An Experimental Study on the Influence of Micrographite on the Improvement of Mechanical Characteristics of A356 Microcomposites Processed via the Stir Casting Route ^†

Abstract

A composite is a combination of two or more insoluble materials that have greater characteristics than all of their individual constituents. The current study focuses on manufacturing and testing the mechanical characteristics of A356 alloy composites reinforced with micrographite particles at different weight fractions of 0%, 5%, 10%, and 15%. According to the findings, including micrographite particles up to 10% wt. increases the bulk hardness, tensile strength, and compression strength of the A356 alloy matrix. In addition, scanning electron microscopy (SEM) was employed to inspect the tensile-fractured surfaces of the fabricated composites.

1. Introduction

Composite materials have taken over a large section of the automobile industry due to their light weight and ability to endure harsh environments [1]. Applications for aluminum metal matrix composites (AMMC) include cylinder liners, pistons, rotors, and brakes [2]. Due to their affordable production costs and balanced properties, they are widely utilized in the marine and aerospace industries [3]. A variety of methods, including liquid and solid metallurgical processing, can be used to fabricate AMMC. Some examples of liquid metallurgy processing include liquid infiltration, stir casting, and compos casting [4]. Among all techniques, stir casting is the most flexible and least expensive process, which ensures a greater production rate [5]. Ravindran et al. [6] utilized the powder metallurgy technique to produce hybrid composites with Al2024 alloy/0 to 20% SiCp/5% graphite particles. The addition of silicon carbide reinforcement in the aluminum composite resulted in enhanced hardness and wear resistance. Rao and Das [7] found that increases in SiC particle content in the Al matrix resulted in a reduced wear rate. The study evaluates the mechanical properties of an aluminum alloy composite reinforced with graphite, focusing on its performance and suitability for its intended application.

2. Materials and Methods

The base metal chosen was the A356 alloy, which has been reinforced with graphite (Gr) particles with an average size of about 50 µm. The composition of the composites is displayed in

Table 1. Composition of manufactured composites.

Composite Sample	% of the Material
	A356	Graphite
G1	100	0
G2	95	5
G3	90	10
G4	85	15

. The composites were manufactured via the stir casting technique. In the initial stages, the furnace was used to heat and melt the A356 alloy rods at 700 °C. The molten metal was stirred with a graphite stirrer at 500 rpm for five minutes prior to adding the reinforcements. After being heated to 130 °C, the reinforcing particles were added to the molten metal. The composite blend was dispensed into a mild steel die after being stirred.

With the help of the universal testing (UTM) machine, tests for ultimate tensile and compression were carried out in accordance with ASTM Standards (Tensile: E8/E8M and Compression: ASTM E9). The Brinell hardness test was performed using a 50 kg force and a 2.5 mm ball indenter.

3. Results and Discussion

3.1. Bulk Hardness Evaluation

Figure 1 illustrates that the hardness values (BHN) indicate distinct trends in the mechanical properties of the composites. In the base A356 alloy without micrographite (0% composition), the hardness is measured at 71 BHN. With the addition of 5% micrographite, the hardness increases to 74 BHN, suggesting a strengthening effect. Further incorporation of micrographite up to 10% results in a slight hardness improvement to 75 BHN. However, the composite with 15% micrographite exhibits a decreased hardness of 68 BHN compared to the 10% composition. At larger quantities of micrographite, the observed loss in hardness may be attributable to the presence of porosity or particle agglomeration. This implies the possibility of a saturation point or difficulties with the even distribution of graphite particles in the composite.

3.2. Tensile and Compression Strength Evaluation

The influence of micrographite reinforcement on the tensile strength and compression strength of A356 alloy composites is shown in Figure 2a,b, respectively. Tensile strength and percentage of elongation were examined across varying percentages of micrographite, which can be inferred from Figure 2a. In the baseline A356 alloy (0% micrographite), a tensile strength of 99 MPa and 3.8% elongation were observed. The introduction of 5% micrographite resulted in increased tensile strength (106 MPa) and elongation (4.0%), suggesting a positive effect on mechanical properties. Further enhancement was evident with 10% micrographite, exhibiting a tensile strength of 110 MPa and 4.1% elongation. However, at 15% micrographite, while tensile strength remained relatively low (102 MPa), a slight decrease in elongation (3.8%) was noted compared to the composite with 10% micrographite. From Figure 2b, it is observed that in the base metal A356 alloy (0% micrographite), the compressive strength is noted at 175 MPa. With the introduction of 5% micrographite (A356 + 5% micrographite composite), the compressive strength increases to 188 MPa, representing a positive reinforcement effect. This trend remains with 10% micrographite (A356 + 10% micrographite composite), where the compressive strength further improves to 192 MPa, suggesting an enhanced resistance to compressive forces. However, at 15% micrographite (A356 + 15% micrographite composite), while the compressive strength remains relatively high at 178 MPa, there is a slight reduction compared to the 10% composition. This decline may be indicative of factors such as challenges in particle dispersion or diminishing returns with higher graphite content. Overall, the results show that micrographite reinforcement enhances the tensile and compression strength of A356 alloy composites, with the 10% composition performing best.

Microstructural analysis was used to perform a detailed examination of the tensile fracture mechanism of the tested composite specimens. Figure 3a,b show SEM (scanning electron microscopy) micrographs of the fractured Al-356/10Gr specimen. Upon closer inspection of the SEM image, distinctive features such as tear ridges and dimples are observed. Tear ridges are visible at the grain boundaries, which can be attributed to the resistance produced by the uniformly dispersed reinforcing particles acting against the applied tensile loading.

4. Conclusions

A356 aluminum matrix composites with varying weight percentages of graphite (Gr) were fabricated using stir casting. Tensile properties improved with up to 10 wt.% Gr, exhibiting ductile failure with tear ridges and dimples. Matrix strengthening by the primary particle Gr was the dominant mechanism. Bulk hardness increased with up to 10 wt.% Gr, declining with 15 wt.%. A356/10% Gr outperformed A356 and other composites, attributed to uniform particle dispersion, superior bonding, hard particle presence, and the absence of porosities. A356/10% Gr exhibited enhanced compressive properties, with a 9.8% higher strength (192 MPa) compared to the alloy.