Development and Testing of Tailored Tool Surfaces for Sheet Metal Forming

This thesis describes measures taken to minimize or substitute environmentally hazardous lubricants applied in sheet metal forming processes by less harmful lubricants or not applying lubricant at all. The breakdown of lubricant film often leads to galling, and therefore application of the hazardous lubricants has spurred industrial interest.

In order to face a serious challenge in trying to stimulate less consumptions of such hazardous lubricants, the PhD project was intended to lead to improvements in resistivity towards galling in sheet metal forming by studying three different subjects; compressibility of lubricants, application of structured tool surfaces and application of anti-seizure tool coatings.

In order to analyze the mechanisms of lubricant entrapment and escape, knowledge of the lubricant bulk modulus characterizing the compressibility of lubricant is required. Two methods were studied to achieve this purpose. A simple laboratory test consisting of upsetting a specially designed metal cylinder with a lubricant reservoir together with elasto-plastic, numerical modelling of the metal cylinder is carried out in order to determine the bulk modulus at low pressure regimes of approximately 100 MPa.

The above mentioned simple experimental procedure for determining lubricant bulk modulus gives a first rough estimate, and it is supplemented by a more advanced laboratory test based on a newly designed equipment. The lubricant compressibility experiment with a direct pressure measurement inside the high-pressure container allows for the direct determination of the bulk modulus at various pressure levels with no influence from friction in the sealing between punch and container.

Using water as a reference, a good agreement between the experimental bulk modulus and values suggested in literature was found. Testing of liquid lubricants has revealed a nonlinear relationship between the bulk modulus and the pressure. While texturing of workpiece surfaces to promote lubrication in sheet metal forming has been applied for several decades, tool surface texturing is rather new.

A detailed background investigation and fundamental analysis of different textured tool surface arrangements have been carried out by Strip Reduction Test (SRT). Low as well as high viscosity oils were tested at varying sliding speeds. Micro-textured surfaces on the tool were made using an in-house micro-milling machine for the manufacturing.

The SRT tools were manufactured with longitudinal, shallow pocket geometries oriented perpendicular to the sliding direction. The pockets have small angles to the workpiece surface and varying distance. The experiments show an optimum distance between the pockets to exist that creates a table mountain topography with flat plateaus and narrow pockets in between.

If the flat plateaus are too narrow, an increase in drawing load and pick-up on the tool plateaus is observed. The same occurs for too wide plateaus. The tool textures were advantageous at larger sliding speeds when using higher viscosity oils, which facilitates the escape of trapped lubricant by micro-plasto-hydrodynamic lubrication.

Large lubricant viscosity results in higher sheet plateau roughness and prevents pick-up and galling. A theoretical friction model for a soft workpiece deforming against a textured tool surface was proposed. The friction model takes into account the plastic wave motion appearing, when the workpiece material flows into and out of local pockets between the flat plateaus of a table mountain tool surface topography.

The friction model supports the experimental findings of an optimum distance between the pockets, where the contribution to friction by mechanical interlocking of the strip in the pockets is limited and lubrication of the plateaus is enhanced by micro-plasto-hydrodynamic lubrication. It was found that an optimum amount of tool texture exists which reduces friction and thus drawing load for the table-mountain tool surface topography.

Stamping of sheet metal components without lubrication or using minimum amount of hazard free lubricant is a possible solution to diminish health hazards to personnel and environmental impact and to reduce production costs. Adopting SRT, which emulates industrial ironing production of deep drawn, stainless steel cans, Diamond-Like Carbon (DLC) coating were deposited on SRT tools.

The DLC coated tools with multi-, double- and single-layer coating structures were tested under severe tribological conditions, i.e, high normal pressure and temperature. A screening test campaign on a manually operated sheet tribo-tester is carried out to identify promising candidates. The screening tests revealed that the double layer coating worked successfully, i.e. with no sign of galling using no lubrication even at elevated tool temperature, while the other coatings peeled off and resulted in severe galling unless lubrication was applied.

The next test campaign on an automatic sheet tribo-tester examines the durability of the promising candidate as regards persistence towards pick-up. It is shown that the double-layer coating, DLC/Hyperlox®, can function effectively if a minimum quantity of hazard free lubricant is applied and hence, avoid peeling off of the coating leading to galling.

Numerical simulation using a thermo-mechanical analysis supports the experimental findings, where lubrication lowers the temperature at the tool/workpiece interface by reducing the friction.