Identification of Intermetallic Compounds and Its Formation Mechanism in Boron Steel Hot-Dipped in Al-7 wt.% Mn Alloy

In laser welding and hot stamping Al-Si-coated boron steel, there is a problem that the strength of the joint is lowered due to ferrite formation in the fusion zone. The purpose of this study is to develop an Al-7 wt.% Mn hot-dip coating in which Mn, an austenite stabilizing element, replaces the fe...

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Bibliographic Details
Published inCoatings (Basel) Vol. 7; no. 12; p. 222
Main Authors Kwak, Sung-Yun, Yun, Jung-Gil, Lee, Jae-Hyeong, Shin, Dong-Ik, Kang, Chung-Yun
Format Journal Article
LanguageEnglish
Published Basel MDPI AG 01.12.2017
Subjects
Aluminum base alloys
Boron compounds
Boron steels
Constitution
Decomposition reactions
Diffusion layers
Ferrites
Hot dip aluminizing
Hot stamping
Immersion coating
Intermetallic compounds
Iron oxides
Laser beam welding
Manganese
Microstructure
Silicon steels
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Summary:In laser welding and hot stamping Al-Si-coated boron steel, there is a problem that the strength of the joint is lowered due to ferrite formation in the fusion zone. The purpose of this study is to develop an Al-7 wt.% Mn hot-dip coating in which Mn, an austenite stabilizing element, replaces the ferrite stabilizing element Si. The nucleation and formation mechanism of the reaction layer was studied in detail by varying the dipping time between 0 and 120 s at 773 °C. The microstructure and phase constitution of the reaction layer were investigated by various observational methods. Phase formation is discussed using a phase diagram calculated by Thermo-CalcTM. Under a 30 s hot-dipping process, no reaction occurred due to the formation of a Fe3O4 layer on the steel surface. The Fe3O4 layer decomposed by a reduction reaction with Al-Mn molten alloy, constituent elements of steel dissolved into a liquid, and the reaction-layer nucleus was formed toward the liquid phase. A coated layer consists of a solidified layer of Al and Al6Mn and a reactive layer formed beneath it. The reaction layer is formed mainly by inter-diffusion of Al and Fe in the solid state, which is arranged on the steel in the order of Al11Mn4 → FeAl3 (θ) → Fe2Al5 (η) phases, and the Fe3AlC (κ) in several nm bands formed at the interface between the η-phase and steel.
ISSN:2079-6412
2079-6412
DOI:10.3390/coatings7120222