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.

了解在电子束粉末床熔合（EBPBF）过程中热裂纹如何在难焊接的合金中传播是制造优质零件所必需的。文献中的当前理解没有深入地介绍裂纹扩展的机理。这项研究的主要部分旨在揭示在EBPBF期间裂纹如​​何扩展。使用难熔型钴基合金的EBPBF制作样品，并通过连续的分段抛光来揭示裂纹图案/网络，这是说明热裂纹网络的一种新方法。发现当观察到表面裂纹时，它们倾向于垂直于扫描方向取向。在每一层之后，扫描方向改变了90°，表面裂纹也相应地改变了方向。观察到表面上的裂纹取向与顶层下方的裂纹网络之间的关系。显示了裂纹网络如何生长以及液化如何促进这种增长的识别。即使顶层下方的裂纹网络不断发展，有时也发现不存在表面裂纹。表面裂纹的缺乏可以通过在低切线熔体轨迹条件下形成的水平树枝状表面层的自然生长来解释。因此，研究的第二部分是尝试利用水平枝晶来防止热裂纹。将显示，受控重熔可以是在每个扫描层中保留水平枝晶晶粒的有效方法，因此不会发生热裂纹。