation of corrosion products in a confined space can lead to ‘oxide jacking’ where the expansion suffered by the components can cause bursting and distortion This has been widespread in some forms of concrete reinforcement where relatively mild corrosion can give rise to serious cracking of the nearby concrete

31.1.5 Chemicals

For data on particular systems the following bibliography may be helpful

SOURCES OF CORROSION RATE DATA

Three main sources of information:

1 data books;

2 national and international standards;

3 scientific journals and abstract literature

I Data books

‘Corrosion Guide’ E Rabald, VDI, Diisseldorf, 1969

‘Corrosion Data Survey’, G A Nelson, NACE, Houston, 1967

‘Werkstoffetabelle’, DECHEMA, Frankfurt am Main, 1980

‘Materials Selector’, The Elsevier/Elsevier Science Publishers, London, 1991

2 See ‘Corrosion Prevention Directory’ HMSO, for extensive list

Bimetallic corrosion 31-3

3 Abstract literature

Metal Abstracts, ASM, Met SOC., London

Corrosion Abstracts, NACE, Houston, Texas

Corrosion Profile (Chem Abstracts), UKCIS, University of Nottingham

31.2 Bimetallic corrosion

Designers require structures and machines to have metals and alloys of differing mechanical properties in close proximity, e.g mild steel backed copper bearing surfaces, lightweight aluminium structure on a mild steel base, and many fasteners, rivets, screws etc

Table 31.1 CORROSION RATE IN mm yr-' FOR BIMETALLIC COUPLING IN SEA WATER O F COM- MON STRUCTURAL MATERIALS'*

39

32

49 0.5 2.0 2.4 2.8 1.6 0.7 2.4 2.2 1.7 2.7 2.2 3.0 1.7

4.5

2-10 2-10 2-10 0.15 0.6 0.6 0.7 0.3 0.06 0.15 0.14

0 I

0.15 0.12 0.16 0.07

0.66

0.4 0.5

0.2 0.09

0.04 0.06

7.4 2.2 2.8 1.8 0.04

2.0 1.4

2.7 0.3 2.8 0.35

1.2 0.3

*

** Based on BS PD6484 (1979)

Includes effect of anode/cathode area ratio,

Table 31.2 CORROSION RATE (mm yr-') FOR CATHODIC MATERIALS CARBON, TITANIUM AND COP-

PER 'COUPLED TO VARIOUS ALLOYS

2

0.3

0.1 0.16 0.02 0.3 0.01

0.16 0.22

1.4 0.2 0.14

0.25 0.05 0.02 0.005 0.004 0.002 0.002

31-4 Guide to corrosion control

In these cases, providing the environment is sufficiently conducting, serious acceleration of corrosion may occur However, compatibility can be achieved

Table 31.1 gives the accelerating effect on coupling various materials with sea water as the conducting electrolyte Comparable results may occur with other conducting chemical solutions For electronic materials with thin metallic coatings in humid conditions, corrosion products may affect performance

31.2.1

Various noble metals are used to provide good conducting, tarnish-free contacts which are reliable over the life-time of the equipment These metals are used generally as very thin films (1-5 pm) are usually porous They are therefore likely to act as a good cathode and induce corrosion even under slightly humid conditions Thus, the connector can become covered with a thin film of

corrosion products which can reduce the performance of the electronic device Frequently this corrosion effect occurs in handling and in storage It can also arise with unsatisfactory packaging, and the bimetallic coupling encourages attack

Table 31.3 contains the combinations which have given satisfactory service and those that have been known to cause corrosion should the environment permit

Table 31.3 SEVERITY OF GALVANIC CORROSION FROM METALLIC COMBINATIONS

Coatings are shown in brackets: (Ni)Cu = nickel-plated copper; (r.Sn)Cu = reflowed tinned copper; (s.d.)Cu = solder- dipped copper

Bimetallic coupling associated with electronic materials

AI-Brass (Au)Cu-(s.d.)Cu

Al-(Ni)Cu AI-(Ni)Brass AI-(Ag)Cu (Sn)Al-(Ag)Cu AI-(Au)Cu

(Sn) AI-(Au) Cu

' No copper undercoat ** Zincate process

31.2.2 Dealloying-selective dissolution as a form of bimetallic corrosion

A special case of bimetallic corrosion occurs for certain alloys where the base metal can be

preferentially dissolved Copper-zinc alloys are prone to this corrosion, and dezincification can be

a serious corrosion problem resulting from the chemical composition of some natural waters In particular, high chlorides and high temperatures lead to corrosion of the zinc alloying element leaving the copper as a porous mass The component may keep its shape but has weak mechanical properties Information about these water supplies may be obtained from the British Non-Ferrous Metal Association The standard test, I S 0 6509, for susceptibility to dezincification requires the immersion of a sample of the alloy in 1% copper chloride at 75°C for 24 h The corrosion can be

reduced by a 1 % alloy addition of Sn or by 0.02-0.06% of Sb or P Copper alloys with resistance

to dezincification are given in Table 31.4

Table 31.4

DEZINCIFICATION

SINGLE AND TWO PHASE COPPER-ZINC ALLOYS AND THEIR SUSCEPTIBILITY TO

Alloy type Dezinciflcation Alloy addition to reduce rate

Copper 85% and above

Copper 70-85%

Copper 60-70%

N o Susceptible Susceptible

- Arsenic up to 0.1%

Tin up to 1.0% with arsenic 0.1%

Crevice corrosion 31-5

31.3 Crevice corrosion

Many engineering designs place metals together for joining, or create narrow slots or pockets where liquids could be retained Such crevices include screw threads, nuts, washers, gaskets, some weldments, heat exchanger rolled in tubes, valve packings, etc In neutral aerated waters there is the strong possibility of corrosion within these crevices, particularly with strongly passive metals

in chloride solutions such as sea water Practically all metals can suffer from this form of attack

and the usual remedy is to remove the crevice by careful design of the fit of the components, or by

sealing or coating Table 31.5 gives an order of resistance to crevice corrosion which shows that some metals show good resistance It is interesting to note that many of the popular stainless steels can bse affected to a serious extent by crevice corrosion

Table 31.5 RESISTANCE TO CREVICE CORROSION

Metal

Moderate Severe Very resistant + corrosion -+ corrosion

Fe l8Cr 13Ni 3Mo 2Si NNG

Fe 18Cr 14Ni 2Mo NiTi

Fe l8Cr 24Ni 3Mo 2Cu

Fe 20Cr 25Ni 5Mo 1.5Cu

Fe l8Cr lONi 2.5Mo 2.5Si

302 13:< Cr

304 16% Cr

* Ti 0.2:; Pt 25 Mo(or 2:; Ni) relatively more resistant

31.4 Corrosion/erosion resistant materials

Flowing electrolytes can increase corrosion rates that are dependent on diffusion This usually applies to aerated solutions Corrosion rates in mm yr-' for flowing sea water are given in Table 31.6 and for copper central-heating tube in Table 31.7