Alsife ternary diagram
The formation of AlxFey intermetallics at the interface between a steel substrate and a hot-dipped Al-base coating has been intensely investigated during the last two decades. The formation of these AlxFey intermetallics is strongly influenced by the chemical composition of the Al bath, coating thickness, as well as by the dwell time and the austenitizing temperature prior to stamping. During the hot dipping and austenitizing processes, diffusion processes occur at the interface between the steel substrate and the Al coating, resulting in the formation of intermetallic AlxFey phases at the interface. The protective effect of this Al-base coating is attributed to the formation of a thin and dense Al2O3 layer. To prevent strong oxidation of the steel sheets during austenitization at temperatures of 880 to 950 ☌, a protective Al-base pre-coating is commonly deposited on the steel sheets by hot dipping. įormed steel sheets have a fully martensitic microstructure with a strength greater than 1350 MPa. Subsequently, the soft steel sheets with an austenitic microstructure and low yield strength (200 MPa) are transported into a tool and then formed and quenched in one step. During direct press-hardening, steel sheets are austenitized at temperatures that exceed AC3 by 30 to 50 ☌ (i.e. This problem can be avoided by the use of ultrahigh-strength steels, such as boron-alloyed steel (22MnB5), which can be processed by direct press-hardening. However, the formability of these steels decreases as the material strength increases, promoting the appearance of material defects (cracks, corrugation, stretcher strain marks) during cold stamping. The last decades saw the development of several steel grades, such as dual-phase (DP), TRIPsteels (TRansformation Induced Plasticity), complex-phase (CP), and martensitic high-strength steels, which are currently used successfully in the automotive industry, especially for security-relevant component parts such as longitudinal rear rails, B-pillar reinforcements, inner roof rails, and other crash protection applications.
One promising strategy to fulfill the aforementioned aim is to use high-strength steels. Introduction To meet the requirements of lowering CO2 emissions in the automotive industry by reducing weight which means a fuel saving and to provide a simultaneously high crash safety, emphasis has been incorporated on the development of new material concepts as well as new joining methods.
Higher austenitization temperatures and longer dwell times support the formation of more ductile FeAl intermetallics but also lead to grain growth thus having a negative effect on the mechanical properties of the steel. Formation of FeAl intermetallics and thus the mechanical properties of the AlSi10Fe3 coating can be influenced by heat treatment. Simultaneously, Al diffusion into the substrate changed the microstructure of the steel substrate near the coating interface. As a result of the diffusion processes, Al-rich intermetallics in the coating transformed to more Fe-rich intermetallics. Increasing the dwell time led to Fe diffusion into the Al-base coating as well as Al diffusion into the substrate.
Phase identification by EBSD and XRD confirmed the formation of Al-rich intermetallics during austenitization.
The coated steel substrates were austenitized at 920 ☌ for several dwells, and phase formation at the steel/coating interface was investigated by means of ex-situ phase analysis with synchrotron radiation and EBSD. Theisen Lehrstuhl Werkstofftechnik, Ruhr-Universität Bochum, 44801 Bochum, GermanyĪrticle history: Received 22 October 2012 Accepted in revised form 27 March 2013 Available online 6 April 2013 Keywords: Al-Fe Diffraction with synchrotron radiation EBSD IntermetallicsĪ b s t r a c t Al-base coating (AlSi10Fe3) was applied to a steel substrate (22MnB5) by hot dipping. Surface & Coatings Technology journal homepage: Phase formation at the interface between a boron alloyed steel substrate and an Al-rich coating M. Surface & Coatings Technology 226 (2013) 130–139Ĭontents lists available at SciVerse ScienceDirect