Wow... yeah, titanium.
Back when I used to design parts but farm out all of the machining I was often frustrated by the (what I perceived to be ridiculously high) quotes I would get back. Even accounting for material cost it was still just expensive to get parts made out of Ti. Now though, I'm beginning to understand why.
Aluminum is a wonderful material. Light-weight, strong and easy to machine. Corrosion resistant and even fairly scratch resistant when Type II or Type III hard anodized. It's also cheap, readily available and easy on the tooling. In fact, solid carbide tooling running only on aluminum seems to last forever as long as you don't cook it or break it. TiAlNi coated tools last even longer. Plus you can run aluminum as fast as your spindle speed allows (just make sure to clear those chips and keep it cooled). There are more parts machined out of aluminum than all other materials combined, that's how useful the stuff is.
Brass comes next. It's not quite aluminum but it's pretty close in terms of machinability. Heavier and much more expensive, it looks wonderful (especially when patina'ed). It scratches easily but it's actually pretty strong all things considered. The same tooling that works in aluminum also works on brass. Two or three fluted endmills cut nicely and take big, fat, juicy chips. A few people I know have run it dry with air blast but I tend to use flood coolant (with everything pretty much). Again, tool life is not an issue. That said, you do have to slow the small cutters down a bit or take smaller cuts than you would in aluminum.
Now we have mild steel. 1018 is literally the benchmark for steel machinability. This stuff is not corrosion resistant and I don't have a lot of use for it outside of cutting work-holding. Much stiffer although not necessarily stronger (depending on the alloy) than aluminum (for stepped jaws or soft jaws) it's a great metal for holding onto small parts and making for rigid work-holding fixtures. It still machines very, very well but it's no longer the same "hot knife through butter" ease of cutting that you get with aluminum.
Chromoly is a bit more complicated. All flavors of chormoly are structurally strong and stiff but machining it annealed runs about half as fast as mild-steel. Supposedly it machines better using solid carbide tooling and air-blast (no coolant) but I haven't really run enough to be sure. Running it post-hardened is another story and I haven't had the pleasure yet but I'm sure it's hell on cutters and slow as molasses. This is where we start to find that the sweet spot for proper feeds and speeds becomes more critical. Everything up to mild steel is very forgiving and even if you don't have your feed rate dialed in, can be cut slow, fast or everything in between. With chromoly that's no longer the case (especially post heat-treat) and running it too hot or taking too big of a cut will destroy cutters or inserts relatively quickly. Tool life really starts to suffer here if things aren't dialed in.
Following up is stainless steel. Obviously, there are many grades of stainless but 303 is the most common (and pretty easy to machine) though not as strong or corrosion resistant as say 420, which is often used for knife blades, or a host of others like 316 and 416 which are often found in watches. Some are heat treatable, some are not. Many work harden which makes machining a bit of a challenge as taking a too-small cut will actually be counter productive and quickly dull your cutter. The sweet spot for feeds and speeds narrows considerably here. Get it right and you'll get decent tool life, get it wrong and you'll find that your cutter or insert will show wear after even one part.
Now we come to Titanium. Lightweight, strong and corrosion resistant, titanium is a wonderful material for both structural as well as decorative parts. Like stainless it has some spring to it (even more than steel) and also work hardens. Unfortunately, it has an even worse characteristic which makes it difficult to machine, the tendency to retain heat. Unlike aluminum which transfers heat readily, or steel which sheds it in the chip, titanium tends to keep heat in the pocket, the immediate area being cut. This means that the tool heats up and beyond a certain point, the cemented carbide tends to soften and break down. Smaller radial cuts are the key here (axial depth doesn't have quite as drastic effect on cutter heating). A good, steady stream of coolant to clear the chips helps quite a bit as well as keep temperatures down but even so I've missed the mark and watched the pocket start glowing. While I've run HSS and cobalt tooling I find that coated carbide tooling works best here. Still, managing the heat means that unlike aluminum or mild steel, we have a very defined "speed limit" beyond which things get too hot and everything goes to shit in a hurry. The sweet spot here is very, very small. Run too slow and things take forever and you're just beating on the tools. Run too fast and you cook the tool and break it down. Miss your chip load and you either dull the tool or start chipping pieces off the cutting edge. Worst of all, even if you get everything right the tools still last only a fraction of the time they do in mild steel or chromoly. All this adds up to an expensive proposition with little margin for error or impatience. Still, the parts are light, strong and they polish out beautifully. Nothing good is cheap, right?
Is there anything worse than titanium? From what I hear some of the exotic nickle alloy steels such as Inconel are much, much worse. That said, they're strong and corrosion resistant but very heavy so I haven't had a use for them (yet).
So with all of the fuss over titanium is it really worth the trouble? I'll let you be the judge but considering that it's as strong or stronger than steel with only 60% of the weight and virtually corrosion proof... yeah, it's not bad:
