Titanium alloy has the characteristics of low density, high specific strength, corrosion resistance, and high and low temperature resistance. Its forming technology was initially developed to meet the needs of the aerospace industry. In the early days, to meet the urgent research and development needs, it was mostly purchased by the government, so it has the characteristics of heavy weight and light cost. In recent years, China's high-tech field has developed rapidly. In order to meet the demand for lightweight structural components, the demand for high-quality titanium alloy components in major aerospace projects such as manned spaceflight, lunar exploration, large aircraft, and high-resolution satellites, as well as new weapons and equipment, has been increasing year by year. At the same time, titanium alloys are gradually being widely used in civilian fields such as ships, chemicals, medical care, nuclear power, and fire protection.

However, the production cost of high-quality titanium alloy components remains high at present, mainly due to two aspects: first, the smelting process requires strict requirements. Titanium is extremely active at high temperatures and is prone to chemical reactions with elements such as oxygen, nitrogen, silicon, and carbon. The smelting and heat treatment processes need to be carried out under vacuum or inert gas protection, making it difficult to control the purity and uniformity of the composition; The second is the difficulty in forming. Titanium alloys have poor deformation ability, high yield/elasticity ratio, and low thermal conductivity, making it difficult to form using conventional cold/hot processing. It is estimated that in the entire production process of titanium alloy components, the cost of preparing sponge titanium raw materials accounts for about 14%, the cost of processing titanium alloy ingots and plates accounts for about 36%, and the cost of forming titanium alloy final products accounts for about 50% (Figure 1). It can be seen that the cost composition of titanium alloy products mainly comes from the preparation of castings and plates, forging, casting, machining and other forming processes.

Taking castings as an example, the production cost of high-end military grade titanium precision castings reaches 1500-2500 yuan/kg, while the production cost of ordinary civilian grade titanium precision castings is also 500-800 yuan/kg. The production cost of stainless steel precision castings is about 50-200 yuan/kg, and the production cost of titanium precision castings is more than 10 times that of stainless steel precision castings. High cost has become a bottleneck restricting the promotion and application of titanium alloy products. Therefore, reducing the production cost of titanium alloy products has become one of the important issues that many research institutions and production units are constantly tackling and solving, and there is an urgent need to develop low-cost titanium alloy materials and their low-cost preparation and forming technologies.

Figure 1 Cost composition and proportion of the entire production process of titanium alloy products

At present, there are three main ways to achieve low-cost production of titanium alloy materials and their forming technologies. One is to use alloy composition design with cheap elements. When designing titanium alloy materials, cheap elements such as Fe, Si, Al, Sn are used instead of expensive elements such as V, Mo, Zr, Nb, Ta, etc., to reduce alloying costs while maintaining the mechanical properties of titanium alloys; The second is to recycle and reuse titanium materials. The price of residual titanium (residual titanium materials) is only 20% to 30% of the price of sponge titanium. Adding recycled materials during the melting and preparation process of titanium alloys can greatly reduce the preparation cost of cast plates. However, there are often problems such as alloy element segregation and impurity element content in recycled materials; The third is to carry out research on low-cost processing and forming technology. The high cost of forming titanium alloy components is the main reason for their high prices. Optimizing and innovating the forming process is an important way to achieve low cost. For titanium alloy ingots, integrated control of melting and refining can be carried out to reduce the melting frequency of titanium alloy ingots. For titanium alloy deformation processing, integrated control of solidification and deformation can be achieved through short process technologies such as continuous casting and rolling. For titanium alloy casting and forming, low-cost refractory materials can be used to replace yttrium rare earth oxide materials to prepare molds.

1. Development of low-cost titanium alloy materials

In order to reduce the production cost of titanium alloy materials, countries around the world have conducted research on new types of titanium alloy materials. The main approach is to use inexpensive intermediate alloys such as Fe, Si, Al, Sn to replace expensive elements such as V, Mo, Zr, Nb, Ta, etc. Based on the above ideas, the main low-cost titanium alloys currently developed domestically and internationally are shown in Table 1.

Table 1 Major low-cost titanium alloys at home and abroad

1.1 Timetal 62S alloy

Timetal 62S (Ti-6Al-2Fe-0.1Si) alloy is a new low-cost titanium alloy designed and developed by Timetal Corporation in the United States for non aerospace applications, belonging to the α+β type alloy. The original intention of this alloy design is to replace Ti-6Al-4V alloy with Fe element instead of V element in Ti-6Al-4V alloy. The addition of an appropriate amount of Si can refine the structure, and the alloy properties are not inferior to Ti-6Al-4V. The cost is reduced by 15% to 20%, and it has excellent cold and hot workability. This alloy is mainly used in high-strength and damage resistant titanium alloy sheet metal structural parts in the civilian field, and has replaced Ti-6Al-4V alloy in the production of valve seat rings.

1.2 Timetal LCB alloy

Timetal LCB (Ti-4.5Fe-6.8Mo-1.5Al) is a high-strength beta alloy developed by Timetal Corporation in the United States. The original intention of designing this alloy is to replace Ti-10-2-3 (Ti10V2Fe3Al) by adding Fe element instead of V element in the form of Fe Mo intermediate alloy. TimetalLCB alloy has high strength and good formability, and can be cold or warm worked like steel. Its performance is comparable to Ti-10-2-3, and its cost is 78% of Ti-6Al-4V. Mo in this alloy is a β - stable element that can form compounds of Fe and Mo. After aging hardening, it has high tensile strength and has been applied in automotive parts, springs, and suspension springs in Japan and the United States.

1.3 ATI425 alloy

ATI425 alloy (Ti-4Al-2.5V-1.5Fe-0.25O) is a β - type low-cost titanium alloy developed by ATI Wah Chang in the United States. It replaces some V elements with Fe elements, reducing costs, and has good mechanical properties and corrosion resistance. Its tensile strength can reach 827-965 MPa, yield strength can reach 758-896 MPa, elongation can reach 6% -16%, and its ballistic resistance is comparable to Ti6Al4V, meeting the material performance requirements of current military armor standards. It has been used in the field of weapons armor plates and military vehicle components.

1.4 Ti-Fe-O-N series alloys

The Ti-Fe-O-N series alloy is a beta type alloy developed by Japan Steel Corporation and Toho Titanium Corporation. This type of alloy uses Fe, O, and N elements instead of the V element in Ti-6Al-4V alloy, with 0.5%~1.5% Fe, 0.2%~0.5% O, and 0.05%~0.1% N. The room temperature strength of this alloy system can reach 700-1000 MPa, but its high-temperature performance is poor. The representative of this alloy system is Ti-1% Fe-0.35% O-0.01% N, which has a tensile strength of about 800 MPa and is mainly used in alloys designed for applications other than aviation.

1.5 SP700 alloy

SP700 (Ti-4.5Al-3V-2Mo-2Fe) is a superplasticity titanium alloy material developed in Japan, which can achieve superplastic forming and diffusion bonding at 775 ℃. The superplasticity forming temperature is lower than Ti-6Al-4V alloy, and the tensile strength and fatigue strength are better than Ti-6Al-4V alloy. It can be used to manufacture thin plate shaped aerospace structural components. Due to avoiding the defects of high deformation resistance and poor room temperature plasticity of titanium, the deformation processing cost of titanium materials has been greatly reduced. Japan has applied this alloy to the connecting rod of Honda NSX motorcycle, and RMI Titanium Company in the United States has prepared this titanium alloy into aircraft structural components and rotating parts.

1.6 Ti8LC and Ti12LC alloys

The Northwest Nonferrous Metals Research Institute has developed low-cost titanium alloys, Ti8LC and Ti12LC, through alloy design and performance testing. These alloys are Ti Al Mo Fe series alloys, and low-cost Fe Mo intermediate alloys are added to replace V and Zr in Ti-6Al-4V alloys. At the same time, pure titanium waste (such as titanium chips) is added during the melting process to reduce the amount of sponge titanium used. On the basis of ensuring performance, the raw material cost can be reduced by more than 10%, and the preparation cost of small-sized bars can be reduced by about 30%. After solid solution aging heat treatment, both alloys have good strength, plasticity, and fatigue strength. The tensile strength at room temperature can reach over 1100 MPa, and the strength and plasticity are higher than those of Ti-6Al-4V alloy in GB/T2965. Ti12LC alloy has higher strength and plasticity matching, with a strength of 1200 MPa and plasticity of 20%, which is better than Timetal 62S and Timetal LCB alloys. Ti8LC and Ti12LC can be used to prepare automotive intake and exhaust valves, bicycle torsion bars, etc. Among them, Ti12LC can also be used to prepare the tail nozzle of aerospace solid rocket engines.

1.7 Ti-5322 alloy

Ti-5322 alloy is a Ti-Fe-V-Cr-Al system α+β two-phase titanium alloy developed by Northwest Nonferrous Metals Research Institute for non aviation applications. The alloy fully considers the application of cheap Fe elements and recycled titanium materials, and adds 2% Fe instead of expensive alloy element V, which is lower in cost than Ti-6Al-4V alloy. After heat treatment, the alloy has good strength toughness matching, with room temperature strength reaching 1100~1300 MPa and elongation at 7%~14%. At present, this alloy has been applied in the development of tank armor, with better anti ballistic performance than TC4 alloy.

1.8 Ti-35421 alloy

Ti-35421 alloy is a new type of high-strength titanium alloy developed by Nanjing University of Technology for marine engineering, which meets the requirements of high strength, impact resistance, corrosion resistance, and weldability of titanium alloys. It has a tensile strength of 1313 MPa, a yield strength of 1240 MPa, an elongation rate of 8.62%, a cross-sectional shrinkage rate of 17.58%, and a fracture toughness KIC of 75.8 MPa · m1/2. It has low stress corrosion sensitivity in 3.5% NaCl solution and good corrosion resistance. This alloy has improved the domestic low-cost titanium alloy material system for ships with a strength level of 1000 MPa, which is of great significance for the selection of equipment in the design and construction process.

Low cost melting technology for 2 titanium alloys

Low cost control in the melting process of titanium alloys is mainly considered from two aspects. One is to increase the application of residual titanium to replace sponge titanium. Residual titanium mainly refers to the riser, scrap, and corner materials generated during the melting and machining processes. Scrap parts generated during the melting, testing, and part processing of castings also belong to residual titanium, with a large amount of residual titanium. Primary residual titanium (residual titanium produced from semi-finished products) can reach 30% to 50%, and secondary residual titanium (residual titanium processed from finished products) can reach 20% to 80%. Fully utilizing residual titanium can significantly reduce the cost of titanium products. The second is to improve the melting efficiency and quality, and achieve integrated control of melting and refining. At present, the most widely used titanium alloy vacuum consumable arc melting technology in China is prone to introducing low-density oxide inclusions and high-density TiW inclusions in the electrode preparation process due to the use of argon arc welding technology. At the same time, due to poor composition uniformity during the melting process, 2-3 remelting processes are required, which reduces production efficiency. At present, the melting technologies that can achieve integrated control of melting and refining, as well as residual titanium recovery, mainly include Cold Hearth Smelting (CHR) and Cold Crucible Induction Melting (CCIM).

2.1 Cold bed furnace melting technology

The cold bed furnace melting technology mainly includes Electron Beam Cold Hearth Melting (EBCHR) and Plasma Cold Hearth Melting (PACHM). Electron beam cold bed melting is the use of a concentrated and controllable stable electron beam emitted by an electron gun as a heat source to melt, refine, and remelt metals; Plasma cold bed melting is a transformation of electron beam cold bed furnace melting technology, which uses a plasma gun to emit a stabilized plasma arc instead of a vacuum electron beam as a heating source to melt and refine metals. The structure design of the cold bed furnace divides the melting process into three zones: raw material melting zone, refining zone, and solidification zone. The heating source in the melting area sequentially heats and melts the residual titanium waste on the conveying equipment. The melted titanium liquid flows into the refining area for refining, and finally enters the solidification crucible to solidify into a slab.

The cold bed furnace melting technology has the following characteristics: ① It can effectively eliminate high-density and low-density inclusions during the melting process, and obtain ingots or castings with fine grains and uniform structure. High density inclusions in the titanium liquid in the refining zone fall into the low-temperature solidification zone due to gravity, and are removed by sedimentation. The low-density inclusions float to the surface of the melt pool and are melted and eliminated by high-temperature heating. The intermediate density inclusions flow in the cold bed and gradually melt and eliminate by continuous heating in a complex flow field environment; ② Low requirements for the state of raw materials, no need to prepare electrodes, and 100% utilization of recycled residual titanium as raw materials, while vacuum consumable arc melting technology can generally only utilize less than 30% residual titanium; ③ It can be melted into alloy ingots at once, which greatly improves the melting efficiency compared to vacuum consumable melting technology, has good composition uniformity, and can save processing costs by 20% to 40%; ④ Easy to replace the crystallizer, by adjusting the crystallizer structure, it is possible to prepare various castings such as flat ingots, hollow ingots, and round ingots, improve the production efficiency of sheet and pipe production, and reduce product costs.

Advanced foreign enterprises widely use cold bed furnaces for titanium alloy melting to solve the problem of ingot inclusions. Cold bed furnace melting technology is an important way to achieve near zero defect pure purification technology for titanium alloy materials. The current aerospace material standards in the United States require the use of cold bed furnace preparation technology for important titanium alloy components. GE began using the cold bed furnace melting and vacuum consumable arc melting technology in 1988 to produce titanium alloy ingots for key rotor components of aircraft engines. TIMET company in the UK used electron beam melting technology to prepare Timetal 6-4 alloy ingots and made titanium alloy tank armor. Researchers tested the titanium alloy armor using tail fin stabilized armor piercing shells, and the results showed that low-cost Timetal 6-4 alloy can completely replace traditional Ti-6Al-4V alloy for armored vehicles. Baotai Group imported a 2400 kW electron beam cold bed melting furnace from ALD Company in Germany, and used this advanced equipment to build the first domestic return material recycling and processing production line with an annual output of 5000 tons. It can add up to about 80% of TC4 titanium alloy return material and can melt various specifications of TC4 titanium alloy ingots.

2.2 Cold crucible induction melting technology

Cold Crucible Induction Melting (CCIM) is a special melting method that uses induction heating combined with segmented water-cooled copper crucibles for melting. Specifically, the segmented water-cooled crucible is placed in an alternating electromagnetic field, and the eddy current generated by the electromagnetic field is used to melt the metal. Due to the fact that the molten metal in this method generally forms a layer of solidified shell at the bottom of the crucible, it is also called Induction Skull Melting (ISM). The biggest feature of this method is that the side wall of the water-cooled crucible is divided into more than 20 sections. Under an alternating magnetic field, the gap between every two adjacent crucible sections will produce a magnetic field enhancement effect, which will cause strong stirring through magnetic compression effect. The alloy composition and melt temperature reach equilibrium, achieving uniform melting of refractory metals. At the same time, the magnetic field generated by the induced current of each petal on the side wall of the crucible and the electromagnetic repulsion generated by the induced current on the surface of the material maintain a soft or non-contact state between the material and the crucible side wall. When melting titanium alloy ingots using this technology, the form of raw materials is basically unrestricted, and titanium alloy waste can be continuously remelted and recycled without the need for refractory materials as crucibles or welding electrodes. Therefore, its theoretical residual titanium utilization rate is 100%, and high-quality titanium and titanium alloy ingots can be obtained without pollution. CCIM equipment has undergone decades of development in countries such as the United States, Germany, Russia, and France. Currently, the equipment has a melting capacity of over 200 kg (calculated as titanium), a crucible diameter of over 500 mm, and a melting temperature of over 3000 ℃. Major foreign equipment manufacturers include CONSARC, RETECH, and ALD in the United States, and have obtained commercial applications. Currently, the capacity of the equipment developed domestically is mostly below 50 kg. In recent years, cold crucible induction melting technology has gradually combined with other material preparation technologies, developing cold crucible electromagnetic continuous casting technology, cold crucible directional solidification technology and spray deposition technology with cold crucible as auxiliary device. The biggest problem with the cold crucible induction melting technology is that the formed crust is relatively thick. Due to the integral connected structure at the bottom of the crucible, although sufficient crucible strength is ensured, the contact between the melt and the crucible at the bottom causes significant heat loss. The formed crust often exceeds one-third of the total volume, greatly reducing the melting efficiency and uniformity, especially for the melting of multiple element alloys and high melting point materials.

Cold Crucible Levitation Melting (CCLM) technology increases the electromagnetic repulsion at the bottom of the melt, eliminates the connection structure at the bottom of the crucible, and adopts a conical bottom crucible structure with completely separated upper and bottom sections. Each section is independently water-cooled, and the external coil structure is changed to increase the frequency of the electromagnetic field, the number of cutting seams, and the input power of the power supply, achieving semi suspension or full suspension of the melt. The principle structure is shown in Figure 2. Compared with cold crucible induction melting, the melting process has better overall stirring effect and composition uniformity, making it more suitable for the preparation of highly active metals, multi-component alloys, and refractory metals. The material utilization rate is high, and the scrap material still maintains high purity, which can greatly reduce the preparation cost of materials. The technical requirements for CCLM are higher, and there are relatively few developments related to it both domestically and internationally. Abroad, technologies such as CONSARC from the United States and ALD from Germany are the most advanced. Currently, multiple shell less suspension melting furnaces have been introduced from both countries in China, with a reported maximum capacity of 20 kg (in titanium). At present, the main units engaged in related research and development in China include Shenzhen Saimaite Suspension Metallurgy Technology Co., Ltd., Beijing Iron and Steel Research Institute, Shenyang Foundry Research Institute Co., Ltd. Shenzhen Saimaite currently has an induction suspension melting furnace with a maximum crucible capacity of 25 kg (calculated as titanium) and a maximum melting temperature of 2600 ℃. Shenyang Foundry Research Institute Co., Ltd. has successfully developed an induction suspension melting furnace with a crucible capacity of 30 kg (calculated as titanium), and is currently designing and developing larger capacities. The capacity of titanium alloy induction suspension melting furnaces developed by other units in China is usually less than 30 kg (calculated as titanium).

Figure 2 Schematic diagram of induction suspension melting principle

Low cost casting technology for 3 titanium alloys

Titanium alloy casting technology itself is a production process technology that improves the utilization rate of titanium alloy materials and controls production costs. Based on experience estimation, the production cost composition of titanium alloy precision casting process is shown in Figure 3, and its molding cost reaches more than 20%. In recent years, in order to meet the research and development needs of large and complex thin-walled titanium alloy precision castings, precision casting processes such as graphite, metal, and ceramic molds have been continuously improved and developed, providing a foundation and space for the advancement of low-cost and efficient titanium alloy casting processes.

Figure 3 Production cost composition and proportion of titanium alloy precision castings

3.1 Graphite Casting Process

The molding materials used for titanium and titanium alloy casting must have good high-temperature stability, and graphite material is one of the earliest and most stable molding materials in application. The most widely used graphite molding method currently is the machining graphite molding process, which has the advantages of simple operation and high internal quality of castings. But since 2017, the price of graphite electrodes has skyrocketed, and the production cost of titanium alloy castings has sharply increased. Compaction or compression of graphite sand casting effectively overcomes the above problems. This process uses graphite powder to prepare graphite molds through a method similar to sand casting. Zhu Guang used residual graphite powder from mechanical processing of graphite molds as refractory materials, mixed phenolic resin and anhydrous ethanol to make binders, and prepared graphite molds by compacting. He also poured titanium alloy globe valves and centrifugal pump castings, which had no defects such as sand adhesion, cold isolation, cracks, etc. on the surface of the castings. The thickness of the surface pollution layer was about 0.1 mm, and the mechanical properties and chemical composition of the castings met the relevant national standards. Foreign countries have adopted this process to prepare military torpedo ejection pumps, large seawater pumps, ball valves, butterfly valves and other products. Compared with the machine-made graphite type, the compacted graphite type has good breathability and setback, which can save 30% to 40% of graphite material. Moreover, the graphite fragments can be reused after being crushed, greatly reducing production costs.

3.2 Ceramic Casting Process

At present, the most widely used inert oxide process in titanium alloy investment ceramic casting technology is represented by yttrium oxide. Abroad, companies such as PCC in the United States, HOWMET in the United States, and TITAL in Germany Companies such as Shenyang Foundry Research Institute Co., Ltd., Beijing Aerospace Materials Research Institute, and Guizhou Anji Foundry in China extensively use yttrium oxide as the surface layer shell material. Although the cost of inert oxide surface layer technology has been significantly reduced compared to tungsten surface layer technology, the price of yttrium oxide is still more than 30 times higher than other ordinary refractory materials. For titanium alloy precision castings produced using inert oxide surface layer materials, the cost of molding materials accounts for more than 30% of the casting cost. The expensive cost of molding materials has become an important factor restricting the rapid development of titanium precision casting technology. The use of low-cost oxide refractory materials to replace yttrium oxide has become an important research direction at present.

Al2O3, as a refractory material, has been widely used in the field of investment casting. Ordinary Al2O3 needs to be transformed into stable corundum powder through high-temperature calcination or electric melting. However, when pouring titanium alloy using a surface layer material prepared by mixing corundum powder and conventional binders, the quality of the poured titanium castings is poor. Xiao Shulong from Harbin Institute of Technology independently developed a binder that does not contain impurities such as Na2O, mixed with corundum powder to produce a coating with good coating properties. He successfully cast titanium alloy castings with a contour size of 376 mm × 205 mm × 142 mm, with a surface roughness of 1.6~3.2 μ m and dimensional accuracy of CT6-CT7 levels. CaO material has good chemical stability for molten titanium and is inexpensive. LaSall et al. used a mixture of water-based alkaline binders such as calcium carbonate and silica sol to prepare a coating for the surface layer shell. After calcination at 1000 ℃, the calcium carbonate in the back layer was converted into calcium oxide. The shell was then insulated and poured at around 800 ℃, resulting in high-quality titanium alloy castings. However, this type of shell is not easy to store for a long time due to the tendency of calcium oxide to absorb water and moisture, which limits its large-scale application. CaZrO3 is generally sintered at high temperatures by mixing CaO and ZrO2, and this type of shell has great application value. CaO is extremely inexpensive and widely available, and has great potential for development as a casting material for active metals. Kim et al. prepared ceramic shells using CaZrO3, which have good water resistance, do not deliquesce in water, and have good thermal stability. No obvious reaction layer was found in the titanium alloy samples cast with this shell. Klotz et al. conducted comparative casting tests using CaZrO3 shell and silica shell, and the results showed that the surface oxygen content of the titanium alloy sample cast with CaZrO3 shell was lower and no alpha layer was formed.

Short process preparation and forming technology for 4 titanium alloys

The technical route for preparing titanium alloys using traditional ingot metallurgy process is: sponge titanium - multiple vacuum melting - casting billet - multiple modified forging - forging billet - forming - deep processing - titanium finished product. The complex and tedious processes in the preparation process greatly increase production costs. Therefore, developing short process preparation technology for titanium alloys can effectively reduce costs and improve efficiency.

4.1 Continuous casting and rolling technology

Continuous casting and rolling technology (CC+HDR) has been widely used in the production of steel and aluminum sheets. It connects melting, solidification, and deformation, and achieves integrated control of microstructure, properties, and shape. It plays a significant role in reducing production energy consumption, improving production efficiency and product yield, and improving product uniformity. The Japan Institute of Metal Materials has conducted experimental research on the basic process of continuous casting and rolling of Ti-15-3, Ti-6242, Ti-10-2-3, and NiTi. The research shows that titanium has good thermoplasticity and lower thermal strength above 1200K, and its high-temperature workability is better than steel. As long as the bending deformation does not occur above temperature T β, traditional continuous casting and rolling processes can produce titanium alloy sheets. The US Army has developed continuous casting and rolling technology based on electron beam cold bed melting, and verified the application of Ti-6Al-4V alloy. The research found that only the C content in the prepared plate is slightly higher than that of conventional melting process, while the rest of the components are basically similar. The mechanical properties of the three thicknesses of plates tested (24.6 mm, 38.2 mm, 63.6 mm) are higher than the military standard requirements of MIL-T-9046J, and have excellent anti ballistic performance, fully meeting the needs of tank armor use. Chang Hui and others from Nanjing University of Technology in China have conducted exploratory research on titanium alloy continuous casting and rolling, producing billets with a diameter of 30 mm and bars with a diameter of 10 mm. They are currently conducting more in-depth research on this technology.

4.2 Powder Near Net Forming Technology

Powder near net forming technology is a technique that uses powder as raw material and achieves final product formation through injection, extrusion, hot isostatic pressing, cold pressing, laser additive manufacturing, and other forming methods with minimal or no processing. It has the advantages of high raw material utilization and simple process flow. The near net forming technology of titanium alloy powder solves the melting problem of titanium alloys, avoids the preparation of ingots and the forging process, and is the most rapidly developing short process forming technology for titanium alloys in recent years. In general, the material utilization rate of forgings is only 10%~15%, while that of castings is 45%~60%. However, the material utilization rate of powder near net forming technology can almost reach 100%, greatly improving the material utilization rate. Table 2 compares the advantages and disadvantages of commonly used powder near net forming technologies. Researchers both domestically and internationally are continuously improving and optimizing these technologies.

Table 2 Advantages and disadvantages of titanium and titanium alloy powder forming technology

At present, titanium alloy powder near net formed parts have not yet achieved large-scale industrial production to replace forgings and castings, except for small-scale applications in high-end equipment fields such as aerospace. The reasons for this are twofold: on the one hand, the internal quality and mechanical properties of the products have not yet been fully recognized by the industry, and on the other hand, the high cost is mainly affected by the high preparation and forming technology of high-performance titanium powder. At present, high-quality titanium powder is mainly prepared through methods such as atomization and rotating electrodes, appearing spherical or nearly spherical. However, the sintering properties of spherical powder are poor, and high-density titanium alloy components can only be obtained through pressure sintering or laser sintering, greatly increasing the production cost of powder metallurgy. The preparation process of hydrogenated dehydrogenated (HDH) titanium powder involves hydrogenation fragmentation dehydrogenation of sponge titanium to produce irregularly shaped titanium powder. This process is simple, low-cost, and easy to scale up, but due to its irregular morphology, its flowability and Poisson's ratio are relatively poor. Guo Zhimeng et al. developed a technology for preparing ultrafine low oxygen HDH titanium powder, and achieved low-cost powder pressing and forming through cold isostatic pressing technology and vacuum sintering technology. The ultrafine low oxygen titanium powder used significantly reduced the sintering activation energy and significantly improved the sintering density. Different specifications of TC4 parts with a relative density of ≥ 99% were obtained after vacuum sintering.

5 Outlook

(1) The research on low-cost titanium alloy materials at home and abroad mainly focuses on using cheap elements such as Fe, O, N to replace expensive metals in alloys. However, the comprehensive performance of the materials is also limited, making it difficult to meet the increasingly developing requirements of high-end titanium alloy equipment in aerospace. Especially in terms of fatigue strength and high damage tolerance performance, there are still problems. Therefore, it is necessary to conduct more in-depth composition design and related research on high-end, high-performance, and low-cost titanium alloy materials to expand their application scope in the field of high-end aerospace products.

(2) The recycling rate of titanium alloy waste is still relatively low, with a single recycling method, severe surface pollution layer, high impurity content, difficult composition control, and poor composition uniformity. Further systematic and in-depth research on relevant melting processes is needed to form a complete set of operable residual titanium recycling and reuse process methods, and establish a sound production line for titanium alloy waste recycling and treatment.

(3) Develop high-capacity induction suspension melting technology. At present, the induction suspension melting technology of titanium alloys at home and abroad is limited by equipment technology capabilities, and the melting amount is relatively small, which is difficult to meet the practical needs of engineering. Therefore, it is necessary to develop new high-capacity induction suspension melting technology to achieve high cleanliness recovery and precision forming of high-capacity titanium alloys.

(4) Develop stable low-cost investment casting ceramic shell preparation technology. At present, low-cost ceramic shell materials such as Al2O3 and CaZrO3 are still in the laboratory research stage and have not been widely used in titanium alloy investment casting. It is necessary to conduct in-depth research on their phase structure, microstructure, and other aspects, and to use neutral binders to truly achieve the preparation of high inertia, high density, and high stability surface layer shells with complex structures, meeting practical production needs.

(5) Develop high adaptability composite manufacturing forming technology. The single type of near net forming technology is limited by its own limitations and cannot meet the high-performance, low-cost titanium alloy components required for all high-end equipment manufacturing. Therefore, it is necessary to combine multiple forming technology process characteristics, such as precision casting+laser additive manufacturing/powder metallurgy, to achieve high-precision, high-efficiency, and high-performance forming of complex structural components.