Laser Processing and Additive Manufacturing of Metallic Alloys: Laser Impact Welding, Laser Shock Peening, and Directed Energy Deposition


August 2022


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The objectives of this work are to develop a more accurate numerical simulation of the laser impact welding (LIW) process, to investigate the effects of the laser shock peening (LSP) process as a post-treatment to LIW, and to examine the effects of interlayer machining (IM) on the microstructure and residual stress (RS) in builds manufactured via the directed energy deposition (DED) additive manufacturing (AM) process. First, a method for capturing the laser-generated plasma pressure load is proposed to increase the accuracy in numerical LIW simulations. LIW is a recently developed technology for the fully-mechanical joining of thin metallic foils. LIW is of great interest as it could be used to join any pair of similar or dissimilar metals without any melting required, thus avoiding the formation of brittle intermetallic compounds. However, the LIW of thin metallic foils is a complex process, and the underlying physical phenomena involved in the mechanical interlocking of the foils are not yet fully understood. Therefore, to benefit from its full potential in the near future, extensive research on experimental implementation and numerical simulation of the LIW process is of paramount importance. Thus far, only a few articles on numerical simulations of the LIW process have been published in the literature. All of these works have made over-simplifying assumptions such as using a pre-defined deformed flyer foil shape with a uniform initial velocity, that diminish the accuracy of the simulation. In contrast, the research in this work proposes the idea that the incorporation of the actual spatial and temporal profiles of the laser beam and modeling of the corresponding pressure pulse based on an LSP approach could provide a more realistic prediction of the LIW process mechanism. In this study, spatial and temporal profiles of an Nd:YAG laser beam pressure pulse are experimentally characterized and fully captured for use in numerical simulations of LIW. Both axisymmetric, arbitrary Lagrangian-Eulerian, and Eulerian dynamic explicit numerical simulations of the collision and deformation of the flyer and target foils are created. The effect of the standoff distance between the foils on impact angle, velocity distribution, springback, the overall shape of the deformed foils, and the weld strength in lap shear tests are investigated. In addition, the jetting phenomenon (separation and ejection of particles at very high velocities due to high-impact collision) and interlocking of the foils along the weld interface are simulated. Preliminary work indicates very similar deformation and impact behaviors in simulation results compared to experiments performed for validation. Next, using the same laser system configuration, an experimental methodology is proposed to investigate the effects of the extremely high strain rates present in the LSP process on the strength and interface geometry of welds obtained from LIW of dissimilar metallic foils. LSP is a processing technology capable of improving fatigue life and performance by inducing plastic deformations and, thus, compressive RS into the near-surface depth of metallic components. Due to this unique ability, in recent years, LSP has been explored as a post- treatment to improve performance in metallic welds fabricated via conventional fusion-based techniques. However, in high-velocity impact welding (HVIW) methods, specifically LIW, LSP has never been explored as a post-welding treatment. Therefore, in this work, LSP’s potential to improve the weld strength and integrity in dissimilar metallic joints fabricated via the LIW technique is investigated for the first time. Single and double LSP shots are applied to LIW foils using three different metallic material combinations. Subsequent lap shear testing show that single-shot LSP increases the average weld strength by 12% to 25%, depending on the flyer and target material combination. In contrast, with double-shot LSP, the average weld strength decreases regardless of the flyer and target materials involved. Scanning electron microscope images reveal wavy weld interfaces and increased interlocking between the foils for the single-shot LSP treatments as compared to the initial “flat” weld interface geometry, thereby leading to greater flyer/target weld strength. In the double-shot LSP treatments, however, separations and melting are observed along the weld interface due to rebounding and excessive plastic heat dissipation of the foils. The findings of this study reveal the first insights and effects regarding the application of LSP as a post-welding treatment beyond conventional fusion-based welding to HVIW methods. Last, an experimental procedure is presented to examine IM’s impact on the microstructure and RS in metallic components manufactured via the powder-based DED AM process. DED is one of the major additive manufacturing processes for producing and repairing large-size and high-value metallic components. At smaller size scales (micro to millimeters), DED could be potentially used in conjunction with LIW to manufacture small devices such as micro/mini robots. However, IM may be necessary to provide a flat surface on the DED build for its successful joining to metal sheets/wires via LIW. Recently, it has been shown that the grain structure of the materials influences the in-situ LIW behavior and likely performance. Therefore, to predict the performance of assemblies manufactured via hybrid processes that combine DED, IM, and LIW, it is important to understand the IM effects on the DED build’s microstructure. Therefore, in this work, for the first time, the influence of IM on the processing-structure-properties relationships in powder-based DED of stainless steel 316L is investigated. Four types of single-track builds are manufactured on stainless steel 316L substrates: single-layer, double-layer, machined single-layer, and double-layer with IM. The effects of IM on the microstructure and residual stress before and after the second layer’s deposition are studied via metallographic imaging and neutron diffraction. In single-layer samples, due to induced plastic strains and heat generated during the machining process, the microstructure undergoes dynamic recrystallization, which results in smaller, more equiaxed grains. In double-layer samples, IM results in greater tensile stresses at the interface of the two deposited layers, where a considerable variation in the microstructure is also observed. This is attributed to the delay caused by IM resulting in the second layer’s deposition onto a cooler first layer and thus a higher temperature gradient. However, the overall build height remains almost unchanged, with a slight reduction in build width. This study’s results show that IM has important and influential effects that should be considered in the design and control of the processing-structure-properties-performance relationships in the DED AM.



Engineering, Mechanical, Engineering, Materials Science, Physics, General