The aim of the present research is to propose a new method for the reduction of the residual stress which was generated by laser cutting. Laser cutting has been expected to be used as an effective and potent method for cutting out blanks from rolled sheet metals because of its high flexibility of the cutting line and high productivity. However, residual stresses, which are inevitably generated by laser cutting, have been the greatest obstacle to the prevalence of laser cutting. One of the problems is the warping that occurs in the sheet metal subjected to bending processes following laser cutting. The authors present a new method for reducing residual stress generated by laser cutting. The method involves applying is to give light indentation on the sheet metal edge. The FEM analyses were carried out to study the feasibility of this method, to determine the method of applying indentation, and to optimize the conditions. Experiments were conducted for verification. As a result, the warp generated by the residual stress was significantly reduced to one-tenth of that without the new treatment, and the magnitude was satisfactorily small for industrial usage.
In a previous study, a new metal flow joining method that enables a shaft and ring to be combined without the need for any specialized punch was examined. In this method, the shaft was loosely fitted on the ring, which has a step, and then pressed. Metal flow caused the shaft to be joined to the ring upon pressing. As a result, the maximum joining efficiency (i.e., the shaft return-proof load / joining load) of the new method was more than 25%. In this study, the method was examined in detail in the experiment using a JIS S45C shaft (soft shaft) and JIS SKD11 ring (hard ring). Specifically, the initial dimensions of the joining hole of the ring were evaluated. First, the effect of the step width at the hole on the shaft-return-proof load was investigated. For different step widths, an 18- or 19-mm-diameter hole was combined with a concentric 20-mm-diameter hole. Second, the effect of the radial clearance between the hole and shaft on the proof load was investigated. The size of the 20-mm-diameter hole was varied to create three clearances ranging from 0.015 mm to 0.055 mm. As a result of this study, the proof load and the joining efficiency were found to be increased by using a ring with a narrower step width. The proof load reached 1.3 times greater than that in the previous study. On the other hand, the proof load and the joining efficiency were not affected by the clearance, the tightness of which would be related to the cost and productivity of joining parts with holes.
The surface texture of a steel sheet exerts a considerable effect on sheet press formability and image clarity after coating. Temper rolling is one of the key processes for determining the surface texture, because the final procedure is printing using an appropriate work-roll surface. To carry out stable control of the surface texture by temper rolling, it is important to maintain the topography of the roll surface and increase the roll service life for industrial use. In this study, electrical discharge coating is applied as a means of improving the wear resistance of the roll surface. First, characteristics of the film coating made by electrical discharge coating using a TiC sintered electrode are investigated experimentally. Then, the influence of the electrical machining conditions on the film thickness, hardness, and surface roughness is evaluated and discussed. Furthermore, a rolling experiment is conducted and the wear resistance was evaluated by comparison with a film fabricated by the conventional hard chromium plating method.
In this study, the effect of a wedge on a piercing punch on hole-expansion ratio has been investigated. As a result, the holes pierced using a wedged piercing punch showed significantly high hole-expansion ratios under certain conditions. Regarding the wedge vertical angles, the hole-expansion ratio becomes high at 80 degrees and low at 45 degrees. The sheet bending effect caused by the wedge shape contributes to these results. The small amount of bending in the case of 80 degrees induces tensile stress around the punch edge without plastic deformation, which accelerates material fracture. This acceleration decreases plastic strain on the pierced surface and leads to the high hole-expansion ratio. On the other hand, a large amount of bending in the case of 45 degrees causes plastic deformation around the punch edge, which deteriorates the hole-expansion ratio. Regarding the material direction when set on a die face, the hole-expansion ratio becomes high when the material direction at which the stretch-flange formability is the highest is at 90 degrees to the wedge tip line. In this condition, the stretch-flange formability at the weakest position resulting from piercing using the wedged punch becomes the highest in the other material directions.
The effect of clearance on tool wear was examined during the blanking of a high-tensile-strength steel sheet. When the clearance was small, at approximately 2%t (relative to sheet thickness), the tool expectedly became more worn during the blanking of the high-tensile-strength steel sheet than during the blanking of a stainless-steel sheet, which is generally considered to induce severe tool wear. However, tool wear was greatly reduced to lower than that for the blanking of a mild steel sheet when the clearance was increased to 10%t or more. This was because the percentage of the shear surface relative to the cut surface decreased with increasing clearance, resulting in a decreased distance over which the sheet slid on the tool surface. For the high-tensile-strength steel sheet, the inner diameter of a hole on the sheet and the outer diameter of the blanked piece also changed greatly as the clearance increased. This prevented the sheet from adhering to the tool surface and suppressed the increase in shearing energy during blanking.