1. Preparation before welding
The pre-welding preparation for titanium alloy welding is very important, mainly including:
(1) Material cleaning before welding
Before welding, the surface of the titanium alloy within 50mm of both sides of the strip needs to be polished until the metallic luster of the material itself is exposed. After polishing, wipe the edge of the strip with a clean white silk cloth and acetone to completely remove the oxide film, grease, water, dust and other impurities in the welding area. But for production lines with a high degree of automation, this method is not practical. Therefore, a deburring device can be installed before forming the welding section.
(2) Debugging equipment
Before welding, check the pressure of each gas cylinder carefully to ensure that the pressure of each gas is sufficient. Adjust and check the automatic pipe welding machine to ensure that the power supply and wire feeder are working properly. During adjustment and inspection, the welding torch can generally be placed over the full length of the welding seam to ensure that the equipment is working properly and the welding torch and the welding seam are in ideal alignment. It is recommended to install a visual weld tracking device in the welding gun working area, which can effectively monitor the weld alignment. After the offset occurs, the weld track is automatically corrected.
(3) Welding materials
When using plasma arc welding (PAW), the ion gas, nozzle shielding gas, support cover and back shielding gas use first-grade pure argon (≥99.99%);
Laser welding (LW) is used, the side blowing gas is pure helium (≥99.99%), and the drag hood and back protection gas are first grade pure argon (≥99.99%);
2. Welding method
(1) Plasma arc welding
For titanium plates with a thickness between 2.5 and 15mm, when the groove is I-shaped, the small hole method can be used to weld through at one time. In order to ensure the stability of the small hole, the size of the gas-filled groove on the back is 30mm×30mm. PAW has many process parameters. When the small hole method is used, it mainly involves nozzle diameter, welding current, ion gas flow, welding speed, shielding gas flow, etc.
(2) Laser welding
The main process parameters of laser welding include laser power, welding speed, defocusing amount, side blowing gas flow rate and shielding gas flow rate. Due to the extremely high speed of laser welding, it is generally impossible to adjust the process parameters during the welding process. Therefore, it is necessary to determine the best combination of parameters through pre-tests before the formal welding, and the interlayer temperature during welding is not more than 100°C. At this time, the standard production process recipe is very important. Hangao Tech (SEKO Machinery) high precision titanium alloy steel tube production line pipe manufacturing machine works with PLC intelligent system, could record and store all the processing data in real-time.
(3) Laser-MIG hybrid welding
When adopting LW-MIG hybrid welding, there are two heat sources, laser and arc, and each heat source has more process parameters to be adjusted. Therefore, a lot of experimentation is needed to make the laser and the arc match harmoniously. The relative position of the laser and the arc should be adjusted appropriately during welding.
3. Inspection after welding
After the welding is completed, the appearance of the weld is inspected and non-destructive testing is carried out. At this time, an eddy current flaw detection device can be added. When the weld is found to be poor or perforated, the device will buzz and alarm. The appearance color of the titanium alloy can indicate the degree of contamination of the weld. Generally speaking, silver white means excellent protection, and there is almost no harmful gas pollution; light yellow and golden yellow welds have little effect on the mechanical properties; other colors such as blue and gray are not of good quality and unacceptable. As long as the protection in the high temperature zone is adequate, the appearance of the weld after welding is basically silvery white or golden yellow. However, since the drag cover cannot be completely secured in the arc starting section, the protection effect at the arc starting point is slightly worse. Under normal circumstances, the appearance of the weld after welding machine process is well formed, and there are no defects such as cracks, lack of fusion, pores, weld bumps, etc.
