*3.2. Stress Field Analysis*

Thermal stress is the main reason for cracks in the process of laser cladding, both laser power and scanning speed will affect the thermal stress value of the cladding layer. In order to further determine the numerical simulation parameter scheme, the thermal stress cloud images of the same position under six schemes are obtained through simulation (shown in Figure 6). The maximum thermal stress of the six schemes is listed in Table 3, it can be clearly observed that when the laser power is 1200 W and the scanning speed is 2 mm/s, the corresponding thermal stress value is the minimum. Therefore, this scheme is proposed as a numerical simulation scheme to study the thermal stress cycle.

**Figure 6.** *Cont*.

**Figure 6.** Von Mises thermal stress distribution at the same location under six schemes.


**Table 3.** Maximum thermal stress of von Mises.

Taking the optimal simulation scheme, the thermal stress and thermal cycle curves are drawn and analyzed as shown in Figure 7. In the figure, the sample points are heated rapidly by laser at 16 s and reach the maximum temperature at 17.5 s. In the vertical direction, the maximum temperature of all points decreases with the increase of the cladding layer depth. The melting point of powder is 1450 ◦C, while the highest temperature of sample points 1–5 is higher than the melting point of powder, so the powder can be completely melted. The melting point of the substrate is 1300 ◦C, while the maximum temperature of sample points 6 and 7 is higher than that of the substrate. The maximum temperature of sample point 8 is 1180 ◦C. Therefore, the junction between the cladding layer and the substrate is located between sample points 7 and 8, sample points 8~9 are the heat-affected zone of the cladding layer. The spacing between sample points is 0.2 mm, the depth of the molten pool is approximately 0.2~0.4 mm.

**Figure 7.** Thermal cycle curve.

Figure 8 shows the von mises thermal stress cycle, thermal stress cycle curves of most sample points have two peaks. When the laser beam is close to the sample point, the thermal expansion of the material around the molten pool exerts pressure on the sample point, so that the first peak of thermal stress occurs at the sample point, at which point the thermal stress of most sample points reaches its maximum value. When the laser beam is located directly above the sample point, the material at the sample point is melted, and the stress at the sample point rapidly decreases to the bottom of the valley. When the laser beam is far away from the sample point, the molten pool starts to solidify as the temperature drops and the stress increases gradually, then the second thermal stress peak appears at the sample point. As the temperature gradually decreases to room temperature, the thermal stress gradually decreases and finally tends to a stable value, namely the residual stress of the sample point.

**Figure 8.** Von Mises thermal stress cycle.

In the vertical direction, with the increase of the depth cladding layer, the thermal stress at the two peak points gradually decreases, the thermal stress at the bottom of the cladding layer gradually increases, but the residual stress at each sample point tends to be the same value. The maximum thermal stress in the cladding process is 996.67 MPa, and the residual stress tends to be 210 MPa. The von Mises thermal stress curve does not have two distinct peaks when the sample point is outside the molten pool, because the material at the sample point did not melt. The shape of molten pool can be judged according to the thermal stress cycle curve. Sample points 8 and 9 in Figure 8 have no two obvious peak points. 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 judgment of the thermal cycle curve.

Based on the von Mises thermal stress cycle, it was found that there were unstable alternating thermal stresses at each sample point. The characteristics of unstable alternating thermal stress are the unstable alternating thermal stress starts and ends at the same time, starting from 18.5 s and ending in 20 s, the unstable alternating thermal stress at sample points 1–4 will occur twice intensively, it is composed of multiple unstable alternating thermal stresses, there is a stable increase of thermal stress between the two times. With the increase of the cladding layer depth, the two unstable alternating thermal stresses gradually approach and connect together at sample point 5. As the cladding layer depth increases, the variation amplitude of alternating thermal stress increases first and then decreases, the maximum stress amplitude is 45.5 mpa.
