Titanium alloy materials are difficult to process, poor thermal conductivity, easy to adhesion with tools, there is a strong notch sensitivity, in the rolling process will produce a strong work hardening.In practice, there are a series of problems in the rolling of full teeth.
Type one, full of teeth in the heart of the rolling of the gap between the two screw rolling wheel full after, screw rolling wheel, such as continue to roll, blank material is nowhere to flow, only back and forth between the rolling wheel by extrusion, thread has a higher surface hardening as a result, when the hardening exceeds the limit when they produce material tensile crack, thread side, teeth hardening is the most serious, therefore produce crack;
Extrusion, second, full participation in rolling thread rolling wheel of each tooth is also affected by the strong cyclic load, the influence of the energy of the roller also ACTS on the thread rolling wheel, make the life of serious decline, in actual production, each pair of screw rolling wheels rolling only 3000-5000 pieces of titanium bolts, produce the phenomenon of dental crown fracture, screw rolling wheel cannot again continue to use, cost is very expensive, in addition, screw rolling wheel teeth the forming process of a crack top make producers unable to determine the eligibility for the rolling thread, when the roller is qualified, when rolling is unqualified;
Third, in the process of forming thread by rolling, both sides rise the fastest, which is bound to produce a folding at the top of the tooth when the thread is full. During flaw detection, the top of the tooth has fluorescent traces.Fourth, modern fastener joints have a high fatigue life, for the bolt hole has a high precision requirements, in the installation process is not allowed to produce hole wall damage, and the tip of the more pointed thread tooth easy to damage the hole wall.In short, full of thread type brought great trouble, to produce not only increases the chance that the crack, reduce fatigue life of parts, also reduces the service life of screw rolling wheel, moreover to detect bring unnecessary trouble, also easy to produce the hole wall damage, so to modify titanium alloy fastener screw big diameter is imperative.
In foreign countries, such as the United States all titanium bolts, the latest titanium bolt standard have been modified to the thread diameter, the thread diameter size moved down, thread are used not full teeth.Force mainly near the thread pitch diameter of thread, and the screw thread of large diameter down estimate will not affect the mechanical properties of the bolt, the author to confirm this view, for titanium alloy fastener screw thread of large diameter correction in our country, for the two kinds of typical HB6563.6, titanium HB6563.10 hexagonal head bolt screw thread of large diameter different mechanical properties were studied.Two kinds of bolts adopt three kinds of large diameter respectively;Mechanical property verification includes tensile, fatigue and stress endurance tests.
Pure titanium has a strength of 800,000 pounds/inch 2(5517MPa) and alloy titanium has a strength of more than 180,000 pounds/inch 2(1241MPa), which is much higher than the strength of many alloy steels and therefore has a very high strength-to-weight ratio.Titanium is twice as elastic as steel, making it an ideal choice for applications with high fracture and fracture resistance requirements.In addition, titanium alloy has higher corrosion resistance and oxidation resistance than stainless steel.Titanium's many qualities make it suitable for most applications, but also make it one of the most difficult materials to process.However, manufacturers who understand the material's characteristics can successfully process titanium parts without having to pay a high price.Most titanium alloys have poor thermal conductivity.The heat generated during processing is not diffused through the parts and machine structure, but is concentrated in the cutting area.In some cases, the temperature reached as high as 2,0000°F(11093 ° c), which may lead to blade breakage and deformation.A blunt blade generates even more heat and further reduces tool life.Cutting temperatures can be so high that titanium chips sometimes burst into flames.The high temperature generated in the cutting process will also cause the workpiece hardening, this phenomenon will affect the surface integrity of titanium, and may lead to the geometric accuracy of parts is not accurate, and seriously reduce the fatigue strength.Elasticity of titanium alloys, which is useful and necessary for finished products, adds fuel to flexure and vibration in heavy-duty cutting.Under cutting pressure, the "elastic" material is removed from the tool.Therefore, the cutting edge does not cut but friction, especially in the case of a relatively small amount of feed.This friction process also generates heat, exacerbating problems caused by poor thermal conductivity of the material.
Adopt positive cutting geometry to reduce cutting force, heating and parts deflection.Constant feed is used to prevent work hardening.Never stop feeding while the tool is cutting.Maintain thermal stability with a large amount of coolant and prevent temperature rise problems that may lead to subsurface irregularities and possible tool failures.Keep the knife sharp.A blunt tool accelerates temperature rise and causes wear and tear that can lead to tool failure.Process 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 as they form second-phase particles.As far as possible to use a larger radius of the tool end or circular blade, so that the tool more into the cutting.This can reduce the cutting force at any point and prevent local damage.