This paper reports an investigation into the effect of silicon content on phase transformations, microstructural evolutions, and mechanical behavior of Nd:YAG laser welded dual-phase steels. The experimental investigation was conducted on laboratory-processed 0.15 C-1.7 Mn steels with different silicon contents. The heat treatment cycle to develop a dual-phase structure consisted of austenitization at 790 ◦C for 15 min followed by water quenching. The fabricated steels were welded in a butt joint configuration to obtain a fully-penetrated weld joint. Optical microscopy, scanning electron microscopy, and uniaxial tensile test were performed to evaluate microstructural changes, phase transitions, and mechanical properties, respectively. The fracture behavior of the welded joints was also investigated using a scanning electron microscope. The results showed that the volume fraction of martensite was increased with an increase in silicon content. The samples with higher silicon content also had a finer structure, which was due to the higher driving force created during previous cold working. All welded joints were found to consist of a nearly fully martensitic structure, and the heat-affected zone (HAZ) contained a low-temperature region (with partially or fully tempered martensite) and a high-temperature region (with newly-formed martensite). The specimen with the lowest silicon content had the minimum elongation and toughness modulus owing to an incomplete tempering process in HAZ. The specimens containing higher amounts of silicon were fractured in the base metals because of the presence of the martensite phase as a major microconstituent in the fusion zone. The fracture mechanism of the laser-welded dual-phase steels with a lowvolume fraction of martensite and small ferrite grain size was micro-void formation whilst the separation and de-cohesion were observed in dual-phase steels that had a high-volume fraction of martensite.
