Research On The Characteristics Of Quenching Droplet Growth In Plasma Spray Forming

Spray forming, with the characteristics of technological simplicity, cost efficiency,manufacturing rapidity as well as the unique technical advantages in the manufacturing of moldsand parts of high-melting materials, has been put on more and more emphasis by researchers.However, during the process, the droplets flatten, quench and solidify within a very short time afterthey impinge on the substrate mold with extremely high velocity under the action of plasma, whichconstructs the mold shell by irregular accumulation and dynamic growth. Therefore, the process isvery difficult to observe with conventional experimental methodology. With the influence ofcomplicated factors such as the involvement of surrounding air and unstablization of anode spot,the plasma fluctuates swiftly, which affects the droplet properties, melting state, oxidization degreeand landing position, and results in the transient change of the droplet accumulation and growth.The characteristics of the technology mentioned above frequently bring the failures such aswarping, cracking, spalling etc. These failures dramatically degrade the forming quality andmanufacturing accuracy of the molds and parts, sometimes even prevent the process fromproceeding and cause the great waste of energy, materials and human resource. Nevertheless, theresearch on the characteristics of the dynamic growth of quenching droplets is still insufficientinternationally. Therefore, it will be of great significance to research on the characteristics of thequenching droplet flattening and dynamic growth systematically and deeply, and to build thecoupling relationship between the spraying parameters and the coating properties.Based on the analysis of the characteristics of droplet flattening, solidifying and irregularlyaccumulating, the simulation concept of flattening behavior and growing process of the quenchingdroplets in spray forming are presented in this paper. The research emphases include: the influenceof droplet diameters, temperatures, velocities, substrate temperatures and interface thermal contactresistance on the droplet flattening process; the coating growing model based on the multi-dimensional statistical features of the droplets; the splat temperature field and thermal stresseswhen it solidifies and cools; the layer-by-layer accumulation model based on the metallographicalphotograph of the coating, and the temperature distribution of the coating with pores; the predictionof inner-relationship between spraying parameters, droplet temperatures and velocities, coatingporosities and micro-hardness with support vector machines.With the fluid dynamic model and fluid-solid coupled heat transfer model including thesubstrate, the flattening behaviors of stainless steel droplets are simulated visually, which providesthe basis for the study of droplet dynamic growth and coating formation. During the flattening ofthe droplet, the interface contact pressure attenuates with time in a fluctuating way, and themaximum pressure is found to be at the spreading front. After about one quarter of the fattening time, the droplet exhibits completely horizontal spreading. Droplet diameter, velocity, substratetemperature and interface thermal contact resistance are the important influencing factors of theflattening time and flattening ratio. Both flattening time and flattening ratio increase with theincrease of droplet diameters, substrate temperatures and interface thermal contact resistance.While the flattening time decreases with the increase of droplet velocities, the flattening ratioshows an opposite tendency with the change of droplet velocities.A digital splat is constructed based on the characteristics reflected from the photograph. Withmulti-dimensional statistical features of the droplets and the discretized cross sections of the splats,the microstructures of the coatings formed by the irregular accumulation of the splats are simulatedand studied. The influencing laws of droplet diameters, velocities and gun scanning velocities onthe porosities and micro hardness of the coatings are analyzed. The increase of droplet diameterswill result in the decrease of the coating porosities and increase of the surface roughness whendroplet velocity and gun scanning velocity remain unchanged. Both coating porosity and surfaceroughness increase with the increase of droplet velocities if the droplet diameters and gun scanningvelocities keep constant. The study results are of great importance to the analysis of coatingmicrostructure.With the splat obtained from simulation serving as the prototype, the temperature field andthermal stresses of a 3Cr13 splat when solidifying and cooling are simulated by finite elementmethod. As the splat appears disk like which is thick at the rim and thin in the center, thetemperature at the rim is higher than that in the center at the beginning of solidifying and cooling.The position of the highest temperature moves gradually to the center of the splat withsolidification and cooling. The maximum stresses are found to be at the interface between the splatrim and the substrate, while the minimum stresses found to be on the upper surface of the splat rim.The first principal stresses decrease gradually along the radial direction and along the axialdirection at the interface. The first principal stresses are tensile stresses within the splat andcompressive stresses within the substrate. The increase of the substrate temperature will bring thedecrease of maximum stresses within the splat, which is advantageous to the failure avoidance ofwarping, spalling, etc.Aiming at the shortcoming of existing layer-by-layer accumulation model that no porespresent in the coatings, a new layer-by-layer accumulation model that can handle the pores ispresented based on the digital processing of the metallographical photograph of a coating formedby droplet dynamic growth. The model is then used in the simulation of the temperature filed of thecoating as it grows. The analyzing results show that the pores have significant influence on thecoating temperature field during the process of deposition and cooling. The temperature of the partclose to the pores descends faster than that of the part far from the pores, and the temperaturedifference is more distinct when the pores are bigger. The pores also exert influence on thetemperature field of the substrate. The results can provide basis for the study of residual stressanalysis of coatings with pores and for the solution of coating warping, cracking, spalling, etc. The relationships between spraying parameters, droplet temperatures and velocities, coatingporosities and micro hardness are studied by orthogonal experiments with WC-12%Co as anexample. Support vector machines are then introduced to spray forming technology for theprediction of influencing laws of spraying parameters on the droplet temperatures, velocities andcoating porosities and micro hardness. The increase of argon flow rate will bring the increase ofdroplet velocities and the decrease of droplet temperatures, but its influence on the coatingporosities and micro hardness is less distinct. The appropriate augmentation in hydrogen flow ratewill increase both the droplet temperatures and velocities, which reduces the coating porosities andincreases micro hardness as a result. The influence of electric flow on the droplet temperatures,velocities and coating porosities and micro hardness is less significant. With this method, theprediction of coating porosities and micro hardness by spraying parameters; droplet temperaturesand velocities is much more effective.