Welding and testing of titanium alloys

Weldability Analysis
Due to the high affinity of titanium alloys with O2, N2, and H2, the presence of these gases in the joints will make the joints brittle and reduce the impact performance, plasticity and toughness of titanium alloy welded joints. The presence of hydrogen in titanium alloys can cause delayed cracking in the heat-affected zone. When the amount of oxygen and nitrogen in the weld is high, the weld or heat-affected zone will also crack under the action of large welding stress, and the crack is also a delayed crack. Therefore, inert gas (or vacuum chamber) protection is very necessary. Due to the high cost of using a vacuum chamber, the method of inert gas protection is generally used. The main protective gases are helium and argon. Since the price of helium is higher than that of argon, generally speaking, for welding joints and heat-affected zones of titanium alloys that are not specially required, high-purity argon can be used for protection to prevent oxidation.

Most titanium alloys can be welded using oxyacetylene welding, and all titanium alloys can be welded using solid state welding methods (eg TIG, MIG, plasma arc, laser and electron beam welding). In fact, the tendency of titanium alloy welded joints to crack is much smaller than that of black metals (such as iron alloys, nickel alloys).

Although titanium alloys have such good properties and other excellent welding characteristics, some engineers still think that the welding of titanium alloys is quite difficult, mainly because the welding of titanium alloys has very high requirements for gas protection, and generally only very professional staff can Ensure that the gas protection meets the requirements. In fact, many welding methods can be used to weld titanium alloys.

Since the N2, O2 and carbonaceous substances in the air introduced during the welding process make the molten welded joint of the titanium alloy brittle, the area to be welded must be cleaned and protected with inert gas. The welding material is basically selected according to the characteristics of the material to be welded. The weldability of titanium alloys is generally evaluated based on the ductility and strength of the welded joint.

Laser welding titanium plate
The current application trend of laser welding of titanium alloys is more and more extensive, laser welding has small deformation, high production efficiency, and the degree of automation is higher than electron beam and TIG. Compared with electron beam welding, laser welding does not require complex equipment such as a vacuum chamber, so laser welding is more practical, and laser welding can be directly welded in different welding states.

Due to the high power of the CO2 laser, 25kW/h can be used to penetrate a titanium plate with a thickness of 20mm at one time. Nd:YAG lasers make YAG welding more flexible due to the ability to use optical fibers for energy transfer, but the penetration depth is limited due to the low power. It is easy to generate spatter during laser welding, which makes the surface unclean, and special care must be taken when post-weld treatment is not possible.

Post weld inspection
Visual inspection of titanium alloy welding parts is mainly to evaluate the quality of gas shielding. When the surface is silvery white, it means that the gas protection is very good; and when the surface is light yellow or dark yellow, it means that the titanium alloy is slightly polluted, but it is still acceptable; the surface is dark blue, which means that the pollution is serious, but due to the use of Different working conditions, some can be used; the surface is light blue, the pollution is serious, and it is almost impossible to use; when the surface is gray-blue or gray, the pollution is very serious and cannot be used; the same surface is white, the pollution is very serious, unavailable.

Color detection method
Hardness testing and eddy current testing can be used to detect whether the joint contains impurities. Since the properties of the joint will change when impurities are contained, the hardness and resistivity of the joint will be significantly higher when impurities are contained. The portable hand-held hardness tester can be used to test the hardness of weldments in situ. The application of this technology can easily detect the quality of welding. Under normal circumstances, the occurrence of welding cracks in titanium alloys is relatively small. However, cracks sometimes occur in the weld or due to inclusions. At this time, defects can be found by coloring inspection, and this method also has a certain effect on porosity. It must be noted that the colorant must be cleaned before proceeding with welding.