Abstract: The effects of welding parameters on the forward force of the stir-pin were investigated with the mechanical signal detecting system during friction stir welding LF2 and LY12 aluminum alloys.The results show that the onward force will increase with welding speed and heel plunge depth increasing during welding the same materials,but when the various phenomena occur,the onward force increases at first,reaches the stabilization,and then decreases with rotational speed increasing;the higher the materials hardness the larger the onward force is.The onward force of the stir-pin is connected with the heat generation and the deforming volume of advancing side metal during welding;when the deforming volume increases and the temperature decreases,the onward force will increase.