Effect of deep cryogenic treatment on mechanical properties and residual stress of AlSi10Mg alloy fabricated by laser powder bed fusion

Abstract

During the fabrication of AlSi10Mg alloys by laser powder bed fusion (LPBF), the high-power density incident on the melt pool locally results in rapid heating and cooling rates, which leads to residual stress inevitably. Since the residual stress can lead to defects in the LPBF-built parts, it is necessary to reduce them by post-process treatments. However, the conventional heat treatments (HT) will weaken the mechanical properties while removing the residual stress, which is still one of the major challenges in additive manufacturing. This work proposes a deep cryogenic treatment (DCT) to reduce the residual stress, while maintaining the mechanical performance. The residual stress were measured by the hole-drilling strain-gauge method. The mechanical properties and microstructure were investigated using hardness measurements, tensile tests, scanning electron microscopy, and electron back-scattered diffraction. The results indicate that this treatment can relieve up to 72.7% of the residual stress, which is equivalent to the 76.5% reduction achieved by heat treatment. Moreover, this treatment can improve the tensile strength by 245 MPa and hardness by 56.72 HV0.2 in comparison to the heat-treated samples. It is suggested that plastic deformation occurred at the interfaces between Al matrix and eutectic-Si networks due to the discordant volume shrinkage responsible for the stress relief. Thermal stress was evaluated to verify the occurrence of the plastic deformation. The present work proves that the proposed DCT is an effective method to simultaneously reduce the residual tensile stress and maintain the strength and elongation in the samples built by additive manufacturing.