Key Engineering Materials Vols. 554-557

Abstract: The in-plane torsion test is used to determine plastic flow curves for sheet metals. Very high strains of up to an equivalent strain of 1.0 can be measured since there are no edge effects in a plane torsion specimen. In combination with optical strain measurement, an efficient evaluation method for this test was developed. However, the achievable strain varies for each material. The slippage between the inner clamps and the specimen was found to be one main limiting effect. In order to improve the clamping capability, different surface corrugations are applied at the inner clamping tool. Four sheet materials, DC06, DP600, AA6016, and AA5182 are selected for testing this new clamping setup. While the flow curve of DC06 is determined until a strain of 1.0 and above, such high values cannot be achieved for the other materials. It can be shown that the measurable strain can be increased by the choice of the surface corrugation features at the inner clamping. For the DP steel and the aluminum alloys, the flow curve can be determined until equivalent plastic strains of 0.5 to 0.6, which is also a significant improvement compared to many other sheet metal testing methods.

Abstract: Welding properties of a tailored blank which consists of dissimilar metals are interesting because of the possible benefits for light weight design of materials. Especially, the combination of mild steel and aluminum alloy is expected as a representative of such a light hybrid material.Recently, our research team has developed a butt laser welding method with compression using CO₂ laser. Joint strength of mild steel/industrial aluminum alloy joints welded by this method has not been reported in detail. In this study, variations of joint strength of SPCC/A5052 joints by welding conditions, i.e. laser power, pulse frequency and butt compressive stress, were investigated. Furthermore, the welded interface of a SPCC/A5052 joint obtained was observed and analyzed using FE-SEM and EDS.

Abstract: Material behaviour description frequently used in commercial codes may not be adequate to simulate real forming processes. One of the reasons is the fact that they rarely include the modeling of internal damage of material. This is a decisive feature in order to be able to predict defective parts in processes like forging or to describe processes in which fracture is a part of the process itself as in sheet blanking or metal cutting. In large deformation of metals, when plastic deformation reaches a threshold level, which may depend on the loading, the fatigue limit and the ultimate stress, a ductile damage process may occur concomitantly with the plastic deformation due to the nucleation, growth and coalescence of micro-voids. Although damage and plastic deformation are two distinct dissipative processes, they influence each other. In this paper a numerical benchmark of the uniaxial tensile tests, for aluminium alloy, has been performed using Ls-Dyna and Deform 2D without damage. Then, a numerical uniaxial tensile tests has been studied using a coupled model of elasto-plasticity and ductile damage implemented in LS-DYNA. Experimental material property present in literature has been used.

Abstract: Within the constitutive framework adopted here, the plastic distortion is described by multislips in the appropriate crystallographic system, the dislocation densities $\rho^{\alpha}$ and hardening variables $\zeta^{\alpha}$ in the $\alpha-$slip system are the internal variables involved in the model. The rate type boundary value problem at time $t$ leads to an appropriate variational equality to be satisfied by the velocity field when the current state of the body is known. Numerical solutions are analyzed in a tensile problem when only two physical slip systems are activated in the single (fcc) crystal sheet. The slip directions are in the plane of the sheet, while the normals to the slip planes are spatially represented. At the initial moment the distribution of the dislocation density is localized in a central zone of the sheet and in the tensile problem no geometrical imperfection has been introduced. The plane stress state is compatible with the rate type constitutive formulation of the model. The FEM is applied for solving the variational problem in the actual configuration, together with a temporal discretization of the differential system to update the current state in the sheet. The activation condition, which is formulated in terms of Schmid's law, allows us to describe the spread of the plastically deformed zone on the sheet.

Abstract: The mechanical behavior of DP980 steel sheets of 1.7 mm thickness has been investigated with both tensile and bending tests. Free bending tests were performed on square samples of 60mm side. The bending tool has a sharp radius of around 0.4 mm and the sample simply lies on two rollers. Scanning electron micrography observations were performed in order to check the occurrence of cracks, that indicate the onset of rupture in bending. Moreover, X-ray microtomography observations were performed on smooth and notched tensile specimen, with a specific small-size geometry, and bending specimen. Maximum void volume fractions of 1.5 10^-3 were recorded and the influence of the triaxiality ratio was investigated, by changing the notch radius. In the case of bending, samples were cut in the bent area and void volume fraction distribution was analyzed along the sheet thickness. Material parameters for Gurson-Tvergaard-Needleman (GTN) model, associated with isotropic hardening and von Mises yield criterion, were identified from the tensile tests. Inverse identification was performed over the different sample geometries, showing that GTN model can not capture the triaxiality ratio influence. Finite element simulations of the bending test were then carried out, in order to compare experimental and predicted void volume fractions in the sheet thickness.

Abstract: Oxide dispersion strengthened (ODS) steels, produced by powder metallurgy, are considered as promising material for high burn up cladding tubes for future Sodium Cooled Fast reactors. They present superior radiation resistance compared with austenitic steels and high creep strength due to reinforcement by the homogeneous dispersion of hard nano-sized particles. While the manufacturing route of 9Cr-based martensitic ODS steels are relatively well mastered thanks to the alpha  gamma phase transformation, the cold processing of ferritic ODS steels is more complicated because the material recovery after amounts of cumulative plastic strain is quite difficult. The aim of this study is to investigate several possible cold rolling routes for a Fe-14Cr-1W-0,3Ti-0,3Y2O3 ODS ferritic grade comparing the effects of annealing temperature on cold-workability, microstructure evolution and mechanical properties. A three-roll type HPTR rolling mill was used to manufacture ODS steel claddings. Cold rolling passes and intermediate annealing were repeated until reaching the final geometry: 10.73mm external diameter and 500µm thick. Depending of the cold rolling routes, different annealing temperatures of 1150°C, 1200°C and 1250°C were applied on the mother tube. Each pass was conducted using cross-section reduction ratio varying from at least 15% up to 25%. In each case, intermediate annealing at 1200°C for 1 hour were applied between one or several passes. The optical and SEM observations, hardness measurements, tensile tests were conducted to characterize the manufactured cladding tubes. The highest annealing temperature used on the mother tube enhances the recovery which leads to the lowest hardness level. The intermediate heat treatments applied in the course of the cold processing induces relatively low decrease of hardness. Microstructure characterization of hot extruded mother tubes shows highly anisotropic structures with equiaxed grains in the transverse direction but with significant elongation in the longitudinal direction. The elongated grain structure produced during hot extrusion is retained during cold rolling processes. Tensile tests are carried out on both longitudinal and circumferential directions by mean of respectively tile and ring tensile specimens for temperatures between 20°C and 700°C. The lowest is the annealing temperature applied on the mother tube the highest is the ultimate strength and the lowest is the uniform elongation. For the lowest annealing temperature, the UTS values measured at room temperature are ~1500MPa and ~1300MPa in the longitudinal direction and the circumferential direction, respectively. UTS values around 1000MPa in the both directions are found in case of lower annealing temperature showing a less pronounced anisotropy. For each test temperature, the uniform elongation values are relatively low compared to values obtained by other authors on 12%Cr-ODS ferritic steels. The lowest values of elongation are measured around 400°C.

Abstract: The present paper aims at a theoretical study of the forming limits of a sheet metal subjected to double strain path changes by using as reference material the AA6016-T4 aluminum alloy sheet. The simulation of plastic instability is carried out through the Marciniak-Kuczynski analysis. The initial shape of the yield locus is given by the Yld2000-2d plane stress yield function. The strain hardening of the material is described by the Voce type saturation law. Linear and several complex strain paths involving single and double strain path changes are taken into account. The validity of the model is assessed by comparing the predicted and experimental forming limits under linear and selected one strain path change. A good accuracy of the developed software on predicting the forming limits is found. A sensitive analysis of the influence of the type and value of the double prestain in the occurrence of the plastic flow localization is performed. A remarkable effect of the double strain path change on the sheet metal forming limits is observed.

Abstract: Improved formability has been reported due to stress relaxation when the continuous forming cycle is interrupted with steps by adjusting the punch motion. The contribution of stress relaxation and its parameters on the ductility of materials has not been established so far. In the present work, the stress relaxation behavior of three materials, low carbon steel, DP and TRIP steels are studied. The influence of strain rate and strain on the ductility enhancement due to stress relaxation is analyzed. It is observed that stress relaxation improved the ductility of materials in all the cases and therefore can be used as a potential method to improve formability in sheet metal forming.

Abstract: Increasing acceptance and use of hydroforming technology within the aerospace industry requires a comprehensive understanding of critical issues such as the material characteristics, friction condition and hydroformability of the material. Moreover, the cost of experiments that can be reduced by accurate finite element modeling (FEM), which entails the application of adapted constitutive laws for reproducing with confidence the material behavior. In this paper, the effect of different constitutive laws on FEM of tubular shapes is presented. The free expansion process was considered for developing the FEM. Bulge height, thickness reduction and strains were determined at the maximum bulge height using different constitutive models, including Hollomon, Ludwik, Swift, Voce, Ludwigson. In order to minimize the effect of friction, the free expansion experiments were performed with no end feeding. The simulation results were compared with the experimental data to find the appropriate constitutive law for the free expansion process.