Cracking Behavior of Coating during Hot Tensile Tests of AlSi-Coated Press Hardening Steel

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Abstract

For industrial hot stamping applications, press hardening steel is usually coated with Al-10wt.%Si, in order to prevent substrate decarburization and oxidation at elevated temperatures. However, during hot stamping, the AlSi coating layer fractures, causing severe tool wear, substrate oxidation and increased friction coefficient between the tool and stamped part. The initiation of coating fracture can largely be attributed to the formation of several intermetallic compounds (i.e., FeAl & Fe2Al5) via Fe-diffusion, which also results in void formation throughout the coating layer. These intermetallics are formed mainly during the heating stage, with decreasing Fe-content from the coating-substrate interface. Due to distinctive thermo-mechanical properties of intermetallics compared to the steel substrate, the interaction between different intermetallics, including voids, causes high strain localization around the voids, leading to coating fracture. The goal of this study is to detect the initiation of cracks in a ~45 mu-m coating layer during uniaxial tensile deformation of a 1.5 mm AlSi-coated press hardening steel. For this purpose, isothermal tensile tests were performed at elevated temperatures. The coating cracks were detected by means of acoustic emission (AE) sensors during deformation. The distribution of coating cracks at hot stamping condition was examined via optical measurements. The tensile strain was measured from a strain grid on the sample gauge. The experiment involves heating the coated steel in a furnace to 920 ̊C for 6 minutes, followed by uniaxial tensile deformation (at 600 ̊C and 800 ̊C), and finally quenching at ambient air. The first AE signal from the sample was observed during the tensile deformation at 600 ̊C, indicating that tensile strain initiates fracture in the coating layer. At cooling stage, the temperature change with time triggered more AE signals, which may correspond to substrate phase transformation and additional fractures in the coated steel; the latter is owing to thermal expansion mismatch between the intermetallics in the coating layer, and steel substrate. Interestingly, no AE signals were observed during the heating stage; i.e., no coating cracks occur prior to deformation and quenching.
Original languageEnglish
Pages (from-to)602-607
JournalProcedia manufacturing
Volume47
DOIs
Publication statusPublished - 26 Apr 2020
Event23rd International Conference on Material Forming, ESAFORM 2020 - BTU Cottbus-Senftenberg, Cottbus, Germany
Duration: 4 May 20206 May 2020
Conference number: 23
https://esaform2020.org/

Keywords

  • Hot stamping
  • AlSi coating
  • Acoustic emission
  • coating fracture
  • hot tensile

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