Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research

Summary

Titanium alloy is widely used in aerospace, marine equipment and other fields because of its high specific strength, good corrosion resistance and high temperature performance.

In recent years, with the increasing demand for thick walled titanium alloys, the welding technology of thick walled titanium alloys also shows great application value.

Therefore, this paper summarizes the research progress of fusion welding technology for thick walled titanium alloy materials, mainly including non melting electrode gas shielded welding, electron beam welding and laser welding, and looks forward to the development trend of thick walled titanium alloy welding technology.

Related reading: Manual Arc Welding vs CO2 Gas Shielded Welding

Preface

Titanium alloy is characterized by low density, high specific strength and specific stiffness, excellent corrosion resistance, and good processability.

It is a new functional material with development potential and application prospects.

It is known as the “third metal” after steel and aluminum, and is an important strategic metal material.

It has been widely used in aerospace, petrochemical, national defense equipment and other fields.

In recent years, with the demand for large-scale and lightweight equipment in the national defense industry, the demand for thick walled titanium alloy is becoming more and more urgent, and its corresponding processing technology is also urgent.

In practical engineering applications, the connection of thick walled titanium alloy structures is mainly completed by welding, so efficient and high-quality thick walled titanium alloy welding technology has attracted much attention.

In this article, the research status of thick walled titanium alloy fusion welding technology is summarized, the existing problems of thick walled titanium alloy fusion welding are put forward, and the development prospect and research direction of thick walled titanium alloy fusion welding technology are prospected.

1. Classification and characteristics of titanium alloys

1.1 Classification of titanium alloys

According to chemical composition and content, titanium alloys can be divided into five categories: α titanium alloy, near α titanium alloy (β phase mass fraction ≤ 10%), α-β dual phase titanium alloy (10% ≤ β phase mass fraction ≤ 50%), metastable β titanium alloy and β titanium alloy.

α-β dual phase titanium alloy is the most widely used titanium alloy because of its excellent comprehensive properties, which has both the thermal stability characteristics of α-type titanium alloy and the heat treatment strengthening characteristics of β-type titanium alloy.

1.2 Titanium alloy material characteristics

(1) High specific strength.

Titanium alloy is a light alloy with a density (20 ℃) of 4.54 g/cm3, which is about 56% of that of ordinary steel.

Using titanium alloy to make mechanical parts can significantly reduce the weight and achieve the effect of lightweight.

(2) Good corrosion resistance.

When titanium alloy is exposed to air, it will form a stable, continuous and dense oxide film on the surface, making it in a passive state;

At the same time, the oxide film of titanium alloy has good repair performance.

When it is damaged due to external factors, it can be quickly repaired, so titanium alloy has good 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 resistance and heat resistance.

1.3 Welding characteristics of thick walled titanium alloy

(1) Embrittlement of welded joint:

Without protection, when the heating temperature of titanium alloy reaches 250 ℃, hydrogen absorption starts, oxygen absorption starts at 400 ℃, severe oxidation occurs at 540 ℃, and nitrogen absorption starts at 600 ℃.

These gases dissolve into the molten pool and undergo chemical reaction, which makes the welded joint embrittlement, leading to a rapid decline in the plasticity and toughness of the welded joint.

(2) Welding cracks:

The content of S, P, C and other impurities in titanium alloy is low, the low melting point eutectic compound is less, and the crystallization temperature range is narrow, so it is not easy to produce hot cracks;

However, when thick walled titanium alloy is welded by multi-layer and multipass welding, due to the large constraint stress of the welded joint, there is a large residual stress in the welded joint, and cold cracks are easily generated under the effect of the residual stress.

(3) Porosity:

Porosity is the most common defect in titanium alloy welding.

Titanium alloy itself has active elements and high saturated vapor pressure.

When the surface of base metal and welding material is polluted or the shielding gas is impure (containing oxygen, hydrogen or water), hydrogen porosity is easy to occur.

2. Research status of non consumable electrode gas shielded welding

2.1 Traditional TIG welding

Non consumable gas shielded welding (TIG welding) has been widely used in titanium alloy field due to its advantages of stable arc, less welding spatter and good weld formation.

However, in the traditional TIG welding process of titanium alloy, the high temperature residence time of the welded joint is longer, and the cooling speed of the liquid molten pool metal is faster.

Because of the low thermal conductivity of titanium alloy, the grain coarsening tendency in the weld zone and heat affected zone is particularly obvious;

At the same time, due to the large groove size, multi-layer and multi pass welding is required, which will lead to low welding efficiency, excessive stress and deformation and other problems.

In order to reduce the tendency of grain coarsening of welded joints, Lu Xin uses TIG welding to achieve multi-layer and multi pass welding of 20 mm thick TC4 titanium alloy with a groove angle of 60 °.

The microstructures of welded joints under different heat inputs are shown in Fig. 1.

With the reduction of welding heat input, the weld grains are gradually refined, and the size of martensite inside the grains is smaller and more regular.

Therefore, when using TIG welding to weld TC4 titanium alloy thick plates, the welding heat input should be strictly controlled to prevent the joint grain from being coarse, and to avoid the occurrence of abnormal structures, cracks and other defects.

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 1

Fig.1   Microstructure of weld zone under different heat input

In order to reduce the residual stress and deformation of welded joints, Yang Lu et al. used an X-shaped groove for alternating front and back welding to achieve multi-layer TIG welding of 24 mm thick TC4 titanium alloy.

At the same time, based on the SYSWELD platform, the temperature field, stress field and welding deformation of welded joints were numerically simulated under the condition of completely rigid clamping at both ends of the welding plate, as shown in Fig. 2.

The results show that the stress and deformation of the welded joint can be significantly reduced by using the welding sequence of alternating two sides.

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 2

Fig.2   TEM morphology profile of welding joint residual stress thickness

To sum up, although traditional TIG welding can be used to weld thick walled titanium alloy, the grain size, joint stress and deformation can be reduced by appropriately reducing the welding heat input and using X-shaped groove for double-sided alternate welding.

Related reading: MIG vs TIG Welding

However, there is still a problem that large groove size leads to low welding efficiency, which makes it difficult to popularize it in thick walled titanium alloy welding.

2.2 Narrow gap TIG welding

The groove size of narrow gap welding is small, and compared with the traditional groove filling weld, the volume is greatly reduced, which not only improves the welding efficiency, but also reduces the production cost.

The narrow gap TIG welding process is flexible, the equipment cost is relatively low, and the welding process is stable.

In addition, narrow gap groove can also reduce the number of welding passes, thus improving the welding deformation and controlling the welding stress.

Therefore, narrow gap TIG welding of thick walled titanium alloy has great advantages.

However, the groove gap of narrow gap TIG welding is small, which is easy to trigger the arc to “climb” along the side wall, resulting in insufficient heat input at the bottom corner of the wall on both sides of the weld bead and poor fusion of the side wall.

At present, the narrow gap TIG welding technology for thick walled titanium alloys often uses mechanical swing and external magnetic field to regulate the arc, which can effectively solve the problem of poor fusion of narrow gap sidewalls.

2.2.1 Mechanical swing narrow gap TIG welding

The principle of mechanical swing narrow gap TIG welding is: during welding, the tungsten electrode swings left and right in the groove through the rotation of the tungsten electrode clamp, making the arc point to the groove side wall periodically, ensuring the fusion quality of the groove side wall.

The welding process is shown in Fig. 3.

Mechanical swing narrow gap welding mode has a strong adaptability to the change of welding groove width.

It is not easy to produce side wall non fusion defects in the welding process, and the welding quality is more stable. It has been widely used in narrow gap TIG welding of thick walled titanium alloys.

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 3

Fig.3   Schematic diagram of mechanical swing narrow gap TIG welding process

Jiang Yongchun adopted the mechanical swing narrow gap TIG welding technology.

By selecting reasonable welding parameters and welding protection measures, he realized the high-quality connection of TC4 titanium alloy with a thickness of 52 mm.

The macro metallography and microstructure of the welded joint are shown in Fig. 4.

As the cooling speed is too fast, α ‘martensite is generated in the heat affected zone, the weld strength reaches 90% of the base metal, and the hardness of the fusion zone presents the maximum value.

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 4

Fig.4   Macroscopic metallography and microstructure of welded joint

Li Shuang et al. used mechanical swing narrow gap TIG welding technology to realize 30 mm thick TC4 titanium alloy single-layer filler wire welding, and analyzed the microstructure of the welded joint.

The results show that the grains in the weld zone are seriously coarsened, mainly coarse columnar grains, and their microstructures are all acicular α ‘martensite, which are distributed parallel to each other in the β phase grains;

The grain coarsening degree of heat affected zone near the weld side is greater than that near the base metal side.

To sum up, the mechanical swing narrow gap TIG welding technology has stable welding process and low equipment cost.

The periodic swing of tungsten electrode can effectively solve the problem of poor fusion of thick walled titanium alloy sidewall.

However, due to the large heat input, the grain coarsening tendency of the joint is obvious.

2.2.2 Magnetically controlled narrow gap TIG welding

Magnetically controlled narrow gap TIG welding technology was first proposed by Ukraine’s Barton Welding Technology Research Institute.

In recent years, Guangdong Welding Technology Research Institute has carried out basic research and industrial application promotion of magnetic controlled narrow gap TIG welding technology for thick walled titanium alloys.

The welding process diagram and arc swing of magnetically controlled narrow gap TIG welding are shown in Fig. 5.

During the welding process, the electromagnetic coil is connected with alternating current, the silicon steel sheet passing through the coil becomes a magnet, and the magnetic induction line passes through the electrode and the arc, thus realizing the periodic swing of the arc to the two side walls, facilitating the fusion of the narrow gap side walls, and realizing narrow gap TIG welding.

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 5

Fig.5   Schematic illustration of external transverse magnetic field and arc swing

In order to achieve high-quality welding of magnetic control narrow gap TIG welding, scholars all over the world have carried out relevant research on the influence of magnetic field strength, magnetic field frequency and electrode position on side wall fusion, weld formation and crystallization process.

Kshirsagar R et al. studied the influence of external magnetic field on weld formation, as shown in Fig. 6.

The results show that there is obvious lack of fusion in the side wall when there is no external magnetic field, while the side wall fusion is good when there is an external magnetic field.

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 6

Fig.6   Effect of external transverse magnetic field on configuration and microstructure of welding seam

(a) No external magnetic field

(b) With external magnetic field

Hua Aibing et al. studied the influence of external magnetic field strength on side wall fusion.

The results show that when the magnetic field strength is ≥ 4 mT, the side wall fusion of narrow gap weld can be effectively improved, and the weld fusion is relatively uniform.

Chang Yunlong et al. studied the effect of external magnetic field frequency on side wall fusion. The results showed that with the increase of magnetic field frequency, the penetration depth of weld bottom and arc impact depth increased, and the weld penetration width and side wall penetration decreased.

Yu Chen et al. studied the influence of electrode position on side wall fusion.

The results showed that when the tungsten electrode shifted from the central position, the current inflow intensity of the near side wall increased, while the current inflow intensity of the far side wall decreased.

In order to avoid uneven side wall penetration and poor side wall fusion, the electrode position needs to be strictly controlled.

Sun Jie et al. studied the influence of electromagnetic strength on the crystallization process.

The diagram of the primary crystallization of titanium alloy weld under the action of magnetic field is shown in Fig. 7.

The results show that electromagnetic effect can improve the stability of planar crystallization front area and later formed equiaxed crystals.

With the increase of magnetic field strength, the microstructure near the fusion line gradually changes from columnar crystal to equiaxed crystal.

The stability of the equiaxed crystal generated in the weld center can be significantly improved by the action of magnetic controlled arc.

At the same time, the equiaxed crystal gradually grows in a single direction with the increase of magnetic field strength.

Hu Jinliang et al. used the magnetic control narrow gap TIG welding technology to realize the welding of 120 mm thick TA17 titanium alloy.

The microstructure of the welded joint is shown in Fig. 8.

The results show that the microstructure along the transverse direction of the joint appears significant inhomogeneity, and the microstructure along the thickness direction of the joint has no significant difference, and because of the large welding heat input, the fusion zone is seriously softened.

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 7

Fig.7   Primary crystallization process of titanium alloy weld metal under magnetic field

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 8

Fig.8   Microstructure of 120 mm-thick TA17 titanium alloy joint welded by magnetically controlled NG-TIG welding seam

To sum up, the magnetic control narrow gap TIG welding technology has stable welding process and low equipment cost.

The addition of magnetic field can realize the periodic swing of the arc, effectively solve the problem of poor fusion of thick walled titanium alloy side walls, and obtain uniform weld zone structure.

However, there is still a serious problem of softening the fusion zone of welded joints due to large heat input.

Narrow gap TIG welding technology can achieve stable welding of thick walled titanium alloy.

Compared with traditional TIG welding, it reduces the number of welding passes and improves the welding efficiency.

However, due to repeated remelting and heating of joint grains, there are problems of coarse grains and uneven distribution of microstructure and properties along the thickness direction.

2.3 Submerged arc welding

Submerged arc welding is a special form of TIG welding.

It uses helium as the shielding gas. Its electrode diameter and welding current are both large.

Under the combined action of helium and arc force, it can drain the liquid molten pool metal at the weld position.

The electrode dives into the base metal to be welded.

The arc burns in the electrode and the cavity formed at the bottom of the crater, and the molten metal finally forms the molten pool.

As the burning position of the arc is lower than the surface of the base metal to be welded, it is called submerged arc welding.

The principle of submerged arc welding is shown in Fig. 9.

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 9

Fig.9   Schematic graph of SAW principle

In recent years, scholars have carried out research on submerged arc welding technology applied to large-thickness titanium alloy.

Chen Guoqing and others conducted submerged arc welding butt test on TA15 titanium alloy with a thickness of 29 mm, and obtained well formed welds.

However, due to the large heat input, the weld zone and heat affected zone of the welded joint are relatively wide, and the elongation after fracture of the joint is only 50% of the base metal.

The bending property of the welded joint is poor, and it breaks when it bends to 15 °.

Liu Yanmei and others realized the welding of 58 mm thick TA15 titanium alloy by using the submerged arc welding double-sided welding process.

The macro section of the weld is shown in Fig. 10. The weld zone is columnar crystal with large grain size, and the intragranular is acicular α ‘martensite.

The tensile fracture location of the joint is the weld zone, which is ductile fracture.

The tensile strength reaches 96% of the strength of the base metal.

In order to improve the mechanical properties of the submerged arc welding joint, Duqiang et al. conducted submerged arc welding of 64 mm thick TA15 titanium alloy plate with the addition of TA1 pure titanium interlayer.

The results showed that the hydrogen, oxygen and nitrogen contents in the weld after adding the interlayer were reduced compared with the base metal, and the plasticity of the welded joint was significantly improved.

Hou Qi et al. studied the influence of shielding gas purity on the performance of 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 shielding gas purity.

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 10

Fig.10   Macroscopic cross section of weld

To sum up, submerged arc welding can realize the welding of thick walled titanium alloy, with a relatively stable arc shape, and can obtain better weld formation;

At the same time, helium is used for coaxial protection in submerged arc welding. Compared with argon, helium has high ionization potential and high thermal conductivity.

Therefore, the arc column area of submerged arc welding is narrow and concentrated, and the utilization rate of arc heat is high.

It can realize double-sided welding of thick titanium alloy, and the welding efficiency is significantly improved compared with narrow gap TIG welding.

However, this method has some problems 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 can realize the welding of thick titanium alloy, with relatively stable arc shape, and can obtain better weld formation, which shows high application value in the research of thick titanium alloy welding.

However, there are still problems such as joint softening caused by large welding heat input.

Therefore, it is necessary to carry out research on reducing the heat input of thick plate titanium alloy welding to improve the homogeneity of the structure and properties of thick wall titanium alloy non MIG welding.

3. Research status of electron beam welding

Electron beam welding technology uses high-energy density electron beam to bombard metal materials, which can realize single-sided welding and double-sided forming of large thickness metal materials;

In the welding process, the beam power density is high, the depth width ratio of the weld is large, and the welding deformation is small;

At the same time, electron beam welding needs to be carried out in a vacuum environment, which can well avoid the hazards of hydrogen, oxygen and nitrogen in the welding process.

Therefore, electron beam welding is often used in the welding of large thickness titanium alloys.

The electron beam welding device is shown in Fig. 11.

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 11

Fig.11   Schematic of the electron beam welding

3.1 Joint structure and performance

The microstructure and properties of vacuum electron beam welded joints of titanium alloys have been studied by domestic and foreign scholars.

Hou Jiangtao used electron beam welding technology to realize the welding of 20 mm thick TC4 titanium alloy, and analyzed the grain size of the weld zone and the mechanical properties of the joint along the thickness direction.

The results show that the grain size of the upper part of the weld zone is 1200 µm, and the grain size of the lower part is 200 µm.

The difference in grain size leads to the difference in properties.

Sun et al. used electron beam welding technology to realize the welding of 20 mm thick TC4 titanium alloy, and analyzed the macro morphology of the welded joint (see Fig. 12).

The results showed that the width of the fusion zone and heat affected zone in the upper, middle and lower areas of the welded joint, as well as the morphology and size of the grain structure were significantly different, and the grain size decreased along the depth direction.

Wei Lu et al. used electron beam welding technology to realize the welding of 50 mm thick TC4 titanium alloy plates, and carried out mechanical property tests along the thickness direction.

The results show that the mechanical properties are distributed unevenly along the welding depth.

The yield strength, tensile strength and microhardness of the welded joint are improved compared with those of the base metal, but the plasticity and toughness are decreased.

Song Qingjun used electron beam welding technology to realize the welding of TC4 titanium alloy with a thickness of 60 mm.

The microstructure and properties of the welded joint were analyzed.

The results showed that the microstructure of the welded joint was distributed unevenly along the thickness direction, and the impact toughness gradually decreased from the top to the bottom of the weld.

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 12

Fig.12   Macroscopic appearance of welded joint

To sum up, after electron beam welding of thick walled titanium alloy, the weld metal undergoes a rapid thermal cycle process, and the residence time of each area of the welded joint at high temperature is inconsistent, resulting in uneven distribution of microstructure and properties in each area along the thickness direction.

In order to solve the problems of non-uniform distribution of microstructure and properties along the thickness direction and low mechanical properties of thick walled titanium alloy electron beam welded joints, relevant scholars have adjusted the microstructure and properties of electron beam welded joints through welding process optimization and post weld heat treatment.

Gong Yubing et al. conducted an in-depth study on the nonuniformity of the electron beam welded joint of 20 mm thick TC4 titanium alloy and the evolution of the structure.

The microstructures of different areas of the welded joint are shown in Fig. 13.

The results show that the weld joint of titanium alloy has large 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.

Widmanstatten structure appears in the upper and middle parts of the weld joint, which increases the brittleness of the weld joint and decreases the plasticity;

When large heat input welding is adopted, the nonuniformity of microstructure distribution can be improved.

Li Jinwei et al. realized the uniformity control of 20 mm thick TA15 titanium alloy electron beam weld composition by applying scanning waveforms of certain frequency and deflection amplitude to the electron beam during welding, embedding transition metal materials in the welding interface, and coordinating with the adjustment of welding parameters.

The control effect of weld composition uniformity under different process conditions is shown in Fig. 14.

Compared with traditional electron beam welding, the alloy elements of scanning electron beam welding fluctuate less in the thickness direction, and the composition is more uniform.

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 13

Fig.13   Microstructure of different regions of welded joint

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 14

Fig.14   Uniformity control effect of weld composition under different process conditions

Fang Weiping et al. used electron beam welding technology to realize the welding of 100 mm thick TC4 titanium alloy plates.

The obtained welded joints were recrystallized annealed at 850 ℃ and solution aging heat treated at 920 ℃×2 h and 500 ℃×4 h.

The results show that the microhardness of the weld zone, heat affected zone and base metal zone obtained by solution aging heat treatment is higher than that of the as welded state.

The tensile strength of the welded joint is 11.3% higher than that of the as welded state, and the yield strength is 17.2% higher than that of the as welded state, but the elongation after fracture is only about 50% of that of the as welded state.

Ma Quan et al. studied the effect of heat treatment process on the microstructure and mechanical properties of electron beam welded joints of Ti-1300 alloy.

The results show that different heat treatment before welding has little effect on the microstructure and properties of titanium alloy weld;

The post weld heat treatment process can not change the shape and size of the β grain in the weld zone, and can regulate the content, size and shape of the a phase in the weld zone. However, the distribution of the precipitated a phase tends to form at the stable grain boundary.

The performance of the weld zone depends on the size and number of the precipitated α phase.

When annealing or aging at a lower temperature alone, the strengthening effect of the α phase in the weld zone is better, and the weld strength is greater than that of the base metal.

To sum up, the inhomogeneity of microstructure and properties of welded joints can be improved to some extent by using appropriate welding heat input combined with swinging electron beam;

The mechanical properties of welded joints can be improved by post weld heat treatment.

3.2 Joint residual stress distribution

Welding residual stress is an important factor that causes stress corrosion and fatigue strength reduction of structural components.

Accurate evaluation of welding residual stress is the basis of life assessment of welded components.

Liu Min and others analyzed the residual stress distribution of 75 mm thick TC4 titanium alloy electron beam specimen based on the thermal elastoplastic finite element theory.

The residual stress test results are shown in Fig. 15.

The results show that, in the area 10 mm from the starting and ending ends and about 1/4 of the thickness, there is a three-dimensional residual tensile stress with a high value, which has an important impact on the mechanical properties of welded joints and should be paid enough attention.

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 15

Fig.15   Calculation results of residual stress

In order to reduce the residual stress of welded joints, Wu Bing et al. measured the residual stress distribution of 50 mm thick TA15 titanium alloy electron beam welded joints after vacuum annealing by using the blind hole method.

The results showed that the heat treatment process made the transverse and longitudinal stresses of welded joints tend to be consistent, and the stress of the whole welded joint tends to be uniform.

Yu Chen et al. measured the residual stress distribution of 100 mm thick TC4 titanium alloy electron beam welded joint after 600 ℃×2 h heat treatment by X-ray diffraction.

The results show that after heat treatment, the residual stress of the welded joint is reduced to a certain extent, and the distribution on the upper and lower surfaces of the welded joint is obviously different.

The horizontal and longitudinal residual stresses on the upper surface are reduced to a certain extent.

The longitudinal residual stress in some areas is changed from tensile stress to compressive stress, and the longitudinal residual stress on the lower surface is eliminated effectively, and some positions are in a compressive stress state, the horizontal residual stress relief effect is general.

Hosseinzadeh F et al. measured the distribution of residual stress in electron beam welded joints of 50 mm thick TC4 titanium alloy after heat treatment using the contour method.

The results showed that the maximum tensile stress at the initial end 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 could be reduced to 30 MPa.

To sum up, post-weld heat treatment can significantly reduce the residual stress of thick walled titanium alloy welded joints.

3.3 Summary

To sum up, electron beam welding can realize thick walled titanium alloy welding with high welding efficiency, and can obtain welded joints with small deformation and good shape.

However, due to large temperature gradient and narrow melting area, three-way stress is easily formed in the structure after thermal cycling, resulting in a sharp decline in joint plasticity and toughness.

The appropriate heat treatment process can improve the structure and performance of the welded joint to a certain extent, but it has not been completely solved.

There are still some problems such as uneven structure, performance and stress distribution along the thickness direction, laying hidden dangers for later service work.

Moreover, the heat treatment process not only increases the production cost, but also reduces the production efficiency.

At the same time, the vacuum chamber also limits the application of electron beam welding in large titanium alloy components.

Therefore, it is necessary to carry out research on the microstructure, properties and stress distribution uniformity of welded joints, especially in the direction of local vacuum electron beam welding.

4. Research status of laser welding

After decades of development, laser welding technology has made great progress, especially with the birth of fiber lasers and the development of photoelectric modules, the output power of lasers has been increasing, and the beam stability has been improving, laying a solid foundation for its application in the field of thick wall component welding.

Compared with the traditional thick wall arc welding technology, laser welding has the characteristics of high welding efficiency, small welding deformation and residual stress, narrow heat affected zone, and good adaptability for welding large and complex structures.

The above advantages make laser welding technology gradually become one of the main research focuses of thick wall component welding in recent years.

At present, laser welding technology of thick walled titanium alloy mainly includes laser filler wire welding and vacuum laser welding.

4.1 Narrow gap laser welding with filler wire

Narrow gap laser filler wire welding is to push the filler metal to the laser focus spot by the wire feeding mechanism.

The filler metal melts and fills the weld under the action of the laser beam, and finally completes the welding.

The schematic diagram of narrow gap laser welding with filler wire is shown in Fig. 16.

In recent years, narrow gap laser welding with filler wire has developed rapidly.

However, there are still some problems in thick walled titanium alloy welding, such as lack of fusion of side wall, welding porosity, welding deformation and high stress, and poor plastic toughness of welded joints.

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 16

Fig.16   Schematic diagram of narrow gap laser wire-filling welding

In order to solve the problem of side wall non fusion and welding porosity, Li Kun et al. used a swinging laser beam to suppress titanium alloy porosity and analyzed its mechanism.

The results show that the swinging beam has a significant effect on restraining the porosity of titanium alloy keyhole welding.

The main reason is that the swinging beam increases the stability of keyhole during welding, and then reduces the keyhole porosity.

Xu Kaixin et al. used laser beam circular swing to weld 40 mm thick TC4 titanium alloy.

When the swing amplitude is 2 mm and the swing frequency is 100~200 Hz, the weld seam has no obvious pores and the side wall is well fused.

Analysis of the microstructure and properties of the welded joint shows that there are densely arranged acicular α ‘martensite and dispersedly distributed granular αg phase in the columnar crystal of the weld seam.

α’ preferred orientation in the same β grain, and the proportion of large angle grain boundaries is high, the welded joint has high strength, but poor plasticity and toughness.

To sum up, swinging laser beam can effectively solve the problems of side wall non fusion and welding porosity.

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 17

Fig.17   Morphology and microstructure of narrow gap section of 40 mm thick TC4 titanium alloy

In order to solve the problem of poor plasticity and toughness of thick walled titanium alloy welded joints, relevant scholars have improved the microstructure and properties of welded joints by controlling welding heat input and regulating weld alloy elements.

Fang Naiwen et al. analyzed and studied the influence of welding heat input on TC4 titanium alloy laser welding with filler wire.

The results showed that appropriate welding heat input could ensure the weld joint to have good plasticity.

At the same time, using the in-situ observation method of high temperature laser confocal microscope, the microstructure formation characteristics and transformation laws of the self-developed Ti-Al-V-Mo series titanium alloy during the cooling process under the welding thermal cycle were analyzed.

The results show that the addition of Mo decreases the initial transformation temperature, decreases the aspect ratio of acicular α ‘martensite and initial α phase, and improves the impact toughness of welded joints.

Therefore, controlling the heat input in the welding process and reasonably designing the alloy element ratio of metal powder cored flux cored wire can improve the plastic toughness of the welded joint.

The process of ultra narrow gap laser filler wire welding of thick titanium alloy plate is the accumulation of heat from a single pass of multi-layer filler metal, and the multiple thermal cycles in the process of multi-layer welding will inevitably make the weld structure extremely complex and have an extremely uneven temperature field.

At the same time, uneven distribution of residual stress and welding deformation will also occur in the welded joint.

In addition, titanium alloy has a large linear expansion coefficient and low thermal conductivity.

Therefore, the tendency to produce welding residual stress and welding deformation will be greater.

Welding residual stress has a very adverse effect on the static load strength, low cycle fatigue strength and corrosion resistance of titanium alloy welded joints;

Welding deformation will seriously affect the appearance of welded joints, reduce the bearing capacity of the structure, and reduce the assembly accuracy of later welding components.

In order to deeply analyze the influence of groove forms on the residual stress of welded joints, Fang Naiwen et al. used ANSYS simulation software to conduct numerical simulation analysis on the stress and strain of different groove forms of 40 mm thick TC4 titanium alloy laser welded joints.

The longitudinal stress distribution of the two groove forms is shown in Fig. 18.

The results show that the stress distribution of single U-groove welded joint is different from that of double U-groove welded joint.

The obvious stress concentration appears on one side of the end weld of single U-groove welded joint, while the stress distribution of double U-groove welded joint is symmetrical along the wall thickness direction.

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 18

Fig.18   Longitudinal residual stress distribution stress distribution

To sum up, the narrow gap laser welding with filler wire can obtain thick walled titanium alloy welded joints without welding defects such as porosity and incomplete fusion of side wall by periodically swinging the laser beam.

The plastic toughness of the welded joint can be improved by reasonably controlling the heat input in the welding process and the alloy element ratio of the metal powder cored wire.

However, in the field of thick walled titanium alloy narrow gap laser welding with filler wire, it is still necessary to continue to explore the control of the microstructure and properties of welded joints, especially in the field of laser filled metal cored flux cored wire with multi alloy system.

4.2 Vacuum laser welding

In recent years, high-power industrial fiber lasers have reached the level of ten thousand watts.

How to use high quality high-power laser efficiently and improve the penetration ability of laser welding by increasing laser power without sacrificing the welding quality of laser welding is a difficult problem faced by the engineering application of high-power laser welding.

In recent years, research shows that the penetration depth can be significantly increased in vacuum environment, and the porosity of weld and weld formation can also be greatly improved.

Reisgen U, Technical University of Aachen, Germany, compared the penetration ability of laser welding, vacuum laser welding and electron beam.

The results show that under the same line energy, the weld penetration obtained by laser welding in vacuum environment is about 2.5 times higher than that in atmospheric environment, and is similar to that obtained by electron beam welding.

However, the vacuum required for laser welding in vacuum environment is only 10 Pa, while the electron beam requires at least 10-1 Pa, which shows that the cost of vacuum laser welding is lower.

Therefore, relevant scholars have carried out research on low vacuum laser welding technology for thick wall structures.

Meng Shenghao et al. studied the vacuum environment laser welding characteristics of TC4 titanium alloy for medium and thick plates.

The results show that the vacuum environment laser welding has a better weld formation, which can significantly improve the weld penetration, increase the weld depth width ratio, inhibit the spatter in the welding process, and greatly reduce the gas hole defects in the weld.

Harbin Welding Research Institute Co., Ltd. realized the welding of 40 mm thick TC4 alloy by using low vacuum (vacuum degree 10 Pa) laser welding technology.

The microstructure and mechanical properties of different positions were compared and analyzed.

The macro morphology of the welded joint is shown in Fig. 19.

The results show that the microstructure of heat affected zone is α phase, residual β phase and α ‘martensite.

The microstructure of weld melting 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, the strength values at the top and bottom are larger, and the strength values at the middle upper part and middle lower part are smaller, but the overall difference is small.

Fusion Welding of Thick Walled Titanium Alloys: Current Status of Technical Research 19

Fig.19   Macromorphology of 40 mm thick titanium alloy welded joint

4.3 Summary

To sum up, vacuum laser welding can realize the welding of thick wall titanium alloy.

Compared with electron beam welding, the welding process requires low vacuum, no ray pollution, low welding cost and high efficiency.

It is a potential welding method of thick wall titanium alloy.

However, in the field of low vacuum laser welding titanium alloy thick wall technology, it is still necessary for relevant scholars to conduct in-depth research on the characteristics of laser energy transmission under vacuum conditions and the control of welding joint microstructure and properties.

5. Conclusion

In order to meet the requirements of high quality welding and manufacturing of thick titanium alloy components in aerospace, marine equipment and other fields, the research progress of fusion welding technology in thick wall titanium alloy welding is mainly introduced.

After more than ten years of development, the fusion welding technology of thick walled titanium alloy has made many achievements in welding technology, welding quality control, joint structure and property control, etc.

Combined with the current research status, the fusion welding of thick walled titanium alloys mainly has the following research directions:

(1) Stress control of thick wall titanium alloy welding.

Due to the characteristics of titanium alloy with small thermal conductivity and large linear expansion coefficient, during the welding process of thick walled titanium alloy, three-way stress is easily formed in the structure after thermal cycling, resulting in a sharp decline in the plasticity and toughness of the joint.

Therefore, for different welding methods, post weld heat treatment, groove optimization design and ultrasonic impact treatment can be used to control the stress and strain of thick walled titanium alloy welded joints.

(2) Development of multi heat source welding technology.

At present, the fusion welding technology of thick walled titanium alloy is mainly a single heat source, such as conventional arc welding, electron beam welding and laser welding. These welding methods have some limitations.

Therefore, multi heat source (such as TIG-MIG hybrid welding, laser arc hybrid welding, etc.) welding technology development can be carried out for thick wall titanium alloy welding.

(3) Control of microstructure and properties of welded joints.

At present, there is little research on the microstructure control of thick walled titanium alloy welded joints.

It is possible to develop multi alloy system welding materials and control the weld microstructure, so as to improve the mechanical properties of thick walled titanium alloys.

Leave a Comment

Your email address will not be published. Required fields are marked *