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

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by Guido Berti and Manual Monti
Spring manufacturing by cold forming of pre hardened steel is critical when the material ultimate tensile strength is above 1800 MPa. The quality and the thickness of the oxide layer on the external surface of the wire affect the success/failure of the coiling process as the oxide acts as a sort of solid lubricant and its removal leads to an increase of friction phenomena. The higher friction forces increase both the required forming force and the feeding force, which can introduce some damages in the wire and finally can cause the breakage of the coil. The numerical model developed for the coiling process can take in account such effects and the implementation of ductile isotropic damage based on Lemaitre formulation allows the correct prediction of the failure during coiling. The extensive utilization of the model permits to explore different interactions between machine setup and friction, determining the configuration that should be adopted to avoid the risk of failure.
University of Padua, DTG
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