The heat treatment phase, which lasts several hours, is in place to make sure that the material properties of the future wheel are exactly how they should be. The three-phase heat treating of the casting blank helps to improve the firmness of the aluminium alloy. Step one: solution annealing which takes place following the alloying stage in accordance with DIN 1706 and at temperatures of between 480 and 550 degrees Celsius. The temperature is selected so that a large enough amount of alloying elements are dissolved into the mixed crystals to increase the strength of the alloy. Step two: quenching. During the quenching phase, the casting is cooled down as quickly as possible to around 70 degrees Celsius so that it is ready for the next stage. Stage three: warm ageing. Warm ageing is in place to ensure that the material obtains the right strength. The process requires absolute precision. If, for example, the solution annealing is conducted at a temperature in excess of 550 degrees Celsius, the casting can no longer be used and has to be melted down.
