Using a broken baseball bat can be dangerous. In any organized game at the high school level and above, it is simply not done. Whether it's an aluminum bat or a wooden one, repairs to the barrel of the bat are almost never made. Those bats are simply replaced. Perhaps a big-league hitter will not want to lose a favorite bat with a hairline crack in it. He may repair it with glue, clear nail polish or liquid cement. But this will only work for minor flaws. However, repairs have to be made to youth baseball league bats and for games that are less organized in order to keep a game alive.
Use liquid cement or a very strong adhesive when a piece of the barrel has been sheared off a wooden bat. This type of repair can be very effective if the adhesive is spread evenly and then the bat is put in a vice for at least 24 hours. A bat can split on one of the grains of the barrel and as long as that piece is sheared at a thickness of less than an inch, the bat can be repaired to fairly good condition. Once cracked the bat may no longer have the longevity it once did, but it should still be able to hit the ball nearly as far and as hard as an undamaged bat.
Use electrical tape to repair any crack to the handle of the bat. If a chunk of the handle has broken off, do not try to repair the bat if it will be used in a baseball or fast-pitch softball game. There is too much force applied to the bat upon impact with the ball for the bat to hold up. However, if there is just a slight crack, strong tape can work wonders for allowing the bat to stay in service. The repair should not be more than an inch or an inch and a half. Any more than that is too big a flaw to repair. Take the tape and start wrapping an inch above the crack and keep going until you are an inch below the crack.
Use clear nail polish to repair any surface cracks in the barrel of a wooden bat. A small crack or chip does not mean a wooden bat has to be thrown away. Use clear nail polish to fill the hole and then allow it to dry. Take sandpaper to smooth out any rough spots to make sure the bat is back to its natural shape.
Find a two-piece tool-and-die device that uses compression to repair dents in an aluminum bat and return in to its original shape. You should never attempt to use a cracked aluminum bat, but a bat that has been in use for several seasons is bound to have a few dents and dings. Using the compression device can return the bat to its original condition.