Microstructure feature of friction stir processed ductile cast iron

Abstract

In the present investigation, the ductile cast iron was successfully fiction stir processed at a tool rotational rate of 1000rpm and a traveling speed of 90mm/min, and an obvious defect-free processed zone was obtained. The macro- and micro-structures, phase evolution, thermal history and microhardness of the processed zone were investigated. The results revealed that the high density of ultra-refined and nearly equiaxed graphite particles (0.5–1μm) uniformly were distributed in most parts of the stir zone. The flake-shaped or tadpole-shaped graphites distributed along flow line due to the flow of the plasticized material in the thermo-mechanically affected zone. The distribution of fine graphite particles at various locations of the stir zone exhibited different plasticized material flow characteristics. The gradual increase of shear zone from the advancing side to the retreating side was clearly detected in the edge of the stir zone just beside the thermo-mechanically affected zone due to the shear flow of softened material. The numerous dense martensite structures were formed in the processed zone as the peak temperature exceeded the eutectoid transformation temperature (A1), which in turn favored the improvement of hardness of the base material. The microhardness test results show that the maximum hardness of the stir zone reached 817HV0.2.