The microstructural response of delta-stabilized plutonium to pulsed laser welding
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7/10/04
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The Microstructural Response of Delta-Stabilized Plutonium to Pulsed Laser Welding J.O. MILEWSKI and D.A. JAVERNICK The effects of the welding parameters on weld-bead morphology and the microstructural evolution of autogenous conduction-mode melted laser welds in gallium delta-stabilized plutonium were investigated. This investigation demonstrates that delta-stabilized plutonium is easily welded with a pulsed Nd:YAG laser. Variations in the welding parameters created a range of heat inputs that had a significant effect on the size and morphology of the resultant weld and associated heat-affectedzone (HAZ) microstructure. Unlike other low-melting-temperature materials such as aluminum alloys, neither fusion-zone porosity nor hot cracking was observed. The reactivity of plutonium promotes the rapid formation of surface oxides that inhibit complete weld-joint fusion. The high viscosity of the plutonium molten pool impeded the breakup of the surface oxide by fluid motion, thus further impeding joint fusion. Modifications to input parameters to affect changes in the weld-pool flow were only partially successful in improving joint fusion. Analysis of the weld region using optical and electron microscopy and electron microprobe analysis revealed fusion-zone microstructures consisting of fine acicular delta grains, with a limited amount of retained alpha phase. The Pu6Fe, the low-meltingpoint intermetallic phase, was observed in the base material and the partially melted HAZ. Further investigation into the solid/solid phase transformations, specifically the effect of the cooling rate, during the cooling of the weld will be required to describe fully the microstructural development of the fusion zone.
I. INTRODUCTION
FEW materials behave similarly to plutonium due to its unique physical properties; the nature of this metal creates unique challenges related to welding. Published information on the welding of plutonium and plutonium alloys is very limited. The Plutonium Handbook,[1] a reference of nearly 1000 pages, contains only ten sentences related to the welding of this material. The lack of published information on plutonium welding is not surprising, given that no civilian or commercial applications exist for this material. However, a fundamental understanding of plutonium welding metallurgy is warranted because of the unique properties of this material and because it will extend the fundamental understanding of the laser-weld processing of metals. Historically, the joining of plutonium has been performed since the late 1950s. Gas tungsten arc welding (GTAW) methods were first used to join pure alpha- and delta-stabilized plutonium.[2,3] These studies showed that delta-phasestabilized plutonium exhibits acceptable weldability; however, pure alpha-phase plutonium has a tendency to crack during welding. Delta-phase plutonium has also been welded to a variety of other metals including uranium, aluminum, nickel, steel, copper, molybdenum, niobium, and Hastelloy.[3] The GTAW welding processes pro
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