High Temperature Tribology in Hot Stamping

Abstract: Many automotive components are made of Al-Si coated ultra-high strength boron steel (UHSS) and are produced by hot stamping process. In this process, the workpiece is heated to an austenitizing temperature and is then formed and quenched simultaneously between the tools to achieve the desired shape and high strength. During hot stamping process, friction and wear occur which affect formability and maintenance intervals for tool replacement and repair. To repair worn tools, metal is deposited by fusion welding technique. The tribological behaviour of repair welded tool steel sliding against Al-Si coated UHSS has not been studied in detail and there is a need to investigate if the modified tool surface will affect friction and wear. Hot stamping, similar to many manufacturing processes, is affected by the global mega trend of digitalization and Industry 4.0. To monitor the process and optimize the control and operation are the main aims. In view of this, tribological condition monitoring is a promising approach that can allow measurement of physical properties such as vibrations, temperatures, and acoustic emission to be coupled to the tribological response of the system. The aim is to monitor the hot stamping process and enable early detection of changes in friction and wear which can be used for e.g. optimized maintenance and minimized scrap. The aim of this M.Sc. thesis was to improve the robustness of hot forming processes by studying the tribological behaviour of repair welded tool steel sliding against Al-Si coated UHSS under conditions relevant for hot stamping. Another aim was to obtain more predictable tool maintenance by the implementation of acoustic emission measurement system on a hot-strip tribometer and correlating condition monitoring signals to friction and wear phenomena. The tribological tests were carried out using a hot-strip tribometer in conditions representative of a hot stamping process of automotive components. Acoustic emission during sliding between hot work tool steel and different automotive component material surfaces was measured at room temperature in the same strip drawing tribometer and correlated to friction and wear of the surfaces to get more predictable maintenance intervals. Tool steel specimens were welded with the same material as the base material QRO90. Before conducting the tribological test, the repair welded tool steel pin cross-section was polished, etched, and observed under optical microscope and SEM to analyze the effect of Tungsten Inert Gas (TIG) welding process on the microstructure. The analysis was completed with EDS to study the elements in the microstructure. Microhardness was measured to obtain the microhardness profile from the repair welded tool steel pin surface to the bulk in order to study the effect of different microstructures on the mechanical properties. The weight and surface roughness of the pins were measured before the tribological test. After the test was finished, the weight of the pins was measured to calculate the weight difference. The sliding surface of the pins and the strips were photographed. The sliding surface of the pins was also observed and analyzed using SEM and EDS after the test to study wear characteristic of the repair welded tool steel at high temperatures. Acoustic emission signal from the sliding was studied using Toolox44 pins with surface roughness 300-400 nm and with lay direction parallel and perpendicular to sliding direction. Toolox44 pins were sliding against uncoated UHSS, as-delivered Al-Si coated UHSS, and heat-treated Al-Si coated UHSS strips. Acoustic emission was measured during the sliding at the same time as COF measurement. Weight of the pins was measured before and after the test and the wear damage on both surfaces was photographed. COF, AE signals in the time and frequency domain, and wear damage were compared and analyzed. It is found that repair welded tool steel has similar COF compared to the original hot work tool steel with the largest weight gain from the test at 700 ⁰C due to compaction galling mechanism with slower lump formation and the presence of wear particles, transfer layer, and formation of lumps. The weight gain is smaller from the test at 750 ⁰C due to faster lump formation. The weight loss from the test at 600 ⁰C is due to abrasive wear mechanism. SEM micrographs revealed that the repair welded tool steel surface and transfer layers can be found beneath a transfer layer. Wear particles adhered on the repair welded tool steel surface come from broken transfer layer or directly from Al-Si coated UHSS. A change in wear mechanism is indicated by acoustic emission burst signals or gradual amplitude change in the time domain. Frequency analysis of AE signals revealed a change in wear mechanism due to the formation of transferred material in the form of a lump causes AE signals with peaks at higher frequencies above 0.3 MHz to shorten.