The most stunning change in baseball bats in the past thirty years started in the 1970s, when bats made from tubes of aluminum began to appear. These tubes are machined to vary the wall thickness and the diameter, and produce bats that are light, strong, and hollow, as opposed to the solid wood. At first, the aluminum bat was just a metal copy of a wooden bat. They were just more durable, so they were cheaper to use. But manufacturers and players soon discovered that there were other differences as well.
Aluminum bats are quite different than wooden ones.
They're much lighter, more than five ounces. The barrels are bigger, and because they are lighter they can be swung faster than a wooden bat. In addition, the hardness and resilience of aluminum can result in much greater speeds when the ball comes off the bat. Major League Baseball has required that its players use wooden bats, but the aluminum bat has come to dominate the lower levels of baseball, from Little League to American Legion to the college game. The walls of the bat are thin enough that they deform, or flex when the ball hits the bat.