| Aluminum alloy which has light weight, high strength, high corrosion resistance andother characteristics is widely used in aviation, shipbuilding, aerospace and otherindustries. Heavy launch vehicle need to develop large propulsion systems, causing therocket storage bins with the characteristics of huge diameters and lengths. It may formassembly at vertical position. Horizontal welding of medium thickness aluminum alloycomes into sight because of assembly at vertical position welding. Variable polarityplasma welding which can provide high energy density and arc power can achieveone-side welding with back forming. It has became the preferred welding method foraluminum alloy. However, the literatures about horizontal position welding by VPPA arefew. There are many question include the characteristics of welding parameters and thestability of welding pool in horizontal position welding. The welding process and thewelding defects in horizontal position welding of aluminum alloy by VPPA were studiedin this paper. Moreover the behavior of keyhole pool was analyzed.Forming properties aluminum alloy with different thicknesses were mainlydiscussed. Aluminum alloy with6mm thickness in horizontal welding have betterforming properties and stability, but there are serious welding defects on the front andback side. The more input energy is beneficial for suppressing the welding defects inbottom side. In contrast, the less input energy is beneficial for reduction of the weldingdefects in top side. Through adjusting the welding speed, welding current, plasma gasflow rate, aluminum alloys with8mm thickness is difficult to forming. Therefore, itcannot obtain welded seam only by parameter adjustment.In order to reveal the reason for the difficulty of forming, the characteristics ofmolten pool of horizontal welding was studied by numerical simulations. With numericalsimulation of internal metal flow in molten pool by the FLUENT software, metal flow issymmetrical on both sides of the pool in vertical welding, while metal flow on the upperand lower side of the pool are different under the influence of gravity in horizontalwelding in which more metal flows to the bottom. With increase of welding input heat,the pool size increases, the asymmetry of flow is more apparent. Therefore, the volume ofwelding pool can be reduced by decreasing the welding input energy so that weldingpool was lightly affected by gravity.Through traditional control measures of horizontal welding to control the forming ofVPPAW at horizontal position, the effect is not obvious. Although some of the measuressolve partial problems, but the weld quality is poor. But Plasma welding in keyhole modeis different from ordinary horizontal welding. Reducing the amount of input heat cannot solve partial problems. The force of welding pool in vertical and horizontal position wasanalyzed. Gravity procures molten metal flow downward in vertical welding, consistentwith liquid flow form. In the horizontal welding, surface tension mainly overcomesgravity preventing the metal to separate from the seam. Different state makes VPPAW athorizontal position forming difficultly. Through force analysis by welding pool, it showsthat keyhole with small size and flat sidewall makes the stable pool easily to keep.Therefore, change of arc pressure obtains good quality welding welds. The maximumthickness of aluminum alloy in horizontal welding achieves the breakthrough.Microstructure of weld obtained in horizontal and vertical position is similar, butthere are differences between the upper and lower horizontal seam. Tensile strength ofgood welding forming in vertical and horizontal position is basically the same. Therefore,aluminum alloy with8mm thickness can successfully achieve joints without defects inhorizontal welding by VPPA, its mechanical properties is same as vertical welding.. |
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