to keep constant the plunging force you need a system composed by load cell to measure the plunge load and a retrofit systems which comoares the actual force with the target force and automatically moves the vertical position of the tool.

Generally, the plunge for slightly varying in entire FSW process. But you can reduce this variation a lot and make it almost constant by using hydraulically generated plunge force. This you can generate by using a hydraulic system.

thanks, researchers and brilliant minds!! for such a precise and helpful answers.

for a modified semi-automated vertical milling machine, it will be very difficult to make such mechanism. CNC friction stir welding provides easy to handle plunge force and plunge depth. is there any sort of mechanism used at vertical milling machine for such problem? 

This depends upon the FSW machine which you use for your research work. Generally, there will be large variation during initial plunge and dwell period. The variation will slowly subside and come to a narrow range during traverse moment. Machines like I-STIR FSW have a special mode to keep either load or plunge depth constant throughout the welding process.