Abstract:  Using the crossbeam of rail vehicles as the carrier, different specifications of mixing heads were adopted for different plate thicknesses of the lap joint. The influence of the shape of the mixing head, welding process parameters, and welding offset on the formation of hook defects was deeply studied. The research shows that after the corner at the end of the mixing head is chamfered, the size of the hook can be reduced, and even the direction of the hook may change; when the length of the needle of the mixing head becomes shorter, the size of the hook will increase, and this factor has the greatest impact on the size of the hook; with the increase of the welding speed, the size of the hook tends to decrease, but the rotation speed has almost no significant effect on the size of the hook; under the same welding process conditions, when the offset is not large, the size of the hook decreases, but when the offset is too large, the size of the hook will increase sharply. By controlling the length, shape and process parameters of the mixing head, a qualified welding process window for the lap joint can be obtained.