Abstract: Experiments with stir tool kept rotating at the same position on 6056-T6 aluminium alloy sheet were carried out,and the torque of stir-head,down force and welding temperature were all recorded during this process.The experimental results show that the torque will increase with the increasing of down force;the reason for torque increase is that the down force strengthens the material in stir zone.Although the torque will decrease with the increasing of rotation speed,the product of torque and rotation speed which corresponds to the heat generation will increase with the increasing of rotation speed.The rotation speed influences the torque through two ways of welding temperature and strain rate of material in stir zone,and the influence from the former way is more significant.