Challenges in Tribometry for Warm and Hot Sheet Metal Forming of High Strength Aluminum with Tool Lubrication

Abstract

For conventional sheet metal forming at room temperature, numerous tribometers were developed in the 20th century. At the present state of the art, unsolved issues for tribometry remain for temperature-supported forming processes of high strength aluminum (e.g. EN AW-7075), in which the sheet is heated to temperatures between 200 and 480 °C. The tribological design of these processes remains a major challenge, which needs to be addressed by investigations with adapted tribometers. In this study, a recently adapted strip drawing test for aluminum warm and hot forming is presented – including a newly developed strip heating unit, a die lubrication system and a die tempering system for efficient tribological testing. The contribution is completed with both, experimental results and a numerical investigation of temperature gradients in the strip drawing test. Finally, it is discussed whether transient process conditions of non-isothermal forming processes with die lubrication should be considered in tribometers for warm and hot sheet metal forming.

Tribology in Warm and Hot Aluminum Sheet Forming: Transferability of Strip Drawing Tests to Forming Trials

“…[22] During a single strip drawing test, the mean temperature in the friction zone Thermal conductivity (20-500 °C) Unimax dies: 27-29 W m À1 K À1 , cf., [34] Specific heat (30-500 °C) Unimax dies: 460-656 J kg À1 K À1 , cf., [34,36] 7075 sheet: 121-158 W m À1 K À1 , cf., [35] 7075 sheet: 863-1102 J kg À1 K À1 , cf., [35] Contact behavior Exponentially softened kinematic COF Constant; 0.01, 0.1, 0. increases due to the draw-in of hot aluminum strips. [21] A similar behavior, although in a weakened form, is expected for the polymer RT strip drawing test at 20 MPa due to frictional heat. In the case of the polymer lubricant, the decrease in COF with temperature and the increase in COF with wear partially offset each other.…”

“…[ 22 ] During a single strip drawing test, the mean temperature in the friction zone increases due to the draw‐in of hot aluminum strips. [ 21 ] A similar behavior, although in a weakened form, is expected for the polymer RT strip drawing test at 20 MPa due to frictional heat. In the case of the polymer lubricant, the decrease in COF with temperature and the increase in COF with wear partially offset each other.…”

“…The dies are temperature controlled from 20 to 450 °C with heating cartridges in their base plate in combination with water cooling channels. [ 21 ] The COF values used in this article were determined in a previous publication in flat die strip drawing tests. The initial strip temperatures were 425 °C (hot), 225 °C (warm), and room temperature (RT) at the start of the drawing process.…”

“…An adapted strip drawing test for warm and hot forming processes has already been presented by the authors. [ 21 ] A comprehensive study of several lubricant classes at various temperatures in the strip drawing test demonstrated differences in coefficient of friction (COF) and wear performance. [ 22 ] In the present article, the transferability of these results to a real forming process of a hat geometry is investigated.…”

A Comparative Study on the Production of a Hat Profile by Roll Forming and Stamping

“…For the discontinuous heat treatments, experiments are conducted in order to apply the temperature curves from the temperature‐supported process routes to the material. Therefore, the W‐Temper experiments are conducted in a contact‐heating device according to another study [ 23 ] and the experiments for the O‐process route are conducted in the Nabertherm NA 15/65 furnace. Figure shows the influence of the heat treatments on the mechanical properties, dependent on the SA time.…”