Hot machining of aluminium sheet metal

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To widen the industrial use of the superplastic forming, research efforts aim at the reduction of forming times. the most striking result was the development of the quick plastic forming technology, particularly studied in the united states automotive sector.

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[1] Phases of the superplastic and quick plastic forming processes.
The superplastic forming represents a process where metal materials in the form of fine-grained sheet metal are pressure-formed inside high-temperature portable moulds to supply products, often with complex shape and high added value. In figure 1, the pressure forming process is schematically represented. Among the material properties, super-plastic alloys must feature a high value of the strain rate sensitivity, m. The high values of m, required to increase the tensile strength, are obtained at relatively low strain rates, making the super-plastic forming a slow process compared to the conventional sheet metal working techniques [1]. The advantages of the super-plastic forming can be synthesized in the fact that SPF represents a net shape forming process. Reducing the use of successive machining, it minimizes the total scrap produced, it can lead to appreciable cost reductions if a complex component, which is generally implemented by joining several parts, can be formed as if it was a single part. Besides, it decreases the weight of the realized component by eliminating joints, rivets and fixing elements in general, it reduces the general costs by annulling assembling phases, it makes the spring-back phenomenon negligible and it requires the use of a single equipment, thus reducing costs.

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fig.2 Plant for the superplastic forming and the quick plastic forming installed at the Laboratory of Technology and Machining Systems of the Cassino and Sothern Lazio University.

Quick plastic forming
The quick plastic forming (QPF) technology refers to the hot pressure forming process developed by General Motors Company [4]. The process allows implementing panels with complex shape starting from sheet metals of aluminium-based alloys and in the standard production volumes of the automotive sector. QPF forms the aluminium alloys at higher strain rates than those linked with the SPF that is designed to maximise the material ductility. The process needs higher automation than conventional SPF processes in order to produce, directly from a forming cell, dimensionally correct components. The process is highly optimized to minimize the cycle time and to maximise the profitability of huger investments. Besides, QPF produces parts of minor complexity that those attainable if longer cycle times are permitted. At the Laboratory of Technology and Machining Systems of Cassino and Southern Lazio University, they have setup a plant able to perform superplastic forming and quick plastic forming tests (figure 2). The used equipment allows executing forming tests at constant pressure inside a mould previously heated up to the required temperature. During the tests, they accurately detect the…

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