Use of Aluminum Bats
The Major League has never permitted the use of aluminum bats for safety and competitive reasons. However, high school and college baseball teams are allowed to use them, and aluminum bats well outnumber wood bats in all amateur softball and baseball play. Players trained to swing with aluminum bats, however, have trouble swinging wooden bats well enough (due to muscle memory) to be successful in Major League baseball (http://www.hsbaseballweb.com/aluminum_bat.htm).
Advantages of Aluminum Bats
In the 1970s, when funding for nonrevenue-producing sports was nonexistent, teams cut costs by purchasing aluminum instead of wooden bats. Though aluminum bats are more expensive, they are more durable and need not be replaced as often. As manufacturing processes matured and aluminum was alloyed with various minerals, aluminum bats also became lighter and easier to swing. Some players think that hitting a ball with an aluminum bat increases ball speed.
Aluminum Bats and Ball Speed
The study that proved balls hit with aluminum bats travel faster than balls hit with wooden bats (Crisco-Greenwald Batting Cage Study of 1997-1998) was not published until 2000-2001, and the bats used in the study are not legal under today's National Collegiate Athletic Association (NCAA) rules. Therefore, it has not yet been proven that balls hit with aluminum bats used in today's college baseball games travel faster. However, aluminum bats not used by NCAA teams may indeed contribute to a ball's speed.
Double-wall and Single-wall Aluminum Bats
Aluminum baseball bats continue to be improved upon. In the late 1990s, double-walled aluminum bats were made using two thin aluminum tubes, the smaller set inside the larger. Thinner walls improved the bat's performance; using two of them increased the bat's strength. However, as progress was made in aluminum alloy production and the manufacturing process became more complex, "single-walled aluminum bats have reached a compromise between maximum possible performance and durability" (http://www.kettering.edu/~drussell/bats-new/titanium.html).
Scandium Aluminum Bats
When alloyed with aluminum, the mineral scandium "increases the strength and resilience of the aluminum without adding to the weight" (Baseball Bat Innovations, http://minerals.usgs.gov/minerals/pubs/commodity/scandium/). Most higher-quality aluminum bats today are made from scandium-aluminum.