The transfer stamping process is composed of the drawing, the ironing and the shearing steps, each of which requires for a pair of die and punch with the different diameters and clearances. Hence, the initial brass sheet is subjected to various reduction in thickness and straining.
In this metal-forming simulation, the whole dies and punches were duplex-coated by the common procedure. After plasma nitriding of SKD11 dies and punches for prehardening up to 1500 Hv for surface hardness, the metal chromium bottom layer was coated and followed by the graded chromium nitride coating as an interlayer. Then, they were nano-laminate-DLC coated by L = 10 nm up to 100 bilayers; finally, the normal DLC coating is coated onto this nano-laminated DLC as a top coat.
Figure 10 depicts the series of dies and punches to be used from the initial blanking step to the final drawing steps. Besides for the initial blanking dies and punches, the diameter of drawing die-holes and punch is reduced in step-by-step with consideration of reduction in brass sheet thickness. With aid of the developed coating technique [15], the nano-laminate DLC is coated into the depth of die holes even when the die-hole diameter is less than 5 mm.
Figure 11 shows the series of brass products from the brass blank to the final shape of hook through the drawn brass cups. During the fine blanking step, the punch and die corners are subjected to high stress and friction by shearing the brass sheet. In the following deep-drawing steps, the brass cup diameter as well as the sheet thickness are gradually reduced and followed by the neck-formation step. When N = 600, no galling or adhesion are observed on the brass blank and cup surfaces in any transient step of this semi-dry transfer stamping.
The product quality of brass hooks is determined by their metallic shining on their head and neck. Even without the failure in brass-cup shaping or the fatal galling during the series of steps, the neck and head surfaces of brass hook often roughen themselves and lose their metallic shining once the brass work surface experiences locally a metallic contact with the SKD11 substrates. In other words, the appearance of metallic shining surfaces for brass-hook neck and head after stamping proves that the smooth surface quality of dies and punches should be preserved in the transfer process to form the neck and bottom surfaces in the final product. Figure 12 shows the final brass-hook after continuously transfer stamping up to 100,000 shots. This highly qualified brass-hook is just a proof that the present duplexed dies and punches have sufficient engineering endurance under the minimum quantity lubrication condition in practice.