Titanium alloy pipeline welding is usually on-site full position welding, its difficulty is much higher than stainless steel pipe welding, titanium and alloy weld zone above 400℃ must be protected, because of its chemical properties are particularly active, easy to absorb harmful gases, resulting in material performance decline, even cracks. So pay special attention to the following matters when welding:
When welding titanium and its alloy pipes, the welding material selection is basically equal to the principle of strength, can not find suitable welding material, can be used to cut from the base metal method to solve; the titanium tube manufacturer believes that the selection of argon will directly affect the hardness and toughness of the weld, and also have an obvious impact on the generation of welding cracks, so it is required to select (especially pure titanium) high purity argon (99.99%).
Titanium tube cutting and fracture production should be cold processing method, do not make it overheated and discoloration. Polish the metal luster with grinding wheel or stainless steel wire wheel within the range of not less than 20mm on the inner and outer surfaces of the groove. Polish the burr of the groove with a scraper before polishing it smooth, avoid hanging up cloth wire during cleaning.
Cleaning quality directly affects the generation of welding cracks and pores. Usually, pickling (3%HF+35%HNO3+H2O) is used first, and then washed with clean water, and finally dried, and wiped with acetone or alcohol before welding. The welding joint must be finished immediately after cleaning, not more than 4h, otherwise it needs to be cleaned again. Do not use rubber gloves when cleaning, use clean white cotton gloves.
Titanium alloy pipe welding area protection is very important, similar to stainless steel pipe welding, but the gas protection requirements are more strict, the tube should be filled with argon until the end of the welding, until the welding area temperature drops below 350℃ can be stopped; the argon gas protection outside the front of the tube adopts a large diameter nozzle and a lengthening trailing cover. In order to improve the protection effect, the trailing cover does not end when the welding is finished. Only when the temperature drops below 200℃ can the gas supply be stopped and the trailing cover be taken off.
Welding parameters of titanium tubes are very important. The welding process parameters of common titanium alloy tubes are shown in Figure 1.
The operation technique of titanium alloy tube TIG welding is basically similar to that of stainless steel. In the case of ensuring complete penetration, the smaller welding line energy is used as far as possible, that is, the rapid welding of straight line gun and filling welding wire. Welding operations mainly pay attention to the following points:
Before welding, it is replaced for a period of time. The open flame detection method can be used to check the welding joint and confirm that the gas in the pipe is completely argon before welding. Adopt short arc welding, do not swing the welding gun and wire. When covering the surface, it is to swing, but also only a slightly slow swing. The end of the wire should not be removed from the protection range in the welding process, otherwise the end of the wire will be oxidized. If it is found that the end of the wire has been oxidized, the oxidized part must be cut off before use. After stopping welding, gas should be delayed until the temperature of the welding zone drops below 300℃. When welding, the arc-starting and arc-closing points must be carried out in the break and not on the base metal.
When the wire is sent, the welding wire can not be quickly sent to the filling wire, and the welding wire should be sent slowly and evenly to the melting pool, otherwise it will cause gas disorder and affect the protection effect. During welding, the distance between the nozzle and the nozzle mouth should be as low as possible without affecting the line of sight. The angle should be about 75°, but the tungsten pole should not collide with the base metal and cause tungsten clamping; the angle between the welding wire and the pipe mouth is about 12°. In the welding process, the inter-layer temperature must be strictly controlled. In case the layer should wait for the temperature to drop to room temperature before continuing to weld the next layer. There should be a gap of 0.5 ~ 1mm between the rubber sealing plate and the inner diameter of the tube. During welding, it can replace the air in the tube before welding, release excess argon and cool down during welding. When welding, the weld surface color should be observed at all times, with silver as the best, followed by golden, purple, blue, gray, gray.
The welding environment is very important. The welding site must be kept dry and clean. The ambient temperature during welding should be controlled above 18°. There is no draft in the welding area. Water, water vapor or other gases are absolutely prohibited from infiltrating into the welding pool and heat-affected zone. Otherwise, shielding measures must be taken to avoid contamination of the weld and affecting the welding quality. In short, to weld titanium and its alloy pipes well, from the selection of welding materials, groove grinding, cleaning and drying, environmental measures, gas protection, process parameters, welding operation and other links are very important, seriously treated, so as to weld qualified welded joints.