Titanium alloys have low density, high strength-to-weight ratio, good corrosion resistance, low thermal conductivity, non-toxicity, non-magnetism, and can be welded. They are widely used in the aerospace, aviation, chemical, petroleum, energy, medical, construction and sports industries.
1. Features of welding titanium and titanium alloys:
(1) Embrittlement caused by contamination by impurities:
Due to the high chemical reactivity of titanium, the welding thermal cycle can cause the weld pool, the fusion zone metal above 350℃ and the heat-affected zone to easily react with hydrogen, oxygen, nitrogen as well as contaminants such as oil, moisture, etc. present in the air or in the material and welding wire.
Titanium rapidly absorbs hydrogen above 300°C, oxygen above 600°C and nitrogen above 700°C. When it contains a large amount of carbon, it can develop a brittle TiC network structure. These conditions significantly reduce the ductility and toughness of titanium and its alloys, resulting in a deterioration in the performance of the welded joint.
The color of the oxide film formed on the titanium surface depends on the production temperature.
Below 200℃, it appears silvery white; at 300°C, it turns pale yellow; at 400°C, it turns to gold; at 500℃ and 600℃ it presents blue and purple colors, respectively; and from 700°C to 900°C, it appears in various shades of gray.
The color of the oxide film can be used to determine the temperature of the unprotected area during the welding process.
(2) Performance degradation caused by phase transformation during welding:
There are two crystal structures of titanium: above 882℃, it has a body-centered cubic lattice structure known as β-titanium, and below 882℃, it has a compact hexagonal lattice structure called α-titanium. Titanium used for vessels contains very few β-stabilizing elements and is mainly α-iron alloys.
During welding at high temperatures, the weld and parts of the heat-affected zone transform into the β-crystalline structure, leading to a significant grain growth tendency.
As titanium has a high melting point, high specific heat capacity and low thermal conductivity, the residence time at high temperatures during welding is approximately 3 to 4 times longer than that of steel.
This results in a wider high-temperature heat-affected zone, causing noticeable grain growth in the weld and in the high-temperature heat-affected zone, leading to a significant decrease in ductility.
Therefore, when welding titanium, it is generally advisable to use lower welding heat input and faster cooling rates to reduce residence time at high temperatures, minimize the extent of grain growth, decrease the size of the heat-affected zone at high temperature and mitigate the decrease in ductility.
(3) Inert gas protection in the welding area is required:
At high temperatures, titanium has a strong affinity for oxygen in the air. Therefore, it is necessary to use inert gas protection above 200°C to prevent oxidation.
(4) Significant welding distortion:
The modulus of elasticity of titanium is only half that of carbon steel. Under the same welding voltage, the welding distortion of titanium will be twice that of carbon steel.
Therefore, when welding titanium, it is generally recommended to use backing plates and clamping plates to minimize welding distortion.
(5) Prone to porosity:
Porosity is a common defect in titanium welds. The pores formed during titanium welding are mainly hydrogen pores, but there may also be pores formed by CO gas.
(6) Cracking potential:
Impurities such as sulfur, phosphorus and carbon in titanium have a low melting point and a narrow solidification temperature range with titanium at grain boundaries.
As a result, the shrinkage of the weld during solidification is small and thermal cracks are generally not produced in the weld. Cracks in titanium welds are typically hydrogen-induced cold cracks.
(7) Incompatibility with fusion welding steel:
Iron dissolves in titanium at very low mass fractions, ranging from just 0.05% to 0.10%.
Therefore, titanium and steel cannot be fusion welded directly.
2. Welding methods for titanium and titanium alloys
The main welding methods used for titanium and titanium alloys are tungsten inert gas (TIG) welding, melting electrode inert gas (MIG) welding, and plasma arc welding.
Brazing can be used to weld sealed structures that are not load-bearing. Explosion welding can also be employed for composite welding of titanium and steel composite plates.
3. Welding materials for titanium and titanium alloys:
(1) Welding wire:
The selection of titanium and titanium alloy welding wire is generally based on the combination with the base material, but must also pass the welding process evaluation qualification.
When choosing welding wire, there is a challenge in finding a suitable combination because the impurity content of the wire is only controlled within an upper limit. In most cases, there is no control over the lower limit.
Furthermore, each batch of welding wire produced only guarantees the chemical composition, but not the mechanical properties after welding. There is a possibility that some production batches of welding wire have an unusually low impurity content, making them qualified products.
However, its welding strength may be lower, which may not meet the minimum tensile strength requirement below the annealed state standard of the base material.
In these cases, it is necessary to switch to another production batch of the same brand of welding wire or even a higher quality wire (such as pure industrial) to carry out a new evaluation of the process until it qualifies before finalizing the selection of the welding wire. wire.
For welding wire and filler wire used in containers made of titanium and titanium alloys, the chemical composition (melt analysis) can be found in Table 4-29.
When performing chemical composition retests on samples of finished welding wire and filler wire, the allowable deviations for analysis can be found in Table 4-30. The recommended types of welding wire and filler wire for titanium materials commonly used in containers can be found in Table 4-31.
Table 4-29 Chemical Composition (Melt Analysis) of Titanium and Titanium Alloy Welding Wire and Filler Wire for Containers.
| Wire Model | Chemical composition (mass fraction, %) | ||||||||||
| Main components | Impurity elements | Residual elements | |||||||||
| You | Mo | No | P.D. | Faith | O | W | N | H. | Single | Total sum | |
| ERTAIELI | Rem. | – | – | – | ≤0.08 | 0.03~0.10 | ≤0.03 | ≤0.012 | ≤0.005 | ≤0.05 | ≤0.20 |
| ERTA2ELI | Rem. | – | – | – | ≤0.12 | 0.08~0.16 | ≤0.03 | ≤0.015 | ≤0.008 | ≤0.05 | ≤0.20 |
| ERTA3ELI | Rem. | – | – | – | ≤0.16 | 0.13~0.20 | ≤0.03 | ≤0.02 | ≤0.008 | ≤0.05 | ≤0.20 |
| ERTA4ELI | Rem. | – | – | – | ≤0.25 | 0.18~0.32 | ≤0.03 | ≤0.025 | ≤0.008 | ≤0.05 | ≤0.20 |
| ERTA9 | Rem. | – | – | 0.12-0.25 | ≤0.12 | 0.08~0.16 | ≤0.03 | ≤0.015 | ≤0.008 | ≤0.05 | ≤0.20 |
| ERTA10 | Rem. | 0.2-0.4 | 0.6-0.9 | – | ≤0.15 | 0.08~0.16 | ≤0.03 | ≤0.015 | ≤0.008 | ≤0.05 | ≤0.20 |
Table 4-30: Analysis of chemical composition and allowable deviations of finished titanium and titanium alloy welding wires and filler wires
| Component Elements | Chemical Composition (Mass Fraction, %) | |||||||||||
| Mo | No | P.D. | Faith | O | W | N | H | Individual residual element | ||||
| ≤0.20 | ≤0.30 | ≤0.10 | 0.10~0.15 | ≤0.25 | ||||||||
| Allowed deviations | ±0.03 | ±0.03 | ±0.02 | +0.05 | +0.10 | +0.02 | ±0.02 | +0.03 | +0.01 | +0.01 | +0.002 | +0.02 |
Table 4-31: Recommended Wire and Filler Wire Models for Titanium Materials Commonly Used in Containers
| Titanium Grade | Yarn and Filler Yarn Models |
| TAI | ERTAIELI |
| TA2 | ERTA2ELI |
| TA3 | ERTA3ELI |
| TA4 | ERTA4ELI |
| TA9 | ERTA9 |
| TA10 | ERTA10 |
(2) Shielding Gas:
Argon gas is commonly used as a shielding gas for welding iron and titanium alloys. The purity of argon gas (volume fraction) must not be less than 99.99%.
The volume fractions of other gas components must be as follows: oxygen less than 0.002%, nitrogen less than 0.005%, hydrogen less than 0.002%, and moisture content less than 0.001 mg/L. The pressure in the gas cylinder must not be less than 0.5 MPa.
When using, the air in the shielding gas system such as gas hoses, welding torches and welding masks must be replaced with clean gas. Helium gas or argon-helium mixed gas can also be used as shielding gas.
(3) Tungsten electrode:
Commonly used tungsten electrodes are pure tungsten electrodes and ceriated tungsten electrodes. Ceriated tungsten electrodes contain cerium oxide as an impurity (mass fraction no more than 0.1%).
Ceriated tungsten electrodes have working function of low electron emission, high chemical stability, high allowable current density, no radioactivity and better performance than pure tungsten electrodes. They are tungsten electrodes widely used today.
4. Pre-Welding Preparation
(1) Pre-Welding Cleaning:
Before welding titanium and its alloys, the surface must be carefully cleaned to remove oxides, nitrides, oil, moisture, etc. Acid pickling or grinding with a grinding wheel or sandpaper is generally used.
For parts that are difficult to strip, such as longitudinal welds, container corner welds, and welding of tubes and plates in heat exchangers, the sides of the chamfer can be ground with a grinding wheel or sandpaper, and care must be taken to clean off the sand and remaining dust.
For welding wires, heads, expansion joints and other parts that are not easy to sand, acid pickling should be carried out before welding, followed by rinsing with clean water.
If stripping is not feasible, a hard alloy scraper can be used. After the above cleaning process, the welding area should be cleaned with solvents such as acetone or anhydrous alcohol before welding, and should not be touched with hands to avoid recontamination. If recontamination occurs, it must be cleaned and washed again.
(2) Manufacture of Other Protection Devices in the Welding Zone:
When welding titanium and titanium alloys, the welding gun nozzle protects the weld pool, the welding mask protects the front of the welded joint during cooling, and the back plate protects the back of the welded joint.
The welding gun used for welding titanium and titanium alloys is different from that used for welding aluminum or stainless steel and generally employs a large diameter nozzle.
For manual welding, the nozzle diameter is normally 14 to 20 mm, while for automatic welding it is 16 to 22 mm. The welding mask can protect the weld and the heat-affected zone above 400°C.
The shape and size of the welding mask must be determined based on factors such as workpiece thickness, cooling method, welding current, and weld shape. The welding mask must move together with the welding gun over the welding zone.
Copper backing plates can be used on the back of the weld to accelerate cooling and isolate air. Shielding gas can also be blown through the copper backing plate, or a welding mask can be applied to the back of the weld, accompanying the welding process.
