Tribological Behaviour of PVD Coated Tool Steels in Hot Forming of Aluminium Alloys

Aluminium alloys are commonly used as light-weight materials in the automotive industry. This non-ferrous family of metal alloys offers high versatility of properties and designs. In order to reduce weight and improve safety, grades with high strength-to-weight ratio, such as the 6XXX and 7XXX alloy series, are increasingly implemented in vehicles. These alloys, however, exhibit low formability and experience considerable springback when formed at low temperatures, and thus have to be formed at elevated temperatures. Severe adhesion and galling are known to be critical tribological challenges in hot forming of aluminium. During the forming operation, adhesion and transfer of aluminium onto the die surface take place. This phenomenon has a detrimental effect on the surface properties, geometrical tolerances of the formed parts and maintenance of the dies. The influence of surface engineering as well as lubricant composition on adhesion and galling has not been sufficiently investigated. Diamond-like-Carbon and Chromium Nitride PVD coatings applied on the tool steel have shown promising results for reducing aluminium transfer at high-temperatures, especially in the presence of a lubricant. However, the interaction between lubricants and PVD coatings during hot forming of aluminium alloys is not yet fully understood. The present study thus aims at characterising the high temperature tribological behaviour of selected PVD coatings and a lubricant during sliding against an aluminium alloy. The objectives are to select promising lubricant-coating combinations for the given application and to study their tribological response in a high-temperature reciprocating friction and wear tester. The tests were carried out at 300°C, under dry and lubricated conditions, in order to study the friction and wear performance. Uncoated tool steel reference tests were performed under dry and lubricated conditions and lubricated tests using DLC, CrN, CrTiN and CrAlN coated tool steel were performed and compared to the reference results. The initial and worn surfaces were analysed with white light 3D optical interferometry, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) with a view to understand the wear mechanisms.