## 3 Results and Discussion
### Mechanical Properties of Joint  
Figure 5 compares the tensile strength of the welded joints for friction times of 35 s, 40 s, and 45 s with the base metals. Comparison of tensile strength of welded joints for friction time of 35 s with preheating of 100 \({}^{\circ}\)C, 150 \({}^{\circ}\)C, and 200 \({}^{\circ}\)C with base metals. Increased welding friction time diminished the joint strength. With a friction time of 35 s, a rotational speed of 1450 rpm, a friction pressure of 3.9 MPa, and a forging pressure of 6.9 MPa, the maximum tensile strength of the joint achieved was 53.93 MPa. For a friction time of 35 s with preheating of 100 \({}^{\circ}\)C, a rotational speed of 1450 rpm, a friction pressure of 3.9 MPa, and a forging pressure of 6.9 MPa, the maximum tensile strength of the joint achieved was 59.70 MPa. All joints were fractured at the interface of the welds. The identical situation occurred with the AZ31-SS 316L welded junction. The fracture on the Al alloy side of the AA 6103-SS 202 joint was documented by authors in a prior work [30]. [PERSON] et al. (2016) described three types of failure for friction-welded joints, of which this is one [44]. Figure 4: The morphology of the welded joint and removal of flash on sample after friction welding process. Figure 5: Tensile strength of SS 316-pure Zn joints rotary friction-welding (friction time of 35 s, 40 s, 45 s, 35 s—100 \({}^{\circ}\)C, 35 s—150 \({}^{\circ}\)C, and 35 s—200 \({}^{\circ}\)C, rotation speed at 1450 rpm, friction pressure at 3.9 MPa, and forging pressure at 6.9 MPa) with base metals. As demonstrated in Figure 8, based on the SEM picture of the fracture surface, there is still diffusion of Fe and Cr elements on the zinc side. As friction duration increased, heat deformation occurred, resulting in the creation of a layer on the joint surface. The reason is that when friction time increases, excessive heat input occurs on the material surface, forming a coating of oxide and carbon, which reduces the joint tensile strength. The same results were achieved for a friction time of 35 s at preheating of 100 \({}^{\circ}\)C to increase the weld strength. In particular, higher preheating lowered the weld strength. Preheating also affected the increase of Fe, Cr, O, and C elements which caused the formation of layers at the interface. According to [PERSON] et al. (2016), oxide and carbide particles are defects that form a film at the interface [45]. To clarify the joint properties in detail, bending tests were used to determine the bond strength of friction-welded joints [46]. Bond strength measurements provided information on the mechanical quality and integrity of joints between base metals. The welded joints' bending strength evaluations were conducted utilizing a three-point bending test on a Universal Testing Machine (TIME WDW-100E, China). Figure 6 shows the results of the three-point bending test for three replicates of specimens made by using friction-welded joints. The friction welded joints' maximum bending strength values were achieved with a friction time of 35 s. The friction welded joint bending strength values were insignificant with friction times of 40 s and 45 s (unreadable on the test machine) and they were not included on the graph. The obtained bending strength of 14.68 MPa was significantly lower than the base metal bending strength (Figure 6). The joints' low bending strength may be attributed to the poor bond quality and non-diffusion bonding between base metals. The research found that friction time was an essential criterion for determining the quality of welded joints and their strength. According to [PERSON] et al. (2010), the increase in bending strength is related to heat input and high plastic deformation that occurred at the components' interface as a result of the increased rotational speed and axial pressure [46]. Increasing the temperature of the base metal by providing preheating is expected in the sense that it can accelerate the heating of SS 316 material.