**5. Analysis of through-thickness RS in aluminium FSW butt joints**

The HMs can be also used for the analysis of the through-thickness RS in friction stir welding on metals. As an example in [19] the HDM have been advantageously used for the analysis of the RS in FSW butt joints between plate made by aluminium. In detail, using the HDM the in-depth RS distribution in the zone close to the tool shoulder border of the joint advancing side, has been accurately determined by considering also different aluminium alloys (AA1050 O, AA2024 T4, AA6982 T6, AA7075

#### **Figure 6.**

*Comparison between the RS computed by the HDM in point 3 of Figure 4 and the RS profile obtained by accurate numerical simulations.*

**Figure 7.**

*Comparison between the RS computed by the HDM in point 4 of Figure 4 and the RS profile obtained by accurate numerical simulations.*

*Use of Hybrid Methods (Hole-Drilling and Ring-Core) for the Analysis of the RS on Welded… DOI: http://dx.doi.org/10.5772/intechopen.102051*

**Figure 8.** *Sketch of the FSW butt joint considered, during the welding process.*

T6) and different specific thermal contribution (STC) that characterizes the velocity of the welding process. **Figure 8** shows the sketch of the FSW butt joint considered, during the welding process.

In detail, it is considered the case in which the FSW of a butt joint is obtained by inserting a specially designed pin, rotating with velocity *Vr* into the adjoining edges of the sheets to be welded, and then moving it all along the joint with velocity *Vf* (**Figure 8**). The realization of the weld bead leads to significant changes of the mechanical properties and of the microstructure of the material near the weld bead. In detail, as it shown in **Figure 9**, it is possible to distinguish the following areas:


**Figure 9.**

*Material microstructures in a typical transversal section of an aluminium AA6082-T6 FSW butt joint.*

#### **Figure 10.** *(a) Rosette installation on a welding specimen and (b) details of the rosette.*

In order to analyse the most relevant residual stress distribution that occur through the thickness of the welding joint, a special rectangular strain gauge rosette type MM EA-062RE-120, have been bonded into the welded specimens, with the 1 grid aligned with the tool shoulder border of the joint advancing side (see **Figure 10**), that is the zone of the welding seam where the maximum values of the RS are expected [19].

After the strain gauge rosette installation, it has been linked to a proper multichannel strain gauge monitoring machine (HBM UPM 100), and the three strains relaxed after each successive increase of the hole depth, have been collected by using a software [20] properly developed by the maker of the milling machine shown in **Figure 3**. In detail, such an advanced machine have an high speed air turbine and a special microscope, that permit respectively to limit the residual stresses due to machining as well as the rosette eccentricity. Also, the hole has been drilled by 25 successive steps of 0.1 mm (total hole depth of 2.5 mm), by using a tungsten carbide mill. In accordance with the ASTM standard [14], as well as with the good practice indications reported in Refs. [11, 12, 16], the measuring procedure have been performed in such a way to minimize the spurious residual stress induced by the hole drilling.

#### **5.1 Residual Stress evaluation**

The evaluations of the residual stresses through the thickness of the examined welded bead, have been carried out by using the calculation process exposed in chapter 2, and the discrete results have been fitted with simple polynomial functions. Then, it permits to perform the uniformity test prescribed by the ASTM standard [14] and then to compute uniform or non-uniform residual stresses by the Integral Method exposed in detail in the previous chapter 2. As an example, considering the case of the aluminum AA1050-O FSW joints with medium specific thermal contribution (MSTC), **Figure 11** shows the typical curves of the three relaxed strains (ε1, ε2, ε3).

From **Figure 11** it is seen how, the surface relaxed strains take initially typical negative values due to the relaxation of positive RS; successively they increase in

*Use of Hybrid Methods (Hole-Drilling and Ring-Core) for the Analysis of the RS on Welded… DOI: http://dx.doi.org/10.5772/intechopen.102051*

**Figure 11.** *Typical relaxed strain curves relative to AA1050-O aluminum joints, with MSTC.*

**Figure 12.** *Typical residual stresses relative to AA6082-T6 joints, with MSTC.*

module (up to 140 to 280 μm/m) until hole depth of 1.5–2.0 mm, at which they take a flat trend (saturation of the surface relaxation phenomenon). Similar relaxation curves have been acquired for all the aluminium alloys considered and for all the manufacturing conditions (STC) studied.

As an example, **Figure 12** shows the RS computed by considering the case of AA6082 T6 FSW joints with medium specific thermal contribution (MSTC). Similar trends have been obtained for the other cases examined.

**Figure 12** permits to observe that in all the three examined cases (AA6082 T6, AA1050-O, AA2024-T4), both the main RS components, acting on x and y directions respectively, take similar values and trends (increasing with depth). In detail, they range from negative value (on surface, from 8 MPa to 30 MPa), to positive values of about 60 MPa for MSTC (40 MPa and 90 MPa for LSTC and HSTC respectively) at depth of about 0.5 ÷ 1 mm. Therefore, for this aluminum alloy in the STC examined range (from LSTC to HSTC) the maximum stresses decrease if the STC value increase. Very interesting is the comparison of RS evaluated for the different materials and for the three different STC levels considered, which shows that unlike the traditional welded joints, in the examined FSW butt joints, the maximum RS do not occur on joint surface but inner to the weld at a depth from surface that varies from 0.5 to 1.25 mm. In more detail, for the aluminum alloys commonly used in the structural design (AA6082 T6 and AA2024-T4) the surface residual stress *σ<sup>s</sup>* takes in practice

**Figure 13.** *Residual stresses measured on surface of the FSW joints.*

**Figure 14.**

*Maximum residual stresses measured along the thickness of the joints: (a) absolute values and (b) percentage of the parent material yield stress value.*

always negative values ranging from 20 to 40 MPa (see **Figure 13**), i.e. it takes always values less than 20% of the yielding stress *σo*.

Additionally, the HDM has allowed to highlight that the maximum residual stresses *σmax* varies from about 50 to 150 MPa (see **Figure 14a**), i.e. from about 15–30% the yielding stress, depending to the particular material type and the STC level (see **Figure 14b**).

*Use of Hybrid Methods (Hole-Drilling and Ring-Core) for the Analysis of the RS on Welded… DOI: http://dx.doi.org/10.5772/intechopen.102051*

Finally, it is important to note that if the use of the HMs leads to significant plasticity effects at the bottom of the geometry variation (hole or groove) due to high RS levels, than values of the principal RS computed by the above exposed procedure should be corrected by using the procedure reported in literature [21, 22], to which the reader is addressed.
