Friction drilling is a method that is used to make holes in metals. This process involves pushing the material out of the way with the help of the heat that comes from the friction. The process is also known as flow drilling or thermal drilling and relies on some properties and behaviors that can also be seen in friction welding.
Flow drilling is different from the other methods in that it helps increase the cross section of thin walled tubing much easier. The principle is quite simple. As mentioned before, you need a hard metal and friction. The heat of the friction of the hardened bit brings the metal to its melting point and by using pressure from the milling machine, it makes a perfect hole.
In the video below, the YouTuber AvE walks us through various test drills and gives us a thorough explanation of the process and the metals involved in it. Check it out!