Showing papers by "Tarasankar Debroy published in 2015"

Evolution of solidification texture during additive manufacturing

Abstract: Striking differences in the solidification textures of a nickel based alloy owing to changes in laser scanning pattern during additive manufacturing are examined based on theory and experimental data Understanding and controlling texture are important because it affects mechanical and chemical properties Solidification texture depends on the local heat flow directions and competitive grain growth in one of the six preferred growth directions in face centered cubic alloys Therefore, the heat flow directions are examined for various laser beam scanning patterns based on numerical modeling of heat transfer and fluid flow in three dimensions Here we show that numerical modeling can not only provide a deeper understanding of the solidification growth patterns during the additive manufacturing, it also serves as a basis for customizing solidification textures which are important for properties and performance of components

Spatial variation of melt pool geometry, peak temperature and solidification parameters during laser assisted additive manufacturing process

Abstract: 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.

Mitigation of root defect in laser and hybrid laser-arc welding: A model is developed that predicts the range of processing conditions that produce defect-free, complete-joint-penetration welds

Abstract: Even though laser and hybrid laser­arc welding processes can produce single­pass, complete­joint­penetration welds in excess of 12 mm, root defects, such as root hump­ ing, have been observed at these greater plate thicknesses. The competition between the surface tension and the weight of the liquid metal in the weld pool is expected to govern root­defect formation. A series of laser and hybrid laser­gas metal arc welds has been completed in which each force is independently varied. The internal morphologies of the resulting root defects are characterized by X­ray computed tomography and found to vary significantly when welding with either the laser or hybrid laser­arc process. In order to compute the surface tension and liquid metal weight, a model based on the approximate geometry of the weld pool is developed and successfully predicts the range of processing conditions where root defects form. Process maps are then constructed for low­carbon steel and 304 stainless steel alloy systems. These maps can then be used to select welding parameters that produce defect­free complete­joint­ penetration welds over a wide range of plate thicknesses.

Employing microsecond pulses to form laser‐fired contacts in photovoltaic devices

Abstract: Laser-fired contacts (LFCs) are typically fabricated with nanosecond pulse durations despite the fact that extremely precise and costly control of the process is necessary to prevent significant ablation of the aluminum metallization layer. Microsecond pulse durations offer the advantage of reduced metal expulsion and can be implemented with diffractive optics to process multiple contacts simultaneously and meet production demands. In this work, the influence of changes in laser processing parameters on contact morphology, resistance, and composition when using microsecond pulses has been fully evaluated. Simulated and experimental results indicate that contacts are hemispherical or half-ellipsoidal in shape. In addition, the resolidified contact region is composed of a two-phase aluminum–silicon microstructure that grows from the single-crystal silicon wafer during resolidification. As a result, the total contact resistance is governed by the interfacial contact area for a three-dimensional contact geometry rather than the planar contact area at the aluminum–silicon interface in the passivation layer opening. The results also suggest that for two LFCs with the same size top surface diameter, the contact produced with a smaller beam size will have a 25–37% lower contact resistance, depending on the LFC diameter, because of a larger contact area at the LFC/wafer interface. Copyright © 2014 John Wiley & Sons, Ltd.