Deformation Behavior and Ductility in Free Forging with Convex Die.

1993 Fiscal Year Final Research Report Summary

• Project/Area Number: 04650624
• Research Category: Grant-in-Aid for General Scientific Research (C)
• Allocation Type: Single-year Grants
• Research Field: 金属加工(含鋳造)
• Research Institution: Akita University
• Principal Investigator: MORITOKI Hitoshi Akita University Mining College Professor, 鉱山学部, 教授 (60005246)
• Co-Investigator(Kenkyū-buntansha): OKUYAMA Eiki Akita University Mining College Assistant, 鉱山学部, 助手 (80177188)
• Project Period (FY): 1992 – 1993
• Keywords: Forging / Limit of Ductility / Plastic Instability / Forming Limit / Localized Necking

Research Abstract

Improvment for spring characteristic and strength of sheet metals has been brouhgt by giving them plastic deformation through free forging with convex shape dies. Here, the deformation behavior and ductility in sheet metal forging with cylindrical dies are investigated. Slab method under plane strain is used in order to evaluate deformation behavior. Forging region of a sheet increases with forging process at the early stage of forging, but after the middle stage its increase changes to decrease. At the center of the forging sheet the sterss along longitudinal direction is tensile, in many cases, thouhgh depending upon tool radius and friction coefficient, and it becomes maximum near middle stage, after which it decreases and shifts to compressive. When the maximum tensile stress is too high, ductile fracture may be possible to arise.
We proposed general criteria for ductility based on plastic instability, and tried to predict the formability of various metal formings. They correspond t o modified diffuse necking by Swift and to modified localized necking by Storen & Rice. The materials used are strengthened aluminium, phospher bronze and stainless steel. The cracking of the aluminium and steel occurs at the interior of materials under plane strain, but that of the bronze does at sheet edge under plane stress. Ductiltiy limit obtained by experiment is compared with analytical estimation on the aluminium, because the bronze shows different view of fracture, and the fracture of the steel and a part of the bronze occurs after very large deformation where very high hydrostatic pressure arises, which rejects all of application of any criteria proposed on phenomenological basis and plastic instability.
The ductility on the aluminum shows, on the whole, good agreement between experiment and analysis, though experimental values are a little higher than analytical ones. The cracking appearing after very large deformation should be inverstigated considering all possible items producing effects on the change of stress state, for example, the flatting of dies under high pressure.