マグネシウム合金展伸材の機械的性質と成形性に及ぼす集合組織の影響

Worldwide, crystal plasticity models are currently developed to predict texture development during processing of material. Such models require a precise knowledge of the active deformation mechanisms. The activation energy for certain deformation modes will change with temperature and also depend on the chemistry of the alloy as well as the microstructure. Deformation mechanisms were studied in ZIRLO™ during room and high-temperature uniaxial compression testing. Materials with a strong crystallographic basal texture and a more random texture due to β-quenching were investigated with the aim of establishing the effect of temperature, microstructure, and texture on the active deformation modes during the initial stages of deformation. First, specimens were strained at room temperature, 180°C and 300°C to 2 % and 5 % or 10 % total strain and subsequently analyzed by Electron Back Scatter Diffraction (EBSD) to determine the texture evolution. It was found that a dramatic texture change was observed for all testing temperatures in the strongly textured specimen after only 5 % total strain, which can only be understood in terms of tensile twinning of {10 1¯2} ⟨1¯011⟩ being active mainly at room temperature and compressive twinning of {11 2¯2} ⟨1¯1¯23⟩ being operational at room and elevated temperature. The β-quenched specimens did not show any evidence of texture change when strained to 10 %. In-situ intergranular strains were measured by time-of-flight neutron diffraction during continuous compressive loading. This information enabled the development of a crystal plasticity finite element model (CPFEM), which was subsequently used to predict the stress state in individual grains. It was found that in the strongly textured material the spread of intergranular strain in the {0002} grain family (normal pointing towards the ND direction) results in some grains being in compression even though the mean stresses are tensile, which could explain the activation of the observed compressive twinning. The crystal plasticity model also demonstrated that the observed texture changes in the strongly textured material, including those at high temperature, cannot be explained by slip alone even when ⟨c+a⟩ slip is considered. In addition, the model showed that the dramatic difference in yield strength of the two conditions studied here cannot be solely attributed to the difference in texture but that grain size plays an important role.

Worldwide, crystal plasticity models are currently developed to predict texture development during processing of material. Such models require a precise knowledge of the active deformation mechanisms. The activation energy for certain deformation modes will change with temperature and also depend on the chemistry of the alloy as well as the microstructure. Deformation mechanisms were studied in ZIRLO™ during room and high-temperature uniaxial compression testing. Materials with a strong crystallographic basal texture and a more random texture due to β-quenching were investigated with the aim of establishing the effect of temperature, microstructure, and texture on the active deformation modes during the initial stages of deformation. First, specimens were strained at room temperature, 180°C and 300°C to 2 % and 5 % or 10 % total strain and subsequently analyzed by Electron Back Scatter Diffraction (EBSD) to determine the texture evolution. It was found that a dramatic texture change was observed for all testing temperatures in the strongly textured specimen after only 5 % total strain, which can only be understood in terms of tensile twinning of {101¯2} ⟨1¯011⟩ being active mainly at room temperature and compressive twinning of {112¯2} ⟨1¯1¯23⟩ being operational at room and elevated temperature. The β-quenched specimens did not show any evidence of texture change when strained to 10 %. In-situ intergranular strains were measured by time-of-flight neutron diffraction during continuous compressive loading. This information enabled the development of a crystal plasticity finite element model (CPFEM), which was subsequently used to predict the stress state in individual grains. It was found that in the strongly textured material the spread of intergranular strain in the {0002} grain family (normal pointing towards the ND direction) results in some grains being in compression even though the mean stresses are tensile, which could explain the activation of the observed compressive twinning. The crystal plasticity model also demonstrated that the observed texture changes in the strongly textured material, including those at high temperature, cannot be explained by slip alone even when ⟨c+a⟩ slip is considered. In addition, the model showed that the dramatic difference in yield strength of the two conditions studied here cannot be solely attributed to the difference in texture but that grain size plays an important role.

Deformation and fracture behaviors of AZ31 and E-form Mg alloys sheets were investigated during Erichsen test. Formability of Mg alloys was discussed in terms of Erichsen index (IE) and tests were conducted at room temperature using conventional Erichsen tester. The role of difference in initial textures and grain sizes in Mg alloys sheets was investigated to understand the deformation and fracture mechanisms in Mg alloys during Erichsen test. The evolution of the microstructure and microtexture of the deformed Mg alloys was analyzed via an electron back-scattered diffraction (EBSD) technique. Crystal plasticity finite element method (CPFEM) was used to predict the micromechanical deformation behavior of Mg alloys during Erichsen test. EBSD analysis revealed that deformation twins along with shear localization by dislocation slip were the main deformation mechanisms during Erichsen test. E-form Mg alloys with a weaker basal texture show higher IE compared to AZ31 alloy with a stronger basal texture.

Role of grain structure, grain boundaries, crystallographic texture, precipitates, and porosity on fatigue behavior of Inconel 718 at room and elevated temperatures

Evolution of Crystallographic Texture of Deformed Mg at Room Temperature

The effect of changing strain paths on forming limits of aluminum alloy 2008-T4 has been investigated by determining forming limit diagrams (FLDs) after prestraining. Sheets were pre- strained to several levels in uniaxial, biaxial, and plane-strain tension parallel and perpendicular to the prior rolling direction (RD). Abrupt changes in the strain path can be used to increase the forming limits. Prestraining in biaxial tension generally lowers the forming limits for the entire FLD, whereas prestraining in uniaxial tension raises the limits on the right-hand side of the FLD without much effect on the left-hand side. Prestraining in plane-strain tension raises both sides away from the minimum. Finally, it was found that after prestraining, the amount of the additional plane-strain deformation possible depends on the effective strain during prestrain.

Deformation twinning in Ti-6Al-4 V during low strain rate deformation to moderate strains at room temperature

The superplastic blow forming limits for a 5083 aluminum alloy were investigated at a strain state from the plane strain to the balanced biaxial at a temperature of 773 K and a strain rate of about 1 � 10� 3 s� 1. It was discovered that the equivalent strain at the forming limit was in a range of 1:3 � 0:1 for every strain state and was consistent with that obtained by the uniaxial tensile tests. Namely, the forming limit for the blow forming could be estimated from the equivalent strain-to-fracture in the uniaxial tensile test. The cavity volume fraction and the cavity growth rate at the forming limit, however, increased by changing the strain state from uniaxial to balanced biaxial through a plane strain. The relationship between the forming limit and the cavity volume fraction is dependent upon the strain state. Most research on the deformation characteristics of super- plastic materials has been carried out using uniaxial tensile tests to evaluate the flow stress, strain rate sensitivity, elongation and cavity volume fraction at various temper- atures and strain rates. The uniaxial tensile tests at high temperatures facilitated a comparison of the phenomena of creep and superplasticity and enabled the elucidation of the scientific characteristics of superplasticity. However, it is difficult to accurately estimate the capability of the super- plastic blow forming from the results of the uniaxial tensile test. The superplastic blow forming is usually performed under the biaxial tensile conditions. Therefore, the deforma- tion characteristics of the materials under the biaxial tension at high temperatures are required in order to compare them with those under the uniaxial tension. For instance, the prediction of the forming limit for the biaxial tension from the uniaxial tension is expected. There are many studies on the experimental and theoretical research regarding the cold press-forming limit for steels and aluminum alloys. 1-7) The theoretical and experimental results for the cold press- forming limit diagram showed that the minor strain at the plane strain state was lower than that at the biaxial strain state because the effect of the strain rate sensitivity on the plastic stability was not expected at room temperature. There are a few reports on the superplastic-forming limit. 8) Mahoney et al. carried out the superplastic deformation for the 7475 aluminum alloy at the uniaxial tension, the plane strain and the balanced biaxial strain state. 8) They showed that a forming limit diagram was developed to relate the damage due to cavitation during forming to a corresponding decrease in the mechanical properties and that a major strain at the plane strain was as large as that at the biaxial strain state. They also described that the strain state has no measurable influence on cavity growth rate during the superplastic forming. The result is somewhat surprising since the mean stress is known to influence cavitation in creep and super- plasticity. 9) In this study, a superplastic blow-forming machine was constructed for laboratory use in which the pressure-time path was precisely controlled by a personal computer. Using this equipment, the superplastic forming for a 5083 aluminum alloy was performed in a range of strain state from a plane strain to a balanced biaxial strain. The purpose of this study is to show the forming-limit diagram for a 5083 alloy and a relationship between the forming limit and cavitation.

A different approach for considering the effect of non-proportional loading path on the forming limit diagram of AA5083

In this study, experimental and numerical analyses of Forming Limit Diagram (FLD) for Advanced High Strength (AHS) steel grade 980 were performed. Forming limit curve was first determined by means of the Nakazima stretch-forming test. Then, analytical calculations of the FLD based on the Marciniak-Kuczynski (M-K) model were carried out. Different yield criteria, namely, Hill’48 (r-value and stress-based), Yld89 (r-value and stress-based) and Barlat2000 (Yld2000-2d) were investigated. The strain hardening law according to Swift was applied. To identify parameters of each model, uniaxial tension, balanced bi-axial bulge test and in-plane biaxial tension test were performed. As a result, predicted plastic flow stresses and plastic anisotropies of the AHS steel by various directions were evaluated. In addition, effects of the anisotropic yield functions, strain rate sensitivities, imperfection values and work hardening coefficient on the predicted FLD were studied and discussed. It was found that the FLD based on the Yld2000-2d yield criterion was in better agreement with the experimental curve. Accuracy of the FLD predictions based on the M-K theory, especially in the biaxial state of stress, significantly depended on the applied yield criteria, for which yield stresses and r-values of different loading directions were required.

Failure analysis of AZ31 magnesium alloy sheets based on the extended GTN damage model

The formability of aluminum alloy 6061-T6 sheet was evaluated, and the effects of temperature and strain rate on the formability were analyzed. Uniaxial tension tests and Nakajima tests were conducted at room temperature to obtain the constitutive parameters of AA 6061-T6 and establish the forming limit diagram (FLD), respectively. Moreover, uniaxial tension tests were performed at the temperatures ranging between 180 and 380 °C and the strain rates ranging between 0.0005 and 0.05 s−1, and the constitutive equations of AA 6061-T6 were established. Nakajima tests at temperature 330 °C and two forming speeds (15 and 150 mm/min) were carried out to evaluate the formability of AA6061-T6 at elevated temperatures. In consequence, FLDs under different forming conditions were established and compared. Experimental results showed that the forming limit of AA 6061-T6 increased with the increasing temperature and the decreasing forming speed. The present investigation presented the formability of AA 6061-T6 under different forming conditions, which provided a guidance to design the warm/hot forming of AA 6061 sheet. The FLDs and constitutive equations established through these experiments will be used to predict the forming defects in the forming process design.

The forming limit diagram (FLD) is an important tool to be used when characterizing the formability of metallic sheets used in metal forming processes. Experimental measurement and determination of the FLD is time-consuming and therefore the analytical prediction based on theory of plasticity and instability criteria allows a direct and efficient methodology to obtain critical values at different loading paths, thus carrying significant practical importance. However, the accuracy of the plastic instability prediction is strongly dependent on the choice of the material constitutive model [1–3]. Particularly for materials with hexagonal close packed (HCP) crystallographic structure, they have a very limited number of active slip systems at room temperature and demonstrate a strong asymmetry between yielding in tension and compression [4, 5]. Not only the magnitude of the yield locus changes, but also the shape of the yield surface is evolving during the plastic deformation [4]. Conventional phenomenological constitutive models of plasticity fail to capture this unconventional mechanical behavior [4, 6]. Cazacu and Plunkett [6] have proposed generic yield criteria, by using the transformed principal stress, to account for the initial plastic anisotropy and strength differential (SD) effect simultaneously. In this contribution, a generic FLD MATLAB script was developed based on Marciniak–Kuczynski analytical theory and applied to predict the localized necking. The influence of asymmetrical effect on the FLD was evaluated. Several yield functions such as von Mises, Hill, Barlat89, and Cazacu06 were incorporated into analysis. The paper also presents and discusses the influence of different hardening laws on the formability of materials with HCP crystal structures. The findings indicate that the plastic instability theory coupled with Cazacu model can adequately predict the onset of localized necking for HCP materials under different strain paths.

Production of high strength, light weight automotive components is one of the major thrust areas in the automotive sector, which is achieved by using advanced high strength steels. However, problems associated with these steels are lower formability and high springback effect at room temperature therefore hot stamping process is widely used to address those challenges. Hot stamping or press hardening process is a high temperature deformation process where specimen is heated above austenization temperature and held at this temperature for appropriate time for homogenization. Once temperature of specimen is uniform, specimen is transferred to forming press where forming and quenching take place simultaneously. Due to elevated temperature and quenching involved in the process, determination of formability is difficult. For determination and prediction of formability of 22MnB5 steel, experiments are performed using Gleeble-3800 at various temperatures and strain rates. In Gleeble-3800 uniaxial movement is possible only and therefore in this work notch and simple tensile test are performed to obtain Forming Limit Diagram (FLD) in drawing region. Thickness gradient-based necking criterion was used for FLD determination. ANN model was developed to predict flow behavior and FLD. It was found that in drawing region of FLD formability increases with increase in temperature and strain rate. The ANN model developed was able to accurately predict the constitutive behavior and limit strains in drawing region.

Heterogeneity in deformation and twinning behaviors through the thickness direction in E-form Mg alloy sheets during an Erichsen test