Due to the high friction and wear caused by the development of high-speed and heavy-duty railway transportation, parts and components are prone to failure in the forms of wear, peeling, fatigue cracks, and so on. Higher requirements for the uniformity of the work piece's hardened layer after laser quenching are put forward to improve the wear resistance and service life of the surface of parts and components. The surface hardening effect of laser quenching is closely related to the light intensity distribution of the beam. When the commonly used flat-top rectangular light spot acts on the material, the topography of the hardened layer presents a "crescent" shape, deep in the middle and shallow at both ends, as the consequence of the obvious lateral heat loss at the edge of the light spot. To increase the depth of the hardened layer at the edge, a specific light field distribution is required to increase the energy injection on both sides of the spot, so that the temperature field under the action of the laser is as uniform as possible. As a result, developing a broadband laser optical system with adjustable light intensity distribution to realize the real-time output of a broadband beam with a specific light intensity distribution is an effective way to improve quenching quality. We present a laser quenching optical system based on galvanometer variable-speed scanning in this paper. The light field and temperature field distributions in the quenching process are regulated by changing the variable-speed scanning mode of the galvanometer, so that more heat is injected into the quenching regions at both ends to overcome the energy loss caused by lateral transfer, improving the uniformity of the phase transformation hardened layer. This study provides a reference for the high-quality surface strengthening of high-end equipment parts under heavy load conditions.
The ANSYS software is used to establish the thermophysical model of the laser quenching process, and the initial conditions and boundary conditions are set according to the working conditions. A static rectangular light field with an equivalent thermal effect is established in the simulation model as a result of the complex calculation of the moving heat source model scanned by the galvanometer. The heating curve at the characteristic point and the maximum surface temperature distribution confirm the equivalence of temperature field evolutions in the two laser quenching methods. The 45 steel is selected as the base material, and the model considers the thermophysical parameters of the material at different temperatures. Because the temperature gradient in the laser quenching process is large enough to meet the cooling rate requirements of self-cooling quenching, the hardened layer distribution can be obtained by making isotherms based on the material’s phase transition temperature. The effects of the variable speed coefficient and variable speed range of galvanometer scanning on the uniformity of the phase transformation hardened layer are investigated, and a quenching experiment is performed using the equal power, equal quenching area, and variable scanning mode scheme.
When the scanning frequency of the galvanometer is above 333 Hz, the surface temperature field distribution and cyclic heating process after repeated scanning by a small spot are essentially the same as those after static heating by an equivalent rectangular large spot (Figs. 6 and 7). Two types of laser quenching modes have equivalent temperature field characteristics. The light field distribution and hardened layer distribution after galvanometer scanning with different variable speed coefficients are simulated (Figs. 8 and 9), and the light field distribution and hardened layer distribution after galvanometer scanning with different variable speed ranges are simulated (Figs. 10 and 11). The variable-speed coefficient or variable-speed area in the galvanometer variable speed scanning is increased, allowing more heat flow into the two-stage quenching area, compensating for the loss of heat transfer in the lateral direction, and improving the hardened layer uniformity. A laser quenching experiment with the same quenching parameters as the simulation ones is carried out using the developed system device. Taking the position where the depth of the hardened layer is reduced to 90% of the maximum value as the boundary of the homogenization area, the quenching area width is 10 mm, and the homogenization area width after galvanometer scanning at constant speed is 3.6 mm. When the variable-speed area at both ends is 2.5 mm wide and the variable-speed coefficient is 0.8, the homogenization area width in the hardened layer is 5.1 mm, which is approximately 42% higher than the homogenization area width (Fig. 13).
In the present study, a laser quenching optical system based on variable speed scanning of a galvanometer is proposed. The system consists of a QBH (quartz block head), a collimating and focusing integrated mirror, a single-axis galvanometer, and a galvanometer variable-speed scanning control system. The "saddle" shape light field with low energy in the middle and high energy at both ends is realized by setting the widths of the galvanometer variable-speed scanning areas at both ends and the width of the galvanometer constant speed scanning area in the middle, as well as the variable-speed coefficient. The laser quenching process based on galvanometer variable-speed scanning is studied, and the equivalent thermal light field model based on repeated variable-speed scanning is established. The simulation is used to examine the effects of the variable-speed coefficient and galvanometer variable-speed scanning area on the morphology of the hardened layer, and the laser quenching test based on repeated variable speed scanning is performed through the system. The results show that increasing the variable-speed coefficient and galvanometer variable-speed scanning area can improve hardened layer uniformity, which can be used to guide high-quality quenching of equipment parts under heavy load conditions.
