Intro: How to Form Memory Wire Nitinol
Memory wire, a.k.a. Nitinol, a.k.a. Muscle Wire, a.k.a. Flexinol, is a nifty metal alloy (mixture) of Nickel (NI) and Titanium (TI). The NOL stands for Naval Ordinance Laboratory, where the memory properties were discovered by William J. Buehler and Fredrick Wang. If you are interested in buying Nitinol or the history and its many uses, check out these links:
Step 1: Material List
Read all instructions first!
Metal tweezers or clamps of some sort
Water and container
Rod for a spiral, anything else you would need for different shapes
Body heat or battery holder pack
Nitinol wire, ($20 to $30 per meter)
Step 2: Annealing Explination
Nitinol, the non-brand-specific name, comes in an as-drawn form, which is what a Maker would want, because it allows one to set the memory. As-drawn refers to the drawing (not picture drawing) process, which is how a bar of alloy is continually pulled through reduced sized openings until it is a wire. When I received the Muscle Wire, it was very springy and difficult to bend. This is because the drawing down process presses the atoms in the metal closer and closer together. To make the wire easier to form, it has to be annealed.
Annealing the wire is the same type of annealing that is used for all other metals, as well as glass. The material is brought to a temperature that allows the atoms to relax and rearrange, but not too hot as to deform or melt. A way to anneal wire is to heat it to a dull red glow along the entire length, and then quench it by putting it into water. It would be a good idea to find a container that will fit your metal, I used a bread pan. It is also possible to anneal the wire while it is looped, although it will try to straighten out at first. It is not necessary to use a pickle, for you jewelry smiths out there, although it would help to clean the wire. I chose not to, simply because I do not have the setup for that.
Step 3: Annealing
I am not responsible for anything that you do with these instructions (including burning down your house or your neighbor’s house), so now that we have that out of the way, I used two large tweezers to hold the wire in front of my flame. I am using a glass melting torch that burns oxygen and propane, but you can use any type of flame, as long as it can get the wire to a dull glow, i.e. propane, acetylene, oxy-acetylene, your gas stove top, etc., etc. Be careful as to not burn yourself, bystanders, things in your kitchen, or melt the wire (Which is pretty easy, remember, only heat up to a dull red glow). Once every section has been brought up to a dull red glow at least for a couple of seconds, quench the entire wire in water to cool it off. You can also quench the ends of the tweezers, as they tend to get hot enough to burn the unwary.
Step 4: Don't Burn Through the Wire!
This is what happens when you get the wire too hot. I now have a shorter wire to work with.
Step 5: Annealed and Bendy
Now the wire is easy to bend, because the atoms have been relaxed. The problem with this is that whatever shape you bend it into, once heated to a lower heat than the flame, it will return to the straight shape. The goal here is to have it return to a shape that you set. Somehow for whatever shape you are trying to set, it has to be able to be held while it is heated to approximately 950 degrees.
Step 6: Wrap Wire
To make a spring, I wrapped the wire around a small metal rod. I have used brass and tungsten, steel should be fine, but aluminum has such a low melting point that it would probably be a bad idea. I used small clamps to hold the wire onto the rod. Be aware that the wire will try to straighten on you in the lower heat ranges. Try using steel needles in a kiln brick to wrap the wire around if you want to make a different shape or words. Just remember to clamp the ends so the wire can't escape. It may be possible to bend the section and hold the bend while you heat/cool to set the shape.
Step 7: Set Spring Shape
Heat the wire in the shape that you have bent and clamped it into. Quench it before handling. I do not know if it is necessary to quench while it is still very hot, but since I am impatient and do not feel like waiting 5+ minutes for it to cool down enough to handle, that is what I do. Although it looks like I am torching the handles on the clips, they are not actually in the flame. My flame is set very low with minimal oxygen to make it easier to not burn the wire up. I would use pure propane, but it is a dirty gas and will clog the ports on my burner, not to mention getting soot all over.
Step 8: Set Shape
Your wire is now set to your shape, and still soft enough to easily bend. Bend it, mash it, etc. into a different shape. You can apply heat with your hands or use hot water to get it to move back to your set shape. Or you can use electricity to heat the wire up until it moves. The resistance of the wire to the electricity causes it to heat up. Do this with a battery holder and two alligator clip leads. I used a 4 AA battery holder, which provided 6 volts (1.5volts per battery x 4 batteries). You can also use a 9 volt battery. Either way, don't leave it connected too long, or it will reset your hard earned efforts, and possibly burn out the Nitinol. Connect one wire to one side of the wire, and connect the other live wire to the other side of the Nitinol wire. It will spark and get hot, but it will also move back into your shape. It would probably be a good idea to attach this setup to a resistor and a momentary switch (button) to close the circuit till you release the button (momentary part).
In these 3 pictures, the first is the set shape, second I have stretched it out, and third it has gone back to the set shape (mostly) after slightly heating it.
Step 9: Movie
Step 10: Tips
If you are using Nitinol for a project where you need it to move something, it can pull some serious weight with the shrinkage of the straight wire. I have read variously 2 to 10 pounds. The wire has to be pulled back long, and if you are using this as a design element, you should buy Flexinol, which has been cycled through the increase/shrink cycle enough times that it will consistently shrink by 10%, which is quite enough to pull a lever to increase the movement potential. A project that I made with the Muscle Wire brand uses a spring to pull the wire back into the stretched out shape before the heat/electricity was applied. Also, the wire will cool slowly once the heat is removed, and will gradually pull back out of the set shape if it is under tension.
Soldering this wire takes a little bit of extra work. Because of the movement of the wire, it will quickly break any solder joins. the wire will first need to be crimped, and then the crimp can be soldered. For more information on that, click on the Makezine blog link in the next section.
I hope this is helpful to anyone who wishes to experiment with the fascinating properties of Nitinol, and if you have any comments or suggestions, please write them up below. This information is to the best of my knowledge, and some things may be a little hazy or incorrect so helpful input is always appreciated.