Chromium-copper materials have been developed to be substituted for W-Cu and Mo-Cu materials in heat-sink applications. These latter materials have poor formability and poor machinability, which raise processing costs. Tungsten and molybdenum powder are both rare metals, very expensive, and have unstable supply. Chromium is a 6A group element, the same as W and Mo, and the cost of its powder is lower and its supply is more stable, but its thermal properties are inferior to those of W and Mo. To realize thermal properties of Cr-Cu close to those of Mo-Cu and W-Cu, rolling is thought to be an effective improvement method. Rolling formability was tested by 50mass%Cr-Cu material. The material showed good rolling formability. However some microcracks, which are detrimental to Ni plating, were observed on the surface of the rolled sheet. These microcracks were shown mainly at the boundaries between Cr particles and the Cu matrix. Warm rolling was effective for reducing the number of cracks. FEM analysis revealed that large roll diameter could reduce the stress at the boundaries. It was confirmed that there is a correlation between the number of cracks on the plate and the amount of stress at the boundaries.
In order to gain the fundamental knowledge of plastic deformability in a Cr-Cu composite material containing a high fraction of Cr particles, tensile tests of 50mass%Cr-Cu composites were performed at 299K to 473K. Although single-phase Cr generally shows brittle fracture at temperatures lower than approximately 350K, the Cr-Cu composite shows ductile fracture at all tested temperatures. The tensile strength and fracture elongation decreased with increasing testing temperature. These tensile properties of the high-Cr-fraction Cr-Cu composite originate from the properties of a matrix material comprising a Cu alloy with a small amount of dissolved and precipitated Cr. However, the reduction in area was increased with increasing testing temperature. The deformation characteristics of the Cr-Cu composites were considered in terms of the re-arrangement, elongation, and rotation of Cr particles in a ductile Cu matrix. In addition, the ductile-brittle transition behavior of Cr particles would affect the tensile property of Cr-Cu composite at 353K to 373K, which is the range of the ductile-brittle transition temperature (DBTT).
In order to expand the practical application of tube hydroforming to automotive parts, it is important to develop the technologies for forming complex shapes and to expand the formable range. To meet these demands, the hydroforming technology using axial movable dies is proposed. This method consists of two stages. In the first stage, similar to conventional tube hydroforming, internal pressure and axial feeding on tube edge are applied. In the second stage, the movable die is fed in synchrony with axial feeding cylinders under internal pressure. It is experimentally confirmed that by this method, small corner radius can be achieved with low internal pressure, and thickness deviations in the expanded portion can be made uniform. Furthermore, the application of this method to products with long expanded areas is discussed. This method is effective in suppressing the increase in thickness near the edge of the tube and reducing the thickness at the center of the expanded area. By FE analysis, the effect of friction on the material flow and the deformation behavior in this process is clarified.
In this study, numerical and multivariate statistical analyses are performed for the re-evaluation of each individual factor reported to have an effect on the estimation of the void closure behavior by FEM analysis in past studies. The following results were obtained. 1) True strain in the stroke direction and hydrostatic stress ratio are the most effective factors for evaluating the void closure behavior. Void closure can be predicted using true strain alone with R2 = 88% accuracy regardless of void position, and the accuracy can be raised by adding the hydrostatic stress ratio factor to the calculation. 2) A regression equation is obtained from the closure behavior at various positions in a cylinder model under the condition of one stroke upsetting, but the equation can also be applied to other cases such as side upsettings and V-die coggings with high accuracy. 3) With the use of the hydrostatic stress ratio, consideration of the difference in deformation resistance is not required for the evaluation of void closure.
Surface roughness formation of tin plates during skin pass rolling with dull work rolls of 165mm and 480mm diameter has been investigated through rolling experiments and elastic-plastic finite-element (FE) analyses considering surface asperity and elastic deformation of the work rolls. Surface roughness predicted by two-dimensional rolling FE analysis exhibits fairly good agreement with experimental results obtained using smaller work rolls of 165mm diameter, but not with results obtained using larger work rolls of 480mm diameter. Discussion has been carried out on the characteristics of surface roughness formation and the validity of the two-dimensional rolling FE analysis in view of the surface roughness of the rolled plates, utilizing three-dimensional die press FE analysis simulating a generic piece of the rolled plate and a unit of asperity of the work roll. It is concluded that surface roughness formation is governed by mean stress in the rolling direction and elongation of the rolled plate, and that two-dimensional rolling FE analysis inevitably induces substantial error in predicting surface roughness for two reasons. One is the neglecting of shear stress acting from adjacent material in the width direction. The other is that, when mean stress in the rolling direction is small, unrealistic nonplastic regions appear in the rolled plate at the concave portion of the work roll. For a practical solution to these problems of the two-dimensional rolling FE analysis, a combination of two-dimensional rolling analysis with three-dimensional die press analysis connected with mean stress in the rolling direction and elongation of the rolled material is proposed for surface roughness prediction.
In order to gain insight for developing fine-grained magnesium alloy materials through plastic deformation, Mg-Zr, Mg-La, and Mg-La-Zr alloys, prepared from pure magnesium and lanthanum metals, and a Mg-33Zr master alloy were tensile tested at room temperature and 150ºC. The microstructures were examined by optical microscopy, X-ray diffraction analysis, and EBSD analysis to establish the characteristics of deformed portions of the tensile-tested specimens. Whereas the elongation values of Mg-La and Mg-La-Zr materials tested at room temperature decreased with lanthanum content, the materials tested at 150ºC maintained a similar level of elongation regardless of lanthanum content. EBSD analysis of the deformed materials of the Mg-La-Zr alloys tensile-tested at room temperature and 150ºC revealed that the microstructures of primary αMg consisted of approximately 50% ultrafine grains of 0.5 to 1.5 μm in size. About 70% of the eutectic αMg grains was found to be approximately 0.1 μm in size, suggesting that microstructural transformations that take place through deformation processes may help to enhance ductility.