The way in which the tube is formed is called bending, and the way in which the square tube is formed is called bending. There are two basic methods of bending forming: solid bending and empty bending.
The solid bend, as the name implies, is the compaction of the bend, and the inner and outer rolls and the inner and outer walls of the tube blank are biaxially compacted during the solid bending.
The advantage of the solid bend is that the rebound is small, the molding is accurate, and as long as the roll shape is accurate, the R formed at the inner corner is relatively accurate.
The disadvantage of solid bending is the stretching/thinning effect. First, the solid bend causes the bend to be stretched, and the stretch effect shortens the longitudinal length of the bend line. Second, the metal at the bend bend becomes thinner due to stretching.