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Laser Powder Bed Fusion of Scalmalloy (Al-4.6Mg-0.6Sc-0.3Zr): Formation Mechanism of Bimodal Grain Structure and Fatigue Behaviour of the Alloy

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Thesis(26.61 MB)

Date

2025

Authors

Supervisor

Chen, Zhan
Singamneni, Sarat

Item type

Thesis

Degree name

Doctor of Philosophy

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Publisher

Auckland University of Technology

Abstract

Scalmalloy® is a relatively new and high strength aluminium alloy with a distinctive bimodal microstructure consisting of equiaxed and columnar grains, when processed through Laser Powder Bed Fusion (LPBF) additive manufacturing or 3D printing. The formation of this bimodal grain structure during LPBF allows the alloy to be 3D printable, without forming hot cracks. Hot cracking during LPBF of the traditional high strength aluminium alloys largely prevents these alloys from being 3D printable. However, the mechanism of the bimodal grain structure of Scalmalloy during LPBF has not been fully understood. For applications of high strength aluminium alloys, fatigue properties are particularly important for ensuring reliability in industries like aerospace and automotive, where lightweight and durable materials are essential. However, despite the significant research on Scalmalloy LPBF, the fatigue behaviour of the alloy is not fully understood. Thus, the aim of this PhD research is to reveal and thus to understand the solidification behaviour during LPBF of Scalmalloy leading to the formation of the bimodal grain structure and the crack growth behaviour of the alloy under cyclic loading. In the first part of this research (on solidification behaviour), the partitioning of elements in the alloy and forming of various particles during LPBF solidification were experimentally determined. It is found that strong and weak segregation of Mg and Sc, respectively, occurs in the final solidification areas of the fine- and equiaxed-grain regions. The coarser and columnar grain regions show weak segregation of Mg and no Sc segregation. A priori knowledge on the Al–Sc eutectic reaction based on the known phase system and its dependence on cooling rates, and the well-known thermal and solidification conditions related to the track location during LPBF is used to ascertain the mechanism of formation of the bimodal grain structure. The mechanism suggested is substantiated by the location-dependent elemental distributions and the various particles that are observed. In the second part of this research, how LPBF defects affect the fatigue life of the alloy in both as-built (AB) and heat-treated (T5) samples and also in various crack directions (CD) and build directions (BD) has been studied. This part in turn has necessarily been divided into two sections to form a more complete understanding of the fatigue behaviour of the alloy. In section one, threshold stress intensity factor (ΔKₜₕ) values of the alloy were determined. This mechanic approach provides a base that the boundary of fatigue limit/strength as a function of defect size can be determined. In section two, tests were conducted so that S-N (stress and number to failure) curves can be drawn. Following the tests, surface and sub-subsurface defects (referring to the distance to the sample surface) on fracture surfaces were observed and the sizes were measured, for the understanding of how the surface and subsurface defects of the samples affect the fatigue strength of the alloy processed by LPBF. For the ΔKₜₕ study, experimentally, FCG tests with R=0.1 have been conducted using samples with crack growth direction normal, parallel or 45° to build direction, meaning CD⊥BD, CD//BD and CD/BD=45°, respectively. Tested sample conditions include LPBF with a room temperature base plate or a heated base plate and as-built state or heat-treated state. FCG tests reveal a narrow range of ΔKₜₕ values, 1.3–1.4 MPa·m¹/², across all loading directions relative to the build direction, attributed to minimal orientation effect of roughness-induced crack closure. Heat treatment (T5) and variations in build plate temperature (180°C and room temperature) show negligible effects on FCG rates, as the grain morphology has changed little and thus crack paths remain smooth. Paris law parameters (C and m) are comparable to those of conventional aluminium alloys. In the S-N study, fatigue strength tests with R=0.1 are carried out using two different groups, namely, AB group and T5 group. For each group, there are CD⊥BD and CD//BD samples. Thus, in total, there are four types of samples. As expected, defects affect the fatigue strength significantly. It has been found, however, that LD and T5 have not affected the fatigue strength significantly, although CD//BD may have displayed a lower fatigue strength due to them possibly having defects of large sizes as the size of a defect is orientation dependent. Using the Kitagawa-Takahashi (K-T) approach, given that ΔKₜₕ is not affected significantly by T5 and BD and given the sizes of defects on fracture surfaces measured, how the defect size affects the fatigue life of the samples has been explained. It has been further found that, due to the presence of defects, the fatigue strength at N≥10^7 range 95-125MPa, defects size dependent. The defect size range of mostly 60-150µm measured in the fracture surfaces of the S-N tested is consistent with the defect size range that is LPBF track size specific. It can be suggested that, through examining the data in the K-T diagram, the fatigue strength of a short crack sample is ~140MPa (stress range).

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http://hdl.handle.net/10292/19910

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