Processing defect, microstructure evolution and mechanical properties of laser powder bed fusion Al-12Si alloys

Abstract

The laser power and scan speed are significant processing parameters influencing the defects and microstructures due to affecting the thermal state within the melt pool during laser powder bed fusion (LPBF). To this end, these process parameter combinations under a constant laser energy density are correlated with defects (balling phenomenon and pores) and microstructures (melt pool arrangement and cell morphology). Setting the parameter combination (laser power and scan speed are 200 W and 910 mm/s, respectively) yields large balling droplets (average size of 150 μm) due to the relatively high melt viscosity resulting from the low melt pool temperature. In this case, a limited surface tension difference acting on a liquid surface cannot provide sufficient driving forces against viscous drags to promote the efficient spreading of the molten alloy. Cavity regions are formed between the large balling droplets and the previous layer, forming lack-of -fusion pores. By increasing the laser power and scan speed gradually, samples with relative densities greater than 99.5% are produced. This phenomenon can be attributed to the increases in laser power and scan speed, which improves the spreading ability of the molten alloy. With increasing laser power and scan speed, the cell size decreases (from 0.51 μm to 0.42 μm) because of the increase in the cooling rate. The sample (laser power and scan speed are 350 W and 1590 mm/s, respectively) has the highest performance (310 MPa yield strength, 470 MPa ultimate tensile strength, and 6.6% fracture strain) due to its highest relative density (greater than 99.5%) and fine microstructure (average cell size of 0.45 μm). All four samples exhibit anisotropic mechanical properties resulting from the combination of the melt pool boundary and lack-of-fusion pores.