Experimental Investigations on Hybrid Surface Composite of AA5083 Produced by Friction Stir Processing

Abstract

In recent years, Friction Stir Processing (FSP) has emerged as a very promising method for attaining superplasticity and fabricating surface composites. The use of FSP has been extensively employed to modify the mechanical, microstructural, and tribological characteristics of many materials. The fundamental idea behind the operation of FSP is rooted in the principles of Friction Stir Welding (FSW). In FSP, the workpiece is introduced by employing a rotating tool with a pin in it. This aids in material movement, which in turn induces changes in the workpiece's microstructure that improve the desired qualities of the material. Plastic deformation of the material can be observed when the rotating pin contacts the workpiece. The tool shoulder plays a crucial role in enhancing friction generation and enabling forging action, resulting in increased heat generation and subsequent microstructural alterations. In the process of producing surface composites, reinforcement particles are added to the cavities in the base matrix. The reinforcing particles are then dispersed and evenly distributed throughout the base matrix by the stirring movement generated by the rotating tool during FSP. Surface composites have shown the potential as a viable approach for enhancing characteristics like hardness, microstructure, corrosion resistance, tribological properties etc. This enhancement in these properties helps the industry to use surface composite materials in various applications like marine, aerospace, automobile, etc. Due to its excellent strength-to weight ratio and ability to resist corrosion, the aluminium alloy AA5083 is popular among marine industries. However, this alloy lacks microhardness and tribological properties. Therefore, there is a need to produce a surface composite of this alloy so that these properties can be enhanced. In order to enhance these properties at the same time, it is necessary to use hybrid reinforcement. Consequently, very limited studies have been reported in the literature that show the production of surface composites on the AA5083 matrix by using hybrid reinforcement. Moreover, there is limited availability of literature related to the determination of the optimal process parameters for FSP and the investigation of the impact of FSP process parameters, such as tool rotating speed, tool traverse speed, hybrid reinforcement volume fraction, size of reinforcement particles, number of FSP passes, and alteration in tool travel direction (ATTD), on the microstructure, microhardness, and tribological properties of AA5083-based composites. In this study, FSP was used on A5083 aluminium alloy in order to generate a surface composite. This was done by incorporating a hybrid reinforcement consisting of Silicon Carbide (SiC) and Graphite (Gr). The surface composite was produced by using the different combinations of FSP process parameters and then studied for their effect on microstructure, microhardness, and tribological properties. The production of surface composite by FSP is a difficult task because of the reason that in FSP, a proper balance has to be maintained between different process parameters of FSP like tool traverse speed, tool rotating speed, hybrid reinforcement volume fraction, size of reinforcement particles, number of FSP passes and ATTD. The first step included the optimisation of fundamental parameters in the FSP process, such as tool rotation speed, traverse speed, number of FSP passes, and volume ratio of hybrid reinforcement. This optimisation was carried out utilising response surface approach. Box Behnken design was used to conduct experiments, and the optimum solution was found after the test. The optimal process parameters for the FSP were determined to be a traverse speed of 50mm/min, a rotation speed of 1000rpm, 3 passes of FSP, and a reinforcement volume ratio of 75:25 (SiC:Gr). An FSP experiment on AA5083/(SiC-Gr) was conducted using the optimum solution and was investigated to find its influence on microhardness, wear rate, and coefficient of friction (COF). It was found that the predicted and experimental results were within the range of ±10%. Furthermore, the effect of the hybrid reinforcement volume fraction and the size of the reinforcement particles were investigated. Different ratios of reinforcement were mixed together, viz. 25:75, 50:50, 75:25 (SiC:Gr) and was investigated to study their effect on the different properties. The experimental results indicated that the hybrid reinforcement volume fraction of 75:25 (SiC:Gr) exhibited the most favourable outcome because SiC acts as the hard ceramic particles and Gr acts as the lubricant particles, and together, they help to enhance both microhardness and tribological properties of AA5083 surface composite. Similarly, the effect of the reinforcement size was also investigated. Both SiC and Gr, having their particle size in microns and nanometres, were investigated, and it was found that the reinforcement particle size in nanometres showed the best dispersion and has enhanced properties of the AA5083. It was found that the reinforcement ratio of 75:25 (SiC:Gr) nano hybrid reinforcement enhanced the microhardness of the FSPed specimen by 25.6% in comparison with the base matrix. It reduced the wear rate by 69.2% and Average COF by 28.8% in comparison with the as received AA5083 and thus enhanced the wear resistance of the material. In both tests of volume percent and reinforcement size, FSP process parameters were chosen based on the optimal solution obtained from the optimisation findings. Furthermore, the process parameters of the FSP, such as the tool traverse speed, rotational speed, number of passes, and ATTD, were examined. The previous FSP process parameters were selected, and the study was done to find its effect on various properties of the prepared surface composite. Three different rotating speed and traverse speeds ere combined and analyzed for enhancement in the material properties. Also, 1, 2, 3, and 4 numbers of FSP passes were investigated with the same and with ATTD. It was found that 3 number of FSP passes with ATTD produced the best results. Also, the tool rotational speed of 1000rpm with a tool traverse speed of 50mm/min showed the best-enhanced microhardness, wear rate, COF, and microstructural properties. The produced surface composite enhanced the microhardness by 9.6%, reduced the wear rate by 175.9%, and COF by 33.8%. The investigation facilitated the identification of the optimal FSP process parameters and the improvement of the material properties. The study revealed that the use of the FSP technique resulted in the development of the AA5083/(SiC-Gr) hybrid surface composite, which displayed improved microhardness, tribological, and microstructural characteristics. Hybrid reinforcement helped the AA5083 to simultaneously enhance the wear and microhardness properties. This type of surface composite can be useful in a variety of applications, including marine and automobile industries applications.