Selective laser melting of Al7050 powder: Melting mode transition and comparison of the characteristics between the keyhole and conduction mode

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. (C) 2017 Elsevier Ltd. All rights reserved.