Friction Stir Spot Welding model

Hi,

   In my fssw modeling, I'm using Johnson cook material model and C3D8RT element. I checked the consistancy of units also.

   My job is running upto 7frames out of 20frames and suddely aborting saying "the defomation speed exceeded the wave speed due to the excessing temperature and badly distorted elements" 

   Some times it also terminating saying "excess temperature occurs in solving the heat transfer equations"

can any body help me to complete my job please.

Your help will be greatly appretiable 

Thanks in advance