Abstract: Radial resistant force against the moving tool was measured dynamically and influences of some factors on the force were analyzed. It shows that the force rises up sharply at the beginning of friction stir welding As soon as reaching the top it decreases down rapidly, and then goes to a value at which the welding procedure is steady. If increasing the rotation speed of tool, but keeping all other parameters and factors fixed, the top force and the steady force are both decreased, and the times taken for reaching the top force and steady force are shortened. If increasing welding speed only, the top force goes up greatly, and the steady force increases or decreases in a rather small range, and the time taken for the top force changes a little, but the time for the steady force is prolonged; The influences of the diameter of pin on the force are complex, because the change of the diameter of pin leads to the variation of frictional heating power and the cooling power simultaneously.

Key words: Dynamic measuring, Analysis, Friction stir welding, Radial resistant force