**1. Introduction**

Aluminum matrix composites (AMCs) have the advantages of light weight, high tensile strength, high specific stiffness and specific strength, better fatigue strength, corrosion resistance and a low thermal expansion coefficient, etc. [1–6]. They have broad application prospects in electronics, new energy vehicles, aerospace and other fields [7–9]. Traditional cast iron and cast steel brake discs are not conducive to improving the braking performance of vehicles due to defects such as heavy weight, poor thermal conductivity and poor fatigue performance [4,10]. Aluminum-based composite materials are ideal substitutes for traditional brake disc materials [4,11]. The application in the brake disk has been widely considered [12]. For example, Venkatachalam et al. [13] prepared Al6082 composite material by stir casting, and verified through performance research that the composite material had a small friction coefficient and wear rate, and could be used to prepare automobile brake discs. Firouz et al. [14] prepared Al-9Si-SiC composite automobile brake discs with 10% and 20% SiC volume fractions by stir casting, and conducted thermal fatigue research. The results showed that the thermal fatigue performance of composite brake

discs was better than that of cast iron brake discs. Sadagopan et al. [15] prepared the Al 6061 metal matrix composite brake rotor with 20% SiC volume fraction by stir casting, and verified through experiments that the composite brake disc had better efficiency in terms of braking distance and heat dissipation than the cast iron disc. Daoud et al. [16] prepared the A359 particle composite brake rotor with 20% SiC volume fraction by sand casting, and verified that the AMC brake disc had the advantages of wear resistance and higher thermal conductivity compared with the cast iron brake disc, lighter weight and a more uniform coefficient of friction; these characteristics reduced braking distance and braking noise. In addition, there are many studies on the friction and wear behavior of AMCs. For example, Jin et al. [5] conducted a high-temperature friction and wear test on as-cast SiCp/A356 composites. The results showed that the wear rate of the as-cast SiCp/A356 was very sensitive to temperature changes, and the friction stability decreased sharply with the increase in temperature. Hekner et al. [17] used molecular dynamic simulations to study the nanoscale wear behavior of SiC particle-reinforced AMCs (SiC/Al NCs), and the results showed that the wear mechanism was changed during high temperature.

Particle-reinforced composite castings prepared by traditional casting technology have defects such as uneven particle distribution and large porosity [18–20], and do not have high strength and ductility compared with the base material [21], resulting in the inability to fully exert the performance of composite materials. To overcome this problem, Friction Stir Processing (FSP) is widely used in the preparation of composite materials [22–24]; through the continuous stirring motion of the stirring tool, the reinforced particles are evenly distributed throughout the matrix, which reduces the porosity and improves the friction and wear properties of the composite [25]. Based on the above research results, this study proposed a new method for the preparation of composite brake discs, which used friction stir welding to lap the AMC sheet on the aluminum alloy substrate, and at the same time used FSP to modify the AMC, to prepare a functionally graded brake disc material with both wear resistance and toughness. At present, there is much research on FSW for AMCs. For example, Avettand-Fènoël et al. [26] reviewed the microstructure, friction stir welding performance and other indicators of the FSW joints of various AMC materials, and proposed ways to improve them. Zuo et al. [27] reviewed the weldability, macrostructure and microstructure of joints, mechanical properties of joints, tool wear and monitoring of SiCp/Al composites, and looked forward to the future development direction. In addition, the research on the use of FSP to prepare AMCs is also a hot topic that has attracted much attention. For example, Vijayavel et al. [28] used FSP to process the surface of the lm25 composite material with a volume fraction of 5% SiC. The experimental results showed that when the shaft-to-shoulder ratio of the stirring pin was 3.0, the obtained equiaxed grains were finer [29] and the microstructures processed at a tool traverse speed of 40 mm/min showed excellent wear resistance. Mohamadigangaraj et al. [30] evaluated the effects of friction stir processing parameters on the properties of A390-10 wt% SiC composite using response surface methodology, and the results showed that the speed of rotation had a higher impact on hardness than other parameters. Kumar et al. [31] investigated the mechanism for improving the tensile properties, wear properties and corrosion resistance of stir-cast Al7075–2 wt.% SiC composites by friction stir processing (FSP); the results showed that nanoparticle-reinforced composites after FSP exhibited better wear resistance than microparticle-reinforced composites. Kurtyka et al. [32] studied the effect of the plastic deformation generated in the FSP process on the concentration and distribution of SiC particles in the cast composite A339/SiCp, and the study showed that the FSP process significantly improved the distribution of reinforced particles in the composite. Butola et al. [33] conducted pin-on-disk friction and wear tests on AA7075–2 wt.% SiC composites prepared by FSP, and the results showed that FSP can produce surface composites with no defects and the uniform distribution of reinforcement materials, which helped to improve the wear resistance of composites. Aruri et al. [34] studied the effect of tool speed on the wear properties of aluminum-based surface hybrid composites manufactured by FSP, and the results showed that reducing the tool speed appropriately

could reduce the wear rate of Al-SiC/Al2O3 surface composites. Devaraju et al. [35] studied the effects of rotational speed and reinforced particles such as SiC and Al2O3 on the wear and mechanical properties of aluminum alloy-based surface hybrid composites prepared by FSP, and found that the size of the reinforced particles was reduced, and the wear resistance was greatly improved after FSP. Rana et al. [36] used FSP to prepare Al 7075-T651-B4C surface composite material, and found that the wear resistance of the composite material increased by 100% compared with the base material. The FSW process research on AMCs mentioned in the above research rarely involves the FSLW process. The existing research on the preparation of AMCs by FSP mainly focuses on the optimization of process parameters, and the volume fraction of the reinforcing phase is not high (≤10%). The research on the friction and wear properties of the prepared surface composites is also mainly concentrated on the normal temperature in the environment. Since the sliding friction will generate a lot of heat during the braking process, the temperature of the brake disc will change sharply, which will affect the braking effect (thermal stability, the vibration of the braking system, braking noise, braking safety and so on [37,38]). The above research results rarely involve the influence of temperature on the friction and wear properties of FSPed composite material. The friction and wear properties of this brake disc material at different temperatures are of great significance for exploring the wear mechanism of the brake materials at different temperatures and evaluating their braking performance.

ZL101 has excellent casting performance and good weldability, and is widely used in the preparation and welding of AMCs [39–44]. In this study, the 20% volume of the stircasted SiCp/ZL101 composite sheet and the ZL101 sheet was used for the preparation of the composite material using the FSLW. The friction and wear performances and mechanisms of the SiCp/ZL101 and ZL101 composite material at 30 ◦C, 100 ◦C, 150 ◦C, 200 ◦C, 250 ◦C and 300 ◦C were studied, providing a theoretical basis for the evaluation of the braking performance of the brake disc material.
