Simulation is not a very good tool for advanced simulations dealing with the HAZ and gradient material properties.
The simplest way to do this is to draw the component as it is welde. For a pipe this will include the two pipes and a ridge on the outside and inside. If the base metal and weld material are very dissimilar in elasticity and poissan's ratio (example case would be brazing or soldering) you will need to create the two base parts/bodies and create a seperate part/body of the weld bead.
Best of luck