As to zone A, the velocity differences isneglectable. Meanwhile, the velocity differences increase with thestrain rate sensitive exponents of viscous medium. The maximumvelocity difference reaches to 24.5% in zone C when m is selectedas 0.25. In this case, viscous medium moves more rapidly thanmetal blank. So material of metal blank in zone C is stretched alongmeridian direction. It can be used to explain the excessive wallthickness thinning in lateral expansion zone. Velocity differencesdecrease evidently for the conditions of m = 0.15 and m = 0.05, inwhich cases metal blank and viscous medium deform almost syn-chronously. Thus the tensile stress subjected by ring-shaped blankalong meridian direction reduces sharply as a result. And the wallthickness distribution of formed parts tends to be uniform.