Common welding methods for titanium and titanium alloys include argon arc welding, submerged arc welding, and vacuum electron beam welding. For thicknesses below 3mm, tungsten inert gas (TIG) welding is used, while for thicknesses above 3mm, gas tungsten arc welding (GTAW) is employed. The purity of argon gas must not be below a certain level, and the content of air and water vapor in the argon gas must be strictly controlled. Surface treatment, including degreasing, descaling, and removing oxidation films, is performed prior to welding. Due to the high chemical reactivity of titanium and titanium alloys, they are prone to contamination from oxygen, nitrogen, and hydrogen, and therefore, welding methods such as shielded metal arc welding, oxygen-acetylene (or oxygen-propane, etc.) gas welding, CO2 welding, and atomic hydrogen welding cannot be used.
Titanium's Impact
The Effects of Oxygen and Nitrogen Folding
Oxygen and nitrogen interstitially dissolve in titanium, causing lattice distortion, increasing deformation resistance, and enhancing strength and hardness. However, it also reduces ductility and toughness. The presence of焊接 oxygen and nitrogen in weld seams is detrimental and should be avoided.
Impact of Folding Hydrogen
Increased hydrogen content can drastically reduce the impact toughness of the weld metal in titanium, with only a slight decrease in plasticity, and hydrides can cause the joint to become brittle.
The Impact of Folded Carbon
At room temperature, carbon dissolves interstitially in titanium, enhancing strength while reducing ductility, though not as significantly as oxygen or nitrogen. When the carbon content exceeds the solubility limit, hard and brittle TiC forms, distributing in a network pattern, which is prone to cracking. The national standard stipulates that the carbon content in titanium and its alloys should not exceed 0.1%. During welding, the oil污 on the workpiece and welding wire can increase the carbon content, thus necessitating a thorough cleaning during the welding process.
Weldability of Titanium
The Creation of Folding Pores
Common defects in welding titanium and its alloys are gas pores, mainly occurring near the fusion line. Hydrogen is a significant cause of gas pores, as titanium has a strong ability to absorb hydrogen during welding. With the decrease in temperature, the solubility of hydrogen decreases significantly, leading to the hydrogen dissolved in the molten metal often not having enough time to escape and form gas pores.
Foldable Connector脆ification issue
At room temperature, titanium reacts with oxygen to form a dense oxide film, thereby endowing it with high chemical stability and corrosion resistance. During the welding process, the welding temperature can reach as high as 5000 to 10000°C, leading to rapid reactions between titanium and its alloys with oxygen, hydrogen, and nitrogen. Tests have shown that titanium alloys can rapidly absorb hydrogen at temperatures above 300°C, oxygen at temperatures above 450°C, and nitrogen at temperatures above 600°C during welding. When these harmful gases侵入 the molten pool, the plasticity and toughness of the weld joint undergo significant changes, especially above 882°C, where the grain size of the joint becomes severely coarse. During cooling, martensite structures form, resulting in a decrease in the joint's strength, hardness, plasticity, and toughness, with a severe tendency towards overheating and significant brittleness. Therefore, when welding titanium alloys, reliable gas protection should be implemented for the molten pool, droplets, and high-temperature areas, both on the positive and negative sides. This is crucial for ensuring the quality of titanium and its alloys' welding. Delayed cracking occurs in the near-surface area of titanium and its alloys for some time after welding, caused by the diffusion of hydrogen from the high-temperature molten pool to the lower-temperature heat-affected zone. As the hydrogen content increases, the amount of titanium hydrides precipitated also increases, leading to greater brittleness in the heat-affected zone. Additionally, the volume expansion of the precipitated hydrides generates structural stresses, which result in the formation of cracks.
