Effects of nickel on the sintering behavior of Fe-Ni compacts made from composite and elemental powders
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INTRODUCTION
NICKEL has been shown to enhance the sintering of iron compacts,[L2,3]and the degree of enhancement depends on the quantity and the method of its introduction. Since nickel has been known to be a beneficial sintering aid for conventional powder metal iron compacts, it has also been employed since the early development stage of powder injection molding (PIM). Several studies [4,S,61 examined the Fe-Ni system using elemental powder mixtures with the nickel content varied from 0 to 8 wt pct. These studies indicated that nickel could enhance sintering but that its distribution in the microstructure was not uniform. It would be intuitive to think that to improve the homogeneity in microstructure and mechanical properties, fine powders or, even better yet, composite powders would be ideal starting materials. Several studies using the conventional press-andsinter process have indeed shown that composite Fe-Cu, FeNi, and Fe-Ni-Mo powders give higher sintered densities and better mechanical properties than do mixed powders3'-~~ With this idea in mind, nickel-coated iron powder was also evaluated for PIM by this group, and the results with regard to mechanical properties showed that composite powders produced higher strength and ductility.r~t] One of the explanations for the role of nickel in enhanced sintering of iron compacts is that nickel is present in the grain boundaries of the iron matrix during sintering, which impedes grain growth. I~,~~ Another hypothesis is that with nickel present in the grain boundaries, the grain boundary diffusion rate of iron is increased.lg,~o~ Countering to these benefits of nickel additions, it was also found that Kirkendall porosities formed in the iron side of the Fe-Ni diffusion
couple,t ~2,~31which led to dimensional expansion and poor sintered density for Fe-Ni compacts. Mitani et aL f~21 showed that the maximum sintered density was obtained at 2 wt pct Ni for mixed coarse electrolytic iron and electrolytic nickel powders. When more nickel was added, expansion occurred and reached a maximam at 50 wt pct Ni. If fine carbonyl iron powder was used, only densification was noted, and it increased with an increasing amount of nickel. Zhang and German :4] also reported a maximum density at 2 wt pct Ni using the PIM process with 4-/~m carbonyl iron powder and 9-#m nickel powder. Kohara and Tatsuzawa P~Jused - 1 0 0 + 325 mesh composite powder and the press-and-sinter technique and found that densification increased as nickel content increased. Hanatatet~~ used - 1 5 0 + 200 mesh composite powder and noted that the density reached a plateau at 0.08 wt pct. Further addition o f nickel did not improve densification. These experiments showed that the method of nickel introduction, the size of the iron and nickel powders, and the processing method used in making the compacts are all critical factors in determining the optimum nickel addition to obtain the maximum sintered density of Fe-Ni compacts. The effect of nickel coating on sintered densities and the mechanical properties
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