Residual stresses in an electron beam weld of Ti-834: Characterization and numerical modeling
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Residual Stresses in an Electron Beam Weld of Ti-834: Characterization and Numerical Modeling J.R. CHO, K.T. CONLON, and R.C. REED Neutron and synchrotron X-ray strain scanning techniques are used to characterize the residual stresses in an electron beam weld in the near-alpha titanium alloy Ti-834. Measurements are made using the {101 3} and {112 0} lattice planes in both the longitudinal and transverse directions, i.e., both along and normal to the welding direction. The lattice strains are large and positive (several thousand microstrains) in the longitudinal direction; in the fusion zone, the strains measured on the two planes differ by about 30 pct. The strains in the transverse direction are significantly smaller. A numerical model based upon the finite-element method (FEM) is developed in order to rationalize these results. Account is taken of the transfer of heat to the metal and subsequent conduction into the heat-affected zone; convection and radiation effects are neglected. The residual stresses are then estimated using an elastic-plastic analysis with temperature-dependent properties. Despite the limitations of the modeling approach, there is good agreement between experiment and theory, i.e., the sign and extent of the stress field. The plastically upset region extends about 3.5 mm from the weld centerline. The model is used to examine the adequacies of the experimental approach. It is demonstrated that the difference between the lattice strains measured for the two reflections in the fusion zone occurs as a consequence of the micromechanical response of the material, e.g., the partitioning of stress between the grains and the accumulation of intergranular microstrains.
I. INTRODUCTION
ELECTRON beam welding can be used to join materials that might otherwise be impossible to process using conventional methods, e.g., arc welding. The technique has found widespread application in the aeroengine industry, since the alloys used have high strength at elevated temperatures so that distortion and cracking might conceivably occur unless great care is taken. Levels of defects such as pores and microcracks are also minimal, and this is important since great emphasis is placed on assuring the structural integrity of the joints produced. Using a triode-type electron gun, an electron beam is accelerated at voltages up to 150 kV under a high vacuum, typically 103 to 104 torr. Focusing is achieved by an electromagnetic lens, which reduces the diameter of the electron beam considerably; energy densities of up to 1013 W m2 can then be achieved.[1] In practice, deflecting coils provide the flexibility to move the focused spot onto the workpiece. The kinetic energy of these electrons is rapidly converted into thermal energy. The high power density plus the extremely small intrinsic penetration of electrons in a solid workpiece results in local melting and vaporization of the workpiece material such that an approximately cylindrical cavity or “keyhole” is produced. As the workpiece is m
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