H. J. Hu

Physical fields evolution and microstructures for compound extrusion of AZ31 magnesium alloy

To research physical fields evolution and microstructures for compound extrusion of AZ31 Alloy, A kind of compound extrusion technology including extrusion and following shears for AZ31 magnesium billets have been explored. Three-dimensional finite element simulations of extruding AZ31 magnesium alloy billets into small rods at certain ram speed have been performed by compound extrusion with extrusion ratio 28 and channel angle 150°. Parameters including workpiece material characteristics and process conditions have been taken into consideration. High cumulative strains and low temperatures rise was the main reason for grain refinement during compound process. The observation results of microstructures and rod surface quality showed that the compound process effectively refine the grains of AZ31 magnesium and product magnesium alloy rods.

Dry sliding wear behavior of ES-processed AZ31B magnesium alloy

The friction and wear properties of AZ31B magnesium alloy fabricated by a new severe plastic deformation called extrusion-shear (shorted for ES) have been investigated in dry sliding condition. Dry sliding tests for AZ31B magnesium alloy processed by direct extrusion have been performed and compared with that of the alloy prepared by ES process. The results show that the wear resistance of AZ31B alloy was improved by Extrusion-Shear process, compared with direct extrusion. This was mainly attributed to the grain size refinement in Extrusion-Shear process. The main wear mechanisms of both alloys are abrasion slight deformation, oxidation delamination and melting under different frequencies. The effective stresses for the surface region during wear process have been calculated by using FE (Finite Element) method. The simulation results indicates that a higher effective stress is accumulated in the direct extrusion alloy than in the ES alloy, which may cause a larger volumetric wear rate of AZ31B magnesium alloy.

Comparisons of extruded magnesium rods by hyperbolic curve die and parabolic curve die

In order to predict defects of extrusion especially the surface cracks caused by hyperbolic curve die and parabolic curve die in direct extrusion process, three-dimensional (3D) thermo-mechanically coupled finite element simulations of extruding AZ31 alloy into small rod at certain ram speeds have been performed using DEFORM™-3D. A half of the geometries including billet and dies with hyperbolic curve die and parabolic curve die have been designed and meshed due to symmetry. Simulation parameters including work-piece characteristics and process conditions (e.g., billet temperature, extrusion ratio, and ram speed) have been taken into consideration. The simulation results have been verified in extrusion experiments under identical conditions as simulation conditions. It can be found that hyperbolic curve die could improve homogeneity of the metal flow, avoid the formation of the dead zone, decrease additional tensile stresses and increase uniformity comparing with parabolic curve die. The parabolic curve die may cause continuous cracks on the surface of the extruded rod. Theoretical results obtained by the simulations agree well with the experiments data. The obtained results provide the fundamentals and practical guidelines for the choice and design of die structures to produce magnesium rods with finish surface.

The simulation researches on hot extrusion of super-fined tube made of magnesium alloys

To research the deformation mechanisms of extrusion for artificial blood vessel. A series of experiments were done to obtain the simulation parameters: stress-strain curves, friction factors and heat transfer coefficient, etc. Three-dimensional, coupled thermo-mechanical finite element simulations of extruding a wrought magnesium alloy were performed. Computed parameters including workpiece material characteristics and process conditions (billet temperature, reduction ratio, and ram speed) were taken into consideration. The temperature distribution of the transient-state extrusion is different from the steady-state extrusion. There exists heat flow in two reverse directions. The maximum temperature is found to be at the center of small tube along the longitudinal direction, while the minimum temperature is found at the edge of the billet in contact with both the container and the die face. The material at the center of the profile has a lower strain rate and therefore it flows through the die with less severe deformation. It is clear that the metal flow velocity field along the negative Y axis remains steady during tube extrusion, so artificial blood vessel with homogeneous microstructures would be obtained. The maximum damage occurs at the billet surface in the exit region of the die. This is because critical damage occurs at the point of maximum tensile stress in the tube. The extrusion simulation was the reliable predictions of strain rate, effective strains, effective stresses and metal flow velocity in an AZ31 billet during direct extrusion.

Relationships between the process conditions and microstructures evolution for extrusion-shear of magnesium alloy

It is widely recognized that grain refinement has significant influences on the strength and ductility of metals. To improve the industrialization of the severe plastic deformations (SPD) technologies for magnesium alloy, a new extrusion-shear (ES) method has been explored and widely investigated because it can manufacture ultra-fine grained microstructures in magnesium alloys. It is crucial to research the effects of process parameters on the deformation behaviors of ES process. Three-dimensional (3D) finite element modeling of ES processes with different billet temperatures and channel angles have been researched. Different billet temperatures have been regarded as the initial conditions in DEFORMTM-3D software. The strain rates evolutions have been gained with different ES process conditions. The microstructure evolution has been analyzed by the simulation and experimental results. The decreases of billet temperatures and channel angles could improve the grains refinements of magnesium alloys which have been prepared by ES process. The simulation and theoretical analysises results showed that reasonable billet temperatures and channel angles could promote grains refinements. Research results could present the fundamentals and practical guidelines for the designs of ES dies and optimization of process parameters to manufacturing high-performance magnesium rods.

Relationships between process parameters and temperature field of roll casting for wide sheet made of AZ61 magnesium alloy: Finite element method

In recent years, wide sheet made of AZ61 wrought magnesium alloys has been widely studied and applied in industry. Thin roll-casting technology for the new wrought magnesium alloy can provide acceptable quality wide and thin sheet made of AZ61 magnesium alloy. To study the influences of roll-casting process parameters on temperature field for wide and thin sheet made of AZ61 magnesium alloy plates, some simplification and assumptions have been done by characteristics of magnesium alloy. Two-dimensional FEM model for roll-casting has been established along casting direction. Simulations of temperature fields of the plates have been done by using finite element analysis ANSYS software. A series of researches on the temperature distributions under different process parameters (pouring temperature, heat-transfer coefficients and casting speeds) have been done. The simulation results and the literature about the casting process of the relevant theory are the same. The simulation results show that the process parameters of rapid-casting process for AZ61 magnesium alloy are mutual influenced on the temperature fields of wide sheet made of AZ61 magnesium alloy.