What are the processing technologies of titanium alloy wire?

Titanium and titanium alloy wires are not only widely used in aerospace and other high-tech fields, but also are increasingly entering various civil fields because of their good corrosion resistance, high specific strength, non-magnetism, good affinity with human body and shape memory function. For example, titanium alloy wire fasteners are widely used in the aerospace field, which can not only achieve the purpose of weight reduction and corrosion resistance, but also be necessary connectors for structural parts such as titanium alloy and carbon fiber composite materials. Compared with steel springs, springs made of titanium alloy wires in automobile field can reduce the weight by 60% ~ 70%. Medical titanium alloy wire is favored by doctors and patients because of its non-toxicity, light weight, biological corrosion resistance and good biocompatibility. In mariculture, the culture net made of titanium wire is still intact after 15 years of use.

Titanium and titanium alloys are difficult to machine. Titanium has high yield strength, generally 0.70~0.95, good elasticity and high deformation resistance, but its elastic modulus is relatively low, so it has high deformation resistance and serious rebound during processing. Moreover, the adhesion problem during processing also has a very bad influence on the surface quality of products. At present, the preparation technology of titanium alloy wire has been continuously improved and perfected, and various new technologies have been adopted, which has rapidly improved the quality of titanium alloy wire products, increased the types and further expanded the application fields. Drawing is still the most commonly used method to produce titanium alloy wire. Usually, the production process of wire rod is as follows: raw material → ingot melting → forging → rolling → drawing → heat treatment → inspection → finished product. Starting from the production technology of wire rod, this paper mainly introduces the drawing technology of wire rod, and briefly introduces the preparation technology (melting, forging and rolling) and processing technology of wire rod blank.

Preparation technology of 1 silk blank

1. 1 melting process

Titanium is a very active metal, which reacts with oxygen, nitrogen, hydrogen and carbon very quickly in liquid state, so the melting of titanium alloy must be carried out under the protection of high vacuum or inert gas (Ar or Ne). Melting technology mainly includes vacuum consumable electrode arc furnace melting, vacuum consumable electrode shell furnace melting, electron beam cold bed furnace melting, plasma cold bed furnace melting, vacuum induction furnace melting and so on. From the comparison of power consumption, melting speed and cost, the first two are still the most economical and applicable melting methods at present. However, the ability of vacuum arc melting to eliminate high-density inclusions and low-density inclusions in titanium alloys is limited, and cold hearth furnace melting has unique advantages in this respect. The quality of ingot will affect the subsequent processing technology and the quality of finished products. By carefully selecting raw materials, selecting reasonable melting process parameters (melting current, arc voltage, vacuum degree, air leakage rate, cooling rate and stirring magnetic field intensity) and strictly controlling the process, high-quality ingots can be obtained. Due to the small wire size and complex processing technology, the sensitivity to metallurgical defects (segregation and inclusion) in the alloy is increased, so the melting process is very critical to accurately control the composition, reduce the impurity content in the alloy and ensure the excellent performance of the wire.

1.2 forging process

The purpose of forging is to improve the microstructure, improve the comprehensive properties of metals and provide blanks for rolling process. The basic process flow is as follows: ingot casting → heating → cogging forging → cooling → surface cleaning → blank deformation → heating → forging rod → inspection → finished product.

Appropriate heating temperature, heating speed and heating time should be selected, and the atmosphere in the furnace should be controlled to ensure the product quality. The heating temperature should be within the temperature range of good deformation plasticity, high forging quality and low deformation resistance. Ingot cogging heating is above (α+β)/β transition point 100 ~ 200℃ (except β titanium alloy); After forging, the coarse as-cast structure has been broken to some extent, the internal structure has been improved, and the plasticity has been improved, so the heating temperature of re-forging can be gradually reduced with the increase of annealing times. In order to prevent β embrittlement and obtain good microstructure and comprehensive properties, for α-alloy and α+β-alloy, forging and heating before finished products should be carried out at a temperature below the transformation point, while for β-alloy, it is actually heated and forged in β-zone. Due to the low thermal conductivity of titanium, it is 0.0397 k/cm s℃ at room temperature, which is about 1/4 of that of medium carbon steel, but it is similar at high temperature. Therefore, slow heating should be adopted at low temperature to avoid excessive temperature difference between the surface layer and the central layer during heating. At high temperature, the thermal conductivity of titanium increases, so it can be heated at a slightly faster speed.

In the process of forging, deformation temperature, deformation amount and deformation speed have important influence on the quality of forgings and must be controlled correctly. As mentioned above, the ingot before forging is generally heated above the transformation point, because the deformation resistance is low and the plasticity is high at this temperature. However, if the ingot deformation is too low, the as-cast structure can not be effectively destroyed, and its performance is poor, which directly affects the subsequent processing. In the forging process, if the deformation is not properly selected, the microstructure and properties of the alloy will be seriously affected. Such as TC4 alloy, when the heating temperature is higher than the transformation point and the deformation is not large enough, coarse flaky or acicular α -β structure, also known as coarse widmanstatten structure, will often be obtained. The strength of this structure changes little, but the plasticity decreases significantly. When the deformation increases, the strip-shaped α+β structure with different degrees of distortion appears, which is called basket structure. The high-temperature properties and fracture toughness of the microstructure are improved, but the plasticity is decreased. Appropriate deformation should be selected to obtain finer β structure and a certain amount of equiaxed primary α+transformation. This kind of structure has good comprehensive properties. The deformation speed also has a very important influence on the quality of forgings. When the deformation speed is too fast, not only the deformation resistance is improved, but also the local or overall temperature of the forging is too high due to the thermal effect of deformation, which leads to the deterioration of the microstructure and comprehensive properties of the forging. Finally, it must be pointed out that deformation temperature, deformation speed and deformation amount are by no means isolated and affect the quality of forgings. For example, if the heating temperature is slightly higher, but there is enough deformation and low deformation speed, better microstructure and properties can be obtained.

1.3 rolling process

Rolling processing mainly provides wire blank for wire drawing, further improving alloy structure and improving comprehensive properties of metal. Like forging process, it has an important influence on the microstructure and surface quality of wire rod. The main process parameters are heating temperature, rolling speed and hot rolling rate.

(1) heating temperature

After forging, the uniformity and compactness of the billet structure have been greatly improved, so the heating temperature can be slightly lower than the forging temperature. The heating temperature of α+β alloy before rolling is generally slightly lower than the (α+β)/β phase transition temperature, that is, heating is carried out in the (α+β) phase region, so that the rolling process is completed in the (α+β) phase region, and the structure and properties of the product are better. The heating temperature of α -type alloy is also in the (α+β) phase region, at which time it has good hot workability and room temperature performance. The heating temperature of β -type alloy is higher than β phase transition temperature, so that its deformation is completed in β phase region. At this time, the alloy has small deformation resistance and good plasticity. Different heating temperatures have great influence on the microstructure and properties of the alloy. For example, when TC9 bar is rolled at 1050℃, the needle-like structure is obtained because the rolling temperature is above the β transformation temperature, and the properties are poor. When rolling in α+β phase region (below 980℃), the equiaxed structure is obtained and the properties are good.

(2) Rolling speed

At present, titanium and titanium alloys are not suitable for high-speed rolling because of the small output and short length of titanium products, and most of them are manually operated. Moreover, if the rolling speed is too fast, the rolled piece will heat up rapidly, which will affect the microstructure and properties of the final product. Theoretical calculation shows that when the rolling speed is greater than 12m/s, the temperature rise of the rolled piece increases in direct proportion to the rolling speed. When the rolling speed is more than 30m/s, the final rolling temperature has nothing to do with the heating temperature.

(3) Hot rolling rate

Due to the different deformation, the microstructure and properties of the alloy are obviously different. For example, when TC4 bar is hot rolled at 920℃ and rolled with 28% deformation, its structure is basically that α phase is divided into equiaxed shapes by β phase mesh, and its structure and properties are poor. When the deformation is 44%, the mesh of β phase has been broken, and the particle size of α phase is larger, so the microstructure and properties are also poor. When the deformation is 66%~78%, it has almost the same structure. The microstructure is based on α phase, with fine dispersed α+β structure and good properties.

In order to fully process and refine the structure and improve the material properties, the stepped rolling process was invented in the 1970s, which is a processing method combining the deformation characteristics of rolling and forging, and has the characteristics of large deformation of forgings and high rolling speed. Drawing lessons from a few foreign advanced countries, the preparation process of wire rod is: ingot casting → cogging forging → hot continuous rolling into wire rod. Qin et al. studied the process of producing 10mm high-speed pure titanium wire rod with large coil weight by alloy steel hot tandem mill, and analyzed and discussed the structure, properties, shape and dimensional tolerance of the product. The research shows that the products produced by this method have good mechanical properties, uniform structure and good surface quality.

2 Drawing process

2. 1 drawing temperature

Titanium alloy with poor cold working performance is usually processed by hot drawing, and the drawing temperature has an important influence on the microstructure, properties, interstitial element content and surface quality of wire rod. Zhu et al.' s research on drawing method of Ti2Cu titanium alloy wire shows that Ti2Cu titanium alloy wire is not suitable for cold drawing, but qualified Ti2Cu titanium alloy wire can be successfully drawn by hot drawing method. The increase of carbon, oxygen, nitrogen and hydrogen can be eliminated by alkali, pickling and vacuum annealing. Figure 1 shows the tensile properties of Ti2Cu titanium alloy wire under cold drawing and hot drawing. It can be seen that in the cold drawing process, the tensile strength of steel wire increases with the decrease of diameter, and the elongation decreases rapidly with the decrease of diameter. In the range of 8mm~6. 19mm, with the decrease of diameter, the tensile strength increases rapidly and the elongation decreases significantly, because only partial recrystallization occurs, and the hardening effect is greater than the softening effect. In the range of 6.19mm ~1.15mm, the tensile strength and elongation remain basically unchanged, which is due to the dynamic balance of hardening caused by deformation and softening caused by recrystallization.

2.2 Drawing Pass Rate Processing Rate

In the process of hot drawing, the processing rate of each pass mainly depends on the processing temperature and wire diameter. For cold drawing at room temperature, the processing rate of each pass mainly depends on oxidation, coating quality and lubricant quality. Table 1 is a general specification for the distribution of pass processing rate with diameter change during drawing at room temperature.

2.3 tensile stress

When drawing, the tensile stress should be less than the yield strength of the drawn metal material, which is the basic condition to realize the drawing process. There are many factors that affect the tensile stress, such as drawing temperature, drawing speed, machining speed and die cone angle. The increase of machining rate, the decrease of drawing temperature, too large or too small cone angle will all cause the increase of tensile stress; In linear drawing, the drawing speed has no significant change on the tensile stress, but when the wire passes through the die hole and is wound on the traction winch, if the drawing speed exceeds a certain range, the tensile stress will increase. In order to reduce the tensile stress in the tensile process, methods such as lubrication, reducing deformation and improving metal deformation plasticity can be adopted. For this reason, people have studied a variety of processing technologies, including roller die stretching, ultrasonic vibration stretching and other methods.

2.4 Stretch lubrication

Because titanium alloy tends to stick to die during drawing, it is difficult to draw, so besides good lubricant, other measures to enhance lubrication such as coating and oxidation should be taken. Most titanium alloys are oxidized and coated before drawing. The coatings used are graphite emulsion, salt lime and calcium-based coatings. The basis for selecting the coating should not only be closely combined with the processed silk, but also have good wettability with the lubricant and be easy to remove. Under different drawing process conditions, the lubricants used are different. In the process of titanium wire drawing, the lubricants used are industrial soap powder, graphite emulsion and the mixture of soap powder and other substances, so the lubricant with good wettability and thermal stability should be selected. For example, in the processing of TB2 titanium alloy wire, choosing calcium-based coating as coating and adding self-made lubricant (HTK-SM) can obtain satisfactory wire surface. In order to enhance the lubrication effect, pressurized dies are often used to improve the surface quality of wires.

2.5 drawing die

The main materials of wire drawing die are cemented carbide, natural diamond, artificial diamond and polycrystalline diamond. Single crystal natural diamond molds are often used in filament production. Although the cost of natural diamond mold is high, it is durable, with small size change, and it is not easy to cause adhesive wear and steel wire scratch. In order to make the wire to be processed pass through the die smoothly, realize the purpose of deformation, and form the required specifications and sizes, the shape of the die to be processed is required to be conducive to lubrication, reduce the phenomenon of wire breakage, and facilitate the rapid dissipation of deformation heat. The result of stretching for a period of time is that the die surface is worn, that is, something falls off the die surface due to friction and tearing, which will scratch the wire surface. Therefore, it is necessary to improve the mold finish, reduce mold defects and strengthen mold alignment.

With management control.

2.6 surface treatment

In the process of wire drawing, surface treatment is also a factor affecting the surface quality and microstructure and properties of wire. Its methods include pickling, mechanical polishing, electrolytic polishing, phosphating, oxidation, electroplating and so on. The surface treatment of Ti-Ta alloy wire and Ti-Ni alloy wire was studied by Northwest Research Institute of Nonferrous Metals and Youyan-Jinjin New Materials Co., Ltd. The results showed that the tensile samples of pickling, mechanical polishing and electrolytic polishing all showed ductile fracture, but electrolytic polishing effectively improved the mechanical properties of Ti-Ni alloy wire because of reducing the source of surface cracks, while pickling showed better comprehensive properties than mechanical polishing because of reducing the influence of surface inclusions on tensile. Phosphating oxidation treatment can effectively ensure that the surface of wire is not scratched during drawing due to the high hardness of phosphating layer and oxide layer, but the deformation of surface and core will be uncoordinated during drawing, which will easily lead to cracks on the surface and fracture of materials. Although the surface of the plating line is smooth, due to hydrogen embrittlement, the sample presents brittle fracture and the mechanical properties of the material are significantly reduced.

2.7 heat treatment process

Annealing is widely used in the heat treatment of titanium and titanium alloy wires, including intermediate annealing and finished product annealing. Its purpose is to improve the processing plasticity of wire rod and achieve the required finished product properties. When making the annealing process, we should not only consider the specific conditions of production, but also consider the relationship between the mechanical properties of metals, deformation degree and annealing temperature. For example, industrial pure titanium, with the increase of processing rate, the elongation decreases, while the tensile strength increases, indicating that cold working hardening is fast and intermediate annealing must be carried out. The annealing temperature of wire products should be selected according to the required properties of finished products to achieve the best performance matching. For example, the optimum vacuum annealing temperature of Ti-2Al-2.5Zr wire is 700~850℃, and the elongation and tensile properties can meet the requirements of the wire. Tables 2 and 3 are general annealing specifications for titanium and titanium alloy wires. It can be seen that the annealing system of wire should also consider the size of wire. In practical application, the best annealing process should be selected through experimental research according to the alloy composition and processing technology.

In addition to annealing process, heat treatment such as solution aging is usually needed to obtain the required properties for various purposes. For example, Ti-22V-4Al alloy wire for spectacle frames is annealed at 780℃×30min, and the structure is uniform, and the elongation is above 20%. After aging at 520℃×4 h, the Vickers hardness reaches 2800MPa, which meets the technical requirements of the hardness of glasses frame wires.

3 processing technology

The traditional fixed die drawing (that is, conventional drawing) has its inherent defects, and its outstanding problem is the friction between the die and the deformed metal contact surface and the accompanying thermal effect. To this end, people invented a variety of processing technologies to solve the above problems.

(1) Roller die drawing: This technology combines the characteristics of traditional rolling and drawing, reduces the drawing force, improves the pass processing rate and reduces the work hardening degree. Because the roller die is stretched in the pass composed of non-driven and freely rotating rollers, the sliding friction between the material and the die hole is mostly converted into small rolling friction during the stretching of the fixed die, thus greatly reducing the stretching friction. The disadvantage of roller die drawing is that the dimensional accuracy is not as high as that of fixed die drawing, which is suitable for rough drawing, while fixed die drawing is used for finishing in fine drawing.

(2) Ultrasonic vibration stretching: This method was developed in 1950s. When drawing, applying ultrasonic vibration to the drawing die can effectively reduce the drawing force and improve the processing rate of each pass.

(3) Die-less wire drawing: This process is to locally heat and soften the wire by using induction coil or laser, and apply tension to make the wire thinner. Its advantages are no need of drawing die and lubricant, high deformation rate and high efficiency, but its disadvantages are poor dimensional uniformity and unstable finished product quality.

(4) Booster die drawing: This process refers to the method of installing booster nozzle device in front of the drawing die, which can cause the effect of automatic pressurization and forced lubrication during wire drawing. Its advantages are: the broken wire rate is reduced by 4/5, the service life of wire drawing die is increased by more than 20 times, and the surface quality is improved.

(5) Coating-coating bundle stretching: firstly, the surface of titanium wire is coated with a layer of low-carbon steel, then the plated titanium wire is bundled into a low-carbon steel tube, and then bundle stretching processing and intermediate annealing are carried out. After processing to the final size, the low-carbon steel sheath and coating are removed by sulfuric acid pickling. Its advantages are high efficiency and low production cost.

(6) Cladding-crumb extrusion: This process was developed by Tohoku University in Japan and is mainly used to process TiNi shape memory alloy wire, which can improve product quality and reduce production cost. Firstly, multi-layer composite plates composed of different metal plates are prepared by cladding rolling, and the thickness ratio of various metal layers depends on the determined chemical composition. Then, the rolled clad sheet is cut into pieces, and the cut pieces are put into containers to make blanks, which are extruded into thin rods and then processed into filaments. Finally, the composite wire is transformed into the required intermetallic compound wire by thermal diffusion treatment.

(7) Four-high wire rod mill for producing wire rod by continuous rolling: This mill is composed of four rollers with round holes, and one driving roller drives the other three rollers to rotate during work. A number of such stands form a continuous rolling mill, which can produce titanium alloy wire, thus greatly improving the productivity and yield of wire.

4 conclusion

Titanium and titanium alloy wires are widely used, but their high price is the main obstacle to their application. It is necessary to develop and popularize new wire preparation technology to reduce the processing cost of wire. There are many reports about wire processing technology abroad, and many new technologies have been adopted, so foreign titanium alloy wire products have good quality and many specifications. However, the domestic production technology of titanium alloy wire rod is still relatively backward, and the problems to be solved at present are long production process, low efficiency and high cost. Therefore, China should increase the research investment in titanium alloy wire processing, improve the technical level and equipment level in this field as soon as possible, and produce titanium alloy wire products with high quality and low price to meet the market demand.