Everything You Wanted To Know About Titanium
If steel and titanium are both equal in strength, then aluminum is half the strength. When you get into the 7000 series of aluminum they get to about two thirds the strength of titanium. The problem is that the ductility of aluminum is less than steel. This is what comes into play when designing parts, especially the axles and some bolts, like cylinder head bolts, the stretchiness becomes an issue with titanium. And the technical term for that is modulus of elasticity. That's why a when looking at a titanium spring, it has less coils in it. With the modulus number of titanium being half of steel it's going to flex twice as much for the same amount of load. 
So let's give an example. Let's say it's a steel bolt and you give it 1,000 pounds of force on the bolt, the bolt is going to stretch a certain amount. A titanium bolt with that same 1000 pounds of force is going to stretch twice as much. Because of stretching it will still carry the load as long as you are within the elastic limit. That's why people say they have a problem with the bolt stretching, but they both stretch until they come up to the clamp load that holds the assembly together. In motorcycle terms we see this in the triple clamps. There have only been a few riders we know that have the ability to tell the difference between steel bolts and titanium bolts. That same rider can tell the difference between 15 foot-pounds and 18 foot-pounds of torque. There are only a few riders that are that sensitive to tell the tiny differences in the triple clamps. Across the board right now we pretty much have everybody running titanium triple clamp bolts and titanium front axle pinch bolts.
The precursor to it, everyone talks about the factory teams having the advantages. Back in the ‘60s and ‘70s, with the titanium coming out of the aerospace industry, a lot of the factory teams had access and the budget to be able to build titanium fasteners. Back in the ‘70s, going into the early eighties, the machining of titanium was quite tricky. The CNC machines and the quality of the cutting tools to cut the material were still relatively expensive and hard to handle. If you talk to anyone that has been around motocross for a long time like Roger DeCoster, back in the ‘70s he had bikes totally decked out with titanium fasteners with the factory programs. But for privateers it was really hard to get their hands on it. Then CNC machines were more available, but this is where a lot of the bad wrap started with titanium axles and fasteners. CNC machines were coming along and people could take titanium bar and machine it and make a fastener by just purely machining it out of bar stock. The problem with that is if you want to make an aircraft quality fastener, it needs to be hot forged, meaning it needs a forged head to get the proper grain structure, followed by the proper heat treating and then you need to have a rolled thread. 
When you put a piece of bar stock into a CNC machine and machine it, you build up stress risers in the surface and they look like little Grand Canyon fishers on the microscopic level and then they would get fatigue cracks under the head or they get fatigue cracks in the threads and the bolts would break. Titanium seems to be more sensitive to the finish of the surface. This is where people were having random breakages with titanium and they were calling it brittle or saying it didn't have the strength of steel. But this was a processing problem, not a problem with the actual material.
When it comes time for assembly, we recommend to use anti-seize of some type. It can be an aluminum or copper anti-seize, but better than that, we like to use moly assembly paste. So the same moly paste that you'd put on your cam lobes. Look for one with a 40 percent moly content. The molybdenum gets ground into the surface and prevents any galling between the titanium and aluminum, or ti on titanium nut.  With axles, a thin layer of grease is recommended so the bearing isn't beating on the dry axle.  Then, when you're doing your normal maintenance pull them out and drop them in 409 or Simple Green to soak them overnight or if you have an ultrasonic cleaner they'll come out super bright and clean again. In some places you need to use Loctite like on rear sprockets, you can use blue Loctite on it and it actually also keeps the bolts from galling. 
Titanium screw(Most of titanium alloy screws are titanium Gr.5, Ti-6al4v) have high specific strength, tensile strength up to 100-140kgf/mm2, and density is only 60% of steel.
The medium temperature strength is good, the working temperature is several hundred degrees higher than aluminum alloy. Titanium alloy screws can maintain the required strength at moderate temperature, can work for a long time at 450 ~ 500 ℃.
Good corrosion resistance, titanium alloy screws will form a uniform and dense oxide film on the surface of the atmosphere. It has the ability to resist a variety of media erosion. It has good corrosion resistance in oxidizing and neutral media and excellent corrosion resistance in seawater, wet chlorine and chloride solutions. However, in a reducing medium such as hydrochloric acid, the corrosion resistance of titanium is poor.
Titanium alloy screw has good low-temperature performance, and titanium alloy with extremely low gap element can maintain a certain plasticity at -253 ℃.
Titanium alloy screws has low elastic modulus, low thermal conductivity, no ferromagnetism, high hardness, poor stamping property and good thermoplasticity.
Using plus cathodic polarization to reduce or prevent metal corrosion is called cathodic protection that could be achieved by two methods: impressed current protection and sacrificial anode protection.Impressed current protection makes the whole surface of the protected metal structure work as cathode, source negative pole will be connected to the metal while source positive pole connected to auxiliary anode to protection a metal equipment. Cathodic polarization occurs when current of auxiliary anode goes through the electrolyte solution and concentrates on the metal cathode, the current goes back to the source and the total potential of the metal will be reduced. If the protective current is big enough, anode of the metal structure becomes insoluble, meanwhile, the cathodic reduction reaction only occurs on the metal surface, the impressed current could achieve full protection. Anode materials develop and become various under a variety of anode working conditions. MMO titanium anode that using titanium as substrate and high catalytic activity of platinum group metal oxide as the coating with good electrical conductivity, small output resistance and good formability without corrosion applied widely in seawater, fresh water and soil environments. The protected metal connecting metal with smaller negative potential to form anode and the protected metal in the electrolyte solution to form a big source, this is called sacrificial anode protection, current flows through anode, electrolyte solution and then goes to the metal equipment to make metal equipment cathodic polarization protection.
Normally, titanium anode in the soil bed works with current density of 100A/m2, life span of 20 years and consumption rate of 0.1mg/A. With 100A/m2 and more than 20 years’ life span in the shallow soil and deep soil environments and without anodic passivity and dissolution, titanium anode is the most ideal auxiliary anode material.
Grade 2 titanium is considered the workhorse of the titanium family and is suitable for most applications. If greater corrosion resistance is required, welded tube can be produced in grades 7, 12, 16, or 26 according to ASTM B-338. If greater strength is required, grades 3 and 12 tubing are available.
Not only is standard-size grade 2 titanium pipe and tube available from inventory or quick production runs, larger sizes also can be produced by independent fabricators. Many of them also can design and produce complete piping systems, heat exchangers, and pressure vessels. This geographically diverse fabrication base has more than 25 years of engineering and design experience.
Titanium as a fastener material
Titanium is banned in the current Formula One engine regulations from being used for threaded fasteners, despite its attractive attributes for such components. The rules specify that threaded fasteners must be made from alloys based on one of three elements - iron, cobalt or nickel - and this is planned to be carried forward for the new V6 turbo engines we will see in use from 2014 onwards. It should be noted though that there is no similar regulation governing the use of titanium fasteners on the chassis.
Besides titanium's obviously attractive property of low density, its elastic modulus is the other property that makes it a good candidate material for fasteners, both of the internally and externally threaded varieties. The use of nuts with lower modulus than the male fastener is known to reduce the stress concentration effect at the first thread, and improves the distribution of load over the length of the engaged threads. Where high-modulus materials are specified for both internal and external threads, one way to achieve the same effect is to use combinations of male and female parts with very slightly differing thread pitches.
When considering the design of a stud or Gr5 titanium bolt used for cyclically loaded fastener, it is important to consider both the fastener stiffness and the stiffness of the parts being clamped. A simple formula involving these quantities dictates how the service load is shared between the unloading of the joint, and the extra load borne by the fastener. This relationship has been covered in one of the early RET Monitor articles on fasteners and in a past article in Race Engine Technology magazine*.
The smaller the stiffness of the fastener is compared to the stiffness of the joint, the less of the service load that is borne by the fastener. Ideally, what we want from a fastener material is for it to be strong - fatigue strength is the significant strength in a cyclically loaded fastener - and to have low stiffness. Titanium can score well here, and its lightness is a bonus, although that shouldn't come as a surprise. Most metallic materials fall within a pretty narrow range of specific modulus (modulus divided by density) and so any material with a low modulus is likely to have a low density. There are some notable exceptions to this 'rule of thumb', such as beryllium, but most common aluminium, magnesium, titanium and steel alloys we are likely to commonly use have very similar specific modulus values.
There are some technical problems though with the use of titanium as a male fastener. Its tendency to gall at low levels of load when sliding means it needs to be installed with special grease, or needs to have its surface treated to prevent the problem, especially where it is used in conjunction with a titanium nut. However, the problem is far from insurmountable, and racing motorcycles of 20 years ago were festooned with such fasteners throughout the engine and chassis, as are many racecars, motorcycles and boats today. It seems strange that they are now outlawed in bespoke race engines at the highest levels of motorsport, but are affordable to low-budget racers.
Titanium bolts
About titanium bolts
Bicycle titanium bolt come closest to steel in terms of strength but Ti is 47% lighter.
Note, although Ti bolts can be as strong as mild steel bolts,they are no substitute for high tensile steel bolts. Syntace sell some high quality Ti bolts and here is what they say about high-tensile bolt replacement.
Why titanium’s the best option
There are a few materials approved by the Association of Professional Piercers (APP), but implant-grade titanium is the one most piercers recommend for initial piercings.
Here’s why:
It’s nickel-free. Nickel is the most common contact allergen in the world, according to the European Centre for Allergy Research Foundation. It’s regularly found in jewelry for piercings. Titanium doesn’t contain any nickel, which makes it safe for people with sensitive skin or a nickel allergy.
It has a high strength-to-density ratio. In other words, titanium is considerably less dense than stainless steel and other metals, but just as strong (if not more so). This makes it durable and less likely to bend or break.
It’s lightweight. That low density we just talked about is what makes titanium studs lighter than those made with other metals.
It can be anodized. Titanium’s dark metallic color is cool as is. But unlike other stainless steel, you can get titanium in other colors. This is done through anodizing, an electrochemical process that changes the surface color while maintaining safety.
The runners-up
As long as you don’t have a known metal allergy or extremely sensitive skin, you have other safe options outside of titanium.
The following are jewelry materials approved by the APP for fresh piercings.
Surgical steel
Surgical steel is a popular choice for piercings because it’s affordable, durable, and safe for most. It does contain some nickel, but thanks to a low rate of transfer, your skin is unlikely to notice.
Just remember that not all steel jewelry is of the same quality. Only a few specific grades are biocompatible, meaning the jewelry won’t oxidize, tarnish or react with skin.