Why my laptop adapter is getting over heat in 5 minutes?

I'm imagining that your adapter a solder joint was damaged, or a wire is burnt. There is also some circuitry inside the adapter that regulates the power output to prevent this exact overheating problem (plus preventing the lithium battery from charging too fast and overheating). Heat is not only dangerous to components, but also to yourself, and suggests there's more damage than just "letting it cool down between 5-minute charge cycles" will fix.

If you're adamant about fixing the adapter you have, the best check I can suggest is opening the case of the adapter (which is not easy as Dell tends to glue them together), and seeing what is damaged. Hopefully you'd only find charred wires or a number of blown components that, if you have soldering skills, could be relatively easy to access, remove and replace so long as they're not encapsulated inside a block of epoxy.

However it may be easier and less risky to just find yourself a replacement adapter. Dell is okay about making theirs universal, so one should be easy to find. You just need to make sure the replacement has the same voltage and current ratings, and that you're not getting a cheap knock-off which will more than likely break in different ways in a few months.

In any case, good luck!