On microstructure and early stage corrosion performance of heat treated direct metal laser sintered AlSi10Mg

Abstract

This study examines the impact of low-temperature heat-treatment on the microstructure and corrosion performance of direct metal laser sintered (DMLS)-AlSi10Mg alloy. Differential scanning calorimetry (DSC) was used to determine the phase(s) transition temperatures in the alloy. Two exothermic phenomena were detected and associated with the Mg2Si precipitation and Si phase precipitation in the as-printed alloy. Based on DSC results, thermal-treatments including below and above the active Si precipitation temperature at 200 °C and 300 °C, respectively, and 350 °C as an upper limit temperature for 3 h were applied to the as-printed samples. Scanning electron microscopy and X-ray diffraction analysis confirmed that heat-treatment from 200 °C to 350 °C promotes the homogeneity of the microstructure, characterized by uniform distribution of eutectic Si in α-Al matrix. To investigate the impact of the applied heat-treatment cycles on corrosion resistance of DMLS-AlSi10Mg at early stage of immersion, anodic polarization testing and electrochemical impedance spectroscopy were performed in aerated 3.5 wt.% NaCl solution. The results revealed more uniformly distributed pitting attack on the corroded surfaces by increasing the heat-treatment temperature up to 300 °C, attributed to the more protective nature of the spontaneously air-formed passive layer on the surface of the alloy at initial immersion time. Further increase of the heat treatment temperature to 350 °C induced severe localized corrosion attacks near the coarse Si particles, ascribed to the increased potential difference between the coalesced Si particles and aluminum matrix galvanic couple. In comparison, the corrosion of the as-printed and 200 °C heat treated samples was characterized by a penetrating selective attack along the melt pool boundaries, leading to a higher corrosion current density and an active surface at early exposure, associated with the weakness of the existing passive film on their surfaces.