Yeong Maw Hwang

A Study on Friction Stir Process of Magnesium Alloy AZ31 Sheet

Friction stir processes (FSP) are important for enhancing mechanical properties of metal sheets, such as the tensile strength, the elongation, etc. The stress distribution of the tool pin is affected by the thermo-mechanical characteristics of the workpiece in FSP. Recently, magnesium alloy AZ31 is widely used in machine industries due to the light-weight material property. In this paper, a thermo-mechanical model for FSP using three dimensional FEM analyses is proposed for exploring temperature distributions, strain distributions and stress distributions of the workpiece. The heat generated from the plastic deformation and the friction between the head tool and workpiece is considered as the heat source in the simulation of the FSP process. A commercial finite element code – DEFORM 3D is used to carry out the simulation of the plastic deformation of AZ31 sheets during the FSP. The analytical results of temperature, strain and stress distributions of the workpiece and head tool can provide useful knowledge for tool pin design in FSP

Study on Forming Limit of Tubular Materials

The objective of this study is to establish the Forming Limit Diagram (FLD) of tubular materials AA6063 analytically and experimentally. A self-designed hydraulic bulging equipment with capacity of 100 MPa is used to carry out bulging tests for obtaining the forming limit of the tubes. Whereas, stretching tests of circular sheets with various notch radii, which are made from the tubes tested, are also conducted to obtain their FLD. Furthermore, plastic instability criteria with Hill’s new yielding function are used to predict the forming limit curve (FLC) of the tubes and sheets. The predicted FLCs are compared with the FLDs from the tube bulging tests and sheet stretching tests. The experimentally obtained FLD from the tube bulging tests are coincident with that from the sheet stretching tests. Also, the predicted FLCs by the plastic instability criteria with Hill’s new yielding function agree quite well with the experimental results. Introduction Nowadays, hydroforming processes have been widely applied to manufacturing parts in various fields, such as automobile, aircraft and aerospace, and ship building industry, due to the increasing demands for lightweight parts [1]. Concerning the studies on tube and pipe hydroforming processes, Sokolowski et al. [2] have carried out a series of simulations and experiments on tube formability tests. Dohmann & Hartl [1] have also undertaken a lot of investigations on tube hydroforming processes, such as manufacturing axisymmetrical parts and T-shape parts by expansion and feeding. Asnafi & Skogsgardh [3] proposed a mathematical model to predict the forming pressure and the associated feeding distance needed to hydroform a circular tube into a T-shape product without wrinkling and bursting. The present authors [4] have developed a model considering sticking friction mode to predict the forming pressure and thickness distribution of the formed parts during expansion in a rectangular die. Some of the studies concerning the forming limit diagram of tubular materials have been reported. For example, Fuchizawa et al. [5] designed a hydraulic bulge-forming machine with axial feeding to establish the FLD of AA6063 tubes that is obtained by changing the stress ratios. Kuwabara et al. [6] also established the FLD of AA5154 tubes subjected to a combined axial load and internal pressure. It’s found that conventional strain based forming limit curves are dependent on the stress paths. Thus, a forming limit stress diagram (FLSD) was proposed. Nefussi and Combescure [7] used two different Swift’s criteria to predict the FLCs of sheets and tubes, and it is said that Swift’s criteria can not be used alone to predict instability in the case of tube hydroforming. So far, among published literature, there were few works that are involved in comparisons of the forming limits from tube bulging tests and sheet stretching tests. Determination of Flow Stress and Anisotropy of Tubes At first, tubes used for tensile tests are fully annealed to avoid crack occurring in the subsequent pressing operations. Then, the annealed tubes are cut into two halves and pressed into flat sheets using a press machine. The specimens for tensile tests are made by cutting the flattened sheets with a CNC Key Engineering Materials Online: 2004-10-15 ISSN: 1662-9795, Vols. 274-276, pp 391-396 doi:10.4028/www.scientific.net/KEM.274-276.391

Processing Fine-Grained and Superplastic AZ31 Mg Tubes for Hydroforming

The microstructures and mechanical properties of the AZ31 Mg tubes fabricated by one-pass forward piercing tube extrusion operated at 250-400 o C and 10 -2 -10 0 s -1 are examined. The grain size is refined from the initial ~75 μm grain size down to ~1.5 μm. The room temperature tensile elongation along the extrusion direction also increases from ~13% for the as-received billet up to 51%. The highest superplastic elongation of 610% was obtained as tensile loaded at 300 o C and 2x10 -4 s -1 , and high strain rate superplasticity of 406% and 502% was achieved at 300 o C and 400 o C with a high strain rate of 1x10 -2 s -1 . Preliminary hydroforming or tube bulging at room temperature has demonstrated the feasibility. Hydrofoming at elevated temperature of 200 o C or above should exhibit much more promising results, utilizing the capability of LTSP and HSRSP of the extruded tubes.

Analyses of Plastic Flow and Die Design during Hot Extrusion of Magnesium Alloy Strips

This study involves analyses and experiments of hot extrusion of magnesium alloy strips. Hot compression tests were firstly conducted to obtain the magnesium alloy’s plastic flow stresses at high temperatures. These data are used in the finite element simulations of the thin strip extrusion process. Using the FE simulations, the flow pattern of the magnesium alloy billet within the die, the temperature variation and the thickness distribution at the die exit were analyzed. The effects of different die bearing height design, initial billet temperatures and ram speed on the extrusion load, the temperature at the die exit and the product thickness distribution were also discussed. Finally, hot extrusion experiments were conducted and the experimental values of the extrusion load and dimensions of the products were compared with the analytical values to validate the analytical model. Sound products were obtained using the best designed bearing heights and other appropriate extrusion conditions.

Study of Large-Expansion-Ratio Tube Hydroforming with Movable Dies

In this paper, finite element codes LS-DYNA and DYNAFORM are used to analyze the plastic flow pattern of a tube hydroforming into a product with large expansion ratio and eccentric axes. Tube hydroforming with a movable die is proposed to enhance the forming capacity of tube hydroforming technology. The relative speed of the axial feedings to the movable die for obtaining a sound product is determined by a geometric analysis. The whole forming processes are divided into two stages. At the first stage, an internal pressure is applied on the inner surface of the tube and two axial feedings and a movable die move forward simultaneouly. At the second stage, one of the axial feedings keeps moving forward, whereas, the movable die moves backward. With this forming schedule for the axial feedings and movable die, products with more uniform thickness distributions are obtained. Finally, experiments of tube hydroforming with a movable die are conducted. Low-carbon steels are used as the tube specimen in the experiments. The simulation results of the product shape and thickness distributions are compared with experimental results to verify the validity of the finite element modeling and the proposed forming schedules.

FEM Simulation and Experimental Verification on Rotating Compression Forming of Double-Layer Clad Ring Considering Shear Friction

This study uses FEM simulation based on Deform 3D commercial software to explore the plastic behaviors in the rotating compression of double-layer bounded clad ring under constant shear friction. The effective stress, the effective strain, the velocity field, the compression force under various forming conditions can be determined from FEM simulation. The realistic experiment has been performed to verify the acceptance of FEM simulation. From the comparisons, the compression force and free surfaces of outer and inner diameters shows a good agreement with the experiments, it proves the FEM simulation can be accepted as industry references.

An accurate upper bound solution for plane strain extrusion through a wedge-shaped die

An upper bound method for the process of plane strain extrusion through a wedge-shaped die is derived. A technique for constructing a kinematically admissible velocity field satisfying the exact asymptotic singular behavior of real velocity fields in the vicinity of maximum friction surfaces (the friction stress at sliding is equal to the shear yield stress on such surfaces) is described. Two specific upper bound solutions are found using the method derived. The solutions are compared to an accurate slip-line solution and it is shown that the accuracy of the new method is very high.

Study on Heading and Thread-Rolling Processes of Magnesium Alloy Screws

This study is to investigate the effects of the process parameters on the heading load and metal flow pattern during heading and thread-rolling processes of LZ91 magnesium alloy screws. A heading process composed of two stages is proposed. The material flow pattern of the billet inside the die is analyzed using the finite element analyses. The effects of the upper die velocity, temperatures and friction factors on the heading loads and product quality are discussed. On the other hand, in the thread-rolling process, the effects of the friction factor on the effective stress, effective strain, and tooth height are investigated. Finally, heading and thread-rolling experiments are conducted using a self-designed die set and a lubricant of MoS2. The experimental values are compared with the simulation results to verify the validity of the finite element models and the proposed heading procedures.

Adaptive simulations of T-shape tube hydroforming processes

A successful THF process depends largely on the loading paths for controlling the relationship between the internal pressure, axial feeding and the counter punch. In this study, an adaptive algorithm combined with a finite element code LS-DYNA 3D is proposed to control the simulation of T-shape hydroforming with a counter punch. The effects of the friction coefficients at the interface between the tube and die on the loading path and thickness distribution of the formed product are discussed. Experiments of protrusion hydroforming are also conducted. The final shape and thickness distribution of the formed product are compared with the simulation results to verify the validity of this modeling.

Manufacture of Magnesium Tubes with Gradient Hardness Distribution Using a Two-Stage Porthole Extrusion Die

This paper aims to manufacture magnesium alloy tubes with gradient hardness using hot extrusion process. A two-stage porthole die together with a mandrel is designed to have a straight channel part combined with a conical part. Materials are pushed through this specially-designed die cavity to generate a non-uniform velocity distribution at cross sections between the mandrel and the die and result in different strain and strain rate distributions. Accordingly, a gradient microstructure or hardness product can be obtained. Using the finite element analysis, the effective strain distributions inside the die cavity and at the die exit are firstly discussed for various inclination angles in the conical part of the mandrel. Then, hot extrusion experiments with a two stage porthole die set are conducted to obtain magnesium alloy products with gradient microstructures and hardness. Using a die set of mandrel inclination angle of 10o and die inclination angle of 25o, gradient microstructures of grain sizes of 4.30μm, 5.92 μm and 3.67μm at the outer surface, center zone, and inner surface, respectively, are achieved.