Summary
Titanium alloy is widely used in aerospace, marine equipment and other industries due to its high specific strength, excellent corrosion resistance and high temperature performance.
In recent years, thick-walled titanium alloy welding technology has gained significant application value due to the increasing demand for such alloys. Therefore, this article aims to summarize the progress in fusion welding technology for thick-walled titanium alloy materials. It mainly includes meltless electrode gas shielded welding, electron beam welding and laser welding. In addition, this article also gives insight into the development trend of thick-walled titanium alloy welding technology.
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Preface
Titanium alloy is characterized by low density, high specific strength, specific stiffness, excellent corrosion resistance and good processability. It is a new functional material with vast development potential and promising application prospects. Known as the “third metal” after steel and aluminum, it is an essential strategic metallic material, widely used in aerospace, petrochemical, national defense equipment and other fields.
In recent years, with the increasing demand for lightweight and large equipment in the national defense industry, the need for thick-walled titanium alloys has become more urgent, along with its corresponding processing technology.
In practical engineering applications, welding is the main method used to connect thick-walled titanium alloy structures, making efficient and high-quality thick-walled titanium alloy welding technology essential and attracting considerable attention.
This article summarizes the research status of thick-walled titanium alloy fusion welding technology, identifies the existing problems with thick-walled titanium alloy fusion welding, and explores the development prospects and research directions of thick-walled titanium alloy fusion welding technology. fusion welding of thick-walled titanium alloys.
1. Classification and characteristics of titanium alloys
1.1 Classification of titanium alloys
Titanium alloys can be classified into five categories based on their chemical composition and content: α-titanium alloy, α-proximate titanium alloy (with a β-phase mass fraction of ≤10%), dual-phase titanium alloy α-β (with a phase mass β fraction of 10% ≤ β ≤ 50%), metastable β titanium alloy and β titanium alloy.
α-β two-phase titanium alloy is widely used due to its excellent comprehensive properties. It combines the thermal stability characteristics of α-type titanium alloy with the heat treatment strengthening characteristics of β-type titanium alloy.
1.2 Titanium alloy material characteristics
(1) High specific resistance.
Titanium alloy is a light alloy with a density of 4.54 g/cm3 at 20℃, which is about 56% of the density of ordinary steel. Using titanium alloy to manufacture mechanical parts can significantly reduce weight and achieve a light weight effect.
(2) Good corrosion resistance.
Titanium alloy forms a stable, continuous and dense oxide film on the surface when exposed to air, which makes it passive. Furthermore, the titanium alloy oxide film has excellent repair performance. In case of damage caused by external factors, it can be readily restored, endowing the titanium alloy with remarkable corrosion resistance.
(3) High temperature performance.
The melting point of titanium alloy is 1667 ℃, which can work stably in the environment of 500 ~ 600 ℃ and has high creep and heat resistance.
1.3 Welding characteristics of thick-walled titanium alloy
(1) Embrittlement of the welded joint:
Without adequate protection, the heating temperature of the titanium alloy can trigger several chemical reactions. Hydrogen absorption begins at 250℃, oxygen absorption begins at 400℃, severe oxidation occurs at 540℃, and nitrogen absorption begins at 600℃.
These gases dissolve in the molten pool during welding and undergo chemical reactions, which can cause weakening of the welded joint. As a result, the plasticity and toughness of the welded joint decrease rapidly. Therefore, it is crucial to protect the welding process to prevent such reactions from occurring.
(2) Welding cracks:
Titanium alloys have low levels of impurities, including S, P, C and other contaminants. They also contain fewer low-melting eutectic compounds and have a narrow crystallization temperature range, making them less susceptible to hot cracking.
However, when welding thick-walled titanium alloys using multilayer and multipass welding techniques, the welded joint is subjected to high levels of restraint stress, resulting in significant residual stress in the joint. Under the influence of this residual stress, cold cracks are easily formed.
(3) Porosity:
Porosity is a common defect that can occur when welding titanium alloys. This is due to the high saturation vapor pressure and active elements present in the titanium alloy. Hydrogen porosity can occur when the surface of the base metal and welding material is contaminated or when the shielding gas contains impurities such as oxygen, hydrogen or water.
2. Research status of gas shielded welding with non-consumable electrode
2.1 Traditional TIG welding
Gas shielded non-consumable TIG welding is widely used in the field of titanium alloys due to its benefits such as stable arc, less welding spatter and good weld formation. However, the traditional TIG welding process for titanium alloys results in a longer high-temperature residence time of the weld joint and faster cooling of the liquid metal from the molten pool.
This is due to the low thermal conductivity of titanium alloys, which leads to a notable tendency for grains to coarsen in the weld zone and heat-affected zone.
Furthermore, the large groove size requires multi-layer and multi-pass welding, resulting in low welding efficiency, excessive stress and deformation. To reduce the grain coarsening tendency, Lu Xin employed TIG welding to achieve multi-layer and multi-pass welding of 20mm thick TC4 titanium alloy with a groove angle of 60°. Figure 1 shows the microstructures of the welded joints under different heat inputs.
As the welding heat input decreases, the size of the martensite within the grains becomes smaller and more uniform, and the weld grains become progressively finer. Therefore, strict control of welding heat input is necessary when using TIG welding to weld thick TC4 titanium alloy plates to avoid coarse grains at the joints and prevent the occurrence of abnormal structures, cracks and other defects.
Fig.1 Microstructure of the weld zone under different heat inputs
Yang Lu et al. employed an X-shaped groove and alternating front and back welding to perform multilayer TIG welding on 24 mm thick TC4 titanium alloy, aiming to minimize residual stresses and deformation in welded joints.
Simultaneously, using the SYSWELD platform, the researchers carried out numerical simulations of the temperature field, stress field and welding deformation in welded joints. The simulations were carried out under the assumption of fully rigid fixation at both ends of the welding plate, as illustrated in Figure 2.
The results indicated that the use of the alternating two-side welding sequence could significantly reduce the stress and deformation in the welded joint.
Fig.2 TEM morphological profile of the residual stress thickness of the welding joint
In summary, although traditional TIG welding is suitable for welding thick-walled titanium alloys, grain size, joint stress and deformation can be minimized by reducing welding heat input appropriately and utilizing a groove-shaped of X for bilateral alternating welding.
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However, there is still a problem with large grooves leading to low welding efficiency, making it difficult to popularize the technique in welding thick-walled titanium alloys.
2.2 Narrow gap TIG welding
The slot size for narrow gap welding is small, resulting in a significant reduction in volume when compared to traditional slot fill welds. This reduction not only increases welding efficiency but also reduces production costs.
Narrow gap TIG welding is a flexible process that features relatively low equipment costs and a stable welding process. Additionally, narrow slots can reduce the number of welding passes, which in turn improves welding deformation and allows better control of welding stress.
As a result, close gap TIG welding for thick-walled titanium alloys offers significant advantages.
However, the small slot gap in narrow gap TIG welding can result in the arc “climbing” along the sidewall, leading to insufficient heat input at the bottom corner of both sides of the weld bead and poor fusion. of the side wall.
Currently, narrow gap TIG welding technology for thick-walled titanium alloys generally employs mechanical oscillation and external magnetic fields to regulate the arc. These methods effectively solve the problem of poor fusion of side walls with narrow gaps.
2.2.1 TIG welding with mechanical oscillation narrow gap
The principle of mechanical close oscillation TIG welding is as follows: during the welding process, the tungsten electrode moves back and forth within the groove by rotating the tungsten electrode clamp, which causes the arc to point periodically to the side walls of the groove, ensuring the fusion quality of the side walls.
The welding process is represented in Figure 3.
The mechanical oscillation narrow gap welding mode is highly adaptable to changes in the width of the welding groove. Minimizes the occurrence of non-fusion defects on sidewalls during welding, resulting in more stable welding quality. This technique has gained wide use in close-gap TIG welding of thick-walled titanium alloys.
Fig.3 Schematic diagram of TIG welding process with close and mechanical oscillation
Jiang Yongchun used TIG welding technique with close and mechanical oscillation to obtain a high-quality connection of TC4 titanium alloy with a thickness of 52 mm. This was possible through the selection of appropriate welding parameters and welding protection measures.
Figure 4 illustrates the macro metallography and microstructure of the welded joint. Due to the rapid cooling rate, α' martensite is formed in the heat-affected zone. However, the weld strength reaches 90% of the base metal, and the hardness of the fusion zone has the maximum value.
Fig.4 Macroscopic metallography and microstructure of the welded joint
Li Shuang et al. used narrow gap mechanical swing TIG welding technology to achieve 30mm thick TC4 titanium alloy single layer filler wire welding and analyzed the microstructure of the welded joint.
The results revealed that the grains in the weld zone were significantly coarse, predominantly coarse columnar grains, and their microstructures consisted of acicular α'martensite, which were dispersed parallel in the grains of the β phase.
The heat-affected zone adjacent to the weld side exhibited a more significant degree of grain coarsening than the base metal side.
In conclusion, TIG welding technology with close oscillation and mechanical oscillation has a stable welding process and low equipment cost.
The periodic rocking of the tungsten electrode effectively solved the problem of inadequate melting of the thick-walled titanium alloy sidewall.
However, due to substantial heat input, the joint demonstrated an apparent tendency for grain coarsening.
2.2.2 TIG welding with magnetically controlled narrow gap
The concept of magnetically controlled narrow gap TIG welding technology was first introduced by the Barton Welding Technology Research Institute of Ukraine. In recent years, Guangdong Welding Technology Research Institute has conducted fundamental research and promoted the industrial application of this technology for thick-walled titanium alloys.
Figure 5 illustrates the diagram of the welding process and the arc oscillation of TIG welding with magnetically controlled narrow opening. During the welding process, the electromagnetic coil is connected to alternating current and the silicon steel sheet passing through the coil becomes a magnet.
The magnetic induction line then passes through the electrode and arc, resulting in a periodic oscillation of the arc toward the two side walls. This makes it easier to fuse sidewalls with narrow gaps, making TIG welding with narrow gaps possible.
Fig.5 Schematic illustration of external transverse magnetic field and arc oscillation
Academicians from all over the world have conducted extensive research on the impact of magnetic field intensity, magnetic field frequency and electrode position on sidewall melting, weld formation and crystallization process in order to achieve high-quality welding of TIG welding with narrow magnetic control range.
Kshirsagar R et al. investigated the impact of an external magnetic field on weld formation, as illustrated in Fig.
The findings indicate that there is a significant lack of sidewall fusion when there is no external magnetic field. However, sidewall fusion is satisfactory when an external magnetic field is present.
Fig.6 Effect of external transverse magnetic field on the configuration and microstructure of the welding seam
(a) No external magnetic field
(b) With external magnetic field
A study conducted by Hua Aibing et al. examined the impact of external magnetic field strength on narrow gap weld sidewall fusion. The results indicate that a magnetic field intensity ≥ 4 mT can effectively improve sidewall fusion, resulting in relatively uniform weld fusion.
Another study by Chang Yunlong et al. investigated the effect of external magnetic field frequency on sidewall melting. The results showed that as the magnetic field frequency increased, the weld bottom penetration depth and arc impact depth also increased, while the weld penetration width and sidewall penetration decreased.
Yu Chen et al. conducted a study on the influence of electrode position on lateral wall fusion. The results revealed that when the tungsten electrode was displaced from the central position, the current flow intensity of the near sidewall increased, while the current flow intensity of the opposite sidewall decreased. To avoid uneven sidewall penetration and poor sidewall fusion, strict control of the electrode position is necessary.
Sun Jie et al. carried out a study on the influence of electromagnetic force on the crystallization process. Figure 7 illustrates the primary crystallization of the titanium alloy weld under the action of the magnetic field.
The results indicate that the electromagnetic effect can increase the stability of the planar crystallization front area and the subsequently formed equiaxed crystals.
As the magnetic field intensity increases, the microstructure near the fusion line gradually changes from columnar crystal to equiaxed crystal. The controlled magnetic arc significantly improves the stability of the equiaxed crystal generated at the center of the weld. Furthermore, the equiaxed crystal grows in a single direction with an increase in magnetic field intensity.
In another study, Hu Jinliang et al. used narrow gap TIG welding technology with magnetic control to weld 120 mm thick TA17 titanium alloy, and Fig. 8 shows the microstructure of the welded joint. The results indicate that the joint microstructure exhibits significant inhomogeneity along the transverse direction, while no significant differences appear along the thickness direction. Due to the large welding heat input, the fusion zone is seriously softened.
Fig.7 Primary crystallization process of titanium alloy weld metal under magnetic field
Fig.8 Microstructure of 120 mm thick TA17 titanium alloy joint welded by magnetically controlled NG-TIG welding seam
In summary, narrow gap TIG welding technology with magnetic control offers a stable welding process at a lower equipment cost. By adding a magnetic field, the technology allows periodic oscillation of the arc, which effectively solves the problem of poor fusion associated with thick-walled titanium alloy sidewalls and results in a uniform structure of the weld zone.
However, the technology still faces a significant challenge in smoothing the fusion zone of welded joints due to the high heat input. Narrow gap TIG welding, on the other hand, can achieve stable welding of thick-walled titanium alloy. This technology reduces the number of welding passes and improves welding efficiency compared to traditional TIG welding.
However, tight gap TIG welding also has its problems. Due to repeated remelting and heating of jointed grains, it causes problems such as coarse grains and uneven distribution of microstructure and properties along the thickness direction.
2.3 Submerged arc welding
Submerged arc welding is a distinct form of welding, separate from TIG welding.
This method uses helium as the shielding gas, and the electrode diameter and welding current are large.
Through a combination of helium and arc force, it is able to drain liquid metal from the molten pool at the weld position.
The electrode submerges into the base metal to be welded, and the arc burns at the electrode and in the cavity formed at the bottom of the crater, ultimately resulting in the formation of the weld pool.
Because the burning position of the arc is below the surface of the base metal, this is called submerged arc welding.
The principle of submerged arc welding can be seen in Figure 9.
Fig.9 Schematic graph of the SAW principle
In recent years, scholars have conducted research on the application of submerged arc welding technology to large-thickness titanium alloys.
Chen Guoqing and colleagues performed a butt test using submerged arc welding on a 29 mm thick TA15 titanium alloy and obtained well-formed welds.
However, due to the high heat input, the weld zone and heat-affected zone of the welded joint are relatively wide, and the elongation of the joint after fracture is only 50% of that of the base metal.
The bending property of the welded joint is weak and it breaks when bending at 15°.
Liu Yanmei and others carried out welding of a 58mm thick TA15 titanium alloy using a double-sided submerged arc welding process.
The weld macrosection is shown in Fig. 10. The weld zone has columnar crystals with large grain size, and the intragranular is acicular α'martensite.
The location of the tensile fracture of the joint is the weld zone, which is a ductile fracture.
Tensile strength reaches 96% of the strength of the base metal.
To improve the mechanical properties of submerged arc welding joint, Duqiang et al. conducted submerged arc welding of a 64 mm thick TA15 titanium alloy plate with the addition of TA1 pure titanium intermediate layer.
The results showed that the hydrogen, oxygen and nitrogen contents in the weld, after the addition of the interlayer, were reduced compared to the base metal, and the plasticity of the welded joint was significantly improved.
Hou Qi et al. studied the effect of shielding gas purity on the performance of a TA15 titanium alloy plate submerged arc welding joint.
The results showed that the mechanical properties of the welded joint could be improved to a certain extent by increasing the purity of the shielding gas.
Fig.10 Macroscopic cross section of the weld
In summary, submerged arc welding is capable of welding thick-walled titanium alloys with a relatively stable arc shape, resulting in better weld formation. Helium is typically used for coaxial shielding in submerged arc welding due to its high ionization potential and high thermal conductivity when compared to argon.
As a result, the arc column area in submerged arc welding is narrow and concentrated, leading to a high utilization rate of arc heat. This welding technique can perform bilateral welding of thick titanium alloys, significantly improving welding efficiency when compared to narrow gap TIG welding.
However, there are some problems associated with this method, such as excessive heat input, coarse grain structure, and uneven distribution of microstructure and properties in the thickness direction.
2.4 Summary
Non-consumable inert gas arc welding is capable of welding thick titanium alloys with a relatively stable arc shape, resulting in better weld formation. This technique demonstrates high application value in welding research of thick titanium alloys.
However, there are still problems such as the softening of joints caused by a high welding heat input. Therefore, it is crucial to carry out research on reducing heat input during welding of titanium alloys into thick plates. This can improve the homogeneity of structure and properties of non-MIG welded thick-walled titanium alloys.
3. Research status on electron beam welding
Electron beam welding technology uses high energy density electron beams to bombard metallic materials, enabling one-sided welding and two-sided forming of thick metallic materials.
During the welding process, the beam power density is high, resulting in a large weld depth-to-width ratio and minimal welding deformation.
In addition, electron beam welding must be carried out in a vacuum environment, which effectively avoids the negative effects of hydrogen, oxygen and nitrogen during the welding process. As a result, electron beam welding is commonly used to weld large thickness titanium alloys.
Figure 11 shows the electron beam welding device.
Fig.11 Schematic of electron beam welding
3.1 Joint structure and performance
Domestic and foreign scholars have studied the microstructure and properties of vacuum electron beam welded joints of titanium alloys.
Hou Jiangtao used electron beam welding technology to weld 20mm thick TC4 titanium alloy, analyzed the grain size of the weld zone and the mechanical properties of the joint along the thickness direction.
The results revealed that the upper part of the weld zone had a grain size of 1200 µm, while the lower part had a grain size of 200 µm, leading to differences in properties.
Sun et al. also used electron beam welding technology to weld 20 mm thick TC4 titanium alloy and analyzed the macromorphology of the welded joint (see Fig. 12).
The fusion zone and the heat-affected zone in the upper, middle and lower areas of the welded joint showed significantly different widths, as well as differences in the morphology and size of the grain structure, which decreased along the depth direction.
Wei Lu et al. Welded 50mm thick TC4 titanium alloy plates using electron beam welding technology and carried out mechanical property tests along the thickness direction. The results revealed that the mechanical properties were unevenly distributed along the welding depth.
The yield strength, tensile strength and microhardness of the welded joint improved compared to those of the base metal, while the plasticity and toughness decreased.
Finally, Song Qingjun used electron beam welding technology to weld TC4 titanium alloy with a thickness of 60mm and analyzed the microstructure and properties of the welded joint. The results showed that the microstructure of the welded joint was unevenly distributed along the thickness direction, and the impact toughness gradually decreased from the top to the bottom of the weld.
Fig.12 Macroscopic appearance of the welded joint
In summary, during electron beam welding of thick-walled titanium alloys, the weld metal undergoes a rapid thermal cycling process, which results in uneven distribution of microstructure and properties in different areas along the thickness direction due to to inconsistent residence times at high temperatures.
To solve the problems of non-uniform distribution of microstructure and properties and low mechanical properties in electron beam welded joints of thick-walled titanium alloy, relevant researchers have optimized the welding process and conducted post-welding heat treatment to adjust the microstructure and the properties of the joints.
Gong Yubing et al. conducted an extensive study on the non-uniformity of the electron beam welded joint of 20 mm thick TC4 titanium alloy and the evolution of the structure. Figure 13 shows the microstructures of different areas of the welded joint.
The results indicate that the titanium alloy welded joint exhibits significant non-uniformity in the direction of fusion width and penetration depth. The average grain size of the upper weld joint is larger than that of the middle and lower parts.
The Widmanstatten structure appears in the upper and middle parts of the welded joint, increasing the fragility of the joint and decreasing its plasticity. By using welding with large heat input, the non-uniformity of microstructure distribution can be improved.
Li Jinwei et al. achieved uniformity control of electron beam welding composition of 20mm thick TA15 titanium alloy by applying sweep waveforms of certain frequency and deflection amplitude to the electron beam during welding, incorporating metal materials of transition at the welding interface and adjusting welding parameters.
Figure 14 shows the control effect of solder composition uniformity under different process conditions. Compared to traditional electron beam welding, scanning electron beam welding results in less fluctuation of the alloying elements in the thickness direction, leading to a more uniform composition.
Fig.13 Microstructure of different regions of the welded joint
Fig.14 Effect of uniformity control of weld composition under different process conditions
Fang Weiping et al. used electron beam welding technology to weld 100mm thick TC4 titanium alloy plates. The resulting welded joints were subjected to recrystallization annealing at 850 ℃ and solution aging heat treatment at 920 ℃×2 h and 500 ℃×4 h.
The results revealed that the microhardness of the weld zone, the heat-affected zone and the base metal zone obtained through solution aging heat treatment was higher than that in the welded state. Furthermore, the tensile strength of the welded joint was 11.3% higher than that of the as-welded state, and the yield strength was 17.2% higher than that of the as-welded state. However, the elongation after fracture was only about 50% of that in the soldier state.
Ma Quan et al. investigated the impact of heat treatment processes on the microstructure and mechanical properties of electron beam welded joints of Ti-1300 alloy. The results showed that different heat treatments before welding had little effect on the microstructure and weld properties of the titanium alloy. In contrast, post-welding heat treatment processes could not change the shape and size of the β-grain in the weld zone, but could regulate the content, size and shape of the a-phase in the weld zone. However, the distribution of the precipitated a phase tended to form at the stable grain boundary.
The performance of the weld zone depended on the size and number of the precipitated α phase. When annealing or aging only at a lower temperature, the strengthening effect of the α phase in the weld zone was better and the strength of the weld was higher than that of the base metal.
In summary, appropriate welding heat input combined with an oscillating electron beam could somewhat improve the inhomogeneity of microstructure and properties of welded joints. Furthermore, post-welding heat treatment could improve the mechanical properties of welded joints.
3.2 Joint residual stress distribution
Welding residual stress is a critical factor that can lead to stress corrosion cracking and reduced fatigue strength of structural components.
An accurate assessment of welding residual stress is crucial to determining the service life of welded components.
Liu Min and colleagues analyzed the residual stress distribution of an electron beam sample made of 75 mm thick TC4 titanium alloy based on thermal elastoplastic finite element theory.
Figure 15 shows the results of the residual stress test.
The results indicate that there is a three-dimensional residual tensile stress with a high value in the area located 10 mm from the initial and final ends, covering approximately 1/4 of the thickness. This stress can significantly affect the mechanical properties of welded joints and therefore requires appropriate attention.
Fig.15 Residual stress calculation results
Wu Bing et al. conducted a study on reducing residual stress in welded joints by measuring the residual stress distribution of electron beam welded joints of 50 mm thick TA15 titanium alloy after vacuum annealing using the blind hole method. The results showed that the heat treatment process made the transverse and longitudinal stresses of the welded joints more consistent, and the stresses of the entire welded joint became more uniform.
Similarly, Yu Chen et al. measured the residual stress distribution of electron beam welded joints of 100 mm thick TC4 titanium alloy after 600 ℃ × 2 h X-ray diffraction heat treatment. The results demonstrated that heat treatment reduced to a certain extent the residual stress of the welded joint, and the distribution on the upper and lower surfaces of the welded joint was noticeably different.
The horizontal and longitudinal residual stresses on the upper surface decreased, and the longitudinal residual stress in some areas changed from tensile stress to compressive stress. The longitudinal residual stress on the bottom surface was effectively eliminated, and some positions were in a state of compressive stress. The horizontal residual stress relief effect was medium.
Furthermore, Hosseinzadeh F et al. used the contour method to measure the residual stress distribution in electron beam welded joints of 50 mm thick TC4 titanium alloy after heat treatment. The results showed that the maximum tensile stress at the leading edge of the weld was 330 MPa, the maximum compressive stress was 600 MPa within 10 mm of the rear end of the test plate, and the tensile stress at the weld centerline after heat treatment it could be reduced to 30 MPa.
In summary, post-welding heat treatment can significantly reduce the residual stress of thick-walled titanium alloy welded joints.
3.3 Summary
In summary, electron beam welding is capable of achieving high welding efficiency and producing welded joints with minimal deformation and good shape when welding thick-walled titanium alloys. However, due to the narrow melting area and large temperature gradient, thermal cycling can lead to the formation of triaxial stresses in the structure, resulting in a marked decrease in joint plasticity and toughness.
Although an appropriate heat treatment process can partially improve the structure and performance of the welded joint, the problem has not been fully resolved. Hidden dangers remain for further maintenance work, such as uneven structure, performance and stress distribution along the thickness direction. Furthermore, the heat treatment process not only increases production costs but also reduces production efficiency.
Furthermore, the vacuum chamber also limits the application of electron beam welding to large titanium alloy components. Therefore, research should be conducted on the microstructure, properties and stress distribution uniformity of welded joints, particularly in the direction of local vacuum electron beam welding.
4. Research status of laser welding
After decades of development, laser welding technology has made significant progress. With the birth of fiber lasers and the development of photoelectric modules, the output power of lasers has increased and beam stability has improved, laying a solid foundation for its application in the field of welding thick-walled components.
Compared with traditional thick-wall arc welding technology, laser welding offers high welding efficiency, minimal welding deformation and residual stress, narrow heat-affected zones and excellent adaptability for welding large and complex structures.
These advantages have made laser welding technology one of the main research focuses for welding thick-walled components in recent years.
At present, laser welding technology for thick-walled titanium alloys includes laser filler wire welding and vacuum laser welding.
4.1 Narrow gap laser welding with filler wire
Narrow gap laser welding with filler wire involves using a wire feed mechanism to push the filler metal to the laser focus point. The molten filler metal fills the weld through the action of the laser beam, finally completing the welding process.
Figure 16 illustrates a schematic diagram of narrow gap laser welding with filler wire. This technique has experienced rapid development in recent years.
Despite its progress, narrow-gap laser welding with filler wire still faces some challenges, especially when it comes to welding thick-walled titanium alloys. Such problems may include lack of sidewall fusion, welding porosity, welding deformation and high stress, and low plastic toughness of welded joints.
Fig.16 Schematic diagram of narrow gap laser wire fill welding
Li Kun et al. used an oscillating laser beam to suppress porosity in titanium alloy and analyzed its mechanism to solve the problem of sidewall non-fusion and welding porosity.
The results showed that the oscillating beam had a significant effect on reducing the porosity of titanium alloy keyhole welding. This was due to the increased stability of the keyhole during welding, resulting in a reduction in keyhole porosity.
Xu Kaixin et al. used a circular laser beam swing to weld 40mm thick TC4 titanium alloy. When the oscillation amplitude was 2 mm and the oscillation frequency was 100-200 Hz, the weld seam had no visible pores and the sidewall was well fused.
Analysis of the microstructure and properties of the welded joint showed that the columnar crystal of the weld seam contained densely arranged acicular α'martensite and dispersedly distributed granular αg phase. The preferred orientation α' was found in the same β grain, and the proportion of large-angle grain boundaries was high. The welded joint exhibited high strength but low plasticity and toughness.
In conclusion, an oscillating laser beam is an effective solution to the problems of sidewall non-fusion and welding porosity.
Fig.17 Morphology and microstructure of the narrow section of 40 mm thick TC4 titanium alloy
To solve the low plasticity and toughness of thick-walled titanium alloy welded joints, researchers improved the microstructure and properties of welded joints by regulating the welding heat input and welding alloy elements.
Fang Naiwen and colleagues investigated the impact of heat input on laser welding of TC4 titanium alloy with filler wire. Their findings indicated that adequate welding heat input could ensure good plasticity in the welded joint.
Furthermore, using the in situ observation method of high-temperature laser confocal microscope, they analyzed the microstructure formation characteristics and transformation laws of the self-developed Ti-Al-V-Mo series titanium alloy during the process of cooling under the welding thermal cycle. The results demonstrated that the addition of Mo decreased the initial transformation temperature, decreased the aspect ratio of the acicular α' martensite and the initial α phase, and improved the impact toughness of the welded joints.
Therefore, by controlling the heat input in the welding process and reasonably designing the alloy element ratio of metal powder core flux-cored wire, the plastic toughness of the welded joint can be improved.
The ultra-narrow gap laser fill wire welding process of titanium alloy thick plate is the result of heat accumulation from a single pass of multilayer filler metal. The multiple thermal cycles in the multilayer welding process will inevitably create an extremely complex weld structure with an uneven temperature field.
During welding, the welded joint may experience uneven distribution of residual stress and welding deformation. In addition, titanium alloy has a high coefficient of linear expansion and low thermal conductivity, which further increases the probability of residual stress and welding deformation.
The negative impact of welding residual stress on the static load resistance, low-cycle fatigue resistance and corrosion resistance of titanium alloy welded joints is significant. Furthermore, welding deformation can significantly affect the appearance of welded joints, reduce the bearing capacity of the structure, and decrease the assembly accuracy of subsequent welding components.
To gain deeper insights into the influence of groove shapes on the residual stress of welded joints, Fang Naiwen et al. used ANSYS simulation software to perform numerical simulation analysis on the stress and strain of different groove shapes of 40mm thick TC4 titanium alloy laser welded joints.
Figure 18 represents the longitudinal distribution of stresses of the two groove shapes. The results indicate that the stress distribution of the single U-slot welded joint differs from that of the double U-slot welded joint. In the single U-slot welded joint, an obvious stress concentration appears on one side of the final weld, while the stress distribution of the double U-slot welded joint is symmetrical along the wall thickness direction.
Fig.18 Longitudinal distribution of residual stresses Stress distribution
In summary, filler wire narrow gap laser welding is capable of producing thick-walled titanium alloy welded joints without welding defects such as porosity and incomplete sidewall fusion by periodically oscillating the laser beam.
The plastic toughness of the welded joint can be improved by controlling the heat input in the welding process and the alloying element ratio of the powder core metal wire.
However, in the domain of narrow-gap laser welding of thick-walled titanium alloy with filler wire, it is essential to continue exploring the control of microstructure and properties of welded joints, especially in the domain of laser-filled metal core flow. . wire with multi-alloy system.
4.2 Vacuum laser welding
In recent years, high-power industrial fiber lasers have reached the 10,000-watt level. However, efficiently using high-quality, high-power lasers and improving the penetration ability of laser welding without sacrificing its quality is a difficult problem in engineering applications.
Recent research has shown that penetration depth can be significantly increased in a vacuum environment, improving weld porosity and weld formation. Reisgen U of the Technical University of Aachen, Germany, compared the penetration capabilities of laser welding, vacuum laser welding and electron beam welding.
The results showed that under the same line energy, the weld penetration obtained by laser welding in a vacuum environment is about 2.5 times greater than that in an atmospheric environment and is similar to that obtained by electron beam welding. However, the vacuum required for laser welding in a vacuum environment is only 10 Pa, while electron beam welding requires at least 10-1 Pa, making the cost of vacuum laser welding lower.
Therefore, scholars have carried out research on low-vacuum laser welding technology for thick-walled structures. Meng Shenghao et al. studied the characteristics of laser welding in a vacuum environment of TC4 titanium alloy for medium and thick sheets.
The results showed that laser welding in a vacuum environment has better weld formation, significantly improves weld penetration, increases the weld depth-to-width ratio, inhibits spatter in the welding process, and greatly reduces gas hole defects in the solder.
achieved welding of 40mm thick TC4 alloy using low vacuum laser welding technology (vacuum degree 10 Pa). They compared and analyzed the microstructure and mechanical properties of different positions.
The macromorphology of the welded joint is shown in Fig. 19. The results showed that the microstructure of the heat-affected zone is α phase, residual β phase and α' martensite.
The microstructure of the weld fusion zone mainly includes α' martensite of different sizes and distribution states and α phase formed at low cooling rate. The tensile properties along the thickness direction are uniform, and the strength values at the top and bottom are larger, while the strength values at the top middle and bottom middle are smaller, but the overall difference is small.
Fig.19 Macromorphology of a 40 mm thick titanium alloy welded joint
4.3 Summary
In summary, vacuum laser welding allows the welding of thick-walled titanium alloys. This welding process has several advantages over electron beam welding, including lower vacuum requirements, no radiation pollution, lower welding costs and greater efficiency. As a result, vacuum laser welding represents a promising method for welding thick-walled titanium alloys.
Despite these advantages, there is still a need for more research by relevant scholars in the field of low-vacuum laser welding of thick-walled titanium alloys. Specifically, there should be an in-depth investigation into the characteristics of laser energy transmission under vacuum conditions and the control of the microstructure and properties of welding joints.
5. Conclusion
This article mainly discusses the progress made in fusion welding technology for welding thick-walled titanium alloys, aiming to meet the high-quality welding and manufacturing requirements of equipment in aerospace, marine and other related fields.
Over the past decade, significant advances have been made in fusion welding technology for thick-walled titanium alloys. These achievements cover diverse areas such as welding technology, quality control, joint structure and property control.
Combined with the current research status, fusion welding of thick-walled titanium alloys mainly has the following research directions:
(1) Voltage control in welding thick-walled titanium alloy.
Due to the small thermal conductivity and large coefficient of linear expansion of titanium alloy, three-directional stress can easily form in the structure after thermal cycling during the welding process of thick-walled titanium alloy. This can result in a marked decline in joint plasticity and toughness.
To solve this problem, various techniques can be employed, such as post-welding heat treatment, groove optimization design and ultrasonic impact treatment, depending on the welding method used. These techniques can help control stress and deformation of thick-walled titanium alloy welded joints.
(2) Development of welding technology with multiple heat sources.
At present, fusion welding technology for thick-walled titanium alloys mainly involves single heat sources such as conventional arc welding, electron beam welding and laser welding. However, these welding methods have certain limitations.
Therefore, to resolve these limitations, the development of multiple heat source welding technology such as TIG-MIG hybrid welding and hybrid laser arc welding can be pursued for welding thick-walled titanium alloys.
(3) Control of the microstructure and properties of welded joints.
Currently, there is limited research into controlling the microstructure of welded joints in thick-walled titanium alloys.
However, it is feasible to develop welding materials with multi-alloy systems and regulate the microstructure of the welds. This can potentially improve the mechanical properties of thick-walled titanium alloys.
