MIM工件之尺寸穩定性與緻密性之研究

Abstract

金屬粉末射出成形製程適合製作形狀複雜的零件，但是其產品的尺寸穩定性卻不如傳統粉末冶金容易控制，其中最主要的原因為工件在燒結過程中會產生10~20%的線性收縮。為了改善金屬粉末射出成形工件之尺寸穩定性，本研究分析了在燒結過程中之尺寸變化。在實驗過程中，藉由熱膨脹儀的協助，分析在燒結過程中試片的in-situ尺寸變化並探討其可能對尺寸穩定性造成的影響。 羰基鐵粉的試片由於在燒結過程中會發生alpha-gama相變態，造成試片在發生相變態時產生急遽的尺寸收縮，使得其尺寸不易控制。此問題可以藉由添加合金元素來避免或減緩相變態發生來改善。在不

Powder injection molded (PIM) parts usually incur large amounts of shrinkage after sintering due to their low solid content and resulting poor dimensional stability. This problem is further aggravated when a high shrinkage rate occurs or when the furnace temperature is not uniform. To alleviate this dimensional control problem, the effects of the phase transformation, sintering temperature, and heating rate were investigated. The results show that when an abrupt volume change occurs, as happens during the alpha-gama phase transformation of iron, the dimensional stability deteriorates. This problem gets worse when the density of the part is low. By slowing down the heating rate in the region where the high shrinkage rate occurs, avoiding the phase changes, and adding alloying elements to broaden the temperature range of the phase transformation, the dimensional control of ferrous PIM compacts can be improved. 316L stainless steels usually use relatively coarse atomized powders. Thus, high-temperature sintering and, sometimes, liquid phase sintering is needed in order to get densities greater than 95%. With the formation of the liquid phase, the dimensional control problem occurs, despites of its beneficial effect of increasing the sintered density. To improve the dimensional control, a slow heating rate and dual phase sintering are recommended. Carbonyl iron powders are the most widely used raw powder in PIM components owing to their high driving forces for sintering. However, the cost of this powder is relatively high. To improve the competitiveness of the PIM process, coarse iron powders, which are much more economical, were mixed with fine carbonyl iron powders in an optimum ratio of 6/4 in this study. This replacement of fine carbonyl iron powders did not change the kneading and molding behaviors significantly. The solvent and thermal debinding rates of the compacts that contain 100% and 40% fine powders also showed little difference. Such debinding results, which are contrary to the general belief, suggest that the particle size is not the critical factor in the debinding of PIM compacts. The debinding rate is more likely controlled by the diffusion of the soluble binder in the solvent (for solvent debinding) and the decomposition rate of the backbone binder (for thermal debinding). High sintered densities can still be attained in the compact with mixed powders after