Surface coating for the lubricant-free deep drawing of high-alloy stainless steels

Abstract

Sheet metal forming is today using environmental harmful lubricants which will be applied and removed in additional process steps. In this research non-lubricated deep drawing of high alloy stainless steel sheets was successfully carried out by a metal matrix composite (MMC) tool surface with a high-low surface structure featuring protruding fused tungsten carbide (FTC) particles. To obtain such a hard particle reinforced surface, a process chain was successfully implemented where in a first process step a laser melt injection (LMI) was conducted to reinforce the aluminum bronze tool matrix material by hard particles. The geometry and hard particle content of the LMI tracks can be influenced especially with the process parameters laser power, process velocity and powder feed rate which allows for a hard particle content up to 55 %. To further in-crease the contacting area and reduce the local contact pressure in the radius area of the tool, a novel local laser particle fusion (LLPF) process is introduced. In LLPF, FTC particles are fused together in a second local LMI step under additional powder supply to produce large FTC nuggets in the tool surface. Characterization of the FTC nuggets shows that the chemical composition is comparable to tungsten carbide with a hardness of up to 2500 HV0.5. LLPF can be applied for regions of higher local contact pressures where one nugget can cover an area of more than 1 mm2. Surface finishing of the MMC tool is realized by laser ablation which was carried out to set back the matrix to obtain a protruding particle surface area. Depending on the scan repetition, a depression of up to 25 μm between the matrix and particle plateau can be adjusted. Dry forming of sheet metals showed that adhesive wear can be prevented by using MMC tools with a high-low structured surface. An increase in the hard particle content leads to a reduction of the friction coefficient and forming force, and to lower local contact pressures. However, in dry forming with MMC tools a penetration of the hard particles into the metal sheet surface may result in scratches and grooves. This can be prevented, and the surface quality of the metal sheets can be significantly improved by reinforcement with FTC nuggets in all high local pressure regions of the tool. Application of FTC nuggets covering the top area, the leading edge area and the radius center area of the drawing die can minimize local pressure and reduce the forming force even below the forming force of the aluminum bronze reference tool. Thus, it was shown that a dry metal forming with MMC tool surfaces is feasible.