**1. Introduction**

Lightweight materials, especially aluminum (Al) and its alloys, are increasingly utilized in different industries because of their uniqueness, including high corrosion resistance, outstanding strength to weight ratio, excellent flexibility, thermal properties, and remarkable finishing properties [1–3]. As such, Al and its alloys are finding increased use in the fields of automotive, construction, marine, and aerospace. Solid-state and liquid-state processes are widely implemented to fabricate metals to obtain Al and its alloys that could be applied in the abovementioned fields [1, 4–7]. However, there is an increased interest in the solid-state process and practice due to its versatility, greater control of starting materials proportion, chemical

homogeneity, potential to fabricate near net shape with complex parts, and cost-effectivity [5–8]. The studies by Ji et al. [8] and Feijoo et al. [9] documented a noteworthy improvement in the ultimate tensile strength (UTS) of around 180 megapascals (MPa) and 377 MPa of an Al alloy system with an optimum reinforcement addition of 5 weight percent (wt.%) silicon carbide (SiC) and 0.5 wt.% multiwall carbon nanotubes, fabricated through powder metallurgy technique.

However, despite its advantages, the sintering of Al and its alloys is problematic due to a thermodynamically stable oxide layer on the particle surfaces. The scenario often hinders effective bond formations between the powder particles. Therefore, the accomplishment of desirable physical and mechanical performances could not be realized. Moreover, the presence of the unwanted oxide layer frequently promotes a nonwetting diffusion barrier for effective sintering of Al and its alloys [10–16]. Consequently, it is crucial to disrupt at least partially the oxide film, known as aluminum oxide (Al2O3) or alumina, on the surfaces of Al particles. Therefore, the addition of sintering additives such as tin (Sn), zinc (Zn), and magnesium (Mg) to facilitate the liquid-phase sintering of Al and its alloys were introduced to minimize the issues [11–19]. Azadbeh and Razzaghi [15] reported that when 5.5 wt.% of Zn was introduced to an Al system, its densification and mechanical strength were improved at 119 HV and 564 MPa, respectively. Furthermore, a study performed by Liu et al. [16] documented that the addition of Sn (4 wt.%) could only provide sufficient liquid formation for effective liquid phase sintering of Al after the oxide layer on the Al surface has been interrupted via external load application or Mg addition. In this context, the authors recorded that the Al-Sn liquid development proceeded via melting of Sn followed by a repetition process of cracking and repairing owing to reoxidation and thermally induced stress of the exposed Al particles. Consequently, as the outer layer of Al particles scavenged the oxygen from the Argon (Ar) gas, the oxygen concentration in the core was reduced, accompanied by the creation of an Al-Sn liquid phase via the dissolution of the Al in the liquid Sn. Finally, sacrification of outer layers took place hence these layers remained porous. Additionally, the authors also suggested that an alternative mechanism by means of selection of irregular shape of Sn element could ensure maximum liquid formation during sintering of Al.

Therefore, the sintering of the Al cores proceeded through a typical liquid-phase sintering mechanism. Similarly, the utilization of sintering additives (Mg and Sn) enhanced the sintering response of sintered Al, which consequently improved its tensile strength up to 118 and 300 MPa, as discussed in the studies by Sercombe et al. [13] and MacAskill et al. [17]. Although there were extensive works on the sintering response of Al and its alloys fabricated via powder metallurgy technique, additional exploration is necessary, particularly with different formulations of Al and sintering additive. Consequently, the behavior of liquid-phase sintering of Al alloys was investigated in the current study by exploiting Sn constituent as sintering additive, developed through powder metallurgy technique. Moreover, Mg constituent at fixed content was also utilized in the current study to optimize the liquid phase formation by Sn constituent. Therefore, the reliability of the sintering response of Al alloys was systematically examined via its physical characteristics, oxygen content levels, compressive strength, density, and microstructure of the sintered Al alloys. The investigation aimed to provide an alternative solution to minimize the problematic sintering response of Al and its alloys.
