titanium katana for Z-day

[PhoenixForger]

Hello,
The zombie apocalypse is real popular. I had an idea for a katana that I think would be perfect. I know that the inner section (sorry forget the more technical term) is generally made to be softer so that it will absorb shock better, while the edge is diamond hard.
So what if the softer steel were to be replaced with a strong titanium. I figure that i would have to fold the steel and titanium separately (I'm assuming that folding titanium will make it stronger) then I can shape them and finally forge weld the edge to the body. I have 0 experience and less than a year with steel so any info is valued.

Question 1- can steel and titanium be succesfully welded?
Question 2- how strong is folded titanium?
Question 3- would this be a viable sword?

[Steve Sells]

Question 1- can steel and titanium be succesfully welded?
Question 2- how strong is folded titanium?
Question 3- would this be a viable sword?

1... Yes depending on alloys used
2... Depends on the alloy
3... No

[MattBower]

(1) If you made it to "standard" katana dimensions, it would be weaker than a fully hardened, properly tempered, monosteel blade of a good quality, modern alloy steel. It would be lighter than the all steel blade, but it wouldn't be stronger. As recently mentioned here, Ti alloys (generally speaking) are stronger than steel by weight. But they're considerably less dense than steel, and by volume they're generally weaker.

(2) Forge welding Ti to steel is extremely advanced stuff, and quite dangerous to boot. Can it be done? Yes. Can it be done now, by you, with "zero experience"? No.

(3) Folding modern, essentially homogeneous metals does not make them stronger. At best it'll do no major harm to steel. At worst it'll result in severe grain growth, intergranular oxidation, decarburization and weld flaws, and you'll end up with something far inferior to what you started with. Folding and welding was a way to improve the crude, slag-laden steels and irons of the past.

[PhoenixForger]

Guess I should have mentioned that A) this is not a project I will be attempting any time soon, I know it won't be stronger, I wanted to know how much weaker it would be so I could weigh the different values of weight to strength. ^_^

[MattBower]

That depends what Ti alloy you would be using. You could get a rough idea by comparing the yield strength of your particular Ti alloy to the yield strengths of some steel alloys that you might use instead. 6Al4V titanium -- a common alloy -- has a "typical" yield strength of 120,000-130,000 psi. 1075 carbon steel at RC 53 has a yield strength of around 255,000 psi. That makes it about about twice as strong as 6Al4V. (Yield strength is the point at which the material plastically deforms -- i.e., takes a permanent bend. Tensile strength is the point at which it breaks. Since you don't want your sword to bend in use, I chose to compare yield strengths.)

To my knowledge the people who have managed to forge weld Ti to steel have done it in a sealed canister filled with an inert gas, like argon. You would need a press, preferably hydraulic. If you misjudge the temperature there is a very real possibility of burning, liquid titanium squirting out of the canister. Ti burns at around 4000 degrees F and is very difficult to extinguish.

[ThomasPowers]

A good weight for a Katana is about 2.5 pounds, make it lighter and it won't "chop" well as there is not enough inertia.

Ti loves to absorb gasses when hot turning it brittle.

Great if you are making swords for people you don't like; but be sure to collect payment in full *before* they try to use them!

It is quite funny that many people talk of european swords as being overly heavy, almost crowbars rather than swords. Yet for nearly 1000 years the average weight of a standard European sword was about 2.5 pounds---the same weight as a katana *and* European swords tend to be much thinner than a katana in cross section so which one is the crowbar? Don't believe what you see from Hollywood or Anime!

[PhoenixForger]

Thanks for the advice guys. This was probably just a pipe dream, I was interested to hear that titanium can become really brittle and now that I think about it the weight would help with the "chop".

P.S. Thomas Powers- yeah, I never will understand people who say crowbar while looking at a European sword. In the long run I had wanted to make a claymore like this but I think I'll just stick to steel. ^_^

[MattBower]

People do incredible stuff with steel. You can spend more than a lifetime on it. Don't feel as if you're somehow "settling."

[PhoenixForger]

I didn't mean to imply steel was inferior, I meant that after hearing about the weaknesses of titanium that steel really is the best choice. I suppose thats why it has never been replaced. Thanks for all the advice guys. ^_^

[Calvin T McAllister]

nice blades

[MattBower]

Not mine, by the way! Just to be clear on that. I wish I had that kind of skill. Or half of it.

[Rich Hale]

Diamond hard edge? hmmmmmmm

[Steve Sells]

Diamond hard edge? hmmmmmmm

I have heard that some people like having their blades chip or shatter while in use.. go figure

[caotropheus]

Some literature from bicycle pedals technology (Bebop company) that explains engineering aspects of metals widely used in modern bicycles and helped me understand better why "steel is real" and why I always preferred steel for my bicycle frames...

"applied metallurgy 101

Following is an excerpt from a recent lecture by professor Drønon Andon, noted metallurgist and consultant to Bebop.
Why doesn't Bebop make titanium spindles?

Lots of folks have asked us whether we are going to make titanium spindles. Most of our competitors do, so it might seem like a logical move. Our reasons for avoiding ti spindles are a little technical, but bear with us.

A pedal spindle has to be strong because it is stressed as a cantilever: the load is at one end of the spindle (your foot); the spindle must transmit force to the other end (the crankarm). If you use plain sleeve bearings (aka bushings) and/or loose ball bearings like Ritchey, Shimano and most Taiwanese pedals, strength is pretty much all you need to look at. But cartridge ball and needle bearings require that that the pedal body stay precisely parallel to the spindle. That requires the spindle to be stiff as well as strong. This is where most titanium spindles fail Engineering 101. To understand why, you need to look at a few key properties of steel and titanium and understand a couple of basic engineering concepts.

Engineers have a buzzword for stiffness: modulus. It is a convenient, quantifiable way of comparing the rigidity of different materials. In terms of modulus, the differences between steel, titanium and aluminum are not very interesting. Titanium is half as stiff as steel, and half the density. Aluminum is one-third as stiff, and one third the density. (The buzzword for stiffness as a function of a material’s density is “specific modulus.” All three materials have essentially the same specific modulus.) So a titanium spindle that is the same size as a steel spindle will weigh half as much, but will bend twice as much. Even if that bending doesn’t eventually cause the spindle to break, the constant bending will cause rapid bearing failure (just ask SRP, which recalled all its Ti spindles a few years ago for that reason).

If titanium is so flexible, why does it work so well in other places, like frames?

OK, here is the part you don’t hear so much about. You may have read about the modulus of materials before, but even more important is the inherent stiffness of different shapes and structures, sometimes called “section modulus.” This is probably the most important (and misunderstood) concept in bicycle frame design. A lot of folks have been talking lately about the “typically stiff aluminum frame ride." This is hooey. Aluminum is only 1/3 as stiff as steel, and anybody over six feet tall who ever tried to ride an early Vitus frame knows it. Folks like Klein and Cannondale make mondo stiff frames by taking advantage of the section modulus of a cylindrical tube. Here’s the basic deal: when you double the diameter of a tube, its stiffness increases by roughly eight times. So the stiffness of the basic structure of Cannondale’s 2 1/2” down tube is about 8x stiffer than a normal 1 1/8” down tube. Aluminum is only 1/3 as stiff as steel, but because of the vastly increased section modulus, you still end up with an aluminum tube that is more than twice as stiff as the smaller steel tube. So by trading off section against material you can end up with any degree of stiffness you want -- with almost any material. So talk about the “typically stiff big-tube frame ride” and you can impress your friends even if they have plastic pen pockets.

Are you telling me that my Klein isn't stiff?

Of course not. But it isn't stiff because it is made of aluminum. It is stiff because it has way oversize tubes. A bike with tubes that size made of titanium would be even stiffer, and in steel it would be so stiff you wouldn't want to get near it. Capish?
How does that relate to spindles?

You’ve probably guessed that a titanium spindle will work fine -- it just needs to be bigger than the steel spindle it replaces. Unfortunately, that approach creates a number of problems. First of all, the crankarms, bearings, pedal body and a bunch of other parts will have to be completely redesigned. Second, making the spindle larger generally means significantly increasing stack height, which we think is a bad idea.

There is one more problem with titanium spindles: hardness. Most pedals, including ours, use the spindle as the inner race for at least one bearing. To function properly, this inner race must be very hard. It is easy to make steel hard enough, but almost impossible with Ti. (Those fancy coatings, like titanium nitride, help somewhat, but if they are much harder than the underlying metal, they tend to separate and flake off.) Some other manufacturers solve this problem by pressing a steel sleeve over part of the spindle, but this reduces the diameter of the structural portion of the spindle, which we think reduces stiffness and requires that the overall diameter of the spindle be increased still further.

We aren’t saying that a good Ti spindle is an impossibility. Like a number of very smart people in this business we’ve spoken to about this subject, though, we’ve concluded that they are a bad idea as direct replacements for steel spindles. So for now, we suggest you ignore Ti spindles and go with the lightest all-purpose pedal system you can buy -- which happens to have a very stiff, strong and light steel spindle.

So what is the right way to save weight in pedal spindles?

Bebop's way is to find a better steel -- our exclusive Aero-S ultra-high strength stainless steel. Aero-S may be the most advanced steel alloy available. Originally created for things like landing gear for navy jets and aerospace fasteners, it is nearly twice as strong as cromoly. It isn't any stiffer than ordinary steel, but because it is stronger, we can use less of it. So you get a spindle that is twice as stiff as any Ti spindle, and only a couple of grams heavier than most of them. Oh, and it it is also super hard, which makes it nasty to machine but great as a bearing surface.

And finally, unlike our competitors who think it is just fine to sell more expensive parts that are not as strong as their cheap stuff, we don't put arbitrary weight limits on our Aero-S parts. I mean think about it -- when they say 170 pound rider weight limit, do they mean a 170 pound roadie profiling past Starbucks, or 170 pounds of human cannonball ripping down a rocky 20% chute? Do you want to bet your insurance deductible on your answer?

(Dr. Andon has recently announced that he will donate $10.87 to the Sierra Club's "Ban bikes from singletrack and non-interstate roads" fund every time some knucklehead claims that aluminum is a "stiff" frame material. Help keep the roads and trails safe for bicycles -- smack the next guy who says it.)"






The same concepts can be translated to cutlery, I think...

Even though today also Bebop presents pedals in the market with titanium spindles...

http://beboppedals.com/prod.htm

[Dogsoldat]

Thanks for that post, Stuff I kinda sorta knew, but presented that way makes for a few AH HA! moments...