An experimental investigation of the superplastic forming behavior of a commercial al-bronze
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I.
INTRODUCTION
THE work of Higashi et
al. tq has shown that a complex Al-bronze based on the composition of Cu-10A1-4.5Fe6Ni-2Mn (JIS 6301) can be thermomechanically processed to develop abnormally large superplastic strains over a range of temperatures and strain rates. The alloy has given elongations of 5500 pct during tensile straining,* and there is commercial interest in the superplastic *Higashi has more recently recorded tensile strains to failure in excess of 8,000 pct (unpublished work).
forming (SPF) of the material in Japan. However, there exists a degree of uncertainty about the optimized forming procedure, i.e., the combination of forming temperature and strain rate which maximizes superplastic formability and minimizes forming costs. Furthermore, the alloy is ideal for the investigation of the effect of strain rate sensitivity, or m value, on SPF, since a relatively stable microstructure can be maintained over a large range of superplastic conditions, tl] A laboratory bulge-forming rig which can operate under controlled temperatures and pressures has been used in the present work. The advantage of a bulge-forming procedure is that it is similar to industrial processes, which often involve the initial shaping of a hemispheric dome. Hence, the results obtained from bulge-forming tests are likely to be of more relevance to commercial practice than those from uniaxial stretching. In particular, the thickness variation of a component may be predicted from the former investigation. Many theoretical and experimental studies have been devoted to SPF during the last t w o d e c a d e s , t2-6!
Z.X. GUO, formerly with the Manchester Materials Science Centre, is Research Assistant with the Division of Metallurgy and Engineering Materials, University of Strathclyde, Glasgow G1 1XQ, United Kingdom. K. HIGASHI, formerly Visiting Scientist, Manchester Materials Science Centre, is Associate Professor, Department of Mechanical Engineering, University of Osaka Prefecture, Osaka 591, Japan. N. RIDLEY, Reader in Metallurgy, is with the Manchester Materials Science Centre, University of Manchester/UMIST, Manchester M1 7HS, United Kingdom. Manuscript submitted February 15, 1990. METALLURGICAL TRANSACTIONS A
During bulge forming, the periphery of the clamped sheet is usually held in a fixed position and does not draw in. (Forming with partial "draw in" of material around the periphery of a dome is beneficial but requires complicated equipment and precise control.) Thus, the alloy stretches into the cavity of a die, and this results in considerable thinning of the sheet for a deep-formed part and also a significant gradient in thickness. Experimental r e s u l t s 13,41 and theoretical analyses [5,6] have shown that the variation in thickness after bulging increases with decreasing m value and with increasing height. However, a systematic experimental investigation of the subject is still lacking. The variations of dome geometry and strain rate have not been thoroughly examined experimentally. An understanding of the
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