Titanium alloy stress corrosion cracking

Stress corrosion cracking (SCC) of titanium alloys was once one of the very active research topics at home and abroad, especially in the aerospace field. Industrial practice shows that most of the titanium welding wire used in the chemical industry are very resistant to stress corrosion. For cracking, stress corrosion cracking may occur only in some special media, such as pure methanol, fuming nitric acid, halide aqueous solution of nitrogen tetroxide, and liquid metal calcium and mercury. Industrial practice has proved that the actual occurrence of stress corrosion cracking accidents is much less than the stress corrosion sensitivity found in the laboratory.

The stress corrosion susceptibility is related to factors such as medium composition, pH value, potential, temperature, and even the viscosity of the medium. The addition of halogen ions improves the stress corrosion sensitivity and increases with the increase of ion concentration. Increased temperature and decreased viscosity can accelerate the rate of cracking.

The mechanism of stress corrosion cracking is still not completely unified. At present, there are two main types of viewpoints: stress-accelerated anodic dissolution and hydrogen enrichment at the crack tip. In fact, this may not necessarily be a tit-for-tat irreconcilable viewpoint. condition. However, a lot of practice shows that hydrogen-induced cracking is a very important cause of stress corrosion cracking, especially the SCC cracking phenomenon found in general industries (such as chemical industry).

Processability of Titanium Alloy and Technology of Grinding and Polishing

Many process parameters of titanium alloys in forging, volume stamping and sheet stamping are close to those in steel processing. But there are some important features that must be paid attention to when pressing working titanium and titanium alloys. When the titanium alloy is cold plastically deformed, regardless of its chemical composition and mechanical properties, the strength will be greatly improved, and the plasticity will be correspondingly reduced. For this reason, annealing treatment between processes must be performed. The low thermal conductivity of titanium and the formation of non-coiled shavings are the reasons for the important characteristics of the machining of titanium and its alloys. When cutting titanium, the tool face will generate high temperature. In order to dissipate the heat, sharpened knives can be used, and a large amount of coolant and lubricant can be supplied. The tools made of carbide BK6M, BK8, BK4, BK2 have good cutting performance in grinding, milling and drilling.

The grinding and polishing of titanium alloys are divided into the following three methods:

(1) Fine grinding

That is to use conventional various emery rubber wheels to grind the surface of titanium and titanium alloy castings. The problem that needs to be paid attention to during grinding is still not to make the casting generate heat, not to cause grinding damage on the surface of the casting, and to make the entire surface smooth and smooth.

(2) Barrel grinding method

The so-called barrel grinding method is to put the processed castings, abrasives, water and additives into the barrel-type grinding tank, and the grinding barrels rotate and vibrate, so that friction is generated between the mixture of abrasives and the processed castings, and the surface of the castings is ground. Smooth and flat. It is characterized by no dust pollution, low labor intensity, and no heat generation in the conventional grinding process. At present, Japan has commercialized barrel grinders and abrasives for grinding Grade 1 Titanium Bar, other metal castings and plastics. It has been confirmed by experiments that the grinding efficiency of pk series abrasives is high, but the surface roughness is also very high. Although the grinding efficiency of SA and B series abrasives is not as good as that of PK, the surface smoothness of the ground titanium castings is higher. At present, the Fourth Military Medical University has successfully developed the first domestic dental barrel grinder.

(3) Mechanical polishing method

The method of polishing the surface of titanium and titanium-containing alloys using soft cloths or black brushes of different specifications, dipped in special polishing pastes for titanium and titanium-containing alloys. When polishing titanium castings, it is necessary to completely clear the contamination layer on the surface of the casting and no new grinding and hardening layer, which will not achieve the ideal polishing effect. When polishing, high speed and light pressure should be used for polishing. The author has also tested the use of green polishing paste to polish titanium and titanium-containing alloy castings, and achieved a relatively ideal polishing effect. Polished titanium and titanium-containing titanium alloy castings cannot be washed with water immediately. The surface oxide film must be completely formed before washing with water, otherwise the surface will be darkened.

Heat transfer and corrosion resistance of thin-walled coil-welded titanium tubes for seawater desalination

With the continuous development of the seawater desalination industry, the evaporation heat transfer tube, which is one of the core components of thermal desalination devices, has also attracted the attention of scientific researchers and technicians. Traditional stainless steel, copper alloy, aluminum alloy, and other materials cannot fully meet the requirements of seawater application environment for material corrosion resistance, mechanical properties, and lightweight, while industrial pure titanium TA2 has excellent corrosion resistance, good plastic toughness and high It is an ideal material for making heat exchange elements of thermal seawater desalination devices. At present, in the large-scale thermal seawater desalination plants constructed in my country, rolled seamless titanium tubes have been applied to a certain extent, but the large-scale promotion of titanium heat transfer tubes is still greatly limited. The high price of seamed titanium pipes leads to high material investment costs.

Many scholars at home and abroad have been committed to the design and development of thin-walled Grade 2 Pure Titanium Tube and the research on corrosion resistance. The application history in the field of horizontal tube falling film seawater desalination technology is short, and the experimental data on the heat transfer coefficient and corrosion resistance of titanium tube falling film evaporators are insufficient, so it is difficult to guide the design of seawater desalination system. In addition, the wall thickness of titanium welded pipes used in seawater desalination projects is currently 0.5mm, which is difficult to meet the control requirements of seawater desalination projects on material costs. , reduce the amount of titanium used in equipment. Based on this, TA2 industrial pure titanium ϕ22mm×0.4mm thin-walled coiled titanium tube was prepared by tungsten argon arc welding (TIG) process, and the application experiment of low-temperature multi-effect distillation seawater desalination was carried out to test its corrosion resistance. performance and heat transfer performance, verify the quality of welded joints, obtain experimental data, and provide technical support for the large-scale production of 0.4mm thin-walled coiled titanium tubes and their popularization and application in seawater desalination projects.

Physical phenomena of titanium machining

The cutting force of titanium alloy processing is only slightly higher than that of steel with the same hardness, but the physical phenomenon of processing titanium alloy is much more complicated than that of processing steel, which makes titanium alloy processing face huge difficulties.

The thermal conductivity of most titanium alloys is very low, only 1/7 of steel and 1/16 of aluminum. Therefore, the heat generated in the process of cutting Grade 9 Ti3Al2.5V Titanium Tubes will not be quickly transferred to the workpiece or taken away by the chips, but will accumulate in the cutting area, and the temperature generated can be as high as 1 000 °C or more, which will cause the cutting edge of the tool to rapidly wear, chip and crack. A built-up edge is created, which quickly causes a worn edge, which in turn generates more heat in the cutting area, further reducing tool life.

The high temperatures generated during the cutting process also destroy the surface integrity of the titanium alloy parts, resulting in a decrease in the geometric accuracy of the parts and work hardening that severely reduces their fatigue strength.

The elasticity of titanium alloys may be beneficial for part performance, but during cutting, the elastic deformation of the workpiece is an important cause of vibration. The cutting pressure causes the "elastic" workpiece to move away from the tool and bounce so that the friction between the tool and the workpiece is greater than the cutting action. The friction process also generates heat, aggravating the problem of poor thermal conductivity of titanium alloys. This problem is even more serious when machining thin-walled or ring-shaped parts that are prone to deformation. It is not an easy task to machine thin-walled titanium alloy parts to the expected dimensional accuracy. Because when the workpiece material is pushed away by the tool, the local deformation of the thin wall has exceeded the elastic range and plastic deformation occurs, and the material strength and hardness of the cutting point increase significantly.

Machining characteristics of medical titanium and titanium alloys

Titanium 6AL-4V ELI is the standard product for the fabrication of hip joints, bone screws, knee joints, plate bones or organs, dentures and surgical devices. However, cobalt-chromium alloys are being used more and more often because of their strength, tighter grain size, and cleanerness than titanium. Machining titanium alloys requires only slightly greater cutting forces than machining steels, however, the metallurgical properties of titanium alloys make them more difficult to machine than steels of moderate hardness. Titanium has a titanium work-hardenability that eliminates consolidated metal (hemming) in front of the cutting tool. This helps to increase the shear angle in machining, thus forming a thin chip that touches the cutting tool surface in a reasonably small area. Because of this work-hardening, the feed should not be suspended during the tool-workpiece motion contact. The large supporting force that occurs during machining, combined with the frictional force generated by the chips in the touch area, causes a large increase in heat in the tool part area. The heat generated by cutting titanium does not dissipate quickly because it is a poor conductor. Therefore, most of the heat is concentrated on the cutting edge and tool surface. The large bearing force and heat form a crater near the cutting edge, causing the tool to be damaged quickly. To make matters worse, Titanium Alloy Threaded Rod have a strong tendency to fuse with the products in the tool to form alloys or revive chemical changes at the working temperature of the tool. damage. These difficulties are multiplied when tools begin to break, so tools used to machine titanium and its alloys should be carefully monitored to ensure sharp edges and replacement before they become dull. The experience of machining titanium and titanium alloys is that if you see any changes during the machining process, you should change the tool immediately, because the change means that the tool will become dull. Another reason to insist on sharp knives is that titanium can catch fire when cutting with a damaged or damaged tool. When incinerated, the metal generates oxygen, so the fire will spontaneously ignite. Therefore, many workshops that process titanium do not report fires, and they are equipped with rescue systems on their machine tools.

Titanium has a moderately low modulus of elasticity and is more elastic than steel and therefore tends to defy the cutting tool when machined unless it is to be cut robustly or used as a proper support. Slender parts tend to deflect under tool pressure, causing problems with tool chatter, tool friction, and tolerance. Through the processing experience, it is believed that the rigidity of the entire system of the tool is very important, and the sharp and accurate shape of the tool should be used. As a result of these pressures, new technologies have been introduced to help shops that make medical parts cope with the competition, and the machining performance can produce these complex parts with very high precision; many innovations in EDM have enabled the production of high-quality parts faster, eliminating many old machining techniques inherent topics.

As a new type of product, titanium has only been developed and used in China's pharmaceutical industry, medical equipment, human implants and other fields for nearly two decades. However, it has achieved great success and achieved significant social and economic benefits, shortening the gap between China and the advanced countries in the world. The use of titanium equipment in the production of many commodities not only solves the equipment corrosion problem that has seriously plagued the company's production and development, but also greatly improves the quality of drugs.

Passivation Characteristics of Titanium and Titanium Alloys

Titanium and Grade 7 Titanium Plate are active metals, which can easily react with oxygen in the air to form a very thin transparent oxide film, which passivates the surface and forms a passivation film. On the one hand, this passivation film protects the base metal and improves the corrosion resistance of the metal surface; on the other hand, because the formation of the passivation film reduces the activity and electrical and thermal conductivity, it is difficult to coat the surface, which will obviously affect the bonding force between the coating layer and the substrate. Therefore, activation treatment must be performed before the surface of titanium and Grade 3 Pure Titanium Plate, which is a critical step to ensure the bonding force between the plated layer and the substrate.

Characteristics and Application of TC4 Titanium Rod

As a new generation of future metal, titanium metal has strong corrosion resistance. The TC4 titanium rod produced, processed, and manufactured with titanium metal as raw material is quite stable in organic media and has a wide range of applications in the medical and chemical fields; machined titanium round rod High-temperature resistance, this feature allows it to be used in high temperature operating environments without deformation and aging; in addition, TC4 titanium rods have high strength, that is, they have super mechanical properties, so they can be used in high-pressure processes; TC4 The regeneration effect of titanium rods is good and the service life is long. There are two kinds of regeneration treatment methods: physical regeneration method and chemical regeneration method. The physical regeneration method includes pure water backflushing, steam backflushing and ultrasonic cleaning; chemical treatment methods include acid washing and alkali cleaning. Wash both.