Cracks and process control in laser powder bed fusion of Al-Zn-Mg alloy

Al-Zn-Mg alloy is receiving a growing number of attentions due to the strategic demand in the aerospace in-dustry. But crack is a headache problem in Al-Zn-Mg alloy fabricated by Laser Powder Bed Fusion (LPBF). This paper investigated the characteristics, formation mechanisms and process control of the cracks in LPBF fabri-cation of Al-3.6Zn-0.6Mg alloy. Cracks in LPBF fabrication of Al-3.6Zn-0.6Mg alloy were found to be solidifi-cation cracks. Solidification cracks were initiated by fracture of Mg contained liquid film. Fracture of liquid film was because the pressure drop reached the critical pressure. The pressure drop increased with the increase of scan speed and decrease of laser power. Small pressure drop is helpful to reduce crack susceptibility. Crack-free Al-3.6Zn-0.6Mg alloy were fabricated by reducing scan speed and increasing the laser power. The optimized laser power was 490 W, scan speed was 400 mm/s, scan space was 0.08 mm, laser focus shift was-3.5 mm. Crack-free Al-3.6Zn-0.6Mg alloy exhibited a tensile strength of 306.7 +/- 7.4 MPa, yield strength of 214.3 +/- 3.1 MPa and elongation of 5.5 +/- 1 %. The study gives a direction to control the solidification crack in LPBF fabrication of Al-Zn-Mg alloy.