**3. Results and Discussion**

## *3.1. Microstructure*

The microstructure of GDC, LPSC, and HPSC samples are shown in Figure 2 using optical (Figure 2a–c) and scanning electron microscopy (Figure 2d–f). The microstructure of the GDC specimen has coarse dendrites as shown in Figure 2a. The microstructure gets refined with the application of pressure. The LPSC and HPSC samples have shown the presence of small dendrites, which are also deformed. They are not in a continuous state like the GDC samples. An increase in the squeeze-pressure increases the cooling rate, resulting in higher nucleation and finer dendritic size with large dendrite spacing. Similar observations were made by Amar et al. [59], where the 2017A alloy was squeeze cast using GDC and at high pressures. Moreover, Amar et al. have shown that with the application of pressure, a refined and homogeneous microstructure was observed, which is in agreement with the present results. The heat inside the mold and pressure have a significant effect on the size of the dendrites, dendritic morphology, and the distribution of microstructural constituents. Increasing the squeeze casting pressure refined all microstructural features (including the size of the microstructural features and arm spacing of dendrites) and modified the morphology of Al-Si eutectic phases. Further, dendrites were small and almost spherical in shape in squeeze cast conditions. In GDC alloys, the dendrites were observed to have an elongated plate-like morphology (Figure 2d), whereas, in the other two alloys (Figure 2e,f), cast microstructures consist of needle-like morphologies. In all of the samples, the Al-Si-Fe regions are constrained within the inter-dendritic regions due to kinetic differences between the phases. These phases were formed as curved crystals and in some regions, it exhibits plate-like morphology joined along with irregular, curved surfaces.

**Figure 2.** (**a**–**c**) Optical micrographs and (**d**–**f**) scanning electron microscopy images of the cast samples fabricated by (**a**,**d**) GDC route, (**b**,**e**) LPSC route, and (**c**,**f**) HPSC route, respectively.

#### *3.2. Hardness*

The hardness analysis was carried out to study the variation of hardness along the cast cross-section from the surface to the middle of the cast sample in all three-process conditions, *viz*., GDC, LPSC, and HPSC, respectively. The results shown in Figure 3 indicate that the squeeze-cast sample with higher pressure exhibits a higher hardness. Lin et al. studies on the Al-based alloys showed a hardness of 75 HV and 85 HV for GDC and highpressure squeeze cast materials [38]. Similarly, Thirumal et al. [39] studies on AA6061 alloy castings as a function of different squeeze-cast pressures show an increase in the hardness of the alloy with an increase in the pressure. The results from Lin et al. and Thirumal et al. are in agreement and are similar to the results from the present study. In addition, there is significant variation in hardness values as observed from the surface to core, indicating the absence of porosity and other casting defects.

**Figure 3.** Microhardness survey taken for the cast samples in all three conditions (gravity die casting (GDC), low-pressure squeeze-casting (LPSC), and high-pressure squeeze-casting (HPSC) taken from surface to center of the casting).

The hardness observations from the surface to the core also indicate that the cast structure is homogenous and uniform. On the other hand, the LPSC sample shows similar hardness values to the high-pressure squeeze-cast sample along the surface. However, the hardness values show some fluctuations when measured from the surface to the core, unlike the high-pressure squeeze-cast samples. This corroborates the presence of defects

(such as porosity) in these LPSC samples. Similarly, the GDC sample shows inferior hardness when compared to the squeeze-case samples due to reduced cooling rates. In addition, the hardness fluctuates between 95 HV to 75 HV as we move from the surface to the core showing the presence of defects/imperfections in these samples. Based on the hardness survey and microstructural correlation, it is evident that squeeze pressure is one of the most significant process parameters for achieving higher material properties with uniform distribution in the squeeze-casting process. This is in good agreement with the discussion carried out by Azhagan et al. [39] and Mohamed et al. [59]. In addition, the hardness of the alloy increases with the application of pressure. This enhanced behavior in HPSC and LPSC alloys in comparison with GDC alloy was due to improvements in heat transfer rates during solidification due to the applied pressure, resulting in refinement of microstructure and the improved contact area between the die and molten metal surface [50,51,60,61].

## *3.3. Tensile Properties*

Tensile properties of the investigated GDC and other two-squeeze cast samples are shown in Figure 4. The HPSC sample has shown a tensile strength of ~540 MPa against LPSC at ~382 MPa and GDC at ~367 MPa. On comparing GDC and squeeze cast alloys, the mechanical properties are superior for the LPSC and HPSC alloys. The results explain that the samples fabricated by the squeeze-casting process exhibit higher yield and tensile strength as compared to samples fabricated by the GDC process. In the squeeze-cast samples, the tensile and yield strength of the alloy increases with increasing pressure. A decrease in the grain size with an increase in squeeze casting pressure results in an increased grain boundaries volume. The increased grain boundary volume increases the resistance to dislocation movement, resulting in enhanced strength properties [62–64]. As pressure was held on molten metal during the squeeze casting process until the end of the solidification process, the rate of heat transfer was increased and macro and microporosity had been eliminated in comparison to the GDC process, resulting in enhanced mechanical properties. The elongation observed for the cast samples (GDC, to be almost similar with LPSC, and HPSC) is similar within the experimental conditions.

**Figure 4.** Engineering stress-strain curves of the cast samples fabricated under different conditions (gravity die-casting, low-pressure squeeze casting, and high-pressure squeeze casting).

#### *3.4. Wear Behavior*

The wear behavior of all three samples fabricated by the three casting routes (namely GDC, LPSC, and HPSC) were studied to understand their tribological behavior. The wear test results in terms of coefficient of friction (COF) and wear loss are shown in Figure 5. The COF increases with an increase in the working temperature. However, with the application of pressure at the same condition, the COF decreases in general (Figure 5a). The results suggest that HPSC samples show better wear resistance compared to LPSC and GDC samples at every given load and temperature combination (Figure 5b). Such improved

tribological performance of the HPSC samples is attributed to the reduction in porosity and shrinkage defects. A higher wear rate is observed for the non-pressurized cast samples due to its high coefficient of friction, which is the result of poor surface quality along with the presence of porosities and shrinkage defects, whereas the coefficient of friction is less in pressurized cast samples, thereby increasing its tribological response. Samples fabricated by squeeze-casting process demonstrated lesser wear rate in comparison to GDC process. Squeeze pressure maintains the molten metal closer to the wall surfaces of the die, which in turn gives a higher cooling rate at the surface. Higher cooling rate results in a more refined dendritic structure, resulting in a smoother surface. Finer microstructures offer improved hardness, which in turn offer higher wear resistance [65–67]. It may be observed that in general, the wear resistance of the HPSC decreases with increasing load and/temperature combination due to accelerated conditions (which is as expected). Ashiri et al. [36] have shown similar wear properties on the Al–Si–Mg–Ni–Cu alloy fabricated by GDC and pressure squeeze cast samples. They have demonstrated that both wear rate and COF decrease with an increase in the pressure at a given load. The wear loss increases with an increase in the applied load. In addition, the COF of pressure squeeze-cast materials is lower than the GDC counterpart, and the results are in agreement with the present study.

**Figure 5.** Tribological performance including (**a**) coefficient of friction and (**b**) wear loss comparison between GDC, LPSC, and HPSC samples as a function of load and operating temperature.

## *3.5. SEM Surface Analysis of the Worn out Samples*

Fracture analysis conducted on worn-out samples by using SEM micrographs is shown in Figure 6. The wear surface of Al-based cast samples through the GDC route (Figure 6(a1,a2)) shows the presence of excessive material loss due to digging and penetration (deeper ploughing grooves [68,69]) of the pin at higher loads applied at elevated temperature. On the other hand, the LPSC samples wear surface shows minor digging and smearing observed due to frequent rubbing of the pin (Figure 6(b1,b2)). In addition, delamination and micro cracking (Figure 6(a1,a2)) may be observed in the samples produced through the GDC route due to its lower hardness compared to LPSC and HPSC samples. However, deep ploughing grooves and considerable delamination were not observed in the samples fabricated through LPSC and HPSC. The HPSC samples wear surface shows minimal rubbing/wear pattern at higher loads applied at elevated temperature. This difference in the wear rate of HPSC samples (Figure 6(c1,c2)) is due to the molten metal being solidified under high pressure, which reduces gas entrapments and shrinkages or gas porosity thereby improving its tribological properties, in addition to the microstructural refinement [70–72]. The present results are very similar to the studies conducted by Ashiri et al. [36], where deeper ploughing grooves are observed for the GDC samples compared with the pressure squeeze-cast samples leading to severe damage in the

GDC samples. Hence, the present results demonstrate the role of pressure during the casting/solidification process and its influence in refining the microstructure and improving their mechanical and tribological performance.

**Figure 6.** Worn out surface of the cast alloys in GDC, LPSC, an HPSC at two different load conditions 20 N (**a1**,**b1**,**c1**) and 60 N (**a2**,**b2**,**c2**) at a higher operating temperature of 200 ◦C.
