Vacuum electron beam welding is an advanced high-energy welding technology that was only used in production in the 1950s. It uses convergent high-speed electrons to bombard the joints of workpieces. The kinetic energy of the electrons is converted into heat energy, which causes the bombarded material to heat up and melt. , To achieve the purpose of welding. In vacuum electron beam welding, the energy density is highly concentrated, the welding heat efficiency is high, the heat input is small, the weld seam and heat affected zone of the workpiece are narrow, the welding deformation is small, and the chemical composition of the welded joint is purified.
First, the necessity of applying electron beam welding technology in titanium alloy welding
In the application and development of titanium alloy structural parts, its structural connection problems are increasingly prominent. Among the various methods of connecting titanium and titanium, titanium and aluminum, titanium and magnesium, titanium and steel, welding is an important method for processing titanium alloys. It has unique advantages in improving material utilization, reducing structural weight, and reducing costs. Therefore, the research work on titanium alloy welding has been paid attention by welding workers at home and abroad. Titanium alloys can be welded by various methods such as electron beam welding, laser beam welding, TIG, MIG and friction welding under the conditions of good protection measures. Traditional welding techniques such as TIG welding, MIG welding, etc., because they have low equipment costs , Easy to operate and other characteristics, is still widely used in the manufacturing industry and occupy the mainstream position. However, they are subject to certain restrictions when welding special materials such as titanium alloys, because they have some shortcomings, such as low energy density; slow welding speed and low efficiency; poor controllability of welding parameters; weld depth-to-width ratio is small and thick Multi-layer and multi-pass welding is required for the plate workpiece; the heat affected zone of the weld is wide, and the weld performance is low; the deformation of the workpiece is large, and correction measures are required to reduce the deformation; the accuracy of the weld is poor, and important parts need to be re-machined. In addition, when welding titanium alloys by TIG, MIG, PAW, etc., due to the limitation of protective atmosphere and purity, the content of oxygen, nitrogen, and hydrogen in the welded joint will increase, and it is easy to form brittle phases or micropores in the weld area after welding , Reduce the plasticity and toughness of the welded joint. Electron beam welding is one of the most mature high-energy beam processing methods at present, and it has been widely used in the manufacture of welding structures in aviation and aerospace. Electron beam welding is usually carried out under vacuum conditions, with good protection conditions, high melting width ratio, small deformation of weldments, and high purity of welds. It is especially suitable for the welding of active metals such as titanium and titanium alloys, so it has a large Research and application potential.
Second, the study of dissimilar titanium alloy electron beam welding technology
(1) Working principle of electron beam welding
Electron beam welding technology is a kind of high-energy beam welding, which uses the heat energy generated when the converged high-speed electron beam bombards the surface of the workpiece to melt, cool and crystallize it for welding. The electron beam is generated by an electron gun in a high vacuum environment. The electron gun is generally composed of a cathode, a beam-forming electrode, and an anode. The cathode is heated to produce a thermal emission effect. The electrons emitted on the surface converge into an electron beam with a high energy density at an accelerating voltage and pass through an electromagnetic lens, and accelerate to fly to the workpiece. And evaporated metals. Under the action of high-pressure metal vapor, the molten metal is discharged, the electron beam penetrates into the workpiece and continues to hit the deep solid metal, and at the same time, a lock-shaped small hole is drilled in the welded workpiece, and the small hole is surrounded by liquid metal . As the depth of the small holes increases, the radius of the heat flow continues to decrease, and eventually a nail-shaped weld with a large aspect ratio is formed, which causes the material to heat up and melt to achieve the purpose of melting the weld metal.
(2) Technical characteristics of electron beam welding
Electron beam as a heat source for welding has two characteristics: First, high power density. When electron beam welding, the acceleration voltage range is 30-150kV, the electron beam current is 20-1000mA, the electron beam focal point diameter is 0.1-1mm, the power density of such electron beam can reach 106W / cm2 or more, which is higher than the high-power argon arc 2-4 orders of magnitude, at the same time, the special energy conversion mechanism of the electron beam has a very high energy conversion efficiency, so that not only can achieve high-speed welding, but also the total energy delivered to the weldment and the welding deformation caused by the welding is almost more than the conventional arc Welding is an order of magnitude smaller, and the thermal impact on the material is also quite small; second, accurate, fast and controllable. Because electrons have a very small mass and a certain negative charge, the electron beam can be quickly and accurately controlled by the electric and magnetic fields.
Third, the analysis of the effect of vacuum heat treatment of dissimilar titanium alloy electron beam welding
- (1) Effect of post-weld heat treatment on fatigue performance
Relevant personnel studied the effect of electron beam local heat treatment on the fatigue properties of 12mm thick Ti-6Al-4V titanium alloy electron beam welded joints. The results showed that the electron beam welded titanium alloy Ti-6Al-4V joints and the locally heat treated joints were in the fatigue test Have undergone softening, the elastic strain is higher than the plastic strain, and the fatigue life of the locally heat-treated joint has increased by 30%, so the local heat treatment of the electron beam can improve the performance of the electron beam welded joint; there are also some people who study the swing electron beam welding The impact of fatigue properties of Ti-6Al-4V titanium alloy welded joints. The experimental results show that the fatigue performance of swing electron beam welded joints is worse than that of unswinged electron beam welded joints. Observation of metallographic structure has found a wide distribution in swing welded joints At the same time, PWHT (post-weld heat treatment) at different temperatures (700 ° C, 900 ° C) was also performed on the welded joints. The experimental results showed that the 700 ° C PWHT samples had better fatigue performance, and the 700 ° C PWHT joints The number of α photo-like tissues in the tissue is less than that of 900 ° C PWHT, and it is much finer. Scanning electron microscopy observations showed that fatigue cracks originated from the weld surface, the crack propagation morphology was rough, and finally the fracture zone showed ductile micropore morphology.
- (2) Effect of heat treatment on mechanical properties of joints
TA15 titanium alloy electron beam welded joint tensile specimen and its corresponding temperature heat treatment, joint tensile specimens are all broken on the base material during the tensile test. Both the tensile strength and the yield strength are slightly higher than those of the unheated base material. The tensile strength is up to 700. The sample group corresponding to the annealing treatment has an average value of 933MPa, which is only 1.95% higher than the base material without heat treatment. The sample group corresponding to the highest strength of 600 has an average value of 883.9MPa, which is 3.23% higher than that of the base material. It can be seen that the annealing treatment has no obvious effect on the strength of the joint. The shrinkage and elongation of the two plastic indicators have changed to a certain extent. Compared with the base material without heat treatment, the elongation of the joint heat-treated samples of each group has decreased, 600 is the most, and the average value reaches 22.41 %, The smallest drop at 750, but also reached 7.64%. The shrinkage rate of the section has changed well, and it has been improved at 650 and 800, of which the ratio to the base material has increased by 4.35% at 650. The change of the heat treatment temperature did not change the relative strength of the welding seam, heat affected zone and base metal of the electron beam welding joint of TA15 titanium alloy. The strength of the weld seam and heat affected zone is still higher than that of the base metal. It is generally believed that TA15 titanium alloy is a near-α titanium alloy with high aluminum equivalent, and its strengthening mechanism is mainly solid solution strengthening of Al and other elements, which cannot be strengthened by heat treatment. While no welding materials are added during electron beam welding, the special welding process increases the alloy concentration of the material at the electron beam weld, that is, the solid solution strengthening effect of the alloy element on the titanium alloy part of the weld joint. Since the weld metal is annealed at the same time, the base material is also subjected to the same thermal cycle heat treatment, so that the strength of the base material is slightly increased after the heat treatment, but the relative strength of the weld, heat affected zone and base material does not change. The strength of the weld and heat affected zone must be greatly improved compared to the base material, but the effect of heat treatment at different temperatures on the strength of the weld and heat affected zone is not obvious.
In summary, the combined technology of electron beam welding and local heat treatment is an electron beam equipment, which completes two processes of welding and heat treatment in a vacuum cycle, and the scanning of local heat treatment passes an electron beam on the deflection coil circuit of the electron gun The scanning generator realizes, appropriately adjusting the scanning waveform, scanning amplitude, scanning frequency, relative movement speed of the electron gun and the workpiece of the electron beam, which can achieve the refinement of the structure, reduce the residual stress and hydrogen content in the weld and heat affected zone, and improve the fatigue performance And other purposes.