Evolution of texture and residual stresses in 2205 duplex stainless steel during laser powder bed fusion

This study uses a custom-designed laser powder bed fusion machine, capable of operating in neutron instruments, to track metallic material evolution during additive manufacturing process.  More specifically, it investigates the development of residual stresses in textured 2205 duplex stainless steel during laser powder bed fusion. In situ and operando neutron diffraction experiments were conducted to study the transient and real-time evolution of stresses and strains during processing, using an AM machine designed for neutron studies. Additionally, Bragg-edge imaging was employed to investigate the crystallographic texture. The results showed that residual stress redistribution primarily occurs in the first set of added layers when further layers are added on top. The cube texture observed in the sample significantly affects residual stress determination, leading to inaccuracies up to 96 MPa if not accounted for. This highlights the need for orientation-dependent diffraction elastic constants in residual stress calculations. Furthermore, variations in texture intensity across the sample dimensions were found to be driven by changes in the local temperature history, which were deciphered from real-time strain measurements. Finally, this study demonstrates the potential of combining laser powder bed fusion with neutron diffraction to investigate the underlying mechanisms of additive manufacturing in the bulk of the sample.

Additive manufacturing (AM), also known as 3D printing, encompasses a broad range of technologies that build parts layer-by-layer and enable the creation of near-net-shaped parts with complex and customized geometries. Laser powder bed fusion (LPBF) is an established AM technology, known for its versatility and ability to manufacture metallic parts with good accuracy and quality. Due to the small heat source, high temperature, and rapid scanning speeds, this process involves local fast cooling rates up to ∼ 10⁶ K/s. Among others, the steep temperature gradients generated under these conditions result in the buildup of noticeable residual stresses. Residual stress formation is a complex interplay between the material properties, manufacturing parameters, part size, and part geometry. They can be detrimental to part reliability by inducing, e.g., distortion, cracking, delamination, and reducing fatigue properties. Therefore, a better understanding of residual stress formation is essential for predicting and mitigating these effects by adjusting processing strategies and parameters, thus facilitating the transferability to industrial applications.

In this study, a combination of advanced neutron techniques have been used to investigate the residual stresses and texture evolution, including in situ and real-time experiments, in a duplex stainless steel. The samples were manufactured at the POLDI beamline (SINQ) with recently custom-designed laser powder bed (n-SLM) additive manufacturing machine that can be operated in neutron instruments.

The results revealed a strong cube texture aligned with the sample geometry throughout the sample with variations in intensities due to changes in the local thermal history during the manufacturing. Residual Von Mises stresses up to 600 MPa were observed along the transverse direction of the sample, representing nearly 60 % of the material’s yield strength at room temperature. Additionally, the principal strain direction in the bulk of the sample aligns with the diagonal of the sample in the transverse plane. Residual stress redistribution was primarily observed in the bottom part of the sample when further sets of layers were added on top. Furthermore, texture has a strong influence on the residual stress determination, with differences up to 96 MPa if not accounted for.

The outcomes of this study highlight the potential of the n-SLM combined with advanced neutron techniques for the study of AM. This approach allows for the gathering of information from the bulk of centimeter-sized samples, in contrast to the typically surface measurements or millimeter-sized samples studied in LPBF. The experimental information obtained from both simple and complex sample geometries will be valuable for testing, validating, and benchmarking thermomechanical and metallurgical numerical simulations at various stages of the process. Furthermore, the use of centimeter-sized samples will also facilitate the transfer of these findings to industrial applications.