Papers by Author: Yun Zhong Liu

Abstract: A novel spray forming process was developed to produce large billets, wide plates or thick tubes with excellent microstructures and high cooling rates. Its uniqueness lies in a combination of the wide-range reciprocating movement and the swing scan of a gas atomizer, and the externally forced cooling of substrate during this spray deposition procedure. Its basic concept is that both good sticking and rapid solidification can be achieved if droplets with high liquid fractions impact a cold substrate, spread fully and then deposit on the surface. In order to control and optimize this new process, the thermal histories of droplets and deposits for spray forming of aluminum alloy billets were simulated with a set of new numerical models. Through shortening spray distance and raising melt superheat properly, the liquid fraction of droplets before deposition will increase and their spread on the deposit surface can improve for good sticking. Simulation results show that the optimal liquid fraction of droplets for deposition is about 0.2 higher than that in the conventional Osprey process. Its optimum spray distance is about 0.25m, which is nearly half as that in the Osprey process. In addition, this new process increases the mushy layer area and the specific surface area of heat extraction during deposition. Together with the forced cooling of substrate, it results in higher cooling rates. A high-quality large billet can be obtained by controlling the atomizer movement, the droplet liquid fraction and the deposit surface temperature properly in this new process.

Abstract: A new process of pulsed electric current sintering was developed. It combines compaction with activated sintering effectively and can manufacture bulky nano-crystalline materials very quickly. Pulsed electric current sintering of high-energy ball-milled nano-crystalline iron-based powders is investigated in this work. A nanostructured steel is obtained with high relative density and hardness by this process. The average grain size of iron matrix is 58nm and the carbide particulate size is less than 100nm. The densification temperature of ball-milled powders is approximately 200°C lower than that of blended powders. When the sintering temperature increases, the density of as-sintered specimen increases but the hardness of as-sintered specimen first increases and then decreases. Microstructure analysis results show that the decrease of hardness is caused by the dramatic grain growth of iron matrix.

Abstract: In conventional studies, different empirical atomization equations are correlated for different kinds of atomization methods or even in the same method. In the present study, it was found that the basic law of melt breakup from bulky liquid into droplets can be universally applied to all atomization methods. Based on theoretical analysis, a new general equation of mean particle size applicable to both conventional atomization methods and new atomization processes is presented. The mean particle size in melt atomization is mainly controlled and decided by two key dimensionless parameter groups representing the liquid stability of melts and the breakup ability of atomizer respectively. Different specific atomization mechanisms result in different formulae in conventional atomization methods. In case of gas atomization, it is equivalent with and can be changed into Lubanska Equation. In case of centrifugal atomization, it can be changed into the equations that are currently the most widely used. In case of water atomization, it is similar to the equation proposed by Grandzol and Tallmadge. According to the universal equation, new correlations for mean particle size in novel atomization processes such as Hybrid Atomization and Multistage Atomization were proposed and agreed with our experimental data well.