**3.5. Fatigue testing**

stresses up to 650 MPa are produced in the weld itself and the immediately adjacent parent material during solidification. These high tensile residual stresses near the weld are balanced by compressive longitudinal stresses further from the weld line. Transverse residual stresses have a similar profile but are an order of magnitude lower than the longitudinal component with a maximum value not exceeding 50 MPa. Our results are in good agreement with that

**Figure 12.** Residual stress distribution in the vicinity of the laser beam welded Ti‐6Al‐4V butt joint. (a) Residual stress‐

es in the as‐welded condition and (b) influence of PWHT on longitudinal residual stress profile.

**Figure 13.** Residual stresses in the BM after milling.

reported by Cao et al. for laser beam welded Ti‐6Al‐4V alloy [33].

128 Study of Grain Boundary Character

The results of room‐temperature high cycle fatigue tests are shown in **Figures 14** and **16**. For reference, base material data are also provided in **Figure 14**. Arrows indicate non‐failures after 107 cycles (run‐outs). The curves shown in these plots represent the mean lines corre‐ sponding to 50% probability of survival. The results highlight the inherent scatter in fatigue test experiments for titanium alloys [36]. As shown in **Figure 14**, the fatigue limit of the BM in the starting as‐received condition is approximately 650 MPa or nearly 65% of the yield strength. Machining both reduces the surface roughness from 3 to 1.5 Rz and results in the formation of compressive residual stresses in the 0.2‐mm‐thick surface layer (see **Figure 13**). Improved surface quality and introduction of favourable compressive stresses in the near surface region have a beneficial effect on the HCF resistance of the Ti‐6Al‐4V BM. As seen in **Figure 14**, the fatigue limit increased to 720 MPa after milling the surface of the specimens. This result should be kept in mind when comparing the effect of milling the weld defects on the HCF properties. The S‐N curve of the milled BM must be considered as the reference for machined weldments.

**Figure 14.** Influence of machining on the fatigue behaviour of the laser beam welded Ti‐6Al‐4V butt joints.

**Figure 15.** Transverse cross sections of fractured S‐N specimens. (a) As‐welded condition, 200 MPa, 889,500 cycles and (b) annealed (FA2) and machined, 575 MPa, 4,953,100 cycles.

**Figure 16.** Influence of PWHT on the fatigue behaviour of the laser beam welded Ti‐6Al‐4V butt joints.

#### *3.5.1. Influence of defects*

**Figure 14.** Influence of machining on the fatigue behaviour of the laser beam welded Ti‐6Al‐4V butt joints.

**Figure 15.** Transverse cross sections of fractured S‐N specimens. (a) As‐welded condition, 200 MPa, 889,500 cycles and

**Figure 16.** Influence of PWHT on the fatigue behaviour of the laser beam welded Ti‐6Al‐4V butt joints.

(b) annealed (FA2) and machined, 575 MPa, 4,953,100 cycles.

130 Study of Grain Boundary Character

The S‐N curves for the as‐welded and machined flush laser beam welded Ti‐6Al‐4V butt joints are given in **Figure 14**. The fatigue limit after milling the weldment flush with the sheet surface was approximately 500 MPa. This value corresponds to 70% of the base material fatigue limit (also machined). It can be seen (**Figure 14**) that the presence of reinforcements and small underfills significantly deteriorates the fatigue of the laser beam welded butt joints. Geometry imperfections such as underfills and reinforcements play the role of stress concentrators (notches). The failure always occurred in the welding seam initiated at the face or root underfill. **Figure 15** shows typical transverse cross sections of fractured S‐N specimens in the as‐welded and machined conditions. In the as‐welded specimen, the crack started from the weld root and propagated through the FZ perpendicular to the direction of applied stress as shown in **Figure 15(a)**.

Thus, machining the weld reinforcements and underfills flush with the sheet surface can be considered as an easy method to improve the fatigue performance of the laser beam welded butt joints. These results are consistent with the work of Squillace et al. [2]. They showed that the fatigue strength of autogenous laser beam welded Ti‐6Al‐4V butt joints is strongly influenced by the value of the underfill radius, and the S‐N curves shift towards the region of HCF as the value of the underfill radius increases. Improved fatigue strength by partially or totally eliminating the underfills, predicted in the above‐mentioned work, was confirmed in the present study. The use of filler wire partly prevented the formation of underfills; however, as seen in **Figure 14**, the synergetic effect of the weld reinforcements and underfills consider‐ ably affected the fatigue performance, although the acceptance criteria in terms of geometrical defects were passed.

In the low cycle fatigue (LCF) region, the S‐N curve of the milled condition approaches the static strength of the laser beam welded joints, which usually equals the strength of the parent material [2, 5]. The specimen tested at the 950 MPa level of maximum stress was fractured in the base metal. This implies that in the LCF region, the laser beam welded Ti‐6Al‐4V flush milled butt joints exhibit a BM level of fatigue strength. All other laser beam welded specimens tested in the current work were fractured in the FZ. The typical location of failure in the machined laser beam welded joint is shown in **Figure 15(b)** for the specimen, which endured nearly five million cycles at the 575 MPa level of maximum stress.

In the HCF regime, the S‐N curve for the flush milled condition is located lower than that of the base material. The fatigue limit decreased by nearly 31%. This result implies the exis‐ tence of internal microstructural features or defects deteriorating the fatigue strength of the joint. In experiments with butt welds in the as‐welded condition, the stress concentration at the weld toes or roots is much more severe than that due to minor defects existing in the welding zone, and these defects are therefore less important. Thus, geometry features can overshadow the microstructural effects and internal defects. The latter are of primary inter‐ est in this work. By removing the stress concentrators from the surface of the welding seam, internal defects become the most important notches in the joint and exhibit their full delete‐ rious effect.
