Microstructural formation and characterization mechanisms of selective laser melted Al–Si–Mg alloys with increasing magnesium content

We elucidated the microstructural formation and characterization mechanisms of Al–4Si–Mg selective laser melted (SLM) specimens with increasing Mg content (~2.5 mass%). First, the effect of Mg content on the processability of Al–4Si–Mg SLM specimens was systematically studied. From Al–4Si to Al–4Si–2.0Mg alloys, densified SLM specimens with a relative density of more than 99.5% were achieved by optimizing scan conditions. The relative density of the Al–4Si–2.5Mg SLM specimen decreased to approximately 99.4% due to the generation of gas pores by Mg evaporation. Then, the relation between the metal structures and mechanical properties and thermal conductivity of the Al–4Si–Mg SLM specimens was investigated. The tensile strength and 0.2% proof stress of the Al–4Si–Mg SLM specimens significantly increased with the Mg content. A multiscale metal structural analysis of the Al–4Si–Mg SLM specimens revealed that this strengthening was ascribed to multiple factors: both the crystal grain refinement and increase of solid-solute Mg in the α-Al matrix obstructed the dislocation movement. On the other hand, the elongation decreased in a stepwise manner with increasing Mg content. The SEM observations of the fracture surfaces in the Al–4Si–Mg tensile SLM specimens indicated that the decrease in elongation was attributed to the gas pores generated by Mg evaporation and shrinkage cavities. The thermal conductivity decreased slightly as the solid-solute Mg in the α-Al increased. These results can provide profound knowledge for the design of novel Al SLM alloys.