Yuandong Li

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.

SOME KEY ISSUES AND ACCESSES TO THE APPLICATION OF MAGNESIUM ALLOYS

The status of Mg alloy application, and then some key issues limiting their applications and the corresponding accesses were briefly discussed. It was supposed that development of new alloys with high performance and low cost, investigating advanced forming technology, and development of credible and effective surface modification technologies and related equipment were the urgent tasks in present. Correspondingly, three aspects of researches were carried out. (1) A new alloy with high strength and elongation, but low RE and Zn contents, named ZW21, was invented. (2) Semisolid ingots of AZ91D, AM60 and ZW21 alloys available for thixoforming were prepared. Thixoforming increased the ultimate tensile strength of AZ91D alloy by 20% compared with permanent mould casting. (3) A new kind of micro arc oxidation equipment with a capacity of treating 6m2 surface was developed and has been used to treating motorcycle hub of magnesium alloy.

EFFECTS OF PROCESSING PARAMETERS ON THIXOFORMABILITY AND DEFECTS OF AZ91D

The thixoformability and defect formation of AZ91D magnesium alloy are studied with different processing parameters including reheating temperature, time and die temperature. The results indicated that the suitable processing parameters should be reheating temperature between 575-595°C, reheating time more than 75 min, and die temperature over 275°C. Four types of defects, coldshut, liquid segregation, microporosity and cracks, have been observed in the thixoformed products if the processing parameters are not controlled properly. Among these defects, microporosity and cracks are always dominant.

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.

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.

Microstructural evolution and high-temperature compressive properties of an extruded Mg–Dy–Zn alloy sheet

Abstract Microstructural evolution and compressive properties of an extruded Mg-2Dy-0.5Zn (at.%) alloy sheet at 350°C were investigated. As the compressive strain increased, the volume fraction of dynamic recrystallization increased, the fine lamellar 14H long period stacking ordered phase precipitated in the dynamic recrystallization grain, and the Mg12ZnDy phase with an 18R long period stacking ordered structure gradually bent. These secondary phases not only acted as nucleation sites to promote dynamic recrystallization but also restrained grain growth by inhibiting dislocation movement and grain boundary sliding. The compressive yield strength, ultimate compressive strength, and compressive strain of the alloy sheet were 161 MPa, 212 MPa, and 12.4% at 350°C, respectively. The high compressive strengths were mainly attributed to grain refinement, kink band strengthening of the 18R long period stacking ordered phase and precipitation strengthening of the fine lamellar 14H long period stacking ordered phase in the dynamic recrystallization grain.

Mathematical model and theoretical research of flow shear constitutive relation during rheo-rolling of semisolid alloy

The mathematical model of flow shear constitutive relation during rheo-rolling process has been established. The distribution of velocity and shear stress in rolling cavity was investigated, and the effects of process parameters on shear stress of Sn-15Pb alloy during rheo-rolling process were studied. In rolling cavity, the nearer the roll is, the bigger the velocity and shear stress are. The shear stress increases with the increment of the roll speed and the roll radius during rheo-rolling process, but deceases with the increment of the thickness of the strip. When the solid fraction of Sn-15Pb alloy increases from 0.3 to 0.5, the shear stress increases slowly, but when the solid fraction increases from 0.5 to 0.6, the shear stress rapidly.

Study of microstructural formation mechanism of AZ91D magnesium alloy melt during semi-solid isothermal holding

The semi-solid slurry of AZ91D alloy, produced by a novel self-inoculation method, is isothermal treated. Microstructure evolution and mechanism of microstructure formation during isothermal holding are investigated. The results show that primary α-Mg grains are in small size and present polygon morphology in the slurry. During isothermal holding, the size of particles increases with increasing holding time, while the roundness turns better. But the excessive long holding time is bad for the size and roundness of particles. Deformation of dendrite arm is the primary cause of microstructure formation during pouring, meanwhile, ripening and coarsening during isothermal holding. The nucleus generated from the procedure of melt treatment by self-inoculation, and competitive nucleation in undercooled melt which is near the liquidus during isothermal holding is the main source of primary α-Mg phase.

Role of self-inoculants in solidification process of AZ91D magnesium alloy

The role of self-inoculants with various original microstructure in solidification process of AZ91D magnesium alloy was studied by quenching. The experimental results show that the melt temperature field is changed because of introduction of self-inoculants, which makes local supercooling in the melt. And thus a feasible place is gained for nucleation. Consequently, nucleation rate of the melt is increased. The self-inoculants appeared the half-cooked residue when the parameters of melt treatment are controlled unsuitably. The dendrite fragments generated from dendrite remelting and distribute in the liduids around remelted dendrites, provid the seed crystals of alloy refining by itself. During self-inoculation process, the ‘second inoculation’ of melt is induced from cooling channel, and which provides the ideal condition of formation of tiny dendrite and small granular grains.