Thermal-mechanical modeling of laser powder bed fusion inconel-718 towards relating processing to properties

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Vest, Alexandra, Marion
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Electronic thesis
Mechanical engineering
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Additive manufacturing (AM) offers a plethora of advantages to modern industry, such as the ability to produce complex geometries and offer on-site part replacement. The drawback of incorporating AM techniques for metal structural components is the uncertainty in the integrity of the parts due to the intense and cyclic thermal-mechanical work to which the part is exposed. This work aims to increase the understanding of the linkage between the processing parameters in a laser powder bed fusion (LPBF) AM process of Inconel-718 (IN-718) to the resultant mechanical properties and residual stress field. As a part of this research, a small number of experimental tests are performed to inform and test the performance of the models developed. An initial set of tensile samples of IN-718 is manufactured with varying processing parameters but maintaining the same energy per unit area. These samples undergo tensile testing, density measurements, and microscopy imaging. The experimental work in conjunction with thermal simulations and relations from the literature are used to develop a porosity indicator parameter. Conclusions derived from the tensile samples are used to guide the processing of an additional set of IN-718 sample cubes with different processing parameters and energy densities from that used with the tensile sample set. The porosity indicator parameter is then computed for the cube sample processing conditions and compared to the measured porosity of the cube samples. The results show the expected correlation between the predicted porosity indicator and the measured porosity levels. The porosity indicator parameter is then made non-dimensional through a relation of meltpool geometry to the linear energy density. The simulation work focuses on developing and implementing a temperature-dependent elastic-viscoplastic model, capable of modeling the mechanical behavior from the melt to room temperature, into an in-house finite element code to allow for the prediction of residual stresses due to LPBF processing. The mesoscale model takes the temperature history from LPBF thermal simulations for its input and predicts the resulting stress field after the process. The residual stress fields predicted with the elastic-viscoplastic framework are verified against semi-analytical solutions for simplified test problems. Simulations forming portions of an IN-718 part and predictions of accumulated residual stresses are performed for one layer of powder. This code is used to examine the effects of individual processing parameters on the residual stress field for a limited amount of test cases. The connections explored in this work bring AM IN-718 one step closer to inclusion in wide-scale production with its increased prediction of failure and performance with respect to processing parameters.
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Rensselaer Polytechnic Institute, Troy, NY
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