Selective laser sintering of intermetallics
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Two p o w d e r mixtures with a composition o f 95 pct (59 pct Ni + 41 pet Sn) + 5 pct ZnC12 were used throughout the course o f this work. One mixture consisted o f commercially available tin ( < 4 5 / z m , spherical) and coarse nickel ( < 1 5 0 /zm, agglomerated spherical) powder. The other mixture was the same tin powder mixed with finer commercially available nickel powder (particle size between 3 and 7 /~m, spherical). The powder mixtures were made by first blending together an appropriate amount o f nickel and tin powder. ZnClz, which was used as a wetting agent, was dissolved in ethyl alcohol and mixed with the powders. The wet powder mixture was then dried in a vacuum oven at 100 °C for 3 to 7 days, depending on the quantity o f powder being mixed. The dried p o w d e r was then sieved to produce ZnC12 coated nickel and tin powders. The powder mixtures were selective laser sintered using a computer controlled, Q-switched, 100 W nominal TEM00 Nd:YAG laser. The laser process conditions were wavelength = 1.06/xm, beam diameter = 0.5 mm, incident p o w e r = 17 to 25 W , and scan speed = 2 to 6 c m / s . The thickness o f each l a y e r was varied between 127 and 178 /.zm. Prior to SLS processing, the powder bed was heated to approximately 150 °C using a resistive ring heater mounted in the work station. Most o f the l a s e r sintering was done in a nitrogen atmosphere to prevent oxidation and for safety reasons; however, some o f the initial experiments were performed in air. A f t e r laser sintering, some o f the samples were postprocess annealed. All heat treatments were done in a hydrogen atmosphere. The annealing schedules were 800 ° C / 2 4 h o r 700 °C/41 h, followed by air-cooling. Multiple l a y e r samples o f 20 to 40 layers were SLS processed using the above range o f l a s e r parameters. Figure 3 is an scanning electron micrograph o f a cross section o f an as-SLS-processed sample (coarse nickel p o w d e r mixture), w h i c h shows very good wetting o f the nickel by the tin. The phases remained elemental. Wetting is evidenced by the flow o f molten tin around the nickel particles with no indication o f tin bailing. The asSLS-processed samples were well bonded and easy to handle. SLS-processed samples from the fine nickel p o w d e r mixture tended to exhibit a lesser degree o f wett i n g , and the fine nickel particles tended to become captured within larger, molten tin spheroids. Upon postprocessing a f t e r SLS, changes in microstructure and phases present were observed for the coarse nickel powder mixture. Figure 4 is a typical fracture surface obtained f o r material annealed 24 hours at 800 °C. The structure became spheroidized, much more porous, brittle, and friable. Intermetallic compounds did form during postprocessing. Ni3Sn, Ni3Sn2, and Sn were the phases identified in specimens annealed using the 24 hour annealing schedule. The sample that was annealed f o r 41 hours at 700 °C contained mostly Ni3Sp2 with some Ni3Sn and a slight amount o f Ni. No Zn or C1 was detected using energy
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