Residual Stress and Distortion in Machined Wrought and Additively Manufactured Metallic Components

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The described research aims to elucidate the factors that influence post-machining residual stress and part distortion in machining of wrought (rolled and heat-treated) aluminum al- loys and additively manufactured stainless steel components. At present, even with carefully designed machining process parameters, part distortions that arise in wrought aluminum alloys due to the presence of inherent residual stress (IRS) makes it difficult to produce high aspect-ratio thin-walled monolithic components requiring increasingly stringent toler- ances. Lack of understanding on the effects of IRS during high-speed machining (HSM) inhibits it’s adaption into the manufacturing work flow to determine appropriate process parameters or tool path strategy necessary for competitive economic production that avoids trial-and-error machining strategies with such parts. This work intends to shed light on the influences of inherent material characteristics, such as IRS and inhomogeneous/anisotropic material properties, on the machining response (due to coupled effects of IRS and MIRS) in both wrought and additively manufactured metallic components. It is first hypothesized that coupling between IRS and MIRS contributes to final part residual stress and distortion. This coupling, contrary to assumptions prevalent in the existing literature, is believed to not simply allow for superposition of the inherent and machining contributions, but rather call for a nonlinear relationship that is dependent on various factors. In the presented work, the effects of IRS and material inhomogeneity and anisotropy are analyzed through computa- tional studies and experimental validations made possible through industrial collaborations. The work described herein, first, studies the effect of IRS and MIRS on the final-state of residual stress (FRS) and post-machining part distortion and suggests that there is indeed a nonlinear coupling that exists between IRS and MIRS. Analyses have been performed using 2D orthogonal cutting and 3D end-milling computational models. The material model, val- idated using the 2D model based on observations from published literature, is used in both the 2D and 3D case studies. IRS in the wrought material has been modeled by employing an iterative stress reconstruction algorithm (ISRA) to generate a compatible IRS field that encompasses the entire wrought material based on limited experimental measurements. The results from the 2D case studies support the existence of a nonlinear coupling between IRS and MIRS that subsequently determines the FRS. The 3D case studies reveal distortion re- sults that elucidate the coupling effect even further by showing that IRS not only influences the distortion of the final part, but that the degree of influence is dependent on the coupling of the IRS profile and the HSM tool path. The presence of said coupling can thus either magnify or reduce the final part distortion. The above established model is further devel- oped to simulate machining of a 2 mm thin-walled part of the same material. To compare and validate the predictive capability of the computational model, corresponding machining experiments are performed, followed by coordinate measuring machine (CMM) distortion analysis on the thin wall. In order to characterize the material IRS and subsequently in- corporate it appropriately in the computational model via ISRA, Neutron Diffraction (ND) stress measurements are performed at various locations on a plate of the same material, from which smaller blocks are extracted for machining. High-speed machining simulations are performed and the results are compared with the CMM measurements. Results indi- cate that the presence of IRS in the material significantly influences the distortion profile. Idealized material removal by element deletion simulation shows that IRS alone does not induce significant distortion in these specific parts whereas the coupling between IRS and MIRS is the prominent driver. The presence of IRS also tends to drive the distortions to- wards experimentally measured result. It is also hypothesized that additively manufactured parts that require post-process machining also experience similar coupling between IRS (in the substrate and the build) and MIRS, which likewise is believed to significantly affect post-machining part distortion. To test this hypothesis, first, a continuously coupled com- putational fluid dynamics (CFD) and finite element analysis (FEA) framework is developed in collaboration with other researchers. This model, involving directed energy deposition (DED) builds, is employed to simulate manufacture of single-layer and double-layer, single- bead DED builds. Corresponding experiments are performed to create samples, followed by geometry scanning and ND stress measurements to compare with the simulation results. The predicted final stress profiles agree well with experimentally measured profiles obtained via ND stress measurements. Subsequently, the geometry and stress profile obtained from the coupled CFD-FEA framework is employed to simulate an interlayer machining operation in order to study the influence of IRS and inhomogeneous microstructure on post-machining residual stress and distortion. A kinetic Monte-Carlo based microstructure prediction model is calibrated and employed to obtain the microstructure in the build. The predicted mi- crostructure is compared with EBSD measurements for validation. The microstructure is then incorporated into the FEA model to create a representative volume element (RVE) with varying strengths for grains of different sizes, based on a combined Johnson Cook - Hall Petch material model. It is found that IRS and inhomogeneous microstructure influences the part distortion after machining. IRS shows a greater influence on distortion compared to microstructure. Finally, the IRS is found to influence the bulk behavior of the material more significantly, whereas, the microstructure influences the local behavior of the material. With this new understanding of the influence of various factors on post-machining stress and part distortion, machining operations can be improved further to reduce waste and improve part compliance to tight tolerances, including in new hybrid manufacturing markets.

Engineering, Mechanical