Causes of black bar defects in the rolling process of TC4 titanium rods!

TC4 (Ti-6Al-4V) is a Martens a/3-type two-phase titanium alloy with good comprehensive properties, and its service temperature can reach 450. It is widely used in important structural parts of the aviation industry, such as wing blades, aero engine discs, etc. Because the grade 5 titanium round rod is a two-phase titanium alloy, if the composition of the microscopic area is not uniform, it will inevitably lead to abnormal macrostructure and microstructure, resulting in a significant difference between the hardness of the abnormal area and the normal area, and the overall performance of the material is uneven. Causes fatigue crack sources, brings great hidden dangers to the safety of parts, and reduces the service life of the alloy. Aiming at the black streak defects found in the low-magnification inspection of a TC4 titanium alloy bar product, in order to accurately determine the defect type, a metallographic microscope was used to observe the metallographic structure and determine the abnormal area of ​​the metallographic structure. Then, a scanning electron microscope was used to analyze the chemical composition segregation defects in the black stripe area rich in molybdenum and poor in aluminum. Through the microhardness test, it is determined that the component segregation in the black stripe area is non-brittle segregation.

The experimental results show that the composition segregation and type of TC4 titanium alloy can be effectively determined according to the above method. And it is determined that this type of defect will not affect the use and can be delivered after removal. Such defects can be reduced or eliminated by controlling the selection of raw materials for titanium alloy ingots, material mixing, electrode preparation and voltage and current during smelting. The segregation of titanium alloys can be divided into hard segregation (hardness higher than normal zone, also called brittle segregation) and soft segregation (hardness lower than normal zone, also called non-brittle segregation) according to the difference between the hardness of segregation site and normal zone. If there is only non-brittle segregation in the product, and all properties meet the requirements of the product standard, the product can still be delivered for use after the segregation is eliminated. Fragile segregation is not allowed to be delivered after removal, and should be discarded in batches.

A. For the black streak defect of grade 5 titanium bar found by visual inspection, the structure is observed through a metallographic microscope. The defect area is not much different from the normal area, and the defect type cannot be judged; in addition, the chemical composition of the defect area of ​​the titanium rod is analyzed by scanning electron microscope. It is found that the defect area is the segregation of chemical elements rich in heavy and poor in aluminum. Finally, combined with the microhardness test, it is determined that the segregation type of TC4 titanium rod is the non-brittle segregation of rich aluminum and poor aluminum. Through microstructure observation, micro-area composition analysis and microhardness test, the composition segregation and type of TC4 titanium alloy can be effectively determined.

B. The segregation in TC4 titanium alloy bar is non-brittle segregation of rich and poor aluminum, which does not affect the use and can be continuously transported after cutting; by controlling the selection of raw materials, mixing and electrode preparation parameters, as well as the voltage and current during the smelting process, it can be Reduce or eliminate such defects.

Cold rolled semi-TC4 titanium alloy tube

Semi-TC4 titanium alloy has the advantages of low density, high specific strength, good corrosion resistance, non-magnetic and low linear expansion coefficient. The small-size thick-walled titanium alloy tube made of this alloy has been successfully used in aerospace vehicles and various civil applications. In the hydraulic and fuel pipeline systems on the aircraft. Aerospace semi-TC4 small size Seamless Titanium Tube Grade 2 are usually thick-walled pipes with different diameters such as 6, 8, 10mm, and the standards have higher requirements for various properties. The effects of rolling process parameters on the surface quality and tensile properties of small-size, thick-walled Grade 3 Pure Titanium Tube are studied, aiming to improve the yield of semi-TC4 titanium alloy pipes in the production process and realize small-size thick-walled semi-TC4 titanium Mass production of alloy pipes. 

Titanium alloy welding cracks and preventive measures

Welding cracks are common quality defects in titanium alloy welding. Because titanium alloys contain fewer impurities such as sulfur, phosphorus, and carbon, low-melting eutectics are generally not common in grain boundaries, and the effective crystallization temperature range is narrow. The shrinkage is small during the process of seam solidification, so hot cracks rarely occur during welding. If the quality of the base metal and the welding wire does not meet the standard requirements, if there are quality defects, and there are harmful substances in the interlayer, hot cracks may occur. Therefore, you should pay attention to the quality of the welding wire when welding. And do a good job in the protection of harmful substances and strengthen the heat treatment after butt welding, which can effectively avoid the occurrence of thermal cracks.

Delayed cracks may appear in the heat-affected zone during the welding of titanium alloys, and such cracks generally appear within a period of time after welding. In order to prevent the occurrence of delayed cracks, the hydrogen content of the welded joints should be reduced as much as possible. You can choose based materials and welding wires with less hydrogen content, and clean up before welding, and eliminate residual stress after welding. Generally, titanium hydride does not appear in titanium alloys with normal hydrogen content, so the probability of cracks in the weld is also very small. However, when using thick-plate α+β Grade 5 Ti-6Al-4V Titanium Plate for multi-layer welding, the use of industrial pure titanium as a filler material will cause the appearance of titanium hydride, thereby inducing hydrogen embrittlement.

Processing characteristics of titanium

In addition to pure titanium, nearly 30 types of titanium alloys have been produced in the world. The most widely used titanium alloys are Ti-6Al-4V, Ti-5Al-2.5Sn, and so on. The industrial method for the production of titanium metal is the Kraul method, and the product is a titanium sponge. The traditional process of making titanium is to melt and cast sponge titanium into ingots, and then process them into titanium. According to this, the main steps of the technological process from mining to making titanium are:

Titanium Ore->Mining->Ore Dressing->Too Concentrated Ore->Enrichment->Titanium-rich Material->Chlorination->Crude TiCl4->Refined->Pure TiCl4->Magnesium Reduction->Sponge Titanium->Melt Casting-> Titanium Ingot -> Processing -> Titanium Material or Titanium Parts In the above steps, if the rutile is obtained by mining, it does not need to be enriched and can be directly chlorinated to obtain crude TiCI4. In addition, the casting operation should be a metallurgical process, but sometimes it is also classified as a processing process.

The processing in the above process refers to plastic processing and casting. Plastic processing methods include forging, extrusion, rolling, stretching and so on. It can process titanium ingots into various sizes of cakes, rings, plates (titanium plates), pipes, bars, profiles and other products, and can also be cast into various shapes of parts and components.

The plastic processing of titanium and titanium alloys has the characteristics of large deformation resistance; normal temperature plasticity difference, high yield limit and strength limit ratio, large rebound, sensitivity to notches, easy to bond with the mold during deformation, and easy to absorb harmful gases when heated, etc. Plastic processing is more difficult than steel and copper.

Therefore, the processing technology of titanium and Titanium Alloy Rectangular Tube must consider these characteristics.

Titanium adopts plastic processing, the size of adding soil is not limited, and it can be mass-produced, but the yield rate is low, and a large amount of scraps and residues are generated during the processing.

Aiming at the above shortcomings of titanium plastic processing, powder metallurgy technology has been developed in recent years. The powder metallurgy process of titanium is the same as ordinary powder metallurgy, except that the sintering must be carried out under a vacuum. It is suitable for the production of large-volume, small-sized parts, and is especially suitable for the production of complex parts. This method requires almost no processing and has a high yield rate. It can not only make full use of titanium waste as raw materials, but also reduce production costs, but it cannot produce large-size grade 5 titanium round rod parts. The powder metallurgy process of titanium is: titanium powder (or titanium alloy powder) -> sieving -> mixing -> press forming -> sintering -> auxiliary processing -> titanium products.

Titanium alloys for large aviation forgings

Titanium alloy has been widely used in the aviation industry due to its high specific strength, specific rigidity, corrosion resistance, and other excellent properties, and it has significant effects on the weight reduction of aircraft structures and the improvement of the service life of parts. Especially for military aircraft, some important structural components such as wing spars, bulkheads, landing gears, etc. are widely processed by titanium alloy forgings, which have high weight, large size, and cross-sectional area.

At present, the main grades of titanium alloys for aviation are TC4, TC18, TC21, TA15, TB6 (Ti 1023 Titanium Thick Plate in the United States), and so on. Among them, TC18 is suitable for manufacturing large forgings due to the highest hardenability and is currently the most widely used in China. Russia developed Ti-55531 titanium alloy on the basis of TC18 alloy based on the demand of civil aircrafts such as Boeing and Airbus by reducing the content of easily segregated element Fe. Ti-55531 has higher strength, superior hardenability, and a wider processing window. It has been used in parts such as A380 aircraft suspension joints and landing gear struts.

The domestic C919 aircraft has also carried out the research and development of domestic materials, and the domestic heavy-duty helicopters are also using this material to develop the central part of the large helicopter hub. The data collected so far show that: Compared with TC21 and TC18, Ti-55531 has higher tensile strength, lower plasticity and fracture toughness, higher fatigue limit, and is suitable for long-life design.

Development and application of titanium alloy materials in bicycle manufacturing

With the change in people's understanding of bicycle functions, especially the increasingly fierce international bicycle competition, bicycle manufacturers continue to seek new materials with lightweight and high strength, so titanium seamless pipe has become their target of attention.

According to the functions of each part of the bicycle, the bicycle can be divided into 13 parts, namely, the body part, the control part, the drive part, the walking part, and the transmission part. Braking part, seat part, loading part, parking part, alarm part, lighting part, accessories and connecting parts. The car body includes the frame, the front car, the front, the suspension, the valve seat and the mudguard. This part accounts for about 27% of the car's weight by its weight. The driving part includes a sprocket, a crankshaft, a pedal, a chain, a flywheel, and a chain box. This part accounts for about 22% of the car's weight. The walking part includes tires, inner Grade 3 Pure Titanium Tube, rims, spokes, and wheels, which account for about 24% of the total weight of the bicycle. It can be seen that the weight of the three parts of the body, the driving part and the walking part account for more than 70% of the total weight of the bicycle.

Therefore, the issue of titanium materialization to reduce the weight of bicycles first started here, and after more than 10 years of development and research, some of the human parts have been made of titanium material.

Surface treatment method of titanium tube

The alloying of the surface of the titanium tube can also be called a kind of coating. Compared with the oxide coating of palladium on titanium, it has the advantages of strong bonding force, wears resistance, and corrosion resistance. Compared with the titanium-palladium alloy, the surface content of palladium is high, which saves the precious metal palladium, reduces the cost, and also has excellent performance.

Aluminizing the surface of the Grade 5 Titanium Tube can reduce the hydrogen absorption tendency of titanium. Mixing 88% alumina, 4% aluminum fluoride, and 8% aluminum powder to coat the titanium workpiece is a good aluminizing condition, the temperature is 810℃. The thickness and structure of the aluminized layer depend on the holding time. Aluminizing by the permeation method, a thin coating about 2 mm thick can be obtained by diffusion in a short time, and the bonding force is good, and the mechanical properties of the substrate are not affected. The coating is mainly composed of titanium oxide phase, which forms a barrier layer that prevents hydrogen penetration very effectively.

After palladium is plated on the titanium tube, it can be heated by different methods to diffuse the palladium-plated layer to the surface area of ​​the titanium tube to form a titanium-palladium alloyed layer on the surface, thereby improving the corrosion resistance. Heating diffusion or laser surface remelting are both feasible in principle. Use a laser to irradiate the titanium surface to quickly melt and condense, which can quickly melt the pre-vacuum-deposited palladium film of 150 microns to form an effective palladium alloy.

The surface treatment of the Titanium Condenser Tubes has many advantages, which can increase the oxidation resistance of the titanium tube and enhance the brightness of the surface of the titanium tube. Different titanium tube surface treatment methods will meet the different performance requirements of titanium tube, which is an important manifestation of the production capacity of titanium tube manufacturers at this stage.