Development and application status of titanium hot extrusion technology

Hot extrusion process is by extrusion on an extruder rod passing high pressure, for closing the extrusion cylinder in the bad material extrusion molding die with the same shape of an article advanced method of plastic working (hot extrusion common metallic The process is shown in Figure 1. The utility model has the advantages of improving the deformation ability of the metal, high comprehensive quality of the product, and wide product range. Titanium and titanium metal are difficult to deform metals, and they are expensive. Therefore, the hot extrusion process is the most expensive for producing large-size, thick-walled or high-demand titanium tubes, titanium rods, and titanium profiles (hereinafter referred to as titanium extruded materials). A promising production method.

Figure 1 Schematic diagram of the hot extrusion process of steel

First, the development of titanium extrusion molding technology

Titanium is a highly active metal that is not only highly contaminated by heating in air, but also has the property of bonding to a mold under certain temperatures, pressures and surface conditions. Titanium has poor thermal conductivity. When hot extrusion, the surface layer and the center of the billet tend to produce a large temperature difference, which promotes the inhomogeneity of the metal flow, so that the surface layer generates a large additional tensile stress, and cracks are easily formed on the surface of the product. In severe cases, large central compression holes may be created on the extruded bars and tubes. At the same time, the thermal effect is significant when extruding titanium and titanium alloys, and the unsuitable extrusion process has side effects on the structure and properties of the extruded product. Titanium has a low modulus of elasticity, a heavy rebound, and is difficult to form. Thus alloy extrusion pressing process is more complex than other non-ferrous deformable aluminum alloy, copper alloy or the like. The hot extrusion process of titanium is different according to whether the blank is covered or not. The main process flow is shown in Figure 2. Since the development of titanium hot extrusion technology, Chinese and foreign related technicians have focused on improving the quality and yield of titanium extruded materials and reducing production costs in billet preparation, billet heating temperature, extrusion ratio, extrusion speed, lubrication and extrusion die. Do a lot of research and exploration work.

Figure 2 Titanium hot extrusion process

(1) Preparation of titanium extrusion ingot

The conventional manufacturing process of titanium and titanium alloy extrusion blanks is generally that a vacuum arc melting ingot is forged or rolled into a blank, and then subjected to cutting or hot pressure perforation to form a light blank having a size and surface quality. Direct extrusion without hot perforation, the quality of the waste pipe is good, but the yield is low. In order to improve the comprehensive yield of titanium extruded materials, the research on the direct extrusion of hollow ingot casting process is a development direction for the future scale extrusion of titanium. The EOPaton Electric Welding Institute of Ukraine has studied the melting of large hollow ingots by electron beam cooling bed. At present, Bao Ti and Baosteel Special Steel have introduced plasma and electron beam cold bed furnaces. The next step is to actively study the smelting process of hollow ingots that can be directly extruded.

(2) Heating of titanium extrusion billets

Titanium is easily contaminated by gas when heated in air, so the extruded billet must be heated to protect the metal surface from or less contaminated by gas. The heating of the extruded billet can be divided into jacket heating, coating heating, salt bath heating, glass melt heating and conventional heating according to the protection method. At present, induction heating is generally used. In order to facilitate rapid extrusion under minimum pressure during the development of the heating process, the extrusion should be carried out at the highest possible temperature while ensuring good mechanical properties of the product. For example, for industrial pure titanium, even if the extrusion temperature is as high as 1038 °C, there is no obvious influence on the mechanical properties. At present, pure titanium, α-type and α+β-type titanium alloys are usually lower than the α+β/β phase transition temperature of the alloy by 20 °C. Squeeze at ~100 °C. Beta type titanium alloy usually

Extrusion is carried out above the phase change temperature.

(III) Determination of titanium extrusion ratio

In extrusion processing, the degree of deformation is generally expressed by the extrusion ratio (λ). In order to improve the structure and properties of the product, many literatures believe that the extrusion ratio should be larger when titanium and its alloys are extruded. In fact, the extrusion ratio of titanium is relatively small, generally less than 30. The research shows that the TC4 titanium alloy is heated in the two-phase region, and the extrusion ratio of 3-10 is used to obtain a product with good comprehensive performance. When the same temperature is used, when the extrusion ratio is 28, the temperature rises due to the deformation heat effect. Up to the α+β/β phase transition temperature, the product has a network structure, and the overall performance of the material deteriorates. In addition to considering the characteristics of the metal itself, it is also necessary to consider the equipment capability and the strength factor of the tooling. At the same time, the extrusion ratio is also affected by the way the titanium is lubricated. Generally, the extrusion using glass lubrication is smaller than the extrusion of the sleeve.

(4) Range of titanium extrusion speed

As with the extrusion temperature and extrusion ratio, the extrusion speed not only affects the performance and surface quality of the extrusion but also the extrusion force. The actual extrusion shaft speed achievable during extrusion varies depending on the composition of the titanium alloy, the extrusion temperature, and the extrusion ratio. Generally, 80-130 mm/sec medium speed extrusion is used. The effect of speed on the thermal effect of the extrusion can be used to keep the temperature of the extrusion constant. According to foreign literature reports, the extrusion speed grade is corrected according to the temperature change of the extrusion extrusion, and the temperature is recorded by a precision instrument. The extrusion speed is adjusted by temperature feedback. In addition, the extrusion speed can also be controlled by a theoretical simulation-program. The temperature rise law is pre-calculated by the computer, and the corresponding program is selected for isothermal pressing according to different products.

(5) Selection of titanium extrusion lubricant

The lubrication problem is a difficult point in the hot extrusion technology of titanium and titanium alloys at home and abroad, and it is also a research hotspot. At present, the lubricants used are mainly three types of grease, glass lubricant and metal coating.

The grease is typically a mineral oil with a thickening agent. It is convenient and practical to use a grease lubricant, and it can extrude titanium materials with excellent surface quality, but the length of the extruded products is often limited. The maximum length of the extruded profile is limited to 3 to 4.5 meters. The end of the long extruded material is prone to bonding defects. Most of the methods are now produced in small batches or in combination with the following two methods.

Glass lubrication extrusion is currently the most advanced lubrication process in the world. It has been widely used since its invention in 1941. Compared with other lubricating materials, glass lubricants have low thermal conductivity, good thermal insulation properties, good high temperature adhesion, high pressure resistance, good chemical properties, no reaction with metals, and can prevent metal from being contaminated by gases. . Therefore, it is the most promising lubricant material. At present, glass lubrication extrusion is commonly used in the world. Although China has carried out research on glass lubricants very early, it has not yet reached the level of industrial application.

Hot extrusion of titanium and titanium alloys can also be lubricated with metal coating. It is mainly coated with copper, mild steel or other metals on the outside of the blank, and it can also be coated with copper. Copper-clad extrusion, when the metal heating temperature exceeds 850 ° C, a Ti-Cu eutectic structure is formed at the interface between titanium and copper. The structure is a brittle substance, which not only does not function as a lubricant, but instead Destroy normal extrusion. Therefore, the method is generally limited to pure titanium extrusion. In addition, the metal coating extrusion process is complicated, the cost is high, and the environment in the pickling process is seriously polluted.

(6) Use of titanium extrusion die

As with other metals, flat molds are commonly used when extruding titanium tubing. In order to improve the service life of the mold and improve the lubrication conditions, the mold is generally preheated to 300 ° C ~ 400 ° C. Under normal circumstances, the service life of each extrusion die is about 20 times. Mold materials and processing costs are very high, so in order to reduce the processing cost of titanium extruded materials, mold materials and mold structures must be studied. For profile extrusion, in order to improve the dimensional accuracy of the thin-walled profile and the wear resistance of the tool, the Russian Light Alloy Research Institute has studied the application of carbides and oxides of different metals by gas flame method and plasma method on the working surface of the extrusion die. The coating showed that the common tool steel was coated with a molybdenum layer of 0.05-0.1 mm thick, and the mold with the zirconia coating by plasma was the best. The thickness of the section unit was 2 mm and the tolerance was 0.5. Millimeter high strength titanium alloy profiles. The use of ceramic-coated molds in combination with glass lubricants has become an important factor in the mass production of thin-walled profiles.

Table extrusion parameters of titanium and titanium alloy bars

It should be pointed out that the extrusion of high-quality products of titanium and titanium alloy requires that the correct production process be established under the condition that the tool has a satisfactory service life, that is, the combination of temperature, extrusion speed, extrusion ratio and lubrication method is required. The table above lists the parameters of a typical steel extruded bar.

Second, the production and application of titanium extruded materials

In the 1950s, with the industrial production of titanium, hot extrusion technology was rapidly applied and developed in the production of titanium. After decades of development, Russia, the United States, the United Kingdom and other countries can use extrusion to produce titanium and titanium alloy tubes and rods, as well as a wide variety of titanium and titanium alloy profiles. These profiles are not only angles, T-shaped, trough-shaped pipes, but also a wide variety of profiles, variable profile profiles, and even dimensional tolerances, with surface finishes that are not machined.

The testing of titanium alloys in Russia began in 1953 and in the 1960s provided a wide range of thin-walled profiles, wing profiles, hollow profiles, large profiles and siding for rapidly developing aerospace technologies. Since then, Russian titanium extrusion profiles technology is at the advanced level in the world. It produces more than a dozen titanium alloy grades with specifications of more than 2,000. For example: the production of OT4, OT4-1, BT20, BT14, BT15 alloy thin-walled profiles, the thickness of the web is 1.5 ~ 5 mm, the thickness tolerance of the web is 0.5 mm. Extrusion pipes, rods and profiles of Russia's Upper Salda Metallurgical Joint Production Enterprise (VSMPO) are also exported to US and European aircraft manufacturers and suppliers in addition to domestic use. In addition to aerospace, VSMPO's Pd, Ru-containing alloy Ti-6Al-4V alloy tube is also used for oil exploration.

The production of large-diameter titanium alloy extruded pipes in the United States is the world's leading level. The United States uses Ti-6Al-4V-Ru alloy tubes of diameter (48-610) mm × 26 mm × 2600 mm for geothermal and offshore drilling pipelines. U.S. RMI's 650 mm × (22 ~ 25) mm × 35000 mm ultra-long Ti-3Al-2.5V-Ru alloy tube is used for subsea oil exploration. In addition, the Ti-6Al-4V ELI alloy tube with a diameter of 600 mm × 25 mm × 15000 mm is used in the Norwegian North Sea drilling support platform riser. Internationally, the development of titanium tubes is slow. Only Titanium Sports Technology of the United States uses extrusion and drawing methods to produce tubes of various shapes such as squares, rectangles, triangles, ovals, pentagons, hexagons and octagons. , became the only company in the world to produce titanium tubes. At present, the application of titanium tubes is not extensive enough, and the dosage is not large, but there are large potential markets in the fields of construction, sports leisure and special engineering.

China's titanium and titanium alloy extrusion production began in the late 1960s. At that time, Bao Titan and Great Wall Steel imported a 3150 ton extrudable titanium alloy hot extrusion machine from Germany. After nearly 40 years of development, Baoti Titanium can extrude various specifications of tubes and bars of titanium and titanium alloys and profiles and composites of simple sections, with dozens of grades. These products have been widely used in various sectors of the national economy such as aviation, aerospace, satellite, energy, and chemical industries. However, it should also be noted that there is still a large gap between China and advanced countries, and profiles of more complex sections cannot be produced. In recent years, with the sharp increase in demand for high-quality titanium tubes in the civil fields such as chemical industry, the companies such as Western Titanium and Zhejiang Wuhuan have successively introduced extruders mainly used for extruding titanium tubes. In October 2009, Baosteel Special Steel introduced the world's advanced 6000T extruder from Germany (Figure 3), which provided the necessary equipment for the production of large-size titanium tubes and profiles in China, marking China's titanium extrusion equipment. Going up to a new level.

Figure 3 Baosteel 6000T hot extrusion machine

Third, the conclusion

China's titanium hot extrusion technology is developing slowly, and there is a big gap with foreign countries. To develop competitive titanium extrusion materials and improve the overall level of titanium extrusion materials in China, it is suggested that we should first solve the following four aspects:

(1) Research on hollow ingot tube blanks using a cold bed furnace. As mentioned above, according to the current method of manufacturing tube blanks, it has not adapted to the development requirements of building a resource-saving society. To this end, it is necessary to actively carry out research on the process of smelting hollow tube blanks in cold bed furnaces, simplifying processes, reducing costs and improving market competitiveness. It is imperative.

(2) Research on high temperature lubricants. Lubricants have an important impact on the quality and production cost of hot extrusion molding. Therefore, it is an urgent problem to solve the problem of lubricants suitable for different materials to improve the overall quality of the products and reduce the wear of the molds.

(3) Research on mold material and mold structure design. When hot extrusion, the mold is subjected to the combination of high temperature and high pressure and strong friction, which seriously affects the service life of the mold, the quality of the product and the production cost. Therefore, research on mold material and mold structure design methods is also one of the problems that need to be solved in the future.

(4) Actively explore the market of titanium extrusion materials. Titanium extruded materials will have great demand potential in aircraft manufacturing, marine engineering, sports and leisure industries. At present, the combination of titanium extrusion production and design and application units is not close. Everyone should work together to improve the overall level of titanium extrusion materials in China.

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