**3.1 Microstructure of the starting materials and elemental powder mixture**

The microstructure of the starting materials and elemental powder mixture is illustrated in **Figure 2(a–c)** and **Figure 3(a–c)**. From the observations, most of the Al and Mg particles were spherical, with some irregularities observed on the Al particles. These particles were calculated to have an average particle size of 45 and 10 μm, respectively. On the contrary, the shape of the Sn particles was found to be mainly irregular with an average particle size of 45 μm, as revealed in **Figure 2(c)**. Since the sintering of Al powder prevented the solid-state sintering process, it was crucial to disrupt the stable

#### **Figure 2.**

*Microstructure of the starting materials of (a) Al powder, (b) Mg powder, and (c) Sn powder as observed under SEM.*

*Assisting Liquid Phase Sintering of Pure Aluminum (Al) by the Tin Addition DOI: http://dx.doi.org/10.5772/intechopen.101507*

oxide film that readily existed on the surface of the Al particles so that strong metallurgical bonds between the particles could be guaranteed. Therefore, the current investigation employed liquid phase sintering of Al alloys with Sn as the sintering additive. Sn was introduced to promote the liquid phase of Al because of its low melting temperature (232°C) compared to Al (660°C). However, the process could only be successful in the presence of Mg. The addition of Mg was reported to reduce the surface oxide of Al by exposing the underlying metal thus resulting in possible wetting of the Al particles by liquid Sn [14, 18–19]. On the other hand, the absence of powder mixture accumulation and homogenous distribution were identified after 12 h of mixing, suggesting optimal mixing condition in preparing elemental powder mixture of Al, Sn, and Mg, as demonstrated in **Figure 2(d)**. Regardless of different Sn content, the resultant particles of the elemental powder mixture were found to exhibit a lamellar structure, as evidenced in **Figure 3(a–c)**, respectively. These Sn particles were also identified to be flattened and elongated, having an average particle size of 50 μm. The resultant microstructure of the elemental powder mixture could be due to the repeated welding, fracturing, and rewelding of the elemental powder particles during mixing via ball milling technique [20]. Nevertheless, the overlapping and intimate contact between Sn particles were also clearly visible with increasing Sn content, confirming an increased Sn content in the elemental powder mixture, as seen in **Figure 3(a–c)**, respectively.
