Common welding methods for titanium and titanium alloys include: argon arc welding, submerged arc welding, vacuum electron beam welding, etc. Tungsten inert gas welding is used for thickness below 3 mm, and consumable inert gas welding is used for thickness above 3 mm. The purity of argon gas is not less than 99.99%, and the content of air and water vapor in argon gas is strictly controlled. Before welding, degreasing, descaling, and deoxidation film surface treatment are performed. Due to the high chemical activity of titanium and titanium alloys, they are easily contaminated by oxygen, nitrogen, and hydrogen, so arc welding, oxyacetylene (or oxypropane, etc.) gas welding, C02 welding, atomic hydrogen welding, etc. cannot be used for welding.
Effects of oxygen and nitrogen
Oxygen and nitrogen are interstitially dissolved in titanium, which distorts the titanium lattice, increases deformation resistance, increases strength and hardness, but reduces plasticity and toughness. It is unfavorable to contain welding oxygen and nitrogen in the weld and should be avoided.
Effect of Hydrogen
The increase of hydrogen will cause the impact toughness of titanium weld metal to drop sharply, while the plasticity will drop slightly. Hydride will cause the brittleness of the joint.
Effect of Carbon
At room temperature, carbon is dissolved in titanium in the form of interstitial, which increases the strength and decreases the plasticity, but not as obvious as oxygen and nitrogen. When the amount of carbon exceeds the solubility, hard and brittle TiC is generated, which is distributed in a network and is prone to cracks. The national standard stipulates that the carbon content in titanium and titanium alloys shall not exceed 0.1%. During welding, the oil stains on the workpiece and welding wire can increase the carbon content, so they need to be cleaned during welding.
The most common defect in the welding of titanium and titanium alloys is pores, which are mainly produced near the fusion line. Hydrogen is an important reason for the formation of pores. During welding, titanium has a strong ability to absorb hydrogen, and the solubility of hydrogen decreases significantly with the decrease in temperature, so the hydrogen dissolved in the liquid metal often does not have time to escape to form pores.
Joint embrittlement problem
At room temperature, titanium reacts with oxygen to form a dense oxide film, which makes it have high chemical stability and corrosion resistance. During 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 experiments, during the welding process, titanium alloys can quickly absorb hydrogen when the temperature is above 300℃, oxygen when it is above 450℃, and nitrogen when it is above 600℃. When these harmful gases invade the molten pool, the plasticity and toughness of the welded joint will change significantly, especially above 882℃, the grains of the joint will be seriously coarsened, and martensitic structure will be formed during cooling, which will reduce the strength, hardness, plasticity and toughness of the joint, and the overheating tendency will be serious, and the joint will be seriously embrittled. Therefore, when welding titanium alloys, the molten pool, molten droplets and high temperature areas should be fully and reliably protected by gas, whether on the front or back. This is the key to ensuring the welding quality of titanium and its alloys. Delayed cracking Within a period of time after welding, the near-seam area of titanium and its alloys is prone to cracks, which is caused by the diffusion of hydrogen from the high-temperature molten pool to the low-temperature heat-affected zone. As the hydrogen content increases, the amount of titanium hydrogen compounds precipitated increases, the brittleness of the heat-affected zone increases, and the organizational stress generated when the precipitated hydride expands in volume leads to the generation of cracks.
Operation tips
1. During manual argon arc welding, the minimum angle (10~15°) should be maintained between the welding wire and the weldment. The welding wire is fed into the molten pool steadily and evenly along the front end of the molten pool, and the end of the welding wire must not be moved out of the argon protection zone.
2. During welding, the welding gun basically does not swing horizontally. When swinging is required, the frequency should be low and the swing amplitude should not be too large to prevent affecting the protection of argon gas.
3. When breaking the arc and finishing the weld, continue to pass argon protection until the weld and the metal in the heat-affected zone cool to below 350℃ before removing the welding gun.
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