Tomoyoshi Maeno

Deep Drawability and Bendability in Hot Stamping of Ultra-High Strength Steel Parts

The deep drawability and bendability in hot stamping of ultra-high strength steel parts were examined. Although the cold drawability is greatly influenced by the blank shape, the limiting drawing depths for the square and circular blanks were equal for hot stamping because of small flow stress. In hot hat-shaped bending using draw-and form-type tools, the effect of the blankholder force generated with the draw-type tools on the springback was small, and the seizure for the form-type tools was smaller than that of the draw-type tools. Since both edges in contact with the electrodes are not heated for resistance heating, cracks were caused at the edges for resistance heating in the transversal directions in hot stamping of an S-rail with form-type tools, and thus it is required to control deformation of the non-heating zones.

Optimisation of condition in hot gas bulging of aluminium alloy tube using resistance heating set into dies

A hot gas bulging process of an aluminium alloy tube using resistance heating set into a die was developed. In the developed process, the tube was heated during the forming, and thus the drop in temperature was prevented. The control of the hot gas bulging was simplified by sealing air in the tube. The tube was bulged by thermal expansion of the air sealed in the tube without control of internal pressure during the forming. Hot gas bulging of an aluminium alloy tube without and with the axial feeding was performed. The deformation behaviour of the tube in the die was observed by a heatproof glass plate inserted in the die. The timing of the axial feeding, the feeding velocity and the amount of the axial feeding were optimised.

Hot Spline Forming of Ultra-High Strength Steel Gear Drum Using Resistance Heating

A hot spline forming process of die-quenched gear drums using resistance heating of a side wall of a cup formed by cold deep drawing and ironing was developed. The side wall having uniform cross-sectional area is resistance-heated by passage of the current in the axial direction, the heated side wall of the drawn cup is ironed and is finally die-quenched. The gear drum was successfully formed and the hardness was between 400 and 500 HV. Not only the formability was improved but also the formed dram was hardly oxidised because of rapid resistance heating.

Development of the Hot Gas Bulging Process for Aluminium Alloy Tube Using Resistance Heating

A hot gas bulging process of an aluminium alloy tube using resistance heating, set into a forming machine, was developed. The tube was rapidly heated by the electrifying to increase the formability and to decrease the flow stress. The tube was bulged by thermally expanding the air sealed in the tube without control of internal pressure during the forming. Hot gas bulging of an aluminium alloy tube without and with axial feeding was performed. The effects of the initial internal pressure and the current on the expansion ratio of the tube were examined. The decrease in temperature around the contact with the electrode was prevented by inserting a stainless steel ring having low thermal conductivity and high heat generation between the copper electrode and tube, and thus the bulging length was increased. It was found that the hot gas bulging is effective in heightening the formability of the aluminium alloy tubes.

Hot stamping of titanium alloy sheets into u shape with concave bottom and joggle using resistance heating

The hot stamping of α+β titanium alloy sheet into U shape with concave bottom using resistance heating were performed. Since both edges of the sheet in contact with a pair of electrodes were not heated, cracks occurred around the corners of the bottom due to the partially high flow stress. The cracks were prevented by slitting both edges before resistance heating because of the elongation of the edges. In addition, the hot stamping of titanium alloy sheet into joggle using partial resistance heating were performed. The distortion of sheet was reduced by reduction in area of resistance heating

Prevention of Local Thinning and Springback in Hot Stamping of Thin Sheets

In hot stamping of thin quenchable steel sheet, local thinning, springback and hardness of stamped parts were investigated. The sheets having 0.6, 1.0 and 1.6 mm in thickness were heated at 900 °C by a furnace and hot-stamped into a hat shape. Local thinning around the bottom corner of the hat-shaped part for a thickness of 0.6 mm was considerably remarkable in comparison with those above 1.0 mm in thickness. Local thinning was relieved for a transfer time of 15 s from the furnace because of the uniformity of the temperature distribution. In addition, the springback was prevented above 5s.

Local Prevention of Hardening for Punching of Hot-Stamped Parts

Punching portions of the sheet are sandwiched between the ceramic billets during rapid resistance heating to prevent hardening of these portions. When the heating temperature is locally lower than that of the austenitic transformation, i.e. below 800 oC, this portion is not hardened without occurrence of martensitic transformation, and thus cold punching of hot-stamped parts becomes easy. The ceramic billets are made of alumina and the heat transfers to the billets. The temperature distribution just after resistance heating, the hardness distribution of the hot-stamped sheet, the cold punching load, the quality of the punched hole, etc. were measured. Hardening of punching portions was successfully prevented by sandwiching between the ceramic billets. The cold punching load for the local prevention of hardening was half of that without local prevention and the delayed fracture was also prevented, whereas the drop in hardness around the sheared edge became larger than that for laser cutting.

Forging of Parts Having Strength Distribution Using Tailor Friction-Welded Billets Composed of Steel Bars Having Different Quenchabilities

Parts having a strength distribution were produced from tailor friction-welded billets consisting of steel bars having different quenchabilities. Chrome steel SCr420 and mild steel S25C bars having high and low quenchabilities, respectively, were joined by friction welding. The tailor friction-welded billets were forged, and then quenched so as to have a strength distribution. The cold formability of the tailored billet was first examined from simple compression in the normal and tangential directions to the joint. No cracks appeared even for an 80% reduction in height in the compression. In a tensile test of the 80% reduced billet, the softer S25C side was fractured and the interface was not fractured. The tailor billets have enough cold formability. The thermal influence around the joint of the tailor friction-welded billets was eliminated by annealing, and thus inhomogeneous deformation around the joint in forging was prevented. A shaft having a high strength flange and a connecting rod having high strength and machinability were produced by forging and quenching of the tailored billet.