When engineers plan the welding process parameters, the weld nugget size is the key factor to optimize. But weld quality is not only about the weld nugget size. It is also important to get high weld strengths and good material properties around the weld. In this case study, we present the influences of the hold time on the hardness distributions after welding. Also, the effects of hardness on the performance of weld strengths were tested.
Hold time affects hardness after welding
Hardness distribution after welding is critical for the performance of welds. The hold time can affect the cooling rate thus change the hardness distribution after welding.
We ran a set of simulations with different hold times (100ms, 150ms, 200ms) to investigate their effect on the microstructures. The material used in this simulation is DP600 with GI coating. The basic welding parameters are weld time=300ms, F=4kN, I=7.75kA (AC). The welding processes were simulated by SORPAS 2D.welding v14.0.
Cooling rate & Hold time
The nodal temperature curves show different cooling rates affected by different hold times. They would result in different microstructures thus different hardness distributions after welding.
Below images show hardness distributions resulting from different cooling rates by different hold times. All three cases used the same weld time, current, and force, so they had the same weld nugget size.
Weld strength and Hardness
To see the performance of different hardness distributions, we simulated tensile shear testing by SORPAS 3D.testing v6.10. The fracture occurred at different locations during the weld strength testing. Different hardness distributions from different cooling rates resulted in different fracture behaviors.
The weld strength curves show different performance of the welds affected by different hold time. Shorter hold time resulted in lower hardness thus lower strength. Longer hold time resulted in higher hardness thus higher strength but more rapid fracture. An optimal hold time would result in the best performance of the weld with large elongation and high enough strength.