On microstructure and corrosion properties of the laser-powder bed fused (L-PBF) AlSi10Mg alloy

Rafieazad, Mehran (2021) On microstructure and corrosion properties of the laser-powder bed fused (L-PBF) AlSi10Mg alloy. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Despite the already existing advantages of additively manufactured AlSi10Mg, there are still process-induced imperfections associated with the Laser-Powder Bed Fusion process, such as microstructural inhomogeneity and high level of porosity. This thesis aims to examine the impact of (i) low-temperature heat-treatment, (ii) recycled feedstock powder, (iii) the laser-powder bed fusion (L-PBF) process parameters, and (iv) post-printing surface treatment on the solidification defects, microstructures, and the resultant corrosion properties of L-PBF-AlSi10Mg alloy. 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 the uniform distribution of eutectic Si in the α-Al matrix. Moreover, the microstructural analysis confirmed that using the recycled powder in the fabrication of AlSi10Mg leads to (i) an increased volume fraction of internal porosities and solidification micro-cracks, (ii) more coarsening of interdendritic eutectic-Si network particularly along the melt pool boundaries, which were correlated to the larger size and irregular shape of the recycled powders compared to the virgin powders, leading to the reduced thermal conductivity of the recycled powders. Additionally, the implemented process parameters modifications were found to be not only effective in reducing the as-printed surface roughness of the components, but also led to the formation of cyclic small-large melt pools (MPs) in the Upskin layers of the fabricated samples. Employing friction stir processing (FSP) as a post-printing surface modification technique was shown to be effective in eliminating the process-induced porosities of the L-PBF AlSi10Mg alloy, and resulted in drastic microstructural homogenization, grain refinement, and uniform dispersion of refined Si particles. To investigate the impacts of the above-mentioned microstructural changes on the corrosion performance of the alloy, anodic polarization testing, electrochemical impedance spectroscopy in aerated and deaerated 3.5 wt.% NaCl solutions, intergranular corrosion, and Mott-Schottky tests were performed. The electrochemical measurements confirmed the improved corrosion resistance of the alloy and reduced susceptibility to penetrating selective attack at initial immersion time in the electrolyte solution by increasing the heat-treatment temperature from 200 °C to 300 °C. Moreover, the corrosion results confirmed a slight degradation of the corrosion properties of the recycled-powder fabricated samples, ascribed to further coarsening of Si-network along their melt pool boundaries. Additionally, optimization of the process parameters confirmed that the fabricated sample at the highest volumetric energy density revealed a degraded corrosion performance resulted from its extended HAZ and coarser microstructure. Moreover, improvement of the corrosion performance of the FSPed sample was confirmed by the positive shift of the pitting potential and reduction in the corrosion rate and corrosion current density as compared to the as-printed samples. This research provides solutions to the existing challenges in the industries for adopting additive manufacturing methods due to the final cost, microstructural homogeneity, high level of porosity, residual stress, and initial surface roughness.

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