Effect of artificial aging on the strength, hardness, and residual stress of SLM AlSi10Mg parts prepared from the recycled powder

Abstract

This study investigated the effect of artificial aging on the hardness, strength, and residual stress of SLM (Selective Laser Melting) AlSi10Mg parts prepared from recycled powder used for 30 SLM printing cycles. First, an analysis was conducted for a comparison of recycled and virgin powders; it revealed that the powders have the same shape and phase and chemical composition. The only significant differences were the slightly increased average particle size and higher oxygen content in the recycled powder, especially on the powder surface. Typical molten pools were found in the microstructure, and a fine cellular type of structure was observed. The peripheral areas of the molten pools contained coarser cells with an average size of 0.6 mu m(2), and the middle part of the molten pools contained finer cells with an average size of 0.17 mu m(2). Fine Si precipitates with an average size of 25 nm were observed inside the cells. In terms of microstructure, no significant differences were observed when compared to the published data relating to SLM using virgin powders. This suggests that recycled powder after 30 printing cycles can be utilized for SLM component preparation to achieve cost-effectiveness. However, comparison of products with the use of virgin and recycled powders showed that the use of recycled powder affects the submicron characteristics (precipitate size increase), density and the resulting mechanical properties. Artificial aging of samples from the recycled powder at 170 ? caused an increase in hardness, Young's modulus and strength, while ductility remained essentially the same after aging. Peak values of hardness and strength were determined after 6 h of artificial aging. The increase in strength of the prepared samples can be explained by the precipitation of fine Si particles and their coarsening inside the cells. However, after longer aging times, the precipitates continued to coarsen, coalesced and displayed a plate-like shape, which led to the reduction in strength. The residual stress measured by the bridge curvature method (BCM) showed a significant decrease with increasing artificial aging time. The lowest residual stress was determined for an aging time of 100 h; however, the difference between residual stress for 6-7 h and 100 h was not significant. In order to obtain higher strengths and remove internal stresses after SLM during one heat treatment, the most suitable time for artificial aging at 170 ? seems to be 6 h.