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Mechanical characterization of Linear Friction Welded Titanium alloys

Abstract : The mechanical properties in terms of tensile behaviour, fatigue strength and toughness of Linear FrictionWelded (LFW) Titanium (Ti) alloys were studied and compared to the parent materials' (PM) behaviour. LFWis a solid state joining process that could be used for assembling heavily loaded structures in the aerospaceindustry. Since the Weld Centre Zone (WCZ) of a similar Ti6242 LFW joint exhibited overmatching strength,its fracture mechanisms and ductility were investigated by micro-tensile tests using notched specimens. Theyrevealed some ductility and a mixture of brittle and ductile fracture mechanisms. Local strain and normalizedstrain rate fields on smooth samples were assessed by Stereo Digital Image Correlation (SDIC) and showedan early plastic activity at yielding in the vicinity of the WCZ, attributed to residual stresses. For the target lifeof 105 cycles, the fatigue strength was slightly reduced but compromised by a strong scatter that was relatedto prior to welding contamination. The LFW self-cleaning mechanism also failed to extrude the contaminantsat the weld interface of a dissimilar Ti17-Ti64 LFW joint that led to pseudo-brittle fracture at a defect layerin the WCZ. For a pre-cleaned dissimilar Ti17-Ti64 LFW joint, ductile rupture was observed sometimes atthe Ti64 PM and sometimes at the Ti17 thermo-mechanically affected zone (TMAZ), due to a depletion ofthe strengthening alpha phase. Fatigue cracks initiated at the weak Ti17 TMAZ with a reduced fatigue strengthcompared to the PM. Fracture toughness was significantly reduced and the fracture surface at the TMAZhighlighted unexpected clusters of parallel and elongated voids. These trends were also observed for a similarTi17 LFW joint. Comparable mechanical properties to those of the Ti17 PM were successfully regained bymeans of a post-weld heat treatment.
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Juan-Manuel Garcia. Mechanical characterization of Linear Friction Welded Titanium alloys. Mechanics of materials [physics.class-ph]. Université Paris sciences et lettres, 2019. English. ⟨NNT : 2019PSLEM045⟩. ⟨tel-02512860⟩

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