1. Introduction
Weight reduction without affecting the safety performance is a great challenge in the manufacturing of automotive in order to improve fuel economy and reduce emissions. It has been reported that fuel devouring can be decreased by 5.5% for each 10% decline in vehicle weight and a 0.45 kg decline in the weight of a car would decline carbon dioxide emissions by 9.07 kg over the life of the vehicle. An automobile consists of outer panels and a platform, which is typically made of steel and contains the drive system, engine system, and exhaust system. The weight of the platform is around 70% of the total weight of an automobile [
1].
Steel has been applied widely in the automotive industry because of its wide range of desirable properties, ease of processing, availability, and recyclability. However, lightweight materials like aluminum have obvious advantages over steel with comparable properties but nearly three times lower density, a resistance of high corrosion, and a high degree of usage reaching 85–95%. Aluminum alloys are promising candidates for replacing equivalent steel assemblies and the use of them in the manufacturing of automotive is increasing recently [
1].
For replacing steel with aluminum in the structure of automobiles, it is necessary to explore joining methods that can be used efficiently. Current panel welding techniques used to join steels involve resistance spot welding and self-piercing rivets. However, these welding techniques cannot be applied easily to aluminum alloy, because of its physical properties, particularly surface oxide film. Friction stir spot welding is a derivative of friction stir joining, which was developed by TWI (Abington, UK) in 1991 as a solid-state method for aluminum alloy joining. This novel joining mechanism is advantageous for producing aluminum joints without contamination, blowholes, porosity, and cracks [
1].
This study is focused on aluminum alloy 5754, which is the most common aluminum series utilized by the automotive industry. There are many studies on friction stir spot welding of aluminum alloys.
Er [
2] studied the optimum welding limits for the friction stir spot welding of AA5005 sheet samples by comparing the mechanical properties of friction stir spot welding and the resistance spot welding method. He found that the samples which are joined by friction stir spot welding have better mechanical properties compared to the samples that are joined with resistance spot weld.
Kulekci and Er [
3], studied the effect of dwell time, tool rotational speed, and tool plunge depth parameters of friction stir spot welding using AA5005 sheet samples. They concluded that the optimum welding parameter ranges for; tool dwell time between 5 s and 10 s, tool rotational speed between 1500 rpm and 2000 rpm, and tool plunge depth between 2.2 mm and 2.6 mm. In addition, it is found that tool plunge depth is the most important parameter that influences welding performance.
Kacar et al. [
4] studied friction stir spot welding of AA5754 sheet samples using copper interlayer. It was found that the tensile shear load carrying capacities of AA5754 (on the top)—Cu (on the middle)—AA5754 (on the bottom) material welded by using copper interlayer was greater than the welded AA5754 (on the top)—AA5754 (on the bottom) material with the same welding limits without using the copper interlayer.
Bilici et al. [
5] searched friction stir spot welding of AA5754-H22 and AA2024-T3 sheet samples by using the Taguchi method. As a result, two types of fracture type; cross-nugget and nugget fracture occurred after the tensile shear test.
Piccini and Svoboda [
6] studied friction stir spot welding of AA5052-H32 and low carbon steel sheet samples and in addition to the mechanical properties they studied the effect of tool geometry and depth limits of the process.
Jeon et al. [
7] explored the mechanical properties of AA5052-H32 and AA6061-T6 sheet samples welded by friction stir spot welding process. The value of the highest tensile shear strength was obtained for the AA5052 (on the top)—AA5052 (on the bottom) sample combination. The value of the lowest tensile shear strength was obtained for the AA6061 (on the bottom)—AA6061 (on the top) sample combination.
Due to its potential in light-weigthing applications of automotive and other relevant industries, friction stir spot welding can be spread in many industries.
For this purpose, three different experimental configurations were created with AA5754 sheet samples (thickness of 1 mm). These samples were welded by friction stir spot welding with three different unstudied tool rotational speeds and most common tool geometry. Then, tensile shear strength test, microhardness test, and macrostructure analysis were performed on these samples.
3. Experimental Results
3.1. Tensile Properties of Joints
During the experiments, the tensile shear load of the joints was obtained, and the average values of the tensile test loads were calculated for all experimental configurations. The graphical presentation of the average tensile shear load values is shown in
Figure 7.
The average tensile shear load values of experimental configurations are 1.1725, 1.35 and 1.1733 kN for 1350, 1850, and 2530 rpm, respectively.
The difference between the tensile shear load value obtained at 1350 and 2530 rpm, was found to be quite low. The maximum tensile shear load was measured as 1.35 kN at 1850 rpm and the minimum tensile shear load was measured as 1.17 kN at 1350 rpm. The tensile shear load was augmented by 15.38% when the tool rotation speed was increased from 1350 rpm to 1850 rpm. On the other hand, the tensile shear load was decreased by 13.33%, while the tool rotation speed was increased from 1850 rpm to 2530 rpm. The obtained results show that the tool rotational speed was an important parameter in lap shear strength.
The tensile shear load was increased for all the experiments, as the tool rotation speed was increased from 1350 to 1850 rpm. According to Güler [
11] explained that an increase in the tensile shear load may be explained with the microstructural changes due to the heat generation at a stirring location. On the other hand, the tensile shear load was decreased when the tool rotational speed was increased from 1850 to 2530 rpm. This situation can be explained by more frictional energy generation due to the increment of tool rotational speed and the fluidity of the plasticized material in the stirring zone being increased, as supported by reference [
12].
3.2. Hardness Values of Joints
The Vickers hardness values were obtained for the three different rotational speeds of the joints welded by the welding tool are shown in
Figure 8.
The hardness values for the friction stir spot welding via the welding tool are shown in
Figure 8. Maximum and minimum hardness values were obtained as 92.2 and 71.6 HV at the tool rotational speeds of 1850 and 1350 rpm, respectively. Minimum and maximum hardness values of all experimental configurations are presented in
Table 6.
In the keyhole area of the joints, greater hardness values were obtained. Regarding this situation, Güler [
13] explained that the plastic deformation in the welding process causes strain hardening, which is the main reason of the hardness increase. During the welding process, fine grains that consisted with the dynamic recrystallizations are the reason for this situation.
According to
Figure 8, from the weld center to the base metal hardness profile, it is firstly reduced and then a gradual increment occurs, and the trend of the hardness profile is in good agreement with the existing literature data [
6,
14,
15,
16].
As a result of the tensile shear test, it is found that test groups with maximum strength values have maximum hardness values. The plastic deformation generates an increment in the hardness and strength and Güler [
13] reported that both of them are raised together, contingent upon dynamic recrystallization.
3.3. Macrostructure of Joints
In the literature, generally, three failure modes are observed after the shear tests for friction stir spot welding. These are, mode 1—shear failure: The failure occurs along the joint line of betwixt the sheets (sheet thickness> 2 mm); mode 2—mixed cleavage failure: The diminish stamina failure begins by cleavage, pursuing the oxide debris describing the surfaces betwixt the sheets; and mode 3—nugget pullout: The joint’s interface doesn’t fail, and the tears circumference the shoulder edge on the upper sheet contact surface, and are separated and connected to the welding nugget on the lower sheet. This failure mode is also stimulated by thinning of the top sheet on the side of the tool shoulder, depending on the tool shoulder plunge process [
13].
In this study, macrostructures of fractured surfaces of the samples were investigated for all experimental configurations.
All the test samples were investigated based on qualitative visual inspection. The failure modes of all fractured welding joints are shown in
Figure 9. In addition, upper sheet top view, upper sheet back view, lower sheet top view, and the average shear load values are given in
Figure 9.
In all fractures, failure mode 3 was observed as a single type of fracture pattern.
A single fracture pattern; failure mode 3 was observed for all experimental configurations and the increase in the tool speed from 1350 to 2530 rpm did not affect the failure pattern.
A nugget area around the tool pin length was formed by plasticized material blending into each other during the friction stir spot welding. The nugget area is given in
Figure 10a,b. The nugget area contains materials of lower and upper sheets due to the physical phenomenon of the process. During the tensile shear test, the fracture occurs at the outer contour of the welding area and proceeds around the welded section, and it keeps proceeding until the lower and upper sheets are separated from each other.
It was observed that the fracture propagation occurs parallel according to the upper sheet surface. Therefore, this failure type is named as a nugget pullout failure and our findings are supported by the existing literature data.