Thermo-mechanical behavior of the Al-Si alloy coated hot stamping boron steel

Hot tensile tests of boron steels with and without an Al-Si coating were performed using a Gleeble 3500 test system, at temperatures of 700-850 degrees C and strain rates of 0.01-1/s. The phase and microstructure of the coating in as-coated and press-hardened conditions were observed under scanning electron microscopy (SEM) equipped with energy dispersive X-ray analysis and X-ray diffraction (XRD). Experimental results indicate that the Al-Si coating gave an unignorable influence on the thermo-mechanical properties of the boron steels. The ultimate tensile strength (UTS) of the Al-Si coated boron steel was almost equal to that of the uncoated under the lower strain rate at the same deformation temperature. At a higher strain rate, the UTS value appeared to be lower than that of the uncoated. Moreover, the UTS difference increased with the decreasing deformation temperature. The ductility of the Al-Si coated steel was lower than that of the uncoated under the described test conditions. Following the tensile tests, extensive cracks were visible in the Al-Si coating layer. SEM observation showed that microcracks and voids appeared after austenization, which may act as nucleation sites for the cracks. The cracks first propagated in the direction perpendicular to the coating/substrate interface and were identified as Type I cracks. The propagation was hindered by the substrate when these cracks reached the coating/substrate interface. This occurred because the interfacial bonding strength between the coating and the substrate was lower than the substrate strength. Following this initial failure, the cracks turned to propagate paralleled to the coating/substrate interface. In addition with the shear stress resulting from the substrate yielding, Type II cracks formed. Eventually, the cracked coatings were accompanied by interface decohesion from the substrate. The width and density of the cracks were found to increase with the decreasing deformation temperature and rising stain rate. (C) 2014 Elsevier Ltd. All rights reserved.