Key Engineering Materials Vols. 535-536

Abstract: This study aims to investigate the microscopic origin of viscosity by simplifying an amorphous system to a mixture of many independent atomic subsystems. The response of the macroscopic system is then taken as an ensemble average of the relaxations of such subsystems. The result shows that with the reduction of temperature, the overall viscosity changes from the harmonic mean of the subsystems, which is dominated by the fast relaxations, to the arithmetic mean governed by the slowest relaxation. The successful application of our model to the amorphous Selenium indicates the model captures the fundamental mechanism of the viscosity variation.

Abstract: Contact simulation involving asperities was developed by assuming that the deformation by asperities is equivalent to the deformation by an indenter in a hardness test. Consequently, depth dependent flow stress curves were derived from the indentation size effect model from Abu Al-Rub and were used to simulate the influence of the number of asperities involved during contact on the distribution of contact pressure and the value of effective friction coefficient. Results from simulations suggested that multiplying the number of asperities in contact, when the size of the asperities is comparable to the size of the apparent contact, is not followed by proportional multiplication of the reaction forces. The competing phenomena observed in the simulation are then proposed as an explanation to friction size effect occurring in microforming.

Abstract: The r–value is defined as the ratio of the width strain to the thickness strain under the uniaxial tensile test of the sheet metal. Based on r-value of grains, a model of plastic deformation of polycrystalline metal and surface roughening after plastic deformation was proposed in the previous paper. Meanwhile, Marciniak and Kuczynski proposed the so-called M-K model which give the analytical estimation of the formability of sheet metal under biaxial stretching considering a certain irregularity of the thickness of the sheet metal. Yamaguchi et al showed that the experimentally measured surface roughness may correspond to the surface irregularity suggested in the M-K model. In the present paper, the formability of sheet metal under biaxial stretching is analyzed based on the previous analysis of surface roughening caused by the difference of the r-value in the sheet metal.

Abstract: It is difficult to obtain 3D trimming line using traditional prediction methods for auto panel parts. An initial geometrical development method with element layer is proposed based on one step inverse analysis theory for this problem. The flange mesh can be unfold onto the die surface layer by layer according to nodal adjacent element relation, then the above development mesh is smoothed by mesh smoothing method with sliding constraint surface in order to delete overlap and distorted mesh, the 3D initial mesh can be obtained for one step inverse analysis method. The accurate 3D trimming line of auto panel part can be achieved by plasticity iteration of one step inverse analysis. A typical real part of 3D trimming line prediction is selected to prove this method, the comparison results between the simulated and experimental values show that this method has enough precision and can handle complex parts, satisfies the engineering practical demands.

Abstract: Laser forming is an advanced process in sheet metal forming in which a laser heat source is used to shape the metal sheet. In this paper, shape correction of stamped product using the laser forming technique is numerically invesitgaed. As a fundamental study, laser irradiated bending process has been analyzed through the thermo-mechanical FE analysis. Laser scanning experiments has been also conducted for varificaiton of numerical simulations. Finally, the laser forming technique is applied to the u-shaped channel member for springback compensation. It is observed that laser irradiation gives the desired bending angle to compensate the springback by controlling the process parameters such as the laser power, scanning speeds and the number of laser scanning.

Abstract: In recent years, many attempts are being made to develop new lubrication coatings for cold forging. In general, an alternative coating is designed based on the results of the evaluation methods that simulate the frictional condition of cold forging. Various evaluation methods have been proposed and used to design an alternative coating according to the processing purpose. However a lot of parts are manufactured by the multistage forging machine such as parts forming machine, complex shape being produced recently by combined processing. A lubrication coating in such an environment is exposed to repeatedly in a variety of processing modes, and received serious damage. At present the performance of an alternative coating isn’t enough to satisfy needs of market in such a multistep forging. Development of lubricant that can satisfy such demands needs a new evaluation method. This comes the purpose of this study. An newly devised tribometer consists of an ironing process after upsetting process. We designed this test method by using finite element(FE) analysis. In upsetting, damage to the lubrication coating is given by the expansion of free surface. In ironing, the performance of lubrication coating destroyed by upsetting is evaluated. By using the upsetting-ironing type tribometer, the lubrication performance under a large surface expansion and a high pressure can be evaluated. In addition, the degree of coating damage can be adjusted by changing upset ratio and height of billet. The difference in the lubrication performance of the bonderised and the prototype alternative coating was examined by this evaluation method.

Abstract: Longitudinal cavities or pipes are usually developed in a metallic slab or billet produced by continuous casting. In the present study, closure behavior of such cavities by side pressing was investigated by finite-element method. Since the contact surface between the dies and the slab was much longer in the longitudinal direction than in the thickness direction, the slab was compressed under the condition of plane-strain deformation. As a result, the effective strain was found to be an indicator for the void closure. The critical value of the effective strain was dependent upon the aspect ratio of the cross section of the cavity, but not upon its size.

Abstract: The application of high strength steel sheet on stamping part extends the range of contact pressure than that of mild steel’s and makes the effect of contact pressure on frictional characteristics of steel sheet a regardable fact. The friction test with hot-dip galvanized high strength steel sheet was conducted under various contact pressures, to investigate the influence of contact pressure on frictional behavior of steel sheet. According to the test result, the contact pressure effect on friction is considerable. The friction between tool and work-piece influences the stress distribution during stamping process. To investigate the influence of frictional behavior on stress distribution at stamped part, the U-draw bending under constant blank holding force with different blank size were conducted and it was verified by finite element analysis with and without considering contact pressure effect on frictional behavior. As the result of study, the variation of frictional behavior depending on contact pressure results changes in the stress distribution of parts that has effects on tendency of springback for high strength steel sheet stamping.

Abstract: This study presents a new type of sheet metal laminate for lightweight products, and investigates its plastic behavior and formability. The sheet metal laminate consists of three layers, i.e. two thin skin sheets and a perforated core sheet with round holes, which are bonded together by diffusion bonding. Pure copper sheets are used for both of the core and the skins. Plastic deformations of the laminate and its component layers under uniaxial and biaxial tension are examined experimentally and analytically. Results of uniaxial stress-strain responses and yield loci (contours of plastic work) show that the perforated core sheet exhibits anisotropic behavior induced by the hole array but the laminated sheet becomes rather isotropic. Forming limit diagrams of the laminate and its component layers are also obtained by performing stretch forming test. Forming limit of the perforated core sheet is markedly lower than that of the monolithic sheet, and that of the thin skin is in between. It is found that forming limit of the laminate is comparable to that of the thin skin.

Abstract: Ironing process is a normal method to improve the surface quality and thickness precision of a drawn cup. Multi-stage processes that combined drawing and ironing, have been developed to obtain the cups with a each surface quality. However, the ironing process is generally used to outside of the drawn cup, and the trial to improve the inside surface property is not carried out very much. The purpose of this study is to clarify the effect of ironing process on the improvement of the surface quality of multi-stage drawn cup. The effects of the ironing reduction, lubricants and punch shape on the improvement of the inner surface properties are investigated by using a series of experiment. It is found that the inner ironing process is effective to improve the inner surface properties of a drawn cup.