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TWB requirements in hot forming steels

Nicklas Egger
Göteborg : Chalmers tekniska högskola, 2018. 76 s.
[Examensarbete på avancerad nivå]

In this thesis, the combination of two Al-Si coated hot forming steels were investigated for the most suitable process in terms of welding by Tailored Welded Blanks (TWB). The research aimed to establish the criterion for the proper material preparation and welding, as well as the establishment of the required joint and material properties both before and after hot forming. The Al-SI coating used on the hot forming steels have detrimental effects on the mechanical properties of the weld. A common method within the automotive industry, to avoid these effects, is to remove the coating prior to welding by laser ablation. In this research, four different laser welding processes regarding the ablation of the coating and the usage of filler wire were used to join the blanks together, and later analysed. The two main welding processes investigated were: laser welding on partially ablated surfaces (called STD), and laser welding together with filler wire on coated surfaces (called FW). A relatively large literature survey was carried out on the subject. A series of 5 experiments were completed: a drop weight test on complete components; hardness profiling on the cross section of the weld; microstructural analysis on the cross section of the weld; analysis of the chemical composition in the cross section of the weld; tensile testing. Segregated areas of ferrite and suspected intermetallic compounds were discovered in the fusion zone on the FW-welding process. The chemical composition of these segregated areas showed a relatively high constituent of aluminium. The tensile testing showed that the STD- and FW-welding process achieved similar or higher tensile strength than the parent material. It was found that the FW-welding process resulted in equally or slightly better mechanical properties than the STD-welding process. The FW-welding process would require changes in the manufacturing chain of the body in white, while the STD-welding process would not. The addition of a filler wire in the laser welding would complicate and affect the robustness of an already complex welding process. Further studies and investigations are needed on larger sample series to verify these results and to clarify exactly what changes are necessary in the manufacturing chain for the FW-welding process. It was also found, that a tensile test together with a hardness profile over the cross section of the weld would be a good enough method to verify the weld properties after hot forming, for judgment of monotonic strength.

Nyckelord: Tailor Welded Blank, Laser welding, Filler wire, Al-Si coating, Laser ablation, 22MnB5

Publikationen registrerades 2018-06-20. Den ändrades senast 2018-06-20

CPL ID: 255158

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