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%0 Thesis %A Komerla, Krishna Chaitanya %T An investigation of microstructure and mechanical properties of low carbon steels subjected to welding %I Rheinisch-Westfälische Technische Hochschule Aachen %V Dissertation %C Aachen %M RWTH-2020-12241 %P 1 Online-Ressource %D 2020 %Z Veröffentlicht auf dem Publikationsserver der RWTH Aachen University 2021 %Z Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020, Kumulative Dissertation %X Welding is a complex thermo-mechanical process in which, under transient and non-equilibrium conditions multiple metallurgical phenomena can occur simultaneously. The weld thermal cycles introduce significant alterations to the microstructure of material and thereby, affect the mechanical properties of the weldments. In order to create a sound joint, it is essential to understand the impact of various process parameters on the weld microstructure and weld mechanical properties. Beyond conventional process parameters like, acceleration voltage, amperage, welding speed and applied external pressure, a deeper understanding of the influence of welding parameters like tool offset, beam oscillations and oscillation trajectories is necessary to produce long lasting and sustainable joints. In this study, the effect of such uncommon process parameters on the quality of welds is presented. To that end, the two following welding methods have been investigated 1.Fusion welding - High energy beam welding 2.Pressure welding - Dissimilar friction stir welding. Low carbon steels like the automotive dual phase steel (DP1000) and deep drawable mild steel DC04 each possessing 1 mm thickness, along with a 5 mm thick structural steel S235JR were investigated. Additionally, for dissimilar friction stir welding, a 1.12 mm thick solution treated and aged aluminum alloy AL6016-T4 was also studied. Results indicate that, for the same weld heat input, the application of uncommon process parameters can yield a range of different weld microstructures that exhibit improved weld mechanical properties. In the case of high energy beam welding, beam oscillations were applied to create a dynamic distribution of power around the stationary position of the beam that enhanced the flow of heat inside the keyhole and created wider fusion and heat-affected zones. A reduction in the size of welding crowns could be achieved by simply oscillating the energy source. As a consequence of this dynamic power distribution, the weld microstructure exhibited large columnar grains in the fusion zone and equiaxed grains of varying size in the heat-affected zone. This variation in grain size across the weld joint could be attributed to the steep temperature gradient produced during electron beam welding. Typically high hardness is observed in welds due to the occurrence of quenched martensite during conventional welding. However, weld samples fabricated using beam oscillations possessed not only lower hardness but also exhibited good tensile strength, lower residual stress and minimal distortion in comparison to the joints produced by stationary beam welding. This decrease in hardness arose from enhanced grain growth and additional indirect tempering like effect caused by beam oscillations. While in the case of friction stir welding, the tool offset resulted in a mode of welding where, minimal shear strain was accumulated in steel and complete dynamic recrystallization of microstructure in aluminum was achieved. Additionally, macroscopic defect free and inter-metallic compound free joints were obtained, with only a small fraction of Fe chips embedded in the aluminum matrix. Through plastic deformation and thermal effects, significant differences in grain size and micro-hardness were created in the weld joint. High temperatures and extreme strain rates inherent to the welding process, activated the dynamic recrystallization of microstructure in the aluminum alloy. The kernel average misorientation map revealed the lack of stored deformation energy, indicating the presence of fully recrystallized grains. However, due to its high thermal stability, steel did not recrystallize but, exhibited marginal grain growth in both thermo-mechanically affected zone and heat-affected zone. It also contained a highly refined grain structure in the weld zone. The texture maps plotted for the weld zone of Fe alloy showed weak shear texture indicating minimal shear strain accumulation. Thus, by adopting such process parameters and understanding their impact on the microstructure, joints with enhanced life can be produced at low costs. However, experimental research of such parameters is both time consuming and expensive. Hence, there is a strong need for numerical models that predict the final mechanical state of the weldments. Therefore, such an attempt is also made towards the development of a metallurgical framework in this project to study and analyse the process of welding, its metallurgy and microstructure. A sequentially coupled finite element formulation was adopted to simulate the fusion welding process. To accurately predict the sharp temperature gradients across the different weld zones, a moving volumetric heat source model, combining a spherical and a conical thermal flux distribution was developed. The temperature profiles computed with this heat source model were found to be in excellent agreement with the experimentally measured data. Additionally, a comparison of the optical micrographs with the simulated weld geometries further strengthened the basis of the developed framework. %F PUB:(DE-HGF)11 %9 Dissertation / PhD Thesis %R 10.18154/RWTH-2020-12241 %U https://publications.rwth-aachen.de/record/808705