Effect Of Laser Remelting On Microstructure Evolution And Properties Of Al₂O₃-ZrO₂ Ceramic Cladding Layer On Titanium Alloy Surface

As a new green manufacturing process,laser cladding has the advantages of fast cooling rate,small heat affected zone,and close bonding between the cladding layer and the substrate.Since its development in the 1970s,laser cladding technology has been widely used in industrial fields such as aerospace.By laser cladding,the Al₂O₃-ZrO₂ceramic coating achieves a good metallurgical bonding with Ti-6Al-4V,which can effectively improve the performance of Ti-6Al-4V under extreme operating conditions.At the same time,as a hard phase,Al₂O₃-ZrO₂ceramic can significantly improve the hardness and wear resistance of the material surface,further expanding the application field of Ti-6Al-4V.However,due to the differences in thermal expansion coefficients between ceramics and metals,cracks are easily generated at the interface between the coating and metal during the preparation of Al₂O₃-ZrO₂ceramic coatings on Ti-6Al-4V surfaces.In order to reveal the evolution of the grain structure of the Al₂O₃-ZrO₂ceramic coating,suppress the bonding cracks of the coating,and improve the hardness,wear resistance,fracture toughness,and thermal shock performance of the coating,this paper simulates the process of laser cladding and laser remelting of the Al₂O₃-ZrO₂ceramic coating on the Ti-6Al-4V surface to establish a thermally coupled model,and studies the effects of laser cladding process parameters on the temperature and stress fields of the Al₂O₃-ZrO₂ceramic coating.The simulation results show that with the increase of laser power,the temperature gradient of Al₂O₃-ZrO₂ceramic coating presents a negative correlation trend,while with the increase of laser scanning rate and spot diameter,the temperature gradient of Al₂O₃-ZrO₂ceramic coating presents a positive correlation trend.Under the same laser cladding parameters,the temperature gradient in the bottom region of the Al₂O₃-ZrO₂ceramic coating is significantly higher than that in the top region,while the cooling rate in the bottom region is lower than that in the top region.After remelting simulation of the Al₂O₃-ZrO₂ceramic coating,it was found that when the laser power was 1200 W and the scanning rate was 2 mm/s,the temperature gradient of the coating significantly decreased.After laser remelting,the residual stress increased in the laser scanning direction and decreased in the vertical direction.At the same time,after remelting,the residual stress curve with distance becomes more gentle,which is conducive to suppressing the generation of cracks.At the same time,orthogonal experiments of laser cladding were carried out near the laser cladding process parameters obtained by finite element simulation.It is found that when the laser energy density is high,the surface of the coating presents a wavy state,with the grain morphology mainly composed of cellular and equiaxed crystals.The content of Ti in the top region of the cladding layer is high,and the distribution of Al,Zr,and O ceramic elements does not change much.As the laser energy density decreases,the grain size of the coating becomes larger.When the laser energy density is low,the content of Ti in the top region is smaller than that in the bottom region,and when the grain morphology is columnar When dendrites occur,the coating is prone to defects such as pores and cracks.The hardness and wear resistance of the Al₂O₃-ZrO₂coating are significantly improved compared to the Ti-6Al-4V substrate,and the fracture toughness is the highest at a laser energy density of 150J/mm²,which is 7.77 MPa·m^<sup>1/2。After that,the cladding coating with fewer cracks and the highest fracture toughness was selected for laser remelting tests.The remelting experiment of Al₂O₃-ZrO₂cladding coating showed that laser remelting can refine the grain size,homogenize the element distribution,and form solid solution phases such as Ti_xAl_yO_zand Ti_xZr_yO_z.The research on the Al₂O₃-ZrO₂remelted coating with a remelting power of 1200W and a scanning rate of 2mm/s found that after the fine grain strengthening of the Al₂O₃-ZrO₂remelted coating,the grain structure of the coating becomes denser,and the grain boundaries are tortuous and complex.Crack propagation requires transgranular propagation,which will consume greater energy.At the same time,laser remelting allows time for the full diffusion of elements,and elements such as Ti,Al,and Zr are uniformly distributed into the coating along with Marangoni convection,homogenizing the element distribution throughout the remelted coating,preventing the occurrence of cracks due to inconsistent local tensile stresses on the material during sudden cooling.After laser remelting,Zr element fully diffuses to the bottom of the coating,which performs phase transformation and crack arrest on the coating bonding area,which can further enhance the bonding strength of the coating bonding area.In addition,laser remelting can avoid coating stress concentration and alleviate the problem of excessive coating cracking energy.Finally,the performance of the laser cladding coating was compared with that of the remelted coating with different laser remelting scanning rates and remelting powers.It was found that after laser remelting,the wear resistance and hardness of the coating were improved.Under the optimal process parameters,the hardness of the two increased by about13%,the wear cross-sectional area decreased by 23%,and the thermal shock cycles failed after 50 times.The fracture toughness of the coating decreases first and then increases with the increase of laser scanning rate,and increases with the increase of laser remelting power.The average fracture toughness of the remelted coating under different process parameters is10.8 MPa·m^<sup>1/2.The Al₂O₃-ZrO₂remelted coating improves the resistance to external forces per unit area of the coating through fine grain strengthening,resulting in improved hardness and wear resistance of the coating.At the same time,matrix Ti and other elements enter the molten pool to strengthen the solution,further improving the performance of the Al₂O₃-ZrO₂coating.