Characterization Of Elastic Modulus Of Laser Cladding Coatings By Laser Ultrasonic Method

Laser cladding,as a new surface modification and parts remanufacturing technology,is widely used in the industrial field.However,it is easy to produce coatings with uneven microstructure because of its linear scanning characteristics The elastic modulus(E)is one of the important parameters to characterize mechanical properties,which is related to the microstructure of the material.Therefore,the uneven microstructure affects the E distribution of coatings.In order to characterize the E of laser cladding coatings,a nondestructive characterization method based on the surface acoustic wave(SAW)using the laser ultrasonic(LU)technology is proposed.The specific research contents include the following aspects:Firstly,two kinds of excitation mechanisms of LU,the thermoelastic mechanism and ablation mechanism,were introduced.The wave equation of SAW in elastic medium was derived.Moreover,the heat conduction theory and the finite element form of thermo elastic coupling equation were introduced in detail.In addition,the finite element model of coating-substrate structure was solved numerically,and the influence of coatings on the SAW velocity was analyzed.Secondly,the theoretical model of SAW propagation in the coating-substrate structure was established.The wave equation of SAW was solved numerically,and the theoretical dispersion curve of SAW was obtained.Furthermore,the SAWs are excited and collected by LU equipment on different areas of laser cladding coating,and the experimental dispersion curves of SAW were obtained by using the two-dimensional fast Fourier transform(2D-FFT).The nonlinear regression method was used to determine the E of different areas of the coating by minimizing the difference between the experimental and theoretical dispersion curves.According to the difference of E on different areas of coating,it can be concluded that the coating prepared by laser cladding technology is not uniform.Then,the SAWs were received on the surface of nickel alloy coating with different thickness by LU C-scan,and the peak time distribution of the scanning area was obtained after data processing.Compared with the results of 45 steel,the peak time of SAW on the laser cladding coating appeared a large dispersion,and the peak time of SAW presented a very similar to the microstructure of coating.The peak time distribution and dispersion of surface wave showed that the E of laser cladding coating was not uniform.Finally,a nondestructive testing method based on LU C-scan was proposed to measure the E distribution of laser cladding.The surface microstructure of a laser cladding coating was observed by an optical microscope,and the E of the coating was evaluated by a micro-indentation instrument.The results showed that large dendrites with small E were mostly distributed in joints,while the smaller equiaxed grains with large E are mainly distributed in beads.There was no obvious difference between the joint and the bead in the energy spectrum measured by an energy dispersive spectroscopy(EDS).The SAW signals were collected on the coating by LU C-scan,and the maximum amplitude and the delay time of SAWs on the scanning area were obtained.Based on the relationship between the SAW velocity and the E,the distribution of the E could be measured by LU C-scan.Compared with the micro-indentation results,the LU C-scan was feasible to measure the E and characterize the uniformity of the laser cladding coating.Through the above research,it is proved that the evaluation of E of laser cladding coating by SAW using the LU technology has a certain engineering application value,and can provide a technical support for improving laser cladding process.