Malleability and Ductility of Metals

On Tuesday before Thanksgiving break, we did an activity on metallic bonding.  Monday we'd done a reading assignment from our textbook with a pre-reading activity and post reading video from YouTube where they identified and explained their "unknown" concepts from the reading.   I started Tuesday's class with a Power Point slide with three images, one of metal ions in the sea of delocalized electrons, one .gif with the metal ions being moved by a force, and a third with electrons flowing through the metal.

We then did two different experiments, one with a Metal Rolling Mill where they smash pennies, and another where they draw wires into smaller and smaller gauges.

In the first experiment we demonstrated malleability.  We have a few rolling mills bolted to 2'x1' plywood that we use C clamps to hold down to the counter.  Make sure that students are aware of the safety precautions, like keeping hands away from the rollers and making sure it's clamped tightly to the desk so it doesn't slip off.  Also, encourage students not to keep pushing if the handle is stuck; last year I had a student break the board that the mill is bolted to because she was forcing it too hard.  (Nobody was hurt, but the board had to be replaced.)

http://www.contenti.com/products/_enlarge.html?itemno=192-321&itemname=100mm+Flat+Rolling+Mill&section=rolling-mills

The Rolling Mills work like this

from http://en.wikipedia.org/wiki/File:Rolling.gif

We place the penny between the rollers and turn the handle to send it through.  The students should only turn the top knob a little each time and the penny will slowly (over 10+ trips through) stretch out and flatten.  If the kids are careful and the rollers are adjusted correctly, they can get the penny to stretch out several feet long.  The kids really get a big kick out of this and are always asking if they can smash other coins.  This year I had a student smash a token from the arcade.  This opens a conversation as to why we can't use this for things like wood or plastic.

My students sending their penny through the first time

After the lab, students diagram what happened to the metal atoms as the force was applied and answer questions about WHY metals are capable of being squashed like this, where an ionic crystal substance would shatter.

After the penny has been through many times.

Another talking point is why once the penny is smashed, it appears a silver color instead of copper.  Have them try both a pre and post 1982 penny to see the difference in the composition of metals.

http://www.progresstool.com/pd-circle-die-plate.cfm

The second experiment involves ductility of metals.  We use jewelry drawplates and take a larger thickness wire and pull it until it's much thinner.  The plates are relatively easy to understand, you just start with the number 1 and work your way up through each hole, pulling with the tongs.
  To get the wire to feed into the next smaller hole, you will need to file the first centimeter or so of the wire so it's thinner.  We used regular chemistry triangular files for this.  During this part it is VERY important to make sure students are wearing goggles to prevent metal dust/splinters from getting in their eyes.  I would also recommend holding the wire with the tongs while filing, as it's more sturdy and you're less likely to file your finger.  (From personal experience... ouch!)

The die plates must be secured in a

http://www.contenti.com/products/_enlarge.html?itemno=190-718&itemname=Drawtongs&section=drawplates

table top vise so that the students can pull the wire through as they walk away from the table.  I used the tall lab counters around the edge of my room and students could apply as much force as they wanted.  I think if it was attached to a regular desk they would just pull the desk around the room, as it does require a good amount of force.  Another technique to get it started is to grip a small amount of the wire and roll the tongs to the side, so it's pulled through with leverage rather than brute strength.


Students also diagram and discuss what is happening to the metal atoms as they turn a 2-3 inch piece of wire into one that's several feet long.


These are things that the students really seem to enjoy and I think it makes the concepts of malleability and ductility so much more real, rather than a definition on page 228.