Additive manufacturing opened new possibilities in fabrication of components. However, at the same time, it introduced new tasks for metalurgists and materials engineers as the microstructure of the components prepared by additive manuffacturing is significantly different than the microstructure of the conventionally fabricated components. One of the characteristics largely influenced by additive manufacturing is the residual stress distribution. Residual stresses play a crucial role in determining the performance and lifetime of engineered components.
Some examples taken from the field of materials fabricated by additive manufacturing are shown. Residual stresses were measured using neutron diffraction in these demonstrations. The use of neutron diffraction is indispensable for the measurement of residual stresses in the bulk of the material. It is a non-destructive method; therefore, the sample can be later used for other examinations.
The impact of manufacturing strategies on the development of residual stresses in Dievar steel is presented. Two fabrication methods were investigated: conventional ingot casting and selective laser melting (SLM) as an additive manufacturing process. Subsequently, plastic deformation in the form of hot rotary swaging at 900°C was applied. Microstructural and phase analysis, precipitate characterization, and hardness measurement—carried out to complement the investigation by neutron diffraction—showed the microstructure improvement by rotary swaging. The study reveals that the manufacturing method has a significant effect on the distribution of residual stresses in the bars. The results showed that conventional ingot casting resulted in low levels of residual stresses (up to ±200 MPa), with an increase in hardness after rotary swaging from 172 HV1 to 613 HV1. The SLM-manufactured bars developed tensile hoop and axial residual stresses in the vicinity of the surface and large compressive axial stresses (−600 MPa) in the core due to rapid cooling. The subsequent thermomechanical treatment via rotary swaging effectively reduced both the surface tensile (to approximately +200 MPa) and the core compressive residual stresses (to −300 MPa). Moreover, it resulted in a predominantly hydrostatic stress character and a reduction in von Mises stresses, offering relatively favorable residual stress characteristics and, therefore, a reduction in the risk of material failure. In addition to the significantly improved stress profile, rotary swaging contributed to a fine grain (3–5 µm instead of 10–15 µm for the conventional sample) and increased the hardness of the SLM samples from 560 HV1 to 606 HV1. These insights confirm the utility of rotary swaging as a post-processing technique that not only reduces residual stresses but also improves the microstructural and mechanical properties of additively manufactured components.
Residual stresses were also measured in samples manufactured by two different AM technologies within one component: the bottom half prepared using either Laser Powder Bed Fusion (L-PBF) or Direct Energy Deposition (L-DED), and the second half of the component vice versa, i.e. using L-DED or L-PBF, respectivelly. A combination of fabrication by different additive technologies is not a commonly used procedure in practice. Cubic regions (25mm × 25mm × 25mm) of 316L steel were printed either by L-PBF or by L-DED on the steel substrate and afterwards finished to a height of 50 mm by the second technology. The aim was to determine the residual stresses that each technology introduces into the samples and a comparison with FEM prediction: the stresses measured by neutron diffraction are to be used for validation of the FEM model, which will be applieded as an optimization tool in the combination of AM production methods.