On the role of the fabrication method on the machinability of aluminium alloys and aluminium-based metal matrix composites

Aluminium alloys and aluminium-based metal matrix composites (MMCs) are key materials in the effort to develop lightweight, high-performance components for the transportation and aerospace sectors. Their combination of low density, high specific strength, and corrosion resistance makes them attractive for sustainable engineering applications. However, the mechanical and machining behaviour of these materials is strongly governed by the fabrication route, which defines microstructural features such as porosity, phase morphology, and reinforcement distribution. This doctoral research investigates the effect of different production methods, laser powder bed fusion (LPBF), stir casting, and hot isostatic pressing (HIP), on the machinability of aluminium alloys and MMCs, highlighting the relationships between process-induced microstructures, mechanical properties, and cutting performance. The first research line focuses on LPBF alloys, aiming to correlate printing parameters and post-processing heat treatments with machining behaviour. Two aluminium alloys, AlSi7Mg and AlSi9Cu3(Fe), were fabricated under controlled LPBF conditions and subjected to turning tests. For AlSi7Mg, the effect of layer thickness (20-25-30 μm) was examined after T6 heat treatment. Samples printed with thicker layers exhibited reduced cutting forces, improved surface finish, and an increase in overall corrosion resistance. For AlSi9Cu3(Fe), post-processing heat treatments (as-built, T5, and T6) were compared in terms of machinability and corrosion resistance. The as-built condition preserved a fine cellular substructure, while T6 homogenisation produced coarser Si precipitates. Although T6 improved machinability by reducing surface roughness, it also increased corrosion current density due to the formation of galvanically active phases and microporosity. The second research line addresses the machinability of aluminium-based MMCs reinforced with 20 vol.% silicon carbide (SiC), focusing on how the fabrication route modulates reinforcement distribution, tool wear and surface finish. Two composites were examined: an A359/SiC produced by stir casting, and an AA6061/SiC fabricated by HIP. Drilling trials under minimum quantity lubrication conditions revealed marked differences in tool wear and surface quality. In the MMC produced by stir casting, the heterogeneity of reinforcement content between the inner and outer zones of the workpiece caused strong variability in both mechanical properties and machinability. The MMC produced by HIP displayed a fine, uniform distribution of SiC reinforcements and limited surface damage, leading to significantly lower tool wear and smoother surfaces. The results demonstrated that reinforcement size, distribution, and interfacial quality are dominant factors controlling machinability in MMCs. Overall, this dissertation establishes that the manufacturing route exerts a fundamental influence on the machinability of both aluminium alloys and MMCs. For LPBF alloys, optimising printing parameters and heat treatments enables a precise balance between mechanical strength, surface quality, and corrosion performance. For MMCs, the transition from conventional casting to HIP markedly enhances homogeneity and tool life, providing smoother surfaces and more predictable behaviour. The study thus provides an integrated framework linking processing parameters, microstructural evolution, and machining response, offering guidelines for the design of lightweight aluminium-based materials with improved manufacturability and service reliability.