Prediction of Microstructure in Laser Deposition of Titanium Alloys

Laser deposition of titanium alloys is under consideration for aerospace applications, and offers significant increases in efficiency and flexibility compared to conventional manufacturing methods. However, its ultimate success will depend on the ability to predict and control the microstructure and resulting mechanical properties of the deposit. In this study, both 2-D continuum finite element modeling and 3-D cellular automaton finite element modeling of a thinwall geometry are used to investigate the effects of deposition process variables on microstructure in laser deposited Ti-6Al-4V. Numerical results for cooling rate and thermal gradient obtained from the 2-D models are used to provide insight into grain size and morphology, while the 3-D cellular automaton models are used to provide direct predictions of deposited microstructure. The numerical model predictions are subsequently compared with observed microstructures in LENSTM deposited Ti-6Al-4V.This work was supported by the Joint AFRL/DAGSI Research Program, project number ML-WSU-01-11, as well as by a grant from Wright State University and the Ohio Board of Regents.Mechanical Engineerin

Measurement of Residual Stresses in Parts Created by Shape Deposition Manufacturing

Residual stress build-up is a concern in any solid freeform fabrication process involving successive deposition of uncured or molten material, due to differential contractions caused by solidification or curing. The most detrimental effect of residual stresses is typically part warping, which can lead to unacceptable losses in part tolerance. In many processes residual stress build-up is a fundamental barrier to the consistent manufacture of high-quality artifacts. In this paper, two methods of measuring residual stresses in parts created by Shape Deposition Manufacturing (SDM) with microcasting are described. First, a technique for measuring warping in deposited plateshaped specimens is detailed, which can be used to determine residual stress resultants as well as to quantify gross effects of processing changes on residual stress magnitudes. Next, x-ray diffraction procedures are described by which residual stresses in deposited layers can be measured at discrete in-plane locations as a function of depth. Measured results for 308L stainless steel deposits determined from each method are interpreted in the context of residual stress modeling results obtained numerically in a separate research effort. The measured results provide insight into the effects on residual stress of both the material deposition path and the discrete droplet-bydroplet nature of the microcasting deposition process. The insights provided here may also be applicable to other processes involving successive material deposition.Mechanical Engineerin

Measurement and Modeling of Residual Stress-Induced Warping in Direct Metal Deposition Processes

Tolerance loss due to residual stress-induced warping is a major concern in solid freeform fabrication (SFF) processes. An understanding of how residual stresses develop and how they lead to tolerance loss is a key issue in advancing these processes. In this paper, results are presented from warping experiments on plate-shaped specimens created by microcasting and welding processes used in Shape Deposition Manufacturing (SDM). Results from these experiments give insight into differences between the two processes, the role of preheating and insulating conditions during manufacture and the influence of deposition path on magnitudes and distributions of warping displacements. Results are then compared to predictions from two types of residual stress models. While the models effectively predict warping magnitudes and the effects of various thermal conditions, they are unable to capture some of the more subtle trends in the experiments. Results from the experiments and numerical models suggest that a combination of initial substrate preheating and part insulation can be applied to SDM and similar SFF processes to limit warping deflections, which is substantially simpler than active control of part temperatures during manufacture. Results also suggest that 3-D mechanical constraints are important in achieving precise control of warping behavior in SFF processes.Mechanical Engineerin