Surface Chromatic Aberration Model of Laser Cladding of Titanium Alloy in Open Environment
Laser cladding has significant advantages for surface modification or surface repairing of materials.Due to the high specific strength,excellent corrosion resistance and good heat resistance,titanium alloys are regarded as a very good cladding material in surface treatment.However,they are extremely likely to be oxidized at high temperature,which results in metallurgical defects(oxide inclusions,cracks and pores)in the cladding layers,so a sealed chamber filling with inert gas is used.This brings big limitations when cladding the part with big size.Laser cladding of titanium alloys in an open environment is a good way to solve this problem.The current researches on oxidation focus on cladding in a sealed atmosphere chamber.However,the oxidation when cladding in an open environment is rarely studied.Oxidation is an inner performance,so it must be cut for evaluation,which also damages the cladding layer.It is also found that the cladding layers appear different surface colors when using different process parameter combinations.Colors can be represented by chromatic aberration.Therefore,the oxygen contents of the cladding layers formed in an open environment are investigated by different surface chromatic aberration values,and the model of surface chromatic aberration is established.TC4 was cladded on the substrate which was also TC4 in the open environment.Response surface methodology(RSM)was introduced to analyze the relationship between the surface chromatic aberration and the process parameters,such as laser power,scanning speed,powder feeding rate and gas flow rate.First,a newly developed coaxial double-layer shielding device was introduced to the cladding system in the open environment,so as to generate a local inert atmosphere around the molten pool.Second,a CCD matrix of 30 runs was designed using Design-Expert 8.0 software.The surface color of deposited samples was detected by HunterLab UltraScan PRO spectrophotometer.Finally,the oxidation degree of the cladding layer was evaluated by oxygen concentration.The oxygen contents under different surface chromatic aberration values were measured by O/N/H analyzer(LECO,836 series).The surface of the cladding layers deposited under different parameter combinations showed various colors,demonstrating different oxidation degree(Fig.2).The surface chromatic aberration was employed to characterize oxidation color.The quadratic regression model for the surface chromatic aberration was developed and given through the least square method(Eq.5).The ANOVA table of the second order regression analysis indicated that the model terms were statistically significant.All the fit statistics R2,adjusted R2 and predicted R2 values were close to 1,indicating a higher adequacy of the model(Tab.4).The normal probability plot of residuals for the surface chromatic aberration and the plot of the predicted and actual value of the response also indicated that the model was desirable(Fig.3 and Fig.4).From the 3D response surface diagram,it could be seen that the effect of laser power on chromatic aberration was greater than that of scanning speed.The surface chromatic aberration increased with the increase of laser power.A small surface chromatic aberration could be obtained by low laser power,high powder feeding rate and high gas flow rate(Fig.5).With the increase of chromatic aberration value,the oxygen content in the cladding layer increased.When surface chromatic aberration was less than 5.09,the oxygen content was less than 0.2vol.％,satisfying the requirement of the national standard(Fig.6).The optimal processing window was obtained based on the desirability approach.The physical experiments showed that the predicted results were consistent with the actual results,which verified the accuracy of the method proposed in this paper(Tab.6).In this paper,cladding layers of titanium alloy were carried out in the open environment based on the response surface methodology.The surface chromatic aberration was defined to characterize the surface color of the cladding layer.A mathematical model between the surface chromatic aberration and the process parameters was established,where laser power,scanning speed,powder feeding rate and gas flow rate were selected as input variables and the surface chromatic aberration was considered as an output response.The oxidation degree of the cladding layer was evaluated by oxygen concentration,and the oxygen contents under different surface chromatic aberration values were investigated.Based on this model,the obtained allowable value of surface chromatic aberration was 5.09.When the process parameter combinations were laser power of 950-1 090 W,scanning speed of 5-7 mm/s,powder feeding rate of 1.15-1.65 g/min,and gas flow rate of 24-32 L/min,the surface chromatic aberration satisfied the cladding conditions.The physical experiments show that the predicted results are consistent with the actual results,which verifies the accuracy of the method proposed in this paper.The model can provide reference for the selection of laser cladding parameters in the open environment.
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