INCREASE IN FORMING LIMIT OF SHEET METALS BY REMOVAL OF SURFACE ROUGHENING WITH PLASTIC STRAIN - BALANCED BIAXIAL STRETCHING OF ALUMINUM SHEETS AND FOILS

When forming a sheet metal, the free surface of the sheet becomes roug h with increasing plastic strain. This is well known as orange peel ph enomena and has a great effect not only on the surface quality of a pr oduct but also on the forming limit. In an attempt to increase the for ming limit of thin sheet metals, in the present paper, a new forming t echnique is proposed in which surface asperities generated during the forming process are polished to eliminate the weak portions of the she et which lead to the onset of a localized necking. Balanced biaxial st retching of aluminium sheets and foils is carried out in this manner, and how the forming limit can be increased is examined. It is shown th at the forming limit of sheet metal can be appreciably improved by a s imple method of polishing the surface roughness during the stretching process. The polishing of surface roughness at the strain somewhat bef ore the onset of localized necking is recommended to efficiently incre ase the forming limit. The effect of polishing surface roughness on th e forming limit becomes large as the thickness of sheet decreases. For aluminium sheets with 0.1 and 0.2 mm in thickness, for example, the f orming limit strains achieved have been about 1.4 times as large as th ose for the ordinary balanced biaxial stretching without the interstag e polishing. When such processes of stretching and subsequent polishin g are repeated in turn 2 or 3 times, further increase in the forming l imit can be achieved.