**2. Experimental Procedure**

#### *2.1. Experimental Materials*

In this experiment, the SiCp/ZL101 composite material with 20% SiCp volume fraction was prepared by stir casting method, and the size of the SiC particles was 1000 mesh. The material composition of ZL101 aluminum alloy is shown in Table 1. The prepared composite materials were cut into sheets of size 6 mm × 180 mm × 90 mm by wire cut, ZL101 sheets were cut into sheets of size 9 mm × 180 mm × 90 mm by the wire cut, and the surface roughness of the two sheets was processed to Ra 0.8 by machining technology.

**Table 1.** Chemical composition of the ZL101 aluminum alloy (wt.%).


The experimental equipment in this study was a FSW equipment modified from a X35K milling machine from Zhengling (Liuzhou, China), and the FSLW process was adopted to weld SiCp/ZL101 composite plate (upper plate) and ZL101 plate (lower plate) to prepare multiple sets of welding samples. Figure 1a is a schematic diagram of the FSLW process. The overlapping rate of adjacent welding passes was 50%. The detailed welding parameters are shown in Table 2. After the welding samples were prepared, natural aging was carried out for 7 days.

**Figure 1.** (**a**) Schematic diagram of FSLW; (**b**) Schematic diagram of the size of the stirring pin; (**c**) Schematic diagram of the sampling location of the friction and wear specimen; (**d**) Schematic diagram of the sizes of the friction and wear specimen.



Six friction and wear test specimens were cut from the weld joint of the sample by wire cut. The sampling location is shown in Figure 1c, and the sample size is shown in Figure 1d. The surface roughness of the specimens was treated to Ra0.8 by the machining technology.

## *2.2. Friction and Wear Test*

The equipment used in this experiment was MMQ-02G ball-on-disk high-temperature friction and wear testing machine from Yihua (Jinan, China), and six specimens were subjected to friction and wear tests in air atmosphere at different temperatures. Figure 2 is a schematic diagram of the experimental device. As can be seen, the specimen was fixed on the rotating disk, and the counter-grinding ball was in contact with the surface of the specimen under the specified load. The distance between the counter-grinding ball and the center of the rotating disc is the friction radius. After the device was heated to the specified temperature in the incubator, the rotating disk drove the friction specimen to perform relative frictional motion with the grinding ball at a specified speed. Si4N3 balls with a diameter of 6 mm were used as the counter-grinding balls, and the test time was 60 min. The test parameters of the six specimens are shown in Table 3. During the experiment, the test parameters such as test force, rotational speed, friction coefficient, temperature, time, etc., were collected and calculated by the computer in real time, and the wear debris was collected after each test.

**Figure 2.** Schematic diagram of the friction and wear test device.


**Table 3.** The parameters of the friction and wear test.

## *2.3. Wear Detection and Structural Characterization*

After cleaning and drying the friction and wear specimens with alcohol, the MS-M9000 multifunctional friction tester from Huahui (Lanzhou, China) was used to measure the wear extent and wear scar depth of the friction and wear specimens by the surface profile method. Each specimen was measured at four symmetrical parts of the friction ring, and the arithmetic mean value of the four groups of data was taken.

In order to study the microstructure of the wear surface and wear debris, a Quanta 400 scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS) (FEI, Eindhoven, The Netherlands) was used to observe the microstructure of the wear surface and wear debris, and the elemental compositions of the wear debris were analyzed by EDS.

## **3. Results**

## *3.1. Macrostructure Morphology of Wear Surface*

Figure 3 is a schematic diagram of the macrostructure of the wear surface at 30 ◦C, 100 ◦C, 150 ◦C, 200 ◦C, 250 ◦C and 300 ◦C. As shown in the figure, the wear scar of the 30 ◦C specimen (Figure 3a) is relatively wide, and the wear surface is rough and uneven, showing a silvery white luster. The wear surface of the specimens from 100 ◦C to 300 ◦C is black; the wavy folds caused by extrusion deformation can be clearly observed on the wear surface of the specimen at 100 ◦C (Figure 3b), and the distribution is relatively dense. The distribution of wavy folds on the wear surface of the 150 ◦C specimen (Figure 3c) is relatively sparse. The wear surface of the 200 ◦C specimen (Figure 3d) is relatively flat, with only one obvious wavy fold observed. The wear surfaces of the 250 ◦C and 300 ◦C specimens (Figure 3e,f) are relatively flat.

**Figure 3.** (**a**–**f**) Macrostructures of the wear surfaces of 30 ◦C, 100 ◦C, 150 ◦C, 200 ◦C, 250 ◦C and 300 ◦C.
