Titanium alloys are widely used in aerospace, chemical industries, and marine engineering due to their high specific strength and excellent corrosion resistance. However, titanium has extremely high chemical reactivity. When the temperature exceeds the range of 500-700℃, it will strongly absorb oxygen, hydrogen, and nitrogen from the air. It will cause weld cracking and plasticity reduction, which seriously affects the joint performance. As a result, When welding titanium alloys, using the argon arc welding (TIG) method to protect the molten pool and the high-temperature areas of the weld seam with temperatures above 400 to 650 degrees Celsius, and a large-sized welding torch is used to expand the gas protection zone. When the nozzle cannot effectively cover the weld and the high-temperature metal near the weld, an argon gas protection cover should be added to ensure complete isolation from the air throughout the process.
Pre-weld Preparation and Groove Design
Surface Cleaning
The surface quality of the weldment and the welding wire directly determines the mechanical properties of the welded joint. Before welding, the test piece and the welding wire should undergo acid washing treatment, followed by rinsing with clean water and drying. Then, welding should be carried out immediately. If acid washing conditions are not available, organic solvents such as acetone, ethanol, carbon tetrachloride, or methanol can be used to carefully wipe the titanium plate groove and the areas within 50 mm on both sides, the surface of the welding wire, and the parts of the tooling fixture in contact with the titanium plate, thoroughly removing oxide scales, oil stains, and other organic substances.
Welding Equipment
For titanium and titanium alloy arc welding, a DC argon arc welding power supply with a descending external characteristic and a high-frequency arc starting function should be selected. To prevent the weld seam from being oxidized or contaminated during cooling, the welding machine should be set to a lagging gas supply time of no less than 15 seconds. In this example, the WSM-315 type IGBT inverter direct current pulse argon arc welding machine is selected, which can meet the above requirements.
Welding Materials
The requirements for the protective gas argon are as follows: the purity must be no less than 99.99%, the dew point should be lower than -40℃, and the relative humidity should be less than 5%. When the pressure inside the argon cylinder drops to 0.981 MPa, it must be stopped from being used to ensure the protection effect. In principle, the welding wire filled should be of the same material as the base metal; to improve the plasticity of the joint, a slightly less alloyed welding wire, such as TC3, can also be selected.
Groove Form
The groove design should minimize the number of welding layers and the amount of filler metal, as an increase in the number of layers will lead to an increase in the cumulative gas absorption of the weld seam, deteriorating the joint performance. Given that the molten pool size is large during titanium alloy welding, the weld should be machined with a single V-shaped groove, with the angle controlled at 70° to 80°.
Welding Process Parameters and Control Points
Welding Process Parameters and Control Points
Reasonable process parameters are the key to ensuring welding quality. During operation, it is necessary to precisely control the flow rate and velocity of argon gas to avoid turbulent flow that weakens the protective effect. Manual tungsten inert gas welding is suitable for welding of thin titanium plates and medium-thick titanium plates. As long as the parameters are selected appropriately, satisfactory welding quality can be achieved. In addition, all cleaning processes should be completed as soon as possible and welding should be carried out promptly to prevent secondary pollution.
Common Defects and Their Repair Mechanisms
Cold Cracks (Delayed Cracks)
When welding titanium and titanium alloys, the probability of the joint generating hot cracks is extremely low. This is because the materials contain very little impurities such as sulfur, phosphorus, and carbon, which are difficult to form low-melting-point eutectics, and the effective crystallization temperature range is narrow, with a small amount of solidification shrinkage. However, cold cracks may occur in the heat-affected zone, which manifest as appearing only a few hours or even longer after welding, hence being called delayed cracks.
The formation of delayed cracks is closely related to the behavior of hydrogen: During the welding process, hydrogen diffuses from the high-temperature molten pool to the lower-temperature heat-affected zone, increasing the hydrogen content in this area. It forms brittle TiH₂ phases and, at the same time, the hydrogen compounds precipitate, causing large microstructural stresses due to volume expansion. Additionally, hydrogen atoms accumulate at high-stress areas, ultimately leading to crack initiation and propagation.
Porosity
Porosity is another common defect in titanium alloy welding. Its fundamental cause is also related to hydrogen. The solubility of hydrogen in α-Ti is extremely low, being only about 0.002% (mass fraction) at room temperature. When the weld or heat-affected zone cools below 300°C, the over-saturated hydrogen precipitates in the form of titanium hydride (γ phase), accompanied by volume expansion and intergranular stress. This may cause intergranular microcracks. These microcracks, under external forces, will expand into macroscopic cracks, and the presence of porosity significantly reduces the fatigue strength of the joint.
In summary, the key to the success of titanium alloy welding lies in pre-weld cleaning, prevention of oxidation and hydrogen absorption throughout the process, high-purity argon gas protection, reasonable groove design, and precise process control. Manual tungsten inert gas welding combined with shielded cover plate protection is an effective solution in current engineering practice, which can stably obtain high-quality welding joints that meet the usage requirements.
Read more:
Analysis Of Cutting And Welding Performance Of TA2 Titanium Plate
Detailed Explanation Of Titanium Alloy Pipe Repair Welding Technology