Objective During the laser cladding process, the heat input of the laser heat source is not uniform, which will cause an uneven temperature field. The residual stress produced by the uneven temperature field will increase the crack tendency and affect the size, structure, and performance of the cladding layer, thereby considerably reducing the quality of the specimen. The cladding scanning method will affect the heat conduction and accumulation, thus affecting the stress and deformation of the work piece. Furthermore, the heat accumulation of the laser heat source considerably influences the morphology of the solidified structure of the cladding layer. An improved scanning method can produce a small heat-affected zone, stress, deformation and a fine and more uniform microstructure when using the same process parameters of laser cladding, thereby improving the manufacturability of laser cladding. The internal temperature change of the molten pool is difficult to measure using the existing test methods. Using numerical simulations to study the temperature and stress fields in the laser cladding process can greatly reduce the experimental cost and time. The temperature and stress field simulation results can be used to explain the influence of different scanning strategies on the characteristics of the cladding solidification structure and the distribution of residual stress.

Methods AISI304 stainless steel and 316L stainless steel powder are used in this study. The 316L stainless steel coating is fabricated on the surface of AISI304 stainless steel via coaxial powder feeding laser cladding using codirectional and reciprocating scanning methods. Then, the cladding layer is sampled for metallographic analysis. The metallographic corrosion solution is the FeCl₃ hydrochloric acid ethanol solution. Furthermore, the temperature and stress fields in the laser cladding process under different scanning strategies are calculated using the finite element simulation software ABAQUS. The influence of scanning strategies on the temperature and stress field distributions is studied. Finally, to verify the simulation results, the residual stress in X and Y directions of the cladding layer is analyzed using an iXRD residual stress analyzer.

Results and Discussions Heat accumulates in the laser cladding process. The peak temperature of the cladding layer under the reciprocating scanning path is considerably higher than that under the codirectional scanning path, and the heat accumulation of the cladding layer under the reciprocating scanning path is more severe (Fig. 8). As the cladding of the first two layers preheats the cladding of the third layer, the temperature gradient and cooling rate in the bonding zone of the second and third layers are less than those in the bonding zone between the bottom cladding layer and the matrix (Fig. 10); hence, the grain size of the third cladding layer is larger than that of the first cladding layer (Fig. 9). Compared with reciprocating scanning, the grain size of the cladding layer under codirectional scanning is smaller and the microstructure is more uniform (Fig. 9). The Mises stress in the reciprocating scanning path is greater than that in the codirectional scanning path, and the maximum residual stress is located at the junction between the cladding layer and the matrix at the beginning of the heat source scanning (Fig. 13). The distribution law of residual stresses σ_x and σ_y of the cladding layer under the codirectional and reciprocating scanning paths is similar. The tensile stress of the work piece is located on the substrate adjacent to the cladding layer, and the compressive stress is located on the substrate far away from the molten pool. The distribution position of the tensile and compressive stresses is relatively vertical (Figs. 14 and 15).

Conclusions In this study, the 316L stainless steel powder is cladded on the surface of AISI304 stainless steel using laser cladding technology. The microstructure and residual stress of the cladding layer are studied, and a finite element simulation is performed. The codirectional and reciprocating scanning methods are used for multi-layer and multi-pass cladding. Based on the cross-section morphology of the 316L laser cladding layer, the composite heat source model is established using the finite element simulation software ABAQUS, and the temperature and stress fields are calculated. The test results of the residual stresses σ_x and σ_y of the cladding layer show good agreement with the simulation results, thus verifying the reliability of the finite element models. The temperature field simulation results show that a larger amount of heat is accumulated using the reciprocating scanning method than using the codirectional scanning method. By combining the temperature field results and microstructure morphological results, the grain size of the cladding layer using the codirectional scanning method is more even than that using the reciprocating scanning method. The stress distribution of the cladding layer and the stress mechanism in the reciprocating scanning method is better than those in the codirectional scanning method based on the temperature field results. The tensile stress of the residual stresses σ_x and σ_y on the work piece is located on the cladding layer and the substrate close to the cladding layer. The compressive stress is located on the substrate with a vertical distribution relative to the tensile stress.