Titanium Series (IV): Characteristics of Titanium Materials and Welding Properties

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1. Classification of titanium and titanium alloys: Industrial pure titanium: TA1, TA2, TA3. Titanium alloys: α-titanium alloys, β-titanium alloys, α+β titanium alloys. 2. Characteristics of titanium materials: They exhibit excellent performance. The main advantages of titanium materials are their low weight (density of 4.51 g/cm3), high strength, good corrosion resistance, and satisfactory performance at low temperatures. They have excellent corrosion resistance in seawater as well as in most acids, bases, and salts. In urea production, their corrosion resistance is 10 times greater than that of ultra-low carbon austenitic stainless steels. There is an example in salt production where preheaters made of red copper tubes suffered severe corrosion after just a few months of use, and had to be replaced after 10 months. Chloride ions have no corrosive effect on titanium equipment; however, carbon steel pipes used for salt brine conveyance get corroded after just half a month, developing a honeycomb pattern, and these pipes need to be replaced 6 times per year ； Using stainless steel, perforation occurs after 4 months. Replaced with titanium tubes; after 4 years, no corrosion or perforation has occurred. Titanium equipment requires a high initial investment, but its life-cycle costs are relatively low, resulting in significant economic benefits. 3. Welding characteristics of titanium products: Welding titanium equipment is highly prone to oxidation, nitridation, and embrittlement. ① Hydrogen absorption begins in large amounts at 400°C. Hydrogen is one of the most harmful elements for titanium, as it reduces its ductility and toughness and leads to brittle fracture. During cooling, hydrogen does not have time to escape, resulting in pores; therefore, it is generally required that its content in titanium materials be less than 0.01%–0.15%. If the base material contains a high amount of hydrogen, dehydrogenation treatment should be carried out beforehand. ② Above 600°C, titanium reacts rapidly with oxygen and nitrogen to form TiO2 and TiN (which have extremely high hardness), which reduces the plasticity and toughness of the welded joint and can cause defects such as pores and cracks. ③ When heated above 800°C, TiO2 dissolves in titanium and diffuses deep into the internal structure of the metallic titanium, forming an intermediate brittle layer with a thickness of 0.01–0.08 mm. The higher the temperature and the longer the time, the more severe the oxidation and nitridation become, and the plasticity of the welded joint drops sharply. The oxygen content in titanium is required to be less than 0.1%–0.15%; moreover, titanium readily forms brittle carbides with carbon, which reduces its plasticity and weldability. ④ It has a high melting point of 1608–1725°C, a large heat capacity, and poor thermal conductivity; as a result, the welded joints tend to overheat. The grain structure is coarse, and this is especially true for β-titanium alloys, where the plasticity of the welded joints decreases significantly. In the case of workpieces with high structural stiffness, welding stresses can also cause cracks to form. ⑤ Under the influence of hydrogen and residual stress, titanium may develop cold cracks; therefore, the hydrogen content in the welded joints must be strictly controlled. Once titanium is contaminated with iron ions, it becomes brittle, which is one of the main reasons for cracks to form in titanium materials. Titanium materials suffer significant welding deformation, making correction difficult. 4. Process measures to improve welding quality: ① Use manual tungsten inert gas welding (TIG welding) or plasma welding; gas welding, CO2 gas shielded welding, and conventional arc welding are not suitable. Practice has shown that tungsten inert gas welding is one of the best methods for welding titanium. When using TIG welding, direct current with positive polarity is employed, and the back side of the workpiece is protected by argon gas, which yields better results. ② Before welding, the oxide scale, oil stains, and gas-rich metal layers on the surface of the workpiece must be thoroughly removed; mechanically cleaning is generally used (it is the simplest method). First, use acetone to remove oil stains, then use fine sandpaper to eliminate oxidation scale, and wipe it with acetone once more. It is forbidden to use grinding wheels designed for carbon steel to grind titanium materials. Practice has shown that once titanium is contaminated, it affects the welding quality, and overheating in color is not allowed during grinding. ③ Pickling solution formula and process. A. In each liter of the pickling solution, there are 55–60 mL of nitric acid, 5 mL of hydrofluoric acid, 350 mL of hydrochloric acid, with the remainder being water ； Soak in acid at room temperature for 15–20 minutes. B. Hydrofluoric acid with a mass fraction of 20%, sulfuric acid solution with a mass fraction of 30%; the solution temperature is 25–30°C, and the acid cleaning process lasts for 5–10 minutes. After pickling, rinse thoroughly with hot and cold water, wipe with a white cloth, and let it dry; welding must be completed within 4 hours. The environment is very important; taking effective measures to prevent iron contamination, dust, and moisture can yield good results. ④ Strictly control the carbon content, as well as the levels of hydrogen, oxygen, and nitrogen, in the chemical composition of the base material; C