Fig. 3 displays the temperature field nephogram at 1.5 s of laser scanning under two process parameters, where the zone with the temperature higher than 1,460 oC (melting point of TiAl alloy) in the Fig. 3 is molten pool zone. It can be seen from the Fig. 3 that in comparison with the uniform rectangular spot, the isothermal zone cladded using convex shape spot is large, in other words, the temperature gradient of cladding zone and non-cladding zone is small.
The cross-sectional temperature field nephogram at the maximum temperature point at 1.5 s is shown in Fig. 4, and the isothermal line presents crescent shape. The substrate melting depth and interfacial metallurgical bonding breadth can be judged according to the 1,460 oC isothermal line. When the uniform rectangular spot is used, the interfacial metallurgical bonding breadth and substrate melting depth are 3.04 mm and 134 μm, respectively, and those when the convex shape spot is used are 3.03 mm and 145 μm, respectively, so they achieve equivalent cladding effect on the whole. It can be seen by comparing the two figures that the maximum temperatures achieved by using the uniform rectangular spot and convex shape spot are 1,906 oC and 1,820 oC, where the latter is slightly lower than the former, but as the convex shape spot is relatively larger, the laser irradiation time is longer, so is the existence time of molten pool, and thus more heat quantity can be transferred towards the depth direction. Moreover, the temperature gradient is small in the depth direction, the practical cladding effects of the two process parameters are equivalent, and only that the ratio of substrate melting depth to interfacial metallurgical bonding breadth is relatively higher when the convex shape spot is used in the cladding process.
The temperature cyclic curve at the midpoint of center line on upper surface of the specimen is shown in Fig. 5. It can be seen that when the uniform rectangular spot is used for cladding, the temperature at the spot is slowly rising before the laser beam scans to this spot; when the laser beam scans to this point, the temperature will be rapidly increased to high temperature, and then rapidly cooled as the laser leaves, so rapid heating and rapid cooling characteristics, which are typical in laser machining, are presented. During the cladding process using convex shape spot, the temperature cyclic curve at this point is similar to the laser spot shape, namely convex shape, and at the front end is equal to local preheating of the specimen at 400 oC. However, influenced by the laser heat action, the temperature is relatively high in the cooling phase, presenting incompletely symmetric distribution. In general, when the convex shape spot is used, the temperature gradient is small no matter at front end or rear end of laser cladding, obvious preheating and slow cooling features are manifested, and this can relieve the adverse effect of fast heating and fast cooling in laser cladding on coating stress to some extent.
The early-stage research indicates that the tensile stress of the laser cladding specimen is the maximum along the laser scanning direction (transverse) [53], and high tensile stress is closely related to the crack formation at clad layer. Therefore, the influences of only the two spots on transverse stress in the cladding process will be discussed in this paper. Fig. 6 shows the transverse stress cyclic curve at the midpoint of the laser scanning center line on the upper surface of the specimen. It can be observed that before and after the molten pool is formed, the compressive stresses are equivalent under the two process conditions, but in terms of the tensile stress playing a significant role in the crack formation at clad layer, the tensile stress formed by using the convex shape spot is obviously lower than that using the uniform rectangular spot in the subsequent cooling process. According to the calculation results, as the cooling proceeds till 300 s, the tensile stress (may be considered as residual stress) of the uniform rectangular spot is 397.66 MPa, while that of the convex shape spot is 355.83 MPa, which is reduced by 10.5% comparatively.
The transverse stress distribution on the laser scanning center line on the upper surface of the specimen at 300 s is presented in Fig. 7. It can be clearly seen that the residual stress distributions under the two process conditions are similar: The transverse stress on the center line of the whole workpiece is tensile stress, the stress is rapidly increased at the initial end of laser scanning, and this very high stress level is kept in the central area until the end point of laser scanning. However, the residual stress of the convex shape spot is lower than that of the uniform rectangular spot, averagely by about 40 MPa in the central area. Through the previous numerical simulation, it is theoretically proved that the convex shape spot is effective for controlling the crack generation in the laser cladding process.