Welding Technology for Machining Titanium Alloy Parts
Key words: welding; titanium alloy; welding wire; argon gas; argon arc welding; machining of titanium alloy parts
Abstract: This paper describes the material characteristics of titanium and titanium alloys and the weldability of titanium alloy parts machining. In addition, weldability tests were carried out on the defects of oxidation, crack and porosity in the welding of titanium and titanium alloys. Can continue to explore the welding process specifications of titanium and titanium alloys, as well as a reasonable analysis of the problems in the test process, summed up the welding process characteristics and operating methods of titanium and titanium alloys.
1. Classification and characteristics of titanium and titanium alloys
Domestic industrial pure titanium has three kinds of TA1, TA2 and TA3. The difference is that the content of impurities containing hydrogen and oxygen is different. These impurities strengthen industrial pure titanium, but the plasticity is significantly reduced. Industrial pure titanium, although not strong in strength, 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, the petroleum industry, etc., and is actually used for working conditions below 350 °C. Titanium alloys can be classified into three types according to the room temperature structure of the annealed state of titanium alloy: Α-type titanium alloy, (α+β) type titanium alloy and β-type titanium alloy. Among the α-type titanium alloys, TA4, TA5, and TA6 Ti-AI alloys and TA7 and TA8 Ti+AI+Sn alloys are widely used. The alloy has a strength of 931 N/mm 2 at room temperature, and is stable at high temperatures (below 500 ° C) and has good weldability. The application amount of β-type titanium alloy is small in China, and its use range needs to be further expanded.
2. the weldability of titanium and titanium alloy
The welding properties of titanium and titanium alloys have many remarkable features. These weld characteristics are determined by the physicochemical properties of titanium and titanium alloys.
(1). Influence of gas and impurity pollution on welding performance
At normal temperature, titanium and titanium alloys are relatively stable. However, in the test, during the welding process, the liquid droplets and the molten pool metal strongly absorb hydrogen, oxygen and nitrogen, and in the solid state, these gases have reacted with them. As the temperature increases, the ability of titanium and titanium alloys to absorb hydrogen, oxygen and nitrogen also increases. At about 250 ° C, it begins to absorb hydrogen, absorbs oxygen from 400 ° C, and starts to absorb nitrogen from 600 ° C. When these gases are absorbed, they will directly cause embrittlement of the welded joint, which is an extremely important factor affecting the quality of the weld.
Hydrogen is the most serious factor affecting the mechanical properties of titanium in gaseous impurities. The change of hydrogen content in weld seam has the most significant effect on the impact property of weld seam. The main reason is that with the increase of hydrogen elasticity in the weld, the amount of flake or needle TiH2 precipitated from the weld increases. The strength of TiH2 is very low, so the action of the sheet-like or needle-shaped HiH2 is notched, and the impact performance is significantly reduced; The effect of the change in hydrogen content of the weld on the strength and the decrease in plasticity is not very obvious.
Oxygen effect
Oxygen has high melting degree in both alpha and beta phases of titanium and can form interstitial solid phase. The crystal wounds of titanium have been severely distorted, thereby increasing the hardness and strength of titanium and titanium alloys, resulting in a significant decrease in plasticity. In order to ensure the performance of welding joint, besides strictly preventing the main oxidation of weld seam and welding according to heat affected zone during welding process, oxygen content in basic metals and welding wires should also be limited.
Effect of nitrogen
At high temperatures above 700 °C, nitrogen and titanium have a violent effect. The lattice distortion caused by the formation of brittle titanium nitride (RIN) and interstitial solid solution between nitrogen and titanium is more serious than that caused by the amount of oxygen. Therefore, nitrogen improves the tensile strength and hardness of industrial pure titanium welds and reduces the plastic properties of welds more than oxygen.
Carbon effect
Carbon is also a common impurity in titanium and titanium alloys. Experiments show that when carbon content is 0.13%, the strength limit of weld seam increases and the plasticity decreases, but the effect of carbon is not as strong as that of oxygen and nitrogen. However, when the carbon content of the weld is further increased, the mesh TiC appears in the weld, and the amount increases with the increase of the carbon content, so that the plasticity of the weld is sharply decreased, and cracks are likely to occur under the welding stress. Therefore, the carbon content of the titanium and titanium alloy base material is not more than 0.1%, and the carbon content of the weld does not exceed the carbon content of the base material.
Weld joint crack problem
When welding titanium and titanium alloys, the possibility of hot cracks in welded joints is small. This is because the content of S, P, C and other impurities in titanium and titanium alloy is very small, and the low melting point eutectic formed by S and P is not easy to appear on the grain boundary. In addition, the effective crystallization temperature interval is narrow, and the shrinkage amount of titanium and titanium alloy is small when solidified, and the weld metal does not generate hot cracks. When titanium and titanium alloys are welded, cold cracks may occur in the heat-affected zone, which is characterized by cracks occurring several hours or more after welding as delayed cracks. The research shows that the crack is related to the diffusion of hydrogen in the welding process. During welding, hydrogen diffuses from high temperature deep pool to low temperature heat affected zone. The increase of hydrogen content increases the amount of TiH2 precipitated from the zone and increases the brittleness of heat affected zone. In addition, the volume expansion of hydride precipitation results in larger tissue stress, and the diffusion and accumulation of hydrogen atoms to the high stress areas in this region leads to the formation of cracks. The way to prevent such delayed cracks is mainly to reduce the source of hydrogen in the welded joints.
3. Porosity problems in the weld
When welding titanium and titanium alloys, pores are a common problem. The root cause of the formation of pores is the result of hydrogen influence. The formation of pore in weld metal mainly affects the fatigue strength of the joint. The main technological measures to prevent blowhole are as follows:
(1) Protecting neon should be pure and its purity should not be less than 99.99%.
(2) Thoroughly remove organic matter such as scale oil on the surface of the weldment and the surface of the wire.
(3) Apply good gas protection to the molten pool, control the flow rate and flow rate of argon gas to prevent turbulence and affect the protection effect.
(4) Correct selection of welding parameters and increase of deep pool dwell time are conducive to bubble escape, which can effectively reduce the porosity.
Abstract: This paper describes the material characteristics of titanium and titanium alloys and the weldability of titanium alloy parts machining. In addition, weldability tests were carried out on the defects of oxidation, crack and porosity in the welding of titanium and titanium alloys. Can continue to explore the welding process specifications of titanium and titanium alloys, as well as a reasonable analysis of the problems in the test process, summed up the welding process characteristics and operating methods of titanium and titanium alloys.
1. Classification and characteristics of titanium and titanium alloys
Domestic industrial pure titanium has three kinds of TA1, TA2 and TA3. The difference is that the content of impurities containing hydrogen and oxygen is different. These impurities strengthen industrial pure titanium, but the plasticity is significantly reduced. Industrial pure titanium, although not strong in strength, 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, the petroleum industry, etc., and is actually used for working conditions below 350 °C. Titanium alloys can be classified into three types according to the room temperature structure of the annealed state of titanium alloy: Α-type titanium alloy, (α+β) type titanium alloy and β-type titanium alloy. Among the α-type titanium alloys, TA4, TA5, and TA6 Ti-AI alloys and TA7 and TA8 Ti+AI+Sn alloys are widely used. The alloy has a strength of 931 N/mm 2 at room temperature, and is stable at high temperatures (below 500 ° C) and has good weldability. The application amount of β-type titanium alloy is small in China, and its use range needs to be further expanded.
2. the weldability of titanium and titanium alloy
The welding properties of titanium and titanium alloys have many remarkable features. These weld characteristics are determined by the physicochemical properties of titanium and titanium alloys.
(1). Influence of gas and impurity pollution on welding performance
At normal temperature, titanium and titanium alloys are relatively stable. However, in the test, during the welding process, the liquid droplets and the molten pool metal strongly absorb hydrogen, oxygen and nitrogen, and in the solid state, these gases have reacted with them. As the temperature increases, the ability of titanium and titanium alloys to absorb hydrogen, oxygen and nitrogen also increases. At about 250 ° C, it begins to absorb hydrogen, absorbs oxygen from 400 ° C, and starts to absorb nitrogen from 600 ° C. When these gases are absorbed, they will directly cause embrittlement of the welded joint, which is an extremely important factor affecting the quality of the weld.
Hydrogen is the most serious factor affecting the mechanical properties of titanium in gaseous impurities. The change of hydrogen content in weld seam has the most significant effect on the impact property of weld seam. The main reason is that with the increase of hydrogen elasticity in the weld, the amount of flake or needle TiH2 precipitated from the weld increases. The strength of TiH2 is very low, so the action of the sheet-like or needle-shaped HiH2 is notched, and the impact performance is significantly reduced; The effect of the change in hydrogen content of the weld on the strength and the decrease in plasticity is not very obvious.
Oxygen effect
Oxygen has high melting degree in both alpha and beta phases of titanium and can form interstitial solid phase. The crystal wounds of titanium have been severely distorted, thereby increasing the hardness and strength of titanium and titanium alloys, resulting in a significant decrease in plasticity. In order to ensure the performance of welding joint, besides strictly preventing the main oxidation of weld seam and welding according to heat affected zone during welding process, oxygen content in basic metals and welding wires should also be limited.
Effect of nitrogen
At high temperatures above 700 °C, nitrogen and titanium have a violent effect. The lattice distortion caused by the formation of brittle titanium nitride (RIN) and interstitial solid solution between nitrogen and titanium is more serious than that caused by the amount of oxygen. Therefore, nitrogen improves the tensile strength and hardness of industrial pure titanium welds and reduces the plastic properties of welds more than oxygen.
Carbon effect
Carbon is also a common impurity in titanium and titanium alloys. Experiments show that when carbon content is 0.13%, the strength limit of weld seam increases and the plasticity decreases, but the effect of carbon is not as strong as that of oxygen and nitrogen. However, when the carbon content of the weld is further increased, the mesh TiC appears in the weld, and the amount increases with the increase of the carbon content, so that the plasticity of the weld is sharply decreased, and cracks are likely to occur under the welding stress. Therefore, the carbon content of the titanium and titanium alloy base material is not more than 0.1%, and the carbon content of the weld does not exceed the carbon content of the base material.
Weld joint crack problem
When welding titanium and titanium alloys, the possibility of hot cracks in welded joints is small. This is because the content of S, P, C and other impurities in titanium and titanium alloy is very small, and the low melting point eutectic formed by S and P is not easy to appear on the grain boundary. In addition, the effective crystallization temperature interval is narrow, and the shrinkage amount of titanium and titanium alloy is small when solidified, and the weld metal does not generate hot cracks. When titanium and titanium alloys are welded, cold cracks may occur in the heat-affected zone, which is characterized by cracks occurring several hours or more after welding as delayed cracks. The research shows that the crack is related to the diffusion of hydrogen in the welding process. During welding, hydrogen diffuses from high temperature deep pool to low temperature heat affected zone. The increase of hydrogen content increases the amount of TiH2 precipitated from the zone and increases the brittleness of heat affected zone. In addition, the volume expansion of hydride precipitation results in larger tissue stress, and the diffusion and accumulation of hydrogen atoms to the high stress areas in this region leads to the formation of cracks. The way to prevent such delayed cracks is mainly to reduce the source of hydrogen in the welded joints.
3. Porosity problems in the weld
When welding titanium and titanium alloys, pores are a common problem. The root cause of the formation of pores is the result of hydrogen influence. The formation of pore in weld metal mainly affects the fatigue strength of the joint. The main technological measures to prevent blowhole are as follows:
(1) Protecting neon should be pure and its purity should not be less than 99.99%.
(2) Thoroughly remove organic matter such as scale oil on the surface of the weldment and the surface of the wire.
(3) Apply good gas protection to the molten pool, control the flow rate and flow rate of argon gas to prevent turbulence and affect the protection effect.
(4) Correct selection of welding parameters and increase of deep pool dwell time are conducive to bubble escape, which can effectively reduce the porosity.
