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Due to their exceptional strength and good corrosion resistance at high temperatures, Ni-base superalloys have become the most extensively used high temperature alloys. They are particularly suitable for high service temperature where ordinary steels and titanium alloys can not withstand the high temperatures and dynamic loads applied to the component. Typical applications are gas turbine components such as disks, bolts, shafts, cases and blades. [1] In recent years the development of new joining technologies such as Friction Stir Welding (FSW) and Linear Friction Welding (LFW) which can be used for the manufacture and repair of a wide range of aerospace components has created a new impetus for the use of Ni base superalloys. Of particular interest for aircraft engine applications is the LFW technique which has the potential to produce highly efficient joints on new components as well as in repair applications thereby increasing significantly component's life cycle. In LFW, an imposed linear reversing motion of the two surfaces to be joined generates frictional heat and plasticizing of the material at the weld interface. When adequate heat and metal flow has been reached, the moving part is brought into alignment while the axial load is maintained or increased to finalize the weld sequence. Wear particles are expelled from the interface as "flash" and a Thermo-Mechanically Affected Zone (TMAZ) expands from the interface into the parent material. The physical principles of the LFW process are very similar to the well known rotary friction welding; however, very little has been published on LFW of Ni-base superalloys. The few reported publications have been focused on LFW of Ti alloys.In the present study LFW of IN-718, a Ni-base superalloy widely used in the aerospace industry, is investigated. Since the mechanical properties of the welded component strongly depend on its microstructure, an accurate knowledge of IN-718 structure evolution during LFW is necessary to optimize the process. A study of these evolutions was therefore carried out at different scales, from visual examination to macro, micro and finally SEM observations. Experimental ProcedureWelding: Commercial IN-718 alloy with the nominal composition indicated in Table 1, was received as blocks of 13 mm × 26 mm × 35 mm (Width × Length × Height).Before testing, contact surfaces were ground and cleaned with alcohol. LFW experiments were performed at ambient temperature and under prevailing atmospheric conditions using an MTS-PDS LFW machine. The facility was comprised of two hydraulic actuators: the in-plane actuator that oscillates the lower work piece horizontally and the forge actuator that applies a downward load through the top stationary work piece. More details about this equipment can be found in a previous study. [2] Main process parameters such as frequency (f), amplitude of oscillations (a), frictional pressure (p) and total shortening (s) where fixed to values previously reported to insure a good quality weld. All observations presented in thi...

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Linear Friction Welding (LFW) of IN-718 Superalloy was investigated under several processing conditions. The influence of process parameters such as frequency (60Hz to 100Hz), amplitude (2mm to 3mm) and frictional pressure (50MPa to 110MPa) on the microstructure and mechanical properties of welded specimens was determined. Optical and scanning electron microscopy, and micro-hardness testing were used to characterize the welded areas as well as the Thermo-Mechanically Affected Zones (TMAZ). In-situ thermocouple measurements were performed to follow temperature evolution in the specimens during the different phases of the LFW process. The analysis of the results indicated that for some specific conditions (f=80Hz, a=2mm and P=70MPa) a maximum temperature of 1200°C was attained during the last stage of the welding process, the burn-off phase. This temperature, very close to the alloy melting range, would be sufficient to cause partial liquation in this zone. Microscopic examinations revealed the presence of oxide particles aligned around the weld interface. Their concentration and distribution, varying with process parameters, affect the weld integrity. The TMAZ characterised by a global loss of strength (from 334HV to 250HV) is associated with temperatures exceeding 800°C and causing γ’ and γ’’ reversion. A narrow band of the TMAZ, exposed to high strains and temperatures, showed evidences of dynamic recovery and recrystallization (up to 67% of reduction in the matrix grain size). Visual and microscopic examination of the flash layer, revealed two distinct zones. Microstructure evolution and microhardness variations were associated to process parameters and the optimum conditions for obtaining defect free weldments were determined.