Defects that are prone to occur in ultrasonic flaw detection of alloy forgings

In the application of various titanium alloy products, titanium alloy forgings are mostly used in applications requiring high strength, high toughness and high reliability, such as gas turbine compressor disks and medical artificial bone. Therefore, not only high dimensional accuracy is required for titanium forgings, but also materials with excellent characteristics and high stability are required. The following mainly introduces the six problems existing in titanium alloy flaw detection.

Titanium alloy ring

1. Segregation defects

In addition to β segregation, β spot, Gr5 Ti-6Al-4V Titanium Wire-rich segregation and strip α segregation, the most dangerous is interstitial α stable segregation (type I α segregation), which is often accompanied by small holes and cracks around it, containing oxygen, nitrogen and other gases , is more brittle. There is also aluminum-rich α-stable segregation (type II α-segregation), which also constitutes a dangerous defect due to cracks and brittleness.

2. Inclusions

Most of them are metal inclusions with high melting point and high density. It is formed by the high melting point and high density elements in the titanium alloy composition that are not fully melted and left in the matrix (such as molybdenum inclusions), and there are also carbide tool chippings or inappropriate electrode welding processes mixed in smelting raw materials (especially recycled materials). The smelting of titanium alloys generally adopts vacuum consumable electrode remelting method), such as tungsten arc welding, leaving high-density inclusions, such as tungsten inclusions, and titanium compound inclusions.

The existence of inclusions can easily lead to the occurrence and expansion of cracks, so it is not allowed to exist defects (for example, the Soviet Union's 1977 data stipulated that high-density inclusions with a diameter of 0.3~0.5mm were found in the X-ray inspection of titanium alloys. record).

3. Residual shrinkage

In the central area of ​​the acid leaching test piece (in most cases), there are irregular folds, cracks or voids, and there are often serious looseness, inclusions (slag inclusions) and composition segregation on or near it.

4. Holes

The holes do not necessarily exist individually, but may also exist in a plurality of clusters, which will accelerate the growth of low-cycle fatigue cracks and cause premature fatigue failure.

5. Cracks

Mainly refers to forging cracks. grade 5 titanium round rod has high viscosity, poor fluidity, and poor thermal conductivity. Therefore, in the process of forging deformation, due to the large surface friction, obvious internal deformation unevenness and large temperature difference between inside and outside, it is easy to produce shear bands inside the forging ( Strain line), which will lead to cracking in severe cases, and its orientation is generally in the direction of maximum deformation stress.

6. Overheating

The thermal conductivity of titanium alloy is poor. In addition to overheating of forgings or raw materials caused by improper heating during the hot working process, it is also easy to cause overheating due to the thermal effect of deformation during the forging process, causing changes in the microstructure and producing an overheated Widmandarin structure.

Analysis of properties, classification and use of pure titanium

Among titanium materials, including titanium rods, titanium flanges,Titanium Hot Rolled Sheet, etc., are divided into pure titanium and titanium alloys. Compared with pure titanium, titanium alloys are added with Al, Mo, Cr, Sn, Mn, V and other alloying elements. Therefore, pure titanium has its unique excellent properties and different uses in terms of mechanical properties.

1. Performance

Ti: 4.507 g/cm3, Tm: 1688°C. It has allotrope transformation, α phase with hexagonal close-packed structure at ≤882.5°C, and β phase with body-centered cubic structure at ≥882.5°C.

Pure titanium has low strength, but high specific strength, good plasticity, good low temperature toughness and high corrosion resistance. Titanium has good pressure processing performance and poor cutting performance. Titanium can burn when heated in nitrogen, so titanium should be protected by argon during heating and welding.

2. Classification

According to the impurity content, titanium is divided into high-purity titanium (purity of 99.9%) and industrial pure titanium (purity of 99.5%). There are three grades of industrial pure titanium, which are respectively represented by Gr + serial number numbers 1, 2, and 3. The larger the number, the lower the purity.

3. Purpose

The content of impurities has a great influence on the performance of titanium. A small amount of impurities can significantly improve the strength of titanium. Therefore, the strength of industrial pure titanium is relatively high, close to the level of high-strength aluminum alloys. , reactors, ship parts, aircraft skins, etc.

Titanium rod filter request at installation

There must be not many people who do not know about titanium rod filters. Seamless Titanium Tube Grade 2 are widely used in our lives, and many operations are inseparable from it.

Its main material is to use titanium powder sintered filter element, which is generally used for pre-filtration or filter room. Therefore, its filtering effect is still good, and it plays a primary role in each operation. So such an important device is also required when it is installed, so how to install it specifically?

The first thing to pay attention to is that before the shell of the titanium rod and the equipment are used, it is necessary to check whether the sealing is in place, and it is very important to ensure the sealing of the equipment. For the equipment of the shell, it is necessary to have 15 minutes to 20 minutes of cleaning, so that it will not affect its filtration quality when it is used. When the normal working pressure of filtration should be controlled at 0.15 ~ 0.25 MPa, generally a low pressure effect is initiated at least 3-5 minutes before the normal use, so it is forbidden to increase the pressure during the operation. If the titanium rod is seriously blocked and the recoil effect is not obvious, then ultrasonic cleaning should be used.

The blank preparation process in the titanium plate processing process

Titanium is a very active metal element that can interact with all elements. When heated, titanium can interact with oxygen to form an oxide film on the surface. At different heating temperatures, the structure and properties of the oxide film are different. The thermoforming of titanium plate is more complicated, and the mechanical properties of each batch are different. In order to ensure the quality of the material, the manufacturer has a certificate and test report.

1. The raw material supply status requirements for the preparation of titanium plates

One-time deformation of the titanium plate-after rolling into a Grade 9 Ti-3Al-2.5V Titanium Sheet, the performance repeatability is poor. The mechanical properties of the same composition of titanium plates produced at home and abroad are not small; the properties of plates produced by different domestic factories are not the same. Even the products of a factory have different mechanical properties with different heat numbers. Therefore, in order to ensure the quality of the parts, the quality of the raw materials should be ensured first. The titanium and titanium alloy sheets received from the warehouse shall have the factory certificate and the factory re-inspection certificate. In order to facilitate verification in the future, the certificate should be kept. When necessary, the workshop can make a record of the basic situation of the parts manufactured by each sheet for inspection.

All plates are required to be supplied in an annealed condition. In order to ensure the quality of the parts, the oxide layer and other pollution on the surface of the plate should be removed. The surface of the Grade 1 Pure Titanium Plate shall not have defects such as cracks, peeling, crushing, inclusions, and traces of pickling. The sheets should be straight and packed with paper out of the warehouse. In the subsequent process of blanking, forming, assembling, transportation and storage, care should be taken to prevent surface scratches.

2. Blanking of titanium plate

There are many similarities in the wool preparation methods used for various sheet metal parts. Since the thickness of the titanium plates used is generally less than 2 mm, and there are few parts with a thickness of 1.5 mm or more, the blanking of the titanium plates can be carried out at room temperature using existing equipment.

When cutting materials, care should be taken to make the edges of the wool as smooth as possible, especially to prevent defects such as cracks, gnaws and cut marks on the edges, and pay more attention when cutting thick plates.

The wool should be deburred and sharp edges removed before forming. As the titanium alloy plate is sensitive to crack propagation, this work is extremely important to eliminate stress concentration and prevent the wool from cracking and scratching the mold during forming. It can be deburred or polished by filing, abrasive belt grinding or sandpaper.

It should be noted that no matter which method is used for blanking, only non-polluting ink and pens can be used when marking the line. Steel stamping is not allowed on the wool, and only offset printing or special labels can be used.

3. Degreasing and cleaning before forming

Before any thermoforming, grease or other contaminants on the surface should be removed to avoid contamination of titanium during heating. In addition, the oxide on the surface of the wool will reduce the plasticity of the material and should be removed before forming. Therefore, in order to reduce various influences in the atmosphere, vacuum quenching furnaces and vacuum annealing furnaces are generally used for heating. The inert gas in the vacuum furnace can protect titanium and titanium alloy materials from pollution during the heating process.

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.