Designing superplastic forming process of a developmental AA5456 using pneumatic bulge test experiments and FE-simulation

摘要

Relatively low tooling costs, high design complexity coupled with low forming speeds make the superplastic sheet metal forming process attractive, especially for smaller lot sizes. Due to the relatively small lot size, the effort and budget for designing superplastic forming processes is usually limited (Kappes and Liewald in J Mater Sci Eng B1:472–478, 2011). For this reason the tool design and corresponding pressure profiles in superplastic forming processes are often based on trial and error (Franchitti et al. in 11th international Esaform conference on material forming, 2008; Barnes in J Mater Eng Perform 4:440–454, 2007). Consequently a process chain should be established to design superplastic forming processes accurately and efficiently. This paper deals with the process chain to form an aluminium part superplastically. At the beginning of the process chain, there is a new, developmental aluminium alloy sheet (AA5456, s0 = 1.6 mm) designed for superplastic forming supplied by Hydro Aluminium Rolled Products GmbH. The relevant material parameters of this sheet are then determined via pneumatic bulge testing with and without in situ measurement of strains. Using these experimentally determined parameters superplastic forming process can be simulated by FE modelling (PAM-STAMP 2G). Due to in situ measurement of strains during pneumatic bulging, the comparison of experiment and FE-simulation results over the whole pneumatic bulging process could be done. This comparison shows good correlation for the observed conditions. Furthermore a cylindrical cup was simulated, evaluated via determined isobar Superplastic Forming Limit Curve (at fracture) and finally formed by pneumatic bulging. Material characterisation of the bottom of this cup showed that excessive cavitation was observed as a result of the iron-silicon particles. Superplastic forming of a bracket usually formed out of AA5083 was also simulated using material parameters of AA5456. The simulation was able to show that this part is not able to be manufactured out of AA5456 under these forming conditions, which was confirmed by forming trials performed at ALU-SPF AG.