Titanium alloy materials are more and more widely used in aviation, aerospace, petroleum, chemical and shipbuilding industries due to their excellent mechanical properties, corrosion resistance and low density. Tungsten arc welding is one of the most commonly used welding methods for titanium alloy welding structures that are widely used in the above industries. This method has the characteristics of large process margin, strong process adaptability, and excellent weld quality, but it also has low arc energy density, poor penetration ability, large heat input during welding, and large thermal damage to the material. Welding stress deformation is relatively large and other deficiencies; especially when welding titanium alloys, defects such as porosity are easily generated, which directly affects the performance of the welded component.
At present, the development of new aircraft requires higher and higher titanium alloy welded structural parts, and there is an urgent need to develop new, high-quality, and efficient welding methods to meet the high efficiency, high performance, and high reliability structural design of advanced aircraft engines and aircraft. The requirements of advanced manufacturing technology for long life and low cost. Activated flux tungsten argon arc welding (A-TIG) technology is developed to meet this requirement. This technology can not only solve the technical deficiencies of the above conventional TIG welding, but also improve the welding quality and service life of components under the same process conditions [1-3], and open up new application prospects for tungsten arc welding technology.
Titanium alloy A-TIG welding technology and characteristics
A-TIG welding technology is a process method of coating a layer of active flux on the surface of the workpiece to be welded before welding, and then performing TIG welding along the flux layer. Compared with the conventional TIG welding process, the penetration ability of the titanium alloy A-TIG welding arc is significantly enhanced, and the heat input, welding deformation and stress are reduced. When welding product components of the same specification, under the same welding current conditions, single-pass welding without beveling or the number of surfacing layers can be significantly reduced, thereby improving welding productivity and product quality, and reducing costs exponentially.
In addition, the active flux can greatly reduce the weld porosity defects generated in the argon arc welding process, thereby directly improving the fatigue performance of the welded joint and welded structure. Tests show that the fatigue limit of TC4 titanium alloy A-TIG welded butt joints is 16% higher than that of conventional TIG welding, and can reach 90% of the base material. At present, the technology of titanium alloy active flux argon arc welding has developed into a new advanced connection manufacturing technology to ensure that weapons and equipment improve quality, improve processing efficiency and reduce costs.
Basic principle of titanium alloy A-TIG welding technology
The existence of the thin film limits the arc’s conduction cross-section, which causes the arc to shrink; secondly, because the surface of the titanium alloy material is covered with an active flux layer before welding, in the arc conduction process, only the arc heat first melts the active flux and the titanium metal, and Only by successfully squeezing away the flux film from the liquid titanium can the arc be successfully conducted and burned stably. Due to the better wettability between the molten active flux and the liquid titanium, the flux film is not easily squeezed away. The less it is squeezed away, the narrower the weld, the more concentrated the heat flux of the arc, and the deeper the penetration depth; third, during A-TIG welding, the active flux molecular vapor enters the arc atmosphere, increasing The thermal conductivity of the plasma in the arc column causes the arc to shrink; fourth, the arc heat decomposes and ionizes the active flux and enters the peripheral space of the arc. The flux ions capture the electrons in the arc and form negative ions, which reduces the voltage in the peripheral space of the arc column, thereby causing The arc shrinks. It is precisely due to the synergy of the above aspects that the welding arc shrinks significantly during the A-TIG welding process, the arc column current density increases, and the welding penetration increases.
Current status of foreign technology development
Active flux was first developed by the Ukrainian Barton Welding Institute in the 1960s. The initial development goal was to improve the porosity in the weld during titanium alloy TIG welding by adding halides in the weld zone. The test results show that the added halide suppresses the pores of the titanium alloy weld, and also affects the formation of the weld: under other conditions, the weld penetration (h) increases and the weld width (b) decreases. The shape factor of the weld (ψ=b/h) also decreases accordingly. In addition, heat input (q/V) during welding is reduced accordingly. In view of the series of positive effects brought by the addition of halides, Barton Institute developed the first multi-component active flux product in 1964, AHT-9A, for welding titanium alloys. At present, its A-TIG welding process has been confirmed by tests and is used in Russian aviation, aerospace, chemical industry, pressure vessels, power equipment, nuclear power facilities and other fields. The United States is relatively behind that of Ukraine in the research of active flux for argon arc welding. However, currently the United States has used the developed active flux for argon arc welding of stainless steel and carbon steel to build catamaran hulls, tankers, nuclear reaction vessels, pressure vessels, etc.; the Navy is using this flux to weld piping systems for ships and submarines And certain parts.