Laser Surface Coating of Bulk Metallic Glass Composition on High Carbon Low Alloy Steel

Abstract

Bulk metallic glass or amorphous alloys are multi-component and approximate deep-eutectic alloys that offer extra-ordinarily high hardness, elastic limit and corrosion resistance but find limited application in bulk structural applications due to their extremely poor tensile ductility and toughness and restricted size/thickness to which they can be directly cast or fabricated. These alloys or bulk metallic glass can be good candidates for wear/corrosion resistant coating on tough metallic components. Laser surface cladding has traditionally been used to produce hard and corrosion resistant surfaces on the steel substrates. Due to rapid cooling rates associated with laser surface engineering, it is logical to anticipate that laser cladding of an alloy with high glass forming ability is likely to produce an amorphous surface or ultra fine grained coating with high hardness and wear resistance. The present study is aimed at developing a wear resistant coating on SAE 52100 steel by laser cladding using the Fe-Cr-Mo-Y-B-C precursor amorphous powder and correlating the laser cladding parameters with surface microstructure and mechanical properties to determine the optimum laser surface cladding conditions.Following coating, the microstructure and phase aggregate were analyzed by scanning electron microscope and X-ray diffraction, respectively. Microhardness and wear resistance were assessed using Vickers microhardness tester and ball-on-plate wear testing machine, respectively.

The coating thickness varied directly with incident laser power and interaction time. Despite trials with wide range of process parameters, the present experiments were unable to retain complete amorphous surface microstructure after laser surface coating. Numerical prediction of the thermal profile and related parameters suggest that the cooling rate and thermal gradient experienced by the coated zone were fairly high. Yet failure to retain amorphous/glassy microstructure of an otherwise bulk metallic glassy alloy suggests that compositional changes (solute redistribution) within the coated zone and across the coating–substrate interface are responsible for nucleation and growth of crystalline phases from the melt. The microstructure was mainly cellular/dendritic in nature with the presence of a very fine carbides/borides precipitates at the cell boundaries/inter-dendritic regions. X-ray diffraction analysis confirmed the presence of -Fe, iron carbide (Fe7C3), chromium carbide (Cr7C3), iron boride (Fe2B) and yttrium boride (YB12) phases in the coated zone. The microhardness of the coated layer was significantly improved to as high as 950 VHN as compared to 270 VHN of the substrate. A significant improvement in wear resistance was achieved due to laser surface coating. The maximum resistance to wear was obtained in the samples lased with an applied power of 1.5 kW and scan speed of 3.5 m/min (double coat). Wear resistance is found to be the highest at the surface and decreased with the depth from the surface.