Triaxial Residual Stress in Laser Powder Bed Fused 316L: Effects of Interlayer Time and Scanning Velocity

Abstract

The control of residual stress (RS) remains a challenge in the manufacturing of metallic parts using the laser powder bed fusion process (LPBF). This layer‐by‐layer manufacturing approach gives rise to complex triaxial RS distributions, which require extensive characterization effort for a broader acceptance of LPBF in industry. This study focuses on the distribution of bulk triaxial RS and surface RS in LPBF austenitic steel 316L. The RS are determined by X‐ray and neutron diffraction to characterize the RS distribution. Variations in the LPBF parameters interlayer time (ILT) and scanning velocity and their influence on the temperature distribution and resulting RS is investigated using thermographic data from in situ process monitoring. The RS in the LPBF 316L is tensile at the surface and compressive in the bulk. The RS is directly related to the thermal history of the part as shown by the in situ thermography data. Shorter ILT leads to higher temperatures of the part during the manufacturing, which decrease the RS and RS formation mechanisms. Interestingly, the surface RS does not agree with this observation. This study highlights the benefit of using multiple RS determination methods and in situ thermography monitoring to characterize the RS in LPBF processed parts.

Artificial Defects in 316L Stainless Steel Produced by Laser Powder Bed Fusion: Printability, Microstructure, and Effects on the Very‐High‐Cycle Fatigue Behavior

“…It has been, however, shown that the relaxation of the RS upon annealing is closely linked to the gradual dissolution of the cellular structure typical for 316L LPBF material. [ 44 ] Therefore, the optimal heat treatment to be performed to fully relax RS in 316L stainless steel without affecting its microstructure is still a subject for debate. Another way to investigate the effect of RS on the VHCF behavior is to embed artificial defects at different locations so that some defects would be located in areas experiencing tensile RS.…”

Effect of interlayer time interval on residual stress distribution in Ti6Al4V alloy manufactured by laser powder bed fusion

“…As a consequence, an increase in sub-grain size up to a factor 6, melt pool depth up to 20% and cooling rates up to a factor 12 were measured. Sprengel et al [10] investigated the effect of interlayer time and scanning velocity and their influence on the temperature distribution and resulting residual stresses in L-PBF manufactured 316L steel. In their experiments, it was found that at the surface, tensile residual stresses developed and these are directly related to the thermal history.…”

“…Heat input from the laser beam and heat transfer by conduction creates thermal cycles that can affect the microstructure and mechanical properties significantly [3–9]. In addition, high cooling rates and thermal gradients present in the LPBF process increase the magnitude of the residual stress [10,11]. This can leads to a geometrical non-conformity [12,13] and delamination of the as-build component.…”

“…Mukherjee et al [21] have found that on the one hand, the thickness reduction of each layer and increased heat input can decrease the residual stresses by more than 20% enhancing the thermal distortion by about 2.5 times. Columnar grains with martensitic α’ microstructure are usually developed in the as-build Ti6Al4V alloy produced by the LPBF method [10] due to high cooling rates (between 12,000°C/s and 40,000°C/s) that are present during the fabrication process [22,23]. The as-fabricated parts exhibit a strong solidification texture {100}β that is aligned nearly parallel with the build direction across many layers [22].…”

Influence of Heat Control on Properties and Residual Stresses of Additive-Welded High-Strength Steel Components

“…These include, for example, the contour method, neutron and synchrotron diffraction, the dissection technique, or the so-called deep-hole drilling [17]. Several investigations of AM components showed that high residual stresses and residual stress gradients could be expected, especially on the component surfaces [18,19], which also were in the range of the material yield strength. Due to this, as well as the fact that high tensile residual stresses; e.g., at notches, have a detrimental effect on the cold-cracking risk and performance of the component, investigations of the residual stresses should initially be analyzed predominantly by means of XRD.…”