The Influence of Surface Hardening by Combined Thermo-Forced Impacts on the Fatigue Life and Damage of the VT22 Titanium Alloy

The influence of electromechanical surface treatment (EMT) and nonabrasive ultrasonic finishing (NAUF) on the titanium alloy and the complex influence of the above treatment methods and subsequent aging on the fatigue life and the change in the surface microhardness is studied. Specimens for research are made of rods manufactured of the transition VT22 alloy after the standard thermomechanical treatment. The EMT is conducted by rolling the surface of the specimen with a roller pressed to it and passing a high-density current between them. As a result, the surface thermomechanical treatment is conducted under fast local heating and cooling of the surface. The NAUF is performed by shock treatment with a striking ultrasonic emitter on the treated surface. The rotational bending loading of specimens at stress amplitude of 0.5σu upon the NAUF results in an increase in fatigue life by a factor of 1.8 in comparison with the untreated initial state, accompanied by a slight increase in the microhardness of up to 16%. The EMT reduces microhardness by almost 20% and fatigue life by 70%. The combined processing by EMT + NAUF insignificantly affects microhardness, but increases the life by 40% compared with the EMT. Aging at a temperature of 450°C for 5 h increases the microhardness upon the EMT by 30–40% with a simultaneous twofold increase in durability. The aging of the specimens subjected to EMT + NAUF practically does not increase microhardness, but it increases durability by almost three times compared with the EMT. A fractographic analysis of the fracture surface has shown that the reduction in the life after EMT is associated with a reduction in the stage of crack initiation, which virtually excludes this stage of accumulation of fatigue damage from the overall durability of the specimen.