An experiment on incremental forming of a sheet metal has been carried out to study the mechanism of deformation. A bar tool with a spherical head was made to travel along an altitude trace on an aluminum alloy sheet, and stopped at a certain point during the trace. By detailed observations of the termination point, a simplified model of the deformation process was proposed. Moreover, equations were prepared to predict the distribution of strain, on the basis of plane strain conditions. Strain and strain increments have been calculated under several forming conditions using this model. As a result, the distribution of strain increment along the section of tool trace was analyzed, and it was presumed that the location of the peak influences the strain concentration and the limitation of deformation. If a peak of strain increment locates close to unformed area, it would prevent the advancement of thickness decrease, and make it possible to cause a large strain without breakage. It is considered one of the basic mechanisms of how a huge deformation can be produced by an incremental forming process.
The endless hot rolling concept was introduced to meet the demand for diverse hot-rolled steel sheet products produced by a process that is cost-effective compared with the conventional batch-type process. Endless rolling involves joining the head and tail ends of two bars after rough rolling. A new solid-state joining technology was proposed to instantaneously obtain a joint with high integrity using relatively simple equipment. The process requires no external heating since the bar temperature on the line is sufficiently high for joining. In this report, we present the super deformation joining (hereinafter referred to as “SDJ”), specifically, the introduction of the developed process and the effects of joining conditions on the characteristics of joints. The new model, the type II SDJ device, improved joint shape and performance compared with the type I SDJ device, mentioned in a previous paper. In laboratory tests, the joint strength of carbon steel was within 70 - 85 % of the base metal strength, which is from 394 to 440 MPa, and the grain size across the joint interface was very fine. Elucidating the SDJ mechanism and evaluating the characteristics of joints to rolling remain to be performed.
The endless hot rolling concept was introduced to meet the demand for diverse hot-rolled steel sheet products produced by a process that is cost-effective compared with the conventional batch-type process. Endless rolling involves joining the head and tail ends of two bars after rough rolling. A new solid-state joining technology was proposed to instantaneously obtain a joint with high integrity using relatively simple equipment. In this report, we present the mechanism of super deformation joining (hereinafter referred to as “SDJ”) and discuss results of our evaluation of the characteristics of joints to rolling. Super deformation by shearing creates new uncontaminated joint surfaces, and a large upsetting force at the completion of joining promotes metal bonding. SDJ was applied to the joining machine of POSCO POHANG #2HOT, and the use of endless hot rolling in commercial production was started in Jan. 2007. #2HOT is now producing approximately a hundred thousand tons of coils every month by endless hot rolling owing to the stabilization of operation.
Grain refinement and texture control in magnesium alloys are an effective way of improving mechanical properties. We have investigated the possibility of microstructure control of metal materials by compressive torsion processing. This process can apply a large strain to a cylindrical work piece by simultaneous compressive and torsional loadings without changing the shape of the work piece. It is expected that not only grain refinement but also texture control can be achieved. In the present work, we applied this process to the microstructure improvement of extruded AZ61 magnesium alloy. The effect of rotation number on microstructure control was investigated. The microstructure of each specimen after processing was analyzed quantitatively and the distribution of grain size was determined. In addition, texture was measured by X-ray diffraction analysis. Grains of several micrometers were observed in the specimens under all conditions. The grains in the specimen processed with a large rotation number were refined homogeneously over the cylindrical work piece. Most basal planes, which were initially oriented parallel to the cylindrical axis, were distributed nearly perpendicular to the axis, the compressive direction, after processing.
A “surface deflection” is a surface defect with 30-100 μm surface undulations. The development of simulation technology capable of accurately predicting surface deflections is crucial for the cost-effective production of high-quality automotive outer body panels. In this study, a panel that can be used to model a handle emboss of an automotive door outer panel is experimentally press formed using dies of high rigidity and the distribution of curvature of the panel is precisely measured. The material used is a 0.69-mm-thick bake-hardenable galvannealed steel sheet. Biaxial tensile tests on the material are carried out using a servo-controlled biaxial tensile testing machine to accurately determine the best-fit yield function for the finite element simulation of the panel. It is found that; (1) the Corus-Vegter yield function, which is capable of accurately reproducing the deformation behavior of the material, is superior to Hill’s quadratic yield function in the predictive accuracy of surface deflection, (2) the coefficient of friction and blank-holding pressure more strongly affect the predictive accuracy of surface deflection than element size, element type, forming speed set in the analysis and the number of integration points in the thickness direction.
The fine-blanking process is used in the production of automobile parts and other metal components. Although the fine blanking process can produce higher-precision sheared surfaces than the punching process, rounding edges on cut surfaces are also formed, as in the punching process. It is important to determine the causes of the formation of rounding edges. However, it is difficult to clarify the mechanisms of the formation of rounding edges by experiments. The finite element method is adopted to study the causes of the formation of rounding edges. The cut surfaces in the present experiments have fine sheared surfaces but no fracture surfaces. Although a combination of the fracture criterion and element kill methods is used for many simulations of the fine-blanking process, fine sheared surfaces are not evaluated by a combination of these methods. A mesh adaptive technique for the finite element method is used to create fine sheared surfaces in the present calculation. The rounding edge is associated with the initial compression and the subsequent clearance of the punches and dies. Results are obtained for various clearances and initial compressions in the fine-blanking process for high-tensile steel. The results of the experiments are compared with those of the calculation. In the present study paper, we show that the rounding edges are affected by the clearance and initial compression of the punches and die.
A punching process using local resistance heating near a circular shearing band was developed to shear ultra high strength steel sheets. The shearing band was heated by the electrification between the sheet holder and the knockout in order to decrease the flow stress in the shearing, and the heating of the die and punch was prevented by eliminating contact with the sheet during the electrification. Electrode pins having a spring were employed to attain uniform heating of the shearing band. The degree of welding of the head of a Cu-W electrode pin to the sheet by the electrification was smaller than those of Ag-W, Ag+WC and W pins, and thus the Cu-W pins were employed in a punching experiment of 980 MPa level ultra high strength steel sheets. The punching load was considerably reduced by the heating, e.g., about 1/5 of the cold load at 800 °C. As the heating temperature increased, the depth of the shiny burnished surface on the sheared edge increased and that of the rough fracture surface decreased.