I will try to answer this interesting question, hopefully without going into too much detail.
Regarding strength at room temperature, I will give you analogies with AISI316L stainless steel.
AISI316L: Density: 8 g/cc – Yield strength: 290MPa.
Gr.5 Titanium: Density: 4.4 g/cc – Yield strength: 880MPa.
What can be inferred from the above? It means that by using Grade 5 titanium you can reach 3 times the strength while the part will be about half the weight! You can easily get steel alloys that will reach Gr5 Titanium’s yield strength, but again the density will always be 2 times that of Titanium!
Titanium in its pure commercial form is usually softer than steel, that’s true. CP (commercially pure) Titanium comes in different grades: Grade 1, 2, 3, 4. The first 3 grades are the softest. They are called "commercial pure" because the main element is titanium and we only have a minor percentage of oxygen. Grade 1 has the lowest oxygen content, while 4 has the highest. There is no other metal in them, so they are actually “pure” or “unalloyed” titanium. Grades 7, 11 and 12 are also considered unalloyed but not pure as they contain some other metals but in a very low percentage. All of the above have similar mechanical properties.
Grades 1 to 3 are “gummy” when you machine them, exactly because they are soft. The “gummiest” material is copper and I cannot comment if pure titanium is worse or better than copper in that respect, as I have not worked with pure titanium before. Gummy materials are indeed a headache for a machinist as they tend to build up in the cutting edge, the chips are not breaking easily and can be a mess in CNC machining. But they are soft and you can cut them easily with the right geometry tools.
Grade 5 is a different thing altogether. It contains 6% aluminum and 4% vanadium. May I note here that the vanadium used in alloying Grade 5 is a lab-spec clean vanadium that costs around $1500 per kg. These 2 additional elements change the crystalline structure of the alloy and make it very hard and tough.
Here are some points for the Gr.5 titanium making it hard to machine:
- It’s difficult to “CUT” because it’s very “strong”. Yield stress is the “pressure” you need to apply to make the part start to fail. The more the yield stress, the more energy it takes to actually “fail” it or cut it. The above are in pure layman's terms so you get the point!
- It has very poor thermal conductivity. That’s why Gr.5 titanium is used in aviation and in turbine blades in particular. It can withstand extremely high temperatures quite easily. But this is a minus in machining. High cutting temperatures are generated during machining of this alloy as a result of its low thermal conductivity. About 80% of the heat generated when machining this alloy is conducted into the tool material because of the low thermal conductivity of Ti-6Al-4V alloy. This means that most of the heat you introduce to the metal will stay there without spreading. This will work harden the part at the point of cutting. The cutting tool itself will get too hot at its tip and this causes quick wear and degradation! Chips coming away from the work-piece can be hot as hell and if they catch fire you cannot put it down with a normal CO2 extinguisher!
- It has a low modulus of elasticity, which means the material is “springy”. Which means that if you “push” it hard with the tool it will spring back, oscillate and chatter, leading to bad surface finish and lower dimensional accuracy!
Some highly advanced machine shops are following special methods to overcome the issues, such as cryogenic treatment, preheat of the work-piece and hot machining, or use of some special cutting tools, like CBN etc.
