**1. Introduction**

According to the International Energy Association report of 2019, the transportation industry is the second largest contributor to global CO2 emissions (at 27%) [1]. The automotive industry globally is striving to reduce CO2 emissions by light-weighting, improving the efficiency of internal combustion engines, usage of alternate fuels, etc. A 10% reduction in mass results in a 6% improvement in fuel efficiency, by which CO2 emission will be reduced significantly over the lifetime of the vehicle [1].

**Citation:** Chandra, V.S.; Krishna, K.S.V.B.R.; Ravi, M.; Sivaprasad, K.; Dhanasekaran, S.; Prashanth, K.G. Mechanical and Tribological Behavior of Gravity and Squeeze Cast Novel Al-Si Alloy. *Metals* **2022**, *12*, 194. https:// doi.org/10.3390/met12020194

Academic Editor: Wenming Jiang

Received: 3 January 2022 Accepted: 19 January 2022 Published: 21 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Aluminum, being light in comparison to steel, is an excellent choice of material for weight reduction in automobile and aerospace sectors (where it is available in both sheet and cast forms) [2–5]. The aluminum industry offers a wide range of aluminum alloys with various combinations of strength, ductility, wear, and corrosion resistance [6–8]. Many elemental combinations are used to alloy with aluminum as solute [9,10]. Small quantities of elements such as silicon, manganese, iron, chromium, molybdenum, etc. are added in aluminum to enhance its mechanical and physical properties and to improve some of the specific properties required in strategic applications [11–13]. In almost all production industries and in day-to-day life, Al-Si cast alloys play a vital role due to their ease in castability, corrosion resistance, and high mechanical properties [14–16]. Al-Si alloys are widely used in various industries due to their excellent mechanical properties, improved wear and thermal behavior, supreme corrosion resistance, and excellent castability [8,17–19]. The addition of Cu and Fe to this alloy further enhances the mechanical and wear behavior of the material without heat treatment [20–22]. The addition of Copper imparts strength and hardness to the casting [20,22]. The properties achieved by the addition of Fe are comparable to those alloys with various heat-treated and aged Aluminum alloys. Optimized Fe alloying aids for the possibility to reduce the aging time without the addition of Mg, resulting in significant cost saving which is the need of the hour in any industry [23,24].

Iron tends to form intermetallic with other alloying elements resulting in strengthening of the alloy with enhanced wear and thermal behavior [24]. The intermetallics formed are usually hard and brittle with a superior high-temperature behavior. Thus, novelty in conventional Al-Si casting alloys, by the addition of Cu and Fe during the casting process, can result in enhanced performance of the cast product [20–23]. Further, the addition of Fe improves fluidity, a vital requirement to produce a sound casting. Taylor et al. suggested having a critical percentage of Fe based on the silicon percentage in the alloy. If Fe exceeds the critical percentage, it would influence loss of ductility due to shrinkage porosity [23,25]. Enhanced fluidity provides an opportunity for the production of thin-walled castings [26]. Intricate shapes with a near-net finish are possible due to the improved fluidity. 4XXX series wrought alloy has a UTS of ~134 MPa and YS of ~64 MPa and novel Al-Si alloy is expected to have the UTS of ~385 MPa and YS of ~240 MPa [18,19,27–29]. There is a significant enhancement of mechanical properties by the addition of an optimum percentage of Fe, without any thermal treatment leading to huge cost savings [24]. Hence, such novel Albased alloys can be utilized in a variety of applications, due to their superior mechanical and thermal properties. In addition, it can find a prominent place in automotive industry, where light-weighting at a lower cost is a beneficial advantage. Some of the remarkable advantages of die-casting over conventional sand casting are an increase in productivity, dimensional accuracy of as-cast components, and better mechanical properties as a result of improved microstructural features [30].

Squeeze casting is both economical and has the potential to create cast components with minimal defects, often achieving near-net-shaped components [31–33]. In addition, die-casting reduces the metal wastages which arise due to the use of feeders and risers as in conventional sand casting [34]. Squeeze casting is a combination of the casting and forging processes where the solidification of molten metal takes place under pressure, thereby reducing the casting defects created due to gas entrapments as well as increasing the ductility of the resultant component. The squeeze casting process parameters play an important role in determining the microstructure of the cast components. The process parameters such as squeeze pressure, squeeze pressure duration, pouring temperature, and die temperatures have overall control on the microstructure [35]. The squeezing pressure increases the heat transfer rate in between the mold interfaces that enhance the surface finish and also help to create a uniform microstructure from surface to core [36]. For any new alloys or modified alloys, process parameters have to be optimized for better microstructure and mechanical properties [37]. In the squeeze casting process, the desirable mechanical and microstructure features are based on the combination of mold casting and die forging due to the fact that the molten metal is solidified under hydrostatic

pressure. This would have better control over mechanical properties as we all as lead to a homogenous microstructure [38]. In addition, the squeeze-casting process creates a very fine microstructure and also eliminates the gas and shrinkage porosities [39]. Reports on the Fe-addition to Al-Si alloy show that for the Al-Si alloy with Fe, the mechanical properties depend not only on chemical composition but also on the microstructural morphology of the phases (such as the Al-rich alpha phase and eutectic Si phase) [40].

Since the addition of alloys elements to Al lead to the formation of coarse microstructure in the as-cast condition and to obtain finer microstructure suitable strategies need to be followed [41–45]. Fine microstructure can be the addition of grain refiners [44–46], severe plastic deformation [47,48], cryomilling [49], high pressure solidification [50,51], laser processing [52,53], etc. Accordingly, the present investigation aims to study utilize the low-cost fabrication technique (squeeze pressure casting) and explore the influence of applied squeeze pressure on the microstructure during the solidification of a molten Al-Si-Cu-Fe alloy. The influence of squeeze pressure on the mechanical and wear behavior of the alloy is investigated in detail.

#### **2. Experimental Procedure**

Samples considered in the present study are processed through gravity die casting (GDC), low-pressure squeeze casting (LPSC) at 5 MPa, and high-pressure squeeze cast (HPSC) at 12 MPa respectively using a cylindrical mold made up of H13 steel. The chemical composition of the alloy in various cast conditions are determined using an optical emission spark (OES) spectrometer and are listed in Table 1**.**


**Table 1.** Chemical composition of novel alloy designed.

Microstructural analysis of the samples (under various casting conditions) was observed using an optical microscope (LEICA DMLM, Mumbai, India; 50× to 1000× range). The hardness measurements were carried out using a Zwick Roell Vickers microhardness tester (from Zwick, Gurugrum, India) at a test load of 0.1 kgf with a dwell time of 10 s. Tensile testing was performed using a Tinius Olsen H25KL tabletop tensile testing unit (from Tinius Olsen, Noida, India) with a strain rate of 5 × <sup>10</sup>−4/s as per ASTM-E08-2016 standard using a sub-sized specimen [54]. Wear testing was carried out using a pin on disc wear testing machine (Ducom, Bangalore, India) based on the ASTM G99-05 standard [55]. Wear tests were carried out with a sliding velocity set to 0.314 m/s and measured for the sliding distance of 1000 m [56–58]. The sliding disc diameter is 30 mm, the speed of the machine is held at 200 rpm and the test time is considered to be around 3185 s. The machine disc is made up of EN31 material with a roughness of 10 μm and hardness ~60 HRC. The wear testing was carried out with different test variables to understand the behavior of Al-Si alloys as a function of changing parameters. Three different loads were applied (20 N, 40 N, and 60 N) at a higher operating temperature of 200 ◦C, refer to Table 2. The schematic of the wear testing unit is illustrated in Figure 1. The wear testing machine consists of a specimen in the form of a pin and it is tested against a disc made of EN31 material according to the ASTM G99-05 standards. In addition, the load is applied through the loading panel, and the entire equipment is operated using a computer-based controller. All of the parameters including depth, force, temperature, speed of the disc, time, etc. can be controlled using the controller in an acute fashion. The surface morphological features of all of the tensile fractured samples and worn-out surfaces from wear tests were studied using an FEI Quanta 200 Scanning electron microscope (SEM) (FEI, Bangalore, India).


**Table 2.** Al-Si alloys wear test input parameters.

**Figure 1.** Schematic representation of the pin-on-disc wear testing unit.
