Structural organization and properties of surface layers of WC–Co hard alloys after pulsed laser processing

Abstract

The article presents the metal-physical studies results of the structure formation effects in surface layers in the hard alloys of the WC–Co system under extreme thermal and deformation effects of pulsed laser radiation. It is shown that the structural organization and properties of hard alloys VK6, VK8, VK10 upon radiation treatment with a power density of 175 MW/m2 are determined by state of the zones which are formed around carbide inclusions due to the various kinds of stresses appearance at the “carbide-bond” composition boundaries, including thermostrictive and phase stresses. The result is dissolution of the carbides boundary zones due to contact melting, which is accompanied by mutual mass transfer of atoms at the boundaries in the “carbide-bond” system with the possible formation of a thin amorphous-like super hard shell. These processes make it possible to create compositions in hard alloys with a set of differentiated properties specified by varying the laser treatment process parameters and composition of the starting materials. After laser alloying with a radiation power density of 200 MW/m2, temperature gradients and thermal stresses appearing in the surface layers of hard alloys with coatings (cobalt, nickel) contribute to convective mixing of the molten coating components and their penetration into the hard alloy to a depth of more than 20 μm. Simultaneously, despite the extremely short laser pulse time (10–3 s), mass transfer of tungsten, carbon and titanium atoms from the melted boundary zones of carbides to the adjacent bond zones with their hardening is possible in the irradiated zones. It was established that after high-temperature laser heating, carbides, in contrast to the initial ones, achieve a globular shape of grains. They are dispersed, and stoichiometric characteristics change in the local zones bordering the bond (the complex type carbide CoxWyCz is formed). As a result, due to these processes, the surface layers’ viscosity of hard alloys and the irradiated products performance increase. Compared to non-irradiated samples of hard alloy, the ultimate strength increases by 15 %, strength and durability – by 30 – 40 %.