Welding properties and formabilies of a tailored blank which consists of dissimilar metals are interesting because of the possible benefits for lightening of materials. Especially, the combination of mild steel and aluminum or aluminum alloy is expected as a representative of such a light hybrid material. Until now, some successful examples about welding of steel and aluminum using friction stir welding and laser roll welding have been reported. Recently, our research team has developed a butt laser welding method using CO2 laser. It was already confirmed that, by using this method, it was possible to obtain SPCC/A1100 O-tailored blanks with almost equal joint strength to the tensile strength of base aluminum. However, it has not been reported what was made in welded interfaces of the hybrid sheets. In this study, the welded interface of SPCC/A1100-O tailored blank obtained by butt laser welding was observed and analyzed using FE-SEM and EDS. Furthermore, fractured site in some formability tests were also observed, and it was identified where fracture occurred. In the experiments, the welded interface of steel/aluminum tailored blank, which has joint strength of 80MPa-90MPa, was observed. In the observation of low magnification, no clear intermetallic compound layers were seen. In the observation of higher magnification, it was confirmed that some intermetallic layers existed. From results of EDS analysis, it was indicated that this intermetallic compound layer consisted of FeAl3, Fe2Al5, FeAl, Fe2Al and Fe3Al. Thin Fe2Al5 layers of about 3 micro meters mainly existed between steel side and aluminum side. From this layer, some dendritic FeAl3 layers appeared into the aluminum side. In the steel side, relatively wide FeAl layers and Fe3Al layers were seen in turn. According to references, because welded sheet of steel and aluminum with good mechanical properties has intermetallic layers less than 4 micro meters, it was confirmed that butt laser welding method developed by our team had possibility to provide good steel/aluminum tailored blanks. In the fractured site by Erichsen test, Fe2Al5 layered were left in both steel side and aluminum side. Thus, it was indicated that a crack spread along the Fe2Al5 layer. From results of these experiments, it was confirmed that butt laser welding method developed by our team provided steel/aluminum tailored blanks with thin Fe2Al5 layers of about 3micro meters. And, it was indicated that this Fe2Al5 layer brought about early fracture.
