Y. Hao

Microstructural evolution of previously deformed ZA27 alloy during partial remelting

The microstructural evolution process of previously deformed ZA27 alloy during partial remelting was studied. After being compressed by a ratio of 20% in length at 250 °C, the microstructure of ZA27 alloy changed from developed dendrites to coarsened short dendrites and near-equiaxed grains, then in turn to united pieces, big lumps, small pieces, polygon particulates, and finally to spherical grains that would grow and agglomerate and become larger in size and irregular in shape during following partial remelting at semi-solid temperature of 460 °C. The structure evolution resulted from the eutectics first melted between boundaries of the primary grains and then penetrated into polygon boundaries that formed due to recrystallization, resulting in the separation of the primary grains into small polygon grains, followed by spheroidizing. This process was mainly discussed through analyzing the phase transformations occurred during partial remelting.

Effects of Mg and RE additions on the semi-solid microstructure of a zinc alloy ZA27

Abstract The effects of 2 mass% Mg and 0.8 mass% RE additions on the microstructural evolution of a Zr modified zinc alloy ZA27 during isothermal holding at semi-solid temperature of 460 ˚C have been studied by optical microscope and scanning electron microscope. Results show that these two elements all decrease the coarsening rate of solid primary particles, and improve the uniformity of particle size. The addition of Mg greatly decreases the solid fraction, and then increases the distance between particles. However, the element of RE mainly concentrates at the liquid regions between particles, and hinders both the atom diffusion and the welding of contact particles. In addition, the behaviors of particle coarsening of these two alloys, together with that of the Zr modified ZA27 alloy without the other elements, obey the LSW law after the three semi-solid systems reach at their solid–liquid equilibrium states. The additions of these two elements have no visible effect on the shape factor of particles.

Semisolid Microstructure of AZ91D Magnesium Alloy Refined by MgCO3

AbstractThe microstructural evolution of an AZ91D alloy refined using 1% MgCO3 is investigated during partial remelting at 580C (1076F). Simultaneously, the effect of the initial as-cast microstructure on the semisolid microstructure and the microstructure uniformity of semisolid rods are discussed. The results indicate that the microstructural evolution includes four stages: the initial coarseningstructure separationspheroidizationfinal coarsening The finer the as-cast microstructure, the smaller and more spheroidal the primary particles are in the semisolid microstructure. For alloys with a grain size of 50–100μm, the resulting primary particle size is slightly larger than the initial grain size. But for the alloys having larger size grains (>150μm), the result is just the opposite. The main reason which leads to this difference is the different evolution mechanisms determined by the as-cast microstructure. To obtain a semisolid ingot with uniform microstructure, an ingot with uniform as-cast microstructure must be used.

Die-filling process during the thixoforming of a ZA27 alloy cylindrical rod

The die-filling process during thixoforming of a ZA27 alloy cylindrical rod was deduced by analyzing microstructural characteristics in the semi-solid ingot prior to and after forming, and by analyzing the phenomena occurring during this thixoforming. These characteristics referred to some constituent segregation, such as a liquid-phase segregation and an inhomogeneous distribution of primary solid particles. The results indicated that the die-filling process could be properly deduced by using the method developed in this paper. The inhomogeneous distribution of primary solid particles in the formed rods mainly resulted from the inhomogeneous distribution in the semi-solid ingot. The detailed distribution was determined by the sequence of the die-filling. However, the aforementioned liquid segregation was mainly attributed to the die-filling process. The sequence of the die-filling process was from the bottom of the cylindrical cavity to the top and from the edge to the inner part of the cavity.

Study on the Dimensional Precision of the Polymer SLS Prototype

Selective Laser Sintering (SLS) has many advantages such as fast building speed, wide range of materials, complicate shape of prototype and simple post-treatment. However, the shrinkage and mechanical stresses result in lost of the accuracy. This paper investigates the dimension precision of polymer SLS prototypes and proposes methods to control dimensions in suitable degree. The influencing factors of dimensional precision in the file preparation of CAD model, manufacturing system of SLS prototype and sintering process are analyzed. It indicates that the sintering shrinkage in sintering process is the main reason of dimensional changes. To improve the dimensional precision, the measures are proposed to diminish the shrinkage: improving material of prototype, optimizing the sintering process and compensating the lost of dimension. After analysis the experimental results, the rule of error changes is summarized by fitting equations and feedback to controlling software of SLS system to improve the dimensional precision effectively. The SLS prototype could be controlled in 100±0.20mm through compensation.

A New Method for Production of Nondendritic Semisolid ZA27 Alloy

In this article, a new method for the production of nondendritic semisolid ZA27 alloy available for thixoforming was developed. The results indicated that a semisolid microstructure with small and spherical primary particles could be obtained when the alloy had been treated for 3 h at 370°C and then partially remelted at semisolid temperature of 460°C. This method could not only overcome some inevitable shortcomings resulting from using some present popular approaches but also simplify the procedure of thixoforming. The coarsening of dendritic arms through merging during solid solution treatment and then separating from the dendrites during partial remelting were the main methods by which such a microstructure was formed.

Corrosion Behavior of Thixoformed AZ91D Magnesium Alloy in NaCl Aqueous Solution

The corrosion behaviors of thixoformed AZ91D alloy immersed in 3.5% NaCl aqueous were studied by using the weight lose method and observed by scanning electron microscopy and analyzed by electron probe micro-analysis.The results showed that corrosion rate of thixoformed AZ91D alloy was higher in initial stages and decreased as the immersion time increased. Compared with the thixoformed alloy, the change of corrosion rate of permanent mould cast (PMC) alloy was smoothly. The observation on the corrosion surfaces showed that the primary αphase spherical particles were first suffered from corrosion during corrosion testing, and then the eutecticαgrains. the composition of primaryαphase particles in the thixoformed alloy was quite uniform, and thus the protective film of the corrosion products on the alloy surface was more intact. The amount of the eutectic β phase in the inter-particle eutectics was relatively higher and the distribution was more uniform, so the protective film consisted by eutectic β phase was also more intact. That is to say the protective role from the surface film on the thixoformed alloy was better than that on the PMC alloy which was the cause of lower corrosion rate of the thixoformed AZ91D alloy.