Cracking mechanism of Rene88DT superalloy by laser rapid forming

The forming mechanism on the cracks produced in the laser rapid forming (LRF) processing of Rene88DT superalloy has been investigated. The results show that the cracks belong to the liquefying crack with a characteristic of cracking along grains, forming the liquid films along grains due to the liquefying of γ/γ′ eutectic composition in the pre-layer already solidified by laser scanning. The pre-layer was pulled apart by the increasing residual tensile stress with the laser deposition process. There are columnar dendrites parallel to the laser deposition direction at the bottom of the present laser deposition layer, while at the top, the dendrites haul to the direction perpendicular to it due to the change of heat flux direction. The hauling dendrites exist at the overlapping zone of the subsequent laser deposition layer due to un-melting. As the LRF samples' microstructure showing the epitaxial growth, the grain boundary formed in the overlapping zone between laser scanning paths due to the un-melting hauling dendrites. The γ/γ′ eutectic also solidified in the area, making the zone cracking susceptibly. The liquefying cracks will rapidly expand along the penetrating columnar grain boundary between the laser deposition layers. As a result, the cracks enlarged generally along the central line of the overlapping zone.