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Welding of industrial pure titanium TA2

Preface
Industrial pure titanium TA2, plate standard number: GB/T3621-2007 (titanium and titanium alloy plates), is suitable for parts working below 350 ℃. Although it has low strength, it has good plasticity and toughness, excellent corrosion resistance, especially low temperature impact performance, and good weldability. As an emerging and promising non-ferrous metal material, it is gradually being widely used in the petrochemical industry.
2 Basic characteristics and welding characteristics of TA2
2.1 Physical properties
Pure titanium TA2 has a lower tensile strength value in the annealed state, and its physical properties are characterized by high melting point, poor thermal conductivity, lower specific heat capacity, and lower density than iron. In addition, it also has the characteristics of high electrical resistivity and low coefficient of linear expansion.
High melting point and poor thermal conductivity make it easy to produce coarse grains in the weld and near weld areas, leading to a decrease in weld plasticity and toughness. Therefore, strict control of welding heat input should be implemented during welding.
2.2 Chemical properties
Titanium material has active chemical properties and is easily contaminated by impurities such as gases, which can cause embrittlement in welded joints. As the temperature increases, titanium's ability to absorb oxygen, nitrogen, and hydrogen gradually increases. Heating at 250 ℃ begins to absorb hydrogen, 400 ℃ begins to absorb oxygen, and 600 ℃ begins to absorb nitrogen. For example, pure titanium may contain excessive gas elements in the weld due to improper protection during the welding process. On the one hand, defects such as pores are easily formed in the weld, and on the other hand, it will inevitably cause a significant increase in weld strength, resulting in a serious decrease in plasticity and embrittlement of the weld. Therefore, oxygen, nitrogen, and hydrogen are harmful elements of titanium. Therefore, during welding, the weld pool, welds with temperatures exceeding 400 ℃, and the heat affected zone (including the back of the weld pool) must be protected. In addition, carbon elements also have a significant impact on the properties of the base material and weld seam. Increasing the carbon content leads to a significant decrease in plasticity. According to China's technical conditions, the carbon content of industrial pure titanium and titanium alloy base materials should not exceed 0.1%, and the carbon content of welds should not exceed that of the base material.
Welding process of 3 TA2
3.1 Welding methods
As analyzed earlier, titanium materials have very active properties and ordinary welding methods such as manual arc welding are not suitable for titanium welding. The most commonly used method should be tungsten inert gas arc welding.
3.2 Preparation before welding
Before welding, the surface near the workpiece joint, especially the mating end face, must be carefully mechanically cleaned, and then wiped with acetone or alcohol. Similarly, the welding wire must also be degreased with acetone before use.
3.3 Welding materials
Select welding wires that are of the same quality as the base material based on its chemical composition, and the argon purity should not be lower than 99.99%.
3.4 Key points of welding process
The welding heat input should not be too large to prevent the performance of the weld from deteriorating due to the coarse grain size of the weld. During the welding process, protective measures should be strengthened. For the rear weld of the molten pool and the heat affected zone above 400 ℃, argon gas protection should be applied to the trailing cover, and the same measures should be taken for the back of the weld. The interlayer temperature should be as low as possible. It is best to wait for the previous layer of weld to cool to below 70 ℃ before welding the next layer of weld to prevent overheating. The swing amplitude of the welding torch should be as small as possible to prevent the molten pool from detaching from the argon protection. During the welding process, pay attention to the color changes of the welded joints, and generally ensure that the welded joints turn silver white after welding.
4 Welding Procedure Qualification Test
4.1 Preparation before testing
According to the material procurement situation and welding process qualification test requirements, 8mm TA2 plate is used for testing, and welding wire ERTA2ELI of the same quality as the base material is used as the filling material Ф The 3mm welding wire meets the requirements of Part 7 of NB/T47018.7-2011 "Technical Conditions for Ordering Welding Materials for Pressure Equipment", and the size and type of the groove for the test plate processing are shown in the attached figure.
4.2 Welding process parameters and requirements
Before welding, clean the oil and other dirt within 50mm range on both sides of the groove with acetone to ensure assembly clearance and misalignment. Clean the welding wire with acetone to remove grease. During welding, the nozzle gas flow rate is 15-20L/min, and the argon gas flow rate protected by the back drag cover is 20-25L/min. After welding, the drag cover protection flow rate is 20-25L/min. During the welding process, control the interlayer temperature to be ≤ 70 ℃. The welding process parameters are shown in the attached table.
4.3 Non destructive testing
100% RT after welding, in accordance with JB/T4730.2-2005, Class I qualified.
4.4 Physical and chemical tests
The welded joint has undergone 2 tensile tests and 4 bending tests, all of which meet the requirements of relevant standards.
5 Conclusion
TA2 sheet metal has good weldability. By using tungsten inert gas arc welding and selecting appropriate specifications, satisfactory welding joint quality can be achieved under reliable protection conditions.

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