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In laser assisted additive manufacturing (AM) an accurate estimation of residual stresses and distortion is necessary to achieve dimensional accuracy and prevent premature fatigue failure, delamination and buckling of components. Since many process variables affect AM, experimental measurements of residual stresses and distortion are time consuming and expensive. Numerical thermo-mechanical models can be used for their estimation, but the quality of calculations depends critically on the accurate transient temperature field which affects both the residual stresses and distortion. In this study, a well-tested, three-dimensional, transient heat transfer and fluid flow model is used to accurately calculate transient temperature field for the residual stress and distortion modeling. The calculated residual stress distributions are compared with independent experimental results. It is shown that the residual stresses can be significantly minimized by reducing the layer thickness during AM. Inconel 718 components are found to be more susceptible to delamination than Ti-6Al-4V parts because they encounter higher residual stresses compared to their yield strength.

Although additive manufacturing (AM), or three dimensional (3D) printing, provides significant advantages over existing manufacturing techniques, metallic parts produced by AM are susceptible to distortion, lack of fusion defects and compositional changes. Here we show that the printability, or the ability of an alloy to avoid these defects, can be examined by developing and testing appropriate theories. A theoretical scaling analysis is used to test vulnerability of various alloys to thermal distortion. A theoretical kinetic model is used to examine predisposition of different alloys to AM induced compositional changes. A well-tested numerical heat transfer and fluid flow model is used to compare susceptibilities of various alloys to lack of fusion defects. These results are tested and validated with independent experimental data. The findings presented in this paper are aimed at achieving distortion free, compositionally sound and well bonded metallic parts.

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Additively manufactured parts are often distorted because of spatially variable heating and cooling. Currently there is no practical way to select process variables based on scientific principles to alleviate distortion. Here we develop a roadmap to mitigate distortion during additive manufacturing using a strain parameter and a well-tested, three-dimensional, numerical heat transfer and fluid flow model. The computed results uncover the effects of both the key process variables such as power, scanning speed, and important non-dimensional parameters such as Marangoni and Fourier numbers and non-dimensional peak temperature on thermal strain. Recommendations are provided to mitigate distortion based on the results.Laser assisted additive manufacturing (AM) process produces 'near net shape' parts from a stream of alloy powders in a layer-by-layer manner for use in medical, aerospace, automotive and other industries [1,2]. The parts undergo repeated spatially variable heating, melting, solidification and cooling during AM [3,4]. Due to the transient heating and cooling, the fabricated parts exhibit thermal distortion [5-10]. Thermal distortion results in dimensional inaccuracy and adversely affects performance of the fabricated parts [11]. Previous work has shown that increase in net heat input [7] and reduction in dwell time [8] between deposition of successive layers can increase thermal distortion. It is also known that the alloy properties, the deposit and substrate dimensions, the laser scanning pathway, the hatch spacing between layers and the heating and cooling conditions significantly affect thermal distortion [9,12,13].The existing AM literature does not provide any guidance for selecting process variables to minimize thermal distortion. A quantitative understanding of the effects of process variables on thermal distortion and a practical means to mitigate this problem based on scientific principles are needed but not generally available.Here, we show for the first time how thermal distortion during AM can be minimized by back of the envelope calculations. The procedure involves evaluation of the effects of common process variables such as laser power and scanning speed on thermal strain. In addition, the non-dimensional parameters that are important for heat transfer and fluid flow phenomena in AM such as Fourier number, Marangoni number and non-dimensional temperature are correlated with thermal strain. Based on these results we provide recommendations to minimize distortion of the additively manufactured parts.We have recently shown that thermal distortion is related to a strain parameter, ε* [5]:where β is the volumetric coefficient of thermal expansion, ΔT is the maximum rise in temperature during the process, E is the elastic modulus and I is the moment of inertia of the substrate, the product, EI, is the flexural rigidity of the structure, t is the characteristic time, H is the heat input per unit length, F is the Fourier number and ρ is the density of the alloy powder. Fourier number is the ratio o...