You need to know what specific topic you want to simulate. Mechanical behavior, electric and heat transfer phenomena, metal microstructure. The welding simulatios is very complex coause the welding involves a lot of phenomena and variables, and, the welding process has different variables before and after the welding. You can start with the mechanical behavior and then yo can add complexity to your model.

You need to perform structural-thermal multiphysics analysis using Solid226 or Solid227 depends on the mesh you choose. The keyopt will be 11. You need element birth and death technique to apply the heat as the result of welding process through the welding pass. I put a link to a video that might be helpful as well.

https://www.youtube.com/watch?v=ns2-kf_3UN8

I agree with Hamidreza. Also, I suggest checking this link

https://caeai.com/resources/modeling-welded-connections-ansys-e-learning

dear Enrique Isaza

i want to find heat affected zone and also weld strength

in ansys

Dear Surendran .R

The Heat afgected zone is more a heat transfer and electrical modeling. You need a multiphysic software like Comsol to do that.

The stregth is more a mechanical stress simulation, yo can make this simulation with the welding geometry, but if you want to simulate the material internal transformation due the thermal and electrical efects, you must use Simuweld or use specific papers to develop models and run it over Ansys or Elmer

As far as I know structural-thermoelectric analysis is used for simulating spot weld not for Co2. The best approach for solving co2 MIG is using Thermal-elastoplastic method which is well described in a lot of publications. If you are interested in performing this method in ANSYS, you need to write a python script in order to apply the heat as I mentioned earlier.