Objective Laser cladding involves rapid heating and quenching processes. During rapid cooling, the temperature field distribution is uneven because the molten pool temperature suddenly drops, generating residual stress. Residual stress in the cladding layer directly affects the mechanical and physical properties of the cladding layer, leading to cracks and other defects. To reduce the manufacturing costs, the residual stress in the cladding layer is usually calculated in numerical simulations. However, most of the simulation studies focus on single-pass cladding; the influence of lap ratio on the residual stress under multipass cladding has been little investigated, and the relationship between lap ratio and residual stress has not been concluded. In actual production, multipass cladding is the norm, and the subsequent cladding-layer processing is also based on multipass overlapping cladding layers. To reduce the machining allowance and improve the cladding-layer quality in multipass cladding, we studied the factors influencing the residual stress in the cladding layer and the laws governing those influences in finite element simulations. After determining the residual stress distribution in the cladding layer for different lap ratios, the most suitable lap ratio for subsequent processing was determined.
Methods The matrix is 42CrMo steel and the powder is 3540Fe. Multipass cladding models with constant thickness (1 mm) and varying lap ratio (30%, 40%, 50%, 60%, and 70%) were established in Ansys software. The temperature-rise model of the laser cladding was based on the model of laser-melting temperature rise and powder absorptivity. The accuracy of the analytical model is verified by comparing with the simulated temperature model. The residual stress distributions in the cladding layers with different lap ratios were obtained by simulating the thermal-mechanical coupling in finite element software. The physical properties of the cladding layers were observed in corresponding experiments. The experiments confirmed the macro- and micro-morphologies of the cladding layers with different lap ratios and the physical properties of the cladding layers prepared at different lap rates. Finally, the most suitable lapping ratio of the cladding layer for subsequent processing was obtained.
Results and Discussions As demonstrated in the finite element simulation results (Fig. 4), the temperature of the cladding layer gradually increased with lap ratio increasing. The residual stress distributions in cladding layers with different lap ratios are displayed in Fig. 6. Increasing the lap ratio gradually decreased the residual stress in the cladding layer. In the experiments, increasing the lap ratio obviously refined the grain size of the cladding layer (Fig. 8). At lap ratios below 50%, the cladding layer was strongly bonded with the substrate, but at lap ratios exceeding 50%, the cladding layer presented obvious defects. Increasing the lap ratio gradually increased the microhardness of the cladding layer (Fig. 9), but nonlinearly affected the friction coefficient of the cladding layer (in particular, the friction coefficient decreased before increasing; see Fig. 10).
Conclusions The following conclusions were drawn from the study. Increasing the lap ratio gradually increased the temperature of the cladding layer, mainly because the substrate temperature was increased prior to the next cladding. This phenomenon is equivalent to preheating the cladding layer. Therefore, the temperature of the cladding layer (including its maximum) gradually increased with number of passes. Increasing the lap ratio also gradually reduced the minimum residual stress in the cladding layer, which appeared at approximately 0.2 mm below the top of the cladding layer. The residual stress in the cladding layer became gradually uniform, and the position of maximum residual stress gradually approached the direction of the matrix. As the lap ratio and temperature increased, the elements in the matrix floated toward the cladding layer and formed a hard phase in that layer. Accordingly, the cladding layer demonstrated a gradually increasing microhardness, and a friction coefficient that first increased and then decreased. Among the cladding layers manufactured at different lap rates, the cladding layer formed at the 50% lap rate was well bonded with the substrate, and demonstrated an obvious antiwear effect, moderate average residual stress, and relatively high cladding efficiency. Therefore, this sample was deemed most suitable for subsequent processing.
