Tribological system for cold sheet metal forming based on volatile lubricants and laser structured surfaces

Abstract

A novel tribological system has been developed, in which volatile lubricants (carbon dioxide - CO$_{2}$ or nitrogen- N$_{2}$) are used as a substitute for mineral oil-based lubricants in deep drawing processes. This process enables the introduction of an intermediate medium under high pressure through flow-optimized, laser-drilled micro holes into the contact surfaces. This eliminates the need for subsequent, cost-intensive cleaning processes, as the volatile lubricants evaporate without leaving any residue during expansion to ambient pressure. The design of initial micro hole geometries was based on simulations of the flow behaviour of the lubricants passing through, which in turn were validated using pressure reactor tests. In addition, the wetting behaviour of CO$_{2}$ on relevant surfaces (tool surface and sheet material surface) was investigated experimentally using the captive-bubble-method. Thus, the optimal design of the micro holes (diameter, hole geometry and number of micro holes) could be determined using flat strip drawing tests. The optimal micro hole geometry determined in this way is suited for the use of both CO$_{2}$ and N$_{2}$ as volatile lubricant. Furthermore, extensive investigations for the production of the required micro hole geometry by laser drilling were carried out. The fundamentals for drilling micro holes in steel with high aspect ratios could be developed using an ultrashort pulsed research laser with very high pulse energy. Further experiments were conducted using an ultrashort pulsed prototype laser of the kW-class specially developed to increase productivity when drilling a multitude of micro holes with higher average laser power. The novel tribological system has been characterised by means of strip drawing tests and stretch bending tests. For both, CO$_{2}$ (liquid) and N$_{2}$ (gaseous), relatively low friction values could be achieved compared to conventional lubricants. It could be shown that deep drawing with both CO$_{2}$ and N$_{2}$ as dry lubricants is possible. Here, usage of the volatile lubricants not only allows the replacement of mineral oil based lubricants, but even improves the tribological system with regard to frictional forces in sheet metal forming. The feasibility of the new tribological system has been proven by performing deep drawing tests of rectangular cups. These tests showed a significantly enlarged process window of the forming process, which emphasise the tremendous potential of this new tribological system.