Warm Friction Characterization

Introduction

The University of Waterloo has a developed a warm-friction characterization system, which utilizes a variant of the Twist-Compression Test (TCT). This test is used to quantify the coefficient of friction between a tool and sheet in warm (or room temperature) forming operations.

Twist-Compression Test

The TCT is comprised of an annular tool specimen which rotates against a small sheet specimen. An axial load is applied to the tool specimen, and the reaction torque is measured. From this data, the coefficient of friction can be calculated. Various interface pressures, sliding speeds, and sliding distances can be examined with this test. The Warm TCT has closed-loop heating systems on the tool holder and sheet specimen holder to allow for testing at elevated tool and/or sheet temperatures.

This test allows for very easy comparisons of die forming lubricants with a given tool and sheet combination. This test can also be an economical solution for comparing die coatings, since only a small tool specimen must be coated.

Die “cup” specimen, shown with a die coating. The annular region at the top of the cup makes contact with the sheet specimen.

Die “cup” specimen, shown with a die coating. The annular region at the top of the cup makes contact with the sheet specimen.

Schematic of the Warm TCT apparatus

Schematic of the Warm TCT apparatus

The Warm TCT apparatus can operate within the following ranges of parameters:

Interface Sliding Speed 1 – 20 mm/s
Temperature 430°C (max.)
Interface Pressure 2.5 – 30 MPa

It is also possible to use the Warm TCT for non-isothermal friction testing, to examine, for example, a die-quenching processing route for high-strength aluminum alloys. In this configuration, the die cup is at room temperature and the sheet specimen is preheated in a furnace. The sheet specimen is then transferred to the TCT where it is quenched during the friction test.

The following figure shows the Coefficient of Friction vs Sliding Distance at 170°C with four different lubricants. Examining the curves, it is possible to pinpoint the onset of lubricant breakdown.

Coefficient of Friction comparison at elevated temperature for several lubricants.

Coefficient of Friction comparison at elevated temperature for several lubricants.

Subsequent microscopy of the tested sheet specimen

Subsequent microscopy of the tested sheet specimen.