Bitter cold makes aluminum compliant

The transport sector is one of the biggest problem areas when it comes to climate protection. In Austria, transportation accounts for almost one third of all greenhouse gas emissions - and unlike other sectors, the growth trend is unbroken. One of the thrusts for making progress in this area is the lightweight construction of vehicles. After all, the more weight a vehicle or aircraft has, the more energy must be expended to move it. A reduction in weight therefore lowers the CO2 emissions associated with traffic (or, in terms of e-mobility: increases the range of the vehicles). 

Aluminum will therefore continue to be increasingly important as a material for vehicle construction in the future: since the 1970s, the share of aluminum in German automotive construction has increased from 2.5 kg to 140 kg per new vehicle in 2012 and will reach 160 to 180 kg in 2020. "The biggest advantage over the 'main competitor' steel is definitely its low density. It is only about one third of steel," explains Florian Grabner, a researcher at the AIT Austrian Institute of Technology. Machining and processing is in itself no more difficult than that of steel. However, due to a fundamentally different microstructure, the mechanical characteristic values of aluminum alloys, e.g. strength or maximum possible elongation, are mostly lower than those of steel. "These mechanical characteristic values are, however, decisive in the production of sheet metal components," says Grabner, who heads a project series entitled "KryoAlu" at the LKR Light Metal Competence Center in Ranshofen, a subsidiary of AIT. In this project, a new process for forming aluminum sheets is being developed together with partners from science and industry that circumvents previous difficulties. 

Pressing at up to minus 180 degrees

The state of the art is to form aluminum at elevated temperatures. "At high temperatures, the material is soft, which means that high elongations are possible during forming. However, the force that can be transmitted is lower, which leads to cracks locally in the formed component," Grabner explains. This is because the microstructure is changed during heating, so additional heat treatment processes are necessary to return to the initial state. 

The idea is based on an interesting property of aluminum: even at low temperatures, the metal's formability improves, and the microstructure does not change in the process, the colder the temperature, the better the mechanical properties. In the "KryoAlu" project - "kryo" is Greek and means "cold" - a process was developed in which aluminum sheets are precooled in liquid nitrogen (minus 196 degrees), inserted by a robot into forming dies cooled to minus 180 degrees and removed again after deep drawing. In the future, this innovative process will make it possible to produce much more complex shaped parts from a single piece of sheet metal - for example, several individual parts of a car door can be combined and produced from a single part. This eliminates the need for joints, and a high surface quality can also be achieved. Depending on the specific application, these advantages can compensate for the increased costs of cooling. 

In the completed cryoAlu projects, the semi-industrial small-series suitability of cryogenic sheet metal forming has been demonstrated. Current challenges in the implementation of the process in series production primarily concern the efficient pre-cooling of the blanks with a high throughput and the enclosure of the forming tools to prevent ice formation on the tool surface, which is suitable for series production.