VPPA Heat Source Characteristics Based On Numerical Simulation And Performance On Thin-walled Shell Structure

The concept of variable polarity plasma arc (Variable Polarity Plasma Arc, VPPA)is began in1947, but has not been real development. Until in the late1960s and early1970s, Vancleave in Boeing was proposed to replace GTAW with variable polarityplasma arc welding. Though the aerospace industry has paid more and more attentionto the thermodynamics analysis of the welding structure, but there are rarely study onthe VPPA heat sources, and the simulation of welding aerospace structure also relatesto the way of welding heat source walking on space structure, so demand of VPPAwelding thermodynamics numerical analysis has many problems and hard task. Aboveall, study is based on the VPPA source and thermodynamics process analysis ofaerospace thin shell structure.In this paper, Keyhole plasma arc welding process is studied morecomprehensively, Keyhole plasma arc welding process is characterized by: plasmagas flow increases, the front fusion width decreases and the back fusion widthincreases; the distance from the nozzle end to the workpiece surface increases, thefront fusion width increases and the back fusion width decreases; tungsten shrinkageincreases, the back fusion width increases. The process is built up by the modeldesigned of this paper, Keyhole Plasma Arc Welding heat source with characteristicsof the actual welding process is established, and the average welding current, weldingspeed, plasma gas flow, nozzle height, and tungsten shrinkage are included in this heatsource model with the indentified function relation.According to the different form of welding on cylinder and different structuretypes, the paper puts forward different modeling methods. For a simple cylindermodel, the" top-down" modeling method should be used and for complex structures,the" bottom-up" modeling method should be used. Sweep meshing is an effectivemesh method which is used to mesh intersection weld in complex structure. By usingthe coordinate transformation, paper solves the surface Gauss heat source model andcylindrical heat source model loading on the cylinder. In the paper, we analyses thewelding deformation in different welding processes.In the paper, hemisphere thermodynamics calculation model is obtained by usingsweep mesh method. After the heat source model is simplified, spherical problem istransformed into a conical surface problem. Through the projection method, Gauss heat source is loaded on the surface, and Through the local coordinate, cylindricalheat source is loaded. Finally the stress distribution and deformation in hemispherestructure is obtained. The result of hemispherical structure deformation prediction hasguiding significance in the actual project, and supported the deformation theory.