High speed metal forming processes. Determining a behaviour law

The cold forming of metals is undergoing major developments, and the use of high deformation speeds has developed considerably to obtain both new forming possibilities and also productivity gains. So-called high-speed forming imposes deformation speeds on the material that can be set between 50 s ^-1 and 10 ⁴ s ^-1 , as in rolling, wire drawing, cold heading or metal machining, for example.

The problem faced by the technician when developing an industrial range is to optimize the material-process combination to obtain the most cost-effective result. In order to succeed in such an approach, the difficulties to be solved are controlling the flow of material during deformation, the feasibility of the process and obtaining the required usage properties for the formed part. Numerical simulation of the manufacturing process is therefore a means of studying feasibility and optimizing a range without incurring the cost of actual operations. However, while the resolution method itself is now well mastered, the same cannot be said of the physical models needed to describe material behavior. The complexity of the phenomena involved in high-speed forming processes places the material under conditions of extreme deformation, strain rates and heating. It is therefore essential to obtain a behavioral law valid over the entire range swept by the forming operation.

After a presentation of the various high-speed forming methods, this article describes the behavior laws and mechanical tests that can be used to simulate these processes. One way of characterizing the behavior of a material at high speed is to perform compression tests under an instrumented drop-weight. In this way, the rheology of a material can be analyzed, and the coefficients of a behavioral law determined, with a view to studying a high-speed forming process using numerical simulation. The idea is also to respond to the constraints of the industrial environment, i.e. to identify a rheology in a short time with a minimum of tests and at the lowest cost. We also show that the simulation phase need not necessarily follow the coefficient identification phase. The use of numerical simulation is indeed desirable to know the respective weight of the different coefficients of the behavior law in order to optimize their values. Finally, the article presents the choice of a damage criterion that can be used to specify the severity of a forming operation.

A glossary and table of symbols are provided at the end of the article.