A study of dry coating process using zinc coated balls

Protective coating of the work-piece for the cold extrusion process is essential to decrease the friction between the die and the work-piece under extreme pressure. Without it the abrasion and the adhesion quickly deteriorate the surface quality of the product and shorten the lifetime of the die. Traditionally, the wet type phosphate coating was universal practice for the process. This method consists of many cumbersome and environmentally unfriendly chemical processes. Recently, the dry zinc coating processes similar to the shot pinning using the zinc coated steel ball was introduced.In this study, the collision and coating process using the zinc coated ball was studied to determine the optimum operation velocity for uniform coating and maximum lifetime of the ball. The ball colliding test equipment was developed. It consists of the 30 mm air gun system and the target, made of the same work-piece material. The ball, carried by the sabot, was launched from the gun to the target at a prescribed velocity and impact angle. After the impact, the target and the ball were examined using the optical and electron microscope. It was observed that, at the velocity lower than 30 m/s, the coated area by the single zinc coated ball and the damage of the zinc shell of the ball were minimal. When the velocity was higher than 60 m/s, greater number of fractured piece of the shell was observed. Thus, the lifetime of the ball was shortened. Computer simulation using the explicit finite element program was done to understand the impact-coating process. Even though the simulation could not express the adhesion of the zinc layer onto the work-piece, but the deformation and the fracture of the zinc shell were similar to the experimental results. Massive crack and fracture of the shell were predicted at faster impact velocity. From the experimental and simulation results, it was concluded that the optimum velocity was in the range of 45–50 m/s for effective zinc coating and maximum lifetime of the ball.