Titanium alloy materials are difficult to process materials, poor thermal conductivity, easy to stick with tools, there is a strong notch sensitivity, will produce a strong work hardening during the rolling process. In actual production, there are a series of problems in the rolling of full teeth.
First, after the full tooth shape squeezes the gap between the two rolling wheels during rolling, if the rolling wheel continues to roll, the blank material has no place to flow and can only be squeezed back and forth between the rolling wheels Pressure, resulting in a higher surface hardening of the thread. When the hardening exceeds the tensile limit of the material, a crack is generated. As shown in Figure 1, the side of the thread and the bottom of the thread are hardest, so the crack is generated;
Secondly, during full squeezing, each tooth of the rolling wheel participating in rolling is also affected by a strong cyclic load, and the energy of rolling also acts on the rolling wheel, causing its life to be seriously reduced. In production, each pair of thread rolling wheels can only roll 3000-5000 pieces of titanium bolts, which causes the crest of the tooth crest, and the thread rolling wheel can no longer be used. The cost is extremely expensive. In addition, the crest of the thread rolling wheel is broken. There is a forming process that prevents the producer from judging the qualification of the rolled thread, that is, when the rolled thread is qualified and when the rolled thread is unqualified;
Third, during the process of rolling to form a thread, the two sides rise fastest. It is bound to produce a fold in the crest of the tooth when the thread is full. There is a fluorescent display trace on the crest of the thread during flaw detection. In order to determine the maximum defect depth, anatomy is also required The method can determine whether the defects here exceed the standard; Fourth, modern fastener connection joints have a higher fatigue life, have higher precision requirements for bolt holes, and do not allow hole wall damage during installation. The sharp thread crest is very easy to damage the hole wall. In short, the full thread profile brings great trouble to the production, which not only increases the chance of cracks, reduces the fatigue life of the parts, but also reduces the service life of the rolling wheel, and brings unnecessary trouble to the inspection. It is easy to damage the wall of the hole, so it is imperative to correct the large diameter of the thread of the titanium alloy fastener.
Foreign countries, such as all titanium bolts in the United States, the latest titanium bolt standards have amended the thread diameter, the thread diameter is moved down, and the threads are not full. The force of the thread is mainly near the middle diameter of the thread. The downward movement of the major diameter of the thread is not expected to have an impact on the mechanical properties of the bolt. In order to confirm this view, the author provides a basis for the correction of the major diameter of the titanium alloy fastener thread in China. The mechanical properties of two representative HB6563.6 and HB6563.10 hexagon head titanium bolts with different thread diameters and large diameters were tested. Two types of bolts are used in three kinds of large diameter sizes; mechanical performance verification includes tensile, fatigue, and stress endurance test research.
Pure titanium reaches 800,000 psi (5517MPa), alloy titanium reaches 180,000 psi (1241MPa) or more, which is much higher than the strength of many alloy steels, so titanium has a high strength to weight ratio. Titanium is twice as elastic as steel, making it an ideal choice for applications requiring high resistance to fracture or breakage. In addition, titanium alloys have higher corrosion and oxidation resistance than stainless steel. The many qualities of titanium make it suitable for most applications, but it also makes it one of the most difficult materials to process. However, manufacturers who understand 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 during the machining process will not be diffused through the parts and machine structure, but concentrated in the cutting area. In some cases, the temperature reached as high as 20,000 ° F (11093 ° C), which may cause chipping and deformation; and the dull blade will generate even higher heat and further reduce the tool life. The cutting temperature can be so high that titanium chips sometimes burst into flames. The high temperature generated during the cutting process will also cause the workpiece to harden continuously. This phenomenon will affect the surface integrity of titanium and may lead to inaccurate geometric accuracy of the part and severely reduce its fatigue strength. Titanium alloy elasticity, which is beneficial to the finished product and necessary, is “pouring oil” on deflection and vibration during heavy-duty cutting. Under cutting pressure, the “elastic” material moves away from the tool. Therefore, the cutting edge rubs instead of cutting, especially when the feed amount is relatively small. This friction process also generates heat, exacerbating problems caused by poor thermal conductivity of the material.
Adopt positive cutting geometry to reduce cutting force, heat generation and parts deflection. Constant feed is used to prevent work hardening of the workpiece. During the cutting process of the tool, do not stop the feed. Use a large amount of coolant to maintain thermal stability and prevent temperature rise problems that may cause irregularities and possible tool failures on the secondary surface. Keep the tool sharp. Blunt knives accelerate the temperature rise and cause wear and tear that cause tool failure. Process the titanium alloy as soft as possible. Because many alloys are age hardened—they harden when heated—they become stronger and more abrasive when forming second-phase particles. If possible, use a larger tool end radius or round insert to make the tool enter the cutting more. This can reduce the cutting force at any point and prevent local damage.