Double-walled softball bats were made to solve a fundamental problem in softball bat performance. Before them, metallic bats were thin, springy, resilient and less potent. The double-walled, which is made with aluminium or titanium, is a high performance bat less likely to sustain damage than the metallic version.
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The aluminum double-walled bat was introduced in 1993. The walls were thinner than in previous bats because one reinforced another. The extra aluminum tube gave the bat extra spring, which led to players hitting balls further with the same amount of effort. The second aluminum tube, or "wall," protects the bat from dents.
In 1993, sports equipment manufacturers started releasing bats made out of titanium -- a lighter, stronger metal than aluminum that created a significant trampoline effect. Limited double-walled titanium bats were available in 2005, but titanium bats of all stripes were outright banned in many slow pitch softball leagues. They found a titanium bat could send a ball out 10 mph faster than its aluminum counterparts, which increased its potential to hurt somebody.