Abstract Melting mode acts a significant role in determining the defect, microstructure and mechanical properties of parts fabricated by selective laser melting. In this work, the effects of scanning speed and defocusing distance on the melting mode transition in selective laser melting of Al7050 powder were investigated. The samples under different melting modes were characterized by comparing the surface morphology, crack, microstructure and nano-hardness. The results show that the defocusing distance has a significant effect on the forming of keyhole mode. The calculated threshold scanning speed of forming keyhole mode agrees very well with the experimental result. A metallic luster surface was obtained in keyhole mode due to the lower oxygen content at the surface. The method of using optimal scanning speed can only reduce but cannot eliminate the cracks induced by the process. The keyhole mode has lower crack sensitivity than that of conduct mode. Uniform and fine microstructure occurred under conduction mode. However, due to the fluctuations of flow field and re-melting under keyhole mode, a V-shaped structure was presented on the bottom of the melt pool. Consequently, fluctuating nano-hardness values were exhibited on the bottom of the melt pool under keyhole mode.

中文翻译：

---

Al7050 粉末的选择性激光熔化：熔化模式转变和小孔和传导模式之间的特性比较

摘要 熔化模式在决定激光选区熔化零件的缺陷、显微组织和力学性能方面起着重要作用。在这项工作中，研究了扫描速度和离焦距离对 Al7050 粉末选择性激光熔化中熔化模式转变的影响。通过比较表面形貌、裂纹、微观结构和纳米硬度对不同熔化模式下的样品进行表征。结果表明，散焦距离对小孔模的形成有显着影响。计算出的形成小孔模式的阈值扫描速度与实验结果非常吻合。由于表面的氧含量较低，在钥匙孔模式下获得了金属光泽表面。采用最佳扫描速度的方法只能减少但不能消除加工过程中产生的裂纹。锁孔模式的裂纹敏感性低于传导模式。在传导模式下出现均匀且精细的微观结构。然而，由于流场的波动和锁孔模式下的重熔，熔池底部呈现V形结构。因此，在锁孔模式下，熔池底部呈现出波动的纳米硬度值。