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PhD Defence Shakil Zaman | Fracture characterization of AlSi coating at high temperatures

Fracture characterization of AlSi coating at high temperatures

Due to the COVID-19 crisis the PhD defence of Shakil Zaman will take place (partly) online.

The PhD defence can be followed by a live stream.

Shakil Zaman is a PhD student in the research group Nonlinear Solid Mechanics (NSM). His supervisors are A.H. van den Boogaard and M.B. de Rooij from the Faculty of Engineering Technology (ET).

Hot stamping is a technology widely used in the automotive industry to produce ultra-high strength steel parts. This method combines traditional heat treatment and cold stamping technologies. The coated sheet is first heated in a roller hearth furnace, after which the blank is simultaneously formed and quenched by forming tools. Every stage of hot stamping determines the quality and integrity of the final product. The heating stage ensures full austenitization of steel, the deformation stage gives shape to the blank sheet with low forming loads and quenching generates martensite in steel. Consequently, the final product consists of a high strength steel with good geometrical tolerances. One of the pre-conditions of performing hot stamping is the need for a coating layer, which not only protects the base material from high temperatures but also ensures good surface quality.

To avoid surface oxidation of steel at high temperatures, aluminum-based coated steel sheets are generally used. These Al-based coatings prevent surface oxidation, decarburization and enhance the corrosion resistance of the hot-stamped parts. The ability of such a coating layer to form intermetallic compounds increases its melting point above 1000 ˚C, enabling such a coating to operate at high temperatures without melting. The diffusion of iron from steel to the coating transforms the aluminum-rich coating into layers of iron-rich intermetallic compounds, which change the overall mechanical and fracture responses of the coating layer. Despite such enhancement of the coating, localized cracking and delamination are its major problems at high temperatures. Furthermore, the coating fracture releases debris, which not only exposes the steel but also damages the tool surface. Therefore, it is important to know how the coating evolves throughout the hot stamping process, leading up to the point when coating fracture occurs. This also enables to understand the mechanics of AlSi coating on steel at elevated temperatures and provides a guideline to the hot stamping industrial process such that the coating fracture is prevented.

After the standard hot stamping cycle, the coating layer is found to sustain fracture. It is not clear when these coating fractures occur, as the coated sheet is visually inaccessible inside the furnace. Another method of thin layer inspection is necessary. In such a scenario, acoustic emission sensors are a suitable candidate for continuous inspection of sheet materials. Therefore, with proper signal threshold, the acoustic sensors can be used to acquire an in-situ visualization of coating fracture events, even at high temperatures. Apart from detecting the acoustic signals, it is also possible to trace the location of every individual signal. In this way, a continuous method of inspection is proposed to precisely monitor coating cracking events during the heating, deformation and cooling stages of the hot stamping process.

In this thesis, a novel hot tensile experimental setup with acoustic emission sensors is realized to monitor the AlSi coating fracture at high temperatures. The results indicate that there is a strong correlation between coating fracture and deformation temperature. Meanwhile, to understand the crack initiation and propagation events in the AlSi coating, finite element analyses are performed. According to simulation results, the coating fracture is minimized when the content of Fe-rich compounds is increased or when the number of voids is reduced in the coating. Based on these simulation predictions, hot tensile experiments are performed by modifying the heating parameters such that different coating microstructures are generated. According to the experimental results, an inverse correlation between the content of Fe-rich compounds and coating crack density is observed. Finally, the AlSi coating fracture is investigated for component level hot stamping experiments. The results show that the coating fracture pattern is in accordance with the direction of principal strain vectors while the coating removal is associated with contact pressure and blank--tool relative sliding.