As an advanced surface modification technology, laser cladding technology has the advantages of low coating dilution rate, good bonding between cladding layer and substrate, and fast cooling speed. However, during the rapid heating and cooling process, the coating and substrate often produce high residual stress, resulting in deformation, coating shedding and cracks. This paper first analyzes the mechanism of residual stress generation in laser cladding, and then reviews the numerical simulation research on residual stress in laser cladding, including model establishment, distribution law of residual stress in single-pass and multi-pass cladding, influence of process parameters on residual stress and result verification, and introduces the residual stress regulation method. Finally, the shortcomings and development direction of numerical simulation research on residual stress in laser cladding are summarized.
Laser cladding technology is an advanced surface modification technology for metal materials. It adds special cladding materials to the surface of the substrate, uses a high-energy-density laser beam to make it melt and solidify quickly, and forms a good metallurgical bonding state with the substrate. By using this technology, a surface coating with superior performance can be prepared on the surface of a low-cost substrate, thereby significantly improving the physical, chemical and mechanical properties of the substrate surface. However, due to the fast heating and cooling speeds of the whole process, there is a high temperature gradient in the heated area, so it is very easy to generate a complex residual stress field. A large number of studies have shown that the initiation and expansion of cracks in laser cladding coatings are closely related to the stress generated during the cladding process. The cracks generated by laser cladding coatings not only affect the surface quality of the material, but also directly lead to the failure of the parts. The detection of residual stress of components by experimental methods will not only damage the components, but also its process is complicated, and it is difficult to grasp the evolution of residual stress with time. Therefore, the use of numerical simulation methods to simulate the cladding process can not only save cost and time, but also grasp the evolution of residual stress with time in the process, and provide a reference for predicting the evolution law of residual stress in the cladding process. This paper reviews the research progress in the numerical simulation of residual stress in laser cladding in recent years, and briefly analyzes the problems existing therein.
2 Mechanism of residual stress generation in laser cladding
Residual stress refers to the internal stress that remains inside the object to balance its own structure after the external force or uneven temperature load is removed. During the laser cladding process, due to the rapid effect of high energy density, the material absorbs high-density energy in the laser irradiation area and melts at a temperature much higher than other areas in a short time to form a molten pool. When the laser beam leaves, the molten pool quickly solidifies to form a cladding layer. Whether it is formed by melting under heat or forming a cladding layer by self-excited cooling, a large temperature gradient will be generated in the heated area, and the material is heated extremely unevenly. The uneven plastic deformation of the material due to thermal expansion is the main reason for the complex residual stress inside the laser cladding. Secondly, due to the phase change transformation of the material such as solid-liquid phase change during the laser cladding process, the series of effects related to phase change also have an important influence on the residual stress of laser cladding [6]. In general, the thermal physical properties of the cladding material and the substrate determine the temperature gradient of the heating process, thereby generating uneven thermal expansion, and the magnitude of the residual stress generated depends on the stress-strain relationship (i.e., yield stress and stiffness) and its change with temperature.
During the cladding process, the temperature of the laser irradiation area rises rapidly, the yield strength of the cladding material and the substrate affected by heat decreases, and expansion occurs. When the temperature exceeds the melting point, a molten pool is formed. Figure 1 shows the stress of the material in the heating and cooling stages of laser cladding. The expansion of the molten pool material is constrained by the material in the low temperature zone, so compressive stress is formed; when the molten pool solidifies, the volume of the molten pool material shrinks, and the shrinkage behavior is constrained by the internal formation of the material at the edge of the molten pool, and the molten pool eventually forms residual tensile stress.
Laser cladding is a dynamic metallurgical process involving the interaction between laser, powder and substrate, the interaction of the formed physical fields, and directly affects the quality and performance of the cladding layer. The interaction relationship of the generated physical fields is shown in Figure 2. It can be seen that the residual stress of laser cladding mainly consists of two parts. The first is the thermal stress caused by uneven heating of the material caused by temperature; secondly, since the material changes from liquid to solid, it will inevitably cause the transition between different phases of the material, thus causing phase change stress, and then generating tissue stress.
3 Numerical simulation of residual stress
In order to analyze the distribution of residual stress in laser cladding, it is necessary to model and analyze the thermal-elastic-plastic behavior of the cladding process. At present, the numerical modeling of this process mostly adopts the method of thermal-mechanical sequential coupling, that is, the temperature field of the cladding process is first analyzed, and then the results of the temperature field are applied as loads to the mechanical analysis module, thereby obtaining the elastic-plastic strain and stress of the component caused by the thermal effect.
3. 1 Model establishment
Due to the uneven temperature distribution and large temperature gradient in the laser cladding process, there are many nonlinear conditions, such as the mechanical properties parameters of the material changing with temperature, the elastic-plastic strain model and the yield criterion of the material, which bring difficulties to the numerical analysis of the stress field. The finite element simulation model of residual stress in laser cladding is divided into two parts, the first part is the temperature field modeling, and the second part is the mechanical analysis modeling. Due to the influence of temperature during the cladding process, the material produces elasticity and plasticity. When the yield strength is exceeded, permanent deformation, i.e., residual strain, will occur. Therefore, the modeling process uses the thermoelastic-plastic law [9], and its control equation is: ε = εe+εp+εth+εph+εex See formula (1)
where ε is the total strain, εe, εp, εth, εph, and εex are elastic strain, plastic strain, thermal strain, phase change strain, and external load strain, respectively.
Elastic strain obeys Hooke’s law. For the plastic strain theory, there are currently flow criteria, yield criteria, and hardening law. The von Mises yield criterion is the most commonly used plasticity theory. Few scholars have considered the influence of phase change strain on the total strain, and most scholars have not considered the influence of phase change strain when studying the residual stress of laser cladding. Strain depends on the characteristics of the material. Therefore, when modeling, in order to obtain more accurate stress values, material parameters should be fully considered, such as Young’s modulus, Poisson’s ratio, thermal expansion coefficient, and yield strength. Due to the large amount of residual stress calculation, some scholars did not consider the change of material properties with temperature. In order to obtain more accurate solution results, most researchers use temperature-related material parameters for solution.
3.2 Distribution law of residual stress in single-pass and multi-pass cladding
During the laser cladding process, due to the uneven heating of the material and the large changes in the material properties with temperature, the distribution of residual stress in the component is also relatively complex. In order to obtain the evolution law of stress in the laser cladding process, many scholars have conducted numerical analysis on the residual stress in the single-pass cladding process.
Li Jinhua et al. used COMSOL software to establish a numerical model of the H13 steel laser cladding Ni-based alloy powder process and analyzed the von Mises stress cycle curve. As the material changes with temperature, the stress in the molten pool has two obvious stress peaks, and it is shown that the stress peak point has an important influence on the depth of the molten pool. The stress cycle curve is shown in Figure 3. Gong Cheng et al. used ANSYS to analyze the residual stress distribution of the laser cladding 316L single-pass cladding layer. The results show that the von Mises equivalent stress of the single-pass cladding layer is mainly distributed at the beginning and end of the cladding layer, showing a dumbbell-shaped distribution. The three principal stresses were analyzed and it was found that the distribution of the three principal stresses was closely related to the temperature gradient of the cladding process. However, the laser cladding process is often accompanied by the influence of other factors on the residual stress. In the numerical study of ZHAO Hong-yun et al., it was shown that the three principal stress directions are from large to small, the largest along the scanning direction, the second perpendicular to the scanning direction, and the smallest along the substrate thickness direction, and the maximum stress is at the junction of the cladding layer and the substrate. GAO Z. N. et al. considered the influence of different cladding layer thickness on the residual stress of single-pass cladding and found that the stress distribution law under different thicknesses tends to be consistent, and the peak stress appears near the junction area, and the stress value gradually increases from the top to the bottom of the cladding layer. Zhang Tiangang et al. used ABAQUS software to establish a numerical model of laser cladding Ni-based coating on TC4 surface. In the simulation results, the single-pass cladding stress distribution law is consistent with the above scholars.
Although there have been many studies on the numerical analysis of residual stress of single-pass laser cladding coating, in actual production and manufacturing, multi-pass or even multi-pass multi-layer cladding is required in most cases. In this process, due to the effect of thermal cycles, the distribution law of residual stress is also different from the distribution law under single-pass cladding. VUNDRU C. et al. established a thermal-mechanical coupling numerical model of multi-layer laser cladding CPM9V powder H13 tool steel in ABAQUS and found that the peak amplitude of tensile residual stress in the multi-layer cladding area is lower than the peak residual stress in the single-layer cladding layer of the same height and hardness. ZHANG Qi et al. used the finite element analysis software ANSYS to simulate the stress field of multi-pass cladding layers of Fe-Mn-Si-Cr-Ni shape memory alloy coatings, using the enthalpy change of the material to compensate for the thermal changes of metal melting and solidification, and analyzed the distribution of the transverse residual stress in the center of the coating on the path, showing a tension-compression-tension law, as shown in Figure 4. Wu Ya et al. established a finite element model of unidirectional scanning and reciprocating scanning of AISI304 stainless steel surface laser cladding by thermal-mechanical coupling. The residual stress simulation results showed that due to heat accumulation, the residual stress in reciprocating scanning was greater than that in unidirectional scanning, and tensile stress appeared in the cladding layer and bonding area. CHEW Youxiang et al. also conducted a numerical simulation analysis of the stress in the multi-pass cladding coating, indicating that in the multi-pass cladding process, the second cladding layer can significantly reduce the residual stress of the previous cladding layer.
3.3 Effect of process parameters on residual stress
Laser cladding is a complex metallurgical additive manufacturing process. Its large melting and cooling rate is one of the main reasons for the residual stress inside the workpiece. The key influencing factor is energy input. Since there are many parameters in the laser cladding process, including laser power, scanning speed, laser spot diameter, powder flow rate and workpiece size, in order to describe the energy input, the specific energy calculation formula is introduced: E=P/DV See formula (2)
Where: E is the laser specific energy; P is the laser power; D is the spot diameter; V is the scanning speed.
GAO Z. N. et al. used numerical simulation methods to analyze the cracking mechanism of Fe-CoCrNi HEA coating prepared on TC4 titanium alloy and analyzed the influence of different laser powers and cladding speeds on stress. Their research shows that the influence of different processing parameters on the stress distribution law is basically the same, and the maximum stress value is concentrated at the junction of the cladding layer and the substrate, indicating that cracks are easy to start in this area and then expand. In the numerical simulation study by WANG C. et al., it was shown that when the laser power and scanning speed undergo the same changes, the effect of laser power on temperature and stress evolution is more significant than that of scanning speed. It can be seen that the laser cladding process parameters have little effect on the distribution of residual stress, but have a more serious impact on its size, and the power has a more serious effect on stress than the scanning speed and spot diameter.
Some scholars have studied the effect of processing parameters on the size of residual stress. The research of Deng Dewei et al. shows that when the laser power is within an appropriate range, the residual stress of the cladding layer increases with the increase of laser power, the decrease of scanning speed and the increase of the number of cladding layers, and it is shown that laser remelting and heat treatment processes can effectively reduce the residual stress of the cladding layer. When studying the residual stress of TC4/Inconel718 functional gradient materials in additive manufacturing, ZHAO H. J. et al. discussed the residual stress distribution under different laser powers. The results showed that at the edge of the cladding layer, the residual stress generated by different laser powers was not significantly different, while the residual stress difference at the center of the structure was relatively large. The reason is that the heat dissipation at the center of the structure is worse than that at the edge, and the accumulated heat cannot be dissipated, so the residual stress generated is relatively large.
During the research and engineering manufacturing related to laser cladding, the cladding materials, substrate materials, processing environment and measurement methods used in different scenes and different research contents are different, and the conclusions obtained are also different. Therefore, the influence of laser cladding parameters on the evolution law of residual stress in its process should also be analyzed according to the actual situation.
3. 4 Result verification
Due to the fact that the numerical simulation process uses many assumptions, including material assumptions, boundary condition assumptions and theoretical methods used in the calculation process, errors will be generated. Therefore, it is also extremely important to experimentally verify the numerical simulation results. For the simulation results of temperature field, the simulation pool temperature distribution is usually compared with the experimental cladding layer cross section for verification, while the verification of stress field simulation results is mostly based on experimental detection. SUÁREZ A. et al. used ANSYS software to establish a numerical model of residual stress of cobalt-based alloy clad on austenitic stainless steel AISI 304 substrate, and used synchrotron experimental equipment and EDXRD stress measurement technology to detect the residual stress of the sample. The numerical simulation results showed good consistency with the experimental results. FARAHMAND P. et al. used ANSYS software to establish a sequential thermal-mechanical coupling model of AISI H13 steel laser cladding. In multi-pass cladding, the last cladding layer has higher stress, while the previous cladding layer shows lower residual stress due to the effect of thermal cycle. In order to verify the correctness of the simulation results, XRD was used to test the residual stress of the single-pass cladding sample, and the experimental results were consistent with the simulation results (Figure 5).
4 Residual stress control method Since the residual stress generated by laser cladding will have many adverse effects on the components, proper control will effectively reduce the impact of residual stress and improve the quality of laser cladding manufacturing process. According to the thermal stress calculation formula generated in the uniform coating mentioned in the literature: σT=EcEsts(αc-αs)ΔT/(1-v)(Ests-Ectc) See formula (3) in the figure.
Where σT is the thermal residual stress in the coating, ΔT is the difference between the processing temperature and the stress measurement temperature (room temperature), α is the thermal expansion coefficient, E is the Young’s modulus, t is the thickness, v is the Poisson’s ratio, and the subscripts c and s refer to the coating and substrate respectively.
From the above formula, it can be seen that the residual stress of laser cladding has a great relationship with the material physical parameters and temperature gradient. In order to reduce the residual stress, the main methods currently include reasonable use of material ratio for gradient cladding, heat treatment before and after cladding, and reasonable combination of auxiliary processing methods. ZHAO H. J. et al. used gradient materials with different compositions to laser clad TC4/Inconel718 to achieve a gradual change from the substrate to the cladding layer material, thereby achieving a slow transition of the thermophysical parameters to adjust the residual stress during the cladding process. Waqar Saad et al. studied the effect of different preheating temperatures on the residual stress distribution of 316L stainless steel manufactured by selective laser melting. The results showed that preheating can significantly reduce the residual stress. When preheated at 400 °C, the residual stress was reduced from 353.57 MPa to 27 MPa. In addition, relevant scholars have also used new auxiliary processing technologies to reduce residual stress, such as dynamic magnetic field assisted laser cladding, electromagnetic compound field (ECF) assisted laser cladding, etc., which shows that the use of auxiliary processing technology can reduce the residual stress in the cladding process, thereby reducing crack sensitivity and improving the quality of the cladding layer.
5 Conclusion
This paper mainly discusses the generation mechanism of residual stress in laser cladding, the numerical simulation of residual stress, and the regulation method of residual stress. Laser cladding is a process of rapid heating and cooling. Local high energy input will inevitably lead to uneven heating of the workpiece, thereby generating large residual stress and having adverse effects on the components. Numerical simulation can monitor the stress distribution that is difficult to grasp in the cladding process in real time, and provide a reference for actual production. How to regulate it has always been the research direction of relevant scholars. The following suggestions are proposed:
(1) Numerical simulation methods can effectively grasp the stress evolution law that is difficult to determine experimentally, but the existing numerical simulation methods still have many shortcomings, such as imperfect boundary conditions, incomplete material parameter settings, and single physical field simulation. Therefore, establishing a more complete boundary condition, material property parameter system, and multi-physical field simulation is still a research hotspot.
(2) At present, there is still a lack of unified standards and unified theories for the numerical simulation of residual stress in laser cladding. A more complete numerical theoretical basis for residual stress in laser cladding should be established, and a more complete result evaluation system should be given.
(3) The regulation of residual stress is still a major difficulty at present. In order to regulate the stress of the construction process to prevent defects such as cracks and deformation, optimizing process parameters, preparing cladding materials that are more compatible with the substrate, and adopting more advanced auxiliary processing technologies are still the main means. In the process of numerical simulation, the use of multi-physical fields to study the influence of auxiliary processes on residual stress is still rare.
