Hot deformation behavior and hot working characteristic of Nickel-base electron beam weldments

The electron beam welding (EBW) of Nickel-base superalloys was conducted, and the cylindrical compression specimens were machined from the central part of the electron beam (EB) weldments. The hot deformation behavior of EB weldments was investigated at the temperature of 960-1140 C and the strain rate of 0.001-1.0 s-1. The apparent activation energy of deformation was calculated to be 400 kJ/mol, and the constitutive equation that describes the flow stress as a function of strain rate and deformation temperature was proposed for modeling of the hot deformation process of EB weldments. The processing map approach was adopted to investigate the deformation mechanisms during the hot plastic deformation and to optimize the processing parameters of EB weldments. It is found that the true strain has a significant effect on the efficiency of power dissipation (η). The η value in the safe processing domain (1140 C, 1.0 s-1) increases from 0.32 to 0.55. In the unsafe processing domain (1080 C, 0.001 s-1), however, the η value greatly decreases with the increase of strain. When the strain is 0.40, the efficiency of power dissipation becomes negative. The flow instability is predicted to occur since the instability parameter ξ(ε̇) becomes negative. The hot deformation of EB weldments can be carried out safely in the domain with the strain rate range of 0.1-1.0 s-1and the temperature range of 960-1140 C. When the height reduction is about 50%, the optimum processing condition is (Topi: 1140 C, ε̇opi: 1.0 s-1) with the peak efficiency of 0.55 for the processing of EB weldments.