Titanium alloy is a kind of difficult-to-process material with poor thermal conductivity, easy to adhere to tools, strong notch sensitivity and strong work hardening during rolling. In actual production, there are a series of problems in rolling full teeth. Firstly, after filling the gap between the two rollers in the rolling process, if the rollers continue to roll, the blank material can not flow and can only be extruded back and forth between the rollers, resulting in higher surface hardening of the threads. When the hardening exceeds the tensile limit of the material, cracks occur. The hardening of the side and bottom of the threads is the most serious, so cracks occur. Secondly, when full extrusion, every tooth of the roller involved in rolling is also affected by strong cyclic load, and the energy of rolling also acts on the roller, so that its life is seriously reduced. In actual production, each roller can only roll 3000-5000 titanium bolts, resulting in the phenomenon of crown fragmentation. The roller can no longer be used, and the cost is extremely expensive. The cracking of the top of the roller tooth has a forming process, which makes it impossible for the producer to determine the eligibility of the rolling thread, that is, when the rolling is qualified and when the rolling is unqualified; thirdly, in the process of rolling forming the thread, the two sides rise fastest, which is bound to produce a fold on the top of the tooth when the thread is full, and there are fluorescent marks on the top of the thread during flaw detection, in order to determine the maximum. The depth of defect still needs to be determined by anatomical method. Fourthly, modern fastener joints have higher fatigue life and higher accuracy requirements for bolt holes. Hole wall damage is not allowed in the installation process, and the top of the more pointed thread is very easy to damage the hole wall. In a word, full thread profile brings great trouble to production. It not only increases the chance of cracking, decreases the fatigue life of parts, but also reduces the service life of the roller. Moreover, it brings unnecessary trouble to testing, and also easily causes damage to the hole wall. Therefore, it is imperative to correct the thread diameter of titanium alloy fasteners.

In foreign countries, such as all the titanium bolts in the United States and the latest standard of titanium bolts, the thread diameter has been revised, the thread size has been moved down, and the thread has adopted the unsaturated tooth type. In order to confirm this viewpoint and to provide a basis for the correction of the thread diameter of titanium alloy fasteners in China, the mechanical properties of two representative hexagonal head titanium bolts, HB6563.6 and HB6563.10, with different thread diameters, were tested. Two kinds of bolts are adopted three kinds of large diameter size, and mechanical performance verification includes tensile, fatigue and stress endurance test.

Pure titanium reaches 800,000 pounds/inch 2 (5517 MPa) and alloy titanium reaches 180,000 pounds/inch 2 (1241 MPa), which is far higher than the strength of many alloy steels, so titanium has a high strength-weight ratio. Titanium has twice the elasticity of steel and is an ideal choice for applications requiring high fracture or fracture resistance. In addition, titanium alloy has higher corrosion resistance and oxidation resistance than stainless steel. Many properties of titanium make it suitable for most applications, but at the same time lead to its becoming one of the most difficult materials to process. However, a manufacturer who understands the characteristics of this material can successfully process titanium parts without having to pay a high price. Most titanium alloys have poor thermal conductivity. The heat generated in the machining process does not diffuse through the parts and machine tool structure, but concentrates in the cutting area. In some cases, the temperature reached as high as 2,0000 F (11093 C) may lead to edge collapse and deformation, while the blunt edge will generate even higher heat and further reduce tool life. Cutting temperatures can be so high that titanium chips sometimes burst into flames. The high temperature produced in the cutting process will also cause the workpiece to harden continuously, which will affect the surface integrity of titanium, and may lead to inaccurate geometric accuracy of parts, and seriously reduce their fatigue strength. The elasticity of titanium alloys, which is beneficial and necessary for the product, adds fuel to the deflection and vibration in heavy load cutting. Under cutting pressure, “elastic” material is removed from the tool. Therefore, instead of cutting, the cutting edge rubs, especially when the feed is small. This friction process also generates heat, aggravating problems caused by poor thermal conductivity of materials.

The forward cutting geometry is adopted to reduce cutting force, heat and part deflection. Constant feed is used to prevent work hardening. Never stop feeding in the cutting process. Use a large amount of coolant to maintain thermal stability and prevent temperature rise problems that may lead to irregular secondary surfaces and possible tool failures. Keep the tool sharp. The blunt knife will accelerate the temperature rise and cause the wear and tear phenomena which lead to the failure of the knife. Processing titanium alloys in as soft a state as possible. Because many alloys are age-hardened — they harden when heated — they become stronger and more abrasive when forming second-phase particles. As far as possible, larger radius of tool end or circular blade are used to make the tool enter the cutting more. This reduces the cutting force at any point and prevents local damage.