How to weld titanium and its alloy?
Titanium metal has been used for various fields due to its unparalleled advantages, such as lightweight, high strength, good resistance to high and low temperature, excellent crack resistance and corrosion resistance in wet chlorine gas. Welding titanium poses an especially significant challenge to many welders since the metal itself is a rather novel one for most industrial sectors. While many materials can be used in welding, none have the combination of durability, flexibility and strength that are found in titanium. This combination of characteristics makes the material extremely difficult to work with and it poses particular challenges even for skilled workers who are trained and experienced in welding. This is what makes titanium welding extremely demanding. Here we will discuss the welding of titanium and its alloy, if interested, please read on!
Weldability analysis
- Embrittlement caused by contamination of interstitial elements
Titanium is an active chemical element at high temperatures. Titanium can absorb hydrogen rapidly above 300℃, absorb oxygen rapidly above 600℃, and absorb nitrogen rapidly above 700℃. If no effective protection is obtained during the welding and post-welding cooling process, plasticity will decrease and brittleness will increase. The carbon of titanium material is generally controlled below 0.1%, because when the carbon exceeds its solubility, it generates hard and brittle TiC with network distribution, which is easy to cause cracks.
- Hot crack
Due to the titanium and titanium, alloy impurities content is less, it is not easy to produce hot cracks, which have high-quality requirements for the welding wire, unqualified welding wire will cause cracks, interlayers and other defects, a large number of impurities may cause welding hot cracks.
- Delayed cracking may occur in the HEAT affected zone
During welding, the hydrogen in the pool and the base metal in the low-temperature zone diffuses to the HEAT affected zone, which leads to the accumulation of hydrogen in the heat-affected zone and causes cracks under adverse stress conditions.
- Porosity
Porosity is the most common defect in welding titanium and titanium alloys. Generally is the weld porosity and fusion line porosity, porosity is generally located near the fusion line when the welding line energy is larger, but mainly in the welding area especially when the welding surface is polluted by water and oil.
Welding Technology
- Welding method
GTAW welding method, direct current connection, using high-frequency arc ignition and attenuation of the arc extinguishing device welding machine.
- Welding material
The selection of welding wire should make the tensile strength of the welding seam is not lower than the lower limit of the standard tensile strength of the annealed base metal, the plasticity and corrosion resistance of the welding seam after welding state is not lower than the annealed base metal or similar to the base metal, and the weldability is good.
The chemical composition of ER Ti-2 wire is shown in the table below.
| Welding wires | Ti | Fe | C | N | H | O |
| ERTi-2 | Balance | 0.3 | 0.1 | 0.05 | 0.015 | 0.25 |
- Selection of shielding gas and weld color
The purity of argon for welding should not be lower than 99.99%, the moisture should be less than 50mL /m³, and the dew point should not be higher than -40℃. It should not be used when the pressure of bottled argon is lower than 0.981MPa. The welding pool and the area where the internal and external surface temperature of the welding joint is higher than 400℃ are protected by argon gas.
| Weld joints color | Silver | Light yellow | Dark yellow | Purple (metallic luster) | Blue (metallic luster) | Off-white, yellow-white |
| Argon gas purity | 99.99% | 98.7% | 97.8% | 97.5% | 97% | 96% |
| Welding quality | High quality | Good | Qualified | Qualified | Unqualified | Unqualified |
- Weld Preparation
Effective measures should be taken to avoid mutual dissolving between steel and titanium in the welding process, keep the site clean and avoid using iron tools.
Groove processing. After cutting the titanium pipe, the grinder is used to polish the groove. The groove Angle is 30°±2.5° on one side and the blunt edge is 0.5 ~ 1.5mm. The processing of the groove should not cause the base metal to produce overheating discoloration. The inner and outer surfaces of the groove and its sides within 25mm shall be cleaned by the following procedure: polishing by polishing machine — polishing by sandpaper wheel — cleaning by acetone. Clean the welding wire with a sponge dipped in acetone, and carefully check whether there are cracks and interlayers near the base metal groove and the welding wire, and wait for the dry end of the groove before operation. If welding cannot be done in time, self-adhesive tape and a plastic sheet should be used to protect the groove. The time from cleaning to welding is not more than 2 hours, welder’s gloves should be clean before use must be cleaned with anhydrous ethanol (or acetone), avoid cotton fiber attached to the surface of the welder.
- Welding process parameters
|
Wall thickness |
Welding layer |
Tungsten electrode diameter |
Welding current |
Wire diameter |
The argon gas flow |
The nozzle diameter |
||
|
Welding handle |
Drag cover |
Tube |
||||||
|
3-4 |
2 |
2.4 |
75-95 |
2.5 |
11-13 |
20-22 |
11-22 |
12 |
|
5-6 |
3 |
2.4 |
90-120 |
2.5 |
12-15 |
20-22 |
11-22 |
18 |
|
7-8 |
3-4 |
3.0 |
120-160 |
3.0 |
12-15 |
20-22 |
11-22 |
18 |
It is worth noting that, under the condition of ensuring good weld formation, small line energy welding should be selected as far as possible, and the interlayer temperature should not be higher than 200℃ to prevent the grain from growing up for too long at high temperature. The welding process shall be carried out under the protection of argon: the welding torch nozzle shall be used to protect the molten pool, the welding torch drag cover shall be used to protect the hot weld and the outer surface of the near joint area, and the pipe shall be filled with argon to protect the welding seam and the inner surface of the near joint area. When the large-diameter titanium pipe is welded, the welder shall use a gas mask and a hand-held protective cover to protect the back of the welding pool.
When welding tubes with a small diameter or fixed orifice, the soluble paper should be used at the place where the surface of the titanium tube is 150-300mm away from the groove (a larger value should be taken according to the operability) to prevent the seal soluble paper from being damaged by excessive pressure in the tube, and then argon gas should be filled to exhaust the air in the tube. Argon must be fully precharged before welding, and argon should be delayed after welding to fully cool the high-temperature area and prevent surface oxidation.
Welding inspection
The welder shall clean the bead surface to a good appearance.
The width should be 2mm over the edge of the groove. The height of the fillet weld toe should meet the design requirements and the shape should be smooth. The surface quality shall meet the following requirements: no defects such as edge biting, crack, non-fusion, porosity, slag inclusion and splash are allowed; Weld residual height: when the wall thickness is less than 5mm, 0 ~ 1.5mm; When the wall thickness is greater than 5 mm, it is 1 ~ 2mm; The amount of staggered edge on the surface of c weld shall not be greater than 10% of the wall thickness, and not greater than 1mm.
The bottom welds shall be penetrant inspected and shall be deemed to be free of cracks and any other surface defects. Check the color of the surface of each weld, which indicates the color change of the surface oxide film at different temperatures, and their mechanical properties are not the same. (See Table 3) Note: The pickling method should be used to distinguish low-temperature oxidation from high-temperature oxidation.
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