Titanium

Tips and descriptions of materials you use

Titanium

Postby Jerry » Mon Jul 19, 2010 3:25 am

Titanium is a misunderstood metal. Rarely is titanium machined in its pure form. Its rather gummy. Probably the most popular alloy is 6Al4V, 6% Aluminum and 4% Vanadium. This alloy makes it much harder and easier to work. People tend to think because of its high strength that it is hard to work.

Well, its not. As long as you keep some things in mind when machining it, it will machine no worse than stainless.

First, Titanium is a terrible conductor of heat. What does this mean? Heat builds up on the cutting tool edge and is not sinked away by the material. To alleviate this you need to use a slow SFM to keep the rate down. Also carbide tooling will help here if you want to get things done in a reasonable amount of time. For example, when I machine a 1" bar of 6Al4V I start out about 700 RPM using carbide tooling. 300, for High Speed Steel. Coolant helps if you have it.

Seconds, dont dwell. Titanium work hardens in the blink of an eye. So if your tool rubs and does not cut the material will get hard and now you are in for some fun. If you machine has power feeds use them.

Last, watch out, titanium swarf is a fire hazard. Dont let it pile up under the lathe. Keep sand or a dry type fire extinguisher around.

It also helps to have a rigid machine. Not absolutely necessary though.

Machining Titanium on my CNC mill:




Finished ring:

Image


Anodizing Titanium:

One neat thing about titanium and niobium is they both can be anodized to just about any color without any dyes. Titanium takes on a color depending on thickness of the oxide layer on the surface. There are two ways to do this. One is to heat the item. Second is with electricity. I use electricity for the titanium rings I make for people. To do this you need a power supply that will go up to about 150v DC. You want an isolated supply as well. If your power supply is not isolated you can use an external isolation transformer. My supply consists of a 0-140v, 3A, variac fed by an isolation transformer. The output of the variac goes through a bridge rectifier and is filtered by a large electrolytic capacitor. The capacitor is not necessary. All of this is mounted in an old 19" rack mount case.

I use TSP as an electrolyte. Use a small non-metalic container with water and a little bit of TSP. You will need a cathode, I use some Ti swarf from the lathe, clip it to the negative terminal. I then use a hanger made from Ti wire to hold the object I am going to anodize, this is attached to the positive output. Put the item in the solution with the power off and the variac at 0. Turn on the power and start turning up the voltage. Around 16v or so you will see the Ti start to change color to a kind of yellow-tan. As you increase the voltage the color will continue to change. Stop when you find a color you like. Bubbles will form on the surface of the item being anodized so I agitate the solution while anodizing.

Here are some rings I have anodized:

Image

Image
Jerry
 
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Re: Titanium

Postby Doug Coulter » Mon Jul 19, 2010 12:37 pm

Truly beautiful, Jerry -- and now my wife is hitting on me for one of these...That engraving is definitely well past what I can do here.

I am using some pure Ti, which as Jerry mentions, is hard to machine. So I don't use those class of tools on it for what I'm doing. I've found it is a good grid material for fusors, and is easily obtainable in sheet/wire/rod form from McMaster, not too expensive if you don't need too much.

In fusors, I've not noticed any serious hydrogen embrittlement, and it will take the heat, while having a low secondary electron output and low thermionic emission, so it's a good material for that. I do usually do the pumpdown after a run while the grid is still hot, which may help with the embrittlment issue.

I use a hole punch in a press to make holes and washers from sheet Ti, usually .020" or .032" thick, and .032" wire for grid rods. I use a spot welder from Harbor Freight to stick it together, or a TIG welder for big pieces. In the case of spot welding thin stuff, I use the spot welder on a big variac at reduced voltages - I usually see something like 60 or 70 on the dial when it's really working right.

Due to the ductility of the pure stuff, and a not way-off tempco, I have made successful glass to metal seals with either soda-lime or lead glass (As is sold for neon signs) with small wire -- .032" and down works well.
Since this has a tempco in the 90's (cm/cm/degC * e-7), and so do soda lime and lead glass, this works. I've not gotten it to work with pyrex glass (33) -- too different, or quartz(5.5), either.
Lead glass for neon signs is sold by Glantz.

One big caveat -- I stretch the wire a bit to make it really straight for my cylinder grids, and it doesn't stretch perfectly evenly, or relax uniformly the next time it is heated. I've had that trouble with the sheet when I make vane grids as well, even though I just sheared it to cut it. What this means is if you make a grid or other complex shape with this, all welded up, the first time you heat it it will warp and if it's fully welded, there's not much you can do about that. So either design for some slip to accommodate that, or pre-heat it well with say, a propane torch to let it assume its final length before putting something like that together. Best is just to do the mechanical design so that there is a way for one end to slip.

The thin sheet drills easily with PCB small drills at high speeds in a dremel drillpress. You might want to spot it first with a punch to keep the bit from wandering, though. I cut the slots for the big vane grid below on a regular shop metal bandsaw, then cleaned up the burrs with a file.

Here is a picture of some grids, which illustrates the do and don't do things I mention. Click the pic for a larger view.

TiGrids.jpg


As you can see, the one in the lower left has sagged from heat - at bright yellow hot, it wasn't strong enough to hold the heavy end washer I made. So, don't do that!
The one just above warped a bit because I didn't do the pre heat before I welded the wires at both ends, but that one has had hours at high heat and at least didn't sag.
The small vane grid to the right illustrates a couple don't-dos -- I TIG welded both ends of the vanes, and it's very warped now. The larger one above, however, has one slip fit end and is fine still.
I used spot welding for that one and all the wire grids, works great. The sharp-eyed might notice that the big vane grid has a non rectangular vane design. The tip for that is to shear them all, then clamp them on the milling machine between two sacrificial pieces of say, scrap steel, then mill them all at once. This makes it fairly easy to do, and they all come out identical.

The big guy was made out of a few different diameter wires, to get strength/weight in the right region. All fat wire and it would intercept too much, but need a few to make it stay together and straight.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Titanium

Postby MrBain » Tue Dec 14, 2010 8:13 am

Makes me wonder what grade of titanium I actually used on the wifes and mine wedding bands. I used low rpm (200 or so) and HSS, the cut was gorgeous. Used 2000 grit and mothers to bring to almost mirror finish.
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Re: Titanium

Postby Jerry » Tue Dec 14, 2010 3:57 pm

The most common titanium in bar stock is Ti6AL4V whuch is 90 percent Ti, 6 percent Aluminum and 4 percent Vanadium.
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