Damage of forming tools at elevated temperatures is a major problem in metal forming processes such as hot stamping. Of special importance is the occurrence of adhesion and gross material transfer from the work-piece to the tool surface, a phenomenon known as galling. This type of damage adversely affects the quality of the produced parts and process economy due to frequent maintenance for refurbishing and/or replacing the tools.
In view of its importance, several studies on galling have been conducted. However most of these studies pertain to conventional or cold forming processes.
In this work, a systematic analysis of damage mechanisms of actual hot stamping tools and extensive high temperature tribological tests to investigate the tool-workpiece interaction have been carried out. The analysis of worn hot forming tools revealed that the main damage mechanisms encountered during hot forming of Al-Si coated ultra-high strength steel (UHSS) are fatigue, corrosion and material transfer. Amongst these mechanisms, material transfer (galling) is the most important from quality and productivity points of view.
It was found that galling onto the tool steel is caused by accumulation and compaction of wear debris. Lumps of transferred material are formed through agglomeration of wear debris. The preferential sites for the accumulation of debris are defects on the tool surface or grinding marks generated during refurbishing. Hence, the surface topography of the tool is of great importance for controlling the material transfer. The most relevant parameters that influence galling during the interaction of Al-Si coated UHSS and tool steel were studied through laboratory tribological tests. It was found that controlling parameters such as Rvk, Rpk, Rsk and Sm can minimise galling on untreated tool steels. Furthermore, it was observed that sliding parallel to the surface lay of the tool results in reduced galling as the debris can escape the contact through the valleys of the tool surface.
The usage of different PVD hard coatings, such as AlCrN, TiAlN and DLC, were considered as a possibility to alleviate galling. Additionally, the tribological response of plasma nitrided tool steel with and without a post oxidation treatment was also studied. The use of hard PVD coatings on the tool steel resulted in severe adhesion and material transfer during sliding against Al-Si coated UHSS at elevated temperature. This was due to the affinity between the constituents of the coating of the tool and the Al-Si coating.
Plasma nitrided tool steel with post oxidation treatment showed negligible galling mainly due to the formation of protective oxide layers on the surface of the Al-Si coated steel. The formation of these layers reduced wear of the Al-Si coating and consequently the generation of Al-Si wear debris. The protective oxide layers are formed primarily by wear debris from the outermost layer of the post oxidised plasma nitrided tool steel. Heat treatments of the Al-Si coating revealed that the phases present in the coating are directly linked to the galling behaviour. Without sufficient temperature or time, stable and harder phases are not formed. This greatly increases the occurrence of galling by severe adhesion as the outermost layer of the coating (unalloyed Al) and the tool steel have high affinity. If the harder phases are formed, the mechanism for galling changes to accumulation and compaction of wear debris from the Al-Si coating and direct adhesion between the Al-Si coated UHSS and the tool steel is reduced.