Original Title: Research Progress of Titanium Alloy Additive Manufacturing Technology 1. Preface Titanium and titanium alloys have excellent physical and chemical properties such as low density, high temperature resistance and corrosion resistance, so they have broad application prospects in various industrial fields, including shipbuilding, aerospace, automobile manufacturing, and they are also one of the important materials in national defense industry. The application of titanium alloy has played a huge role in promoting industrial development. Its performance superior to traditional materials has greatly improved the quality of its products, met the development requirements of industrial development for new materials and new processes, and accelerated the development of modern industry. With the continuous improvement of titanium productivity, titanium alloys have become the third metal in industrial production [2]. Additive Manufacturing (AM), also known as "3D printing", is a digital manufacturing technology that can realize dieless forming of components, which has the characteristics of integration of design and manufacturing, high processing accuracy, short cycle and excellent physical and chemical properties of products. Additive manufacturing technology has developed rapidly since the 1970s. Because of its great difference from traditional manufacturing technology, it has become a research hotspot in the industrial field and has developed rapidly in many fields of modern industry [3]. With the rapid development of additive manufacturing technology, any single or multi-metal composite structure can be realized in theory, which provides a new method for the manufacture of complex structural parts. The additive manufacturing technology of titanium alloy solves the processing problem of precision structural parts and further enlarges the application scope of titanium alloy [4]. With the rapid development of industrial society, titanium alloy additive manufacturing technology is changing with each passing day. According to the different heat sources of additive manufacturing technology, titanium alloy additive manufacturing technology can be divided into laser/electron beam additive manufacturing, fusion welding additive manufacturing and solid state welding additive manufacturing. Experts and scholars at home and abroad optimize the process method, stabilize the additive manufacturing process, reduce or avoid the structural defects of additive manufacturing through different means of additive manufacturing technology, so that the titanium alloy additive manufacturing technology will continue to develop in the direction of green, efficient and stable. 2. Laser /Electron Beam Additive Manufacturing Laser beam and electron beam, as high-density beam sources, have high energy density and can be controlled, and are known as the most advanced manufacturing technology in the 21st century. At present, laser/electron beam additive manufacturing is mainly divided into laser metal deposition (Laser Mental Deposition, LMD) technology, laser selective melting (Selective Laser Melting) technology, SLM technology, Electron Beam Fuse Deposition (Electron Beam Free Form Fabrication, EBF3) technology and Electron Beam Melting (EBM) technology have been widely studied in the field of titanium alloy additive manufacturing. 2.1 Laser metal deposition (LMD) M a H amo o d et al. [5] used LMD technology to study the Functionally graded materials (FGM) of Ti6Al4V/TiC, optimized the process according to the early empirical model, and obtained the optimized FGM. The microstructure, microhardness and wear resistance were characterized. The results show that the FGM fabricated by the optimized process parameters has higher performance, and the hardness of the FGM is four times that of the matrix, up to 1200HV. Silze et al. [6] conducted an experimental study on additive manufacturing of Ti6Al4V using a new semiconductor laser using LMD technology. The LMD device consists of six 200 W semiconductor laser heads circularly encircling the feed gun (see Figure 1). The laser beam diameter is 0.9 mm, which can realize a direction-independent welding process, and the microstructure is defect-free. The results show that the cooling time increases and the grain thickness decreases with the increase of interlaminar residence time, which is helpful to improve the mechanical properties of the material.The minimum yield strength and tensile strength of forged Ti6Al4V can be met by LMD additive manufacturing. Expand the full text Heigel et al. [7] studied the thermal and mechanical evolution process of Ti6Al4V laser deposition additive manufacturing process by using in-situ temperature and stress real-time measurement and thermal-mechanical model combined with finite element thermal-stress sequential coupling model, and found that the maximum residual stress appeared below the center of the additive layer, and the stress decreased to both sides. With the increase of residence time, the temperature difference between layers becomes larger, and the residual stress increases. Zuo Shigang [8] used TA15 titanium alloy spherical powder to study the manufacturing process of TC17 titanium alloy additive repair by laser deposition technology, and studied the influence law of microstructure characteristics and mechanical properties of repaired parts. The results show that there are no welding defects in the TA15/TC17 specimens repaired by laser deposition technology, and the tensile strength of the repaired specimens is 1 029 MPa.After annealing treatment, the mechanical properties of the repaired specimens are significantly enhanced, and the tensile strength can basically reach the standard of TC17 forgings, while the elongation is better than the standard. To sum up, the LMD technology additive manufacturing for titanium alloys is relatively stable, and the mechanical properties of additive parts basically meet the minimum standard of forgings. For some titanium alloys with specific requirements, the heat treatment after additive manufacturing should be carried out to meet the use requirements. 2.2 Selective Laser Melting (SLM) Tang Siyi et al. [9] used SLM technology to prepare Ti6Al4V titanium alloy specimens (see Fig. 2),titanium filler rod, and analyzed the microstructure, mechanical properties and densification behavior. The results show that when the laser power increases from 360 W to 400 W, the relative density increases significantly.When the laser power continues to increase after 400 W, the relative density is greatly affected by the laser scanning speed, and the quality of the sample under the optimal process parameters is much higher than the forging standard. Polozov et al. [10] used SLM technology to fabricate Ti-5Al, Ti-6Al-7Nb and Ti-22Al-25Nb bulk alloys, annealed the Ti-Al-Nb system, and systematically characterized the samples. The results show that Ti-5Al can be fabricated by adding SLM, while Ti-6Al-7Nb and Ti-22Al-25Nb need to be heat treated at 1350 ℃ to completely dissolve the Nb particles, but the oxygen content of the samples is higher and the mechanical properties are reduced. Fan et al. [11] studied the microstructural stability of Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) titanium alloy fabricated by SLM technology under standard aging (595 ℃/8 H). The results show that with the increase of laser scanning speed, the relative density increases to 99. 5% and then decreases sharply to about 95. 7%, the tensile strength of Ti-6242 after aging treatment increases from 1437 MPa to 1510 MPa, and the ductility decreases from 5% to 1. 4%. At the same time, the hardness increases from 410 HV to 450 HV, which is attributed to the precipitation hardening effect of β phase particles. Ren et al. [12] studied the microstructure and properties of Ti-Ni shape memory alloy by using SLM technology for additive manufacturing, and prepared equiatomic Ti 50 Ni 50 (mass fraction) samples. The results showed that almost completely dense samples could be manufactured at a laser power of 40 J/mm3 and a scanning speed of 1000 mm/s. The effect of different scanning speeds on the phase composition, phase transformation temperature and Vickers hardness is limited.Compared with traditional castings, SLM technology has higher vacuum compression and fracture strength. To sum up,titanium bar gr7, the SLM technology additive manufacturing of Ti6Al4V is relatively easy to realize, and the SLM technology additive manufacturing of titanium and other element alloys needs to be further studied. Preheating or other heat treatment means and oxygen content control means are needed to enhance the mechanical properties of other titanium alloys SLM technology additive manufacture. High quality research samples were obtained. 2.3 Electron Beam Fuse Deposition (EBF 3) Jin Wenying [13] studied the additive repair technology of electron beam fusion deposition of TC4 titanium alloy, and compared the additive repair performance of ordinary TC4 welding wire and self-made TC4EH welding wire. The results show that the tensile strength (905. 23 MPa) of the self-made TC4EH welding wire is significantly higher than that of the common TC4 welding wire (809. 04 MPa), the hardness and impact toughness are also high, and the elongation is more than 90% of the raw material, which shows that the self-made TC4EH welding wire has excellent mechanical properties. Chen et al. [14] studied the deformation control of Ti6Al4V by electron beam fuse deposition (see Figure 3). When the electron beam works at a scanning current between 100 and 150 ma and a speed of less than 100 mm/s, a thin-walled part can be formed. The scanning form has little effect on the distribution of residual stress, and the deformation of unidirectional scanning is greater. Shrinkage deformation is more obvious in the case of back and forth scanning, and is proportional to the current change. At the same time, it is found that the deformation is improved under the constant temperature constraint at the bottom of the substrate. Yan et al. [15] studied the residual stress and deformation of Ti6Al4V stiffeners deposited by electron beam fuse. It was found that both stiffeners caused adverse deformation to the plate. The longitudinal track caused greater deformation than the transverse track. The deposition track of the stiffener had a great influence on the deformation. The maximum displacement occurs at the inner bottom edge of the stiffener associated with the longitudinal rail, and the high residual stress area is mainly concentrated at the root of the stiffener. To sum up, there are relatively few studies on additive manufacturing of titanium alloys by electron beam fuse deposition, which mainly focus on deformation control with the help of finite element analysis software. The analysis shows that the electron beam fuse deposition additive manufacturing can overcome the disadvantages of the traditional titanium alloy processing method, and the finite element analysis software can provide theoretical guidance for the practical application process. 2.4 Electron Beam Melting (EBM) Murr et al. [16] studied the relationship between stiffness and density of porous Ti6Al4V foam fabricated by EBM additive manufacturing. The results show that the foam has solid and hollow cell structure, and the strength of hollow cell structure is proportional to the hardness, which is 40% higher than that of solid and compact EBM, and the stiffness is inversely proportional to the porosity. The foam materials made by EBM have great potential in biomedical, aerospace and other fields. Xu Fei et al. [17] carried out experimental research on high-power high-speed fiber laser welding of TC4 titanium alloy prepared by electron beam selective melting technology. The results show that due to the grain size difference of TC4 fabricated by EBM, the β-columnar grains near the upper and lower surfaces of the fusion zone are relatively fine. The microhardness of the weld zone is higher than that of the additive zone, and the hardness of the top is higher. Seifi et al. [18] studied the microstructure and properties of Ti-48Al-2Cr-2Nb fabricated with EBM additive and found that the strength and hardness values of the deposited material exceeded those obtained with conventionally cast Ti-Al, which is consistent with the presence of finer microstructures in the additive materials tested so far. Surmeneva et al. [19] studied the microstructure and properties of Ti – 10% Nb (mass fraction, the same below) added by EBM technology. As a result, it was found that the Ti-10% Nb alloy was in-situ produced in the powder mixture of the EBM technology elements Nb and Ti, the largest Nb particles were retained in the EBM fabricated sample, and Nb was only partially diffused into Ti, as shown in Fig. 4. More studies should be made on the parameter optimization of the EBM process to achieve a more uniform alloy microstructure. To sum up, the EBM research on Ti6Al4V is relatively extensive, and it is found that the EBM additive manufacturing of Ti-Nb alloy is still difficult to solve the diffusion problem of Nb particles, which will lead to uneven microstructure. Therefore, more process optimization tests are needed to improve the material properties for the additive manufacturing of Ti-xNb alloy. 3. Fusion welding additive manufacturing Compared with other additive manufacturing methods, fusion welding additive manufacturing has stronger operability and lower cost, but its structural reliability is relatively low. Fusion welding additive manufacturing typically employs wire additive manufacturing, but due to the large thermal gradient between the substrate and the initial deposited layer, as well as radiative and convective heat losses, a fine-grained structure is observed at the bottom of the fabricated component. Due to the lower thermal gradient, the heat transfer rate is lower, which hinders the formation of a fine-grained structure in the middle layer of the additive process, ti6al4v ,3d titanium wire, while only long columnar grains are formed in the middle of the fabricated part. 3.1 CMT ARC Additive Manufacturing Li Lei et al. [20] used CMT arc additive TC4 thin-walled structure to study the microstructure and properties of its additive layer. The results show that the original β columnar grain boundary, horizontal layer band stripe, martensite structure and basketweave structure appear in the additive layer due to the repeated action of thermal cycle in the additive process, and the microhardness of the upper additive layer is slightly lower than that of the middle and lower parts due to the strengthening effect of aging effect on the middle and lower parts (see Figure 5). Chen Wei [21] studied the microstructure and mechanical properties of CMT arc additive TC4. The results show that the original β grain section area is the smallest when the wire feeding speed is 3. 0 m/min and the welding speed is 0. 48 m/min. The heat treatment of CMT arc additive manufacturing TC4 titanium alloy is 870 ℃, 1 H/solid solution furnace cooling (FC) + 600 ℃, 2 H/solid solution air cooling (AC). The obtained microstructure in each region is more uniform, and the plasticity of the material after solution treatment is higher. 3.2 Plasma arc additive manufacturing Lin et al. [22] employed PAW additive fabrication of Ti6Al4V, which was investigated in terms of microstructure and microhardness. It was found that the epitaxial growth of the previous β columnar grains was inhibited by the pulse perturbation, which led to the formation of columnar grains with nearly equiaxed grains, low microhardness in the early stage of deposition due to insufficient thermal cycling, and increased hardness in the subsequent deposition, which was not affected by continuous thermal cycling at the top of the deposition layer, resulting in a decrease in the volume of the second phase and a decrease in hardness values. Ma Zhaowei [23] studied the microstructure and properties of titanium alloy fabricated by bypass hot wire plasma arc additive manufacturing (see Fig. 6). The results show that the transverse tensile strength of the titanium alloy additive component is 977 MPa, which is equivalent to the tensile strength of TC4 base metal, and the fracture location is in the tail area of the additive straight wall structure. This is because the transverse weld is continuous melting-solidification, and there are fewer defects and impurities in the weld, so the vertical tensile strength of the titanium alloy additive component with good strength performance of the transverse weld is 936MPa, the fracture location is in the upper part of the straight wall structure, and the performance is slightly worse than that of the transverse weld. The hardness of the heat affected zone near the base metal is relatively low, and there is a small range of softening zone, and the overall vertical hardness difference is not obvious. 3.3 Composite arc additive manufacturing Pardal et al. [24] performed a structural stability study of laser and CMT hybrid welding additive fabrication of Ti6Al4V. It was found that the laser can be used to stabilize the welding process, reduce welding spatter, improve the arc drift, improve the weld shape for single and multi-layer deposition, and increase the deposition rate of Ti6Al4V additive manufacturing from 1. 7 kg/H to 2. 0 kg/H. To sum up, the additive manufacturing of titanium alloy by fusion welding is mainly concentrated in the research of TC4, which mostly uses CMT, plasma and other efficient fuse processes, while using other heat source assisted welding methods to stabilize the welding process and carry out the additive manufacturing of titanium alloy. The analysis shows that for the development direction of fusion welding titanium alloy additive manufacturing, the research and preparation of titanium alloy functional materials should be developed to facilitate the application and promotion in various fields, and the additive mode of composite heat source or other controllable heat input should be stable. Additive method will become a hot research direction of welding additive. 4. Solid-state welding additive manufacturing 4.1 Friction Stir Additive Manufacturing (FSAM) Friction stir additive manufacturing is a solid phase additive technology developed from friction stir welding technology, and its principle is shown in Figure 7. The method has the advantages of high additive efficiency and low cost; no metal is melted and solidified in the additive process, so that the problem of metallurgical defects caused by a molten pool can be avoided; and meanwhile, the plastic deformation in the stirring friction process can also play a role in grain refinement, so that low-cost and high-quality additive products can be obtained. Zhang Zhao et al. [25] established two finite element models of friction stir additive manufacturing Ti6Al4V based on Abaqus birth and death element method and moving heat source method to study the temperature distribution and grain growth of friction stir additive. The results show that the peak temperature of transverse addition is higher than that of longitudinal addition, and the grains are coarsened and transformed from β phase to α phase during cooling and accumulation in the stirring zone. Due to the influence of different thermal cycles, the grain size in the lower stirring zone is larger than that in the upper stirring zone. 4.2 Ultrasonic Additive Manufacturing (UAM) Ultrasonic additive manufacturing (UAM) is a new rapid prototyping process for fabricating metal matrix composites at or near room temperature. The lower processing temperature enables the composite to generate recovery stress by utilizing highly pre-strained shape memory alloy (SMA) fibers embedded in the matrix. Hahnlen et al. [26] studied the interfacial strength of NiTi-Al composite structure fabricated by UAM technology, and the strength of fiber-matrix interface is the limiting factor of UAM composite. The results show that the average interfacial shear strength is 7. 28 MPa, and the bonding mode between the fiber and the interface is mechanical bonding, not chemical bonding or metallurgical bonding. In order to improve the load-bearing capacity of carbon fiber reinforced polymer (CFRP), so that it can be further promoted and applied in the aerospace and automotive industries, James et al. [27] conducted a study on the shear failure strength of CFRP/Ti in ultrasonic additive manufacturing, and found that the structure of CFRP/Ti can be manufactured by UAM technology. Both ultrasonic energy and surface roughness have a positive effect on the shear strength of the structure made of UAM, and increasing the surface roughness of the interface before welding helps to increase the shear failure load of the final weld. To sum up, there is little research on ultrasonic additive manufacturing of titanium alloys, and the main research is on metal matrix composites to enhance the specific properties of composites to meet the actual production and application. It is believed that in the future research, we should focus on improving the mechanical properties of composites. 5 Concluding remarks With the rapid development of modern industry, lightweight design has become the development direction of structural parts, and the requirements for the performance and quality of structural parts have become more and more stringent. The rapid development of titanium alloy additive manufacturing technology can further expand the application scope of titanium alloy structural parts, improve the performance of titanium alloy additive parts, and enhance structural stability. Based on the research of titanium alloy additive manufacturing technology at home and abroad and the development direction of modern industry, the future titanium alloy additive manufacturing technology is destined to develop in the direction of green, economic, stable and rapid. 1) From the perspective of green development, friction stir additive manufacturing started late and is still in the experimental research stage. In the future, additive manufacturing of composite structures of multi-metallic materials to achieve special properties of specific structures will be a research direction of this technology. 2) For the economic and stable development direction, it is necessary to explore the stability process of arc additive manufacturing, especially the stability of new composite arc additive manufacturing. 3) For the development direction of rapidity, the laser/electron beam additive manufacturing process is relatively mature at the present stage, so we should continue to explore the economic applicability of laser additive manufacturing, from the assembly accuracy in actual production to the process optimization process in production and manufacturing, so as to reduce production costs and lay a foundation for the large-scale production and application of titanium alloy additive manufacturing structural parts. References: [1] Chen Guolin, Wu Pengwei, Leng Wenjun, et al. 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About the Authors: Dong Chunlin, Tan Jinhong, Lin Zhicheng, Wang Chungui, Zhao Yunqiang, Guangdong Welding Technology Research Institute (Guangdong Zhongwu Research Institute) Source: Metalworking, Hot Working, Vol. 7, 2020, pp. 16-21 About Metalworking (Hot Working): Founded in 1950, Metalworking (Hot Working) is a state-level academic scientific and technological journal published by the Metalworking Editorial Department, which is managed by the China Machinery Industry Federation, sponsored by the Machinery Industry Information Research Institute, and published on the 1st of each month. Metal Processing (Hot Processing) has been included in China Core Journal (Selection) Database, CNKI China Journal Full-text Database,Titanium 6Al4V wire, Wanfang Database, VIP Journal Database, Superstar Discovery Database and other important databases. Return to Sohu to see more Responsible Editor:. yunchtitanium.com