Development of a novel differential velocity sideways extrusion process for forming curved profiles with fine grains and high strength

The aim of this study is to develop a novel process, differential velocity sideways extrusion (DVSE), for forming curved profiles with fine grains and high strength. In this new forming-bending-refining process, billets are used as the work-piece material to directly form curved profiles with certain cross-sections in order to increase the manufacturing efficiency and decrease the bending defects in conventional bending process. The DVSE process has been studied in this thesis by using forming experiments, microstructure characterisation experiments, finite element (FE) modelling and theoretical modelling. A tool set enabling sideways extrusion to be performed using opposing punches moving with different velocities was designed and manufactured. Plasticine was used as a model work-piece material and a series of compression tests were undertaken, to determine its constitutive properties and gain an estimate of work-piece die friction for use in process simulation. Feasibility studies for the DVSE process were carried out through a series of designed experimental programmes on plasticine, in which punch/extrusion velocity ratio, extrusion ratio and die land length were process parameters. Ultimately, trial tests using AA1050 at room temperature and AZ31 at elevated temperatures were conducted. Effects of extrusion velocity ratio, extrusion ratio, die land length, forming temperature and strain rate on profile curvature were studied. The microstructure evolution of the formed curved AA1050 bar by DVSE at room temperature was studied through EBSD. The evolution of grain structure and texture of formed curved AZ31 bars at different DVSE process conditions (temperature and strain rate) was investigated through optical microscopy and EBSD, and the optimum temperature and strain rate condition for obtaining fine equiaxed and homogeneous microstructure was identified. The different grain refinement mechanisms of AA1050 and AZ31 during the DVSE process were revealed. Micro-hardness of formed curved AA1050 and AZ31 bars was examined. Process mechanics of DVSE were modelled using FE modelling and upper bound theorem. The extent of work-piece flow velocity gradient across the die exit orifice, which causes curvature, was identified. A dead zone of roughly triangular shape, which exists on the chamber wall opposite the die exit orifice, was determined. The effective strain of the formed curved profiles was studied to confirm the rise of severe plastic deformation (SPD). The effective strain rate in the intersection regions of the channels was investigated to identify the source of severe plastic deformation. An analytical upper-bound-based model has been developed with the consideration of the determined dead zone. The extrusion force and curvature predicted by the analytical method agreed reasonably well with results from experiments and FE modelling. Discussions were made about the correlations between experimental and modelling approaches and results. The relationships between mechanical properties (yield strength, ultimate tensile strength, and elongation to failure) and microstructures (grain size, micro-texture) of formed curved profiles were correlated. From the experimental and modelling work, it has been demonstrated that the DVSE process proposed in this thesis is an effective way to efficiently form curved aluminium and magnesium profiles with controlled curvature and improved properties.Open Acces

Microstructural and texture evolution of Jethete M152 flanged-test pieces during cold rotary forging

Rotary forging is an attractive incremental metal forming with many advantages over any other processes, requiring smaller deformation force and providing high accuracy (near-net-process). The main applications of rotary forging process include families of bevel and helical gears, and flanged components for transmissions such as disk, rollers, wheels, etc. The main aim of this work is to study the impact of rotary forging process on the microstructural and texture evolution of high-strength materials, and martensitic stainless steels in particular, during cold rotary forging process. Jethete M152 alloy is a cold formable 13%-Cr martensitic stainless steel used in the aerospace industry. Jethete M152 flanged test-pieces were rotary forged at room temperature. The process was interrupted at 4 intermediate steps, providing flange reductions of 25, 30, 50, 65 and 70 %. A complex grain flow and inhomogeneous deformation patterns are developed during rotary forging, characterized mainly by the formation of a strong deformation band which run parallel to the bottom die. A transition from asymmetrical bulging (inverted mushroom) to symmetrical bulging was observed as a result of the initial lower contact area of the preform with the bottom die. From microstructural analysis by EBSD, the lath structure of Jethete M152 is gradually reoriented and changes it shape in a direction parallel to the compression plane, developing a lamellar/pancake structure in those positions with maximum deformation. These microstructural changes are accompanied with the development of a strong texture formed by a duplex + fibers aligned with the compression axis, being the fiber the stronger one. These findings are in good agreement with uniaxial compression for bcc metals. The analysis of the Orientation Distribution Figures (ODF) reveals that 4 main texture components are formed in the course of the rotary forging process: Brass {110}〈112〉, L {110}〈110〉, I {112}〈110〉, and Cube {001}〈100〉. In contrast with reported literature for bcc metals, no texture component associated to the γ-fiber ({111} ‖ ND) was found

Characterization and Modeling of Forged ZK60 Mg Alloys under Quasi-static and Fatigue Loadings

In response to impending changes to the environmental regulations on the vehicles’ gas consumption rate, the transportation sector is motivated toward the widespread adoption of lightweight materials in the manufacturing of its products. Magnesium (Mg) alloys, being the lightest commercial metals available in the industry, can play an integral role in this scope with offering huge mass saving comparing to aluminum and steel. With the development of multi-material vehicle architecture concept in the automotive industry philosophy, ultra-light materials such as Mg alloys should be exploited in the component in which they perform the best. For vehicle parts which are driven by fatigue, e.g., suspension control arm, wrought Mg is a suitable candidate as a substitute for the current structural metals, due to its excellent fatigue performance in addition to opening mass saving windows. Of the commercially available Mg alloys, ZK- series, and in particular, ZK60 Mg alloys, have shown superb mechanical properties and formability. Moreover, amongst several prevalent industrial manufacturing techniques, forging is of particular interest because it has shown its promise to produce components with complex geometry and high strength. However, the mechanical behavior of forged ZK60 has been largely unknown so far. The current research work has aimed to fill this gap by characterizing the mechanical behavior of forged ZK60. The focus has been to establish a link between the material, structure and performance. The discovery-level knowledge, established through this project, develops a better understanding of the fatigue performance of this alloy and provides a wealth of mechanical performance data on this material in order for industry to make better use of it in real-world applications. Initially, the effects of open-die forging on the mechanical behavior of cast ZK60 was studied. A partially recrystallized microstructure with sharp basal texture was developed in the material. Also, the porosity volume was dramatically reduced after the forging. As a result, the tensile yield strength and elongation were increased by 21% and 72%, respectively. Under cyclic loadings, the forged material exhibited a better response in both the low cycle fatigue (LCF) and high cycle fatigue (HCF) regimes in the light of its higher ductility and strength, and lower content of porosities and intermetallic particles, which can play as crack nucleation sites. The fracture surfaces of the samples tested at various strain amplitudes were analyzed using the scanning electron microscope (SEM), and different crack initiation mechanisms were identified. At low strain amplitudes, corresponding to the HCF regime, persistent slip bands (PSB) and second-phase intermetallic particles were defined as the major causes of crack initiation, whilst at high strain amplitudes, ascribed to the LCF lives, the interactions between twin-twin bands besides twin-dislocation were recognized as the key reasons for cracking. Next, the quasi-static and strain-controlled fatigue characteristics of ZK60 extrusion have been investigated along different directions, namely, the extrusion direction (ED), the radial direction (RD), and 45° with respect to the extrusion direction (45°), in the scope of process-structure-property-performance relationships. In contrast to the asymmetric quasi-static behavior of extrusion direction, radial and 45° directions manifested symmetric responses. Also, ED samples showed higher strength compared to the other two directions. The strain-controlled fatigue performance of ZK60 extrusion was insensitive to the material direction in the LCF regime. However, in the HCF regime, ED displayed fatigue responses superior to the RD and 45°. The texture measurement indicated a sharp basal texture along ED. Also, microstructural analyses revealed binary microstructure, explaining the ED’s asymmetric behavior and higher strength comparing to the other directions. Higher tensile mean stress and less dissipated plastic energy per cycle for the ED samples, acting as two competing factors, were the principal reasons for ED’s identical fatigue response to that of RD and 45 in the LCF regime. The fracture surface in the ED direction was dominated by twin lamellae and profuse twinned grains, whereas that in RD was dominated by slip bands. Lastly, Smith-Watson-Topper and Jahed-Varvani models were employed to predict the fatigue lives along all directions using a single set of material parameters. The energy-based model yielded acceptable predictions for ZK60 extrusion with anisotropic behavior. Finally, the multiaxial fatigue characteristics of as-extruded and close-die “I-beam” extruded-forged ZK60 were investigated. Quasi-static tension and shear tests in addition to uniaxial, pure shear, and multiaxial cyclic tests under variety of loading paths as well as texture measurements and microstructural analysis were delivered to characterize the material’s behavior and understand the effects of forging on the performance of the alloy. It was discovered that the imparted thermo-mechanical process modified the initial sharp basal texture in the flanges of the I-beam. Secondly, following forging, the microstructures in the two flanges of the I-beams were similar. Furthermore, following the quasi-static tests, it was revealed that the axial behavior of the forging was superior to that of the starting extruded material, whereas the shear responses were comparable. Multiaxial fatigue tests demonstrated that non-proportionality does not change the fatigue life tremendously; however, it does affect the shear response remarkably. It was concluded that at low shear strain amplitudes that the shear strain is accommodated by slipping, the multiaxial behavior is somewhat dominated by the axial component. The microstructure of both undeformed and deformed samples after 20% shear strain under quasi-static loading was studied using the EBSD technique. The deformed sample showed considerable amount of {101 ̅2} tensile twins in the microstructure; hence, the developed texture plays an integral role in the material’s shear behavior under large strains where appreciable extension twin occurs. Finally, an energy-based fatigue model was employed that effectively explains the different damage contributions by the axial and torsional loadings at different strain amplitudes, and accurately predicts the proportional and non-proportional multiaxial fatigue lives for both as-extruded and forged alloys

Characterization of the Quasi-Static and Fatigue Behavior of Forged AZ31B Magnesium Alloy

Light-weighting is being aggressively targeted by automotive manufacturers as a method to reduce emissions. Magnesium (Mg) alloys due to their low density and high specific strength have therefore gained significant interest for automotive applications. As cyclic loading is a major concern for load-bearing automotive components, this thesis aims to characterize the fatigue response of AZ31B Mg alloy that may be forged to its final component geometry. Initially, the effect of open die forging on cast AZ31B alloy was investigated. A partially refined microstructure with strong basal texture was developed in forged material. Tensile yield and ultimate strengths increased by a remarkable 143 percent and 23 percent respectively whereas the compressive yield strength was unchanged owing to the activation of extension twins for both conditions. Nevertheless, the ultimate compressive strength for the forged material increased by 22 percent compared to the as-cast material. The influence of open-die forging on the low cycle fatigue (LCF) behavior of cast AZ31B was also investigated. The forged material was generally found to exhibit longer fatigue life and also achieved significantly higher stresses at the same level of total strain amplitude compared to cast AZ31B. The Smith-Watson-Topper (SWT) model and Jahed-Varvani (JV) energy model were employed and both models accurately predicted the experimentally obtained fatigue life of both cast and forged alloy conditions. Next, the effect of processing temperature and pre-form condition on the quasi-static response, and stress-controlled fatigue behavior of closed-die forged AZ31B Mg alloy was investigated. The forging process was conducted on billets of cast as well as extruded AZ31B at temperatures of 250°C and 375°C and a forging rate of 20mm/s. The obtained mechanical test results showed a strong influence of processing temperature and initial material condition on the mechanical response of the forged product. For a given starting condition (either cast or extruded) a lower forging temperature resulted in superior quasi-static and cyclic properties whereas for a given processing temperature, the extruded starting billet displayed better mechanical properties compared to its cast counterpart post-forging. Extruded AZ31B forged at 250°C was observed to yield the best overall mechanical properties. Finally, quasi-static tension and shear tests in addition to cyclic axial and cyclic shear tests were also performed to characterize the uniaxial cyclic response of the previously determined optimal forgings. The cyclic axial behavior of the forging was superior to the starting extruded material while the cyclic shear behavior was comparable to low cycle behavior of extruded AZ31B in literature. Multi-axial fatigue tests revealed an increase in cyclic hardening as a result of non-proportionality however, overall fatigue life showed only a weak sensitivity of phase difference. A modified SWT model and Jahed-Varvani (JV) model were developed using the uniaxial cyclic test results. The modified SWT model was found to provide good life prediction for all the tested multi-axial load cases. The multi-axial formulation of the JV model yields good life predictions for in-phase and 45° out-of-phase multi-axial scenarios but slightly over-predicts fatigue life for certain 90° out-of-phase loading cases

Hot Compression Behaviour of Two-Phase Ti-6A1-4V: Experiments and State-Variable Modelling

The flow stress behaviour of two-phase Ti-6A1-4V and its individual α and β phases has been characterised during isothermal forging at temperatures of 850-1050°C and strain rates of 0.3-0.003/s. The influence of initial pre-form microstructure has also been investigated by heat-treating the as-received globular microstructure to produce an acicular β-transformed microstructure. It is found that flow stress behaviour exhibits a strong dependence on working temperature and imposed strain rate for the two-phase globular, two-phase acicular and single α-phase materials. However, for the single β-phase, steady state stress is found to be relatively constant with increasing temperature for the range of 925-975°C and strain rates of 0.3-0.003/s. A pronounced discontinuous yielding phenomenon has been observed for globular Ti-6A1-4V at high temperature (≥900°C) and high strain rates (≥0.01/s). On the other hand, for acicular Ti-6A1-4V, flow stress curves reveal a broad peak stress level at low strains followed by moderate to extensive flow softening until a steady-state stress is reached. For the individual phases in Ti-6A1-4V, flow stresses of the single β-phase is found to be much lower than for the α-phase and most of flow stress curves of the β-phase are of a steady-state type with negligible flow softening. There is an indication of initial work hardening behaviour after the onset of plastic deformation at low strains for deformation of single α-phase at 925-950°C and 0.3/s, whilst a pronounced flow softening is exhibited particularly at 975°C and strain rates of 0.03-0.3/s. A semi-empirical, history-dependent constitutive model for the prediction of the flow behaviour of Ti-6A1-4V which incorporates the temperature-dependent volume fraction and flow properties of the individual α and β phases is presented. It is found that a modified iso-strain approach can be employed in order to better predict the entire flow stress curve of acicular Ti-6Al-4V including the post-peak softening behaviour. This is achieved by introducing structural variable to represent the strain accumulation resulting from gross interaction mechanisms between the α and β phases during deformation. The structural interaction variable has been linked with microstructure information from deformed cylindrical work-pieces, allowing derivation of a set of microstructure relationships. To provide the basis for more general process simulations, the modified iso-strain model has been implemented as a user-routine in DEFORM-2D finite element software and validated for an isothermal forging of a complex-shape double-truncated cone specimen. Excellent agreement was observed between the predicted forging load and the measured load-displacement data and trends in the evolved microstructure were also well-predicted

Experimental and Finite Element Study of Residual Stresses in Zircaloy-4(r) and OFHC Copper Tubes.

Non-homogeneous plastic deformation associated with metal-forming operations results in a state of residual stress in the final product. Complete evaluation of the mechanical behavior of the product under operating condition requires a knowledge of these stresses. In order to evaluate the magnitude and distribution of residual stresses, existing experimental techniques are complemented by computer-aided simulation of the forming process using elastic-plastic finite element method (FEM). It is found the FEM can be effectively utilized to simulate metal-forming operations in order to determine the combination of process variables which optimizes the residual stress distribution in the final product. In the experimental phase of this study, residual stress distribution in Zr-4(R) and copper tubes is determined by electropolishing the outer(or inner) surface of the tubes while measuring the developed strains at the inner(or outer) surface. Material removal by electropolishing proved to be an efficient and suitable technique due to its constant mass removal rate under conditions which do not alter the pattern of residual stresses in the specimen. In the case of Zr-4(R) specimens, experiments were conducted on the as-received(stress relieved) as well as specimens annealed at 500 C for one hour. The effect of various degrees of cold working on the residual stress patterns of drawn copper tubing is determined by performing similar experiments on soft, 1/4-hard, 1/2-hard, and hard temper OFHC copper specimens. In the second phase of this investigation, an elasticplastic finite-element code (ABAQUS) is employed in a parametric study to determine the optimum processing conditions for drawing copper tubes. Various die/plug angles along with different area reductions are considered to simulate the drawing process. Comparison of the simulation results yields that for a given area reduction, there exists a specific combination of die and plug angles which minimizes the drawing stress. The results demonstrate that the magnitude and distribution of residual stresses are greatly influenced by the die and plug angles. It is concluded that proper selection of theses parameters increases the efficiency of the drawing process, enhances the integrity of the final product; all resulting in greater productivity and reduction in the operating costs