Laser assisted Fe-based bulk amorphous coating: Thermal effects and corrosion

The present study focuses on synthesizing composite coatings for corrosion resistance using laser surface alloying (LSA). Amorphous powder with nominal composition (Fe48Cr15Mo14Y2C15B6) is used as the precursor powder on AISI 4130 steel substrate and processed with a continuous wave ytterbium Nd-YAG fiber laser. A multi-physics based heat transfer model was developed to evaluate the thermal histories experienced during processing. The thermodynamic parameters like peak temperatures and cooling rates are evaluated using the computational model and correlated to the evolution of microstructure. Phase and microstructural characterization of the coatings was conducted using XRD, SEM and TEM. Anodic polarization tests conducted in HCl medium indicated the enhancement in corrosion resistance of the laser processed samples. The laser processed samples showed better corrosion resistance than the substrate and among the processed samples, the corrosion resistance decreased with increasing laser energy density. The reduction in the corrosion resistance can be attributed to the formation of Cr23C6 nano crystals in the amorphous phase. The operating corrosion mechanisms are discussed with the aid of the thermal modeling results.