**Abstract**

 Aluminum is one of the rare earth metals that has a high strength to weight ratio. Aluminum alloys are used for many different applications. Aluminum is a highly reactive metal with oxygen at the recrystallization temperature when applying conventional joining processes. The friction stir welding process was used in this research work to join AA 7075 T6 aluminum alloy plates. We used different process variable parameters such as tool pin geometry, tool rotation speed, and tool travel speed (welding speed). We also analyzed the effect of different process variable parameters on the hardness of the welded plates.

**Keywords:** friction stir welding, AA7075 T6, hardness, aluminum alloys, tool geometry

#### **1. Introduction**

 Friction stir welding (FSW) is categorized as a solid state joining process. This process is performed below the recrystallizing temperature. It was invented by Wayne Thomas at TWI United Kingdom in 1991 [1]. FSW needs less energy input, is an environmentally friendly process, with repeat mechanical processes able to produce a very high strength welding process in a wide range of materials. This process is capable of producing high-quality and defect-free welds of 2xxx and 7xxx series aluminum alloys [2]. Different materials can be welded together with different aluminum alloys as there is no melting during the welding process [2]. The effect of different tool rotation speeds along with the different process parameters on AA 6063, T5, and T4 work pieces has been previously observed. The researcher observed that different tool rotation speeds did not result in significant differences in the hardness of welded plates by FSW, except for the width of the softened/partially melted area in the weld of AA 6063-T5 [3]. Regarding the mechanical properties of similar and dissimilar AA5182-H111 and AA6016-T4 metals, the researcher reported that both alloys showed a hardness deviation unswerving with the microstructure evolution across the thermo mechanical affected zone (TMAZ) [4, 5]. The overall mechanical properties depend on the ratio of the tool rotation speed to the welding speed [6, 7]. When the fracture location is at the body of the base metal or at the heat affected zone (HAZ) then the hardness value is high, but is lower hardness in comparison with the stirred zone.

*Analyzing the Effect of Variable Process Parameters in Friction Stir Welding of Aluminum… DOI: http://dx.doi.org/10.5772/intechopen.81083* 

The fracture location of the joints was in the HAZ of the AA6061 side due to a lower hardness value [8, 9]. The reason for the lower value could be the poor fusion of the two plates. This research work presents the effects of various process variable parameters on the hardness of welded aluminum plates by FSW.

### **2. Experiment**

 Aluminum alloy AA 7075 T6 was selected for experimental joints using FSW process. The experimental set up and typical welded plate is shown in **Figures 1** and **2**.

 The plates are sized at 150 × 100 × 6 mm for butt joining. **Tables 1** and **2** show the chemical composition and mechanical properties of the plate material. The FSW is carried along the plate surface horizontally. The process parameters that are highly influential on FSW joints are tool rotating speed, tool travel speed, and tool pin profile. Three different pin profile tools are used to fabricate the joints that are made from the H13 tool steel. They are shown in **Figure 3**. The tool was held in the vertical arbor using a suitable collate. The edge of the plate was clamped on the bed with a zero root gap. The clamping of the plate was done to avoid vibration and translation force. After that, the rotating tool plunged in the butt joint until the shoulder touched the surface of the plate, then the tool ran at a pre-determined speed. The selected range of process parameters were divided into nine experiments based on previous research work. Nine experiments and their selected welding parameter are shown in **Table 3**.

**Figure 1.**  *Experimental setup of FSW.* 

#### **Figure 2.**  *Welded plate.*


#### **Table 1.**

*Chemical composition of plate material (according to ASTM E 8).* 


**Table 2.** 

*Mechanical properties of plate material.* 

**Figure 3.** 

*FSW tools (a) straight cylindrical pin, (b) threaded straight cylindrical pin, (c) square pin.* 


**Table 3.**  *Process parameter.* 

#### **3. Result and discussion**

 Hardness is the property of the material that enables it to resist plastic deformation and penetration. As our material was nonferrous and an aluminum alloy, we measured the HRB of the material at different locations such as at the weld zone, at the HAZ, and at the parent material. The test was carried on the Rockwell hardness testing machine at ambient temperature. The hardness results are shown in **Table 4**.

 As we can see in **Table 4**, the highest hardness at the weld zone was achieved when the tool rotation speed was high with tool traverse speed at 85 mm/min with the square pin profile. As the tool rotation speed decreases, the hardness with different tool pin profile also decreases. The hardness value at the HAZ is highest when


*Analyzing the Effect of Variable Process Parameters in Friction Stir Welding of Aluminum… DOI: http://dx.doi.org/10.5772/intechopen.81083* 

#### **Table 4.**

 *Hardness testing results.* 

**Figure 4.** 

*Graph of experiment VS hardness.* 

 the tool rotation speed was high with the straight cylindrical pin profile and it was achieved with a high traverse speed of tool during welding.

 **Figure 4** shows the hardness at different zones in the different experiments. As the value of the traverse speed decreases, the value of hardness at the heat affected zone also decreases and at a lower traverse speed, the hardness at the weld zone is also lowest. The highest hardness was obtained at the weld zone with the square pin profile and at the HAZ with the straight cylindrical pin profile. In the threaded straight cylindrical pin profile, the hardness was lower when compared to the other two pin profiles.

#### **4. Conclusion**

The hardness was measured at the weld zone, the heat affected zone, and the parent metal and from the results, we can say that using the SP profile with a higher tool rotation speed results in a high hardness at the weld zone and the heat affected zone. Welding with SCP at a lower tool traverse speed results in lower hardness at

the weld zone. Using threaded straight cylindrical pin (TSCP) at a higher tool rotating speed and welding speed (tool traveling speed) results in a hardness of the weld zone somewhat similar to the hardness of the parent metal.
