Residual stresses in metal additive manufacturing

In crystalline materials, residual stresses can be measured non-destructively using X-ray and neutron diffraction. Due to the limited penetration depth of X-rays ranging from a few microns (with laboratory sources) to a few millimeters (using high-energy synchrotron radiation) only stresses at or close to the surface are accessible. Neutron diffraction on the other hand allows measurements up to several centimeters into the volume of the samples and serves as a nondestructive probe of the bulk microstructure in engineering solids.

The engineering materials science diffractometer STRESS-SPEC, one of the most powerful neutron diffractometers for residual stress, texture and phase analyzes is routinely available at MLZ. Within the scope of the user program of the MLZ, since the diffractometer was put into operation in 2005, more than 260 external user experiments have been carried out, which were mainly based on questions relevant to industry.

Laser Powder-Bed Fusion (LPBF), as one of the most popular metal additive manufacturing techniques, demonstrates great potential in producing complex-shaped components with improved properties. However, the microstructure heterogeneity and residual stress associated anisotropy and defects remain as a most critical problem, limiting its further applications. Based on the powerful neutron facility to solve the current urgent problem of the residual stress on LPBF parts, the scientific goal is to study first the relationship between the residuals stress and LPBF parameters, and then further to understand the mechanism of the complex thermo-mechanical laser melting processes. On the other hand, the technical goal is to develop sound LPBF parts used in thermal and nuclear power plants for contributing significantly to their long-term operability and life extension.