The semi-solid rheomolding AZ91 D auto tire active safety device parts were prepared. The effects of severe rolling friction on the surface layer microcracks of the semi-solid rheomolding AZ91 D auto parts were investigated by “the fatigue experiment”

“25 000 km road test of cars equipping with the sample parts”

and “50 km road test of cars after sudden deflation of their tires”. The results indicate that according to the formation mechanism

the microcracks in the surface layer can be divided into plastic strain initiation microcrack and fatigue microcrack. The key factor is that severe rolling friction could cause the increase of temperature of the parts sharply and the plasticity of the parts

and the part can be subjected to great external load

leading to local violent plastic strain incongruity to generate microcracks. The microcracks in the surface layer with a distance more than 20 μm from the surface

is mainly distributed as micro-crack group

where most of the microcracks are non-propagation microcrack with the length less than 33 μm. The whole microcrack in plastic state is the key factor for the formation of the non-propagation microcracks in the surface layer.