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 above 3mm, gas tungsten arc welding (GTAW) is employed. The purity of argon gas must not be below a certain level, with strict control over the content of air and water vapor in the argon. Surface treatments such as degreasing, descaling, and removing oxidation films are conducted prior to welding. Due to the high chemical reactivity of titanium and titanium alloys, they are susceptible to contamination by oxygen, nitrogen, and hydrogen, so welding methods such as stick electrode arc welding, oxygen-acetylene (or oxygen-propane, etc.) gas welding, CO2 welding, and atomic hydrogen welding are not suitable.
The Impact of Titanium
The Effects of Oxygen and Nitrogen Collapse
Oxygen and nitrogen interstitially dissolve in titanium, causing lattice distortion, increasing deformation resistance, and enhancing strength and hardness. However, this results in a decrease in ductility and toughness. The presence of oxygen and nitrogen in weld seams is detrimental and should be avoided.
The Impact of Folding Hydrogen
The addition of hydrogen significantly reduces the impact toughness of titanium weld metal, with only a slight decrease in plasticity, while hydrides can cause the joint to become brittle.
The Impact of Folding Carbon
At room temperature, carbon dissolves interstitially in titanium, increasing strength and decreasing ductility, though not as significantly as oxygen or nitrogen. When the carbon content exceeds the solubility, hard and brittle TiC is formed, distributing in a reticulate pattern, which is prone to cracking. The national standard stipulates that the carbon content in titanium and its alloys must not exceed 0.1%. During welding, the oil contamination on the workpiece and welding wire can increase the carbon content, hence it is necessary to clean them thoroughly during the welding process.
Weldability of Titanium
The creation of collapsible pores
Common defects in welding titanium and titanium alloys are porosity, mainly occurring near the fusion line. Hydrogen is a significant cause of porosity formation, as titanium has a strong ability to absorb hydrogen during welding. However, as the temperature drops, the solubility of hydrogen decreases significantly, often resulting in insufficient time for dissolved hydrogen in the molten metal to escape and form pores.
Foldable Connector Brittle Issue
At room temperature, titanium reacts with oxygen to form a dense oxide film, endowing it with high chemical stability and corrosion resistance. During the welding process, the welding temperature can reach up to 5000-10,000°C, and titanium and its alloys undergo rapid reactions with oxygen, hydrogen, and nitrogen. Tests have shown that titanium alloys can rapidly absorb hydrogen at temperatures above 300°C, oxygen above 450°C, and nitrogen above 600°C during welding. Upon the intrusion of these harmful gases into the molten pool, the plasticity and toughness of the weld joint undergo significant changes, particularly above 882°C, where the grain structure becomes severely coarse. During cooling, martensite structures form, leading to a decrease in joint strength, hardness, plasticity, and toughness, with a high tendency for overheating and severe embrittlement. Therefore, during titanium alloy welding, reliable gas protection should be applied to the molten pool, droplets, and high-temperature zones, both on the positive and negative sides. This is crucial for ensuring the quality of titanium and its alloy welding. Delayed Cracking: In the period following welding, the area near the weld seam of titanium and its alloys is prone to cracking, 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, resulting in the formation of cracks.
