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A Study on the Selective Laser Melting of Scalmalloy and the Influence of Cold Rolling, Heat Treatment and Gas Tungsten Arc Welding on its Mechanical Properties and Microstructure

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Thesis(35.68 MB)

Date

2023

Authors

Supervisor

Schaefer, Marcel
Nomani, Junior

Item type

Thesis

Degree name

Doctor of Philosophy

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Publisher

Auckland University of Technology

Abstract

Scalmalloy is an Al-Mg-Sc-Zr modified aluminium alloy specifically designed for the laser powder bed fusion (LPBF) additive manufacturing process of Selective Laser Melting (SLM). This thesis investigates the processing and manufacture of Scalmalloy in an Additive Manufacturing (AM) 400 Renishaw laser beam melting (LBM) machine and the effect post processing treatments (heat treatment, cold rolling and a combination of heat treatment and cold rolling) and gas tungsten arc welding (GTAW) has on the mechanical properties and microstructures of Scalmalloy when compared to its as-built structure. Heat treatment is well reported in the literature, however there is opportunity to explore different heat treatment conditions. Cold rolling has not been widely reported for Scalmalloy and offers a novel area to study its effect on Scalmalloy. Lastly, GTAW of Scalmalloy has not been reported in the literature and offers solutions to join complex additively manufactured parts together and to join Scalmalloy parts to pre-existing structures. Hence, this thesis is comprised of three main studies: 1. Experimental works were carried out to determine optimal SLM parameters for Scalmalloy using an AM Renishaw LBM machine with constant parameters of 400W laser power, layer thickness of 30µm and hatch distance of 150µm. Energy volume densities were then modified to establish an optimum range. Findings showed that samples manufactured with an energy volume density between 70J/mm3 and 110J/mm3 resulted in samples with >99% relative densities. Furthermore, it was shown that the influence of the layout position of each sample on the build platform had very little impact on relative densities. Therefore, optimal processing parameters were found with 400W laser power, allowing productivity to be increased when compared to current literature employing 200W laser power. 2. Scalmalloy samples manufactured using the optimal SLM process parameters from the first study were subjected to cold rolling, heat treatment and a combination of both. Scalmalloy in its as built condition can possess reasonably high tensile strengths, which can be further increased by heat treatment, and, in turn, the microstructure is also refined. When heat treatment was applied an immediate increase of 14.6% occurred, while subsequent heat treatment had slightly lower values due to slight over ageing but remained higher than the as built condition. Once cold rolling was applied to the as built condition almost no increase was evident thought to be due to the low rolling reduction, however when cold rolling and heat treatment was applied an immediate increase of 28.4% occurred. The grain size distribution was consistent with literature for the as built condition, however, it was increased by the cold rolling due to elongation of the grains. Overall, the application of heat treatment and cold rolling had a positive effect on the microstructure of Scalmalloy reflected in its increased mechanical properties. 3. GTAW was performed to join Scalmalloy to itself and to extruded aluminium alloys using standard welding parameters employed to weld conventionally manufactured aluminium alloys. High porosity and defects were found in the welded joint leading to poor mechanical properties likely due to release of hydrogen content within the Scalmalloy base metal. Hence, standard welding parameters are feasible to be used for GTAW of Scalmalloy, but only as a starting point, highlighting that parameters cannot be simply transferred from one alloy to another, but rather have to be modified in order to achieve optimum results. The main output of this thesis is that the academic community would have a better understanding of the effect that different heat treatment methods, cold rolling, and GTAW have on the microstructure and mechanical properties of Scalmalloy. Hence, providing a modified heat treatment than the typical heat treatment reported in the literature, and providing info on cold rolling of additively manufactured components, which has not been widely reported. In addition, to evaluating the feasibility of welding Scalmalloy using standard welding parameters for conventionally manufactured aluminium alloys, consequently, aiming to eliminate size limitations in additive manufacturing and reduce manufacturing costs.

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http://hdl.handle.net/10292/17665

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Doctoral Theses