Influence of laser process parameters on the densification, microstructure, and mechanical properties of a selective laser melted AZ61 magnesium alloy

We present a detailed, systematic study of how laser process parameters affect the formation characteristics, densification, microstructure, and mechanical properties of AZ61 magnesium alloy specimens prepared by selective laser melting (SLM). The results allow us to develop a method to control defects and optimize the SLM parameters for the fabrication of a dense, bulk AZ61 magnesium alloy. A bulk AZ61 magnesium alloy prepared by SLM is used to investigate how hatch spacing, large-scale scanning speed, and energy input affect the macrosurface morphology (before and after polishing) and how the surface roughness and the relative density of the specimens affect the microstructure and mechanical properties of the dense bulk material. The results show that process parameters strongly influence the macrosurface topography, which we divide into four stages: a strong surface balling pore region, weak surface balling pore region, rough scan track region, and flat smooth region. In addition, the surface roughness changes from 18.95 to 7.49 μm. Two types of pore shapes (meniscus and circular) are governed by the scan speed, and pore size is a function of hatch spacing. We also obtain unusual X-ray diffraction results that depend on input energy in the range of 138.89–208.33 J/mm3. The α-Mg diffraction peak shifts to the right and reaches a maximum at 156.25 J/mm3 (scanning speed of 400 mm/s and hatch spacing of 0.06 mm), and the relative density reaches a maximum of 99.4% because the amount of Al in solid solution is maximized. The microstructure results show that the microstructure evolution involves dispersed α-Mg grains (refined equiaxed α-Mg grains) and coarsened equiaxed α-Mg grains with an average grain size ranging from 1.61 to 2.46 μm. The tensile properties of the as-built AZ61 Mg alloy at room temperature are superior to those of an as-cast AZ61 Mg alloy. The ultimate tensile strength of the as-built AZ61 is 93% greater than that of the as-cast AZ61 alloy, and the yield strength is increased by 136%.