| With the development of huge aluminum alloy welding structures, welding in place technology has been paid a lot of attention on. One of the most typical applicaitons is the welding of propellant tanks in heavy lift vehicles. However, aluminum alloy horizontal welding restricts the progress of this technology. Horizontal welding is difficult to execute due to the bad welding quality and the low welding efficiency which are caused by the gravity and the aluminum alloy physical properties. In order to solve the problems, keyhole mode ordinary variable polarity plasma arc(‘ordinary plasma arc’ for short) horizontal welding of aluminum alloy was carried out. Based on the conditions for a stable keyhole, soft variable polarity plasma arc(‘soft plasma arc’ for short) was proposed from changing arc characteristics perspective. Soft plasma arc promoted the establishment of a stable keyhole by the change of arc energy distribution and arc pressure distribution. 2A14 aluminum alloy plates with thickness 8mm were welded successfully by soft plasma arc and the weld formation was good with high mechanical properties.First, ordinary plasma arc horizontal welding experiments for aluminum alloy plates with thickness 6mm and 8mm were carried out to make clear the problems. The problems in horizontal welding were the serious undercut and porosity defects when the plate thickness was 6mm. The bottleneck in horizontal welding was the collapse of keyhole in molten pool when the plate thickness was 8mm. The reasons causing the above problems were investigated. The undercut defect in weld top side was caused by higher heat input or greater arc pressure. The undercut defect in weld bottom side was formed by lower heat input or greater arc pressure. The observation of dynamic behavior of molten pool indicated that the establishment process of keyhole in molten pool was determined by the bridging of molten metal in the upper side and the lower side, the filling of molten metal in molten pool top side could prevent from keyhole collapsing and the fluid flow around keyhole was an important mark of the stable molten pool. Based on the above analysis, some preliminary control methods were proposed. Undercut defect was eliminated by the decrease of heat input and preheating method when plates thickness was 6mm. The establishment of keyhole in molten pool was improved by groove geometry design when plates thickness was 8mm, but the keyhole stability was not good. The problems in keyhole mode horizontal welding were not solved completely.Based on the conditions for a stable keyhole in molten pool, a research thought that the keyhole stability could be controlled by change of arc characteristics was proposed. Using the water cooled nozzle with three orifices, soft plasma arc was obtained. Its energy density distribution and pressure distribution were changed. With soft plasma arc, the weld formation and keyhole stability were improved highly. Good results were attributed to the special characteristics of soft plasma arc. Comparing with ordinary plasma arc, the shape of soft plasma arc was expansion and the compression degree decreased. Its static characteristic curves moved down and its impedance characteristic decreased. Arc energy density distribution was tested by split anode method and arc pressure distribution was obtained by a pressure sensor imbeded in a water cooled block. The arc energy concentration degree of soft plasma arc was higher than that of ordinary plasma arc due to the use of three orifices mode nozzle and great plasma gas flow rate. The molten pool shape was changed into elliptical shape due to the change of arc energy density distribution on plates top side. The arc pressure amplitude and its gradient both decreased greatly. From the analysis of forces acting on molten pool and the fluid flow, the mechanism of soft palsma arc in horizontal welding was summarized in two aspects. One was the decrease of arc pressure made the maximum permitted kyhole diameter increase and promoted the keyhole closure process. The other was the thermal characteristics of soft plasma arc improved the flow ability of moltem metal, the bridging and filling of molten metal in molten pool could be accomplished easily and then the stability of keyhole mode welding was improved.Numerical simulation of keyhole mode welding process was carried out by computational fluid dynamics software Fluent to investigate the influences of arc type and arc pressure on molten pool. In order to describe the characteristics of soft palsma arc accurately, double ellipitic surface and cone hybrid heat source model and force source model were established. By the UDF(User defined function) re-development of the software and the VOF(Volume of fluid) free surface tracking mehtod, three dimensional instantaneous simulation for keyhole mode plasma arc welding achieved and the reliability of model was verified by experiments. In the forming process of molten pool, the perforation rate of soft plasma arc was slow, heat conduction effect was apparent in plate width direction, the temperature in molten pool was high and the temperature field was asymmetric. With increase of arc pressure, the keyhole diameter increased, the molten metal film around keyhole became thinner and the flow velocity to molten pool top side decreased. In closure process of molten pool, the bridging and filling of molten metal in molten pool with soft plasma arc could be carried out rapidly and the molten pool solidification rate was slow. Based on the simulation resluts, the criteria for keyhole closure was given. One condition was that the surface tension additional force should be bigger than the resultant force of the gravity and arc force. The other condition was that the time used for the molten metal bridging and filling in molten pool should be smaller than the time used in molten pool solidification. The keyhole closure conditions revealed the balance of heat and force of molten pool in horizontal position.At last, keyhole mode soft plasma arc horizontal welding technology for aluminum alloy and the microstructures and mechanical properties of the weld were investigated to test its reliability compared with that of the ordinary plasma arc welding in vertical-up position. Welding parameters window for soft palasma arc horizontal welding was wide and its stability in welding process was high. Porosity defect could be eliminated by using higher heat input, scraping the oxide film and preheating. The tensile strength and elongation at fracture of the horizontal weld by soft palsma arc were higher than that of the vertical-up weld by ordinary plasma arc. The fracture mode of the weld was plastic fracture. The micro-pores and divorced Al-Cu eutectic were the main crack sources. In the upper side heat affected zone(HAZ) of the horizontal joint, the grain size was smaller and the hardness value was higher compared with that in the lower side HAZ. This revealed the asymmetry of microstructures and mechanical properties caused by asymmetric thermal field distributions. The asymmetric fluid flow around the keyhole caused by the gravity in molten pool was considered as the fundamental reason. |
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