Troubleshooting bandsaw cutting at an angle (through aluminum)
Cheaper bandsaws tend to drift in the cut more, but any bandsaw can suffer from it. You can watch the line it is cutting along, and adjust to it, or you can try to resolve the problem. There are a few issues.

If you need to push the material through the cut too hard, then it tends to twist the blade. This will cause the cut to go at a angle. Why might you be pushing?


A dull blade
Thicker material than normal to cut
Impatience


Another factor is the blade tension. A bandsaw has a spring that you tighten to keep the blade under tension. Your saw will probably have instructions for how tight to make the blade. This spring can loose its ability to properly tension the blade after some time, allowing the blade to twist in the cut. Replacing the spring can thus help you to bring your saw back to optimum cutting performance. A sign that it is time to do this is if your saw tends to drift when it did not do so in the past on a similar cut, with a good blade installed.

Next, check the blade guides. If your blade guides are damaged, then they may be allowing the blade to drift off line. Most saws have a set of lower and upper guides. Some are small rolling bearings (on more expensive saws) but on a tabletop saw these are probably metal, held in place with a set screw.

The blade can actually wear the guides away from friction. The metal guides can often be reversed before they need to be replaced, but they are inexpensive to replace. You can usually find a non-metallic (slippery) replacement for those guides, which runs a bit cooler. This is good because less heat means your blade will run more cooly, so live longer before it dulls.

Finally, check the tires. Your saw has rubber (or plastic/polyurethane) tires that the blade runs on. There will be either two or three such tires. After a few years time, these tires dry out. The saw will then start to behave poorly. Those tires can be pulled from the wheels and replaced.