Objective Although metal surfaces can be effectively improved by laser cladding, the cladding process is affected by many factors. Thus, the research limited to a single experiment on this topic is inefficient and wastes resources. A combination of computer simulation and experimentation can greatly reduce the research period and improve the study efficiency. Numerous studies in computer simulation have provided a strong reference. However, the research on the thermal stress and thermal cycle in laser cladding is still rare. Here, a plane continuous heat source model and the COMSOL Multiphysics software are used to conduct a numerical simulation of the single-channel laser cladding process of H13 steels. The thermal stress and thermal cycle curves are drawn and analyzed to study the influence of the thermal stress cycle on the cladding layer under the optimal process parameters, and the laser cladding experiments are conducted to verify these simulation results.
Methods Using a plane heat source, the numerical simulation on laser cladding of H13 steels with Ni-based alloy powder was conducted using COMSOL. The simulation data were determined according to the results from previous researches, and the simulation scheme of the thermal stress cycle including the melting temperature and the influence of the parameters was determined on the basis of the substrate and powder process. A curve was then drawn, and the results were analyzed. The proposed simulation scheme was selected for the laser cladding experiment to verify the accuracy of the simulation model, in which various dimensions of the cladding layer were measured. A horizontal screenshot of the cladding layer was then compared with the simulation results to verify the accuracy of the simulation model.
Results and Discussions The optimal simulation scheme is determined and verified by experiments. According to the melting temperature requirements of the substrate and powder process and the influence of the parameters on the thermal stress, the laser power and scanning speed are set as 1200 W and 2 mm/s, respectively, for the simulation scheme of thermal stress cycle. The simulation scheme proposed here is selected for the laser cladding experiment to verify the accuracy of the simulation model. The cross-section of the cladding layer compared with the simulation results reveals essentially the same morphology, which verifies the accuracy of the simulation model. The thermal stress and thermal cycle are analyzed by drawing these parameter curves. The maximum temperature at various points in the vertical direction decreases with the increase of cladding depth. The top of the cladding layer shows the highest temperature of 2748.1 ℃, the heating rate of about 1632.1 ℃/s, the cooling rate of 699.5 ℃/s, and the matrix melting temperature of 1300 ℃. The maximum temperatures of sample points 6 and 7 are higher than the substrate melting temperature, and the highest temperature at sample point 8 is 1180 ℃ (Fig. 7). Therefore, the junction between the cladding layer and the substrate is located between sample points 7 and 8, which is consistent with the thermal cycle curve. The distance between the two sample points is 0.2 mm, and the depth of the molten pool is 0.2--0.4 mm. The shape of the molten pool can be determined according to the peak point of the thermal stress cycle curve. Sample points 8 and 9 in Fig. 8 do not show two obvious peaks. The lower side of the junction between the cladding layer and the matrix is located between sample points 7 and 8, which is consistent with the evaluation results of the thermal cycle curve (Fig. 8). In the von Mises thermal stress cycle, unstable alternating thermal stresses are identified at each sample point. All begin at 18.5 s and end at 20 s. Lots of unstable alternating thermal stresses at sample points 1--4 occur twice in concentration, with a steady increase in thermal stress occurring among them. The occurrence approaches each other gradually as the depth of cladding layer increases and joins together at sample point 5. As the depth of cladding layer increases, the variation amplitude of the alternating thermal stress first increases and then decreases, with the maximum stress amplitude of 45.5 MPa.
Conclusions The optimal processing parameters are laser power and scanning speed of 1200 W and 2 mm/s, respectively. Laser cladding is conducted under the parameters such as the maximum temperature of about 2748.1 ℃, the depth of 0.28 mm for the molten pool, the maximum heating rate of 1632.1 ℃/s, and the maximum cooling rate of 699.5 ℃/s. The cross-section information of the molten pool is roughly consistent with the simulation result, which verifies the accuracy of the model. The laser power and scanning speed are proportional to the thermal stress at the sample point, and the thermal stress increases with the increase of laser power and scanning speed. Because laser cladding involves a solid-liquid transition, the thermal stress curves of most of the sample points show two peaks. When the sample is outside of the molten pool, the powder at the sample point does not melt, and the von Mises thermal stress curve of the sample does not show two obvious peaks. The duration of the unstable alternating thermal stress differs slightly at each sample point. With the increase of the cladding layer depth, the amplitude of the alternating thermal stress first increases and then decreases, and its duration increases continually. The unstable alternating thermal stresses at most sample points occur twice with the same beginning and end points that join together when the cladding layer reaches a certain depth.
