Spotweld Models for Advanced High Strength Steels
Weld Microstructure and Property Modeling
The SWE requires detailed microstructure and property information in the weld region to properly formulate the constitutive equation. An integrated electrical-thermal-mechanical-metallurgical welding process model for electric resistance spot welding was used in this project, to predict the microstructure and property gradient in the spot weld and the adjacent region. It is based on the earlier work with further refinement of the microstructural model for AHSS.
The RSW process model predicts the weld size, microstructure and residual stresses in a spot weld based on the following user inputs:
- Steel chemistries and base metal microstructure
- Surface coating
- Sheet stack-ups
- Welding conditions (current and electrode force)
- Electrode geometry
A key feature of this integrated weld process model is that it predicts the microstructure evolution based on the calculations of the thermodynamics and kinetics of steel phase transformation processes. There is no need to experimentally measure the continuous cooling transformation (CCT) curves for a given steel, an impossible task for all possible thermal cycles experienced in different locations of the weld and HAZ of a spot weld.
Modeling Approach
In principle, resistance spot welding utilizes Joule heating (I2R) to produce a molten nugget at the contacting surface of the workpieces. Force is applied before, during and after the application of electric current, to maintain the electric current continuity and to provide the pressure necessary to produce defect-free spot welds. In addition, microstructural changes occur during welding of steels. In this regard, the resistance welding is a multi-physics process encompassing electrical, thermal, mechanical, and metallurgical disciplines. Furthermore, there are strong interactions (coupling) among the electrical, thermal and mechanical aspects of the process that must be included in simulation of the resistance spot welding process, as shown in Figure 25.
Results
The integrated weld process model was applied to the two steels and different welding conditions used in Phase I. The simulation results compared well with measurement results of the weld microstructure and microhardness distributions in the spot welds. Figures below show examples of the weld process modeling results and comparisons with the experimental hardness measurement results. The predicted microhardness results were then converted to the yield strength and used in the SWE formulation.
