Aluminum bats first appeared in the 1970s and quickly became the bat of choice among American baseball and softball players. Not only are they more durable than wood; they're also lighter, meaning they're easier to swing, and they're more precisely balanced, giving hitters a larger "sweet spot" for making contact. Aluminum bats are the standard at all levels of softball competition and in all of baseball, save for the major and minor leagues.
Material
An aluminum bat contains more than aluminum. The metal is actually an alloy---mostly aluminum, but with small amounts of elements such as magnesium, nickel, scandium and zinc. These additives can be adjusted to make bats lighter, stronger, more durable or more resilient. Bat makers don't cook up the alloys themselves; rather, they buy them from aluminum suppliers such as Alcoa or Kaiser. The metal arrives at a bat maker's factory as lengths of aluminum tubing, 2 to 3 inches in diameter.
Fabrication
At the bat factory, workers cut the aluminum tubes into pieces 22 to 26 inches long. These pieces go into a machine called a swager, which stretches the tubing lengthwise to thin the walls---the thinner the wall, the greater the "bounce" the ball will have off the bat. The tubes go through the swager several times, with the final pass tapering the tube to create the handle. By the time the swager is done with it, a 25-inch tube will have been stretched to about 40 inches.
Treatment
Once fabricated, the tubing is heat-treated to harden the metal. First, the tubes soak in a salt bath---a sodium nitrate solution---at about 900 degrees Fahrenheit, then they go immediately into cold water. Afterward, they spend several hours in an oven to essentially "lock in" the hardness.
Finishing
The bats are polished, cut to their final length and printed with labels and graphics. Workers add the knob at the handle end of the bat, then cap the barrel end. Capping can be done either by adding a separate cap piece, or by heating the metal at the barrel end and creating a little dome on the end of the bat. The final step is to add the rubberized grip material.
Testing
The fabricated tubes undergo testing after heat treatment to make sure that they're the right level of hardness. If a bat is too hard, it may crack when used; if it's too soft, it can dent. Tubes that aren't within the acceptable range are discarded and recycled.
If the bats are baseball bats, some will be selected for another test once finished: Ball Exit Speed Ratio, or BESR. This measures how fast a ball comes off the bat, as a function of the speed of the pitch and the speed of the batter's swing. BESR is a critical safety issue. The faster the ball comes off the bat, the less time fielders have to react to it, and the greater the potential for serious injury. To be certified for use in competition, a baseball bat must have a BESR in a range comparable to wood bats.
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