Yohei Abe

Initially, the effects of sheet combinations for joining two sheets, including 780 MPa steel and A5052 aluminum alloy sheets, on the joined cross-sectional shapes of the sheets in a clinch-bonding process and the tension-shear load of joined sheets were investigated. The effect of an adhesive on the amounts of the interlock and the minimum thickness in the upper sheet was not large, whereas the effect of the sheet combination was observed. Subsequently, for joining the upper 980 MPa ultra-high-strength steel and lower aluminum alloy sheets in the clinch-bonding process, the effects of the die shape, punch velocity, and sheet holding force on the joinability were investigated. As a result, defect-free conditions were narrowly constrained. Finally, a method that involved controlling material flow using an adhesive with fine particles to increase friction between the sheets was introduced. The upper 980 MPa steel and lower aluminum alloy sheets were successfully joined using this approach.

In the V-bending of sheet metals using a pair of plastic punch and die manufactured by a 3D printer, the effects of two different dimensions designed with the same tool geometry on the deformation behaviors of the punch, die, and sheet were evaluated. The deformation behavior and strain distribution of the punch, die, and sheet were analyzed using a digital image correlation method. Sheets from pure aluminum to ultra-high-strength steel were bent using the two tools with different spans; one was designed on the assumption of tool steel material, and the other was designed on the assumption of plastic material. In both tools, the large compressive strain appeared around the center of the punch tip and on the corners of the die. The tools with a long span for the plastic material gave a lower bending force and small deformation of the plastic tools. The angle difference between a bent sheet at the bottom dead center and a tool was smaller for the tools with the long span, although the springback in the bent sheet appeared. It was found that the design method on the assumption of the plastic material is effective for the V-bending plastic tools.

To increase the joint strength in hemmed aluminium alloy and carbon fibre-reinforced thermoplastic (CFRTP) sheets, two kinds of hemming processes were proposed. In a hemming process, hemming was combined with protrusions with structures on the surface on the aluminium alloy sheet. The protrusions with structure were firmly joined to the surface on the sheet by laser cladding with fine ceramic powders, and then the heated and softened CFRTP sheet material was compressed and infiltrating into the protrusions in hot pressing. In the other hemming process, hemming was combined with the aluminium alloy sheet with drilled holes. The heated and softened CFRTP was flowed into the holes in hot pressing. Each hot press condition was investigated. And then, the strength in the pull-out direction and the strength in the peel direction were measured and compared. The joint strengths by the proposed two hemming processes were larger than that by hemming only.

The effects of a sheared edge and overlap length on the reduction in the tensile fatigue limit before and after hydrogen embrittlement of resistance spot-welded ultra-high-strength steel sheets were investigated. Ultra-high-strength steel sheets with sheared and laser-cut edges were subjected to resistance spot welding followed by hydrogen embrittlement via cathodic hydrogen charging and subjected to static tensile shear and fatigue tests. The distance between the resistance spot weld and the sheared and laser-cut edges was changed by changing the overlap length, and the influence of the weld position was investigated. In the tensile shear test, the maximum load decreased with decreasing overlap length and the maximum load decreased with hydrogen embrittlement, but the effect of hydrogen embrittlement was smaller than that in the fatigue test. In the fatigue test, the fatigue mode changed from the width direction to the sheared edge direction with the increase in the repeated load. Even if the overlap length was reduced, the fracture changed to the sheared edge direction. In the specimens with sheared edges, the effect of fatigue limit reduction due to hydrogen embrittlement was greater than in the specimens with laser surfaces. In particular, the effect was greatest when the fatigue mode was changed via hydrogen embrittlement.

Seizure during ironing negatively affects the quality of parts and die life. To prevent seizures, lubrication has to be improved. In this study, laser-textured dies with lubricant pockets were utilized to improve seizure resistance in the ironing of aluminum alloy sheets and stainless steel cups. The effects of array patterns of micro-pockets, such as grid and crossing array patterns with circular pockets, as well as a grooved array patterns on seizure resistance, were experimentally examined by strip ironing. The sheet and die materials were the A1050-O aluminum alloy and JIS SKD11 tool steel, respectively. Moreover, the underlying physics of the lubricant flow influencing the load-carrying capacity were investigated using three-dimensional computational fluid dynamics simulations. The optimum array patterns of the micro-pockets were then utilized on a tungsten carbide-cobalt (WC-Co) die surface for ironing SUS430 stainless steel cylindrical cups. The strip ironing results showed that the grid array pattern was successful in ironing sheets with a high ironing ratio. The grid array pattern increased the load-carrying capacity of the lubricant more than the crossing pattern, as demonstrated by the simulations, thereby improving the ironing limit. The subsequent ironing of stainless steel cups revealed that when using a textured die with a grid array pattern and lubricant without the extreme pressure additive in comparison to an untextured die, the ironing limit increased by 6% and the average ironing load decreased by 35%. The seizure resistance was improved because the pockets on the surface structured by laser surface texturing improved the load-carrying capacity during ironing.

The changes in the magnetization properties of high-strength steel and ultra-high-strength steel sheets are investigated, and then the sheared edges and the scattering behavior of slivers in shearing of the ultra-high-strength steel sheets are observed. The maximum magnetic flux density of the magnetized sheet is increased with the increasing tensile strength of the sheet. The maximum magnetic flux density in the magnetized blanks decreases, whereas the density in the demagnetized blanks increases. In the sheared edges, the ratio of the fracture surface becomes larger with the increasing tensile strength of the steel sheet. In shearing, the shearing slivers are observed at the time of crack penetration and at the time of punch rise. The mass of the slivers generated from the blank in shearing increases with the increasing tensile strength of the steel sheet. Two-thirds of the generated shearing slivers stick to the blank in the magnetized blank, whereas two-thirds of the slivers in the blank without magnetization scatter to the outside of the die.

The forming limit of stainless steel cups was improved by ironing using a pulsating motion generated by Numerical Control (NC) servo press. Although the limiting ironing ratio of an austenitic stainless steel SUS430 cup for a conventional crank motion using a press oil including a chlorine extreme-pressure additive was about 17%, the ratio was improved to 30% by the pulsating motion. The limiting ratios with and without the additive for pulsating ironing were similar. Pulsating ironing was also effective in improving the limiting ironing ratio of austenitic stainless steel SUS304 cups. A ring-shaped pattern was formed on the sidewall of the ironed cup by the pulsating motion, caused by the difference in surface roughness between portions ironed once and twice. Pulsating ironing not only improves the ironing limit but also eliminates the use of additives with high environmental impact.

Self-pierce riveting of three thin sheets of 980 MPa steel and 5052 aluminum alloy was performed to investigate the effect of sheet configuration on the deforming behaviors of the sheets and the rivet and joint strength. When the lower sheet was aluminum alloy, the joining range was relatively wide, i.e., the interlock hooking the rivet leg tended to be large. In the sheet configuration in which the upper and lower sheets were A5052 and the middle sheet was 980 MPa steel, the rivet leg spread out moderately and the joint without defects was obtained. In the lower 980 MPa steel sheet, fracture tended to occur due to the low ductility of the lower sheet, and the joining range was narrow with the small interlock although the three sheets were joined by an appropriate die shape. In joint strength of joined three sheets, fracture occurred in the lower-strength aluminum alloy sheet if interlocks of about 300 μm and 150 μm could be formed in the lower aluminum alloy sheet and 980 MPa steel sheet, respectively.

A punching test for simply estimating the tensile strength and total elongation of steel sheets and formed parts was proposed. The tensile strength and total elongation were estimated from the shear stress at the maximum punching load and percentage of the burnished depth at the sheared edge of the slug measured without cutting, respectively. For a variety of steel sheets with a range of the tensile strength from 360 to 1500 MPa, linear functions for the estimation were experimentally obtained. The correlation of the estimated tensile strength of the steel sheets with the measured one from the uniaxial tensile test was considerably high, and the correlation of the estimated total elongation was high. The distributions of tensile strength and total elongation for hot- and cold-stamped parts were estimated. The proposed punching test is available under not only a laboratory environment but also a factory environment.

To increase the usage of high-strength steel and aluminium alloy sheets for lightweight automobile body panels, the joinability of sheet combinations including a 780-MPa high-strength steel and an aluminium alloy A5052 sheets by mechanical clinching and self-pierce riveting was investigated for different tool shapes in an experiment. All the sheet combinations except for the two steel sheets by self-pierce riveting, i.e., the two steel sheets, the two aluminium alloy sheets, and the steel-aluminium alloy sheets, were successfully joined by both the joining methods without the gaps among the rivet and the sheets. Then, to show the durability of the joined sheets, the corrosion behaviour and the joint strength of the aged sheets by a salt spray test were measured. The corrosion and the load reduction of the clinched and the riveted two aluminium alloy sheets were little. The corrosion of the clinched two steel sheets without the galvanized layer progressed, and then the load after 1176 h decreased by 85%. In the clinched two galvanized steel sheets, the corrosion progress slowed down by 24%. In the clinched steel and aluminium alloy sheets, the thickness reduction occurred near the minimum thickness of the upper sheet and in the upper surface on the edge of the lower aluminium alloy sheet, whereas the top surface of the upper sheet and the upper surface of the lower sheet were mainly corroded in the riveted joint. The load reduction was caused by the two thickness reductions, i.e., the reduction in the minimum thickness of the upper sheet and the reduction in the flange of the aluminium alloy sheet. Although the load of the clinched steel without the galvanized coating layer and aluminium alloy sheets decreased by about 20%, the use of the galvanized steel sheet brought the decrease by about 11%. It was found that the use of the galvanized steel sheets is effective for the decrease of strength reduction due to corrosion.