Welding and wire selection of titanium and titanium alloys

The material characteristics and weldability of titanium and titanium alloys were studied, and weldability tests were conducted to address the welding defects of oxidation, cracking, and porosity that are prone to occur during the welding of titanium and titanium alloys. Through continuous exploration of welding process specifications for titanium and titanium alloys, as well as reasonable analysis of problems that arise during the testing process, summarize the characteristics and operational essentials of welding processes for titanium and titanium alloys.
Classification and Characteristics of Titanium and Titanium
There are three types of industrial pure titanium: TA1, TA2, and TA3, which differ in the content of hydrogen, oxygen, and nitrogen impurities. These impurities strengthen industrial pure titanium, but significantly reduce its plasticity. Although industrial pure titanium has low strength, it has excellent plasticity and toughness, especially with good low-temperature impact toughness; Simultaneously possessing excellent corrosion resistance. So, this material is mostly used in the chemical industry, petroleum industry, etc., and is actually mostly used in working conditions below 350 ℃.
According to the room temperature microstructure of annealed titanium alloys, they can be divided into three types:
Alpha type titanium alloy, (alpha+beta) type titanium alloy, and beta type titanium alloy.
Among the α – type titanium alloys, the Ti AI series alloys of TA4, TA5, and TA6 types and the Ti+AI+Sn alloys of TA7 and TA8 types are commonly used. At room temperature, the strength of this alloy can reach 931N/mm2, and its performance is stable at high temperatures (below 500 ℃) with good weldability.
The application of β – type titanium alloy in China is relatively small, and its scope of use needs to be further expanded.
Weldability of titanium and titanium alloys
The welding performance of titanium and titanium alloys has many significant characteristics, which are determined by the physical and chemical properties of titanium and titanium alloys.
The influence of gas and impurity pollution on welding performance
At room temperature, titanium and titanium alloys are relatively stable. However, during the welding process, liquid droplets and molten metal have a strong absorption effect on hydrogen, oxygen, and nitrogen, and in the solid state, these gases have already interacted with them. As the temperature increases, the ability of titanium and titanium alloys to absorb hydrogen, oxygen, and nitrogen also increases significantly. They start absorbing hydrogen at around 250 ℃, oxygen at 400 ℃, and nitrogen at 600 ℃. After these gases are absorbed, they will directly cause embrittlement of the welded joint, which is an extremely important factor affecting the welding quality.
1.1 Effects of Hydrogen
Hydrogen is the most significant factor affecting the mechanical properties of titanium among gas impurities.
The change in hydrogen content in the weld seam has the most significant impact on the impact performance of the weld seam. The main reason is that as the hydrogen content in the weld seam increases, the amount of flake or needle like TiH2 precipitated in the weld seam increases. The strength of TiH2 is very low, so the effect of sheet or needle shaped HiH2 is mainly due to notches, resulting in a significant decrease in impact performance; The effect of changes in hydrogen content in welds on the improvement of strength and the reduction of plasticity is not very significant.
1.2 Effects of Oxygen
Oxygen has a high degree of melting in both the alpha and beta phases of titanium, and can form interstitial solid phases. The crystalline wounds of titanium are severely distorted, thereby increasing the hardness and strength of titanium and titanium alloys, while significantly reducing plasticity. In order to ensure the performance of the welding joint, in addition to strictly preventing the main oxidation of the weld seam and the heat affected zone during the welding process, the oxygen content in the base metal and welding wire should also be limited.
1.3 Effects of Nitrogen
At high temperatures above 700 ℃, nitrogen and titanium undergo intense reactions, forming brittle and hard titanium nitride (TiN). The degree of lattice distortion caused by the formation of interstitial solid solution between nitrogen and titanium is more severe than that caused by the amount of oxygen. Therefore, nitrogen has a more significant effect on improving the tensile strength and hardness of industrial pure titanium welds and reducing their plasticity than oxygen.
1.4 Impact of Carbon
Carbon is also a common impurity in titanium and titanium alloys. Experiments have shown that when the carbon content is 0.13%, due to its deep presence in alpha titanium, the strength limit of the weld seam is slightly increased and the plasticity is slightly reduced, but not as strong as the effect of oxygen and nitrogen. However, when the carbon content of the weld seam is further increased, a network of TiC appears in the weld seam, and its quantity increases with the increase of carbon content, causing a sharp decrease in the plasticity of the weld seam and making it prone to cracking under welding stress. Therefore, the carbon content of titanium and titanium alloy base materials shall not exceed 0.1%, and the carbon content of welds shall not exceed that of the base material
2? Welding joint crack problem
When welding titanium and titanium alloys, the possibility of hot cracking in the welded joint is very small. This is because the impurities such as S, P, and C in titanium and titanium alloys are very low, and the low melting point eutectic formed by S and P is not easy to appear at the grain boundaries. In addition, the effective crystallization temperature range is narrow, and the shrinkage during solidification of titanium and titanium alloys is small, so the weld metal will not produce hot cracks.
During the welding of titanium and titanium alloys, cold cracks may appear in the heat affected zone, characterized by delayed cracking occurring several hours or even longer after welding. Research has shown that this type of crack is related to the diffusion of hydrogen bombs during the welding process. During the welding process, hydrogen diffuses from the high-temperature deep pool to the lower temperature heat affected zone. The increase in hydrogen content leads to an increase in the amount of TiH2 precipitated in this zone, increasing the brittleness of the heat affected zone. In addition, the volume expansion during hydride precipitation causes significant tissue stress, and hydrogen atoms diffuse and aggregate towards high stress areas in this zone, resulting in the formation of cracks. The main way to prevent the occurrence of delayed cracks is to reduce the source of hydrogen in the welding joint. When invoicing, it is also necessary to carry out flame suppression treatment.
3? Pore problem in weld seam
Pores are a common issue encountered during welding of titanium and titanium alloys. The fundamental reason for the formation of pores is due to the effect of hydrogen. The formation of porosity in weld metal mainly affects the fatigue strength of the joint.
The main process measures to prevent the formation of pores include:
3.1 The protection of neon gas should be pure, with a purity of not less than 99.99%
3.2 Thoroughly remove organic matter such as oxide scale and oil stains from the surface of the welded parts and welding wires.
3.3 Apply good gas protection to the molten pool, control the flow rate and velocity of argon gas to prevent turbulent flow and affect the protection effect.
3.4 Correctly selecting welding process parameters and increasing the retention time of deep pools to prevent the escape of bubbles can effectively reduce porosity.
Conclusion
The gas protection issue in titanium and titanium alloy welding is the primary factor affecting the quality of welded joints.
When welding titanium and titanium alloys, small heat input should be used as much as possible.
When performing TA2 manual tungsten inert gas welding, the source of hydrogen should be strictly controlled to prevent the occurrence of cold cracks, and attention should be paid to preventing the formation of pores.
As long as welding is strictly carried out according to the welding process requirements and effective gas protection measures are taken, high-quality welding joints can be obtained.