Experimental and numerical analysis of the cold forming process of automotive suspension springs

Nowadays in automotive industry great efforts are spent in achieving weight reduction of cars components. The coil springs are not exempt. For this component weight reduction can be obtained by reducing the spring wire diameter. However, to assure that springs maintain the required mechanical properties, it is necessary to adopt material with high strength. In order to speed up the manufacturing process of coil springs, the trend is to produce them by cold forming of hardened wires. In this case the wires are subjected to the heat treatments of hardening and tempering before the coiling process. This leads to an improvement of productivity since it is not required a heat treatment after forming. However forming a high strength material already hardened and tempered is critical. The material presents low ductility and requires high forming forces. Moreover lubrication, which is more critical in cold forming, is demanded to the oxide layer present on the wire surface. The combination of such factors can lead to coil's failure during forming process. To improve the knowledge on these phenomena, different production batches of hardened 54SiCr6 wire have been analysed, characterized in laboratory and correlated to the corresponding successes/failures recorded in the manufacturing of the springs. The process is sensitive both to the process conditions (such as friction), and to the set up of the spring making machine. The further step of the investigation consisted in the development of a numerical model of the coiling process able to predict the possible failure during spring coiling. Ductile isotropic damage formulation has been adopted and different friction conditions, as well as different configurations of the coiling machine, have been considered and replicated by the model. The predicted results confirmed the failures recorded in the shop floor, as well as the high contribution of friction, which is strictly related to the external oxide layer, to the success/failure of the cold forming of the spring.