Improved mechanical properties and microstructural development of microwave sintered copper and nickel steel PM parts

The application of microwave technology to a diverse range of materials and processes has resulted in a wide spectrum of materials that are commercially processed using microwaves, from the heating of food to the vulcanisation of rubber to the sintering of specialty ceramics. Microwave sintering of elemental or alloy metal powders has gained significance in recent times as a novel processing method since it offers many advantages over the conventional sintering method. Despite substantial R&D investment in this area in the past two decades, no competitive microwave technology has yet emerged for powder metallurgy (PM) sintering. In sharp contrast, because it is 'obvious' that microwaves are reflected by metals, it is not uncommon to be unable to locate many journal papers or literature, wherein metal powders have been sintered in a microwave field. This paper reports the improved mechanical properties and microstructural development of microwave sintered copper and nickel steel PM parts as compared with that obtained using conventional sintering technique. The paper describes the fabrication details of the FC-0208 and FN-0208 composition steel PM parts, and the in house modified commercial microwave oven used for sintering. Microwave sintering resulted in higher sintered density and improved mechanical properties for both Cu and Ni steel PM parts as compared with that processed using conventional sintering under identical conditions. The improved mechanical properties can be attributed primarily to more uniform distribution of the alloying elements, which resulted in greater material homogeneity at the nano- and microlevels as revealed by the Cu and Fe X-ray maps using high spatial resolution scanning transmission electron microscopy (STEM). The optical micrographs of both the etched and unetched samples clearly showed development of novel sintered microstructures having distinct characteristics for the porosity distributions: smooth and rounded pores with low stress concentration regions for microwave sintering as against sharp, triangular and wedge shaped pores with high stress concentration regions for conventional sintering.