Is the tool shoulder in contact with the workpiece surface during the FSW process?

Yes, in FSW welding, the shoulder must be in permanent contact with the surface of the workpiece. It is the friction of the shoulder against the workpiece that provides the heating necessary for the friction stir welding operation.
The shoulder is one of the components of the FSW tool. Its main function is to heat the material by friction in order to soften the parts to be welded. To do this, the rotating FSW tool is immersed in the material until the shoulder is in direct contact with the top surface of the workpiece.