Nowadays, the running speed of China's high-speed railway has reached 350 kilometers per hour, which is in the lead in the world. In order to keep China's high-speed railway leading, we must constantly improve the bogie manufacturing. The bogie is the part connecting the wheel and the vehicle body, which is known as the two legs of the high-speed railway. The bogie and its components are related to the speed and safety of the high-speed railway. Therefore, a new type of titanium alloy bogie with lightweight and high strength should be developed.
At room temperature, titanium reacts with oxygen to form a dense oxide film, which makes it have high chemical stability and corrosion resistance. In the welding process, the welding temperature is as high as 5000 ~ 10000 ℃, and titanium and its alloys react rapidly with oxygen, hydrogen, and nitrogen.
According to the test, titanium alloy can rapidly absorb hydrogen when the temperature is above 300 ℃, oxygen when the temperature is above 450 ℃, and nitrogen when the temperature is above 600 ℃. When these harmful gases are intruded into the molten pool, the plasticity and toughness of the welded joint will change significantly, especially above 882 ℃, the grain of the joint will be coarsened seriously, and martensite structure will be formed during cooling, which will reduce the strength, hardness, plasticity, and toughness of the joint, serious overheating inclination and serious embrittlement of the joint. Therefore, when welding titanium alloy, comprehensive and reliable gas protection should be carried out for the molten pool, droplets, and high-temperature area, both front, and back. This is the key to ensuring the welding quality of titanium and its alloys. Delayed crack generation in a period of time after welding, cracks are easy to occur in the near seam area of titanium and its alloys, which is caused by the diffusion of hydrogen from a high-temperature molten pool to a low-temperature heat-affected zone. With the increase of hydrogen content, the precipitated titanium hydrogen compounds increase, the brittleness of heat affected zone increases, and the microstructure stress produced by the volume expansion of precipitated hydride leads to the generation of cracks
Weld surface cleaning, mechanical cleaning, butt welding according to the weldment with low-quality requirements or difficult pickling, it can be wiped with fine sandpaper or stainless steel wire brush, but it is best to scrape the titanium plate with cemented carbide yellow to remove the oxide film. The selection of welding equipment for TIG welding of titanium and titanium alloys shall be a DC argon arc welding power source with falling external characteristics and high-frequency arc striking, and the delayed gas delivery time shall not be less than 15 seconds to avoid oxidation and pollution. In order to improve the welding efficiency, semi-automatic argon arc welding can be used for large surface welds, mainly instead of manual wire feeding. For the selection of welding materials, the purity of argon shall not be less than 99.99%. When the pressure in the argon cylinder drops to 0.981mpa, it shall be stopped to prevent affecting the quality of welded joints. In principle, titanium wire with the same composition as the base metal shall be selected. Sometimes, in order to improve the plasticity of weld metal, welding wire with slightly lower strength than the base metal can also be selected.
