Abstract: A physical model of the forming process of equiaxed grain in weld nugget zone during friction stir welding of aluminum alloy was established.Experimental results showed that thermal cycle caused material softening near rotation tool.Softening material flowed as plastic state under the mechanical stir of rotation tool.The speed of different flowing layer in softening material was different,resulting in speed gradiends and sticking friction shear stress at the interface between adjacent layers.Under the shear stress,the banded structures of parent material were highly elongated and bended.When the degree of bend exceeded the limit of grain boundary,the original banded structures boundary began to degrade,and the equiaxed grains forming in banded structures were arrayed by the energy principle,resulting in no order and direction pattern of equiaxed grains.