Microstructure and residual stress analysis as heat treatment of additive manufactured A356.2 alloy by powder bed fusion

Abstract

Aluminum alloys are commonly used in additive manufacturing (AM) because of their high specific strength and low weight. However, the high energy used in the AM process to overcome heat reflectivity induces thermal gradients and residual stress in the products. This study, A356.2 alloy was used for AM, and the residual stress and microstructure generated during this process were observed. X-ray diffraction (XRD) and nanoindentation were used to measure the surface residual stress, while the residual stress inside the material was measured using neutron diffraction. During the nanoindentation tests, we varied the indentation load conditions and compared the results with the residual stress values measured using XRD. To relieve the residual stress, we conducted a heat treatment and analyzed the changes. For surface residual stress, tensile residual stress (27∼91 MPa) in the as-built samples changed to compressive residual stress (‐28∼49 MPa) after heat treatment, with higher heat treatment temperatures producing stronger compressive residual stress. As for the internal residual stress, it was generally relaxed after heat treatment. As indentation depth increased, the residual stress results became similar to the residual stress values measured by XRD. In addition, the electron channeling contrast imaging (ECCI) technique was used for observation of lattice defects within precipitates generated and coarsened by heat treatment. We assessed the efficiency of each measurement method by comparing the results obtained through different techniques.