Surface Engineering of Metals - Principles, Equipment and Technologies Part 7 pdf

3.36 Microhardness profile in superficial layer: a 1045 grade steel, laserbeam hardened with overlapping of hardened zones; b gray pearlitic cast iron; 1 - after remelt laser hardening;

Fig 3.36 Microhardness profile in superficial layer: a) 1045 grade steel, laser

beam hardened with overlapping of hardened zones; b) gray pearlitic cast iron;

1 - after remelt laser hardening; 2 - after transformation laser hardening and tempering at 450ϒC for 1 h Fig b - from Straus, J., and Burakowski, T [51] With

permission.)

– surface layer of non-metallic phases composed mainly of metal oxides

and compounds formed as the result of chemical interaction of the atmo-sphere and thermal reaction of the laser beam with the steel, with gases dissolved in it and with components of the absorption coating; its layer usually does not exceed several micrometers;

– remelted and hardened from the melt surface layer, dendritic, with a

martensitic structure; carbides present in the steel underwent total or par-tial melting; within the area adhering to the intermetallic phase layer, the melted zone has a diminished carbon content;

– hardened from the solid phase subsurface layer with a non-uniform

struc-ture: martensitic with retained austenite and carbides in the vicinity of the remelted zone and martensitic with elements of initial structure near the core These elements are ferrite in hypoeutectoid steels and cementite in hyper-eutectoid steels Throughout the layer, a dispersion of martensite occurs, 1.5 to 2.0 times greater than after conventional hardening;

– tempered core zone, also called intermediate, with a structure of tempered

martensite or sorbite

Remelt hardening by laser beam causes similar effects as experienced

in electron beam hardening: deterioration of surface roughness (the sur-face after remelting has the appearance of a weld or overlay) and an enhancement of service properties such as: tribological, fatigue and anti-corrosion

The microhardness profile along the laser beam path is approximately uniform, with clearly lowered hardness due to the tempering effect of beam overlap (Fig 3.36a)

In the remelted zone of martensitic stainless steels and tool steels, re-sidual tensile stresses prevail after resolidification [233]

By remelting the surface layer of the material it is possible to obtain a fine-grained structure and partial or total dissolution of precipitation phases and contaminations in the form of carbides, graphite or oxides which are usually present in the microstructure Rapid crystallization (with cooling rates reaching 105 K/s) causes that after dissolution they do not precipi-tate again or precipiprecipi-tate in a different form Strongly oversaturated solu-tions are obtained For this reason, pure surface remelting is usually ac-companied by a strong refinement of dispersive phases, e.g., ledeburite, as well as cleaning of grain boundaries The latter effect is of particular sig-nificance to corrosion resistance [50]

Remelt hardening has found application primarily in the treatment of gray cast irons [50, 150−154], as well as stainless and tool steels [151−161] Laser remelting of gray cast iron causes total dissolution of graphite and the occurrence of hard spots in the surface layer A layer of fine-grained and non-etching quasi-ledeburite is formed on the surface It is composed of very fine carbide precipitations, austenite and martensite, as opposed to ledeburite

in gray cast irons which is composed of pearlite and carbide precipitations Under the hardened layer there is an intermediate zone with only partially dissolved graphite flakes and a hardened layer [60, 153]

As the result of remelting of the surface of gray cast iron, a lower hardness is obtained in the zone hardened from the melt than in that

hardened from the solid phase (see Fig 3.36b, curve 1) After tempering at

approximately 400 ϒC, an increase of hardness is obtained in the zone

hard-ened from the melt (Fig 3.36b, curve 2) [50].

The depth of remelt hardening may reach several millimeters Surface hardness of cast iron may even reach 1200 HV0.1 In the hardened condition the cast iron is resistant to wear (Fig 3.37) [152] and to corrosion [151] In the hardened layers compressive stresses usually prevail [154, 155]

Remelt hardening of gray cast iron has been broadly utilized in the automotive industry to harden slip rings and engine cylinders, compo-nents of turbines, cams and gears, obtaining a severalfold extension of service life [50, 150, 154]

Remelt hardening offers clear advantages in the case of chromium-bearing medium carbon steels [156], tool steels [157], including high speed

Fig 3.38 Hardness distributions in superficial layer on low carbon structural steel:

1 and 2 - carburized; 1’ and 2’ - carburized and laser remelt hardened (From Straus, J., and Burakowski, T [51] With permission.)

Interesting results have been obtained from remelting hardening of low carbon structural steel (0.14% C; 1% Cr; 4% Ni) after carburizing to a depth of 1 mm (Fig 3.38): a 25% increase in hardness and a doubling of layer depth [60] Laser remelting has also been applied to titanium after prior glow discharge nitriding [172]

Laser remelt hardening tests have also been conducted on carbon and low alloy steels, containing up to 0.2% C, carburized to a level of 0.7 to 0.9% C and coated with a TiN layer, thereby obtaining good adhesion and a 0.5 mm layer hardened to 880 to 900 HV1 [173]

After remelt hardening, carbon steel grades 1020 and 1045 exhibit a lowering of the fatigue limit, quite opposite to the effect after remelt-free hardening

Remelting of pure nickel [174] and aluminum alloys containing silicon, titanium, manganese, nickel and iron [175] has also been researched, yield-ing an improvement of abrasive wear resistance

Remelt hardening causes insignificant deterioration of surface

rough-ness Where roughness with Ra<10 to 15 µm is required, laser treatment

should be carried out prior to grinding [5, 11, 29, 51]

Glazing Laser glazing processes (vitrification, amorphization) are among

the least researched Glazing, i.e obtaining of amorphous layers, requires cooling rates which are greater by an order of magnitude than those typi-cally obtained by continuous CO2 lasers For that reason, Nd-YAG or excimer pulse lasers are often used, hence the obtained amorphous layers are very thin (typically: 20 to 40 µm) and the surface relief is not uniform The applica-tion of special cooling methods makes utilizaapplica-tion of continuous CO2 lasers

Fig 3.39 Schematic representation of continuous operation laser glazing: 1 - laser

beam; 2 - introduction of protective gas; 3 - glazed material; 4 - rotating and sliding stage (ensuring required covering by laser paths) (From Grigoryantz, A.G., and Safonov A.N [29] With permission.)

possible (Fig 3.39) which, in turn, allows better surface and a greater depth

of the vitrified metal layer The power densities employed here are 106 W/cm2

and treatment rates of 1 m/s and higher [29]

By ensuring very high cooling rates, the viscosity of the molten metal may be caused to rise sufficiently high to prevent the formation of crystal-lization nuclei The alloy does not crystallize but solidifies in a disordered form, thus becoming amorphous with properties of a glassy mass Not all alloys exhibit a tendency to amorphization [176] Those that do must have a certain chemical composition and exhibit an amorphization rate, related to that composition For example, for the PdSiCu alloy this rate is approximately 100 K/s, while for pure germanium or nickel, it is approximately 1010 K/s The amorphization rate is determined by a correla-tion between viscosity and temperature, the situacorrela-tion of the crystallizacorrela-tion range, the rate of crystal nucleus growth and other factors [29]

The obtaining of the amorphous or fine-crystalline state is possible in the following cases [29]:

1) Alloys with compositions close to eutectic with a deep eutectic, com-posed of:

– metal - non-metal These are formed by metals of the group I of the periodic table (Ag, Au) and group VII (Fe, Ni, Co, Pd, Pt, Rh) with non-metals such as Si, Ge, P and C, the content of which in the eutectic is usually 20 to 25% Such alloys become amorphous with the application of relatively small cooling rates: 105 to 107 K/s;

– metal - rare earth metal These are formed by metals with normal valences (Ag, Au, Cu, Al, In, Sn) with rare earth metals (La, Ce, Nd, Y, Gd), the content of which reaches 20%; the amorphous state is formed easily; – metals - refractory metals, e.g., Fe, Cu, Co, Ni with Ti, Zr, Nb, Ta Amorphization takes place when cooling rates exceed 107 K/s

2) Hypereutectic alloys:

– on a base of tellurium with Ag, Ga, Cu, In; they do not form a low-melting eutectic and are characterized by non-metallic bonds;

– on a base of lead and tin: Pb-Sn, Pb-Si, Pb-Ag, Pb-Au, Sn-Cu; the eutectic

is situated near the low-melting component; amorphization is possible only when the cooling rate is greater than 108 K/s

Amorphous alloys exhibit high strength and hardness As an example, FeBSi alloys, prior to amorphization, have a hardness of 3500 to 5800 MPa and 7100 to 11700 MPa after amorphization, while retaining significant ductil-ity Although brittle under tensile loading, they allow substantial deformation

- up to 50% - under compressive and bending stresses At low temperatures, their strength drops substantially and the alloys exhibit a very good resistance

to corrosion Some alloys also exhibit special magnetic properties Amorphous alloys without phosphorus exhibit high resistance to radiation In some laser-amorphized alloys a crack network appears

To date, many different alloys have been successfully vitrified by a pulse laser Among these are FeCSn [177], CuZr, NiNb, FeBSi (Fe80B16Si4,

Fe77B19Si4), FePSi (Fe83P13Si4, Fe79P17 Si4), Fe72B14C10Si4, Fe73P12C11Si4 [29], FeB (e.g.,

in triple laser-glazed Fe83B17 alloy, a tri-zone structure was obtained: homog-enous crystallites, heterogeneous crystallites and metal glass at the surface [178]), Pd77Si17Cu6, Fe74.5Cr4.5P7.8C11Si2.1, Fe81B13.5Si3.5C2 [50].

There are also known methods of reglazing, consisting of laser remelt-ing of e.g., strip ready-made from metallic glass [50]

A big future is predicted for alloying of steel and cast iron with ele-ments which enhance their tendency to amorphize (e.g., with boron or sili-con) The process may be carried out in two stages The first pass of the laser beam is for alloying and the second pass (with different parameters) for glaz-ing [29]

Laser glazing is seldom applied but a significant development is fore-seen in this area, mainly for raising resistance to tribological wear, includ-ing primarily magnetic elements, as well as components of assemblies and instruments working in conditions of severe corrosion hazard

Densifying (healing) Densifying consists of remelting of the surface

layer or a deposited coating (or coating and superficial layer - Fig 3.40)

to a certain depth in order to obtain a material of greater density which usually is associated with a decrease of porosity, but also involves the liquidation of surface defects in the form of scratches, delaminations, cracks and open pores [179] This is accompanied by homogenization of micro-structure which is important in the case of material prior subjected to plastic deformation It is also accompanied by a change of residual stresses and, in the case of coatings, obtaining of a better metallic bond between coating and substrate than by spraying alone [180] In densification pro-cesses, relatively low power densities and low treatment rates are applied This allows gases present in the melted material to escape to the surface

of the laser-melted pool A usable effect of densification is an increase of hardness and improvement of surface smoothness, an enhancement of

Fig 3.41 Effect of laser remelting on properties of plasma sprayed coatings from

mix-ture of 80% powdered GSR-3 material (composition: Ni, Cr, B, Si) and 20% powdered

TiC: a) hardness distribution; b) abrasive wear resistance; 1 - plasma sprayed coating;

2 - plasma sprayed and laser remelted coating; 3 - detonation sprayed coating.

and WC-Ni [188] For a significant improvement of corrosion resistance it is sufficient to remelt the thermally sprayed coating to a depth equal to 20 to 50% of its thickness [50]

– Electrodeposited coatings (primarily to remove scratches and cracks) [11]

Smoothing Laser smoothing of surfaces is carried out with the use of

the same range of process parameters as remelt hardening In the micro-structure of the material subjected to smoothing, the same phase and structural transformations take place as in remelt hardening These trans-formations are not, however, the main technological aim of the process, but rather the reduction of surface roughness and a change in the profile of surface unevenness They occur under the influence of hydrodynamic mix-ing of the molten material, due to thermocapillary forces which brmix-ing about convection In the pool of the molten material, a high temperature gradient is formed, along with related gradients of surface tension This causes a rapid circulation of the liquid, but only limited to a zone thinner than the entire melted layer (Fig 3.42) For example, the rate of circulation in molten iron may even reach 150 mm/s [29] Pressure changes within the molten pool are compensated by a change of shape of the pool surface

The strongest effect on smoothing is that exhibited by power density Within the range 5°103 to 5°104 W/cm2 it is possible to obtain a surface which is smoother than after machining [29] The recommended practice calls for low power densities and big diameters of the laser spot, in order to remelt the surface layer only to a shallow depth This is because in such conditions convection whirlpools are broken down into a series of smaller vortexes, conducive to a smoother surface Relatively low treatment rates also lead to the obtaining of a more favorable surface profile: lower asperities and greater asperity peak radius

introducing an alloying additive into the surface layer of the material is by remelting with hydrodynamic mixing before solidification

Laser Surface Alloying (LSA) consists of simultaneous melting and mixing

of the alloying and the alloyed (substrate) material The action and pressure

of the laser beam cause both materials to melt; a pool of molten material is formed in which intensive mixing, due to convection and gravitational move-ments, forms a flash at the pool surface (Fig 3.42) At the interface between solid (substrate) and liquid (alloy), a very thin diffusion zone appears, usu-ally not exceeding 10 µm Only in some rare applications do the alloying components diffuse to depths of 200 to 300 µm This takes place by diffusion

by narrow canals of the molten phase along solid grain boundaries and grain blocks or, in the case of displacement of atoms by dislocations, due to local deformations

When the action of the laser beam ceases, the alloy thus formed solidi-fies while the substrate material in its direct vicinity becomes self-hard-ened Structure, chemical composition, and physical as well as chemical prop-erties of the alloy are different than those of the substrate or of the alloying material First of all, the layer of the alloy does not, in principle, exhibit the characteristic layer structure, typical of diffusion processes Due to convec-tion mixing of the alloy, there are no transiconvec-tions from phases with a higher concentration of the alloying element to phases with lower concentration All phases in the remelted layer are uniformly distributed along its entire depth

An exception to this is the earlier mentioned very thin diffusion zone at the interface between solid and liquid The alloy layer is bound metallurgically with the substrate

The alloy layer, rich in alloying components, usually exhibits a higher hardness than that of the substrate, a higher fatigue strength, better tribo-logical and corrosion properties, but at the same time with poorer smooth-ness of the surface in comparison with the condition prior to alloying These properties depend to a very high degree on the uniformity of mix-ing of the alloy in the molten phase, which, in turn, depends on the inten-sity of convection exchange of mass in that zone [193]

Depending on the method of introducing the alloying additive to the

molten pool, we distinguish remelting and fusion (Fig 3.43)

Remelting Remelting is a two-stage process, consisting of prior

depo-sition of the alloying material on the substrate and subsequent remelting it together with the surface layer of the substrate material (Fig 3.43a) Usu-ally, the thickness of the remelted surface layer is comparable with the

thick-ness of the deposited alloying material, i.e., the mixing coefficient k p is ap-proximately 0.5 The process of remelting begins from the alloying coating and propagates by convection and conductivity into the surface layer of the substrate The alloying material dissolves completely in the substrate ma-terial

Alloying is accomplished with the employment of power densities in the range of 5°104 to 106 W/cm2, which are greater than those used in harden-ing, and exposure times from tenth to thousandth parts of a second

[29, 192] The greater the power density, the bigger the depth of remelting High power densities may lead to the formation of plasma and vaporiza-tion of material (Fig 3.44)

In principle, remelting is always accompanied by the occurrence of plasma and vaporization of material On the one hand plasma screens the surface from further laser heating; on the other, however, it interacts with the sur-face of the melted metal pool exerting pressure and causing the displace-ment of components of the molten material In the pool, precisely at the site

of penetration of the laser beam into the material, a conical pit is formed The surface of this funnel is acted upon by the hydrostatic pressure of the liquid from below and by vapour pressure from above Between the two, an unstable equilibrium is formed, constantly disturbed by, among other fac-tors, relative movement of the beam and the treated object The pit moves toward as yet unmelted material (in a direction opposite to that of the object relative to the beam) Behind the displaced pit, vapour pressure causes a filling in of the discontinuity In consequence, on the molten surface there appears a characteristic waviness, similar to that which is typical of a weld seam

Because of the above-described two-directional interaction of plasma on the molten pool, different methods of slowing down this action on the molten material are used Among these are blowing away of the plasma cloud by a neutral gas, heated in order not to impair the energy effect There are also methods of enhancing the action of plasma, e.g., by blowing away the plasma cloud but with the simultaneous recycling of the reflected laser radiation back to the treatment zone by a set of plane mirrors or a mirror dome Natu-rally, the flow of protective gas always protects the optics of the laser head against the deposition of gases, vapors and solid particles, created during treatment

Alloying is accomplished with the application of one or several passes

of the laser beam The alloying material is deposited on the substrate by [40, 41, 48, 53, 54]: painting, spraying of suspensions, covering by adhe-sive powders or pastes (containing P/M ferrous alloys of alloying metals, boron carbides, tungsten and titanium carbides and borax), thermal spray-ing (flame, arc, plasma and detonation), vapour deposition, electrodeposi-tion, thin foil, plates, rods or wires, or by E.D.M The thickness of the deposited coating ranges from several to more than 100 µm

In the case of P/M materials, the efficiency of laser heating is greater than for solid materials, on account of the higher coefficient of absorption

of laser radiation through powder, usually approximately 0.6 A relatively significant role is played by substrate surface roughness Its growth causes

an improvement of adherence of the powder mass to the substrate and thus

an improvement in the passage of alloying components into the molten pool, attributed to rapid melting of asperities

Alloying components can also be introduced to the substrate from the melt (Fig 3.45) The alloyed part is placed in a liquid; the laser beam reaches the surface through a vapour/gas channel formed in the liquid

surface covered by them) or in varnishes, e.g., bakelite together with activa-tion additives, such as ammonium chloride or borax, or in liquid hydrocar-bons or liquids containing carbon, e.g., hexane, acetone, toluol, carbon tetra-chloride, mineral oil, etc Carburization is applied in order to raise the hard-ness of plain carbon steels (4500 to 14000 MPa);

– laser nitriding: in pastes containing ammonium salts, urea (NH2)2CO,

in gaseous or liquid nitrogen Nitriding is applied to steels, as well as tita-nium, zircotita-nium, hafnium or alloys of these metals, in order to increase hardness, resistance to tribological wear and to elevated temperatures; – laser siliconizing: in pastes containing silicon powder or in liquids (e.g.,

in a suspension of silica gel H2SiO3) in order to enhance thermal, corrosion and tribological resistance of steel;

– laser boriding: in pastes constituting mixtures of boron powders, an-hydrous boric acid B2O3, boron carbide B4C, borax Na2B4O7·10H2O, ferro-boron with filler material, e.g., glue This process is carried out in order to increase hardness and abrasive wear resistance of metals

2) Metals: Co, Cr, Sn, Mn, Nb, Ni, Mo, W, Ta, V or their alloys, e.g., Cr-Mo-W,

Ni-Nb An unfavorable property of remelt alloying with metals is the formation

of supersaturated solid solutions, significantly exceeding solubility in equilib-rium conditions The formation of intermetallic compounds is also possible The utilization of metals and their alloys leads to changing of mechanical properties of ferrous, aluminum, titanium and copper alloys

3) Different compounds, mainly carbides of refractory metals: TiC,

NbC, VC, TaC, WC, Nb2C, Ta2C or alloys of carbides of these metals, de-posited by thermal spraying and by electrodischarge, as well as in the form of pastes (powder + liquid glass, powder + silicate glue, etc.) Alloying is applied to metals and alloys, mainly to steels and cast irons (Fig 3.46 and 3.47) by single elements raising heat resistance, corrosion resistance and abrasion or erosion wear resistance Among these are Mo, W,

C, Cr, B, Mn, Ni, Co, Zn, Cd, Si, Al and composites of elements, e.g B-C, B-Si, Co-W, Cr-Ti, Fe-Cr, C-Cr-Mn, Al-Cr-C-W and alloys, e.g., Cr2C3, Cr3C2-NiCr2, WC-Co, oxides Cr2O3, TiO2, B2O3 [29, 192, 195, 201], all allowing the obtaining

of a better set of properties than by alloying with only single elements Alloying is most often applied to different types of steels [194-196]: – Structural carbon, e.g., 1045 [197-200] and low alloy grades with car-bon, chromium, molybdenum [198-200], P/M carbides, e.g., WC, TiC or mix-tures of WC-Co [197], chromium pastes [201], boron, deposited electrolyti-cally or in the form of paste As an example, the microhardness of carbon (0.2% C) steel is increased by alloying from 2.5 GPa to 8.5 GPa, with a layer thickness of 0.4 mm [202]

– Tool steels: with boron [202], boron carbide or its composites with chromium (e.g., 75% B4C + 25% Cr [208]), by different composites of carbides [209], by tungsten, tungsten carbide and titanium carbide [210], by chromium

or vanadium boride [211], by vanadium carbide [212] or by Mo-Cr-B-Si-Ni composites [213]

Remelt alloying is often applied to cast irons [214], particularly gray [215-217] and high strength [218] These are alloyed with the use of Fe-Si powder packs [215], carbon (up to a content of 22% C) in order to enhance resistance to erosion wear[216], with boron [218], silicon, nickel and its al-loys [217] and with chromium [214]

Good results have been obtained by remelt alloying of aluminum alloys [219], including Al-Si alloys [220, 221] For example, the Al25* grade, alloyed with the application of pastes based on powders of NiCr, FeCuB, or NiCrMo, exhibits a significant increase in hardness and resistance to abrasive wear [219], in a way similar to the D16* grade, alloyed by carbides, e.g., B4C, Cr3C2,

B4C+Cr, B4C+Cr3C2, or by a composite B4C+Cr2O3+CaF2 [224] Powder pack alloying of Al-Si alloys by nickel, chromium, iron, silicon and carbon clearly raises their heat resistance [221] Similarly, alloying by Fe, Fe+B, Fe+Cu, Fe+Cu+B powders, predeposited by painting, in a mixture with zapon var-nish significantly raises hardness, although the distribution of alloys in the remelted zone is not homogenous [220]

Alloying of titanium by remelting of electroplated chromium, manga-nese, iron or nickel coatings causes a rise in hardness of the surface layer from below 1500 MPa for the titanium substrate to 5500 to 10000 MPa for the alloyed layer [222] The hardness of a laser hardened WT3-1* titanium alloy rises, relative to the initial hardness value by a factor of 1.1 to 1.6 This may

be further enhanced by alloying with powders of Al2O3, FeCr, a-BN and oth-ers [223] or by borides and carbides of transition metals (Mo2C, Mo2B5, WC,

W2B5, VB2, B4C, B4C +CaF2) together with chromium [224]

Research is currently being conducted to study the strengthening of low-carbon overlays by alloying, e.g., by chromium, predeposited by electro-plating [225]

Fusion Fusion is a single stage process It involves creating a pool of

molten substrate material with the laser beam and the introduction into this pool of the alloying material in the form of solid particles (powder or paste) completely or partially soluble in the substrate, or in the gaseous form (see

Fig 3.43b) Fusion is accomplished only with the aid of continuous operation lasers because the alloying material may be introduced to the molten zone only while laser heating is on, and not during lapse between pulses

The aim of fusion is the same as that of remelting, i.e., the obtaining of a surface layer in the form of an alloy or a coating with properties which are better than those of either the alloyed or the alloying material

In the case of powder fusion alloying, the process of melting of both

materials is simultaneous: solid particles of the alloying material are heated and may melt already at the moment of entering the site of the laser beam Not completely melted, they drop into the pool of the simultaneously melting alloyed material

The powder added may be a homogenous material or it may constitute

a mixture of powders of several materials The powder should be introduced

in a stream of protective atmosphere in order to avoid oxidation (Fig 3.48a) However, the gas may cause porosity of the alloyed layer