Abstract
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 the forming tools. 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. However, after the 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. In such a scenario, acoustic emission sensors are a suitable candidate for a continuous inspection of sheet materials, including the coating layer.
In this thesis, a novel 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 temperature. Meanwhile, to understand the cracking 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, this time, 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.
In this thesis, a novel 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 temperature. Meanwhile, to understand the cracking 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, this time, 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.
| Original language | English |
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| Qualification | Doctor of Philosophy |
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| Award date | 28 Oct 2021 |
| Place of Publication | Enschede |
| Publisher | |
| Print ISBNs | 978-90-365-5272-1 |
| DOIs | |
| Publication status | Published - 28 Oct 2021 |