To the theory of laser generation of elastic waves in ferromagnetic metals at the temperature of magnetic phase transition

Laser generation of ultrasound has found wide application in modern technologies: to control the quality of composite materials, to detect layer separation in adhesive-bonded joints, subsurface and surface defects, as well as the quality of a product's surface in the process of manufacture. For excitation of waves in metals, pulse laser is normally used. In metallurgy, as well as in promising 3D technologies, it is necessary to control products at high temperatures (800$^{\circ}$С and higher). To design ultrasound equipment with a pulsed laser radiation generator as the ultrasound source, it is required to conduct theoretical research of the process of laser generation of ultrasound in ferromagnetic metals at the temperature of magnetic phase transition, since the hot metal conversion for ferrum and ferrum-based alloys is, as a rule, performed namely at this temperature of 768$^{\circ}$С. The results of the experimental works allow to conclude that the temperature dependence of normalized amplitude of acoustic pulse in ferrum is of extreme character in the range of magnetic phase transition, i.e., in the range of Curie point. In this work a goal has been set to study the process of laser generation of ultrasound in ferromagnetic metal in the condition of non-linear dependence of the volume-expansion coefficient on the temperature. The task of thermoelastic excitation of longitudinal and transverse waves in ferromagnetic metal by a laser pulse at the temperature of magnetic phase transition has been solved. Diagrams of longitudinal and transverse wave patterns when ferromagnetic metal is exposed to laser pulses of various diameters have been obtained. Recommendations for effective use of laser generation of ultrasound in non-destructive testing and thickness measurement have been given.