Hot Cracking Phenomena in Electron-beam-melted Surface Region of Co-Cr-Mo Alloy

抄録

The cause of hot cracking in Co-Cr-Mo (CCM) alloy induced by surface melting by electron beam (EB) irradiation was investigated for different EB currents. A regular pattern of linear grooves and ridges was formed by horizontal EB scanning with a square raster pattern. However, some irregular-shaped grooves also occurred. As the EB current was increased, these irregular grooves became larger and cracks appeared within them. Cross-sectional observations showed that the cracks occurred mainly in the heat-affected zone (HAZ) along grain boundaries and extended into the fused zone (FZ). The HAZ cracks terminated at grain boundary precipitates. The fractured surface of the cracks exhibited well-developed cellular-dendritic solidification structures in the FZ, indicating that these were solidification cracks. In contrast, the cracks in the HAZ had an immature dendritic structure with a relatively flat surface, typical of liquation cracks. It can be deduced that the cracks were caused by a liquid film remaining at the grain boundaries, and the driving force for crack propagation was shrinkage distortion caused by the fusion-solidification process.