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Fatigue of Aluminium Matrix Composites processed by Laser Powder Bed Fusion

Publication date: 2024-07-08

Author:

Senol, Seren
Vanmeensel, Kim

Abstract:

Laser powder bed fusion (L-PBF), an additive manufacturing (AM) technique, offers opportunities for manufacturing and designing lightweight, near-net-shaped metallic components with extensive geometric complexity and advanced performance capabilities. Despite these possibilities, the utilization of L-PBF for aluminium-based alloys has been constrained by inherent limitations. While aluminium (Al) alloys are highly attractive due to their lightweight, high specific strength, high thermal conductivity, and good corrosion resistance, the adoption of high-strength aluminium alloys, such as those in the 2xxx and 7xxx series, is hindered by their susceptibility to solidification cracking during L-PBF, whereas printable Al-Si alloys exhibit limited strength. Additionally, for fatigue-critical applications, the use of L-PBF processed Al parts is further impeded by the elevated surface roughness intrinsic to L-PBF process. This thesis aims to address these challenges with the development of aluminium matrix composites (AMCs) as high-strength, lightweight solutions tailored for fatigue-critical AM applications. Two primary approaches are pursued: (i) Material development, where the aim is to comprehensively understand the interplay between microstructural features, applied heat treatments, and concomitant fatigue performance, as well as the failure mechanisms for AMCs. Two AMCs are studied in this part: (1) non-commercial, ex-situ reinforced nSiC/AlSi7Mg0.6 (developed as an alternative to Al-Si-Mg alloys commonly used for L-PBF), and (2) commercially available, hybrid ex-situ/in-situ reinforced (Ti+B4C)/Al-Cu-Mg. (ii) Process development, focusing on mitigating the challenge of high surface roughness in L-PBF parts by an innovative in-process surface modification. This includes the utilization of dual laser powder bed fusion (dL-PBF) for surface modification and exploring its impact on surface and sub-surface characteristics and resultant fatigue performance of two AMCs, namely, (1) commercially used, in-situ reinforced TiB2/Al-Cu-Mg-Ag, and (2) commercially available, hybrid ex-situ/in-situ reinforced (Ti+B4C)/Al-Cu-Mg. The characterization efforts involve microstructural analyses, quasi-static and stress-based fatigue tests, aimed at unraveling the process-microstructure-performance interrelationship and discussing fatigue influencing factors. The first part investigates the effect of microstructure, influenced by AMC composition, reinforcement characteristics, and heat treatment, on fatigue performance. Stress-based fatigue tests on surface-machined coupons with varying microstructures are followed by detailed microstructural analyses, revealing microstructural features and their impact on fatigue crack initiation and propagation. The second part focuses on the influence of surface roughness by examining various surface conditions. Surface modification of AMCs via dL-PBF is explored, and stress-based three-point bending fatigue tests are conducted on coupons with different surface conditions (as-built, dL-PBF processed and conventionally machined), highlighting the detrimental effects of surface or near-surface defects.