Fiber laser welding of WC-Co to carbon steel using Fe-Ni Invar as interlayer

• This paper prepared cemented carbide-steel laser welds with Invar interlayer using fiber laser.
• Phase constituents, crystal structures, phase transformation, and bending strength of the as-welded cemented carbide-steel laser welds were identified.
• WC segregation, η-phases nucleation in (γFe, Ni) welds, and cored structure formation was discussed using nickel-iron phase diagram.

A fiber laser procedure was developed for joining cemented carbides to carbon steels (3-mm-thick plates) in the butt joint configuration using fcc Fe-Ni invar as interlayer. The optimized welding parameters gave a required weld width and penetration. The as-welded microstructure was characterized as a function of welding speed through fiber laser welding. In fusion zone, non-equilibrium fcc (γFe, Ni) was the predominant phase with α-WC and some mixed carbides such asFe3W3C and Co6W6C. In heat affected zone (HAZ) of cemented carbide, the phases consisted of α-WC, (γFe, Ni) and mixed carbides. Bct–martensite was observed in HAZ near steel side. Which were confirmed by XRD, TEM, and nickel-iron phase diagram calculate using Thermo Cal Classic software. Thick and large sized η-phases were inhibited in the HAZ of cemented carbide and replaced by η-phases with carbon depletion. A higher energy input produced more non-equilibrium (γFe, Ni) and martensite. Small amounts of η-phases, non-equilibrium (γFe, Ni) besides the martensite. The thermal cycles associated with laser welding led to a rather small fusion zone, which usually inhibited η-phases formation due to the inhibition of normal grain growth and the decrease of iron/nickel diffusion across WC/Co interface. The results indicated that the maximum bend strength was 980.90MPa. Due to the formation of martensite/carbides and an excess of which led to embrittlement, welded-joint samples were broken at the HAZ or in the fusion zone instead of the base metal. The cracks exhibited the characteristics of hot crack ((γFe, Ni) in the fusion zone) and cold crack (α-WC at the HAZ of cemented carbide).