Mechanism of hot crack propagation and prevention of crack formation during electron beam powder bed fusion of a difficult-to-weld Co-Cr-Ni-W superalloy

Understanding how hot cracks propagate in difficult-to-weld alloys during electron beam powder bed fusion (EBPBF) is necessary for manufacturing quality parts. The current understanding in the literature does not go into sufficient details of the mechanisms of crack propagation. The major part of this study aimed to reveal how cracks grow during EBPBF. Samples were made using EBPBF of a Co-base alloy that is a difficult-to-weld type and crack patterns/networks were revealed through sequential sectioning-polishing, which is a new way of illustrating hot crack networks. It was found that when surface cracks were observed, they tended to orientate normal to the scan direction. The scan direction changed 90° after each layer and the surface cracks also changed orientations accordingly. The relationships between the crack orientations on the surface and the crack networks beneath the top layer were observed. The identification of how the crack networks had grown and how liquation had assisted this growth is shown. Sometimes surface cracking was found to be absent even though the crack network beneath the top layer kept developing. The lack of surface cracking is explained by the natural growth of a horizontal dendritic surface layer that is formed under the condition of a low tangent angle melt track. Thus, the second part of the study was an attempt to utilize horizontal dendrites for hot crack prevention. It will be shown that controlled remelting can be an effective method to preserve the horizontal dendrite grains in each scan layer and thus hot cracking does not occur. It will be further shown also that controlled remelting can occasionally result in equiaxed grain growth.