Fig. 11 Examples of laser-clad microstructures. (a) Tribaloy T- 800 alloy on ASTM A 387 steel. (b) Haynes Stellite alloy No. 1 on AISI 4815 steel. Source: Ref 33 The Fe-Cr-Mn-C cladding of Mazumder and Singh (Ref 41) had a microstructural consisting of M 7 C 3 and M 6 C type carbides in a ferritic matrix, a solid solubility extension of chromium in ferrite by 50%, and a microhardness of 550 HV. The molybdenum-clad coating on gray iron of Belmondo and Castagna (Ref 42) consisted of molybdenum dendrites surrounded by a Cr-Ni matrix containing Cr 2 C 3 , which had a microhardness of 900 HV. Wear Behavior of Laser-Clad Layers. Mazumder and Singh (Ref 41) found that the Fe-Cr-Mn-C cladding on AISI 1016 steel resulted in tribological properties that were superior to Stellite 6. The width of the wear scar was reduced from 3.5 mm (0.14 in.) in the base alloy to about 0.6 mm (0.02 in.) in the laser cladding. Under the same test conditions, Stellite 6, a common hardfacing alloy, developed a wear scar which of 1.4 mm (0.06 in.). Abbas et al. (Ref 43) laser clad a mild steel with Stellite 6, Alloy 4815, and their composites with SiC by using pneumatic powder delivery. Wear tests, conducted by grinding the samples against a revolving alumina disk, showed that the composite clad samples had the best abrasive wear resistance (Fig. 12). Fig. 12 Abrasive wear behavior of mild steel laser clad with Stellite 6, Stellite 6/SiC, and Alloy 4815/SiC. Source: Ref 43 Belmondo and Castagna (Ref 42) performed wear tests on the Mo-Cr 2 C 3 clad cast iron samples using a reciprocating motion testing machine. When compared with plasma-sprayed coatings of similar composition, the performance of laser- clad coatings was far superior under all testing conditions, especially at the highest pressures. With a load of 25 MPa (3.6 ksi) and slider roughness of 0.3 m (12 in.), the wear loss in the laser sample was 0.4 mg (1.4 × 10 -5 oz), compared to 1.3 mg (4.6 × 10 -5 oz) in the plasma-sprayed sample. In addition, the plasma-sprayed coating developed cracks at the interface, which seriously impaired its integrity. Molian and Hualun (Ref 44) performed pin-on-block reciprocating sliding wear tests on Ti-6Al-4V alloy substrate laser clad with BN, both with and without a NiCrCoA1Y addition, and found that the wear resistance improved from 10 to 200 times more than that of age-hardened and laser-melted samples (Fig. 13). The presence of solidification reaction products, TiN and TiB 2 , resulted in a cladding microhardness of 1600 HV, which led to improved wear resistance by preventing ploughing and reducing friction. Fig. 13 Sliding wear behavior of laser-clad Ti-6Al-4V alloy. Source: Ref 44 Boas and Bamberger (Ref 45) determined the abrasive wear characteristics of plasma-sprayed and laser-melted plasma- sprayed coatings of Tribaloy T-400 on AISI 4130 steel using a block-on-cylinder test. The plasma-sprayed coating was prone to rapid wear by delamination, whereas laser consolidation of the plasma coating removed flaws, such as porosity and microfissures, and generated an adherent wear-resistant layer. Figure 14 demonstrates the improvement in wear after laser consolidation of the plasma coating. For comparison, results on a hard D2 steel are also given. Fig. 14 Wear scar growth curves of plasma-coated 4130 steel before and after laser consolidation. Source: Ref 45 Laser-Cladding Applications. Because one of the primary goals of laser cladding is to improve the tribological properties of components, several applications have been found for the technique. Some of them are in the exploratory stage, others are in the pilot-plant stage, and a few have reached the production stage. Bruck (Ref 46) demonstrated cladding within a confined space by coating the inside surface of small-bore pipes (inside diameter of 50 to 100 mm, or 2 to 4 in., and length of 0.3 to 1.2 m, or 1 to 4 ft), as shown in Fig. 15. An oscillating mirror placed in the pipe formed a 12.5 mm (0.5 in.) wide melt pass, into which the clad alloy powder was fed. The pipe was rotated and translated to cover the entire inner surface. Previously, chrome plating was used, but the laser technique appreciably improved galling resistance. Using suitable cooling techniques, the pipe temperature was maintained below 500 °C (930 °F) to avoid deterioration of the core properties, as well as distortion. Fig. 15 Laser cladding of small-bore pipes. Source: Ref 46 La Rocca (Ref 19) describes laser cladding of exhaust valves with stellite at an Italian automotive manufacturer. The stellite powder was fed into the laser melt pool. The laser-clad material was superior to gas tungsten arc welded material in terms of thickness uniformity and uniform microstructure and elemental distributions, and had better adhesion. Powder utilization also was better (30% that of gas tungsten arc welding), and over-metal removal was reduced by 10 to 15%. One British automotive manufacturer uses production-stage laser cladding of a nickel-alloy turbine blade shroud interlock by powder feeding either Tribaloy or Nimonics (Ref 47). Previously, a manual arc melting technique was used. Laser cladding reduced cladding time from 14 min to 75 s, improved productivity and quality, reduced cost by 85%, and reduced powder consumption by 50%. Table 2 lists components that are laser clad for commercial applications. Table 2 Component, cladding alloy, and cladding technique Component Clad alloy Cladding technique Company Ref Gate and seat of steel valves for oil-field, geothermal, and nuclear energy production, and chemical processing Stellite 6 Powder feed W-K-M Div. of Joy Mfg., Houston, TX 48 Diesel engine valve Stellite SF6 Preplaced powder paste Cabot Corp., Kokomo, IN 49 Gate valve seat Deloro 60 Preplaced powder paste Cabot Corp., Kokomo, IN 49 Gas turbine blade "Z" notch Co-base alloy Powder feed Quantum Laser, Edison, NJ 50 Leading edge of turbine blades Co-base alloy Powder feed Quantum Laser, Edison, NJ 50 Pump bushings and impellers Co-base alloy Powder feed Quantum Laser, Edison, NJ 51 Engine valve bases Ni-Cr alloy Powder feed Quantum Laser, Edison, NJ 51 Oil-field valve gates WC and Co alloy Powder feed Quantum Laser, Edison, NJ 51 Tractor bushings Stellite 6 Powder feed Quantum Laser, Edison, NJ 51 Turbine blades, pump valve seats Stellite, Colmoloy, blended powder Preplaced beds, gravity feed Westinghouse, Pittsburgh, PA 52 Valve stem, valve seat, aluminum block CrC 2 , Cr, Ni, Mo/cast Fe Preplaced powder Fiat, Turin, Italy 52 Off-shore drilling and production parts, valve components, boiler firewall Stellite, Colmonoy, alloys/carbides Powder feed Combustion Eng., OH 52 Aerospace components Stellite, Tribaloy Powder feed Rockwell Int., CA 52 Turbine blade shroud interlocks PWA 694, Nimonics Preplaced chip Pratt & Whitney, West Palm Beach, FL 52 Turbine blade "Z" notch Hardfacing Powder feed United Technol., E. Hartford, CT 53 Swivel joints, petroleum valves Hardfacing Powder feed Spectra Physics, San Jose, CA 53 Gas engine turbine blades Hardfacing Powder feed General Electric, SC 54 Laser Melt/Particle Injection Processing. Laser melt/particle injection produces an in situ, metal-matrix/particulate composite surface layer by mixing, but not melting, the second phase with the substrate. The particulate material is injected with sufficient velocity as a spray into the melt pool formed by the laser beam. If the second phase is hard, such as a carbide, the injected layer can be made to resist wear. Figure 16 depicts the laser injection process. An oscillating CW CO 2 laser beam produces a shallow melt pool on a substrate, into which are injected powder particles via a nearby nozzle. As the sample translates, an injected layer forms on the surface. By varying the amplitude of oscillation, melt widths ranging from 3 to 20 mm (0.12 to 0.8 in.) are possible (Ref 55). Processing conditions for laser melt/particle injection are a power density from 10 to 3000 MW/m 2 (6.45 to 1935 kW/in. 2 ) and an interaction time from 0.1 to 1 s. Any inert shielding gas is normally used. Fig. 16 Laser melt/particle injection process The particles are propelled by pressurized helium gas. The pressure depends on the particle size and the relative densities of the powder and the molten substrate. Larger and heavier particles require lower gas pressures. Typically, powder flow rates are from 0.1 to 0.5 cm 3 /s (0.006 to 0.03 in. 3 /s) and particle velocities are a few m/s. Carrier gas pressures typically range from 50 to 120 kPa (7.3 to 17.4 psi). The carrier gas also serves as a cover gas and keeps the particles relatively cool, preventing them from melting or fusing. Particle sizes range from 45 to 150 m (1.8 to 6 mils). Finer particles tend to either fuse or dissolve in the melt, whereas coarser particles do not flow easily. The injection nozzle, usually made of copper, is inclined at 60° to the horizontal and is positioned from 10 to 20 mm (0.4 to 0.8 in.) away from the sample. Its slotted opening is designed to produce a rectangular spray that is of the same size as the melt pool, to ensure that most of the powder is incorporated into the melt. Although minimum interaction between the carbide phase and the melt is desired, some dissolution does occur. It was found that the degree of carbide dissolution, as well as other physical characteristics of the injected layer, such as penetration depth, mounding, and carbide volume, could be reasonably controlled. Variations in these characteristics, as well as hardness, with laser power and powder feed rate, are determined for Ti-6Al-4V alloy injected with TiC (Fig. 17) (Ref 56). The carbide volume can be varied from 15% to a limit of 60%. For improved wear resistance, higher carbide volumes are desirable, but the conditions that produce increased carbide volume also increase dissolution. An estimation of the degree of carbide dissolution is given by matrix microhardness. As the trends in Fig. 17 show, matrix microhardness is higher when the carbide volume is greater. Fig. 17 Effect of processing parameters on injected layer characteristics in Ti-6Al-4V injected with TiC. (a) Laser power. (b) Powder feed rate. Source: Ref 56 Initial work on laser melt/particle injection was done on low-density/high-strength aluminum and titanium alloys, for which few surface-hardening methods exist. Some of the alloy/particulate systems that have been investigated are 5052 Al/TiC, Al bronze/TiC, Ti-6Al-4V/TiC, Ti-6Al-4V/WC, 304 stainless/TiC, 4340 tool/TiC, Inconel 625/TiC, and Inconel 625/WC (Ref 57). Microstructures of Laser Melt/Particle-Injected Layers. The injected layer is a composite of particles surrounded by a metal matrix. It is desirable that the only surface modification be the presence of the particulate phase, and that the surrounding metal should remain unchanged and retain most of the substrate properties, such as corrosion resistance and toughness. Figure 18 shows the cross-sectional view of a WC injected layer on Inconel 625 alloy substrate. The top surface is rough, but is free of deep channels. The light grains are the WC particles, which are surrounded by the dark Inconel. The carbides are uniformly distributed throughout the injected layer and occupy about 50% of the total volume. Because the matrix phase is essentially made of the same material as the substrate, there is chemical continuity across the interface and a strong metallurgical bond. Fig. 18 Cross section of Inconel 625 alloy injected with WC Carbide dissolution products formed during solidification can influence the matrix microstructure. The carbide particles sometimes develop a perturbed or scalloped, highly alloyed interface with the metal matrix. Eutectic and dendritic carbides are some of the dissolution products that appear within the matrix (Ref 58). Figure 19 shows examples of some of the dissolution products. Besides influencing properties such as microhardness and friction wear, these resolidification products can cause matrix embrittlement and microcracking, although with suitable preheating, cracking can be eliminated (Ref 59). Fig. 19 Carbide morphological changes and matrix resolidification products as a result of carbide phase dissolution in Inconel 625 alloy. (a) Injected with TiC, eutec tic carbides. (b) Injected with TiC, dendritic carbides. (c) Injected with WC, eutectic carbides. (d) Injected with WC, dendritic carbides In order to retain some of the substrate properties in the modified surface, it is important to keep carbide dissolution to a minimum. Some of the matrix microstructural changes are inevitable. Depending on the base alloy, melting and rapid solidification can result in microstructural refinement, formation of solid solutions, phase transformations, and precipitation. These microstructural modifications, along with the dissolution products, can harden the matrix by various mechanisms. Carbide dissolution products harden the matrix by a dispersion-hardening mechanism. This effect was observed in stainless steel, tool steel, Inconel, and titanium alloys. Transformation hardening comes into play when martensite forms in steels and titanium alloys. In tool steels, precipitation hardening also plays a role, and in aluminum bronze, microstructural refinement results in modest hardening (Ref 57). Wear Behavior of Particle-Injected Surface Layers. On a macroscopic scale, hardening of particle-injected alloys is due to plastic flow inhibition by the injected phase. The relative increase in macrohardness of the composite surface is from 1.1 to 2.6 times, depending on the alloy (Ref 57). On a microscopic scale, hardening of the metal matrix varies with the type of alloy and has been found to affect friction wear behavior (Ref 60). Friction wear was evaluated using a balt-on-surface test, which measures the coefficient of kinematic friction, k , between a hard steel ball and a polished sample surface. A pressure of 1 N (0.22 lbf) and a slide velocity of 0.0001 m/s (0.02 ft/min) were used. This test is sensitive to minute changes in microstructure. For untreated Inconel 625, k was about 0.7. Particle-injected Inconel 625 had a k that ranged from 0.15 to 0.20 after the first slide, which increased to a range from 0.3 to 0.4 for WC and 0.4 to 0.45 for TiC after several slides (Fig. 20). The wear scar was hardly evident in the optical microscope and there was very little abrasive damage. Although k increased with number of slides, it was not due to continuous wear of the sample surface. Rather, it was due to formation of wear debris. Fig. 20 Coefficient of friction as a function of number of slides in untreated and particle- injected Inconel 625. Source: Ref 60 The wear mechanism was different for the two carbides. With WC injection, the carbide and the harder (600 to 650 HV) metal matrix were strong enough to cause the slider to wear, and the wear debris was mostly that from the steel ball. With TiC injection, the wear debris was a mixture of material from the steel ball and from the softer (400 to 500 HV) metal matrix (Ref 60). From dry sand/rubber wheel tests done on aluminum- and titanium-base alloys injected with TiC, it was found that the wear volume decreased rapidly with modest volumes of the carbide phase. For aluminum, the reduction is from 0.18 to 0.28 cm 3 (0.01 to 0.02 in. 3 ) to 0.02 to 0.025 cm 3 (0.0012 to 0.0015 in. 3 ). For titanium, it is from 0.06 to 0.065 cm 3 (0.0037 to 0.0040 in. 3 ) to 0.01 cm 3 (0.0006 in. 3 ) (Fig. 21) (Ref 61). Examination of the worn surface showed that the softer metal matrix eroded, but that the hard carbide particles prevented further erosion of the surface. The abrasive action did not uproot the carbides from the matrix, but the normally angular carbides appeared slightly rounded, which helped reduce further erosion. Fig. 21 Reduction in abrasive wear rate with increase in vol% carbide in particle- injected aluminum and titanium alloys. Source: Ref 61 Ayers and Bolster (Ref 62) found that abrasive wear with 3 and 30 m (120 and 1200 in.) diamond particles of aluminum and titanium alloy samples was reduced by introducing WC or TiC into their surfaces. The wear resistance of aluminum alloys improved by a factor of 30 in samples containing TiC, whereas for Ti-6Al-4V containing TiC, the improvement was less dramatic (factor of 4). Applications of Laser Melt/Particle Injection Process. Most of the laser melt/particle injection processing work has involved process optimization, microstructure evaluation, and test sample preparation. One application involved fabrication of wear-resistant surfaces on Inconel alloy shaft seal test rings, such as the one shown in Fig. 22 (Ref 63). The injectant is WC and the circular pass is 127 mm (5 in.) in diameter and 10 mm (0.4 in.) wide. The coverage rate was 130 mm 2 /s (0.2 in. 2 /s). Before testing, the rough surface was ground flat. The seal ring was successfully tested against a graphite mating surface using water as the lubricant. 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Molian and Hualun (Ref 44) performed pin-on-block reciprocating sliding wear tests on Ti-6Al-4V alloy substrate laser clad with BN, both with and without a NiCrCoA1Y addition, and found that the wear. ploughing and reducing friction. Fig. 13 Sliding wear behavior of laser-clad Ti-6Al-4V alloy. Source: Ref 44 Boas and Bamberger (Ref 45) determined the abrasive wear characteristics of plasma-sprayed. of plasma-sprayed and laser-melted plasma- sprayed coatings of Tribaloy T-400 on AISI 4130 steel using a block-on-cylinder test. The plasma-sprayed coating was prone to rapid wear by delamination,