Reduce the surface roughness of laser cladding coating. Iron-based laser cladding coating was prepared by laser cladding technology, and the cladding layer was strengthened after cladding by ultrasonic warm rolling coupling heat treatment process. The influence of temperature field parameters on the roughness of the formed surface was studied in detail. The significance of parameters was established by analysis of variance (ANOVA). At the same time, the response surface method (RSM) was used to construct a prediction model for the influence of temperature field parameters on the surface roughness of iron-based coatings, and the parameters were optimized. The heating temperature and holding time have a significant effect on the surface roughness of the formed specimens. Within the experimental parameter range, the surface roughness of the specimens is positively correlated with the heating temperature and negatively correlated with the holding time. The experimental results show that under the same holding time, the surface roughness Ra of the specimens at heating temperatures of 100, 250, and 400 ℃ are 0.237, 0.158, and 0.096 μm, respectively; at the same heating temperature, the surface roughness Ra of the specimens at holding times of 0.5, 1, and 2 h are 0.156, 0.164, and 0.170 μm, respectively. It can be seen that compared with the holding time, the heating temperature has a more significant effect on the surface roughness of the coating. The results of parameter optimization analysis show that within the experimental parameter range, under the conditions of 400 ℃ heating temperature and 0.5 h holding time, the sample has the minimum surface roughness Ra (0.089 μm). Conclusion Compared with turning and normal temperature rolling process, the ultrasonic warm rolling coupling heat treatment process can further reduce the surface roughness of the laser cladding coating. Within the experimental parameter range, the heating temperature of 400 ℃ and the holding time of 0.5 h are the optimal temperature field parameter combination.

As one of the important technical means to achieve sustainable manufacturing, additive manufacturing (AM) technology has received increasing attention in the industry and academia. Compared with traditional processing technology, additive manufacturing technology can directly prepare parts with complex shapes and internal structures, saving processing costs and reducing resource loss [2]. At the same time, it can also be used to repair damaged equipment, realize the recycling and reuse of waste parts, and promote sustainable development [3]. For some high-strength and difficult-to-process materials such as titanium-based, nickel-based and cobalt-based alloys, it can achieve rapid and accurate forming of functional parts, reduce processing difficulty and improve production efficiency[4-5]. After years of basic research, additive manufacturing technology is gradually moving from theoretical exploration to engineering practice[6-7]. For example, in the mining field, laser cladding technology can be used to prepare wear-resistant/corrosion-resistant high-performance alloy cladding layers on the surface of key parts, thereby replacing integral alloy parts, realizing green manufacturing and remanufacturing of high-performance mining parts, saving resources and reducing costs[8]. However, the surface of parts directly formed by additive manufacturing is relatively rough, and defects are prone to occur inside the cladding layer, which reduces the service performance of the parts[9]. Therefore, additive manufacturing parts often need to undergo subsequent mechanical processing to meet the stringent engineering application requirements.

At present, the commonly used post-cladding mechanical processing processes for additive manufacturing parts are mainly traditional turning, milling, grinding, polishing, etc. These traditional processing methods are prone to leave processing marks on the processed surface, and even produce microcracks, etc.[10]. Although heat treatment, laser remelting and other technologies can improve the local microstructure of additively manufactured parts [11-12], it is still difficult to solve the problem of part dimensional accuracy. Ultrasonic rolling technology can effectively improve the geometric accuracy and surface quality of additively manufactured parts [6,13]. Due to the difficult processing characteristics and harsh working conditions of additively manufactured materials [3], the surface integrity improved by ultrasonic rolling is still difficult to meet the high-precision requirements of equipment in service under extreme environments. For the erosion and corrosion environment in the mining industry, a low level of surface roughness means that the parts have stronger corrosion resistance [14]. Excessive roughness can easily lead to stress concentration and promote the initiation of cracks under fatigue conditions [15]. At the same time, high surface roughness means a high contact area between mating surfaces, so it is easy to have high wear [16].

Amanov [17]’s research shows that through the synergistic effect of heat treatment technology and ultrasonic nanocrystalline surface modification technology (Ultrasonic nanocrystalline surface modification, UNSM), additively manufactured parts can be effectively strengthened to obtain a smooth surface. Shen et al. [18] proposed two post-processing methods for additive parts: Ultrasonic warm burnishing (UWB) and Ultrasonic warm burnishing coupled with sequent heat treatment (UWB/HT), and focused on studying the effects of the two processes on the performance of additively manufactured iron-based coatings. It was found that the UWB/HT process has the best coating surface roughness improvement effect. However, in the UWB/HT process, the effect of temperature field parameters on the coating surface roughness is still unclear. Therefore, this paper focuses on exploring the effects of heating temperature and holding time on iron-based coatings during the UWB/HT process, obtains the optimal UWB/HT process, and establishes a prediction model for the effect of temperature field parameters on the roughness of iron-based coatings.

1 Experiment

A 45 steel cylinder with a diameter of 30 mm was selected as the substrate, and an iron-based self-fluxing alloy powder was used as the coating material. The chemical element composition of the alloy powder is shown in Table 1. The LYS-1000 fiber laser generator shown in Figure 1 was used for coaxial powder feeding laser cladding processing to prepare the iron-based cladding layer. The selected cladding process parameters were: laser power of 4 kW, spot diameter of 2.5 mm; scanning speed of 94.2 mm/s, step distance of 0.7 mm; flow rate of protective gas (N2) of 15 L/min, and powder feeding rate of 24 g/min. A two-factor three-level full factorial experiment was designed, and the cladding samples were subjected to UWB/HT treatment after cladding. The selected temperature field parameters are shown in Table 2, and the experimental parameters are shown in Table 3. The working principle of the ultrasonic rolling processing device is shown in Figure 2. The ultrasonic power supply generates a sinusoidal alternating current signal of ultrasonic frequency; the ultrasonic vibrator converts the electrical signal into a mechanical vibration signal of the same frequency, and amplifies the amplitude through the amplitude transformer; the ball-type tool head is in direct contact with the coating surface and pressed tightly; during the processing, the ultrasonic vibrator directly impacts the rolling tool head with high frequency, and indirectly acts this high-frequency mechanical impact on the coating surface; under the combined action of static load and high-frequency impact load, the coating surface and near-surface materials undergo plastic deformation, thereby obtaining a surface strengthening effect. The UWB/HT process specifically refers to a composite process that uses a temperature field to assist ultrasonic rolling, quickly heats the sample to a certain temperature, and performs ultrasonic rolling strengthening under this temperature condition. After rolling is completed, the sample is kept at this temperature for a certain time. During the experiment, a 1 kW halogen lamp was used for heating, and an infrared thermal imager (FlirA315) was used to monitor the temperature of the cladding layer in real time. After the experiment, the sample was cut into blocks by electric sparks, then ground and polished, and ultrasonically cleaned in anhydrous ethanol for 10 min. The roughness and surface texture morphology of different cladding samples were measured by white light interferometer (Contour Elite K).

2 Results and discussion

2.1 Surface roughness

The surface morphology of the samples after turning, room temperature ultrasonic rolling, and ultrasonic warm burning (UWB, 400 ℃, no post-rolling heat preservation process) is shown in Figure 3. As can be seen from Figure 3a, after turning, there are obvious cutting marks on the surface of the sample, the surface peaks and troughs are obvious, and the surface roughness Ra is 0.586 μm. As can be seen from Figure 3b, after room temperature rolling, the surface of the sample is smooth, the turning marks are significantly reduced, and the surface roughness Ra is reduced to 0.350 μm. Due to the disadvantages of high hardness and difficult deformation of the cladding material, there are still problems such as processing marks and pores on the surface of the sample. As shown in Figure 3c, after UWB treatment, the surface turning marks of the sample disappeared, the number of micropores was significantly reduced, and the surface roughness of the sample was further reduced. Under the synergistic effect of warm plasticity and acoustic plasticity, the deformation resistance of the processed material was reduced, the plastic fluidity of the surface and near-surface materials was enhanced, the finishing effect was improved, and the roughness was reduced to 0.169 μm. The results of previous studies also showed that the UWB process is prone to cause obvious waviness on the cladding surface [19], and there is a large height difference between the surface crest and the trough, as shown in Figure 4. At the same time, this surface waviness is not generated during the rolling process, but is attributed to the rapid cooling of the material after rolling and the uneven release of internal stress. During the ultrasonic rolling process, external high-frequency vibration excitation will form high dynamic stress waves inside the processed material, which is particularly obvious during the heating process [20]. It can be seen that when using the UWB process, the rapid cooling of the material will cause the internal stress to be unable to fully relax, thereby forming surface waviness.

The results of previous studies show that compared with the UWB process, the UWB/HT process is helpful for stress relaxation and elastic recovery of the sample after rolling treatment, thus helping to eliminate the waviness of the formed surface. Based on the previous results, the effects of heating temperature and holding time on the roughness of the formed surface in the UWB/HT process were experimentally studied. The surface roughness measurement results of 6 samples under different temperature field parameters are shown in Figure 5, and the three-dimensional morphology of each sample is shown in Figure 6. It can be seen that under the same holding time, with the increase of heating temperature, the coating surface gradually becomes smoother and the roughness gradually decreases after the UWB/HT process, which is attributed to the combined effect of the temperature plasticity and ultrasonic plasticity of the metal material [22-23]. Ultrasonic rolling forces the surface of the metal material to undergo plastic flow, thereby achieving the effect of “cutting peaks and filling valleys”, reducing the height difference between peaks and valleys, and realizing finishing processing [24]. At the same time, under certain temperature conditions, the ductility of metal materials increases, the deformation resistance decreases, the surface material and the near-surface material are more likely to undergo plastic flow, resulting in greater plastic deformation (Severe plastic deformation, SPD), and obtaining better finishing strengthening effect[20]. When the process temperature was increased from 100 ℃ to 250 and 400 ℃, the surface roughness of the coating treated by UWB/HT process decreased by 35.6% and 61.8% respectively at a holding time of 0.5 h; when the holding time was 1 h, the surface roughness of the coating treated by 250 ℃ and 400 ℃ decreased by 34.3% and 59.4% respectively compared with the coating treated by UWB/HT process at 100 ℃; when the holding time was 2 h, the roughness of the sample treated by 250 ℃ and 400 ℃ decreased by 30.4% and 57.1% respectively compared with the sample treated by UWB/HT process at 100 ℃. It is worth noting that at the same heating temperature, the surface roughness of the treated coating increased with the increase of holding time. At 100 °C, when the holding time increased from 0.5 h to 1 and 2 h, the surface roughness of the treated coating increased from 0.233 μm to 0.239 μm and 0.240 μm, respectively, with no significant change; at 250 °C and 400 °C, the surface roughness of the coating sample increased slightly with the increase of holding time. The deformation recovery of the surface material caused by post-UWB/HT heat treatment (holding stage) is the main reason for the increase of coating surface roughness [25-26]. After the sample is treated with SPD, the material is in an unstable state due to the high energy inside, and has a tendency to spontaneously recover to its original state. Under heating conditions, the surface activation energy of the material is enhanced, resulting in deformation recovery and even recrystallization [27-28]. In the UWB process, the holding time is short, and appropriate stress release and elastic recovery can smooth the surface [19]. During the UWB/HT process, as the holding time increases, the material deforms and recovers, resulting in increased surface roughness.

2.2 Temperature field parameters affect roughness prediction model

In the UWB/HT process, the full factorial experimental results of the influence of heating temperature and holding time on the surface roughness of additively manufactured iron-based coatings are shown in Table 4. Among them, K is the sum of all corrosion rates at the same level of processing parameters, Q represents the average of the sum of squares of all K of each processing parameter, S is the sum of squares of deviations, and T is the sum of all roughness. The variance analysis method (ANOVA) is used to accurately verify the influence of temperature field parameters on the surface roughness of the coating, and the variance analysis results are shown in Table 5. Among them, P is a significant factor. When P is less than 0.5, it means that the factor is statistically significant. It can be seen that both heating temperature and holding time have a significant effect on the surface roughness of additively manufactured iron-based coatings, among which the influence of heating temperature on surface roughness is much greater than that of holding time. Based on the second-order polynomial function of formula (1), with heating temperature and holding time as input parameters and surface roughness as output response, a prediction model for the influence of temperature field parameters on the surface roughness of additively manufactured iron-based coatings in the UWB/HT process is established, as shown in Figure 7. The predicted value of surface roughness is in good agreement with the experimental value, indicating that the model is more accurate in predicting surface roughness. Based on this model, a response surface between temperature field parameters and roughness is constructed, and the variation trend of roughness and temperature field parameters is plotted with parameter level as the horizontal coordinate and roughness average value as the vertical coordinate, as shown in Figure 8. It can be seen that for the UWB/HT process, within the range of warm rolling, the surface roughness of the additively manufactured iron-based cladding layer decreases with the increase of heating temperature and increases with the increase of holding time. Among them, for the three selected holding times, the UWB/HT process has the best surface roughness improvement effect under the conditions of heating temperature of 400 °C and holding time of 0.5 h.

3 Conclusions

The influence of temperature field parameters on the surface roughness of iron-based laser cladding coatings in ultrasonic warm rolling coupled heat treatment process was studied, and a prediction model of the influence of temperature field parameters on surface roughness was constructed. The full factorial experimental results show that the surface roughness of the coating is negatively correlated with the heating temperature and positively correlated with the holding time. With the increase of temperature, the deformation resistance of the material gradually decreases, the ductility of the sample is improved, and the fluidity of the material is enhanced, which can form a deeper plastic deformation, thereby obtaining a smaller forming surface roughness. The results of variance analysis show that both the heating temperature and the holding time will significantly affect the surface roughness of the coating, and the influence of the heating temperature is more significant.