AbstractWire arc additive manufacturing (WAAM), a derivative of additive manufacturing (AM), has gathered attention of many researchers due to its numerous advantages such as high metal deposition rate and near-net shape production over traditional manufacturing techniques. Lightweight aluminium alloys were amongst highly experimented for WAAM products aiming widespread applications in automotive sector; however, there are some challenges which are restricting its wide spread use. In view of this, current research was directed towards better understanding of few challenges such as porosity i.e. relation between porosity formation and hydrogen dissolution, microstructural inhomogeneity, and residual stresses in WAAM produced 5183 aluminium alloy parts.
Commonly practiced technique for WAAM i.e. cold metal transfer (CMT) and conventional pulsed metal inert gas (pulsed-MIG) were used for part manufacturing and compared for porosity and microstructural variations. Samples were manufactured using 120 and 280 J/mm heat input, 50 and 100°C interlayer temperature and 30 and 120 seconds interlayer dwell time. For porosity study, computed tomography (CT) scan and dissolved hydrogen test on solid WAAM part were performed. Optical microscope and scanning electron microscope (SEM) were employed for microstructural investigation. Chosen samples were tested for mechanical properties such as tensile and hardness. Further, chemical analysis was performed to investigate possible elemental variations. Residual stress variation was measured using the newly developed contour residual stress measurement method on pulse-MIG produced WAAM samples. Substrate thickness (6 and 20 mm), interlayer temperature (50 and 100°C), heat input (120 and 280 J/mm) and deposit height (18 and 35 mm) were considered as variables for metal deposition.
Samples produced with CMT process showed smaller and lesser pores with reduced overall pore volume compared to samples from pulsed-MIG technique processed with similar conditions of heat input and temperature controls. On the contrary, CMT samples witnessed higher dissolved hydrogen in solid aluminium deposit. A peculiar trend in specific pore size and distribution drawn from probability study confirmed the influence of heat input, interlayer-temperature, and interlayer-dwell-time on WAAM type production. Further, CMT samples showed smaller grains compared to similarly processed pulsed-MIG samples. Moreover, higher heat conditioned samples i.e. processed with 280 J/mm heat input, 100°C interlayer temperature, and 30 seconds interlayer-dwell-time samples revealed relatively larger grains compared to lower heat conditioned samples i.e. 120 J/mm heat input, 50°C interlayer-temperature and 120 seconds interlayer-dwell-time. Chemical analysis of a deposit revealed loss of the elemental Mg. Tensile residual stresses dominated deposit part while substrate revealed compensating compressive residual stresses. Residual stresses with magnitude approaching the yield strength of a deposit were present. Substrate dimensions had major influence on stress distribution such that thicker substrates (20 mm) showed tensile stresses at the adjacent to deposit region and compressive in rest part, however, thinner substrates (6 mm) showed compressive stresses concentrated at extreme ends and majority of substrate portion showed tensile stresses.
High frequency oscillating motion of feed stock wire and arc on-off effects in CMT technique supported rapid reduction in temperature and increased solidification rate at solid-liquid interface that not only resulted into lesser hydrogen absorption but also produced finer grains compared to pulsed-MIG.
Continuous ignited arc of pulsed-MIG method showed increased overall energy, hotter deposit, higher arc penetration and lower cooling and solidification rates that supported in increased hydrogen absorption, easy movement and coalescence of atomic hydrogen, thus formation larger pores compared to CMT. The condition also supported in formation of larger and columnar grains. In either of the metal deposition condition, temperature of at least penultimate layer was increased above the recrystallization temperature of an alloy that affected grain formation in line with the heat flow direction. Effect of columnar grain formation in built direction was reflected in tensile properties. Vertical tensile samples showed lesser strength than horizontal samples. Similar to welding, liquid metal solidification in a layer format exerted tensile stresses. An active bending moment and firmly clamped substrate produced compressive stresses. Residual stresses as high as yield strength could be the result of high strains encountered due to multiple thermal cycles of contraction and expansion. Higher stiffness offered by 20 mm thick substrate compared to 6 mm, difference in heat flow characteristics, positive and negative strains due to repeated thermal cycles and operating bending moment collectively controlled stress distribution in processed WAAM part of 5183 aluminium alloy.
|Date of Award||Nov 2020|
|Sponsors||Lloyd’s Register Foundation|
|Supervisor||Jonathan Lawrence (Supervisor) & Xiang Zhang (Supervisor)|