Characterisation and generation mechanism of residual stress and plastic strain in Ti/AISI304L stainless steel friction welds

Summary

This paper describes detailed FEM modelling performed to clarify the characteristics of residual stress and plastic strain generated during dissimilar metal friction welding operations. The results obtained may be summarised as follows:

1. When bonding problems of dissimilar materials are being numerically analysed by an FEM thermal elastic‐plastic analysis, comparisons undertaken in the radial direction of both the distribution and magnitude of the z axis stress σ_z generated in both base metals adjacent to the bondline, as determined from the uniform cooling model from 800 °C to room temperature, provide a sound basis for a decision as to the suitability of the FEM mesh used. A comparison of the yield stress σ_Y and temperature relationships in both base metals shows that the Ti base metal has the lower zero‐σ_Y temperature (mechanical fusion temperature).

2. Thermal diffusion in Ti/AISI304L stainless steel friction welds mainly occurs within a narrow region of around 20 mm from the bondline, and this strongly affects stress generation. The maximum temperatures attained on either side of the bondline also differ. At a position 3 mm from the bondline, the AISI304L stainless steel has a temperature around 100 °C higher than that of the Ti base metal.

3. In dissimilar materials, the radial direction stress component σ_r generated in the region adjacent to the bondline is tensile in the base metal with a high coefficient of thermal expansion a and compressive in the base metal with a low one. The axial direction component σ _z in the radial direction is also tensile at the centre and compressive at the periphery. Their absolute values are higher than the stress generated during cooling from a uniform temperature and are caused by the temperature gradient in the z axis direction.

4. According to the σ_z z axis distribution, σ_z at the centre is tensile in the region adjacent to the bondline and compressive inside for both base metals. At the periphery, the AISI304L steel is under compression in the region adjacent to the bondline and under tension just inside the bondline. The Ti base metal shows tension over the whole region other than in the region immediately adjacent to the bondline. The stress generated just inside the bondline is caused by the temperature gradient in the z axis direction.

5. The mechanical conditions of friction welding are severe in the radial direction and direction at right‐angles to the bondline and less so in the circumferential direction.

6. When Ti/AISI304L stainless steel friction welds are produced, heavy plastic strain is generated in the region adjacent to the bondline of the Ti base metal. In this region, Ti‐rich intermetallic compounds are formed, and the hardness is here much higher than in the Ti base metal.