On the Occurrence of Liquation During Linear Friction Welding of Ni-Based Superalloys
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AD730 is a recently developed Ni-based superalloy for turbine disk applications[1]; however, many of its properties, especially its weldability, are still unknown for advanced manufacturing applications such as LFW. LFW is an emerging manufacturing technology for joining of blades to disks or repairing turbine disks in gas turbines and jet engines. This process presents significant advantages over the traditional mechanical assembly techniques by providing quality and performance improvements or weight reduction and economic benefits. The LFW process is in essence a hot deformation process, and in this regard, the material undergoes a severe thermomechanical process which brings substantial microstructural changes.[2–4] One of the on-going questions in LFW is whether liquation and consequently cracks occurs during this process.[5,6] The possibility of liquation during LFW plays a key role in controlling the mechanical properties of the joint as micro-cracks can be produced due to weakness of liquid film against thermal and mechanical stresses induced during cooling. Although bulk melting F. MASOUMI, D. SHAHRIARI, and M. JAHAZI are with the Department of Mechanical Engineering, E´cole de Technologie Supe´rieure (ETS), H3C 1K3, Montreal, QC, Canada. Contact e-mail: [email protected] J. CORMIER is with the Physics and Mechanics of Materials Department, Institute Pprime, UPR CNRS 3346, ISAE- ENSMA, BP 40109, Futuroscope- Chasseneuil Cedex 86961, France. B.C.D. FLIPO is with the TWI Ltd, Granta Park, Great Abington, Cambridge CB21 6AL, UK. Manuscript submitted August 13, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
and consequently solidification cracking do not occur in LFW,[2] the occurrence of local melting and possible micro-cracks are still imaginable. Re-solidified eutectic micro-constituent and constitutional liquation of c¢ precipitates were observed in thermo-mechanically affected zone (TMAZ) of LFWed IN 738 material[6] while no liquation was observed in the LFWed Waspaloy components.[5] Constitutional liquation of second-phase particles occurs due to the formation of intergranular liquid films along the particle-matrix interface through a eutectic-type reaction with the surrounding matrix.[7] Having a good understanding of liquation mechanisms is critical for reliable prediction of microstructure evolution and mechanical properties of LFWed joints and correlating it to process parameters for the optimum design of weld parameters. Extensive studies have investigated different mechanisms of grain boundary (GB) liquation during fusion welding of Ni-based superalloys.[7–12] These reports suggest that some metallurgical and mechanical factors affect liquation cracking susceptibility. The main metallurgical factors are the amount of borides, carbides, and c¢ precipitates; grain and particle size; heat input; and segregation of boron, phosphorous, and sulfur. The mechanical factors are thermally induced strains, stresses, and base alloy hardness.[7–10] However, no quantitative data are available on
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