Search

In this research, microstructure and crystallographic features of different phases generated on interstitial free steel hot dipped in Al-6.9Si-1.4Mg alloy melt was examined. Bath temperature was 750 °C for 0.5, 1, 3 and 6 min dipping. A combined arrangement of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and electron backscatter diffraction was used for phase identification. Results indicated that aluminized layer comprised of outer Al-Si alloy top-coat, thin FeAl3 layer and a thick Fe2Al5 layer in contact with steel substrate. Ternary Fe-Al-Si intermetallic compounds of Al2FeSi (τ3) and Al3FeSi2 (τ4) were also identified. Elemental mapping, chemical composition analysis and in-depth composition profile of elements were done by electron probe microanalysis and glow discharge optical emission spectroscopy (GDOES). XRD study confirmed elemental silicon (Si) and magnesium (Mg) in the form of Mg2Si. SEM-EDS and GDOES studies confirm a sudden rise in Si at% at the top and in the intermetallic layer, which was attributed to the existence of Si-bearing phases or elemental pro-eutectic Si. The formability of the coated sheet was assessed by bending the samples at 180° to investigate the peeling-off of the outer coating layer from the steel surface.

Interstitial-free (IF) steel specimens were coated by hot-dip aluminizing process bearing bath composition as Al-6.9% Si-1.4% Mg and bath temperature at 800°C. Dipping time varied for 0.5, 1, 3 and 6min. The objective of the present study is to conduct a systematic investigation to identify and analyse the formation of various phases and compounds in the coating. Studies on aluminizing of steel had indicated the presence of an intermetallic layer adjacent to steel substrate. In this study, the morphological features obtained after hot dipping at 800°C were investigated. For the application of aluminized steel sheets in industry for manufacture of tubes and pipes, the adherence of the aluminized coating must be ensured. This property of the coating is influenced by the morphological features of the coating. Aluminizing being thought of a possible substitution of galvanizing owing to its promise to evolve lesser volume fraction of intermetallic phases and consequently known to generate a ductile coating. The entire coating consists of an outer Al-Si topcoat and a thin intermetallic layer which composed of an outer θ-FeAl3and inner η-Fe2Al5layers. The thickness of intermetallic layers was found to increase with the increase of dipping time in a near parabolic manner.The phases formed on the IF steel substrates were characterized by scanning electron microscopy with EDS, X-ray diffraction, EPMA and EBSD techniques.Interesting findings from XRD include the presence of Al3FeSi2(τ4) and Al2FeSi (τ3) phases, which normally appear after annealing of aluminised sheet. A distinct elemental Si peak was found in XRD study. Presence of Mg in the form of Mg2Si was also found in XRD and supported by EPMA investigation. Presence of several ternary Al-Fe-Si (τ3, τ4, τ5) and binary FeAl3and Fe2Al5intermetallic phases were identified by composition profile analysis from SEM based EDS and GDOES and further confirmed by EBSD. Presence of Si humps in the top and intermetallic layer attributed by the presence of Si-bearing phases or elemental pro-eutectic Si in Al-Si alloy were also indicated in SEM-EDS and GDOES study. Variation of surface topographical features of the HDA steel substrates with increasing dipping durations were examined and reported for the first time by using AFM. Formability tests were conducted by bending the samples 180° over a mandrel. None of the specimens showed any tendency of flaking exhibiting excellent bonding of the coating with IF steel substrate.