A study on laser sintering of Fe-Cu powder compacts
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I. INTRODUCTION
ONE of the most widely investigated systems in conventional sintering (CS) is Fe-Cu, due to its critical expansion behavior. Dilatometry and microstructure studies have shown that Cu penetrates quickly into grain boundaries within and between Fe particles when the temperature surpasses 1083 8C. This penetration, together with the effect of Cu diffusion into the Fe matrix, causes swelling of the compact.[1,2] Further research conducted by Kaysser et al. pointed out that a rigid skeleton of small Fe particles formed as a result of diffusion in solid-state sintering during heating to the temperature of liquid formation, which seriously inhibited the rearrangement of Fe particles.[3] In order to promote shrinkage, Lenel and Hwang suggested using Cu-coated composite powders so that little grain-boundary penetration would occur.[4] Huang and Hwang found that, with injectionmolded Fe-Cu compacts, unlike in the case of conventional pressing and sintering, no swelling was observed because fine carbonyl iron powders had a large number of small interparticle pores, so that copper filled these interstices first before it penetrated grain boundaries.[5] Others suggested that a carbon addition could also lessen the expansion, as carbon increased the dihedral angle.[6,7] Several studies on laser sintering (LS) of some Fe-based and Cu-based powder metallurgy (PM) parts have been conducted by our laboratory in recent years, and some good results were reported.[8,9,10] The previous experiments have demonstrated that LS has some prominent advantages, as follows. (1) It is easy to realize liquid-phase sintering, and the sintering cycle is very short. (2) It favors achieving higher-density PM parts than possible with CS.
PING SHEN, Postdoctoral Student, and JIANDONG HU, Professor and Advisor, Department of Materials Science and Engineering, and ZUOXING GUO, Engineer, and QINGFENG GUAN, Assistant Professor, Microanalysis Center, P/M and Laser Processing Laboratory, are with the Jilin University of Technology, Changchun, 130025, P.R. China. Manuscript submitted May 4, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
(3) It is convenient to integrate with other operations such as hot forging, surface alloying, and quenching. However, in our prior articles, the reasons for rapid densification and higher density after LS were not adequately explained, and the effects of processing variables on densification and property were not sufficiently discussed. Therefore, this article mainly focuses on these two aspects.
II. EXPERIMENTAL PROCEDURES The Fe used in this experiment was an atomized powder, and the Cu was an electrolytic type. Both particle sizes were -200 mesh. About 0.3 cm3 mineral oil per kilogram of powder was added. The compositions were mixed in a twinshell mixer, at a rotating rate of 33 rpm, for 30 minutes. Compaction was at pressures from 500 to 700 MPa, and the sample size was about 18 mm in diameter and 5 mm in height. In order to prevent oxidation, all the specimens were coated with a carbon protection layer
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