AP&T AB is involved as a partner in a EU project testing thixoforming technology for steel parts applications. Already used on aluminum, thixoforming involves heating metal pieces to a temperature just below their melting point. The material becomes soft without flowing and is therefore easy to form. With the right material composition, it is also possible to achieve, in comparison with traditional forming techniques, higher strength at the same thickness and weight. The technique resembles forging in some respects but offers a number of comparative advantages. Finished parts are lighter and can come substantially closer to their final finish, so the number of production steps is reduced. There is said to be little or no scrap and less energy used in the process.
In the EU project “Development of Steel Parts Forged under Semi-Solid Conditions for the Industrial Market,” IFUM (Institut für Umformtechnik and Umformmaschinen) at the University of Hannover has studied potential ways of thixoforming steel. AP&T is a partner in the project, with responsibility for the presses and automation process used during forming.
According to the supplier, a particular challenge with this forming technique, and one that must be solved to achieve successful results, is heat control. The molecular structure of the end piece is determined by the temperature at which it is formed. If the material is too hot, it cannot be formed. If too cold, the desired properties will not be achieved. The strength of an individual part can also vary dramatically if it is formed at the wrong temperature. The ideal temperature range is known as the thixo window and is reported to impose special demands on the forming equipment and process. “The material has to be at the right temperature in the tool. We have to deal with the time factor,” says technical manager Bengt Walkin.
Thixoforming is used commercially to form aluminium, which is easier to handle using the process because its “thixo window” is roughly 500°C to 650°C. The “thixo window” for steel is between 1,370°C and 1,400°C.
“For the actual press process, we have been able to use a control system that we normally use for hydroforming HNC. The most important thing is that the axes are servo controlled, which results in rapid forming. This is essential to coping with the restrictions imposed by the thixo window,” says Mikael Karlsson, project manager in the EU project. “Servo control also gives us high precision. The cylinder has to move fast, up to three meters per second, but we must, at the same time, also be able to control the positioning very precisely, given the state of the material. The HNC system delivers positional precision down to hundredths of a millimeter.”
It is not yet certain that steel thixoforming will become commercially feasible, but if some process issues are solved it is hoped that it will become a reality within a few years. www.apt.se