Hot stamping of aluminium alloys allows for increased formability, decreased springback and the possibility of integrating age-hardening heat treatments into the process. However, it can be challenging due to the occurrence of material transfer of aluminium onto the tool, as aluminium is prone to adhesion even at low temperatures. Hence, lubrication is always necessary when forming aluminium, but lubricants can still fail, leading to direct interaction between tool and workpiece and thus material transfer. This phenomenon reduces the efficiency of the process, as interruptions are necessary for the refurbishment of the tools. Understanding of how material transfer takes place is important in order to find new or improved solutions, in terms of lubrication and surface engineering, to prevent adhesion. Nevertheless, current research in high temperature tribology of aluminium, mainly in terms of material transfer mechanisms, is very limited, as many of the works focus on lubricated conditions and do not look into the fundamental interactions between aluminium alloys and tool steels. In this context, the aim of this work is to investigate the mechanisms behind the occurrence of aluminium alloy transfer onto tool steel during sliding at high temperature and in dry conditions. A hot-strip drawing tribometer was used to perform tests at room temperature, 300°C, 400°C, and 500°C, directly after solubilizing the aluminium alloy at 520°C. Two different topographies for the tool steel were used: ground and polished. Material transfer characterization was performed mainly through scanning electron microscopy. It was found that grinding marks (ground tool steel) and carbides (polished tool steel) act as initiation sites for the transfer to occur. Temperature plays a role on the growth mechanisms of the transfer films during sliding, as thermal softening of the aluminium alloy is the dominant factor in determining the growth direction of the transfer layers. A growth towards the trailing edge (shearing and smearing of the transferred aluminium) or a growth towards the leading edge (build-up of transferred aluminium, leading to a thicker and more localized transfer material).
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:ltu-80118 |
| Date | January 2020 |
| Creators | Macêdo, Gabriel |
| Publisher | Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Student thesis, info:eu-repo/semantics/bachelorThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
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