Microstructure, mechanical properties, and crack formation of aluminum alloy 6063 produced via laser powder bed fusion

Additive manufacturing provides alternative solutions for complex geometry and design freedom for the customization of aluminum alloys, while the unique microstructure as well as the complex solidification behavior is of great concern due to the severe thermal stress in the layer-by-layer accumulated process. In this work, the effects of laser power, laser scanning speed, and hatch spacing on the microstructure and mechanical properties of a laser powder bed fusion (LPBF) fabricated thermal conductive aluminum alloy 6063 (AA6063) were studied. The relationship between processing parameters and metallurgical defects was investigated. The element distribution and misorientation between grains around the crack were characterized. The results revealed that the majority of cracks were parallel to the building direction. The crack density was suppressed by increasing the scanning speed. The orientated cracks lead to the anisotropy of mechanical properties. The strength and ductility of the vertical samples are both higher than those of the horizontal samples. After aging treatment, an ultimate tensile strength of 241 MPa, yield strength of 215 MPa, and elongation of 4.9% were obtained in the vertical specimen of the LPBF processed AA6063. It was demonstrated that cracks tend to stretch along the high-angle grain boundaries because of the concentration of local thermal stress at the final stage of solidification. This work provides guidance for defects suppression and mechanical properties optimization in additively manufactured AA6063 and other aluminum alloys.