Droplet spreading, substrate remelting and variable thermal contact resistance in microcasting.

摘要

A profound understanding of the molten droplet solidification and substrate remelting process in microcasting is necessary to optimize the operation parameters, such as impact velocity, droplet superheat and substrate temperature, thereby to produce good quality products.; A numerical model considering the complex fluid dynamics of droplet spreading, the flow of substrate melt, and the thermal contact resistance between the droplet and the substrate was introduced. The results of numerical simulation were compared with a previous microcasting experiment. It has been found that the fluid dynamics and heat transfer are strongly coupled so that the thermal contact resistance can not only affect the droplet spreading but also the profiles of maximum substrate remelting front. The effects of operation parameters on microcasting were also described in detail.; The variable thermal contact resistance between the solidifying alloy158 and the substrate of the same material was investigated experimentally. A novel inverse heat transfer method based on the measurement of ultrasonic time delay was developed to estimate the variable thermal contact resistance. It has been found that the variation of thermal contact resistance to a large degree determined by the contact interface temperatures. The influences of the initial molten metal temperature and the impact velocity on the formation of thermal contact resistance were investigated. An empirical correlation equation of variable thermal contact resistance, which can be applied to the numerical simulation of molten droplet impact, was developed utilizing the experimental data.; The experimental investigation of the impact of highly superheated molten alloy158 droplets onto alloy158 substrates was conducted. The detailed droplet spreading and recoiling process was captured using a high speed CCD camera. The effects of the impact velocity and the molten droplet temperature were investigated in detail. It has been found that whether the droplet spreading can be arrested by the solidification at the edge of droplet during the spreading or the recoiling period depends on the droplet temperature and the impact velocity.; The numerical simulation with the application of variable thermal contact resistance was conducted to compare with experimental results. It has demonstrated that the numerical simulations can predict the droplet spreading and recoiling process correctly, verifying the feasibility of using a variable thermal contact resistance in the numerical simulation of the molten droplet impact, and making the numerical simulations more powerful in prediction.