Material Forming ESAFORM 2015

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Authors: Dilek Halici, Cecilia Poletti
Abstract: Titanium-based alloys are commonly applied to aerospace, medicine and energy due mainly to their high specific mechanical properties and high corrosion resistance. Reinforcement with particles further improves their specific strength and stiffness. In previous studies, the hot formability of both unreinforced Ti6Al6V2Sn alloy, and reinforced with 12%vol of TiC particles was analyzed by hot compression tests carried out by means of Gleeble device and metallography. It was observed that the hot workability of these materials is limited at given forming conditions by non-desirable shear bands, voids, as well as micro- and macro cracks, especially in the composite. In this work, damage during hot deformation is predicted by damage models coupled to FEM. Therefore, the flow localization parameter α described by thermal and microstructural softening and the strain rate sensitivity are computed and implemented in DEFORMTM 2D to describe the localization of the plastic flow. The results show intense flow localization as a combination of low dynamic restoration (given by small m values) and temperature gradient. The damage analysis combined with the Cockcroft and Latham continuum cumulative stress model.
Authors: Tomoyuki Hakoyama, Toshihiko Kuwabara
Abstract: Multiaxial tube expansion tests (MTETs) were performed to measure the multiaxial plastic deformation behavior of a cold rolled interstitial-free (IF) steel sheet for a range of strain from initial yield to fracture. The testing machine is capable of applying arbitrary principal stress or strain paths to tubular specimens using an electrical, closed-loop servo-control system for an axial force and an internal pressure. Tubular specimens with an inner diameter of 44.6 mm were fabricated from a cold rolled IF steel sheet with a thickness of 0.7 mm by roller bending and laser welding. Many linear stress paths in the first quadrant of stress space were applied to the tubular specimens to measure the forming limit strains and forming limit stresses of the as-received sheet sample, in addition to the contours of plastic work and the directions of the plastic strain rates. It was found that the shapes of the measured work contours changed with increasing plastic work. The observed differential hardening behavior was approximated by changing the material parameters and the exponent of the Yld2000-2d yield function (Barlat et al, 2003) as functions of the reference plastic strain. The hydraulic bulge tests were also conducted to measure the forming limit strain and forming limit stress for equibiaxial tension and to determine the equivalent stress-equivalent plastic strain relation for a larger strain range. The forming limit curve and forming limit stress curve were calculated using the Marciniak-Kuczyński-type approach. The calculated results were in fair agreement with the measurement.
Authors: Henri Francillette, Fabien Marco
Abstract: Channel die compression tests were performed at 175 °C on polycrystalline magnesium specimens AZ31B which present specific initial textures. Deformation stages up to 90% plastic deformation are considered. The specimens were defined initially by a specific angle characterizing the starting position of the majority of the <c> axes of the hexagonal phase in the experimental device. Stress-strain curves and the evolution of crystallographic textures (using X-ray diffraction measurements) are determined. A macroscopic shearing is observed for most of the investigated specimens. A phenomenological law is applied in order to describe the experimental true stress-strain curves. The identification of the parameters of the law indicates its good ability to reproduce the experimental curves.
Authors: Holanyo K. Akpama, Mohamed Ben Bettaieb, Farid Abed-Meraim
Abstract: The main objective of this contribution is to compare the Forming Limit Diagrams (FLDs) predicted by the use of two different vertex theories. The first theory is micromechanical and is based on the use of the Schmid law, within the framework of crystal plasticity coupled with the Taylor scale-transition scheme. The second theory is phenomenological and is based on the deformation theory of plasticity. For both theories, the mechanical behavior is formulated in the finite strain framework and is assumed to be isotropic and rate-independent. The theoretical framework of these approaches will be presented in details. In the micro-macro modeling, the isotropy is ensured by considering an isotropic initial texture. In the phenomenological modeling, the material parameters are identified on the basis of micro-macro simulations of tensile tests.
Authors: Hwi Geon Kim, Jin Woo Lee, Frederic Barlat, Dae Yong Kim, Myoung Gyu Lee
Abstract: The effects of the stress state and temperature on the martensitic phase transformation behavior in TRIP 780 steel were investigated using multi-axial experimental techniques. Various mechanical experiments are performed to differentiate the stress state and temperature effects. For this purpose, five different stress states were considered; i.e., uniaxial tension, uniaxial compression, equibiaxial tension, plane strain tension and simple shear. A temperatures both 25 and 60 °C for each stress state condition except the simple shear test were investigated. In-situ magnetic measurements were performed to mesure the evolution of the martensite content throughout each experiment. Finally, a new martensitic transformation kinetics model for the TRIP 780 steel is proposed to take the effect of stress state and temperature into account.
Authors: Mohamed Sahli, Jean Claude Gelin, Thierry Barrière
Abstract: The metal injection molding (MIM) process allows the manufacturing of small and very complex metallic components. The metal injection molding processing combines the shaping capability of polymer injection molding with the large material variety of metals and ceramics. This paper discusses in detail the development of a numerical model capable of simulating micro-structural evolution and macroscopic deformation during sintering of complex powder compacts. A sintering model based on elastic–viscoplastic constitutive equations was proposed and the corresponding parameters such as bulk viscosity, shearing viscosity and sintering stress were identified from dilatometer experimental data. The constitutive model was then implemented into finite element software in order to perform the simulation of the sintering process. The numerical simulation methods being compared against results of the sintering experiments. The experimental data were obtained from sintering of 316L stainless steel powders.
Authors: L. Courtin, Stephane Urvoy, Didier Bossu, Sophie Bosonnet, Michel Tabarant, Bouzid Kedjar, Patrick Olier, Ludovic Thilly
Abstract: Due to their high void swelling resistance, work-hardened titanium stabilized austenitic steels have been chosen as cladding material for sodium cooled fast reactor. In this study, HPTR cold pilgering process is compared to cold drawing at the last shaping step of the tube processing. The effects of the cold work accumulation and heat treatments are studied in connection with the microstructure (grain size), the hardness and the texture. The following results were found:- As larger amount of cold work can be applied by HPTR cold pilgering, a lower number of intermediate heat treatments are required. In addition, the bending of the tube is significantly reduced after each pass for this process allowing for a limitation of the straightening operations.- For both processes, optical micrographs show micrometric titanium carbide precipitates and the presence of deformation twins on a few grains on the final tube. A significant grain size refinement from ≈45 μm to ≈17 μm can be obtained by reducing annealing temperature from 1403K down to 1353K. For the latter, the precipitated mass fraction measured by selective dissolution of the alloy matrix is the largest, revealing a possible negative impact on swelling under irradiation.- Through-wall Vickers hardness profiles show an increase of the hardness at the outer diameter for HPTR cold pilgering whereas the hardness profile remains continuous for cold drawing. It is found that the Q-factor has an influence on these wall-thickness hardness profiles.- For each process, neutron diffraction measurements on finished tubes reveal two main fiber texture components 〈111〉 and 〈100〉 parallel to the tube axis with differences in their relative intensities.
Authors: Manoj Kumar, Nigel Ross
Abstract: In this paper, we have investigated AW-7921 in various tempers - specifically under aged T4P1-and T4P2-tempers, naturally aged T4 and peak-aged T6-tempers - for their warm formability at 230°C and paint baking response through one step and five step paint baking treatments. It was found that warm formability depends on dynamic recovery and dissolution of precipitates. However, the response to paint baking was dependent on the stability of the precipitates against coarsening. Of the tempers investigated the T4-temper exhibited the best combination of formability and paint baking response. This study also indicated that we can achieve much better performance in the warm forming processing chain by optimization of the temper and paint baking heat treatment procedure.
Authors: Petr Král, Vaclav Sklenička, Květa Kuchařová, Marie Kvapilová, Marie Svobodová, Ladislav Horvath
Abstract: The microstructure of P92 steel pipe processed by hot bending was evaluated in order to characterize the influence of hot bending on microstructure changes in extrados and intrados of bend. The straight pipes, with an outer diameter 350 mm and wall thickness of 39 mm (Productos Tubulares, s.a.u. Spain), were normalized at 1050°C/60min/air and tempered at 740°C/140min/air.The thick-walled P92 pipe was hot bended at 920-960 °C using bending rate 7 mm/min. The bends were normalized at 1050°C/60min/air and tempered at 775°C/140min/air after hot bending. The selected specimens, machined from different locations of pipe, were additionally thermal ageing at 650 °C to simulate microstructure degradation typical for long-term service conditions.Microstructure was investigated by scanning electron microscope Tescan Lyra 3 equipped with NordlysNano EBSD detector operating at an accelerating voltage of 20 kV with specimen tilted at 70° and by transmission el. microscope JEM-2100F.Microstructure investigation revealed that different processing history influenced distribution and size of secondary phases and caused changes in interboundary spacing in straight pipe and bends. It was found that microstructure changes induced during hot bending and subsequent post-bending heat treatment caused deterioration in creep properties of bends in comparison with those of straight pipe.

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