Spreading,Wetting And Splashing Behaviors Of Tin Droplet Impacting Metal Surface

With the rapid development of industries such as manufacturing,aerospace,electronics,and medicine,higher requirements have been placed on the strength,hardness,wear resistance,corrosion resistance,and high-temperature oxidation resistance of materials.Thermal spray technology and metal droplet forming technology are essential means to meet these requirements,but the drawbacks of these two technologies are ultimately related to the wetting and spreading behavior of a single droplet on the substrate surface.This article adopts a combined method of experimental and simulation to study the wetting and spreading behavior and splashing behavior of high-speed tin droplets impacting on a copper substrate surface,analyze the influence of substrate temperature,droplet impact velocity,substrate surface roughness,and intermetallic compounds on the spreading factor and contact angle,and the research results provide important theoretical guidance for processes related to high-speed droplet impact on metal surfaces,and provide a new theoretical basis for the wetting and spreading behavior of metal droplet impact on metal surfaces.The main research results of this article are as follows:(1)For tin droplets impacting a low-temperature(below the droplet’s melting point)copper substrate,the impact and spreading process can be divided into four stages:rapid spreading,slow spreading,oscillation,and solidification.The substrate temperature affects the spreading process of the droplet,and the final spreading factor of the droplet increases with temperature,but the final contact angle is not significantly affected by temperature.An increase in droplet impact velocity causes an increase in both the spreading factor and the final spreading factor during the spreading process,and the change in contact angle during the spreading process decreases with increasing impact velocity,while the final contact angle decreases and then increases with increasing impact velocity.The surface roughness of the substrate has little effect on the entire spreading process,but the final spreading factor increases slowly with increasing surface roughness,while the final contact angle decreases and then increases with increasing surface roughness.(2)For tin droplets impacting high-temperature(above the droplet melting point)metal substrates,when the system is a reactive wetting system(tin/copper system),the impact and spreading process can be divided into four stages:rapid spreading,retraction,contact angle oscillation,and final stabilization.When it is a non-reactive wetting system(tin/stainless steel system),the impact process can be divided into four stages:rapid spreading,retraction,constant contact angle,and final stabilization.The generation of interface IMCs is the main reason for the different dynamic spreading processes of reactive and non-reactive wetting systems,and IMCs have almost no effect on the spreading process but will have a significant impact on the retraction process,which increases with the duration of the retraction process.(3)For non-reactive wetting systems(Sn/stainless steel system),an increase in impact velocity will lead to an increase in the maximum spreading diameter of the droplet and the final spreading factor,while having little effect on the contact angle.An increase in substrate temperature will result in an increase in the maximum spreading factor of the droplet and a decrease in the final spreading factor,with little effect on the contact angle.An increase in surface roughness will lead to a slight increase in the maximum spreading factor of the droplet and a slight decrease in the final spreading factor,while the contact angle will decrease accordingly.(4)When the tin droplet impacts a copper substrate at a relatively high speed,the severity of splashing is affected by the impact speed.The results of droplet impact on the substrate change from a disk shape accompanied by splashing to complete fragmentation as the impact speed increases.Therefore,the effective spreading factor of the droplet cannot continue to increase by increasing the impact speed.However,the contact angle of the droplet does not change significantly with the impact speed.The result of droplet impact on the substrate also changes from no splashing to a large amount of splashing as the substrate temperature increases,and the effective spreading factor does not continue to increase with the substrate temperature.At the same time,the effect of substrate temperature on the contact angle of the droplet is not significant.Additionally,higher substrate temperatures can make droplets more prone to splashing during the oscillatory solidification stage.(5)A physical model of melt droplet impact on a substrate is developed from the perspective of energy conservation,which can predict the maximum spreading factor of melt droplet and the finger shape number of splash,and can also be used to express the energy change of melt droplet impact process.The initial kinetic energy dominates the droplet spreading at the beginning of the droplet impacting the substrate,and then the viscous dissipation and surface energy dominate the droplet spreading,and each droplet splash leads to the loss of overall energy of the droplet.