Influence of rheocasting and rare earth metals on the 3D-microstructure of ZK60 Mg-alloy measured by multiscale X-ray computed tomography

Abstract

Light weighting has been a key issue to meet stringent requirements on greenhouse gas emissions. One possible way of weight reduction is the substitution of heavier components with lighter metals like Magnesium. In this work, the internal architecture of cast and hot rolled ZK60 alloys with and without addition of 1.5 wt% rare earth metals (RE) has been investigated. The alloys were produced by permanent mould indirect chill cast (PMICC), while the material with 1.5 wt% RE was manufactured by additional rheocasting (RC). A multi-resolution procedure using X-ray computed tomography (XCT) was performed to investigate the alloys in size ranges from meso- (cast blocks) to micro-scale (microstructure). The voxel resolutions used were between (115 μm)3 down to (0.8 μm)3. Morphometric parameters such as volume, volume fraction, connectivity, structure separation, structure thickness etc. were calculated to characterize the 3D-microstructure of the various alloys. The alloys showed no detectable cracks in the as-cast condition, while macroscopic defects such as clusters of high-density Zr-rich particles as well as casting pores were identified. The Zr-rich particles as well as other primary intermetallic phases tended to agglomerate at the bottom of the ingots for all investigated alloys. The addition of RE increased the homogeneity of the microstructure in terms of spatial distribution as well as the global and local interconnectivities of intermetallic phases in the PMICC alloy. RC achieved a more homogeneous distribution of Zr-rich particles. Furthermore, RC led to changes of the solidification topology and increased the distance between the higher-density phases. Hot rolling reduced the fraction of pores formed during the casting process. However, pores were not detected in the ZK60 PMICC alloys after forming, while ZK60 + 1.5 wt% RE PMICC and RC showed the presence of voids that were not completely removed by hot rolling until the thickness was reduced about 60%.