## 1 Introduction  
In the last 50 years, aluminium production from recycled sources has grown to more than one-third the size of the entire industry. The secondary aluminium process primarily generates three products: the molten aluminium, an off-gas and a semi-solid skim, better known as drugs that is scraped off the surface of the molten metal [1]. This drops is commonly processed further to extract more metal and to remove the salts from it, which leaves behind a non-metallic waste, known as aluminium smelter waste (ASW). The composition of ASW varies, but it mainly contains alumina and other oxides. It may also contain traces of aluminium nitride, carbide and aluminium [1]. When ASW is left in open air, a chain reaction occurs with the moisture in the air and eventually leads to the production of hydrogen, which is a potential fire and explosion hazard and limits the uses of ASW [1]. There is a need to divert ASW, currently being sent to landfills, and find ways that may reduce carbon emissions either directly through reuse or indirectly via the repurposing of land that would have gone to be used as a landfill site for ASW. The motivation for this paper is to study the feasibility of the use of ASW in the development of a metal matrix syntactic foam (MMSF) for potential applications as a wear-resistant material.  
A syntactic foam is a material consisting of hollow or porous ceramic sphere fillers in a polymer or metal matrix [2, 3]. The hollow particles normally have a lower density compared to the matrix, so their incorporation allows for the manufacture of lightweight products [4; 5; 6]. Because their density, and hence mechanical properties, can be adjusted by controlling the ratio of hollow spheres to the matrix, syntactic foams can be tailored for different applications, hence making them very useful products [7]. Compared to polymer matrix syntactic foams, MMSFs have superior properties that make them more suitable for high-strength, high-hardness, high-stiffness and high-temperature applications [8; 9; 10; 11].  
The properties of MMSFs depend on various factors, including the choice of metal matrix and the type and volume fraction of microspheres, as well as the manufacturing process used to make the foams. MMSFs have been manufactured using several different processes, including pressure melt infiltration [12; 13; 14; 15] and powder metallurgy [16; 17; 18; 19; 20]. Pressure infiltration casting is the most common process for manufacturing MMSFs because of its ease of operation, low cost and reasonably good quality control.  
There has been plenty of research conducted on the mechanical properties of MMSFs since their introduction in the 1960s. The deformation behaviours of MMSFs under static and high-strain-rate compression or impact have been studied extensively [15; 21; 22]. Studies on the wear behaviour of MMSFs [23; 24; 25; 26] are relatively limited. [PERSON] and [PERSON] [27] looked at the development of fly ash-based composites for wear applications and showed that the aluminium matrix syntactic foam with 50 wt% fly ash had consistent coefficients of friction between 0.35 and 0.4. They concluded that MMSFs have potential for use as brake linings with a reduction in weight compared to commercially available brake linings. However, they did not carry out a detailed characterisation of the friction and wear properties.  
The filler materials used in MMSFs so far are either regular microspheres or fly ash. Very little research has been conducted to investigate the feasibility of incorporating a waste stream coming from the metal processing industry as a filler material.  
This study investigates the tribological properties of Al matrix syntactic foams manufactured with ASW by pressure infiltration casting. Dry and lubricated sliding tests are performed to quantify the friction and wear properties. The results are compared with those obtained from Al matrix syntactic foams with E-spheres as the filler material.