A three-dimensional, transient, heat transfer, and fluid flow model is developed for the laser assisted multilayer additive manufacturing process with coaxially fed austenitic stainless steel powder. Heat transfer between the laser beam and the powder particles is considered both during their flight between the nozzle and the growth surface and after they deposit on the surface. The geometry of the build layer obtained from independent experiments is compared with that obtained from the model. The spatial variation of melt geometry, cooling rate, and peak temperatures is examined in various layers. The computed cooling rates and solidification parameters are used to estimate the cell spacings and hardness in various layers of the structure. Good agreement is achieved between the computed geometry, cell spacings, and hardness with the corresponding independent experimental results. V

A three-dimensional heat transfer and material flow model is developed to numerically simulate the temperature and velocity fields in a laser assisted layer by layer deposition process with coaxially fed powder particles. The computed results are tested with independently reported temperature and build geometry for the deposition of multilayered structures of austenitic stainless steel. The results provide detailed insight about the important physical processes and show that the model can be used to understand the effects of process parameters on the thermal cycles, build geometry, cooling rates and solidification parameters in a multilayer additive manufacturing process.

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...

The effects of many process variables and alloy properties on the structure and properties of additively manufactured parts are examined using four dimensionless numbers. The structure and properties of components made from 316 Stainless steel, Ti-6Al-4V, and Inconel 718 powders for various dimensionless heat inputs, Peclet numbers, Marangoni numbers, and Fourier numbers are studied. Temperature fields, cooling rates, solidification parameters, lack of fusion defects, and thermal strains are examined using a well-tested three-dimensional transient heat transfer and fluid flow model. The results show that lack of fusion defects in the fabricated parts can be minimized by strengthening interlayer bonding using high values of dimensionless heat input. The formation of harmful intermetallics such as laves phases in Inconel 718 can be suppressed using low heat input that results in a small molten pool, a steep temperature gradient, and a fast cooling rate. Improved interlayer bonding can be achieved at high Marangoni numbers, which results in vigorous circulation of liquid metal, larger pool dimensions, and greater depth of penetration. A high Fourier number ensures rapid cooling, low thermal distortion, and a high ratio of temperature gradient to the solidification growth rate with a greater tendency of plane front solidification. Published by AIP Publishing. [http://dx.

Laser engineered net shaping (LENS) and other similar processes facilitate building of parts with freeform shapes by melting and deposition of metallic powders layer by layer. A-priori estimation of the layerwise variations in peak temperature, build dimension, cooling rate, and mechanical property is requisite for successful application of these processes. We present here an integrated approach to estimate these build attributes. A three-dimensional (3-D) heat transfer analysis based on the finite element method is developed to compute the layerwise variation in thermal cycles and melt pool dimensions in the single-line multilayer wall structure of austenitic stainless steel. The computed values of cooling rates during solidification are used to estimate the layerwise variation in cell spacing of the solidified structure. A Hall-Petch like relation using cell size as the structural parameter is used next to estimate the layerwise hardness distribution.The predicted values of layer widths and build heights have depicted fair agreement with the corresponding measured values in actual deposits. The estimated values of layerwise cell spacing and hardness remain underpredicted and overpredicted, respectively. The slight underprediction of the cell spacing is attributed to the possible overestimation of the cooling rates that may have resulted due to the neglect of convective heat transport within the melt pool. The overprediction of the layerwise hardness is certainly due to the underprediction of corresponding cell spacing. The application of Hall-Petch coefficients, which is strictly valid for wrought and annealed grain structures, to estimate the hardness of as-solidified cellular structures may have also contributed to the overprediction of the layerwise hardness.