Evaluation of friction reduction and frictionless stress in ultrasonic vibration forming process

In this study, the contact interface friction, its reduction and the frictionless stress in ultrasonic vibration assisted forming process are investigated by a combination of experimental, analytical and numerical methods. Conventional and ultrasonic vibration assisted compression tests of pure titanium without lubrication are implemented. The flow stress is significantly lessened by applying the oscillation and decreasing the diameter-to height ratio due to the friction reduction. The friction coefficient and the real/frictionless stress can be predicted by the relation among the experimental/apparent stress, strain and friction coefficient established from theoretical analysis. Iterative finite element simulation including the estimated coefficient is also performed to correct the compressive stress. By comparing the stress obtained in lubricated compression test, the analytical friction coefficient and the numerical true stress are verified and recommended. Accurate evaluation of the friction and the material behavior using the proposed analytical and numerical methods is of importance for metal forming behavior with or without the ultrasonic vibration.