In this lecture, we'll talk a little bit about how we utilize cold working as well as hot working to modify materials. We're going to look at how plastic defamation actually occurs. We're going to introduce this concept of slip dislocation motion. What we're going to talk about is how we make these various items. We're going to do a connecting rod, aluminum rods, coiled wire, and a steel slab. Let's talk about plasticity. We mentioned it before. Plasticity is the study of plastic deformation. It is the irreversible work done onto the system. Hence, when we plastically deform something, we increase the internal energy. One way to monitor the relative amount of work is we look at the reduction in cross sectional area, we often refer to the percent cold work. If you remember, when we modify the US penny, we used the same equation where we looked at the change in cross section divided by the original cross-section. We refer to that as reduction area. It was a relative measure of the ductility. Typically, when we're going to do cold working, we start off with a ductile part. The first one, we did cold rolling when we did the US penny. You think about the penny, that was when we were modifying it. Now, when they actually stamp the coin, they take rot copper or cladded copper, and they actually forge it, so you have a die and then you take the blank and apply the force. Forging is great for parts that have features that you want to produce. Drawing, typically you have metal you're going to pull it through a die. Often this is very useful for making wires. Extruding, you put a blank material in, then you have a ram, and you push it through the die. Then finally, we have [inaudible] I've got to go here. Now, in each case, if we look at the initial cross-sectional area and the final cross-sectional area, we see that there's a reduction. Forging is smaller, A naught, A_d, smaller A naught, A_d. Each time we cold work, we actually reduce the cross-sectional area. If we do it over and over, we can allow this percent cold work to increase. That means we're doing more and more work onto the system. The more work we do, the higher the increase in the free energy. Let's take a moment for inquiry. In this example, we want you to decide how these parts are made. All right. Well, as we said earlier, we know that the connecting rod has features, unique, very precise features, so we'll go with forging. Drawing is always the mode for wire. Then for the slab, we want to do rolling. Remember the example of penny when we stretched the penny out, and the only one we didn't discuss was the extrusion. We didn't say what it was, but by deduction, you should say that the rod would be fabricated using extrusion. We'll see that extrusion will be used in polymers extensively, when we get to processing of our polymer parts.
