Papers by Keyword: Rolling

Abstract: For many material forming processes steady-state formulations allows reducing numerical simulation time by an order of magnitude with respect to more conventional approaches. In the presented approach, the steady regime is iteratively computed by a free surface algorithm that alternates computations of the metal forming flow over a known geometry and known contact surfaces, with computations of domain corrections to satisfy free and contact surface conditions. Several weak formulations of the second problem equations are investigated to get a robust algorithm suitable for parallel computations with unstructured meshes. Analytical problems show the necessity to introduce an upwind shift within these weak formulations. Contact inequations enforces this necessity by requiring a more dramatic shift. A robust and accurate algorithm is so obtained, which is successfully applied to 3D complex metal forming processes like rolling. In the wire drawing application, computational time is reduced by more than fifteen with respect to the incremental calculation of the steady-state.

Abstract: In aeronautics, economic and environmental aspects become increasingly important. As those are very much influenced by the frictional drag of the airplane, a reduction of skin friction which causes a major portion of total aerodynamic drag is desirable. One possible approach for passive drag reduction is the application of riblets small longitudinal grooves orientated in flow direction. Through an adapted rolling process, riblets can be brought into metal sheets on a large scale. For this process a thin high-strength steel wire is wound around a work roll to structure it with the negative riblet imprint. In a subsequent step the riblet profile is rolled into the sheet material. Different parameters can influence the process and the quality of the resulting riblet structure. Those parameters that depend on the sheets sheet thickness, material strength, and composition of the sheet are discussed in this paper. Form filling is used as an indicator for riblet quality. It is found that decreasing sheet thickness is beneficial for form filling, but a process dependent minimum sheet thickness exists for which this effect will reverse. Material strength is found to have a much smaller influence on form filling. Nevertheless, harder alloys seem to need a slightly smaller thickness reduction, but higher rolling forces and pressures to achieve desired form filling. Using clad instead of bare materials has a positive influence on form filling and riblet structuring. Furthermore, riblet rolling does not reduce the fatigue strength of the clad material.

Abstract: Magnesium alloys are extensively used in electronics, automotive and aerospace industries due to their low densities and high specific strengths. However, limited deformability of magnesium alloys at room temperature restricts the applications. Grain refinement and texture weakening as a result of recrystallization can be used to enhance the deformability of these alloys. In this study, recrystallization behavior of cold rolled and swaged AZ31 alloy is investigated at different temperatures in the range of 200-300°C. Effects of unidirectional and complex deformation modes on recrystallization behavior and microstructure development are studied. Microstructural analyses consisted of the examination of the grain structure and twinned regions by using optical and scanning electron microscopes. The volume fraction of recrystallized grains is determined by image analysis and supported by the hardness measurements.

Abstract: The deformation and hardening mechanisms of magnesium usually lead to a typical basal orientation of crystals during a production of sheets by forming techniques. The basal texture related anisotropic property behavior and especially the further decrease in formability at room temperature is disadvantageous and undesired for subsequent rolling and final forming processes.The objective of this work is to find methods to improve these texture-related properties and the cold forming ability of magnesium sheets. Firstly, rolling at different temperatures and pass reductions, with the goal of weakening the basal texture component in semi-finished products is investigated, based on the advantageous initial texture and microstructure of twin-roll-cast (TRC) magnesium strips. In this context, texture and microstructure development is examined after a particular multi-pass rolling and heat treatment processes. Twin-roll-cast magnesium strips of alloy AZ31, with an initial thickness of 4.5 mm, rolled to a final thickness of 1.2 mm, are used as feedstock.Secondly, a new thermo-mechanical magnesium strip treatment has been developed in order to completely disintegrate the basal texture and intentionally generate only non-basal orientations with high Schmid-factors for the easy-to-activate basal slip systems. This process, which is designed as a final strip treatment, has been investigated regarding its texture change effect on rolled 1.2 mm AZ31 sheets, which also originate from TRC feedstock.It has clearly been found that the developed rolling technology for TRC feedstock leads to a significantly reduced basal texture due to grain boundary rotation and recrystallization at those rotated regions. The application of the separately developed strip treatment effects a complete elimination of the basal texture in a large volume of the sheet. Applying both technologies on magnesium sheets results in a tremendous increase in formability at room temperature as a consequence of the altered texture.

Abstract: While uniform elongation is a measure of ductility of the material, reduction in area in tensile tests is also an important measure of ductility. It was found that the reduction in area - tensile strength balance is far better than the conventional ferrite+pearlite steels and even superior to martensitic and bainitic steels. Formability of ultrafine-grained steel is examined by applying to form a M1.7 micro screw using these ultrafine-grained steels. Screws are formed through the process of cold heading and rolling. Relationship between cold heading, rolling, uniform elongation and reduction in area are investigated to clarify the formability of ultrafine-grained steels. Low-carbon ultrafine-grained steel has excellent cold headability and favorable rolling properties, i.e., excellent formability. Reduction in area is a measure to determine formability on cold heading. Ultrafine grained steel wire with length of several hundred meter were developed with the technology of warm continuous multi-directional rolling. This wire also have a good formability which can form microscrews. High strength microscrew with ultrafine grained structure was obtained.

Abstract: A fast simulation of the inhomogeneous materials evolution during the deformation steps and its effect on subsequent processes is demanded for the development of new technologies for materials with a homogeneous microstructure. In the paper a layer model for flat hot and cold rolling is presented. A deepened understanding of the influence of inhomogeneities in material state and material flow on the whole process can be reached due to the introduction of a new computational concept for variable layer thickness distributions in the layer model. The concept will be explained and the additional information obtained concerning the influence of shear, local residual stresses as well as inhomogeneous stress and strain states in the roll gap on the microstructure evolution will be presented and discussed.

Abstract: Roll bonding is a joining-by-forming operation, in which two or more metallic strips or plates are bonded permanently through the pressure and plastic deformation in the roll gap. Although roll bonding has been successfully used in industrial production over many years, difficulties occur especially when materials of largely different yield strength are roll-bonded, e.g. when hard aluminum alloys are clad with soft commercially pure aluminum. Examples are AA2024 sheets used in wing and fuselage structures of aircrafts, which are clad with AA1050 to improve the corrosion resistance. Likewise, aluminum sheets for heat exchangers consist of a hard base material that is clad with a soft solderable aluminum alloy. In these cases, the strength difference may influence the bonding behavior since the softer face sheet has to transmit the deformation to the harder core material. To analyze and optimize such cases, a bonding model integrated into a numerical framework for the simulation of the roll bonding process is required. In this paper, a finite element model is presented, in which the development of bond strength is simulated using a cohesive contact formulation. The model is used to study the bonding behavior of laboratory-scale roll bonding trials of two aluminum alloys with a large difference in yield strength. It is found that shear stresses are generated towards the end of the roll gap that may exceed the shear bond strength created earlier in the roll gap such that no firm bond is obtained. The conditions under which bonding is successful are analyzed using a finite element simulation study with varying yield stress differences and pass reductions and summarized in a map.

Abstract: Mg-30Ca and Mg-14Li (wt %) master alloys were melted successively in the induction furnace to obtain a Mg-Li-Ca ternary alloy containing 3.99 % Li and 1 % Ca. The as-cast material of thickness 4 mm was homogenised at 350° C for 120 mins and subsequently rolled to 62.5 % reduction in thickness at 300 °C to get 1.5 mm thick sheet. The microstructures of hot rolled samples were examined in as-rolled condition as well as after annealing at 350° C for various lengths of time. The presence of deformation twins was clearly seen in the as-rolled structure, whereas equiaxed twin-free grains were observed in the annealed condition. The average grain size was found to increase from 10 μm to 18 μm by annealing, according to the kinetics that follows a parabolic law. Tensile samples taken from rolled plate were deformed to failure at room temperature and a strain rate of 10^-4 s^-1. Ultimate tensile strength of as-rolled material increased to 213 MPa, while tensile elongation dropped to 6.5 % from the initial values of 134 MPa and 8.5 %, respectively. Annealing after rolling offered a good compromise between the enhanced tensile strength (160 MPa) and tensile ductility (9 %) suggesting viability of the proposed thermomechanical treatment as a means for enhancing both strength and ductility of Mg-4Li-1Ca alloy.

Abstract: SAE J2114 test method has simulated the recurrence of passenger vehicle rollover form in the traffic accident, mainly including the rollover stage and tumbling stage. In this paper, the whole rolling process of passenger vehicle are analyzed in mechanical model by the testing method, such as the critical condition of rolling ; the condition affecting the vehicle back weeks on the air parabolic motion stage; the condition affecting rolling strength on the landing stage. Furthermore, the article has used MATLAB tools to analyze the relationship of four factors ,which are the critical speed, wheel base, center of gravity height and radius of the rolling ,and the results will help us better understand the collision shape, providing theoretical basis for accident reappearance.

Abstract: Aluminium matrix composite with silicon carbide reinforcement has been developed to increase the hardness of the composite. The determining factors in the development of this material are alloy element content, rolling process and heat treatment. In this research, Al-7Si alloy with Zn and Mg element content variation was made as the matrix to strengthen the aluminium. The reinforcement is silicon carbide (SiC) in the form of particle with volume fraction of 520%. Rolling process and heat treatment were conducted to further increase the hardness. Material characterisation is conducted with chemical composition testing, microstructure observation, and hardness testing. Research showed that after rolling process with 10% reduction, the hardness of composite is increased. The improvement of hardness on precipitation heat treated composite after rolling process tend to decline compare to non-rolled composite.