Formation and Evolution of Surface Morphology in Overhang Structure of IN718 Superalloy Fabricated by Laser Powder Bed Fusion

Controlling the overhang surface quality is still a formidable challenge in manufacturing the components with complex structures during laser powder bed fusion (LPBF). This study systematically uncovers the effects of the volume energy density (VED) and overhang angle on the evolution of surface morphology and corresponding surface roughness (Ra) of top and down-skin surfaces of IN718 superalloy samples. The results show that balling, Plateau-Rayleigh instability, open pore and humping caused by the material stacking are the main factors contributing to the apparent deterioration of top surface quality. When the VED is 80–100 J/mm³, the high down-skin surface roughness is attributed to the serious dross caused by recoil pressure and sinking of the melt pool. Using insufficient VED (15–50 J/mm³) can easily lead to poor metallurgical bonding and material spalling on the down-skin surface. In addition, overhang angle also significantly affects down-skin surface roughness due to the stair effect and the adhered unmelted powders. An improvement in the surface quality of down-skin surface is observed when the overhang angle increases. Based on the finding of this investigation, an optical VED (59.5 J/mm³) significantly improves the top and down-skin surface quality and porosity of overhang samples. This study provides an insight into synergy ascension of the top and down-skin surface quality in the overhang structure.