**3.2 Visual inspection of the sintered aluminum alloys**

The visual inspection of the sintered Al alloys at various Sn contents are depicted in **Figure 4(a–c)**. It was observed that the addition of 1.5 wt.% of Sn

#### **Figure 4.**

*Visual inspection of the resultant Al alloys with different Sn content of (a) 1.5 wt.%, (b) 2 wt.%, and (c) 2.5 wt.%.*

(the lowest amount) produced a black-colored Al alloy after sintering, implying oxidation and the ineffective role of Sn in increasing the fluidity of the Al alloy during sintering. However, when the Sn content was increased to 2 wt.%, the black-colored Al alloy changed slightly to silver that appeared only at the top of the alloy, suggesting insufficient liquid formation to enhance the Al alloy fluidity that consequently minimizing oxidation. Similar observation was also reported in the study of Pan et al. [21]. On contrary, the effects were more pronounced when the Sn content was maximized to 2.5 wt.% as the color of Al alloys completely replicated the original silver color of the Al powder, confirming complete interruption of the stable oxide film on the surfaces of Al during sintering. Furthermore, the findings were supported by the results obtained from the oxygen content analysis as tabulated in **Table 1**. A significantly decreased oxygen content of the resultant Al alloys from 0.58 to 0.44 wt.% was recorded when the content of Sn was increased from 1.5 to 2.5 wt.%, respectively. The results showed that the amount of Sn influenced the oxygen content of the resultant Al alloys. Therefore, as the Sn content was increased in the Al alloys, the oxygen level decreased, indicating an improvement of the overall wetting characteristics of the Al alloys. However, the addition of Mg constituent was also essential in assisting the formation of Sn liquid by reducing the oxide layer on the Al surface [4, 18–19]. According to Kondoh et al. [14], Mg addition was found to assist the distribution of Sn liquid below the oxide layer, thus contributing to successful wetting of Al by Sn liquid.

### **3.3 Microstructure of the resultant aluminum alloys**

The cross-section of the sintered Al alloys at 1.5, 2, and 2.5 wt.% of Sn are demonstrated in **Figure 5(a–c)**, respectively. Moreover, the cross section of the pure Al without Sn and Mg addition (reference sample) is shown in **Figure 5(d)**. Based on the observations, the reference sample exhibited poor sintering response; evidenced by visible appearance of individual particles as if no sintering process has been performed. Furthermore, the presence of large number of pores between the individual particles were obvious, confirming ineffective sintering of pure Al. Similarly, pores or inter-particle voids between the grain boundaries were also noticeable when the Sn amount in the Al system was the lowest (1.5 wt.%), portraying a poor sintering response. However, the microstructure of 1.5 wt.% containing Al was slightly improved in comparison to pure Al without Sn addition.

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


**Table 1.**

*Oxygen content reading of elemental powder mixture and sintered Al alloys at various Sn content from EDS analysis. Data are presented in mean ± standard deviation.*

#### **Figure 5.**

*Microstructures of the resultant Al alloys at various Sn content of (a) 1.5 wt.%, (b) 2 wt.%, (c) 2.5 wt.%, and (d) pure Al without Sn and Mg constituents (reference sample).*

Therefore, the results might be associated with insufficient pore rounding as evidenced by the appearance of elongated pores, an abundance of grain boundaries, and negligible inter-particle necking [17–19, 22]. Moreover, the addition of Sn might be inadequate to occupy both the elongated and rounded pores, preventing a more favorable packing grains arrangement that could bring the solid particles closer, which contributed to grain growth and densification failure. On the contrary, improved sintering response was observed with increased Sn content, particularly at 2 and 2.5 wt.%, owing to the increased presence of pores rounding and lesser particle boundaries. Additionally, grain growth development was observed with increased Sn content as the amount of liquid formation was sufficient to fill the existing pores that successfully brought the solid particles together, improving the sintering quality of the resultant Al specimens. According to Aneta [23], a desirable liquid phase enhanced mass transport, which encouraged the rearrangement and fragmentation of solid particles. The phenomenon was especially true as the formation of inter-particle necking became increasingly visible with increased pore rounding, as shown in **Figure 5(b** and **c)**.

Furthermore, successful wetting of Sn elements on the surfaces of the Al particles enhanced the metallurgical bonds between the Al particles, attributable to higher and sufficient liquid phase formation that filled the remaining pores in the Al system [17–19, 22, 24]. Therefore, the addition of higher Sn content produced a permanent liquid phase that improved the densification and sintering quality of the alloys (mainly for Al added with 2 and 2.5 wt.% of Sn). Undetectable clustering of Sn particles represented by the bright phases indicated a uniform distribution of Sn particles along the grain boundaries of the Al alloys, especially with the addition of 2 and 2.5 wt.% of Sn that further promoted effective liquid phase formation. Consequently, densification and compressive strength of the Al alloys were elevated, as evidenced in **Figures 6** and **7**, respectively. According to MacAskill et al. [17], such morphologies confirmed the liquid formation by molten Sn during sintering that was capable of wetting the surface of Al and scattered evenly

**Figure 6.** *The effect of Sn variation on the sintered densities of the resultant Al alloys.*

#### **Figure 7.**

*Stress-strain curve of resultant (a) pure Al and Al alloys with different Sn content of (b) 1.5 wt.%, (c) 2 wt.%, and (d) 2.5 wt.%.*

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

throughout the Al alloys. However, the occurrence could only be accomplished by the existence of Mg due to its potential to stimulate the wetting response of molten Sn. However, higher Sn content of up to 2.5 wt.% slightly reintroduced several particle boundaries as shown in **Figure 4(c)**. The observation might be due to the remaining unreacted Sn that was unable to wet the Al surfaces, which hindered effective liquid phase formation during sintering [25]. Moreover, Sercombe et al. [13] reported a similar observation.

### **3.4 Phases transformation of the resultant aluminum alloys**

The X-ray diffraction (XRD) patterns of pure Al without Sn and Mg addition as well Al alloys at different Sn content (1.5, 2, and 2.5 wt.%) are presented in **Figure 8(a–d)**. The dominant XRD phases of the resultant Al alloys were mainly attributed to the α-Al constituent, characterized by the (111), (200), (220), and (311) diffraction peaks at 38.87, 45.42, 67.16, and 78.54°, respectively. As can be seen in **Figure 8(a)**, the peak intensities of pure Al were found to be lower compared to the 1.5, 2, and 2.5 wt.% Sn containing Al alloys. This is probably due to poor sintering response in the absence of Sn and Mg constituents hence low crystalline formation during complete sintering. In the case of Al alloys, the peak intensities of Sn increased with increased Sn content, confirming the addition of Sn during the process. The absence of the Mg peak in the Al system was attributed to the dissolution of Mg in α-Al [17, 26]. Based on the Al-Mg binary equilibrium phase diagram, Mg addition up to maximum content of 18.6% could be completely dissolved in the parent Al phase at the eutectic temperature of 450°C [27]. Moreover, it has been

#### **Figure 8.**

*Phases transformation of (a) Pure Al and Al alloys with different Sn content of (b) 1.5 wt.%, (c) 2 wt.%, and (d) 2.5 wt.%.*

reported that the creation of brittle intermetallic compounds (β-Al3Mg2 phase) that has associated with poor mechanical performance was observable with increasing Mg content [28]. For this reason, its utilization should be maximized up to 5 and 10 wt.% for wrought alloys and cast alloys, respectively [28]. Moreover, it was documented that Mg vaporized during sintering, and the gas produced served as a reliable gettering agent for effective liquid phase sintering of Al [14, 26, 29–30]. Therefore, it has been proposed that the application of nitrogen (Ni) as a sintering atmosphere was effective in transporting Mg vapor (effective gettering agent) around the pore network via the formation of magnesium nitride (Mg3N2) [29]. On the contrary, Schaffer and Hall [29] documented that Mg could be effectively transported as a vapor in Ar atmosphere which is comparable to the Ni atmosphere considering higher vapor pressure of Mg as compared to Mg3Ni2 formation. On the other hand, weak peaks of Mg2Sn compounds were observed in the current study with a higher Sn amount (2 and 2.5 wt.%), postulating solidification of the Mg2Sn compounds within the Al phase [31]. This is essential for optimum attainment of physical and mechanical performance of the resultant alloys [31]. It is important to note that a lower sintering temperature of 580°C that was set in the current study prevented the formation of higher peaks of the Mg2Sn compounds. Therefore, future studies should analyze the effects of various sintering temperatures other than the current setting on the sintering response of Al alloys.
