Niroj Maharjan

This study evaluates two ultrasonic impact treatment (UIT) strategies for flux-cored arc welded high-strength structural steel: top-surface peening after welding and interlayer peening between successive weld passes. Top-surface UIT increased the surface hardness by more than 130% and introduced compressive residual stresses of up to 582 MPa, consistent with severe plastic deformation and work hardening. In contrast, interlayer UIT did not produce a comparable increase in average microhardness or compressive residual stresses in the interlayers. Instead, it modified the weld metal microstructure by promoting the formation of equiaxed grains between columnar regions. This change indicates that the strain introduced by UIT disrupted columnar grain growth from subsequent weld passes. Interlayer UIT also substantially reduced geometric distortion, decreasing the distortion angle from 5.5° in the as-welded joint to below 1.2°. Top-surface UIT reduced the distortion angle to ~ 3.5°. These results indicate that top-surface UIT may be preferred where enhancement of surface integrity and fatigue resistance is required, while interlayer UIT offers advantages for distortion control and microstructural engineering during weld build-up.

Herein, an alternative way of achieving site‐specific mechanical properties is explored—the hybridization of a directed energy deposition technology with a secondary deformation process (hammer peening (HP)) which acts between deposited layers. By applying the peening in a selective manner, microstructure and hence mechanical properties can be locally varied. Microstructural characterization reveals recrystallization in the HP‐induced deformation zone. The columnar grains of as‐built regions with a grain size of ≈26 μm are transformed into a recrystallized zone with equiaxed grains having a size of ≈8 μm. There is also a highly strain‐hardened region below this recrystallized zone where the dislocation density is more than two times higher than in the as‐built condition. Subsequent tensile testing reveals that these microstructural zones corresponded to local enhancement in tensile strength normal to the build direction. The strengthening mechanisms are identified as Hall–Petch and dislocation (Taylor) strengthening, and their relative contributions are studied. The local strength enhancement comes at the expense of ductility in the build direction, which is studied via finite element modeling and attributed to strain localization into non‐strengthened areas. The results from this work show the possibility of achieving site‐specific properties via interlayer processing.

Control of residual stresses (RS), inherent to fusion-based additive manufacturing (AM), process is important for the satisfactory mechanical performance of components. Recent work has attempted to control the RS profiles in AM components by applying mechanical peening between built layers. During laser powder bed fusion (LPBF), it has been shown that subsequent layer building does not relieve all the peening-induced compressive stresses. In this work, a similar study has been performed on a directed energy deposition (DED) process. It is shown that owing to the vastly different thermal profile in DED compared to LPBF, the compressive RS induced by peening, is completely alleviated during subsequent layer deposition for 316L stainless steel. Irrespective of the magnitude and depth, the peening-induced compressive stresses were not present in the final part. Experimental and numerical analyses revealed that stress relief due to intrinsic heating was insufficient to explain stress relaxation. Rather, the localized heating and constrained expansion from surrounding cold material was the mechanism responsible for strain redistribution and hence stress relaxation.