Through analysis of the microstructure changes during linear friction welding of the near-α titanium alloy Ti-6Al-2Sn-4Zr-2Mo (Ti6242) towards microstructure optimization

Dorick Ballat-Durand, Salima Bouvier*, Marion Risbet, Wolfgang Pantleon

*Corresponding author for this work

Research output: Contribution to journalJournal articleResearchpeer-review

Abstract

Linear Friction Welding (LFW) is a solid-state joining process producing narrow joints mainly developed for the aircraft industry. The thermo-mechanical loads involved in LFW lead to significant local microstructural changes. This study aimed at identifying the mechanisms impacting these changes in order to develop a Post-Weld Heat Treatment (PHWT) optimizing the joint microstructure. The temperature fields showed that a zone of 1mm on either side of the weld center line experienced thermo-mechanical processing in the β-domain for 2 s followed by a rapid cooling to 400 °C. Inspection of the weld by Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) revealed a strongly affected microstructure characterized by a sharp microstructural refinement and the presence of defects at the interface. The joint consists of: 1) the Welding Line (WL) which underwent a complete α→β transformation accompanied by the recrystallization of the prior-β grain and the development of a {110}〈111〉 texture followed by intragranular precipitation of textured α′ Hexagonal Close-Packed (HCP) martensitic laths; 2) the Thermo-Mechanically Affected Zone (TMAZ) characterized by a partial α→β transformation resulting in a microstructure refinement by α variant selection upon cooling. A third zone, the Heat Affected Zone (HAZ), was revealed as having a microstructure indistinguishable from the base material (BM) but being slightly harder. The texture analysis of the reconstructed β phase in the joint core showed that the local deformation conditions were asymmetrical between the forging and the oscillating part and that the WL may have experienced a complex material stirring with turbulent flow. These microstructural changes generate an increase in hardness in the joint with a maximum increase of HV0.3 by 40% in the WL. The PWHT consisting of an α+β annealing followed by ageing resulted in an α′→α+β decomposition and α globularization in the TMAZ leading to a gradual microstructure refinement from the BM to the WL. A rather homogenous hardness was obtained across the assembly after the PWHT.
Original languageEnglish
JournalMaterials Characterization
Volume151
Pages (from-to)38-52
ISSN1044-5803
DOIs
Publication statusPublished - 2019

Keywords

  • EBSD
  • Linear friction welding
  • Microstructure
  • Recrystallization
  • Texture
  • Titanium
  • β reconstruction

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