Materials Science and Engineering Handbook Part 10 pot

Table 5 Resin-matrix composite open-mold processing characteristicsCure operation Process Resin Reinforcement Filler Thermoplastics Thermosets Fiber Undirectional continuous fiber tape

Table 5 Resin-matrix composite open-mold processing characteristics

Cure operation Process Resin Reinforcement Filler Thermoplastics Thermosets Fiber

Undirectional continuous fiber tape

Yes Yes Yes Multidirectional

parallel to mandrel

Braiding Wet resin

impregnation after winding

Continuous fiber

Yes Yes Yes Parallel to braid

mold surface, multidirectional fibers

Yes Yes Yes Multidirectional,

parallel to mandrel surface

Fiber, fabric mat, short continuous

Yes Yes Yes Random,

parallel to mold surface

Fiber-tape

lay-down

Prepreg tape, wet tape

Continuous unidirectional fiber tape

Yes Yes Yes Multidirectional,

parallel to mandrel

Medium Low

Pultrusion Wet resin

impregnation

of fibers during pull through die

Continuous fiber, fiber mat

Yes Yes Yes Parallel to

laminate exterior surface and mold surface

Yes Yes Yes Parallel to mold

Calendering Semiliquid

resin, catalyst, cure agent

Fiber, powder Yes Yes No Parallel to sheet

surface

Medium Low

Source: Ref 6, 7, 8, 9, 10, 11

(a) Pressure ranges: low, 100 kPa ( 15 psi); medium, 100-1725 kPa (15-250 psi); high, up to 100 MPa (15 ksi)

(b) Temperature ranges: low, room temperature to 165 °C (330 °F); medium, 165-190 °C (330-370 °F); high, 190-205 °C (370-400 °F); very high,

205-815 °C (400-1500 °F)

Table 6 Metal-matrix composite processing

P/M process Aluminum, titanium, or

stainless steel powder

Silicon carbide powder Temperature, pressure, and

time sintering

Extrusion Roll sheet, plate Forging

Rotating mandrels

Cospray of molten matrix

and SiC whiskers on

rotating mandrel

Aluminum, titanium, or stainless steel melt

Silicon-carbide whiskers/powder Carbon, boron

Spray pressure Mandrel

rotation Machine finish

Filament wind with fiber

and resin/powder matrix

Aluminum, titanium, or stainless steel powder Resin (thermoplastic, thermoset)

Metal fiber (aluminum, titanium, or stainless steel) prepregged with resin, powder matrix

Pressure and temperature

on mandrel

As wound and surface coated

Filament wind with fiber

and spray with molten

metal matrix (plasma-arc

spray)

Aluminum, titanium, or stainless steel powder pressure molten spray between fiber layers

Metal fiber or man-made fiber (boron, SiC, alumina glass, carbon) filament wound on mandrel

Pressure temperature consolidated Removed from mandrel in sheet form and molded to structural shape

As molded and surface coated

Hot press/diffusion bond

Hot, mold of

metal/reinforced fiber

Aluminum, titanium, or stainless steel foil or thin sheet

Glass, carbon, boron, graphite depending on metal melt temperature

Mold temperature pressure/inside sealed vacuum retort

Machine and finish

Diffusion bond Aluminum, titanium, or

stainless steel thin foil

Metal fiber or man-made fiber (boron, alumina glass, carbon)

Wound on steel tube, sealed and evacuated with metal bag

Isostatic pressure in furnace

Machine and finish

Multiple hot press and

diffusion bond

Aluminum, titanium, or stainless steel thin metal foil

Metal fiber or man-made fiber (boron, alumina glass, carbon)

Thin-pressed metal-fiber sheets diffusion bonded Sheets superplastic formed

to shape with temperature, pressure

Machine and finish

Compressed preform densified

Compressed preform in CVD pyrolytic carbon

infiltrated into preform in

Preform reinforcement of ceramic, metal, or man-made fibers, in particle, whisker,

Vacuum, inert gas purge temperature conversion of organic gas into carbon

Machine and

CVD furnace CVD furnace short- or long-fiber form deposition and H 2 finish

Compressed preform in

hot-melt metal matrix

Molten metal pressed, vacuumed, or wicked into preform reinforcement

Preform reinforcement of ceramic, metal, or man-made fibers, in particle, whisker, short- or long-fiber form

Temperature, pressure, time consolidation

Forge, extrude, or roll Machine and finish

CVD, chemical vapor deposition

Carbon (rayon, pan, pitch base), graphite fiber prior processed to 1650-2760 °C (3000-5000 °F) for fiber weaving dry on a rotating bulk graphite mandrel with radial fibers Filament- wound axial, hoop fibers

Vacuum pressure impregnation with pitch resin 1650 °C (3000

°F) carbonized in inert vacuum

Repeat cycle five times to specific gravity = 1.90 max

Optional final graphitization at

to open voids for the next densification cycle

Two-dimensional

fabric tape-wound

carbon/carbon

Same as three dimensional

Prepreg carbon phenolic fabric laid up, vacuum bagged, and cured at 165 °C (325 °F) and 1720 kPa (250 psi) for up to 18 hours Heat-

up, cure, cool down

Carbonize billet, vacuum pressure impregnate with pitch resin and cure Repeat cycle six times to specific gravity = 1.80 max Final graphitization at

2760 °C (5000 °F) is optional

Machine and finish after each

impregnation and carbonization cycle may be necessary to open voids for next impregnation

Carbon or graphite fiber filament wound shape with helical, polar, or hoop windings with or without coupling agent and binder resin

CVD of pyrolytic carbon into filament wound fiber preform may take one to five cycles depending on woven preform, density desired, and the surface buildup and penetration Final graphitization at 2760 °C (5000

°F) is optional

Machine and finish after each

impregnation may be necessary to open voids for infiltration

Static (male/female) open/closed mold mandrel

Carbon or graphite short fibers are felted into fiber preform

Same as two-dimensional filament wound for rotating mandrels

Same as dimensional filament wound for rotating mandrels

two-Compressed fiber

preform powder,

Resin or CVD impregnation or

Bulk graphite or graphite Multiple resin/CVD Machine and shape

per densification

whisker particles both particles preform densification cycles cycle

Source: Ref 2, 4, 14, 15

Resin-Matrix Processing

Closed mold methods of fabrication are grouped into four families having similar characteristics (Ref 6, 7, 8, 9, 16)

Preform molding in transfer, compression, or resin-transfer molds uses a preform of fiber or fabric in a resin matrix Fiber orientation is parallel to the mold surfaces

Preform flow-die molding by injection, extrusion, or reaction injection uses a preform of fiber or fabric in a resin matrix that is forced through a forming die at high pressure and high temperature and then is cured Fiber orientation

is parallel to the mold centerline or to the mold surfaces

Thin-shell molding by blow molding, rotational molding, or slip casting uses a liquid resin plus a preform of fiber in a resin matrix that is cured with a temperature, pressure, time cure cycle Fiber orientation is parallel to the mold surfaces

Miscellaneous molding processes such as foam, lost core, and thermoforming use short fibers in a resin matrix that is cured using a cure cycle Fiber orientation is parallel to the mold surfaces

Open-mold methods are grouped into the following three families by their characteristics (Ref 6, 7, 8, 9)

Moving-mold mandrel processes (filament winding, braiding, and fabric-tape wrapping) use a continuous fiber or fabric with a resin matrix that is wound on a moving mandrel and is subsequently cured Fiber orientation is parallel to the mandrel surface

Stationary-mold mandrel processes (fabric hand lay-up, chopped-fiber gun lay-up, fiber-tape lay-down, fiber pultrusion through a die, fabric structural-shape lamination) use a resin matrix that is cured after the required thickness is achieved Fiber orientation is parallel to the mandrel surface

Miscellaneous molding processes (such as casting and calendering) use a liquid resin, with or without a fiber reinforcement, shaped to the mold surface by the mold cavity or rollers and cured

Whereas as the closed-mold tooling includes the capacity for pressure and temperature control over time, open-mold processes require additional equipment for the resin-cure cycle such as:

• Electron-beam or ultraviolet light resin-cure systems

The additional curing equipment ensures a uniform distribution of the resin matrix, a highly uniform density, and a quality component

high-Metal-Matrix Processing

Metal-matrix processing involves higher temperatures and pressures for laminate metal-matrix solidification than matrix processing does Both open- and closed-mold presses are used in conjunction with plasma-arc metal-spray equipment The family of processes used for metal-matrix composites are listed below (Ref 4, 12, 13)

resin-Powder Metallurgy. Aluminum, titanium, or stainless steel powder plus fiber reinforcement is compacted at a pressure

of 21 to 28 MPa (3 to 4 ksi) and sintered at temperatures up to 1760 °C (3200 °F) to form solid right-cylinder billets in closed molds Subsequent forming to the final shape is by extrusion, rolling, or forging

Rotating-mandrel processes are used to deposit the following material forms:

shapes

• Filament winding of continuous reinforcement fiber and plasma arc spraying of the metal matrix (aluminum, titanium, or stainless steel) for plates and shapes

plates and shapes

The material is processed on the open mandrel under a low pressure (up to 100 kPa, or 15 psi) and at low temperature (up

to 165 °C, or 330 °F) for final solidification The metal mandrel may or may not be removed, depending on the design concept

Hot Pressing and Diffusion Bonding. A matrix of carbon, boron, silicon carbide, stainless steel, or titanium in the form of thin metal foil is reinforced with continuous fiber to form a flat sheet preform, which is subsequently densified by pressure and temperature diffusion bonding High pressure (up to 100 MPa, or 15 ksi) and ultrahigh metal melt temperature (up to 2760 °C, or 5000 °F) is required for solidification to final shape Shapes are then machined and final finished The temperature used depends on the metal matrix and fiber types used

Compressed Reinforcement Preform with Matrix Infiltrations. The preform reinforcement used is usually a metal or organic short fiber pressed into a sheet or solid billet preform, with infiltration of molten aluminum, titanium, or stainless steel by pressure impregnation or carbon infiltration by chemical vapor deposition (CVD) The pressure used may vary from low (0 to 100 kPa, or 0 to 15 psi) for CVD, to high (up to 100 MPa, or 15 ksi) for molten metal, and temperature is very high (up to 2760 °C, or 5000 °F) The temperature used again depends on the metal matrix and fiber types used

Additional process equipment for the metal-matrix composite fabrication includes:

• Closed molds with temperature, time, and pressure controls

This equipment ensures a matrix laminate having a uniform density with a minimum of subsurface discontinuities

Carbon/Carbon Matrix Processing

Carbon/carbon processing methods involve the use of the highest temperature and pressure open molds and vacuum/inert

carbonization/graphitization of the resin and/or CVD densification cycles (Ref 2, 4, 15)

Rotating mandrels of bulk graphite are used to deposit a fiber (two- or three-dimensional) lay-down of a reinforcement preform that is subsequently pressure impregnated with a resin, cured, and carbonized The resin/carbonization cycles may be repeated up to eight times to achieve the density goals Chemical vapor deposited carbon may also be infiltrated into preform in the late densification cycles as a surface and subsurface strengthening agent and as an oxidation-resistant material

Filament winding, braiding, or fabric-tape wrapping is used to produce two-dimensional fiber preforms or a dimensional fiber shape having radial in-wound or drilled/bonded-in-place radial-rod reinforcements The fiber or fabric can be preimpregnated with resin or post-wind pressure impregnated with resin and then cured The preform is

three-subsequently carbonized and/or carbon (CVD) densified This cycle is repeated three to eight times and may be graphitized to a final density of 1.45 to 1.95 g/cm3

Static (Male/Female) Open/Closed Mold Mandrels. A fiber mat or a fabric-sheet pattern lay-up is resin transfer molded with resin or carbon CVD densification that is carbonized/graphitized and repeat cycled to the density goal

Alternatively, a fiber preform is placed in a closed chamber and vacuum-pressure resin impregnated, cured, carbonized three to eight times, and graphitized A variation of this second process is to carbon CVD infiltrate the preform after 2 to

4 cycles of resin char processing The CVD process is continued for 2 to 3 cycles, with machining/cleaning of the outside surfaces after each cycle to open the preform-billet pores and void areas for further impregnation and densification

Carbon-fiber/carbon-resin-char-matrix processing is accomplished first by a resin-cure cycle at 163 °C (325 °F) and a pressure of 1725 to 6900 kPa (250 to 1000 psi) in a vacuum-bag enclosure Second, the cured resin is carbonized and graphitized in inert vacuum chambers Carbonization requires a 1370 to 1930 °C (2500 to 3500 °F) temperature and a time of approximately one week, and graphitization requires a temperature of 2480 to 3040 °C (4500 to 5500 °F) and a one-week period The reimpregnation of the pores and void cavities of the charred component, after machining the surfaces and cleaning, is at a pressure of 10.3 to 17.2 MPa (1500 to 2500 psi) in an inert vacuum chamber heated to 540 to

1095 °C (1000 to 2000 °F) for less than one week An occasional carbon (CVD) infiltration into the carbonized preform is performed by processing in an inert vacuum furnace at 1370 to 1930 °C (2500 to 3500 °F) for less than one week

Equipment in addition to the open graphite mandrels for carbon/carbon matrix processing includes carbonization and graphitization furnaces and carbon (CVD) infiltration chambers In addition, resin-impregnation pots are used to fill the fiber preforms with a pitch or phenolic-resin system Lathes or large turning machines are used after each densification cycle to prepare a fresh surface for the next resin-impregnation/char-densification cycle and to ensure final dimensional control The open graphite mandrel is used for fabrication of the carbon/carbon fiber filament-wound preform, while closed chambers are used for the resin impregnation, carbonization, graphitization, and carbon (CVD) infiltration densification cycles

Additional information about processing of composites is provided in the article "Design for Composite Manufacture" in this Volume

References cited in this section

2 Engineered Materials Handbook Desk Edition, ASM International, 1995, p 477, 532-582, 1057-1094

4 Composites, Vol 1, Engineered Materials Handbook, ASM International, 1987, p 118, 119, 355, 360, 363,

373, 381, 405, 410, 861, 862

6 R Flinn and P Trojan, Engineering Materials and Applications, Houghton-Mifflin, 1990, p 618, 619

7 Handbook of Advanced Material Testing, Marcel Dekker, 1995, p 945-948, 950, 955

8 E.P DeGamo, Materials and Processes in Manufacturing, 4th ed., Macmillan, 1974, p 190-212

9 C.A Harper, Ed., Handbook of Plastics, Elastomers and Composites, 2nd ed., McGraw-Hill, 1992, sections

1.5, 4.4, 5.31, 11.2

10 R.B Seymour, Reinforced Plastics, ASM International, 1991, p 9, 51

11 "Fiberglass Reinforced Plastics," Owens-Corning Corp., 1964, p 24-30

12 C.T Lynch and J.P Kershaw, Metal Matrix Composites, CRC Press, 1972, p 16, 17, 51

13 K.A Lucas and H Clarke, Corrosion of Alumina Based Metal Matrix Composites, John Wiley & Sons,

1993, p 17, 20, 21

14 Carbon Composite and Metal Composite Systems, Vol 7, Technomic

15 G Savage, Carbon-Carbon-Composites, Chapman Hall, 1993, p 95, 97, 98, 101, 125, 126, 129, 200

16 L Edwards and M Endean, Manufacturing with Materials, Butterworths, 1990, p 73, 145

Effects of Composition, Processing, and Structure on Properties of Composites

R Laramee, Intermountain Design Inc

Mechanical Properties of Composites

Within a composite laminate consisting of a reinforcing fiber/fabric, fillers, coupling agent and resin, metal, or matrix system, the greatest influence on mechanical properties is the reinforcement type and its percentage of the total constituents That is, among composite components that have the same fiber orientation in the laminates and the same matrix material, the component having the highest-strength fiber and the greatest percentage (by weight) of fiber in the laminate will exhibit the greatest strength Likewise, in the component, the highest strength exists in the planes with the highest percentage of fiber (Fig 1) For a three-dimensional block, strength is greatest in the vertical and axial fiber directions For a two-dimensional block, strength is greatest in the axial direction

carbon-The decrease in laminate strength with an increase in temperature, as shown in Fig 4 and 5 for glass and carbon fibers in

an epoxy-resin matrix, is caused by the softening and a weight loss of water and solvent in the resin system Resin weight loss begins at 120 to 175 °C (250 to 350 °F), and resin conversion to a porous char state begins at 315 to 760 °C (600 to

1400 °F) Resin matrix carbon retention for carbon matrix conversion at 760 °C (1400 °F) is 55% for phenolic, 17% for polyester, and 10% for epoxy (novolac) Fiber strength does not degrade significantly until the following temperatures are reached:

Temperature Fiber

Fig 4 Effect of temperature on the strength of S-glass-fiber/epoxy-matrix composites (a) Tensile strength (b)

Elastic modulus Source: Ref 4

Fig 5 Effect of temperature on the strength of carbon-fiber/epoxy-matrix composites (a) Tensile strength (b)

Elastic modulus Source: Ref 4

The strengths of metal-matrix composites using three different nonferrous alloys as the matrix materials are shown in Table 8 As can be seen, the titanium alloy provides higher strength and modulus (and higher density and cost) than the aluminum-alloy matrix Of the available reinforcing fibers, boron, glass, and carbon provide the highest composite strengths, while graphite, silicon carbide (SiC), and aluminum oxide (Al2O3) provide the highest composite modulus

Table 8 Typical mechanical properties of metal-matrix composites

strength(b)

Tensile modulus(b)

Matrix material(a)

T-300 carbon Fiber 35-40 1034-1276 (L) 150-185 (L) 110-138 (L) 16-20 (L)

1490 (L) 216 (L) 214 (L) 31 (L) Boron Fiber 60

(b) For fiber-reinforced materials, properties are reported in the direction of the fiber (L) or transverse to the fiber (T)

(c) Coated SiC fiber

A variety of carbon-reinforcing-material types (fiber, fabric, felt, particles) can be bonded together with a carbon or graphite form of a phenolic or pitch resin-matrix system or a CVD carbon- or graphite-matrix system to form carbon/carbon composite shapes At least six types (having specific gravities of 1.30 to 2.00) exist for the fabrication of laminates, rings, and cones by molding, tapewrapping, lay-up, or filament winding Figure 6 shows the properties of eight such types Of these, the material that uses a carbon fiber in a CVD carbon matrix has the highest tensile strength and percent strain, while the material that uses carbon fiber in a high-melt-temperature pitch-resin matrix exhibits the highest modulus

Fig 6 Room-temperature mechanical properties of carbon-fiber/carbon-matrix (carbonized resin/CVD carbon)

composites (tensile hoop rings) Source: Ref 15

References cited in this section

1 P.K Mallick, Materials Manufacturing and Design, Fiber Reinforced Composites, 2nd ed., Marcel Dekker,

1993, p 16, 71, 213, 289, 301, 373, 390, 476-478, 533

4 Composites, Vol 1, Engineered Materials Handbook, ASM International, 1987, p 118, 119, 355, 360, 363,

373, 381, 405, 410, 861, 862

15 G Savage, Carbon-Carbon-Composites, Chapman Hall, 1993, p 95, 97, 98, 101, 125, 126, 129, 200

Effects of Composition, Processing, and Structure on Properties of Composites

R Laramee, Intermountain Design Inc

Effects of Composition Variations on Properties

A composite material can be "tailored" to a product application by varying the composition and/or percentage of the total composition weight or volume of any or all parts of a composite composition, such as the matrix, fiber reinforcement, and coupling agent

An S or E glass fiber reinforcement will give the highest strength at the least cost, while a graphite fiber provides the highest modulus at the highest cost Fiber content in a matrix can vary from 33 to 66% by weight, can be a unidirectional continuous fiber or a two-directional woven fiber fabric, and the angle orientation between successive plys of material can

be changed for further strengthening A fiber/matrix coupling agent at 1 to 2% (by weight) on the fibers can help the laminate become stable, uniform, continuous, low in void content, and homogenous, thereby enhancing properties up to

25% In addition, a filler can modify the modulus, strength, resin flow (viscosity), uniformity, ease of processing, and the density of a resin-matrix material Hollow microspheres of carbon in the resin matrix can lower the composite density by

5 to 15% and decrease strength levels by 15 to 25% with an addition of only 1 to 10% by weight of spheres Fillers in the resin matrix are usually kept to a low percentage (by weight), but may increase up to 20 to 30% when used in powder or whisker form

It should be noted that two or more different fibers in a resin, metal, or carbon matrix can be formulated successfully as long as they are cure or bond-matrix compatible and their service performance is satisfactory

Variations in composition can include the following significant factors:

Fig 7 Effect of fiber type on the flexure strength and impact strength of fiber/epoxy composites Source: Ref 1

Figure 8 shows the increase in composite strength and modulus with an increase in the percent of glass fiber in a fiber/polyester laminate; this same trend also exists for carbon felt in a CVD carbon- or graphite-matrix material (Ref 12)

The use of longer-length reinforcing fibers in a resin-matrix composite also provides higher strength than shorter fibers (Table 9)

Table 9 Effect of chopped-fiber length on the mechanical and thermal properties of E-glass/phenolic composites

13.8- 3.5

2.0-

241-276

35-40 1.5-2.5

13.8- 2.5

2.0-

241-276

35-40 1.5-2.5

17.2- 3.0

2.5-

207-241

30-35 1.0-1.5

20.7- 3.5

3.0-

172-207

25-30 1.0-1.5

E-glass content, 30 to 40 wt%

Source: Ref 7

Fig 8 Effect of glass fiber content on the longitudinal mechanical properties of pultruded E-glass/polyester

composite sheets Source: Ref 1

As shown in Fig 9 (for a resin-matrix composite) and Fig 10 (for two metal-matrix composites), strength of a composite

is also highest when the fiber direction is parallel to the loading direction (0°) and the lowest when the fiber is oriented perpendicular to the loading direction (90°) (where strength is a measure of resin strength only)

Fig 9 Effect of fiber orientation on the strength of carbon-fiber/epoxy composites Source: Ref 1

Fig 10 Effect of fiber orientation on the creep strength of two metal-matrix composites reinforced with

boron/SiC fiber Titanium-matrix composite has matrix of Ti-6Al-4V; test temperature 425 °C (800 °F) Aluminum-matrix composite has matrix of 6061 aluminum; test temperature 300 °C (575 °F) Source: Ref 17

Another factor that affects mechanical properties is laminate thickness As shown in Fig 11, as the plate thickness of carbon/fiber epoxy resin-matrix composite increases from 12 to 18 plies (13 to 22 mm, or 0.5 to 1.0 in.), strength and modulus decrease as more void areas and/or resin-rich areas of lower local properties are introduced into the laminate

Fig 11 Effect of laminate thickness on the mechanical properties of unidirectional AS4 carbon-tape/Epon 828

epoxy composites processed at 100 psi and 170 °C (275 °F) for 2 h 1 ply = 0.7 mm (0.028 in.) Source: Ref

18

Coupling agents such as silane compounds placed on the reinforcing fiber prior to bonding to the resin matrix will mostly increase the tensile strength of the composite in wet or dry environmental conditions (Table 10) The strength increases will depend on the type of coupling agent and its compatibility with the resin system

Table 10 Effect of silane coupling agents on the strength of E-glass fiber-reinforced polyester rods

Dry strength Wet strength(a)

(a) After boiling water at 100 °C (212 °F) for 72 h

References cited in this section

1 P.K Mallick, Materials Manufacturing and Design, Fiber Reinforced Composites, 2nd ed., Marcel Dekker,

1993, p 16, 71, 213, 289, 301, 373, 390, 476-478, 533

7 Handbook of Advanced Material Testing, Marcel Dekker, 1995, p 945-948, 950, 955

12 C.T Lynch and J.P Kershaw, Metal Matrix Composites, CRC Press, 1972, p 16, 17, 51

17 J.A Lee and D.L Mykkanen, Metal and Polymer Matrix Composites, Noyes Data Corp., 1987, p 113, 150

18 Design and Manufacturing Advanced Composites, Fifth Conference, ASM International, 1989, p 89,

94-95, 143-145, 164-165

Effects of Composition, Processing, and Structure on Properties of Composites

R Laramee, Intermountain Design Inc

Effects of Processing on Properties

For resin-, metal-, and carbon-matrix composites, the basic parameters of temperature, time, and pressure are the key variables of the processing cure, the solidification bonding, or the conversion of resin to carbon char

The cure process used for the resin matrix, with and without a post-cure cycle, can make a difference in the compressive strength and modulus of the matrix material The conventional and optimized cure processes are listed in Table 11 A higher compressive strength and modulus result from the optimized process This is caused by the higher cure pressure and preheat temperature used, plus the longer time used in the vacuum-bag application

Table 11 Effect of processing-cure parameters on the compressive properties of graphite-fabric/epoxy composites

Material preheat

plate, plies

Post

cure

MPa ksi GPa 10 6

Conventional None None 607 88 1 2/none None Yes 270 39.3 42.7 6.2

Material: MXG 7620/2534 (Fiberite Corp.); 0° and 45° fabric orientation; 6.4 mm (0.25 in.) thickness (16 plies)

Generally, use of a post cure for laminates or structural shapes will lower the percent of residual volatiles and increase the properties (interlaminar shear strength), provided the temperature and time is optimized Some post-cure temperatures and duration times can actually lower the properties of the original composite, if caution is not used, as shown in Fig 12(a) The premise of a lower void content enhancing properties is shown in Fig 12(b); plus, a higher autoclave pressure (150 psi) provides a higher shear strength at 0% void content

Fig 12 Effect of interlaminar shear strength of (a) post-curing temperature for a glass/phenolic composite and

(b) void content for a carbon/epoxy composite Source: Ref 1, 7

In metal-matrix processing, the layers of reinforcing fiber in a metal matrix are diffusion bonded with pressure, temperature, and time until the bonding operation is complete However, the right combination of diffusion-bond parameters, depending on the thickness, size, shape, and complexity, needs to be optimized (Fig 13) In this example, 1

h at 425 °C (800 °F) and a 345 MPa (50 ksi) bond pressure provides an optimized tensile strength of 167 MPa (24.2 ksi)

Fig 13 Effect of bond time and temperature on the longitudinal strength of boron-fiber/aluminum-matrix

composites Fiber content, 3.3 vol%; bonding pressure, 345 MPa (50 ksi) Source: Ref 12

The matrix processing of carbon/carbon composites also involves temperature, pressure, and time parameters In this instance, the parameters are those required to complete the conversion of the phenolic, pitch, or furane resin systems to a charred carbon matrix and its eventual densification through multiple impregnation and char conversion cycles After final densification, the composite shape is brought to the full carbonization (1320 to 1870 °C, or 2400 to 3400 °F) or graphitization (1980 to 2980 °C, or 3600 to 5400 °F) temperature to finish the process Carbonization provides a higher final strength, a higher density, and a higher weight loss due to oxidation than does graphitization

A high graphitization temperature (2000 to 2400 °C, or 3630 to 4350 °F) provides a microstructure having a smaller separation (in angstrom units) between carbon platelets, resulting in a form of carbon/carbon composite that is more heat stable and exhibits a higher thermal conductivity, lower thermal expansion, lower density, and a higher oxidation resistance to weight loss Thus, the A and B carbonization composites in Fig 14 display a higher weight loss earlier than the graphitized composites, and the graphitized composites took 20 min longer to reach the same weight loss as the carbonized composites

Heat treat temperature Oxidation temperature

References cited in this section

1 P.K Mallick, Materials Manufacturing and Design, Fiber Reinforced Composites, 2nd ed., Marcel Dekker,

1993, p 16, 71, 213, 289, 301, 373, 390, 476-478, 533

7 Handbook of Advanced Material Testing, Marcel Dekker, 1995, p 945-948, 950, 955

12 C.T Lynch and J.P Kershaw, Metal Matrix Composites, CRC Press, 1972, p 16, 17, 51

15 G Savage, Carbon-Carbon-Composites, Chapman Hall, 1993, p 95, 97, 98, 101, 125, 126, 129, 200

19 S.Y Yang, C.K Huang, and C.B Wu, Influence of Processing on Quality of Advanced Composite Tools,

SAMPE J., April, 1996, p 37

Effects of Composition, Processing, and Structure on Properties of Composites

R Laramee, Intermountain Design Inc

Effects of Design Loadings and Environmental Conditions on Composite Properties

The types of loadings and environmental conditions existing for an applied composite product design must be considered

in advance for the composite to successfully meet the required performance life and operating conditions of the system

Adequate bonding of joints between composites or between a composite and a metal requires a minimum of 1 in or more lap to develop a usable strength level for single- and double-lap joints and from 1.5 to 2.0 in for strap, step, and scarf joints (Fig 15)

Fig 15 Effect of lap length on strength of the adhesive bond (a) Joint configurations (b) Boron/epoxy

composites epoxy bonded to aluminum (c) Fiber glass bonded to fiberglass and to aluminum (single-lap bonds; epoxy adhesive) Source: Ref 1

The presence of a hole in a flat laminate will lower the failure stress by an average factor of three to five, due to the stress concentration caused by the hole This is shown in Fig 16, which also shows the effect of fiber angle in respect to the load

Fig 16 Effects of fiber orientation and cut-out on the failure stress of boron/epoxy composite plates Source:

Fig 17 Comparison of the fatigue strength (tension-tension loading) of an aluminum alloy and several

unidirectional resin-matrix composite materials Source: Ref 17

Fig 18 Effect of type of resin matrix material on the fatigue strength of glass-fabric/resin composites Source:

Ref 20

Fig 19 Effect of resin matrix material content on the fatigue strength of ±5° glass-fiber/epoxy composites

Source: Ref 20

A glass/phenolic exterior panel stored, weathered, or stress weathered in Florida could lose 15% or more in strength after

3 years (Table 12) Table 10 indicates the tensile strength of dry and wet glass/polyester composites The effects of exposure to room-temperature water, boiling water, salt, and acid are given in Table 13 and 14 and Fig 20 and 21 As the resin-matrix composite gains weight in boiling water, it experiences a corresponding loss of properties

Table 12 Weather serviceability data for a glass/phenolic composite

Tensile strength, MPa (ksi) 408 (59.2) 401 (58.2) 367 (53.2) 349 (50.6)

Compression strength, MPa (ksi) 278 (40.3) 291 (42.2) 254 (36.8) 201 (29.1)

Flexure strength, MPa (ksi) 421 (61.0) 408 (59.2) 356 (51.6) 333 (48.3)

Modulus of elasticity, GPa (10 6 psi) 24.8 (3.6) 24.7 (3.58) 22.1 (3.2) 18.6 (2.7)

Material: heat-resistant phenolic glass fabric composite conforming to MIL-R-9299 (outdoor Florida weathering)

Source: Ref 7

Table 13 Effect of exposure to boiling water on interlaminar shear strengths of type P1 glass phenolic laminate

Interlaminar shear strength

Fig 20 Effect of time in boiling water on the weight and volume gain of type P1 glass/phenolic composites

Source: Ref 7

Fig 21 Effect of time on the moisture absorption of glass/phenolic composites exposed to salt, acid, and water

Source: Ref 7

References cited in this section

1 P.K Mallick, Materials Manufacturing and Design, Fiber Reinforced Composites, 2nd ed., Marcel Dekker,

1993, p 16, 71, 213, 289, 301, 373, 390, 476-478, 533

7 Handbook of Advanced Material Testing, Marcel Dekker, 1995, p 945-948, 950, 955

17 J.A Lee and D.L Mykkanen, Metal and Polymer Matrix Composites, Noyes Data Corp., 1987, p 113, 150

20 Composites Materials Testing and Design, STP 497, ASTM, p 98, 146, 378, 507, 509-510, 556, 559,

588-600

Effects of Composition, Processing, and Structure on Properties of Composites

R Laramee, Intermountain Design Inc

Conclusions

Composite material development design concepts should be tested for confirmation of material supplier test properties and design loaded with test fixture criteria for subscale and full-scale application Existing materials and fabrication methods for composite design should be used wherever possible to control the unknown factors involved in advanced-concept applications

It is important to have a thorough understanding of the following factors for composite material applications:

• The material composition and the percentage (by weight) of all constituents and the storing, shipping, and handling requirements

• The cure/forming cycle suggested by the material supplier and the preferred machining, surface treating, and finishing operations (including the bands of acceptability for expected variations)

in the performance service life

If changes to the baseline composition, cure/forming cycle, and properties are required, it is important to stay in close contact with the material supplier's technical representatives and laboratory scientists and their corresponding equivalents within your company and your industry The first few components fabricated should be expected to change in material composition, processing, properties, shape, fabrication step sequencing, and specification criteria However, with a sound engineering approach over the life of the program, the necessary adjustments usually can be made to produce a high-quality product

Effects of Composition, Processing, and Structure on Properties of Composites

R Laramee, Intermountain Design Inc

References

1993, p 16, 71, 213, 289, 301, 373, 390, 476-478, 533

2 Engineered Materials Handbook Desk Edition, ASM International, 1995, p 477, 532-582, 1057-1094

3 Modern Plastics Encyclopedia, McGraw-Hill, 1994, p 233

4 Composites, Vol 1, Engineered Materials Handbook, ASM International, 1987, p 118, 119, 355, 360, 363,

373, 381, 405, 410, 861, 862

5 H.S Katz, and J.V Milewski, Handbook of Fillers for Plastics, Van Nostrand, 1987, p 56, 57, 75

6 R Flinn and P Trojan, Engineering Materials and Applications, Houghton-Mifflin, 1990, p 618, 619

7 Handbook of Advanced Material Testing, Marcel Dekker, 1995, p 945-948, 950, 955

9 C.A Harper, Ed., Handbook of Plastics, Elastomers and Composites, 2nd ed., McGraw-Hill, 1992,

sections 1.5, 4.4, 5.31, 11.2

10 R.B Seymour, Reinforced Plastics, ASM International, 1991, p 9, 51

11 "Fiberglass Reinforced Plastics," Owens-Corning Corp., 1964, p 24-30

12 C.T Lynch and J.P Kershaw, Metal Matrix Composites, CRC Press, 1972, p 16, 17, 51

13 K.A Lucas and H Clarke, Corrosion of Alumina Based Metal Matrix Composites, John Wiley & Sons,

1993, p 17, 20, 21

14 Carbon Composite and Metal Composite Systems, Vol 7, Technomic

15 G Savage, Carbon-Carbon-Composites, Chapman Hall, 1993, p 95, 97, 98, 101, 125, 126, 129, 200

16 L Edwards and M Endean, Manufacturing with Materials, Butterworths, 1990, p 73, 145

17 J.A Lee and D.L Mykkanen, Metal and Polymer Matrix Composites, Noyes Data Corp., 1987, p 113, 150

18 Design and Manufacturing Advanced Composites, Fifth Conference, ASM International, 1989, p 89,

SURFACE ENGINEERING is a multidisciplinary activity intended to tailor the properties of the surfaces of engineering

components so that their function and serviceability can be improved The ASM Handbook defines surface engineering as

"treatment of the surface and near-surface regions of a material to allow the surface to perform functions that are distinct from those functions demanded from the bulk of the material" (Ref 1) Of concern to the design engineer is the availability of surface-specific properties for the component that can provide:

• Protection in a desired environment

• Electronic or electrical properties

Of further concern to the designer is the availability of economical processes to produce the required properties These processes include solidification treatments such as hot dip coatings, weld-overlay coatings, and thermal spray surfaces; deposition surface treatments such as electrodeposition, chemical vapor deposition (CVD), and physical vapor deposition (PVD); and heat treatment coatings such as diffusion coatings and surface hardening

Surface treatments are used in a variety of ways to improve the material properties of the component Coating mechanical properties, for example, hardness, strength, and toughness, can improve the component wear, fatigue, and erosion properties, respectively Electrical properties in circuit design are dependent on the surface-deposition processes Similarly, environmental properties such as resistance to aqueous corrosion and high-temperature oxidation and sulfidation can be improved by selective surface treatments

The bulk of the material or substrate cannot be considered totally independent of the surface treatment Most surface processes are not limited to the immediate region of the surface, but can involve the substrate by exposure to either a thermal cycle or a mechanical stress For example, diffusion coatings often have high-temperature thermal cycles that may subject the substrate to temperatures that can cause phase transformations and thus property changes, or shot-peening treatments that deliberately strain the substrate surface to induce improved fatigue properties It is the purpose of this

article to review information on surface treatments that improve service performance so that the design engineer may consider surface-engineered components as an alternative to more costly materials

Related information is provided in the article "Design for Surface Finishing" in this Volume More detailed information

about the processes described in this article can be found in Surface Engineering, Volume 5 of ASM Handbook (Ref 1)

Reference

1 Surface Engineering, Vol 5, ASM Handbook, ASM International, 1994

Effects of Surface Treatments on Materials Performance

Arnold R Marder, Lehigh University

Solidification Surface Treatments

Solidification surface treatments include hot dip coatings, weld overlays, and thermal spray coatings

Hot Dip Coatings

Hot dip coatings are predominantly used to improve the aqueous corrosion of steel Processing of hot dip coatings involve either batch or continuous processing The continuous process is more advantageous for sheet steels, whereas the batch process is normally used for individual parts Details of the processing techniques are outlined in Ref 1 In the batch galvanizing process, the two types of conventional practices used at the present time are the wet process and the dry process (Ref 2) The wet process involves a flux blanket on the top of the molten zinc bath to remove impurities from the surface of the steel and also to keep that portion of the surface of the zinc bath, through which the steel is immersed, free from oxides In the dry process the steel is usually cleaned, treated with an aqueous solution, dried, and then dipped in the molten zinc bath The molten zinc bath is maintained at temperatures between 445 and 455 °C (830 and 850 °F) and times

in the range of 3 to 6 min The time of immersion is used to control the thickness of the coating, which consists of zinc alloy phases at the interface along with a top coat of pure zinc Good cooling control is necessary because the zinc can continue to react with the substrate to produce further alloying and detrimentally affect the properties of the coating such as the spangle finish (or grain size)

iron-In continuous hot dip processing, welded coils of steel are coated at speeds of 200 m/min The flux or Cook-Norteman line is similar to the batch process in that the sheet is cleaned and fluxed in line prior to immersion The hot-processed continuous line is more complex in that the steel sheet is first cleaned at temperature in a reducing environment, annealed above the recrystallization temperature of about 700 °C (1290 °F) and then immersed in the molten bath As the strip exits the bath, the thickness of the molten metal film is controlled by gas wiping dies that remove excess coating metal After coating, the sheet is either cooled by forced air or subjected to an in-line heat treatment, called galvannealing, before being rewound into coil or sheared into cut lengths at the exit of the line

In general, the coating microstructure consists of the substrate, the interfacial alloy layer, and the overlay cast structure Depending on the type of coating, the microstructure and composition of these constituents changes As expected, the substrate plays a major role in the type of coating obtained, and substrate composition can affect growth kinetics of the phases formed For example, if the substrate contains silicon the well-known Sandelin Effect can influence the iron-zinc phase reaction and consequently the thickness of the coating (Ref 3) Similarly, alloy additions to the steel to improve sheet formability, for example, interstitial-free (IF) steels with titanium, titanium/niobium, and phosphorus, can influence the microstructure of the iron-zinc phases in galvanized and galvannealed steel (Ref 4) Substrate grain size has also been shown to greatly affect the nucleation of the iron-zinc phases In aluminum-containing baths, the structure formed first is

an inhibition layer that is dependent on bath composition and prevents further alloying for a certain short time before the inhibition layer becomes unstable (Ref 5)

When the zinc galvanizing bath contains only a trace of aluminum, zinc attack of the substrate is uniform and the phases that form are governed by the iron-zinc binary phase diagram In zinc baths containing aluminum, the stability of the

inhibition layer governs the amount of iron-zinc phases formed Once the inhibition layer is no longer stable, outbursts or rapid growth of iron-zinc phases occur during hot dipping (Ref 6) During the thermal cycle of the galvannealed process, the inhibition layer dissolves and iron-zinc phase layer growth occurs in a controlled manner until the entire coating is made up of iron-zinc phases (Ref 7) Both galvanized and galvannealed alloy phase growth are determined by Fe-Al-Zn ternary diffusion, and the overlay cast microstructure greatly depends on aluminum content of the bath The pure-zinc and low-aluminum coatings form an overlay of pure Zn ( ) phase Zn-5wt% Al (Galfan) solidifies as eutectic microstructure, and the Zn-55 wt% Al (Galvalume and Zincalume) solidifies as aluminum dendrites with zinc-rich interdendritic regions The aluminum coatings (Type I and Type II) either form overlays of aluminum-silicon or aluminum alloy, respectively

Galvanized coatings are commonly characterized by surface spangles In cross section, an Fe2Al5(Zn) inhibition layer develops first, preventing any iron-zinc intermetallic phase formation The overlay layer is made up of dendrites of pure

Zn ( ) phase and appears as a polycrystalline structure The three surface finishes commonly produced are:

• Regular spangle, where the coating solidifies from the dipping temperature by air cooling

• Minimum spangle, where the coating is quenched using water, steam, chemical solutions, or by zinc powder spraying

• Extra-smooth temper roll finish carried out as an additional operation with regular and minimum spangle material

Aluminum is probably the most important alloying element added to the hot dip galvanizing bath, with different levels required to produce different properties in the bath (Ref 8) Aluminum levels of 0.005 to 0.02 wt% are added to brighten the initial coating surface The effect is related to the formation of a continuous alumina (Al2O3) layer on the coating surface that inhibits further oxidation by acting as a protective barrier layer This effect is also responsible for the reduced atmospheric oxidation of the zinc bath In addition, aluminum in the range of 0.1 to 0.3 wt% is added to the zinc bath to suppress the growth of brittle iron-zinc intermetallic phases at the steel coating interface by forming the Fe2Al5 (Zn) inhibition layer The end of this incubation period is marked by the disruption of the initial layer, followed by rapid attack

of the substrate steel An increase in the incubation period depends on increased aluminum bath composition using a low bath temperature, having low bath iron content, agitation, and the presence of solute additions in the steel Thus, during commercial production, the immersion time is kept below the incubation period in order to obtain a highly ductile product

Zinc coatings add corrosion resistance to steel in several ways As a barrier layer, a continuous zinc coating separates the steel from the corrosive environment By galvanic protection, zinc acts as a sacrificial anode to protect the underlying steel at voids, scratches, and cut edges of the sheet The sacrificial properties of zinc can be seen in a galvanic series where the potential of zinc is less noble than steel in most environments at ambient temperatures In addition, after dissolution of the zinc metal, zinc hydroxide can precipitate at the cathodic areas of the exposed steel, forming a secondary barrier layer Zinc corrodes at a slower rate than the steel substrate, although the corrosion rate of zinc varies depending on the atmosphere to which it is exposed (Ref 9), as shown in Fig 1

Fig 1 Service life (time to 5% rusting of steel surface) versus thickness of zinc for selected atmospheres

Shaded area is thickness range based on minimum thicknesses for all grades, classes, and so forth, encompassed by ASTM A 123 and A 153 Source: Ref 10

During forming, especially stretch-forming operations, increased friction of the zinc can result in less total stretch before fracture In severe forming operations, galling and coating pickoff can also occur Furthermore, coating particulate buildup on die surfaces can lead to poor surface appearance of formed parts Proper lubrication is essential in the design

of any forming process, especially when forming zinc-coated parts Weldability of zinc coatings is also an important property of the coating Spot weldability properties are particularly important because most galvanized product is joined

in this manner Zinc coatings reduce the life of welding electrodes because of the copper electrode alloys with zinc This effect leads to higher resistance, localized heating, and increased pitting and erosion of the electrode tip As a result, manufacturing costs increase because lower tip life reduces productivity due to frequent downtime in the welding operation to redress tips

Although zinc coatings are often used in the as-coated state, some applications call for a painted surface, and therefore paintability is an important design property of the coating It has been shown that large-spangle material is difficult to paint; therefore, most painted products are either minimum spangle or temper rolled It is usually necessary to pretreat a hot dip galvanized coating with a zinc phosphate or complex oxide thin coating before prepainting In the automobile industry, following the pretreatment most automobile bodies are primed with an electrophoretic paint (e-coat), and, as a result, resistance to e-coat cratering is an important property At high e-coat voltages, sparking as a result of exceeding the dielectric properties of the deposited paint film cases localized heat generation, film disruption, and premature curing of the paint After paint curing, these sparked areas form pinpoint craters that result in a paint surface with a detrimental appearance Therefore, resistance to e-coat cratering, expressed in cratering threshold voltage, is an essential paintability property (Table 1)

Table 1 Effects of hot dip coatings on threshold voltages for cratering of cathodic electrophoretic primer Type of surface Cratering threshold, V

Uncoated bare steel >400

be continuous Good process control requires that the effects of heating rate, hold temperature and time, and cooling rate

on the iron-zinc reaction kinetics be well understood Galvanneal coatings have been classified as (Ref 7):

• Type 0: Underalloyed coating containing predominantly -phase

• Type 1: Optimal alloyed coating with less than a 1 m interfacial layer and a top layer containing

-phase interspersed with a small amount of phase

• Type 2: Overalloyed coating with a -layer more than 1 m thick and an overlay of -phase containing

basal plane cracks

Formability is an important property in galvanneal coatings because iron-zinc intermetallic phases are considered brittle

As a result, powdering and flaking of the coating can occur during the forming operation, resulting in reduced corrosion resistance and impaired paintability The type 1 coating was found to have the best formability properties (Ref 12), but as

in most forming operations lubrication to improve metal flow is essential Spot weldability of galvanneal coatings are improved over galvanized coatings because it is more difficult for these iron-zinc phases to alloy with the copper electrode Paintability is also better than that of galvanized coatings because of the microscopically rough surface formed

as a result of the iron-zinc alloy phases throughout the coating However, galvanneal coatings are more prone to cratering during e-coating (Table 1) Conversely, corrosion resistance can be slightly reduced because of the increased iron in the coating from the iron-zinc phases; the galvanic potential is not as great as it is for pure zinc

Zn-5Al alloy coating (Galfan) is near the eutectic point in the aluminum-zinc equilibrium phase diagram Two compositions have been reported based on additions to the eutectic composition: small (up to about 0.5%) mischmetal additions containing lanthanum and cerium and additions of 0.5% Mg These additions are made to improve the wettability and suppress bare spot formation as well as to produce a typical "minimized spangle" structure The microstructure of Galfan is characterized by a two-phase structure, a zinc-rich proeutectoid -phase surrounded by eutectic phase consisting of lamellae of -aluminum and zinc-rich -phase However, the microstructure can be varied depending on the cooling rate In the range of normal bath temperatures, 420 to 440 °C (790 to 825 °F) there is no visible intermetallic layer or at least an extremely thin layer (<0.5 m) at the interface between the steel substrate and the overlay coating Thus, Galfan coatings have excellent formability and cut-edge corrosion protection

Zn-55Al alloy coating (marketed under the tradename Galvalume) contains about 1.5% Si added for the purpose of preventing an exothermic reaction at the coating overlay/substrate steel interface As a result, the coating contains -

aluminum dendrites, zinc-rich interdendritic regions, and a fine dispersion of silicon particles, along with a prominent Al-Zn intermetallic alloy layer at the interface between the steel substrate and the overlay coating The surface of the coating contains characteristic spangles that consist of aluminum dendrites with a clearly measurable dendrite arm spacing Cooling rate after dipping can significantly refine the microstructure of the coating, increasing the number of silicon particles and constraining the growth of aluminum dendrites

Fe-Initially, the atmospheric corrosion of the Zn-55Al coating takes place in the zinc-rich interdendritic regions, enabling the coating to exhibit galvanic protection As the coating continues to corrode, the zinc corrosion products become trapped in the interdendritic regions and act as a further barrier to corrosion Eventually, the aluminum dendrites, which also acted as

a barrier layer, add to the corrosion protection, as does the Fe-Al-Zn intermetallic alloy layer This results in a parabolic type of corrosion as evidenced in Fig 2 Although its galvanic protection is less than that provided by galvanized coatings, Zn-55Al is generally adequate to protect against rust staining at scratches and cut edges of the steel sheet

Fig 2 Corrosion losses of hot dip coatings in the industrial environment of Bethlehem, PA Source: Ref 13

Aluminum coatings are produced as type 1 coating, a thin (20 to 25 m) aluminum-silicon alloy coating, and type 2, a

thicker (30 to 50 m) pure aluminum coating Silicon is present in type 1 coatings in the range of 5 to 11 wt% to prevent formation of a thick iron-aluminum intermetallic layer at the coating/steel substrate interface Instead, a thin Fe-Al-Si intermetallic layer is formed, allowing for good formability and coating adherence These coatings are intended primarily for applications requiring improved appearance, good formability, and resistance to high temperatures, as in automobile exhaust components The type 2 coating has a microstructure containing a pure aluminum overlay and a thick iron-aluminum intermetallic alloy layer Thus, the formability and adhesion of this coating is limited by the poor ductility of the alloy layer Nevertheless, the coating is used for outdoor construction applications (e.g., roofs, culverts, etc.) that require resistance to atmospheric corrosion (Table 2) The aluminum outerlayer offers excellent corrosion resistance because of the good barrier properties provided by the increased thickness of the coating (Fig 2)

Table 2 Coating thickness losses for galvanized steel and type 2 aluminized steel in atmospheric exposure

Middletown, OH Kure Beach, NC

G90 (a) Type 2 G90 Type 2

During weld-overlay surfacing, the coating material is raised to its melting point and then solidified on the surface of the substrate, which means that metals and alloys used for this purpose must have melting points similar to or less than the substrate material The effectiveness of the weld-overlay coating depends mainly on the welding process and the overlay alloy composition The welding process must be selected and optimized to apply protective overlays at high deposition rates and thermal efficiency, with good control over the overlay/substrate dilution and coating thickness The overlay alloy composition must be selected to provide the required properties to prevent coating degradation, and the alloy composition must be readily weldable

A number of welding processes are available for applying protective weld overlays, and many welding parameters must

be considered when attempting to optimize a particular process for a given application The process principles and their characteristics for some processes are summarized for comparison purposes in Table 3 and are described in Ref 15 and

16 The processes can be grouped as torch processes, arc welding processes, and high-energy-beam techniques The torch process, oxyacetylene welding (OAW), is the oldest and simplest hardfacing process and involves simply heating the substrate with the flame and then melting the filler rod to get the hardfacing to melt High-energy-beam techniques use

laser beam welding (LBW) or electron beam welding (EBW) to alloy the surface by adding alloy powders to the weld pool

Table 3 Weld surfacing processes

deposit thickness

(min), mm

Deposition

rate, kg/h

Dilution, single

layer, %

Typical uses

Oxyacetylene (OAW) 1.5 1 1-5 Small area deposits on light sections

Powder weld (PW) 0.1 0.2-1 Small area deposits on light sections

Shielded metal arc

(SMAW)

3 1-4 15-30 Multilayers on heavier sections

Gas tungsten arc (GTAW) 1.5 2 5-10 High-quality low-dilution work

Plasma transferred arc

(PAW)

2 10 2-10 High-quality lowest-dilution work

Gas metal arc (GMAW) 2 3-6 10-30 Faster than SMAW, no stub-end loss; positional work possible

Flux-cored arc (FCAW) 2 3-6 15-30 Similar to GMAW Mainly for iron-base alloys for high

abrasion resistance

Wire 3 10-30 15-30 Heavy section work; higher-quality deposits than FCAW

Strip 4 10-40 10-25 Corrosion-resistant cladding of large areas

Bulk Similar to SAW wire but other alloys possible

Electroslag (ESW) 4 15-35 5-20 High-quality deposits at higher deposit rates than SAW Limited

alloy range

Source: Ref 16

In arc welding, the heat is generated by an arc between an electrode and the workpiece Arc welding processes can be grouped into nonconsumable electrode processes and consumable electrode processes Nonconsumable electrode processes, gas tungsten arc welding (GTAW) and plasma arc welding (PAW), both involve a tungsten electrode and the introduction of the filler metal (in the form of rod or wire in GTAW and powder in PAW) The arc melts the filler metal

to form a molten pool that is protected from the atmosphere by an inert gas shield In plasma arc welding, an additional inert gas flows through a constricted electric arc in the welding torch to form the plasma In general, for consumable electrode processes, the arc is maintained between the consumable electrode and the workpiece In shielded metal arc welding (SMAW), the electrode consists of a core wire surrounded by a flux covering, that upon melting forms a liquid

slag and gas to protect the molten metal pool In flux core arc welding (FCAW), the flux is contained in the core of the metallic tubular electrode, whereas in gas metal arc welding (GMAW) the consumable wire electrode and substrate metal

is protected from the atmosphere by a gas fed axially with the wire through the welding gun nozzle In submerged arc welding (SAW), the arc, which is submerged beneath a covering of flux dispensed from a hopper, melts the electrode, the surface of the workpiece, and some of the flux that protects the molten pool from oxidation Electroslag welding (ESW) uses equipment similar to SAW for strip cladding

There are a large number of processing parameters that must be considered when attempting to optimize welding processes for surface application:

All processes

Voltage across the arc

Current through the arc

Current polarity

Current pulsing parameters

Travel speed of heat source

Shielding gas type (except SAW)

GTAW electrode tip angle (vertex angle)

PAW plasma gas flow rate

However, the important factors considered in terms of arc welding overlay parameter optimization and process performance include arc efficiency, melting efficiency, deposition rate, dilution, and coating thickness (Ref 17) Arc efficiency is only a function of the arc welding process; melting efficiency increases with increasing arc power and travel speed, and the maximum deposition rate is directly related to both the arc and melting efficiency During the deposition of the weld-overlay coating, the base metal and the filler metal are melted and mixed in the liquid state to form a fusion bond Depending on the weld-overlay coating thickness, if a large portion of the substrate is melted and allowed to mix appreciably with the weld overlay, dilution can cause the overall alloy content of the coating to be significantly reduced The level of mixing is quantified as the dilution ratio and is one of the most important parameters in a surface application because the original filler metal mechanical and corrosion properties can be altered The extent to which dilution occurs depends on the surfacing and substrate materials used, the welding process chosen, and the parameters employed Table 3 indicates the range for dilution expected for the various processes employed Figure 3 is a surfacing diagram that relates dilution for various arc welding processes according to filler metal feed rate and melting power (a function of arc and melting efficiency) and can be used to facilitate process selection and parameter optimization in weld-overlay applications (Ref 18)

Fig 3 Effect of processing parameters on dilution with experimental data plotted for SAW process Source: Ref

18

During welding, the base metal is subjected to peak temperatures that are at least as high as the melting temperature of the substrate The properties of the weld and the adjacent heat-affected zone (HAZ) strongly depend on the thermal history as dictated by the heat input Preheating the part may be a necessary step in reducing the residual stress and distortion associated with welding Preheat and maintenance of a specific minimum temperature during the welding cycle can also reduce the cooling rate to prevent the formation of a detrimental transformation region in the HAZ of ferrous alloys Interpass temperature is another important factor needed to be controlled in order to prevent increased dilution and HAZ grain growth at high temperatures Postweld heat treatment can take many forms, depending on whether the weld-overlay coating needs to be stress-relief annealed or must be heat treated for specified properties

Excellent reviews of hardfacing metallurgy and the application of weld-overlay consumables are found in Ref 16 and 19 For overlay coatings, components are designed to provide resistance to various forms of wear, erosion, and corrosion over

a large temperature range Thus, properties such as hardness, microstructure, and corrosion resistance are more important for the coating than tensile strength and elongation, which are usually provided by the substrate material

Generally, coatings selected for wear resistance require high hardness as a characteristic, thus the term "hardfacing." It is believed that most hardfacing alloys develop their wear resistance by virtue of wear-resistance carbides (Ref 20) Almost all hardfacing alloys can be separated into two major groups based on chemical compositions of the primary solidified hard phases:

• Carbide hardening alloys, including cobalt-base/carbide (WC-Co) and some iron-base superalloys

• Intermetallic hardening alloys, for example, nickel-base superalloys, austenitic stainless steels, and

iron-aluminides

However, although increased hardness generally increases wear resistance, different microstructures containing the same carbide type can also have significant effect on wear resistance (Fig 4)

Fig 4 Effect of structure and hardness on abrasion resistance Source: adapted from Ref 21

Erosion resistance of materials is very dependent on the erosion conditions, the effects of which are dominated by a number of variables including particle size, shape, composition, and velocity; angle of incidence; and temperature Unlike wear properties, the erosion rate of weld-overlay coatings generally increases with increasing hardness (Fig 5) However, the erosion resistance of weld-overlay alloys depends on whether the coating can be classified as a brittle or ductile material (Ref 22) Those materials that can be deformed plastically (ductile) produce a large plastic zone beneath the eroded surface, and the increased plastic zone size can be directly correlated to an improved steady-state erosion resistance For those materials that cannot deform plastically (brittle), an increase in coating hardness sometimes may lead

to a decrease in volumetric erosion rate Thus, materials that can dissipate particle impact energy through plastic deformation (plastic zone) exhibit low erosion rates However, for materials that do not deform plastically (no plastic zone) and do not undergo plastic deformation, the ability to resist brittle fracture (i.e., cracking) becomes a major factor that can control the erosion resistance

Fig 5 Volume steady-state erosion rates of weld-overlay coatings at 400 °C (750 °F) as a function of average

microhardness at 400 °C (90° impact angle; alumina erodent) Source: Ref 22

The corrosion resistance of weld-overlay coatings follows the corrosion-resistant properties of the bulk materials and is also dependent on the corrosive environment Weld-overlay coatings provide resistance to oxidation and sulfidation Dilution, as discussed previously, can be expected to modify the behavior of the coating alloy from the properties quoted for the undiluted bulk materials In weld-overlay coatings such as austenitic steels, dilution can affect corrosion resistance because of a reduction in the effective chromium content or an increase in carbon content through carbon pickup from the substrate steel Iron aluminides appear to be potentially important weld-overlay coatings for sulfidation environments Figure 6 shows isothermal weight gain studies for a number of weld-overlay coatings exposed to H2S-H2-H2O-Ar gas mixtures at 800 °C (1470 °F) (Ref 23) This work showed that compositions containing at least 30% Al and 2% Cr had excellent sulfidation resistance and, at increased chromium levels, corrosion rates increased but were still superior to other alloy classes such as stainless steels

Fig 6 Weight change versus time for specimens cut from iron-aluminide weld overlays and isothermally

exposed to H 2 S-H 2 -H 2 O-Ar at 800 °C (1470 °F) The elemental concentrations shown are in at.%; the balance is iron Source: Ref 23

Detailed information about weld overlay coatings is available in the article "Hardfacing, Weld Cladding, and Dissimilar

Metal Joining," in Welding, Brazing, and Soldering, Volume 6 of ASM Handbook (Ref 24)

Thermal Spray Coatings

Thermal spraying is a generic term for a group of processes that apply a consumable in the form of a spray of finely divided molten or semimolten droplets to produce a coating A number of extensive reviews of the topic can be found in Ref 25, 26, 27, 28, and 29 The characteristics that distinguish thermal spray processes from weld-overlay coatings are indicated as follows (Ref 29):

• Substrate adhesion, or bond strength, is dependent on the materials and their properties and generally is characterized as a mechanical bond between the coating and the substrate, unlike the metallurgical bond found in weld-overlay coatings

• Spray deposits can be applied in thinner layers than welded coating, but thick deposits are also possible

• Provided there is a stable phase, almost all material compositions can be deposited, including metals, cermets, ceramics, and plastics

• Thermal spray processes are usually used on cold substrates, preventing distortion, dilution, or metallurgical degradation of the substrate

• Thermal spray processes are line-of-sight limited, but the spray plume often can be manipulated for complete coverage of the substrate

Processes for thermal spray coatings can be classified into two categories, arc processes and gas combustion processes, depending on the means of achieving the heat for melting of the consumable material during the spraying operation

In the lower-energy electric arc (wire arc) spray process, heating and melting occur when two electrically opposed charged wires, comprising the spray material, are fed together to produce a controlled arc at the intersection The molten material on the wire tips is atomized and propelled onto the substrate by a stream of gas (usually air) from a high-pressure gas jet The highest spray rates are obtained with this process, allowing for cost-effective spraying of aluminum and zinc for the marine industry In the higher-energy plasma arc spray process, injected gas is heated in an electric arc and converted into a high-temperature plasma that propels the coating powder onto the substrate at very high velocities This process can take place in air with air plasma spraying (APS), or in a vacuum with vacuum plasma spray (VPS) or low-pressure plasma spraying (LPPS)