According to the material characteristics and weldability of titanium and titanium alloys, the weldability test is carried out on the welding defects of titanium and titanium alloys that are prone to oxidation, cracks, and pores. Through continuous exploration of the welding process specifications of titanium and titanium alloys, and a reasonable analysis of the problems in the test process, the characteristics and operating essentials of the welding process of titanium and titanium alloys are summarized.

Classification and characteristics of titanium and titanium alloys

There are three types of industrial pure titanium, Gr1, Gr2, and Gr3. The difference lies in the content of impurities containing hydrogen, oxygen and nitrogen. These impurities strengthen industrial pure titanium but significantly reduce its plasticity. Although the strength of industrial pure titanium is not high, it has excellent plasticity and toughness, especially good low-temperature impact toughness and good corrosion resistance. Therefore, this material is mostly used in the chemical industry, petroleum industry, etc., in fact, it is mostly used in working conditions below 350 degrees Celsius.

According to the room temperature structure of the annealed titanium alloy. titanium alloys can be divided into three types:

α-type titanium alloy, (α+β)-type titanium alloy and β-type titanium alloy.

Among α-type titanium alloys, Gr4, Ti-5Al-0.005B, Ti-5Al and Gr6, Gr16 alloys are widely used. The strength of this alloy can reach 931N/MM2 at room temperature, and it has stable performance and good weldability at high temperatures (500 degrees Celsius).

βtitanium alloys are less widely used in China, and their use needs to be further expanded.

The welding performance of titanium and titanium alloys has many notable characteristics. These welding characteristics are determined by the physical and chemical properties of titanium and titanium alloys.

1. The influence of gas and impurity pollution on welding performance

At room temperature, titanium and titanium alloys are relatively stable. However, experiments have shown that in the welding process, liquid droplets and molten pool metal have a strong role in absorbing hydrogen, oxygen, and nitrogen, and in the solid state, these gases have already acted on them. As the temperature rises, the ability of titanium and titanium alloys to absorb hydrogen, oxygen, and nitrogen also increases significantly. It starts to absorb hydrogen at about 250 degrees Celsius, absorbs oxygen from 400 degrees, and absorbs nitrogen from 600 degrees. After being absorbed, it will directly cause embrittlement of the welded joint, which is an extremely important factor affecting the quality of welding.

1.1 The influence of hydrogen

Hydrogen is the most serious factor affecting the mechanical properties of titanium among gas impurities. The change in the hydrogen content of the weld has the most significant impact on the impact performance of the weld. The main reason is that with the increase of the hydrogen content of the weld, the precipitation of flake or needle-like TiH2 in the weld increases. TiH2 has very low strength and its impact resistance is significantly reduced. The effect of the change in the hydrogen content of the weld on the increase in strength and decrease in plasticity is not very obvious

1.2 The influence of oxygen

The oxygen content can increase the hardness and strength of titanium and titanium alloys, but the plasticity is significantly reduced. In order to ensure the performance of welding, in addition to preventing oxidation of the weld seam and welding heat-affected zone during the welding process, at the same time, the oxygen content in the base metal and welding wire should also be limited.

1.3 The effect of nitrogen

At a high temperature above 700 degrees, nitrogen and titanium react to form brittle and hard titanium nitride, which is more serious than oxygen. Therefore, nitrogen can improve the tensile strength and hardness of industrial pure titanium welds and reduce welding The plasticity of the seam is more significant than that of oxygen.

1.4 The impact of carbon

Carbon is also a common impurity in titanium and titanium alloys. Experiments show that when the carbon content is 0.13%, the carbon is deep in α titanium, the weld strength limit is somewhat improved, and the plasticity is somewhat reduced, but it is not as strong as oxygen and nitrogen. However, when the carbon content of the weld is further increased, the weld appears to have a network of Tic, and the amount of the weld increases with the increase of the titanium content, which greatly reduces the plasticity of the weld and is prone to cracks under the action of welding stress. Therefore, the carbon content of the base metal of titanium and titanium alloy is not more than 0.1%, and the carbon content of the weld does not exceed the carbon content of the base metal.

2. Welding joint crack problem

When welding titanium and titanium alloys, the possibility of hot cracks in the welded joints is very small. This is because the content of impurities such as S, P, and C in titanium and titanium alloys is small, so the weld metal will not produce hot cracks.

When welding titanium and titanium alloys, cold cracks can appear in the heat-affected zone, which is characterized by cracks that occur several hours or even longer after welding and are called delayed cracks. Studies have shown that this kind of crack is related to the diffusion of hydrogen bombs during welding. The method to prevent this kind of delayed cracking is mainly to reduce the source of hydrogen at the welded joints.

3. Porosity in the weld. Porosity is a frequently encountered problem. The reason for the formation of porosity is the effect of hydrogen. The formation of pores in the weld metal mainly affects the fatigue strength of the joint.

3.1 The protective neon gas should be pure, and the purity should not be less than 99.99%

3.2 The complete removal of the weldment shows that the oxide scale and oil stains on the welding wire show organic matter.

3.3 Apply good gas protection to the molten pool, and control the flow and flow rate of argon to prevent turbulence and affect the protection effect.

3.4 Correctly select the welding process parameters, increase the residence time in the deep pool, and make the bubbles escape, which can effectively reduce the pores.

Conclusion: 1. The gas shielding problem of titanium and titanium alloy welding is the primary factor affecting the quality of welded joints.

2. When welding titanium and titanium alloys, use as little heat input as possible.

3. When TA2 manual argon tungsten arc welding, the source of hydrogen should be strictly controlled to prevent the generation of cold cracks, and attention should be paid to prevent the generation of pores.

4. As long as welding is carried out in strict accordance with the welding process requirements and effective gas protection measures are taken, high-quality welded joints can be obtained.