Metallophysical studies of the effects of stress relaxation in the zones of extreme thermal exposure to laser radiation on alloys

Abstract

The results of metallophysical studies of the specific features of steel and alloy structure after a thermal-deformation exposure to laser radiation are presented. It is shown that the locality of laser treatment in combination with high heating and cooling rates, lack of soaking at the heating temperature, and high temperature gradients along the depth of the irradiated layer lead to the appearance of thermostrictive stresses of up to ~ 320 MPa in value, which play an important role in strengthening the surface of steels and alloys. It has been discovered and confirmed that under the influence of stresses, a localized plastic deformation develops in the microvolumes of the irradiated material, accompanied by the movement of dislocations along the slip planes. Schemes are proposed for the formation of slip lines or bands, kink bands, relief grain boundaries, etc. during high-speed laser treatment. It has been shown that as a result of laser treatment, the defect density of the crystalline structure of the studied single-phase steels and alloys increases to (2–4)×10¹¹ cm^-2. This contributes to the improvement of mechanical properties (including hardness and shear stress) of the surface layers of irradiated materials. As a result of metallophysical studies, structural relaxation effects of thermostrictive stresses in the laser-treated zones of steels and alloys have been confirmed. By using practically single-phase (“model”) alloys, such as copper and nickel alloys, corrosion resistant steels, and technical iron, it was shown that partial stress relaxation occurs due to dynamic polygonization and recrystallization processes that occur within the surface layers of the alloys, resulting in the formation of small fragmented grains measuring 0.5–2.0 μm in size. This reduces the risk of crack formation due to extreme thermal effects of laser radiation. It has been established that the level of hardening and the ultimate structure of steel are determined by the superposition of stresses, generated in the laser treatment zones, and by the processes of relaxation of the laser radiation energy as a result of local plastic deformation, dynamic polygonization, and recrystallization.