Optimization of laser beam welding of PM1000 oxide dispersion strengthened super alloy for high temperature applications

Abstract

The research of this thesis is focused on the optimization of laser beam welding of PMlOOO with respect to its high temperature mechanica! properties. This nickel chromium alloy is used for the thermal protection system of the re-entry test vehicle of ESA; Expert. With respect to the ceramic tiles which are used in conventional thermal protection systems, metals do have a lot of sustainable advantages. For instance the ductility and the self healing capabilities will have a positive effect on the maintenance characteristics of the thermal protection system. PMlOOO makes use of yttrium oxide (1203) for the dispersion hardening which is responsible for the good mechanica! properties at high temperatures. This research focused therefore on the relation between the yttrium oxide in original state in the welds and the mechanica! properties of the welds. To optimize the laser beam wel ding process the wel ding speed ( v) and the laser beam power (P) is varied. The first weld series (weld series I) is investigated with optical microscopy, scanning electron microscopy, hardness tests and morphology research. From these results three weld parameter combinations are chosen ( weld series II) which are used for tensile tests at temperatures from room temperature up to 1250°C. The parameters are chosen such that the infiuence of the welding speed and the welding power is individually measured. There are also tensile tests performed on the PMlOOO base material to set a reference. With optical microscopy it is shown that the macrostructure of the welds exists out of dendrites. In the middle of the weld the dendrites form a center line. The scanning electron microscope showed large amounts of black spots in the weld. With EDX analysis the elements in the spots are identified as titanium, yttrium, aluminum and oxygen. During welding the yttrium oxide is melted and farms yttrium aluminum garnet (YAG: Y3Al50 12 ) in these spots. A part of this material is mixed out at the surface of the weld. The particles do not have a large infiuence on the mechanica! properties of the welds. From the morphology of the welds, the weld efficiency is calculated. The best welds are welded with a heat input between 24 J/mm and 36 J /mm. The efficiency is the highest close to 24 J /mm.