Showing papers by "Tarasankar Debroy published in 2019"

Scientific, technological and economic issues in metal printing and their solutions

Abstract: 3D printing is now widely used in aerospace, healthcare, energy, automotive and other industries. Metal printing, in particular, is the fastest growing sector, yet its development presents scientific, technological and economic challenges that must be understood and addressed.

Three-dimensional grain growth during multi-layer printing of a nickel-based alloy Inconel 718

Abstract: Heterogeneous grain structure is a source of the inhomogeneity in structure and properties of the metallic components made by multi-layer additive manufacturing (AM). During AM, repeated heating and cooling during multi-layer deposition, local temperature gradient and solidification growth rate, deposit geometry, and molten pool shape and size govern the evolution of the grain structure. Here the effects of these causative factors on the heterogeneous grain growth during multi-layer laser deposition of Inconel 718 are examined by a Monte Carlo method based grain growth model. It is found that epitaxial columnar grain growth occurs from the substrate or previously deposited layer to the curved top surface of the deposit. The growth direction of these columnar grains is controlled by the molten pool shape and size. The grains in the previously deposited layers continue to grow because of the repeated heating and cooling during the deposition of the successive layers. Average longitudinal grain area decreases by approximately 80% when moving from the center to the edge of the deposit due to variable growth directions dependent on the local curvatures of the moving molten pool. The average horizontal grain area increases with the distance from the substrate, with a 20% increase in the horizontal grain area in a short distance from the third to the eighth layer, due to competitive solid-state grain growth causes increased grain size in previous layers.

Experiments and simulations on solidification microstructure for Inconel 718 in powder bed fusion electron beam additive manufacturing

Abstract: Previous research on the powder bed fusion electron beam additive manufacturing of Inconel 718 has established a definite correlation between the processing conditions and the solidification microstructure of components. However, the direct role of physical phenomena such as fluid flow and vaporization on determining the solidification morphology have not been investigated quantitatively. Here we investigate the transient and spatial evolution of the fusion zone geometry, temperature gradients, and solidification growth rates during pulsed electron beam melting of the powder bed with a focus on the role of key physical phenomena. The effect of spot density during pulsing, which relates to the amount of heating of the build area during processing, on the columnar-to-equiaxed transition of the solidification structure was studied both experimentally and theoretically. Predictions and the evaluation of the role of heat transfer and fluid flow were established using existing solidification theories combined with transient, three-dimensional numerical heat transfer and fluid flow modeling. Metallurgical characteristics of the alloy’s solidification are extracted from the transient temperature fields, and microstructure is predicted and validated using optical images and electron backscattered diffraction data from the experimental results. Simulations show that the pure liquid region solidified quickly, creating a large two-phase, mushy region that exists during the majority of solidification. While conductive heat transfer dominates in the mushy region, both the pool geometry and the solidification parameters are affected by convective heat transfer. Finally, increased spot density during processing is shown to increase the time of solidification, lowering temperature gradients and increasing the probability of equiaxed grain formation.

Conditions for void formation in friction stir welding from machine learning

Abstract: Friction stir welded joints often contain voids that are detrimental to their mechanical properties Here we investigate the conditions for void formation using a decision tree and a Bayesian neural network Three types of input data sets including unprocessed welding parameters and computed variables using an analytical and a numerical model of friction stir welding were examined One hundred and eight sets of independent experimental data on void formation for the friction stir welding of three aluminum alloys, AA2024, AA2219, and AA6061, were analyzed The neural network-based analysis with welding parameters, specimen and tool geometries, and material properties as input predicted void formation with 833% accuracy When the potential causative variables, ie, temperature, strain rate, torque, and maximum shear stress on the tool pin were computed from an approximate analytical model of friction stir welding, 90 and 933% accuracies of prediction were obtained using the decision tree and the neural network, respectively When the same causative variables were computed from a rigorous numerical model, both the neural network and the decision tree predicted void formation with 966% accuracy Among these four causative variables, the temperature and maximum shear stress showed the maximum influence on void formation

Residual stresses and distortion in the patterned printing of titanium and nickel alloys

Abstract: Since the deposition patterns affect the stresses and distortions, we examined their effects on multi-layer wire arc additive manufacturing (WAAM) of Ti-6Al-4V and Inconel 718 components experimentally and theoretically. We measured residual stresses by hole drilling method in three identical components printed using different deposition patterns. In order to understand the origin and the temporal evolution of residual stresses and distortion, we used a well-tested thermo-mechanical model after validating the computed results with experimental data for different deposition patterns. Distortions were also examined based on non-dimensional analysis. We show that printing with short track lengths can minimize residual stresses and distortion among the three patterns investigated for both alloys. Both Ti-6Al-4V and Inconel 718 had similar fusion zone shape and size and were equally susceptible to deformation and warping, although Ti-6Al-4V was relatively less vulnerable to delamination due to its higher yield strength. A dimensionless strain parameter accurately predicted the effects of WAAM parameters on distortion and this approach is especially useful when the detailed thermo-mechanical calculations cannot be undertaken.

Printability of 316 stainless steel

Abstract: A printability database can help in the selection of a printing process-alloy combination to reduce, and in some cases avoid, common defects in printed parts. The extensive testing of parts...

Harnessing the scientific synergy of welding and additive manufacturing

Abstract: Additive manufacturing (AM) of metallic materials and welding share many similarities in their physical processes and the way the microstructure and properties of the products evolve. This synergy ...

Sir Harry Bhadeshia retires after 24 years as an editor of the journal Science and Technology of Welding and Joining

Abstract: After spending 24 years as a founding editor of the Journal Science and Technology of Welding and Joining (STWJ), Sir Harry Bhadeshia has decided to step down. Having served as founding co-editors ...