The most professional advice on titanium welding

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//Titanium titanium alloy has low density, high specific strength, good corrosion resistance, low thermal conductivity, non-toxic, non-magnetic, and weldable； Widely used in aviation, aerospace, chemical industry, petroleum, electric power, medical care, construction, sporting goods and other fields. 1. Welding characteristics of titanium and titanium alloys (1) Embrittlement caused by impurity contamination. Due to the high chemical activity of titanium, under the action of welding heat cycle, the welding pool, weld metal and heat-affected zone above 350°C can easily react with hydrogen, oxygen, nitrogen in the air, as well as oil stains, moisture, etc. on the weldment and welding wire. Titanium quickly absorbs hydrogen above 300°C, oxygen quickly above 600°C, and nitrogen quickly above 700°C. When the carbon content is high, a network-like TiC brittle phase will appear. The above conditions drastically reduce the plasticity and toughness of titanium and titanium alloy welded joints, causing the performance of the welded joints to deteriorate. The color of the oxide film formed on the titanium surface is related to the production temperature. It is silvery white below 200℃, light yellow at 300℃, golden yellow at 400℃, blue and purple at 500℃ and 600℃, and different shades of gray at 700~900℃. The temperature of the unprotected area during the welding process can be judged based on the color of the oxide film formed on the surface. (2) Performance deterioration caused by welding phase change has two allotropic crystal structures. Above 882℃ to the melting point, it is a body-centered cubic lattice, called β titanium, and below 882℃, it is a close-packed hexagonal lattice, called α. Titanium for containers contains very few β-stabilizing elements and is all α-iron alloy. When these titaniums are welded at high temperatures, the welds and part of the heat-affected zone have a β lattice, and the grains tend to grow rapidly. Titanium also has the characteristics of high melting point, large specific heat capacity, and low thermal conductivity. Therefore, the high-temperature residence time during welding is about 3 to 4 times longer than that of steel. The high-temperature heat-affected zone is wider, which causes the β-grains in the weld and high-temperature heat-affected zone to grow significantly, which will greatly reduce the plasticity of the welded joint. Therefore, when welding titanium, smaller welding heat input and faster cooling rate should usually be used to reduce the high-temperature residence time, reduce the degree of grain growth, shrink the high-temperature heat-affected zone, and reduce the impact of plasticity decline. (3) The welding area needs to be protected by inert gas. At high temperatures, the affinity with oxygen in the air is very strong. In areas above 200°C, inert gas protection must be used to avoid oxidation. (4) Large welding deformation The elastic modulus of titanium is only half that of carbon steel. Under the same welding stress, the welding deformation of titanium will be twice as large as that of carbon steel. Therefore, when welding titanium, backing plates and pressure plates are generally used to compress the workpiece to reduce welding deformation. (5) Porosity is easy to occur. Porosity is a common defect in welds. The pores produced during titanium welding are mainly hydrogen pores, and there are also pores formed by CO gas. (6) Possibility of cracks. The content of impurities such as sulfur, phosphorus, and carbon in titanium that can form a low-melting-point eutectic with titanium at the grain boundary is very small. The effective crystallization temperature range is narrow, and the shrinkage of the weld is small when it solidifies. Therefore, hot cracks in the weld generally do not occur. Cracks in titanium welds are hydrogen refrigeration cracks. (7) Inability to weld with steel. The mass fraction of iron dissolved in titanium at room temperature is only 0.05%~0.10%, so titanium and steel cannot be directly fusion welded. 2. Welding methods of titanium and titanium alloys The main welding methods used when welding titanium and titanium alloys include tungsten arc welding, melting arc welding, plasma arc welding, etc. Brazing can be used to weld sealed structures that do not bear load, and explosion welding can also be used for composite welding of titanium and steel composite plates. 3. Welding materials of titanium and alloys (1) Welding wires Titanium and titanium alloy welding wires are generally selected according to the welding wires corresponding to the base metal, but they should also be qualified by the welding process. When selecting welding wire, there is a problem of matching the appropriate welding wire, because the impurity content of the welding wire is only controlled at the upper limit, and in most cases the lower limit is not controlled. The welding wire produced in each batch only guarantees the chemical composition, and does not guarantee the mechanical properties of the welding wire after welding. There is a possibility that the impurity content in some production batches of welding wire is particularly low and it is a qualified product, but the strength of the weld is low and may not meet the requirement of not being lower than the lower limit of the standard tensile strength of the base metal in the annealed state. At this time, you should change to a welding wire of the same brand in production batches, or even a welding wire with a higher strength (referring to industrial purity), and re-qualify the process until it is qualified, and then you can select the welding wire. The chemical composition (smelting analysis) of titanium and titanium alloy welding wires and filler wires for containers is shown in Table 4-29. For example, when samples are taken from finished welding wires and filler wires for chemical composition retest, the allowable deviations of the analysis are shown in Table 4-30. The recommended models of welding wire and filler wire for commonly used titanium materials in containers are shown in Table 4-31. Picture picture (2) Protective gas Argon is generally used as the protective gas when welding iron and titanium alloys. The purity of argon (volume fraction should not be less than 99.99%), and the volume fractions of other gas components are oxygen less than 0.002%, nitrogen less than 0.005%, hydrogen less than 0.002%, and moisture less than 0.001mg/L. The pressure of the gas cylinder shall not be lower than 0.5MPa. During use, the air in the protective gas system such as gas hoses, welding torches, and drag hoods must be replaced. The protective gas can also be helium or an argon-helium mixture. (3) Tungsten electrodes are commonly used pure tungsten electrodes and cerium tungsten electrodes. Electrodes containing cerium oxide (impurity mass fraction not greater than 0.1%) in pure tungsten are cerium tungsten electrodes. Cerium tungsten electrode has low electron work function, high chemical stability, high allowable current density, no radioactivity, and has better performance than pure tungsten electrode. It is currently the most commonly used tungsten electrode. 4. Preparation before welding (1) Clean titanium and alloy weldments and welding wires before welding. Before welding, the oxides, nitrides, oil stains, moisture, etc. on the surface should be carefully removed. Generally, pickling or polishing with grinding wheels and emery cloths is used. For workpieces that are difficult to pickle, such as the longitudinal circumferential welds of containers, fillet welds, and the welding of tubes and plates of heat exchangers, grind both sides of the groove with a grinding wheel and emery cloth, and pay attention to cleaning up the remaining sand and dust. Welding wires, heads, expansion joints and other parts that are difficult to polish should be pickled before welding and rinsed with clean water after pickling. If the weldment cannot be pickled, it can also be scraped with a carbide scraper. After the weldment has been cleaned as above, the area to be welded should be cleaned with acetone, absolute alcohol and other solvents before welding. It is not allowed to touch or re-contaminate with hands. After re-contamination, it should be cleaned and cleaned again. (2) Preparation of other protective devices in the welding area When welding titanium and titanium alloys, the welding gun nozzle protects the molten pool, the drag cover protects the front of the cooling welding joint, and the backing plate protects the back of the welding joint. The welding guns used for welding titanium and titanium alloys are different from those used for welding aluminum and stainless steel. Large-diameter nozzles are commonly used. The nozzle diameter is 14 ~ 20mm for manual welding and 16 ~ 22mm for automatic welding. The drag cover can protect welds and heat-affected zones with temperatures above 400°C. The shape and size of the drag cover should be determined by factors such as the thickness of the weldment, cooling method, welding current, and weld shape. The drag cover should be attached to the welding area and move with the welding gun. A copper backing plate can be used on the back of the weld to accelerate the cooling of the welding area and isolate the air. Protective gas can also be blown into the copper backing plate, or a drag cover can be attached to the back of the weld area to move along with the welding.