Tribological characterisation of additively manufactured hot forming steels

Sammanfattning: Over the last decade, the application of ultra-high strength steel as safety components and structural reinforcements in automobile applications has increased due to their favourable high-strength-to-weight ratio. The complex shaped components are widely produced using hot stamping. However, this process encounters problems such as galling and increased wear of the tools due to harsh operating conditions associated to the elevated temperatures. Moreover, quenching is a critical step that affects the hot formed components. Slow cooling rates results in inhomogeneous mechanical properties and increased cycle time. Therefore, fast and homogeneous quenching of the formed components in combination with reduction of wear rates during hot forming are important targets to ensure the quality and efficiency of the process. The use of additive manufacturing (AM) technologies opens up potential solutions for novel tooling concepts. The manufacturing of complex shape cooling channels and integration of high-performance alloys at the surface could benefit the tribological performance in the forming operation. However, the research into high temperature tribological behaviour of AM materials in hot forming applications is very limited. The aim of this work is to study the tribological performance of additively manufactured materials. Two steels were used – a maraging steel and modified H13 tool steel. The hot work tool steel H13 is commonly applied for dies in metal forming processes. In this thesis it was used to study additive manufacturing as the processing route instead of conventional casting. The choice of a maraging steel is motivated by a possible application of high-performance alloys as a top layer on dies. The materials were post-machined and studied in milled, ground and shot-blasted conditions. The different post-machining operations were applied to study the effect of surface finish on the tribological behaviour and also to evaluate different methods of post-machining an AM surface. As fabricated dies are usually manufactured with milled surface. During its use, the dies undergo refurbishment after certain number of cycles and the surface condition is changed to a ground surface. These surface finishes are commonly tested for hot forming applications. The shot blasted operation was chosen as alternative surface finish. The process allows to prepare large sized tools easily and the surface has beneficial compressive stresses. The tribological behaviour of AM steels was studied using a hot strip drawing tribometer during sliding against a conventional Al-Si coated 22MnB5 steel. The workpiece temperature during the tests was 600 and 700°C. The results of the tribological performance of AM materials were compared to conventionally cast tool steel QRO90.The results have shown that the friction behaviour of both maraging and H13 steels at 600°C was stable and similar whereas at 700°C the COF was more unstable and resulted in an earlier failure of the tests due to increased material transfer of Al-Si coating from the workpiece surface.The main wear mechanisms for AM materials were galling and abrasion at both temperatures. Abrasion is more severe for the AM steels in comparison to cast tool steel QRO90. The galling formation on milled and ground surfaces showed similar behaviour to cast steel and it increased with higher workpiece temperatures. The shot-blasted surfaces showed less build-up of transferred material on the surface but folding of asperities and entrapment of Al-Si particles within surface defects generated during shot-blasting.