Thermodynamics and structure of solidification in the fusion zone of CO2 laser welds of 15-5 PH stainless steel

The microstructure in the fusion zone of 15-5 PH autogeneous laser beam welds revealed that the solidification mode is primary ferrite. Both primary ferrite and eutectic ferrite existed in the fusion zone. Primary ferrite was found to be in the vermicular morphology and located at the axes of the cells. Eutectic ferrite was roundish in nature and was located at the cell boundaries. Eutectic ferrite was identified with its location in the structure and it is associated with NbC and a niobium-containing Z-phase precipitate.

The formation of a eutectic and the thermal phase stability of 15-5 PH was predicted from the Fe-Ni-Cr phase diagram for the nominal composition of the alloy from equal chemical potentials and differential mass balance equations. These equations were used to predict the solidification path and microsegregation patterns for iron, nickel and chromium for the case of complete mixing in the liquid. It was shown that the solidification mode is ferrite and that the liquid reaches the eutectic valley when the fraction of solid is 0.7.

The precipitation hardening element and the phase were pure copper, and were coherent with the matrix both in the fusion zone and in the base metal. Generally, the fusion zone was found to be harder than the base metal.