On critical surface strain during hot forging of lubricated aluminum alloy

We present a new concept of critical surface strain to evaluate the lubrication-effective limit for process design in metal forming. Finite-element predictions obtained at various fixed Coulomb friction coefficients were compared in hot-forging experiments of a passenger car piston fabricated from short cylindrical aluminum alloy specimens well-lubricated with a solid film of water-dispersed lubricant. The Coulomb friction coefficient was formulated as a function of temperature, pressure, and material strain, and optimized by minimizing the difference between the predicted and experimental results. We established that the effect of surface strain dominated the metal-forming process, and that the optimized Coulomb friction coefficient changed abruptly at the critical surface strain at which the solid lubricating film used in the process is suddenly disrupted. •Friction laws with constant coefficients are criticized in terms of accuracy of plastic deformation of aluminum alloy.•Coulomb friction law with varying coefficient, a function of strain is proposed to solve the problem.•Optimization technique shows that effective strain at the contact surface affects dominantly the friction coefficient.•The optimized Coulomb coefficient changes drastically around a certain surface strain value, called critical surface strain.•This phenomenon is validated by comparing the critical surface strain with damaged lubricated surface in experiments.