**3. Description of the new FSW process setups**

FSW experiments were carried out using a computer numerical control (CNC) machining center.

#### **3.1. DS-FSW method**

As for the DS-FSW method, a conical pin tool geometry (H13 steel of HRC = 52), with a shoulder diameter equal to 12 mm and cone base diameter and height of the pin of 3.5 and 1.7 mm, respectively, with a pin angle of 30°. A 19-mm-diameter rotating tool was used. All the welding experiments were carried out with a nutting angle equal to 2°.

In DS-FSW, the first welding is followed by a second one, performed at the opposite surface, with respect to the first welding operation. Two different sheet positions, with respect to the welding tool, were investigated and are here presented:


In both configurations, the effect of tool configurations on the quality of the DS-FSW joints was also analyzed.

In **Table 1** are reported the different tool configurations and sheet positions used in the DS-FSW. The used blanks were 180 mm in length, 85 mm in width, and 2 mm in thickness. The FSW was performed by fixing the welding line perpendicular to the rolling direction.


**Table 1.** DS-FSW configurations (in terms of tool used and sheet arrangement).

The effect of the process parameters on the conventional and the DS-FSW was inferred using homologous rotational speed values (*ω*), which ranged 1200–2500 rpm, and same welding speed (*v*), equals to 60 and 100 mm/min. The conventional FSW was carried out using a tool sinking of 0.2 mm, while the DS-FSW was performed with a sinking of 0.15 mm in the first pass and 0.05 mm in the opposite surface. These welding parameters were set on the basis of the results obtained by preliminary tests, carried out using different tool sinking values, showed the need to perform the second pass with a sinking lower than that of the first one in order to prevent the occurrence of fracture.

heat treatments, such annealing, prior and after the FSW. The effect of the radius R, pin height, and annealing treatment on microstructure, micromechanical and macromechanical proper‐ ties is here discussed in order to define the process condition and the temper state that allows to obtain defect-free joints, without the occurrence of the oxide defects of kissing-bonds, and

FSW experiments were carried out using a computer numerical control (CNC) machining

As for the DS-FSW method, a conical pin tool geometry (H13 steel of HRC = 52), with a shoulder diameter equal to 12 mm and cone base diameter and height of the pin of 3.5 and 1.7 mm, respectively, with a pin angle of 30°. A 19-mm-diameter rotating tool was used. All the welding

In DS-FSW, the first welding is followed by a second one, performed at the opposite surface, with respect to the first welding operation. Two different sheet positions, with respect to the

**(1)** AS-AS, in which the sheet is placed in the AS, at the first FSW operation, and it is main‐ tained in the same side also at the second FSW passage at the opposite surface;

**(2)** AS-RS, in which the sheet, placed in the AS at the first FSW, to be reversed, in the RS, at

In both configurations, the effect of tool configurations on the quality of the DS-FSW joints was

In **Table 1** are reported the different tool configurations and sheet positions used in the DS-FSW. The used blanks were 180 mm in length, 85 mm in width, and 2 mm in thickness. The

FSW was performed by fixing the welding line perpendicular to the rolling direction.

faint zigzag line pattern in the NZ.

center.

10 Joining Technologies

**3.1. DS-FSW method**

also analyzed.

AS-AS Pin-pin AS-RS Pin-pin AS-AS Pin-pinless AS-RS Pin-pinless AS-AS Pinless-pinless AS-RS Pinless-pinless

**3. Description of the new FSW process setups**

experiments were carried out with a nutting angle equal to 2°.

welding tool, were investigated and are here presented:

the second FSW passage at the opposite surface.

**Sheet position Tool configuration for the first pass – and second pass**

**Table 1.** DS-FSW configurations (in terms of tool used and sheet arrangement).

In **Table 1** DS-FSW AS-AS pin-pin consists of maintaining fixed the AS and RS for both welding procedures; AS-RS pin-pin consists of reversing the AS into RS, from the first to the second welding procedure.

The third and fourth configuration differs from the first two only in the absence of the pin during the second welding process. In the last two (AS-AS, and AS-RS pinless-pinless), the welding process was performed with no pin in both processes. **Figure 1** shows a schematic representation of the three DS-FSW configurations used here.

**Figure 1.** Representation of the three DS-FSW configurations: AS-AS pin-pin (left side); AS-RS pin-pin (center); AS-AS pin-pinless (right side).

#### **3.2. Pin rotation deviation from centerline (RT-FSW) method**

As for the pin rotation configuration method, the innovative approach to the FSW process was defined by authors as RT-type. For this purpose, a conical pin tools in H13 steel (HRC = 52) with a 2.3 mm pin height, 3.9 mm in diameter at the shoulder, a 30° pin angle, and a shoulder diameter of 15 mm (applying a vertical force of 1.7 kN) was used (**Figure 2**).

technique to obtain sound welded joints, either in similar [24, 25] and dissimilar [26, 27]

New Approaches to the Friction Stir Welding of Aluminum Alloys

http://dx.doi.org/10.5772/64523

13

In the present case, the AA5754 sheets were produced by twin roll continuous casting followed by cold rolling to give a H111 (EN485) metallurgical initial status and a thickness of 2.5 mm. The AA6000-series are widely used because of their good weldability, corrosion resistance, and immunity to stress-corrosion cracking. These are known to be among the most used aluminum alloys for extruded components [28]. In fact, AA6082 (Al-Mg-Si) typical application include aeronautics, automotive, and recreation industries. In the present case, cold-rolled

**5. Experimental findings and evidence for sound and better FSW joints**

There is a strong need for an improvement in ductility and formability of FSWed joints. Some previous studies reported significant mechanical improvements by carrying out multipass [29], double lap [30], reverse dual rotation [31] FSW, and FS spot welding [32]. With this respect, the DS-FSW showed better strength, elongation, and formability of FSWed aluminum joints. The DS-FSW was proven to induce the serration of the geometric discontinuities, thus

**Figure 3** shows typical nominal stress versus nominal strain curves of FSWed joints in AA6082 obtained under different values of the rotational speed and welding speed. The joints ductility is shown to be lower in the NZ, with respect to the base metal (BM), irrespective of the welding parameters and process methodology [22]. In general, in terms of both the ultimate values of tensile strength and elongation, the conventional FSWed joints show a tensile behavior better than the one exhibited by the DS-FSWed joints. Actually, the conventional FSW process requires a high sinking value in order to generate the frictional heating allowing the material flow necessary to obtain sound joints, according to Mishra and Ma [7]. Thus, in the first pass, by using the same tool sinking as of conventional FSW produces a step in the blank surface that acts as a notch during the second pass. Therefore, the tool sinking value imposed in the second pass had to be further decreased in order to reduce the formation of surface defects. The pinless-pinless configuration has provided the worst tensile properties. In particular, the AS-AS configuration showed low mechanical properties of the joint, while the AS-RS config‐

The mechanical behavior is strongly improved when welding is performed using the pinpinless configuration. In this case, ductility levels similar to the ones showed by the conven‐ tional FSWed samples were obtained. In this case, the tensile fracture occurred at the HAZ, in the RS zone. The tensile properties of the joints are slightly affected by the rotational and welding speeds. This is not the case in the DS-FSW pinless-pinless tool configuration, which

welding combinations (using AA5757 or AA5083).

**5.1. The DS-FSW method**

*5.1.1. Mechanical properties*

uration did not reach a sound weldment.

sheets of AA6082 were used to show the soundness of the DS-FSW.

promoting a significant microstructure homogeneity at the NZ.

**Figure 2.** Comparison between conventional (R-type) and T-type FSW configurations.

The welding motion combines two different plate-to-pin mutual motion setups:

(1) a pin axial spin rotation sets perpendicular to the sheet blanks, changing the rotation along the plate centerline by a radius equal to R (=0, corresponding to the conventional FSW, 0.5, and 1 mm);

(2) a pin translation along a direction parallel to the welding centerline line.

The RT-type FSW innovative approach was compared with the conventional T-type (linear welding motion, i.e., for *R* = 0). In both the RT-type and T-type FSW processes, the stirring action was exerted by the pin tool rotation around its axis; the pin tilt angle was set at 2°, with respect to the normal direction to the plate surface. The RT-type and T-type FSW were performed using a pin rotational speed, *ω* = 2000 rpm, and a transverse speed, *v* = 30 mm/min. All experiments were carried out with a tool plunging speed of 1.5 mm/min. The above reported setting parameters were chosen by an optimization FSW processing study reported in [22], where the effect of the welding parameters and tool configuration on micromechanical and macromechanical properties of FSW joints in AA5754 sheets were investigated. The AA5754 was subjected to an annealing treatment at 415°C/3 h, both prior (AA5754-O), and after (post-weld annealing, PWA), followed by furnace cooling.
