Formation Mechanism And Process Control Of Typical Defects In Electrospark Deposition Coatings

Due to the convenient operation and practicability of electro-spark deposition technology,it has significant economic value in repairing,strengthening and developing various functional coatings such as thermal barrier coatings and amorphous coatings on metal parts.However,the existence of internal defects(cracks,porositys,incomplete fusion,etc.)of the electro-spark deposition coating severely restricts the development of this technology,and research on coating defect control has rarely been reported.In this paper,the coating was prepared by electro-spark deposition,and the morphology of the coating was observed by scanning electron microscope and the formation of coating defects was analyzed.The coating defects under different processes were studied by changing the process parameters,and the parameter optimization and significance analysis were carried out based on the statistical model.The main research contents and conclusions of this paper are as follows:(1)A TA2 coating having a certain thickness was prepared by electric spark deposition technique.By observing the surface and the cross-sectional morphology,it was found that there were mainly three defects of micro-crack,porosity and incomplete fusion in the coating.The cracks are distributed on the surface of the coating,and some cracks may form secondary cracks when expanding.From the cross section,the cracks can be roughly divided into transverse cracks and longitudinal cracks,and the cracks only appear in the coating.Pore defects are mainly distributed in the lower part of the coating,and most of the porosity defects are pore-like defects,and the shape is basically a regular spherical hole.The shape of the incomplete fusion defect is irregular,and the inside is generally accompanied by the presence of inclusions.(2)The main reason for the occurrence of cracks in the sediment layer is the presence of various stresses mainly based on thermal stress in the electro-spark deposition coating.Crack initiation occurs when the plasticity of the localized portion of the coating is insufficient to withstand the strains produced by the internal stress at that time.The formation of pore defects is affected by both the characteristics of the electro-spark deposition technology and the material itself.When the gas is not removed in time when the coating is cooled and solidified from the molten state,pores are formed in the coating.The formation of incomplete fusion defects is mainly related to the randomness of the discharge of the tip of the spark electrode.The temperature field of the droplets and sediments during the deposition process was analyzed by finite element analysis technique.The temperature curves of the bottom of the micro-bump gap and the material of the droplets were obtained,which explained the causes of the incomplete fusion defects and mutual verifies them with theoretical analysis.(3)The electro-spark deposition process and the deposition environment have a significant effect on the preparation of the coating.As the discharge power and oxygen content increase,the surface of the coating becomes uneven and micro-protruding particles appear;while the larger diameter electrode facilitates the spreading of the droplets,making the surface of the coating more flat.The mass transfer coefficient is mainly affected by the discharge power and the electrode diameter,and the effect of oxygen content is not significant.With the increase of discharge power and electrode diameter,the mass transfer coefficient first increases and then decreases,and the mass transfer coefficient is the highest at a power of 1200 W and an electrode diameter of Ф2.0 mm.(4)Variations in discharge power,electrode diameter,and oxygen content have important effects on coating defects.Crack defects increase with increasing discharge power,electrode diameter,and oxygen content,which is primarily related to the stress state in the coating.The effect of discharge power and electrode diameter on pore defects is similar to cracks,while the effect of oxygen content on pore defects is not significant.The variation of the incomplete fusion defect is inversely proportional to the discharge power and the electrode diameter,and is proportional to the oxygen content.(5)The TA2 coatings were prepared on the TA2,304 and TC4 substrate by electric spark deposition technique.The results show that there is no significant difference in the surface morphology of the three substrates,and the mass transfer coefficient is also small.However,a large number of defects can be clearly seen at the bottom of the coating deposited on the surface of the 304 substrate,indicating that the surface of the substrate is shaped.There is no significant effect on the appearance and mass transfer,but the effect on the quality of the coating is very significant.