The appropriate range of process parameters was selected to obtain a titanium alloy laser cladding layer with good performance in an open environment. The oxidation degree of the cladding layer was measured by oxygen content, and the variation of oxygen content under different surface color difference values ​​was studied. The surface color difference of the cladding layer was detected by a spectrophotometer with laser power, scanning speed, powder feeding rate and gas flow rate as control variables, and the surface color difference of the cladding layer was quantified by using them. The surface color difference was used as a response index, and a mathematical model between the surface color difference of the cladding layer and the cladding process parameters was established based on the response surface methodology. Results The oxygen content in the cladding layer increased with the increase of the color difference value. The effect of laser power on color difference was greater than that of scanning speed. The larger the laser power, the larger the color difference value. Low laser power and high powder feeding rate can obtain smaller color difference values. The interaction between scanning speed and powder feeding rate has little effect on color difference. The change of gas flow rate has a significant effect on color difference. In order to obtain a smaller color difference, the gas flow rate can be increased. According to the surface color difference model, when the laser power is 950~1 090 W, the scanning speed is 5~7 mm/s, the powder feeding rate is 1.15~1.65 g/min, and the gas flow rate is 24~32 L/min, the surface color difference value that meets the cladding conditions can be obtained. Conclusion The color difference can be used to characterize the surface color of the cladding layer. The limit surface color difference value is 5.09, and a suitable laser cladding process window is obtained. The experimental verification shows that the predicted value obtained by the model is consistent with the measured value, which proves the effectiveness of the research method in this paper.

Titanium alloys are widely used in biomedicine and other fields due to their high specific strength, good corrosion resistance, and high heat resistance [1-4]. Compared with traditional processing, laser cladding has the advantages of low dilution rate, small thermal deformation, and metallurgical bonding between the cladding layer and the substrate for surface modification or surface repair of materials [5-8]. However, due to the high chemical activity of titanium alloy, it is very easy to oxidize under the high temperature of laser cladding, resulting in metallurgical defects in the cladding layer, such as oxide inclusions, cracks, pores, etc., which in turn affect its mechanical properties [9-10].

At present, laser cladding of titanium alloy is carried out in a closed inert atmosphere box. By controlling the oxygen content in the atmosphere box, the molten pool can be effectively prevented from oxidizing, but the equipment is expensive, the size of the processed parts is limited by the size of the box, and it is not convenient for the parts to be processed in situ [11-12]. By using a local gas protection device, a local inert atmosphere is formed in the laser processing area to isolate the cladding high-temperature area from the outside air, thereby preventing oxidation and ensuring the cladding quality. It solves the problems of size limitation and inconvenient mobile operation in closed box processing [13]. However, laser cladding titanium alloy is more prone to oxidation in an open environment, and the oxidation degree and forming quality of the cladding layer are closely related to the process parameters. In order to evaluate the performance of oxidized titanium alloy, destructive testing is required, which not only damages the structural integrity of the component, but also consumes a lot of time and money. It is worth noting that when titanium alloy is oxidized, the surface will show different colors [14]. The content of impurities such as oxygen, nitrogen, and hydrogen in the cladding layer, especially the oxygen content, has an important influence on the performance of the deposited alloy [15-16]. Therefore, it is of great significance to evaluate the performance of the deposited titanium oxide alloy without destroying the parts and to establish the relationship between the surface color and the performance of the cladding layer.

The surface color of the cladding layer is used to reflect the degree of oxidation, and the color difference analysis technology can be used to quantitatively characterize the surface color [17]. Long Fei et al. [18] studied the evolution of color difference and surface micro-region composition of copper-phosphorus brazing material under different temperature treatments. It was found that the color difference was caused by the change of surface composition due to high-temperature oxidation, which can reflect the degree of oxidation of the material. Son et al. [19] studied the relationship between the color difference and material composition of 3D printed parts and found that the coating quality would change with different oxidation degrees. Yang Guang et al. [20] studied the effect of oxygen content (volume fraction <0.005%~0.019%) in the forming atmosphere on the microstructure and mechanical properties of titanium alloys and found that with the increase of oxygen content, the color of the deposited sample surface changed significantly, the strength of the sample increased, and the plasticity decreased. Na et al. [21] studied the effect of laser power on the oxygen volume fraction of pure titanium formed parts under a forming atmosphere with an oxygen volume fraction of 0.2%. When the laser power increased from 120 W to 440 W, the oxygen volume fraction increased from 0.121% to 0.214%. Rousseau et al. [22] and Tang et al. [23] showed that when the oxygen volume fraction in the atmosphere chamber is lower than 0.01%, the oxygen content in the formed part will not increase, and its content is related to the oxygen content of the powder. Eo et al. [24] studied the effect of laser intensity and shielding gas flow on oxidation behavior.

At present, there are few reports on the color difference of the surface of laser cladding titanium alloy in an open environment. This paper is based on the laser cladding titanium alloy method in an open environment [25]. First, the color difference is used to quantify the surface color of the cladding layer. Based on the response surface method, a regression mathematical model of surface color difference and process parameters is established, and the influence of each parameter and its interaction on the surface color difference is analyzed. Secondly, the oxygen content of the cladding layer is used as an evaluation index of the oxidation degree and performance of laser cladding titanium alloy. The variation law of oxygen content under different surface color difference values ​​is studied, and the limit surface color difference value is obtained. Finally, the process parameter range is obtained according to the optimization conditions, and the surface color difference that meets the cladding conditions is obtained, which can provide theoretical guidance for laser cladding titanium alloy in an open environment.

1 Laser cladding test

1.1 Test equipment and method

The laser cladding system in an open environment is shown in Figure 1. The system mainly consists of an IPG YLS-2000-TR fiber laser, a KUKA robot, a HUIRUI powder feeder, and an independently developed optical powder feeding cladding nozzle. The optical powder feeding cladding nozzle adopts a light-powder coupling mode of “hollow beam and centered powder beam”. The laser beam is a hollow annular light, which is obtained by external optical path conversion[26]. In order to realize laser cladding in an open environment, a double-layer coaxial protective gas hood[27] is designed to form a local inert atmosphere around the molten pool, isolate the outside air, and prevent the molten pool from oxidation. The double-layer coaxial protective gas hood consists of an inner main gas protective hood and an outer auxiliary gas protective hood. The inner main gas protective hood adopts a curved flow channel structure, which mainly protects the molten pool. The outer auxiliary gas protective hood adopts a small gap straight flow channel structure, which mainly protects the high-temperature cooling zone outside the molten pool. The experimental substrate is hot-rolled TC4 titanium alloy plate, and the cladding powder material is TC4 spherical powder with a particle size of 75~106 µm. The chemical composition of the substrate and cladding powder is shown in Table 1. In order to ensure the same external conditions, the substrate was cut into a small size of 40 mm×20 mm×8 mm (to eliminate the influence of heat accumulation). Each group was subjected to single-layer eight-pass lateral overlap cladding, with an overlap rate of 45%. The length and width of the cladding layer formed were approximately 30 mm and 12 mm, respectively. The carrier gas, collimation gas, and shielding gas all used argon gas with a purity of 99.99%.

1.2 Surface color difference detection and analysis

During the laser cladding process of titanium alloy, the surface of the formed part will show different colors when oxidation occurs. The surface color of the cladding layer is used to reflect the degree of oxidation, and the CIE L*a*b* color space is used to quantify the surface color.

CIELab is a three-dimensional color model that describes all colors visible to the human eye, proposed by the International Commission on Illumination (CIE). Color is represented by three numerical values: L*, a*, and b*. The L* value represents lightness, and the larger the value, the higher the brightness of the color; the a* value represents the red/green coordinate point, with positive values ​​indicating red and negative values ​​indicating green; the b* value represents the yellow/blue coordinate point, with positive values ​​indicating yellow and negative values ​​indicating blue [28]. The HunterLab UltraScan PRO spectrophotometer was used to detect the surface color of the sample. The observation light source was D65, the color temperature was 6500 K, the observer angle was 10°, the wavelength measurement range was 350~1050 nm, and the measurement aperture was 9 mm. The front, middle and rear ends of each cladding layer were measured respectively, and the average value of the three measurements was taken as the measurement result.

The color difference ΔE* is used to represent the surface color difference, that is, the difference in color. Taking the surface color of the polished titanium alloy plate as the standard color, the test results show that L0*, a0*, and b0* are 64.54, 0.44, and 2, respectively. The color difference ΔE* is: see formula (1)-(4) in the figure

1.3 Response surface experimental design

In order to establish a mathematical model between the surface color difference of the cladding layer and the process parameters, the central composite design model (CCD) of the response surface design module in the Design-Expert 8.0 software is used to design four-factor five-level laser cladding process parameters. A total of 30 groups of experiments are conducted, including 16 groups of cube points, 8 groups of axial points, and 6 groups of center points. The laser cladding process parameters are laser power G, scanning speed V, powder feeding rate S, and gas flow rate Q. The experimental factor level coding and the actual parameter values ​​are shown in Table 2. In order to avoid systematic errors, the test order is arranged in a random manner. The specific test parameters and response results are shown in Table 3. Figure 2 shows the results of 30 groups of experiments.

2 Establishment of mathematical model of surface color difference

2.1 Establishment of model

As can be seen from Figure 2, under different combinations of process parameters, the surface of the cladding layer presents different colors and different color difference values. As can be seen from the cladding layer picture, under the process parameters of No. 9, the cladding powder is not completely melted, the melt path is discontinuous, and the surface flatness is poor, so it is not included in the calculation of the model. The analysis modeling module in the Design-Expert software is used to analyze the test parameters and measurement results in Table 3, and the coefficients in the equation are calculated using the least squares method. The obtained second-order regression model is: See formula (5) in the figure

The variance analysis results of the cladding layer surface color difference regression model are shown in Table 4. As can be seen from Table 4, the model coefficient P is less than 0.000 1, much less than 0.05, and the lack of fit term P is 0.172 6, which is greater than 0.05, indicating that the regression equation fits well and the model is significant. The multivariate correlation coefficient R2 is 0.989 3, the predicted fitting coefficient R2Adj=0.978 6 is close to the corrected fitting coefficient R2pred=0.940 7, and the model signal-to-noise ratio AP is 40.289, which is greater than 4, indicating that the model fits well.

Figure 3 is a normal plot of the standardized residuals of the regression model. As can be seen from Figure 3, the scattered points are close to a straight line and the residuals obey the normal distribution. Figure 4 is a relationship diagram between the actual value and the predicted value of the color difference. The scattered points are distributed in a straight line, and the error between the actual value and the predicted value is small, indicating the effectiveness of the established model.

In order to further verify the accuracy of the mathematical model, 3 groups of parameters were designed for verification tests, and the error analysis of the actual test values ​​and the model calculation values ​​was performed. The results are shown in Table 5. The error calculation formula is (actual value ‒ predicted value)/predicted value×100%. As can be seen from Table 5, there is a certain error between the predicted value and the actual value, but the error is within the range of ±5%, indicating that the established model has a high degree of credibility.

2.2 Effect of process parameters on surface color difference

Figure 5 is a three-dimensional response surface diagram of the interaction of various process parameters on surface color difference. As can be seen from Figure 5a, the slope change caused by the change of laser power is greater than the slope change caused by the change of scanning speed, indicating that the effect of laser power on color difference is greater than that of scanning speed. The greater the laser power, the greater the color difference value. As can be seen from Figure 5b, low laser power and high powder feeding rate can obtain smaller color difference values. As can be seen from Figure 5c, the interaction between scanning speed and powder feeding rate has little effect on color difference. As can be seen from Figures 5d~f, the change of gas flow rate has a significant effect on color difference. In order to obtain a smaller color difference, the gas flow rate can be increased.

3 Experimental optimization

3.1 Oxygen content analysis

The oxygen content in the cladding layer with different color difference values ​​was analyzed by the inert melting method using an O/N/H analyzer (LECO 836 series), with a measurement accuracy of 10–8. A measurement sample of about 0.1 g was cut from the center area of ​​the cladding layer, and the test results are shown in Figure 6. As can be seen from Figure 6, when the color difference value is 3.95, the oxygen volume fraction in the cladding layer is 0.093 884%, which is only 0.015 4% higher than the oxygen volume fraction in the original powder (0.078 5%). As the color difference value increases, the oxygen content in the cladding layer increases. When the color difference value is 42.21, the oxygen volume fraction reaches 4.152 9%, and the oxidation of the cladding layer is more serious. The national standard (GB/T 3620.1-2007) stipulates that the oxygen volume fraction in titanium alloy products is ≤0.2%. When the color difference value is 11.23, the oxygen volume fraction exceeds 0.2%.

3.2 Optimization of process parameters

Taking the oxygen volume fraction in titanium alloy forming parts ≤0.2% as the standard, that is, when the color difference value is less than 5.09, it can meet the use requirements. According to the surface color difference model, the color difference value less than 5.09 is used as the optimization criterion, and the optimization results of some process parameters are shown in Table 6. It can be seen from the optimization results that when the laser power is 950~1 090 W, the scanning speed is 5~ 7 mm/s, the powder feeding rate is 1.15~1.65 g/min, and the gas flow rate is 24~32 L/min, the cladding layer with a surface color difference of 1.35~4.91 can be obtained. Through different combinations of process parameters, a cladding layer that meets the color difference requirements can be obtained.

4 Conclusion

The color difference is used to characterize the surface color of the cladding layer. The process parameters such as laser power are used as the input values ​​of the response surface method, and the surface color difference is used as the output value. Based on the CCD design method in the response surface method, a single-layer multi-pass laser cladding test of titanium alloy was carried out in an open environment, and a quadratic regression model between process parameters and surface color difference of the cladding layer was established. The oxidation degree of the cladding layer was measured by oxygen content, and the variation of oxygen content under different surface color difference values ​​was analyzed. According to the critical condition of color difference, the laser cladding process window was obtained. The verification test showed that the predicted value was consistent with the measured value, indicating that the model established in this paper has high reliability and can provide a reference for the selection of titanium alloy laser cladding process parameters in an open environment.