*Article* **Mechanical and Microstructural Characterization of Friction Stir Welded SiC and B4C Reinforced Aluminium Alloy AA6061 Metal Matrix Composites**

**Kaveripakkam Suban Ashraff Ali <sup>1</sup> , Vinayagam Mohanavel <sup>2</sup> , Subbiah Arungalai Vendan <sup>3</sup> , Manickam Ravichandran <sup>4</sup> , Anshul Yadav <sup>5</sup> , Marek Gucwa <sup>6</sup> and Jerzy Winczek 6,\***


**Abstract:** This study focuses on the properties and process parameters dictating behavioural aspects of friction stir welded Aluminium Alloy AA6061 metal matrix composites reinforced with varying percentages of SiC and B4C. The joint properties in terms of mechanical strength, microstructural integrity and quality were examined. The weld reveals grain refinement and uniform distribution of reinforced particles in the joint region leading to improved strength compared to other joints of varying base material compositions. The tensile properties of the friction stir welded Al-MMCs improved after reinforcement with SiC and B4C. The maximum ultimate tensile stress was around 172.8 ± 1.9 MPa for composite with 10% SiC and 3% B4C reinforcement. The percentage elongation decreased as the percentage of SiC decreases and B4C increases. The hardness of the Al-MMCs improved considerably by adding reinforcement and subsequent thermal action during the FSW process, indicating an optimal increase as it eliminates brittleness. It was seen that higher SiC content contributes to higher strength, improved wear properties and hardness. The wear rate was as high as 12 ± 0.9 g/s for 10% SiC reinforcement and 30 N load. The wear rate reduced for lower values of load and increased with B4C reinforcement. The microstructural examination at the joints reveals the flow of plasticized metal from advancing to the retreating side. The formation of onion rings in the weld zone was due to the cylindrical FSW rotating tool material impression during the stirring action. Alterations in chemical properties are negligible, thereby retaining the original characteristics of the materials post welding. No major cracks or pores were observed during the non-destructive testing process that established good quality of the weld. The results are indicated improvement in mechanical and microstructural properties of the weld.

**Keywords:** stir casting; boron carbide; silicon carbide; AA6061 aluminium alloy; tensile strength; friction stir welding

## **1. Introduction**

In many industrial applications, aluminium alloys are reinforced with hard ceramic particles to enhance the mechanical properties of aluminium metal matrix composites (Al-MMCs) [1–3]. Aluminium MMCs are lightweight material accompanied with good

**Citation:** Ali, K.S.A.; Mohanavel, V.; Vendan, S.A.; Ravichandran, M.; Yadav, A.; Gucwa, M.; Winczek, J. Mechanical and Microstructural Characterization of Friction Stir Welded SiC and B4C Reinforced Aluminium Alloy AA6061 Metal Matrix Composites. *Materials* **2021**, *14*, 3110. https://doi.org/10.3390/ ma14113110

Academic Editor: Hideki Hosoda

Received: 25 April 2021 Accepted: 3 June 2021 Published: 5 June 2021

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thermal and electrical conductivity, high stiffness, hardness, strength, melting point and wear resistance [4–6]. Due to their higher processing prices, Al-MMCs are deployed only for military weapons and aerospace applications. Aluminium MMCs have further found applications in automobile products, such as pistons, engine, disk brakes, cylinder liners and drum brakes [7,8]. Fusion welding of Al-MMCs creates brittle intermetallic components within the weld region's matrix and the reinforcement particles. The induced stress in the weld reduces the joint efficiency and reveals porosity and voids at the joint [9,10].

In the friction stir welding (FSW) process, a non-consumable rotating tool having higher toughness than the base material is pitched into the faying/butt ends of the plates to be welded. They are subject to appropriate axial force developed along the line of the joint. Pin and shoulder are the two key portions of the tool. The material experiencing the rotational movement of the tool pin is unstiffened by the frictional heat generated by the tool during the spinning action. The rotating tool drives the plastically distorted material from the front towards the reverse side of the tool. Subsequent forging facilitates the achievement of the weld. Though a solid-state joining process with the absence of material melting, FSW reveals a lack of stimulated melt solidified structure in the weld zone. This addresses the technical lacuna's encountered during the fusion welding of composites [11–16]. A few impactful literature related to the FSW of Al-MMCs has been reported [17–22]. Vijay and Murugan [22] reported that fine grain size and high tensile strength was achieved for the square pin profile of friction stir welded AA6061/TiB<sup>2</sup> composite. Xu et al. [21] stated that the high tensile strength values are obtained for high tool rotating speed. Topcu et al. [20] emphasized that the hardness increases with an increase in the B4C reinforcement. Ali et al. [18] investigated the hardness and tensile properties of weld specimen and linked them with microstructural variation in AA6061/SiC/B4C composites. Palanivelet et al. [17] revealed that the nugget zone and the grain growth are affected when the time of exposure of the FSW tool is high. Wook et al. [19] achieved uniform distribution of reinforced particles in weldment due to friction of FSW tool, and SiC increases the hardness of the Al-MMCs.

Limited literature reports are available on appropriate mixing percentages of SiC and B4C reinforcements on Al-MMCs for which industrially acceptable mechanical property ranges and microstructural integrities for reliability are established. In particular, reports on weldability studies on these Al-MMC's are sporadic, lacking an interdisciplinary treatment. This provides a broader scope for exploration regarding the thermal implications of FSW on microstructures, process parametric influences on weld efficiency and mechanical behavioural analysis. Hence, this work is undertaken to fabricate Al-MMC reinforced with SiC and B4C particles using the stir casting technique in various percentage compositions. FSW is performed on these composites by varying the process parameters to the weld's changes in properties and behaviours. The energy generated during the FSW process in terms of heat is also estimated to examine the thermal behaviour of the weld in different regions of the joint, which governs the microstructural grain sizes and the consequential, mechanical properties.

## **2. Experimental Procedure**

The experimentation is carried out in two phases: the first phase involves stir casting to fabricate Al-MMC's with varying reinforcement composition. The second phase involves the FSW of the stir cast samples, followed by testing and characterization.
