The welding properties of titanium and titanium alloys have many salient features. These welding properties depend on the physical and chemical properties of titanium and titanium alloys. Mixing of gas and impurities, impact on welding performance.
TC4 Titanium Alloy Precision Welding
Currently, TIG welding or plasma arc welding is used for welding with TC4 titanium alloys. However, both methods require filling of the solder material. Due to the limitation, purity and effect of the shield gas, the oxygen content of the joint increases, the strength decreases and the deformation after welding is large. We studied the welding process performance of TC4 titanium alloy by electron beam welding and laser beam welding, and realized precision welding of materials.
The TC4 titanium alloy welding process has the following features.
(1) Tendency to dissolve the pores.
Weld voids are the most common defect in titanium alloy welding. Hydrogen and oxygen present in the arc region of the metal to be welded are the main causes of pores. Electron beam welding of TC4 titanium alloy has almost no void defects in the weld. Therefore, this study focuses on the process factors of hole formation in laser welding.
From the test results, it can be seen that during laser welding, the holes in the weld are closely related to the weld line energy. If the energy of the weld line is moderate, there are only a few holes or voids in the weld. If the line energy is too high or too low, severe void defects can occur in the weld. Furthermore, the presence or absence of void defects in the weld is related to the wall thickness of the weld. Comparing the test results of the samples, it can be seen that as the thickness of the welded wall increases, the likelihood of pores in the weld increases.
(2) Internal quality of welding.
The internal quality of the weld was confirmed using flat weld butt samples, electron beam welding and laser welding. After inspection, the internal weld quality of the weld is checked by X-ray to meet the requirements of GB3233-87 Class II. There are no cracks on the weld surface and inside. The weld has a good appearance and normal color.
(3) Solder depth and its fluctuation.
Titanium alloys are used as engineering parts and have certain requirements for welding depth, otherwise the strength requirements of the part cannot be met. For this purpose, two pairs of butt test rings were individually soldered by electron beam welding and laser welding.
After welding, vertical and horizontal dissections were performed on the test ring to examine the weld depth and variations in weld depth.
As a result, it was found that the average welding depth of electron beam welding was 2.70 mm or more, and the fluctuation range of the welding depth was -5.2% to + 6.0% and did not exceed ± 10%.
The average welding depth of laser welding is about 2.70 mm, and the welding depth varies from -3.8 to + 5.9% and does not exceed ± 10%.
(4) Deformation analysis.
Weld deformation of the joint was detected by the butt test ring, and the radial and axial deformation of the butt ring was examined. As a result, the electron beam welding and laser welding deformation were small. In electron beam welding, the contraction deformation in the radial direction is f 0.05 to f 0.09 mm, and the contraction in the axial direction is 0.06 to 0.14 mm. The amount of radial shrinkage deformation of laser welding is f 0.03 to f 0.10 mm, and the amount of axial shrinkage deformation is 0.02 to 0.03 mm.
Electron beam welding
(5) Analysis of welded structure.
After chemical testing, the weld microstructure is + b and the microstructure is columnar and equiaxed. A small amount of lath martensite appears and the grain size is near the matrix. The heat-affected zone is relatively narrow and the tissue morphology and properties are ideal.
The study can be drawn:
For TC4 titanium alloys, whether laser welding or electron beam welding, the internal quality of the weld will match the national standard GB3233-87II welding conditions as long as the process parameters match properly. Precision welding of TC4 titanium alloy, excellent weld appearance and normal color, very small residual weld height, no defects such as undercuts, dents, surface cracks.
Impact of different gases on titanium welding
Titanium and titanium alloys are relatively stable at room temperature. However, during the test, the droplets and pool metal had strong absorption of hydrogen, oxygen and nitrogen during the soldering process. These gases react with them in the solid state. As the temperature rises, 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. Absorption of these gases directly leads to embrittlement of welded joints, which is a very important factor affecting weld quality.
(1) Effect of hydrogen
Hydrogen is the most serious factor affecting the mechanical properties of titanium in gaseous impurities. Changes in the hydrogen content of the weld have the greatest impact on the impact performance of the weld. This is mainly due to the increased hydrogen content of the junction. There are more flaky or needle-like TiH2 deposits in the weld. Since the strength of TiH2 is very low, the effect of sheets and needles such as HiH2 is a notch, and the impact performance of the weld is significantly reduced. Changes in the hydrogen content of the weld do not significantly affect the increase in strength and the decrease in ductility.
(2) Effect of oxygen
Oxygen has high solubility in both the alpha phase and beta thickness of titanium and can form a gap solid phase. Titanium crystal wounds are severely distorted, which increases the hardness and strength of titanium and titanium alloys, which significantly reduces plasticity. In order to ensure the performance of welded joints, it is necessary to limit the oxygen content of the base metal and wire, in addition to strictly preventing welding and the occurrence of welds in heat-affected zones during the welding process.
(3) Effect of nitrogen
At high temperatures of 700 ° C, nitrogen and titanium play a role in the formation of brittle titanium nitride (RIN). Further, when nitrogen and titanium form a gap solid solution, the degree of lattice strain due to nitrogen is more serious than the degree of lattice strain due to oxygen. Therefore, nitrogen improves the tensile strength, hardness and ductility of industrial pure titanium welds, which is more important than oxygen.
(4) Effect of carbon
Carbon is also a common impurity in titanium and titanium alloys. Experiments have shown that when the carbon content is 0.13%, the carbon content of α-titanium is deeper, the weld strength limit is improved, the plasticity is slightly reduced, but not as strong as oxygen and nitrogen. Is done. However, if the carbon content of the weld is further increased, the mesh of the weld becomes TiC, and the TiC content increases as the carbon content increases. As a result, the plasticity of the welded portion is significantly reduced, and cracks may occur due to the influence of welding stress. Therefore, the base metals of titanium and titanium alloys have a carbon content of 0.1% or less, and the welded carbon content does not exceed the carbon content of the base metal.
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