Fuguo Li

Severe Plastic Deformation Techniques for Bulk Ultrafine-grained Materials

Ultrafine-grained (UFG) metal materials processed by severe plastic deformation (SPD) have attracted the great interest. This overview introduces some attractive SPD techniques. Special attention is paid to two new deformation techniques named Elliptical Cross-section Spiral Channel Extrusion with Equal-area (ECSEE) and Elliptical Cross-section Spiral Channel Drawing with Equal-area (ECSDE). The mechanism and microstructure transformation characteristics of materials in SPD, current problems and ongoing research are also discussed in detail.

Deformation Analysis of Elliptical Cross-Section Spiral Equal Channel Extrusion Technique

Abstract The simplified slice-plain-strain method and the incorporating incremental superposition theory were adopted for the cumulative effective strain (CES) of elliptical cross-section spiral equal-channel extrusion (ECSEE) process. The ECSEE deformation was divided into two basic deformation modes: round-ellipse/ellipse-round cross-section transitional channel deformation and elliptical cross-section torsion transitional channel deformation, through tracking a particle of the cross section. The change laws for the combined CES of the particle with the channel length and the combined effective strain (ES) distribution were obtained by MATLAB software programming, and the results were compared with these via Deform-3D finite element method (FEM). The results show that the ECSEE accumulation torsion strain is greater than that of other forms, and the shear deformation is dominant. The blank cross-section ES presents the gradient decreasing trend from the periphery to the center. The FEM results also verify the accuracy of analytical solution.

Microhardness Distribution and Microstructural Evolution in Pure Aluminum Subjected to Severe Plastic Deformation: Elliptical Cross-Sectioned Spiral Equal-Channel Extrusion (ECSEE)

Abstract Elliptical cross-sectioned spiral equal-channel extrusion (ECSEE), one of the severe plastic deformation techniques, is of great efficiency in producing bulk ultrafine or nanostructured materials. In this paper, the simulation and experimental researches on ECSEE of high-purity aluminum were conducted to investigate the equivalent strain distribution and microhardness distribution on three orthogonal planes, as well as microstructural evolution. Simulation result shows a significant strain gradient on three planes. Microhardness tests comprise the similar results to strain distribution. According to transmission electron microscopy (TEM) results, microstructural evolution ranged from coarse structures to ultrafine structures by undergoing the shear bands, subgrains, high-angle misorientation grain boundaries and equiaxed structures. There are also some distinctions with reference to grain refinement level, grain boundary styles and dislocation distribution on different positions. The TEM investigations are in good agreement with microhardness tests.

Finite Element Simulation of Superplastic Isothermal Forging Process for Nickel-Base PM Superalloy

The Chinese nickel-base powder metallurgy (PM) superalloy FGH96, which was processed through hot isostatic pressing, is very difficult to deform. FGH96 superalloy has better superplasticity in special deformation conditions and superplastic isothermal forging is the best formation method at present. The accurate constitutive equations of the FGH96 alloy was established depended on the isothermal compression experiments. A two dimensional and thermomechanical coupled axisymmetric finite element(FE) model in which both part and die were taken in consideration was established to fully simulate the FGH96 superalloy turbine disk superplastic isothermal forging process. Some physical parameters about the turbine disk forging process, such as load, stress field and strain field were calculated at different temperature within the forging range of FGH96. The regularity of peak equivalent stress acted on die cavity surface, yield limit and ultimate strength of die material during the forging process was found. Based on the regulation, peak equivalent stress acted on cavity surface must be extremely less than yield limit of die material, the optimized processing parameter 1050°C that is the best deformation temperature for the alloy was determined. That was proved better in practice and high quality disk was forged.

Preform Design of Powder Metallurgy Turbine Disks Using Equi-Potential Line Method

In a material hot forging process, rational preform design not only ensures that metal flows properly in die cavity and that final products have excellent quality, but also reduces tooling cost. In the present work, it is proved in theory that the differential equation of electric potential (∇ 2 Φ=0) in the electrostatic field is similar to the differential equations of velocity potential function (∇ 2 φ=0) and velocity stream function (V 2 ψ=0) in velocity field during the material forming process, with all three represented in the form of the Laplace equation. Moreover, the material flow in the plastic stage and the energy in electrostatic field all meet the least-energy principle. Therefore, according to the similarity criteria, an equi-potential line (EPL) method is proposed for the design of the preform shape in material hot forging. Different voltages are applied to the billet shape and the final product shape to generate a proper electrostatic field. One optimal equi-potential line is selected among the innumerable equi-potential lines as the basic shape of the preform shape and is processed into the preform shape following a three-step procedure. The preform design by the EPL method is compared with that by the traditional industrial method. The results show that the proposed method for preform design is feasible and reliable for practical applications.

Microstructure and Microtexture Evolution of Pure Titanium during Single Direction Torsion and Alternating Cyclic Torsion

Systematic experimental studies of microstructure and crystallographic texture of pure titanium during the Single Direction Torsion (SDT) and Alternating Cyclic Torsion (ACT) are carried out at room temperature. The microstructure evolution indicates that the grain size can be refined during SDT, while the grain morphology can be controlled during ACT. Also, lots of {10-12} and few {11-22} twins are observed and their area percentages increase with increasing torsion angles during SDT. The microtexture evolution states that the deformation texture first approaches to the B fiber (0, 90, 0 to 60 deg), and then stays away from B fiber (0, 90, 0 to 60 deg) with increasing plastic strain during SDT. The change of deformation texture is mainly attributed to the appearance of {10-12} twin. However, the deformation texture is always close to B fiber (0, 90, 0 to 60 deg) during ACT. Finally, the effects of different dislocation movements caused by SDT and ACT are discussed. Quantities of subgrains with high density dislocation are observed during SDT while the {10-12} and {11-22} twins intersect with each other, and high density dislocations distribute the twin during ACT.

Micro-structural Evolution in Metals Subjected to Simple Shear by a Particular Severe Plastic Deformation Method

Simple shear (SS) has been considered an optimal deformation method of severe plastic deformation (SPD). To achieve SS, a particular SPD method known as mutative channel torsion extrusion (MCTE) was designed based on the geometric equivalence of SS, and the cavity parameters of a die were calculated according to strain equivalence. To investigate the characteristics of micro-structural evolution subjected to MCTE, simulated and experimental investigations were conducted. The simulated results indicate that equivalent strain distribution on the cross section is relatively uniform, and the metallographic observations confirm the simulated phenomenon. Transmission electron microscopy investigations show that the process of grain refinement undergoes the formation of shear bands, dislocation cells, dislocation forests, large-angle grain boundaries, and recrystallization nuclei. Two types of mechanisms are proposed in view of the different effects of SS on grain refinement. Eventually, MCTE is ensured as an effective method for grain refinement.

Fracture Analysis of Pipe Sleeve on Extrusion Flare-Less Pipe Joints for Aircraft Hydraulic Pipe

The forming process of aircraft hydraulic pipe joints is investigated through experiments and analyses to solve the cracking problem of pipe sleeve on extrusion flare-less pipe joints. It can be concluded that the internal cause of the fracture failure is connected with the content, shape and distribution of δ-ferrite in 0Cr12Mn5Ni4Mo3Al high strength stainless steel, and the external cause is associated with the bearing behavior in the extrusion-bulging process of pipe sleeve. The crack is formed in the criss-crossing section of δ-ferrite on the function of shear stress in the stress concentration position, which belongs to ductile deformed crack. It eventually induces the intergranular fracture of pipe sleeve along the long axis with the expanding of subsequent crack.

Material Factors Analysis of the Cracking in 0Cr12Mn5Ni4Mo3Al Stainless Steel Pipe Sleeve on Aircraft Flare-Less Pipe Joints

It is shown that the discreteness of the chemical compositions of 0Cr12Mn5Ni4Mo3Al steel is large and the range of the Cr/Ni equivalent ratio is wide, which has big impact on the content and morphology of ferrite and easy to occur cracking phenomenon when bearing, based on the chemical compositions analysis of pipe sleeve on aircraft flare-less pipe joints. It can be found that the chemical compositions of cracked pipe sleeve are almost meeting the requirements, but the Cr/Ni equivalent ratio is on the high side. So it can be concluded that the fracture failure of pipe sleeve is closely related to the content, shape and distribution of δ-ferrite. Thus, the available solution of the cracking in 0Cr12Mn5Ni4Mo3Al stainless steel pipe sleeve is to reduce the Cr/Ni equivalent ratio within the permissive of nominal chemical compositions and adjust the hot working process to form the reasonable morphology and distribution of δ-ferrite.

A New Optimization Method of Constitutive Equation for Hot Working Based on Physical Simulation and Numerical Simulation

This paper advances a new optimization method about material constitutive equation on the basis of physical simulation and numerical simulation results which basic thinking can be described as the following: through comparing the results of the material deformation process under actual experimental conditions and virtually simulated by the finite element numerical simulation method with the constitutive equation established on the basis of the physical simulation, the constitutive equation established by the experimental data is optimized in turn. Based on it, this paper advances a visco/plastic constitutive equation to depict the semi-solid thixo-forming and the constitutive equation is analyzed and optimized through coupling of the physical simulation and numerical simulation. It is observed that this method can effectively eliminate the influence of the factor outside material itself on the constitutive equation. So, it can exactly depict the deformation behavior of the materials and improve the accuracy and reliability of the numerical simulation.