Machinability of Resin Impregnated Sintered Iron

It is well known that the machinability of sintered iron is improved by impregnation of thermal curing type resin into the open pore. It is believed that the presence of the resin in the pores results in uninterrupted chip formation and favorable chip-breaking, caused by the duplex microstructure. However, further investigation is needed into the causes of improvement in the machinability. The purpose of this study is to elucidate the effect of resin impregnation on the improvement of machinability. Shear angle and shear strain in the cutting were investigated. It was found that the shear angle was increased and the shear strain was decreased by the resin impregnation. The effect of the resin impregnation on the coefficient of friction was measured by a ring compression test. It was clarified that the coefficient of friction was reduced by the resin impregnation, but that the amount of this reduction was small. A compression test with grooved dies was performed to investigate the nominal stress-strain curve and the formability (workability) of the sintered iron. It was found that the sintered iron and the impregnated sintered iron together showed work-hardening and that the impregnation affected the nominal stress-strain curve little but the formability of the resin impregnated compact was lowered to about 0.5 times compared to that of the non-impregnated sintered iron. The density of the chip in the non-impregnated specimen produced from two-dimensional cutting was increased before the cutting; however, the amount of this increase was small in the impregnated specimen. Vickers hardness of the chip in the non-impregnated specimen was larger than that in the impregnated specimen. These experimental results suggest that the decrease of flow stress corresponding to the embrittlement induced by the resin impregnation is the main cause of drastically improving the machinability.