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Effect of upset pressure on evolution of microstructure and mechanical properties for rotary friction welding of IN713LC superalloy and AISI 4140 steel have been investigated. Microstructure observation shows that with increasing upset pressure, grain refining effect of plastic deformation overrides grain growth effect of heat input, leading to finer prior austenite grains on AISI 4140 side. Dendritic structure of IN713LC transformed into fine equiaxed grains. Weld joints fabricated using low upset pressure showed unbonded regions with curved shape profile. However, weld joints made using high upset pressure showed nearly straight-line joint profile and was found to be favourable for adequate plastic deformation and achieving better bonding. Improvement in mechanical properties of weld joint was observed with increasing upset pressure. [ABSTRACT FROM AUTHOR]