Improved densification of carbonyl iron compacts by the addition of fine alumina powders

Abstract

An investigation of the effect of alumina particles on the sintering behavior of a carbonyl iron powder compact was carried out in this study. Two different-sized alumina, 0.05 and 0.4 m, were added to the iron compact at amounts up to 1.2 wt pct. When 0.4 m alumina particles were added, no sintering enhancement was observed. But, in contrast to previous results reported in literature, the addition of 0.1 to 0.2 wt pct of 0.05 m alumina particles was found to improve the densification. With 0.1 wt pct, the sintered density increased from 7.25 to 7.40 g/cm 3 after the compact was sintered at 1350 ЊC for 1 hour in hydrogen. Dilatometric curves showed that alumina impeded the early-stage sintering of iron in the ␣ phase, but improved densification in the ␥ phase at high temperatures. These results, along with microstructural analysis, suggested that alumina particles exhibit dual roles; their physical presence blocks the diffusion of iron atoms, thus causing inhibition of sintering, while their grainboundary pinning effect prevents exaggerated grain growth of iron and helps densification. It follows that, depending upon the amount and size of the alumina powders, either an increase or decrease in the final sintered density can be obtained.

Enhanced Densification of Carbonyl Iron Powder Compacts by the Retardation of Exaggerated Grain Growth through the Use of High Heating Rates

“…This sintering behavior was very different from those of previous studies using a conventional resistance dilatometer with a heating rate of 1°C/min to 20°C/min. [1,2,21] This suggests either that different parts of the compact reached 912°C at different times or that the phase transformation was not completed within the short time frame, approximately 7 seconds, between 912°C and 1200°C, due to the extremely fast heating rate used. The smooth curve also suggests that the exaggerated grain growth did not happen when phase transformation occurred, if it occurred at all.…”

“…The microstructure thus obtained, as shown in Figure 6(a), indicates that exaggerated grain growth had occurred during the phase transformation, and the mean grain size increased to 15.5 lm, in agreement with previous reports. [1,2,21] When a higher heating rate of 1200°C/min was used to heat the compact to 950°C and then that temperature was held for 5 minutes, the sintered density was only 4.78 g/cm 3 , or 61 pct. As shown in Figure 6(b), the interparticle necks remained small and most pores were attached to the grain boundaries at the interparticle contacts.…”

High Density Powder Injection Molded Compacts Prepared from a Feedstock Containing Coarse Powders

“…9) To inhibit exaggerated grain growth, Al, Ti, and SiO 2 have been added to carbonyl iron compacts to retard the grain boundary movement, and effective results have been reported. [10][11][12] Another intuitive approach to alleviate this grain growth problem is to keep the sintering in the phase and avoid the -phase transformation. This can be achieved by sintering the compact below 1185 K 13) or by adding phase stabilizers such as Mo, W, Si, and P. 4,5,14,15) Various methods of using phase stabilizers to improve the densification of mixed coarse and fine iron powder have been reported in the literature.…”