ISSUE 114 AUTUMN 2024 ADDITIVE MANUFACTURING NOVEL ALUMINIUM MATRIX


COMPOSITES FOR AM APPLICATIONS GIUSEPPE DEL GUERCIO ET AL.*


In recent years Powder Bed Fusion – Laser Beam (PBF-LB) has consolidated its position as one of the most attractive additive manufacturing (AM) processes for an increasing number of critical applications in the aerospace and automotive sectors. Such industries have investigated the adoption of additively manufactured aluminium (Al) matrix composites (AMCs), materials usually characterised by AlSi10Mg and large additions (2-10% in weight) of ceramic particles, including WC, SiC, TiC, TiB2


THE LASER USER


and more [1]. Depending on the additive and its amount, AMCs show increased tensile strength, heat resistance, thermal conductivity, and wear behaviour. Boron nitride (BN) represents one of the most attractive ceramic additions to Al-based alloys owing to its outstanding chemical and thermal stability paired with exceptional tribological properties [2].


In the context of laser-based AM processes, BN is able to enhance not only the base alloy’s strength, but also its fatigue performances and wear behaviour. However, such improvements are often paired with important limitations; firstly, BN has been added to alloys characterised by limited mechanical performance, limiting the potential strength achievable in the AMC. Moreover, high fractions of ceramic particles limit interfacial wettability, promoting porosity formation and a consequent limitation of the AMC’s ductility.


This study investigates the effects of small (0.3 wt%) addition of BN nanoparticles to a bespoke high-strength Al alloy, specifically designed for AM applications, namely AMALLOY3D.


Materials and methods


In the context of this investigation, AMALLOY3D was produced via a mechanical blend of AlSi10Mg and pure Cu powder in weight proportion of 93:7 (Figure 1(a)). Subsequently, AMALLOY3D was mechanically blended with nanosized hexagonal BN particles in a weight proportion of 99.7:0.3, forming AMALLOY3D- HT (Figure 1(b)). The flowability of these mixes was investigated using the Revolution Powder Analyzer.


An AconityMIDI+ PBF-LB system equipped with a resistive platform heating unit was used in this investigation. Cubes with a 10 mm edge were


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Figure 2: (a) Break energy distribution and (b) avalanche angles of AlSI10Mg, AMALLOY3D and AMALLOY3D-HT with images depicting the various powders before and after avalanche.


Figure 1: The scanning electron microscope SEM/backscatter electron BSE images of (a) AMALLOY3D and (b) AMALLOY3D-HT


manufactured to optimise the process window of AMALLOY3D and AMALLOY3D-HT for a total of 25 unique combinations of power and speed. A combination of optical and electron microscopy was adopted to extensively investigate the microstructure of the as-built samples. Finally, optimal parameters were adopted to fabricate 12×25×101 mm3


cuboids that were machined


into tensile bars according to ASTME8/E8M and tensile tested using an Instron 5982 universal mechanical testing machine.


Results and Discussion


The results of the powder flowability assessment are presented in Figure 2. The AMALLOY3D and AMALLOY3D-HT powders showed a flowability improvement with respect to AlSi10Mg; this is illustrated by the distribution of break energy, shifted towards lower values, and by the narrower distribution of the avalanche angle. The improvement of flowability in the AMALLOY3D powders with respect to AlSi10Mg is attributed to the presence of the Cu powder which, acting as


spacers, reduced particle agglomeration. The further improvement of flowability observed in AMALLOY3D-HT is attributed to the lubricative effect of BN nanoparticles, known to reduce friction and therefore improving the fluidisation behaviour of the overall powder mix [3].


Figure 3(a) and 3(b) depict the contour plots of relative density of AMALLOY3D and AMALLOY3D-HT, respectively. In the investigated processing conditions, samples produced with low scan speed and high power showed poor processability and a relative density of about 97%. This was attributed to extensive presence of keyholes, favoured by the higher energy density adopted for fabrication [4]. The samples produced by progressively lower energy densities, achieved adopting lower scan speeds and higher powers, showed optimal consolidation. These samples were characterised by relative densities higher than 99.8% and the minor presence of gas pores. Therefore, the addition of 0.3 wt% of BN to AMALLOY3D


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