Titanium and titanium alloy welding characteristics and methods

- Jun 15, 2018-

Titanium and titanium alloy welding characteristics and methods

Titanium and its alloy welding characteristics

       1 Physical and chemical properties of titanium and its alloys

Titanium has two allotropes, represented by α and β, respectively, and the transition temperature is 882.5°C. The low-temperature crystal α is a close-packed hexagonal crystal lattice, and the β crystal stable above 882.5°C is a body-centered cubic crystal lattice.

Titanium has poor thermal conductivity and its thermal conductivity is slightly lower than that of stainless steel. When titanium contains impurities, its thermal conductivity decreases. Table 1 shows the comparison of the main physical properties of industrial pure titanium with other metallic materials.

2 Titanium alloy welding organization

The welding microstructure of industrial pure titanium and α-titanium alloy is a single phase at room temperature, and a zigzag or acicular structure is generated depending on the cooling rate. There are no major changes in the mechanical properties compared to the base metal, and the welding performance is good. The α+β titanium alloy forms martensite (α' phase) during cooling from the β phase. The number and properties of the α' phase change according to the alloy composition and the cooling rate. Under normal circumstances, as the α' phase increases, the ductility and toughness of the alloy decrease, and even with good weldability of Ti-6Al-4V, when the vanadium content of the β-stabilizing element is greater than 5%, the weldability deteriorates. The martensite production temperature of the β titanium alloy is lower than room temperature, and the weld is a metastable β phase, so the weldability is not deteriorated. However, due to the excessive addition of alloying elements, they often lack extensibility. In addition, the aging and cold working increase the strength of the alloy, and the welding results in a loss of strength, so welding joints are not used.

      3 Titanium alloy welding defects

      3.1 Embrittlement of Welded Joint Area

      Titanium and titanium alloy welds are susceptible to embrittlement due to contamination of gases and other impurities. The main elements that cause embrittlement are O, N, H, C, and so on. At room temperature, titanium and titanium alloys are relatively stable, but as the temperature increases, the ability of titanium and titanium alloys to absorb O, N, and H also increases significantly. Ti absorbs hydrogen from 250°C, absorbs oxygen from 400°C, and absorbs nitrogen from 600°C. Nitrogen and oxygen have great influence on the joint strength and bending plasticity. With the increase of nitrogen and oxygen content in the weld, the joint strength increases, the bending plasticity decreases, and the effect of nitrogen is greater than that of oxygen. Hydrogen mainly affects the impact toughness of the joint.

      3.2 Crack Propensity in Weld Zone

    (1) Hot cracks.

      Since titanium and titanium alloys contain few impurities such as S, P, and C, few low-melting eutectic crystals are formed at grain boundaries, and the crystallization temperature range is narrow, and the shrinkage of the weld when solidified is small, so the thermal cracking sensitivity low.

    (2) Cold cracks and delayed cracks.

      When the weld oxygen and nitrogen content is high, the performance of the weld seam becomes brittle. Under the effect of larger welding stress, cracks will appear. This crack is formed at a relatively low temperature.

      When the titanium alloy is welded, delayed cracking sometimes occurs in the heat-affected zone, and hydrogen is the main cause of delayed crack formation. The method of preventing delayed cracking is mainly to reduce the source of hydrogen at the welded joint, and if necessary, vacuum annealing can be performed to reduce the hydrogen content of the welded joint.

      3.3 weld porosity

      Stenosis is a common defect in titanium and titanium alloy welding. O2, N2, H2, CO2, and H2O may all cause porosity. Titanium and titanium alloy weld pores are mostly distributed near the fusion zone, which is a feature of titanium and titanium alloy pores. The pores in the weld not only cause stress concentration, but also reduce the plasticity of the metal around the pore, and even lead to the fracture of the entire welded joint. Therefore, the generation of pores must be strictly controlled.

      Titanium and its alloy welding methods and research status

      1 TIG welding

      TIG welding is the most commonly used method for titanium and its alloys. It is an excellent method for joining thin plates and for bottom welding. A good welding can be achieved by selecting suitable process parameters. The disadvantages are the slow welding speed, large deformation of the weldment, and the coarseness of the weld seam; the welding holes are easy to produce welding defects such as air holes and tungsten inclusions; the welding process is prone to poor gas protection and affects the weld quality. The pulse frequency of TIG welding has an effect on the grain size and morphology of the titanium alloy. When the pulse frequency is too high or too low, the weld zone is columnar and the strength is low. When the frequency is moderate, it is an equiaxed crystal. The corresponding intensity It is also higher. In recent years, India's TIG welding of titanium alloys has been relatively comprehensive.

      M. Balasubramaniana et al. conducted a pulsed arc welding experiment on a titanium alloy (Ti-6Al-4V) and found that there is a relationship between grain size and hardness and welding parameters as follows:

Grain size GS = 81.43-18.33P -14.17B-10.83F+15T+25.68P2+18.18B2 + 61.93F2 +25.68T2; hardness H=472.15+8.54P-6.87B+4.38F-5.62T 17.57P2-12.57 B2-36.32F2 -15.07T2 + 1.56PF.

      Among them, P represents the peak current, A; B represents the base value current, A; F represents frequency, Hz; T represents time. The experimentally determined, the model predicts the accuracy of grain size and hardness can reach the level of 99%.

      Balasubramanian et al. also found that the impact of pulsed TIG welding parameters on the corrosion behavior also found that with the increase of pulse peak current and the increase of pulse frequency, the corrosion resistance of the joints rises, reaching the optimal value, with the peak current of the pulse and As the pulse frequency continues to increase, the corrosion resistance decreases, and as the crystal purity increases, the corrosion resistance increases. However, the other properties can be kept at a good level while ensuring the minimum corrosion. The grain size and hardness can be calculated by the above formula to predict its possible performance.

      The domestic A-TIG welding method is of great concern. This method is a new technology that has been developed in the past 10 years to increase the welding penetration, improve the weld forming and welding quality, and improve the welding production efficiency.

      For the deep penetration of A-TIG welding, Liu et al. used a single active flux test and found that the penetration depth of A-TIG welding was significantly higher than that of conventional TIG welding under the same process parameters. Further experimental results show that the fluorochemical activator can increase the welding penetration, that is, the fluorochloride is the main factor to increase the penetration of the titanium alloy.

Through experiments, it is confirmed that the effect of active flux on the formation of titanium alloy welds is very obvious: under the same conditions, not only increase the weld penetration depth, reduce the width of the weld, reduce the heat input during welding, but also significantly reduced The grain size of the weld seam; when A-TIG welding is performed on titanium alloys of different thicknesses, the crystallization morphology of the weld seam is uniform, showing the opposite direction from the parent metal on both sides to the center line of the weld; A- The shape of the cross-section of the weld during TIG welding is very different from that of the TIG weld. The shape of the weld shows a cup-like feature with single-sided welding on both sides. This feature improves the mechanical properties of the weld.

      The active agent formulation that is the core of the technology is the bottleneck restricting the development of this technology. Due to the complexity of the study of formulas, the domestic search for suitable materials through the introduction of and orthogonal test and uniform test method, due to the different mechanism of action, the effect of a better mixture of materials alone may decrease, so the study of the active agent Still need further experimental research and exploration.

      2 Plasma arc welding

      As the welding specification of plasma arc welding is narrow, the defects of poor welding stability and repeatability have become a major obstacle to the process of industrial application of plasma arc welding and the development of its own technology. Since the 1990s, due to continuous improvement in the level of plasma arc welding equipment manufacturing and control technology, the stability of plasma arc welding has been greatly improved. Therefore, in the process of perforated plasma arc welding, to grasp the factors that affect the stability of the welding and the role of the law, the use of advanced control technology to further improve the welding automation and control of the degree of precision, must be the focus of future research.

The results of Liao Zhiqian et al. showed that the tensile properties of plasma welded joints are good, and the properties of the welded joints are comparable to those of the parent metal. The weld toughness is lower than that of the parent metal. The weld microstructure is residual β phase and martensite needle-like α phase. The tensile properties, impact properties, and joint hardness distribution correspond to the hardness and strength of these structures that exceed the base metal, but the plasticity is low.

During piercing welding, there is a problem that the perforation arc is unstable and the energy of the perforation line cannot be maintained at a minimum value. This is a problem that needs to be solved to achieve stable perforation welding. Lu Licheng mainly studied the effect of arc-starting parameters on the process of perforating heating and excavating. By controlling the distribution of temperature field at the moment of perforation, the stable forming of arc initiation was achieved. The experimental analysis shows that the welding current is the decisive factor influencing the perforation heating process and the distribution of the temperature field. The ion gas flow rate mainly affects the piercing time in the welding heating process. Welding current and ion gas flow rate are equally important for the depth of penetration and the shape of the hole in the non-penetrating stage. By adjusting the arc starting procedure, the temperature field around the small hole at the time of perforation is close to the steady temperature field distribution of the excellent weld, and the wire feeding time is guaranteed to be 1~2 s ahead of the perforation time, and the stable transition and forming of the arc starting segment can be realized. control.

      Dynamically controlled plasma welding allows plasma arc welding to be switched between perforated welding and penetration welding by controlling the peak current and the ground state current under the condition of ensuring penetration, thereby satisfying the conditions of the weld under the condition of minimum heat input. . Compared with the conventional plasma welding, due to the reduction of heat input, the fusion zone of the joint is reduced and the grain size is reduced. Although the microstructure changes little, the precipitated β-phase grain in the weld is greatly reduced, resulting in martensite. The formation is inhibited and the welded joints have better toughness and higher hardness.

      3 Vacuum electron beam welding

      Vacuum electron beam welding is very suitable for the welding of titanium and titanium alloys. This is mainly because it has a series of advantages: good quality of welding metallurgy, narrow welds, large ratio of depth to width, small distortion of weld angles, fine grain in welds and heat affected zone, good joint performance, welds and heat affected zone. It will not be polluted by air and it will be highly efficient when welding thick parts. The disadvantage is that the pores are prone to occur in the welds. The size of the structure is easily limited by the vacuum chamber and is not suitable for mass production. However, the quality of the small-sized workpieces has an absolute advantage.

      Welding joints will generate considerable residual stress, and with the increase in the thickness of the weldment increases, so the researchers explored the local heat treatment of electron beam to reduce the possibility of residual stress. The experimental results show that the local heat treatment of electron beam can improve the microstructure and properties of the titanium alloy weld, and refine the grain structure in the weld zone. This not only increases the peak value of the longitudinal tensile stress at the center of the weld, but also makes the transverse center of the weld seam. The stress is compressive stress, which greatly improves the distribution of welding residual stress and improves the welding quality. A similar phenomenon has been found for 14.5 mm thick plates, which further proves that for thick plate titanium alloys, electron beam local heat treatment has a significant effect on improving the state of welding residual stress and improving the welding quality.

      Due to the large residual stress of large plates, vacuum electron beam welding is more often used on thin plates. The electron beam welding of large thick plate titanium alloys was subjected to relevant experiments and it was found that the microstructure was a typical TC4-DT containing an α phase and a flake-shaped (α+β) dual phase structure, which could be obtained by electron beam welding. Defective quality welded joints. The fusion zone forms a martensite network of nets, and the precipitated β-phase grain boundaries accumulated in the layers are clearly visible in the upper and middle parts of the weld metal, but not so obvious at the bottom, and the β grain size and the martensite length Gradually decrease from the top to the bottom of the fusion zone. The microstructure of the heat-affected zone is not uniform, and the heat-affected zone near the fusion zone is composed of acicular martensite and a small amount of primary α-phase, while the heat-affected zone near the parent metal is transformed from the primary α-phase and contains acicular α. β phase composition. The boundary of the two heat affected zones depends on the β phase transition temperature during the weld cooling. As the depth in the plate thickness increases, the grain size in the fusion zone decreases, and the microhardness increases. This test provides a good experimental basis for further research, theoretical analysis and application of large thick plate titanium alloys.

     4 Laser welding

      Laser welding is superior to other welding methods in quality and efficiency. Laser easy-to-use mirrors or prisms change the light path and can be soldered anywhere on the workpiece. Laser welding may have wider application prospects for the welding of thin plates and precision parts of titanium and titanium alloys. However, laser welding also has its disadvantages: The penetrating power is not as strong as the electron beam.

      After the performance of the laser welding of titanium alloy plate, relevant studies have shown that the mechanical properties of laser welded joints are affected by weld formation and weld microstructure. When the welding heat input is large, there are densely scattered acicular martensite in the weld, which increases the tensile strength value. When the coarse columnar crystal structure appears in the weld, the yield strength and relative displacement of the welded joint are reduced, which reduces the plastic toughness of the joint. Through reasonable choice of parameters, the tensile strength, shear strength and other properties of the joints can be equal to those of the base metal. The fatigue performance of the joints can be significantly improved after vacuum heat treatment. Although the bending angle improves after vacuum heat treatment, it can only reach 1/2 of the base metal. Therefore, the design of titanium alloy structure should avoid placing the weld at the maximum bending moment.

      The advantages of laser welding are obvious, but at present, laser welding involves the technological issues of gas protection, sample cleaning and photo-plasma control that affect the welding quality of titanium alloys. It is imperative to improve and improve laser welding. Due to the problems of laser welding, the use of laser hybrid welding technology can reduce or even eliminate defects in laser welding, which can improve the welding quality of the weld.

      The quality of the laser after the composite welding, the test results show that, under the appropriate welding conditions can be formed without surface oxidation, porosity, cracks and incomplete penetration welding defects such as excellent weld. Compared with LBW, laser-MIG welded joints have better ductility. Using low-strength TA10 welding wire can improve weld forming quality and reduce micro-hardness, but the heat-affected zone hardness may increase significantly due to large heat input. Laser-MIG welding techniques are considered to eliminate microcracks, hinder the formation of air holes, and improve the composition of the weld. By adding some anti-crack elements to the welding wire, defects such as high susceptibility to hot cracking and reduced strength can be reduced and eliminated. The degree of mixing and diffusion of the wire droplets into the base metal is greatly affected by the liquid flow kinetics in the bath. This requires that in the future research, attention should be paid to finding a more suitable welding wire and a reasonable combination of process parameters so as to ensure good welding quality.

      The reasonable combination of laser and other methods such as MIG or TIG, such as official website, legal community, legal product list, application method, etc., can ensure the optimal performance of hybrid welding. . The study found that the laser energy density determines the formation and disappearance of keyholes. For this purpose, they calculated and quantitatively measured the propagation characteristics and absorption characteristics of the laser beam passing through the arc, thus confirming the view that the laser-TIG composite heat source welding has a finitely enhanced depth of penetration and a change in the welding mechanism. The welding effect provides the basis.

      In addition to reducing defects, the microstructure after composite welding is also different from that after laser welding alone. Both the laser welding and the composite welding technology have α-phase in the weld. The laser welding contains a rough columnar α-phase and a small amount of fine acicular α-phase. The microstructure of the composite welding head includes acicular α, lamellar α, and Twin crystal phase, this microstructure allows the composite joint to have good joint strength and ductility. The EDX analysis shows that the density fraction of oxygen in the weld fusion zone is indeed higher than that of the parent.