Titanium Alloy How To Weld? And Precautions
The welding performance of titanium and titanium alloys has many remarkable features. These welding characteristics are determined by the physical and chemical properties of titanium and titanium alloys. Gas and impurity contamination, the impact on the welding performance.
Currently, for TC4 titanium alloys, TIG welding or plasma arc welding are used for welding. However, both methods need to be filled with welding materials. Due to the limitation of the protection gas, purity and effect, the oxygen content of the joint increases, the strength decreases, and the post-weld deformation is large. Using electron beam welding and laser beam welding, the welding process properties of TC4 titanium alloy were studied and precision welding of the material was realized.
TC4 titanium alloy welding process has the following characteristics:
(1) Weld porosity tends.
The pores in the weld are the most common defects in the welding of titanium alloys. Hydrogen and oxygen present in the arc region of the metal being welded are the main cause of pore generation. The electron beam welding of TC4 titanium alloy has few porosity defects in the weld. For this reason, the research focuses on the process factors for the formation of pores in laser welding welds.
From the test results, it can be seen that when laser welding, the pores in the weld have a close relationship with the weld line energy. If the weld line energy is moderate, there will be only a small amount of pores or even no pores in the weld. Excessive or too small line energy will cause severe pore defects in the weld. In addition, whether or not there is a porosity defect in the weld is also related to the wall thickness of the weldment. Comparing the test results of the sample, it can be seen that as the thickness of the welded wall increases, the probability of occurrence of blowholes in the weld increases.
(2) Weld internal quality.
Using flat butt welding test samples, electron beam welding and laser welding to examine the internal quality of the weld, After inspection, the internal weld quality of the weld was examined by X-rays to meet the GB3233-87 Level II requirements. There were no cracks on the surface and inside of the weld. The weld appearance was well formed and the color was normal.
(3) Solder depth and its fluctuations.
Titanium alloys are used as engineering components and have certain requirements for welding depth, otherwise they cannot meet the requirements of component strength; In order to achieve precise welding, it is necessary to control the welding depth fluctuation. For this purpose, two pairs of docking test rings were respectively welded by electron beam welding and laser welding.
After welding, vertical and horizontal dissections were performed on the test rings to investigate the depth of weld and fluctuations in weld depth.
The results show that the average welding depth of the electron beam welding weld can reach more than 2.70mm, and the welding depth fluctuation range is -5.2% to +6.0%, and does not exceed ±10%.
The average weld depth of laser welding welds is approximately 2.70 mm, and the weld depth fluctuation range is -3.8 to +5.9%, not exceeding ±10%.
(4) Analysis of Deformation joints.
Using butt-joint test rings to examine the welding deformation of the joints, the radial and axial deformations of the docking rings were examined, The results show that both electron beam welding and laser welding deformation are small. The electron beam welding has a radial shrinkage deformation of f 0.05 to f 0.09 mm and an axial shrinkage of 0.06 to 0.14 mm. The radial contraction deformation of laser welding is f 0.03 to f 0.10 mm, and the axial shrinkage deformation is 0.02 to 0.03 mm.
(5) Analysis of Weld organization.
After chemical detection, the microstructure of the weld is a+b, and the microstructure is columnar and equiaxed. A small amount of lath martensite appears, and the grain size is close to the matrix. The heat affected zone is relatively narrow, and the organization form and characteristics are ideal.
The study can be drawn:
For TC4 titanium alloy, whether it is laser welding or electron beam welding, as long as the process parameters are matched properly, the internal quality of the weld can meet the national standard GB3233-87II weld requirements. Realize the precision welding of TC4 titanium alloy; The weld appearance is well-formed and the color is normal; The remaining weld height is very small, and there are no defects such as undercuts, dents, and surface cracks.
Influence of different gases on titanium welding
At room temperature, titanium and titanium alloys are relatively stable. However, during the test, liquid droplets and pool metal have a strong absorption of hydrogen, oxygen, and nitrogen during the welding process. And in the solid state, these gases have reacted with them. With the increase of temperature, the ability of titanium and titanium alloys to absorb hydrogen, oxygen, and nitrogen also increases significantly. It begins to absorb hydrogen at about 250°C, absorbs oxygen at 400°C, and absorbs nitrogen at 600°C. After these gases are absorbed, they will directly cause embrittlement of the welded joints, which is an extremely important factor affecting the quality of welding.
(1) Effect of hydrogen
Hydrogen is the most serious factor that affects the mechanical properties of titanium in gaseous impurities. The change in the hydrogen content of the welds has the most significant effect on the weld impact performance, which is mainly due to the increase in hydrogen content in the joints. There are more tabular or acicular TiH2 precipitates in the weld. The strength of TiH2 is very low, so the effect of sheet or needle like HiH2 is notch, and the impact property of weld is significantly reduced. The change in the amount of hydrogen in the welds does not have a significant effect on the increase in strength and the decrease in ductility.
(2) Effect of oxygen
Oxygen has a high degree of melting in both α-phase and β-thickness of titanium, and can form interstitial solid phase. Titanium crystal wounds are severely distorted, thereby increasing the hardness and strength of titanium and titanium alloys, so that the plasticity is significantly reduced. In order to ensure the performance of the welded joints, in addition to strictly preventing the welds and welds from undergoing oxidation in the heat affected zone during the welding process, the oxygen content in the base metal and the welding wire should also be limited.
(3) Effect of Nitrogen
At a high temperature of 700 deg.] C, the nitrogen and titanium occurs play role in the formation of brittle titanium nitride (RIN). Moreover, when nitrogen and titanium form a gap solid solution, the degree of lattice distortion caused by nitrogen is more serious than that caused by oxygen. Therefore, nitrogen increased the tensile strength, hardness and weld ductility of industrial pure titanium weld more significantly than oxygen.
(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 content is deeper in the α titanium, the weld strength limit is somewhat improved, and the plasticity is somewhat decreased, but it is not as strong as the oxygen and nitrogen. However, when the carbon content of the welds is further increased, the mesh of the welds appears TiC, and the amount of the TiC increases with the increase of the carbon content. As a result, the plasticity of the weld decreases sharply, and cracks are likely to occur under the effect of welding stress. Therefore, the carbon content of the titanium and titanium alloy base metal is not more than 0.1%, and the carbon content of the weld does not exceed the carbon content of the base metal.
Currently, for TC4 titanium alloys, TIG welding or plasma arc welding are used for welding. However, both methods need to be filled with welding materials. Due to the limitation of the protection gas, purity and effect, the oxygen content of the joint increases, the strength decreases, and the post-weld deformation is large. Using electron beam welding and laser beam welding, the welding process properties of TC4 titanium alloy were studied and precision welding of the material was realized.
TC4 titanium alloy welding process has the following characteristics:
(1) Weld porosity tends.
The pores in the weld are the most common defects in the welding of titanium alloys. Hydrogen and oxygen present in the arc region of the metal being welded are the main cause of pore generation. The electron beam welding of TC4 titanium alloy has few porosity defects in the weld. For this reason, the research focuses on the process factors for the formation of pores in laser welding welds.
From the test results, it can be seen that when laser welding, the pores in the weld have a close relationship with the weld line energy. If the weld line energy is moderate, there will be only a small amount of pores or even no pores in the weld. Excessive or too small line energy will cause severe pore defects in the weld. In addition, whether or not there is a porosity defect in the weld is also related to the wall thickness of the weldment. Comparing the test results of the sample, it can be seen that as the thickness of the welded wall increases, the probability of occurrence of blowholes in the weld increases.
(2) Weld internal quality.
Using flat butt welding test samples, electron beam welding and laser welding to examine the internal quality of the weld, After inspection, the internal weld quality of the weld was examined by X-rays to meet the GB3233-87 Level II requirements. There were no cracks on the surface and inside of the weld. The weld appearance was well formed and the color was normal.
(3) Solder depth and its fluctuations.
Titanium alloys are used as engineering components and have certain requirements for welding depth, otherwise they cannot meet the requirements of component strength; In order to achieve precise welding, it is necessary to control the welding depth fluctuation. For this purpose, two pairs of docking test rings were respectively welded by electron beam welding and laser welding.
After welding, vertical and horizontal dissections were performed on the test rings to investigate the depth of weld and fluctuations in weld depth.
The results show that the average welding depth of the electron beam welding weld can reach more than 2.70mm, and the welding depth fluctuation range is -5.2% to +6.0%, and does not exceed ±10%.
The average weld depth of laser welding welds is approximately 2.70 mm, and the weld depth fluctuation range is -3.8 to +5.9%, not exceeding ±10%.
(4) Analysis of Deformation joints.
Using butt-joint test rings to examine the welding deformation of the joints, the radial and axial deformations of the docking rings were examined, The results show that both electron beam welding and laser welding deformation are small. The electron beam welding has a radial shrinkage deformation of f 0.05 to f 0.09 mm and an axial shrinkage of 0.06 to 0.14 mm. The radial contraction deformation of laser welding is f 0.03 to f 0.10 mm, and the axial shrinkage deformation is 0.02 to 0.03 mm.
(5) Analysis of Weld organization.
After chemical detection, the microstructure of the weld is a+b, and the microstructure is columnar and equiaxed. A small amount of lath martensite appears, and the grain size is close to the matrix. The heat affected zone is relatively narrow, and the organization form and characteristics are ideal.
The study can be drawn:
For TC4 titanium alloy, whether it is laser welding or electron beam welding, as long as the process parameters are matched properly, the internal quality of the weld can meet the national standard GB3233-87II weld requirements. Realize the precision welding of TC4 titanium alloy; The weld appearance is well-formed and the color is normal; The remaining weld height is very small, and there are no defects such as undercuts, dents, and surface cracks.
Influence of different gases on titanium welding
At room temperature, titanium and titanium alloys are relatively stable. However, during the test, liquid droplets and pool metal have a strong absorption of hydrogen, oxygen, and nitrogen during the welding process. And in the solid state, these gases have reacted with them. With the increase of temperature, the ability of titanium and titanium alloys to absorb hydrogen, oxygen, and nitrogen also increases significantly. It begins to absorb hydrogen at about 250°C, absorbs oxygen at 400°C, and absorbs nitrogen at 600°C. After these gases are absorbed, they will directly cause embrittlement of the welded joints, which is an extremely important factor affecting the quality of welding.
(1) Effect of hydrogen
Hydrogen is the most serious factor that affects the mechanical properties of titanium in gaseous impurities. The change in the hydrogen content of the welds has the most significant effect on the weld impact performance, which is mainly due to the increase in hydrogen content in the joints. There are more tabular or acicular TiH2 precipitates in the weld. The strength of TiH2 is very low, so the effect of sheet or needle like HiH2 is notch, and the impact property of weld is significantly reduced. The change in the amount of hydrogen in the welds does not have a significant effect on the increase in strength and the decrease in ductility.
(2) Effect of oxygen
Oxygen has a high degree of melting in both α-phase and β-thickness of titanium, and can form interstitial solid phase. Titanium crystal wounds are severely distorted, thereby increasing the hardness and strength of titanium and titanium alloys, so that the plasticity is significantly reduced. In order to ensure the performance of the welded joints, in addition to strictly preventing the welds and welds from undergoing oxidation in the heat affected zone during the welding process, the oxygen content in the base metal and the welding wire should also be limited.
(3) Effect of Nitrogen
At a high temperature of 700 deg.] C, the nitrogen and titanium occurs play role in the formation of brittle titanium nitride (RIN). Moreover, when nitrogen and titanium form a gap solid solution, the degree of lattice distortion caused by nitrogen is more serious than that caused by oxygen. Therefore, nitrogen increased the tensile strength, hardness and weld ductility of industrial pure titanium weld more significantly than oxygen.
(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 content is deeper in the α titanium, the weld strength limit is somewhat improved, and the plasticity is somewhat decreased, but it is not as strong as the oxygen and nitrogen. However, when the carbon content of the welds is further increased, the mesh of the welds appears TiC, and the amount of the TiC increases with the increase of the carbon content. As a result, the plasticity of the weld decreases sharply, and cracks are likely to occur under the effect of welding stress. Therefore, the carbon content of the titanium and titanium alloy base metal is not more than 0.1%, and the carbon content of the weld does not exceed the carbon content of the base metal.
