Temperature gradient induced tough-brittle transition behavior of a high-strength Al-4.2Mg-0.4Sc-0.2Zr alloy fabricated by laser powder bed fusion additive manufacturing

Abstract

The complex process and thermal behavior of laser additive manufacturing will lead to complex microstructure and property evolution. In order to enhance the processing efficiency of a high-strength Al-4.2Mg-0.4Sc-0.2Zr alloy fabricated by laser powder bed fusion (LPBF) additive manufacturing, a temperature gradient induced ductile-brittle transition behavior caused by changes in laser scanning speed for LPBF processed Al-4.2Mg-0.4Sc-0.2Zr alloy was reported in this study. As the laser scan speed increased from 0.4 ms-1 to 2.4 ms-1, a transition in fracture mode from toughness to brittleness was observed, where the tensile strength was decreased from 547.6 MPa to 472.3 MPa while the elongation decreased from 13.8% to 1.9%. Based on transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) characterizations, it was attributed to the cross-scale different microstructures from mesoscopic to microscopic as laser scan speed increased, including (i) more porosity in microscale (~30 μm), (ii) slenderer coarse grains and narrow equiaxed grain regions within melt pools in sub-microscale (~1 μm) and (iii) suppression of the Mg segregation along grain boundaries and formation of higher dislocation densities and stacking faults in the nanoscale (~200 nm). These changes in the microstructure caused the specimens processed with high scan speeds (above 1.6 ms-1) to be more prone to initiation of cracks during tensile, the ability of synergistic deformation between grains to decrease, and the bonding strength between grains to reduce. By simulating the thermal field of the melt pool, the formation mechanism of the different microstructures was ascribed to the different temperature gradients, which further determined the grain structures and element distributions. This work demonstrated that the structure and properties of LPBF processed high-performance Al alloys can be well tailored by processing parameter control.