What is springback? Trade items such as Pipe, Tube, Bar, etc. produced from Steel, Iron, Copper, Brass, Aluminum, or other ductile metals have been hardened by some combination of cold drawing and alloying with other chemical elements to give the stock the qualities of strength and elasticity. Elastic materials in general, when bent, will return to their original condition when released. However, if metal is bent beyond its elastic limit, it will tend to retain the form of the bend. During the bending of stock, the material in the OUTSIDE radius of the bend will be progressively stretched beyond its elastic limit (stressed in tension). The material in the INSIDE radius of the bend will be progressively compressed beyond its elastic limit (stressed in compression). Although these tension and compression stresses are opposite in effect, they never end up being equal in force. The difference between the forces of stress in the inside and outside of a bend is the cause of spring-back.
There is a small hump in my tube at the end of the bend. Why? When you bend, the material stretches, and the diameter will decrease slightly. At the end of the bend, the stock will remain its original diameter. This mostly happens when you bend large diameter tube to a smaller radius. If this is undesirable, limit bends of large stock to larger radii, although the effect may still appear. In softer alloys, this can be even more prevalent.