Friction characterization and application to warm forming of a high strength 7000-series aluminum sheet

Abstract

Tribological conditions are challenging when forming high strength aluminum alloys at elevated temperatures due to the sticking behavior of aluminum and potential material transfer to the die in the form of die pick-up. The current study examines the elevated temperature performance of several lubricants as well as a Physical Vapor Deposition (PVD) die coating, in conjunction with a 7000-series aluminum alloy sheet under conditions corresponding to warm forming of automotive structural components. Lubricant performance is characterized from the coupon-to-component level at temperatures in the range of 25–230°C. The classical coupon-level Twist Compression Test (TCT) is modified to perform elevated temperature experiments and used to rank the lubricants in terms of their steady-state coefficient of friction and resistance to lubricant breakdown as a function of temperature. It is found that both the friction coefficient and lubricant breakdown distance need to be considered for lubricant assessment. The studied PVD die coating is found to successfully mitigate lubricant breakdown for the FORGE EASE AL 278 at 230°C in the TCT. In parallel, warm forming experiments considering deep drawn cylindrical cups and U-channel rail sections (representative of a structural side member) are used to evaluate lubricant performance under real stamping conditions. The trends in forming performance (draw-in length, final blank perimeter, visual scoring, and forming force) confirm the lubricant ranking obtained from the TCT coupon tests, with the Teflon® PTFE film as the most effective followed by the FORGE EASE AL 278, OKS 536, and the LPS® Dry Film PTFE Spray Lubricant. For the highest test temperature, the LPS® Dry Film PTFE Spray Lubricant exhibits breakdown in the TCT at short sliding distances and results in fracture during deep drawing.