On critical surface strain during hot forging of lubricated aluminum alloy

Abstract

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.