1. Chemical and physical properties of titanium and titanium alloys Titanium

The chemical activity is very strong. As the temperature increases, its chemical properties will improve rapidly. In the solid state, it can strongly absorb various gases. For example, when a pure titanium plate is heated to 300°C, the surface of the titanium plate will absorb hydrogen; when heated to 400°C, oxygen will be absorbed. And absorb nitrogen up to 600°C. If pure titanium contains impurities such as oxygen, nitrogen, and hydrogen, the strength of the welded joint will increase significantly, while the plasticity and toughness will drop sharply.

Titanium has a high melting temperature, a large heat capacity and a large resistivity. Its thermal conductivity is lower than that of aluminum and iron. Therefore, the molten pool of titanium has a higher temperature and a larger size, and the heat-affected zone of the metal remains high for a long time. It is easy to cause overheating of the welded joints, which will thicken the grains and reduce the plasticity of the joints. Therefore, welding must be a welding specification with small current and high welding speed.

2. Welding structure of titanium and titanium alloys

The welded structure of industrial pure titanium and α-titanium alloy is a single phase at room temperature, and depending on the cooling rate, a jagged needle-like structure is produced. Compared with the base material, various mechanical properties have not changed significantly, and the welding performance is good.

α+β titanium alloys form martensite during cooling from the β phase, and the amount and properties of the α phase vary depending on the alloy composition and cooling rate. Generally, as α increases, the ductility and toughness of the alloy decrease. Even for Ti-6Al-4V with good weldability, when the vanadium content in the β phase exceeds 5%, the weldability will decrease. Therefore, generally do not use welded joints.

3. Welding defects of titanium and titanium alloys

During the welding process of titanium and titanium alloys, it is easy to produce a large number of pores. These holes are unavoidable in welding. It is composed of many factors and is very complex. On the one hand, due to the surface adsorption of weldments, such as the presence of impure gas, dust, grease or oxides and other direct factors. These pores are formed by impurity gases trapped in the molten metal. On the other hand, there are some indirect factors, such as too much welding current in argon arc welding, too fast welding speed, and the size of the groove angle. Hydrogen is generally considered to be the main cause of porosity.

In order to prevent the formation of pores, the following measures should be taken: 1) Strictly control the content of impurities such as oxygen, nitrogen, and hydrogen in argon; 2) Use plasma arc welding, especially pulse plasma arc welding; 3) Remove the thin plate and welding wire before welding Grease and oxides on the surface; 4) Low-speed welding and weld bead remelting; 5) Vacuum the weldment and welding wire before welding.

Welding method of titanium and titanium alloy

1. Argon tungsten arc welding

Gas tungsten arc welding is one of the most widely used methods for welding titanium and titanium alloys. It is a melting polar gas shielded welding. The metal is melted using an arc between the tungsten electrode and the workpiece to form the weld. During the welding process, tungsten does not melt, but only plays a protective role. At the same time, argon is sent out from the nozzle as a shielding gas. This is known as TIG welding and is suitable for titanium, zirconium and other reactive metals. For the welding of thick workpieces, argon arc welding can be used. For thin walls of weldments, use the pulse tungsten arc welding method.

2. Vacuum electron beam welding

Vacuum electron beam welding is a high energy beam welding. Suitable for welding titanium and titanium alloys. It has a series of advantages: good welding metallurgical quality, narrow weld seam, small welding angle deformation, good performance of welded joints, and high welding efficiency for thick parts. Porosity is prone to appear in electron beam welding, and its structure size is limited by the vacuum chamber. Not suitable for mass production. During electron beam welding, welded joints develop considerable residual stress. The way to reduce residual stress is to complete vacuum annealing after welding.

3. Laser welding

Compared with electron beam and plasma welding, laser welding has the effect of purifying the molten pool and can purify the welding metal. The quality and efficiency of laser welding is superior to other welding methods. According to foreign companies, the efficiency of laser welding is 2 to 3 times that of electron beam welding, and the welding quality is similar to that of electron beam welding. The cost of laser welding is only 1/20 of the electronic technology equipment. Lasers are easy to use mirrors or prisms to change the light path. It can be placed anywhere on the workpiece. For thin sheets, lasers are currently used instead of electron beam welding. Laser welding has broad application prospects in the welding of titanium and titanium alloy thin plates and precision parts.

4. Brazing

Brazing is one of the easiest and most reliable methods of joining titanium and titanium alloys to other metals. It can also be used to join micro-complex parts in titanium and titanium alloys. Solder is an essential brazing material, which is usually divided into two categories: soft solder with a liquidus temperature lower than 450°C, and hard solder with a liquidus temperature higher than 450°C. At present, silver-based, aluminum-based and titanium-based are widely used for brazing of titanium and titanium alloys. Due to the high temperature activity of titanium, brazing is carried out under vacuum or argon protection. Titanium readily fuses with the brazing filler metal, but tends to form intermetallic compounds, resulting in brittle joints. Therefore it is necessary to select a suitable solder,