Strength and fatigue strength of a similar Ti‐6Al‐2Sn‐4Zr‐2Mo‐0.1Si linear friction welded joint

Abstract

Strengths for monotonic and cyclic loadings of similar overmatching Ti‐6Al‐2Sn‐4Zr‐2Mo‐0.1Si (Ti6242) linear friction welds (LFW) were studied and compared with the parent material (PM) behaviour. Non‐destructive synchrotron observations revealed the presence of pores in the weld interface. The weld centre zone (WCZ) showed a higher strength leading to lower macroscopic ductility of the cross‐weld samples. Local strain and normalized strain rate have been assessed by stereo digital image correlation (DIC) and revealed an early plastic activity at yielding in the vicinity of the WCZ attributed to residual stresses. For the target life, the fatigue strength was slightly reduced but compromised by a strong scatter. Indeed, an internal fish‐eye fatigue crack initiation was found on an unexpected dendritic defect that was very different from the PM microstructure and the known martensitic α′ in the WCZ. The dendritic defect was linked to surface contamination prior to welding and led to melting.

Analysis of Microstructure Evolution and Mechanical Properties of Linear Friction Welded Titanium Alloys

“…Similarly, Dorick et al [46] reported intragranular precipitation of α' lathes in the weld center, which was transformed into acicular α+ β microstructure after the PWHT. Hardness results showed an increase of 27% hardness in the weld center compared with the base metal [45]. Also, TMAZ hardness values were slightly lower than the weld center hardness.…”

“…Linear friction welding studies on Ti-6242 show comparable microstructural characteristics as reported in Ti-64 welds. A study conducted by García et al [45] on the strength and fatigue properties of LFW Ti-6242 showed that the weld center was composed of α' martensite, as shown in Figure 2-16. The TMAZ region was characterized by highly thermomechanically deformed grains oriented in the weld direction.…”

“…The fracture location was in the base metal far away from the weld center, suggesting that the weld center and TMAZ regions are stronger than the base metal. Additionally, DIC analysis performed on the as-welded Ti-6242 showed an early plastic activity in the weld center at yielding possibly due to the residual stresses present [45]. Post weld heat treatments have shown that residual stresses are significantly reduced and improved mechanical properties [47].…”

“…Fatigue Fractographic Analysis conducted for both the cases revealed characteristic features of a ductile material. The surfaces consisted of microcracks, striations, and smooth cleavages [45].…”

“…The mechanical integrity of the welds was analyzed by performing hardness, tensile, and fatigue tests. Previous research works [25][26][45][46] on LFW of titanium alloys were mainly focused on the microstructural and mechanical properties of similar welded joints in the as-welded condition. For the first time the effects of stress relief annealing were performed on Ti-64 and Ti-6242 similar welds to study the global and local mechanical properties.…”