Compressive torsion processing(CTP), in which compressive and torsional loadings are applied to cylindrical specimens simultaneously, has the great advantage of providing severe plastic deformation without a change in specimen shape. In this work, CTP is applied to pure aluminum to investigate the effects of processing temperature and specimen diameter on the grain refinement and microstructure homogeneity of a processed specimen. As a result, a lower processing temperature was found to be more effective for obtaining a fine grain size, but was not useful for obtaining homogeneous grain refinement. Increasing the specimen diameter was effective for obtaining homogeneous grain refinement. Grains less than 3 μm in size were observed in room-temperature processing, but the grain size increased with increasing processing temperature.
Tailored blanks are widely used for automobile bodies as a material, that achieves not only weight reduction but also maintains of strength and rigidity for an auto body. High-strength steel whose tensile strength is 590MPa or higher is usually strengthened by transformation hardening. When such occurring high-strength steel is welded, the weld metal becomes harder than the base metal owing to heat input and the subsequent rapid cooling occurring during the welding, while the heat-affected zone (HAZ) is possibly caused by temper softening. In this study, the relationship between a formability and HAZ hardness changes in laser welding was evaluated. As a result, the widths of the weld metal and HAZ became narrower and the Erichsen value was enhanced with an increase in the welding speed. In cases in which the width of the hardened weld metal is constant, when the softening degree and region width of HAZ are large, the Erichsen value deteriorates.
To investigate the damage behavior of DLC (diamond-like carbon)-Si coating under high contact pressure, a series of experiments were carried out using the Vicker's hardness test. As substrate materials, alloy tool steel (SKD11) and high speed tool steel (SKH51) were used. In order to investigate the effect of coating thickness on coating damage, DLC-Si coatings with two thicknesses of 2.1μm and 4.0μm were prepared on SKD11. Coating damage was observed from the coating and cross-section surfaces. From observation of the coating surface, the indentation shapes of DLC-Si-coated tools were able to be sorted into 3 main types in terms of macroscopic coating damage level. For indentation shape type I, which shows no marked damage but a cross mark caused by the Vicker's indenter, cross-sectional observation was carried out, and a crack parallel to the coating surface was observed in all the DLC-Si coatings. It is inferred that the parallel crack in the coatings was generated by a force from the compressive residual stress of the coating.
With the widespread adoption of high-strength steel sheets in sheet metal forming industries, new technologies that can control and suppress three-dimensional springback are highly required. In particular, in the stamping of curved beams, torsional springback is likely to occur, which is very difficult to suppress by conventional technologies such as die-shape change and blank-holder force control. In this paper, we propose a new method of springback compensation by setting the appropriate heights of draw-beads on a die face. We verify this new method by performing a benchmark test using S-rail. In this verification, 1 GPa high-strength steel sheets were utilized as the blank. To determine the appropriate heights of draw-beads, an optimization technique based on finite-element simulation was utilized. Using thus determined draw-beads, we have succeeded in reducing springback markedly from 4 degrees to 0.5 degrees in torsional angle, which was confirmed by the S-rail forming experiment.