Gas welding
First I want to note that the profile metal is produced by cold deformation of a conventional round tube. After this, the pipes are subjected to heat treatment in order to remove the generated stress inside the metal. The profile is made of carbon or low-alloy steel, sometimes using stainless steel. For the welding of profile pipes, the usual methods are used: semi-automatic, contact, gas and arc welding. 
Usually accustomed to believe that a gas burner is only able to cut metal for its subsequent disposal. However, experienced craftsmen can apply it to weld profile pipes using gas welding technology. A suitable gas is a mixture of oxygen and acetylene, which are mixed at the output and form a flame of high temperature, capable of melting the metal. The fused edges of the parts fill the technological gap and the parts are mated. Disadvantage - in the case of gas welding, a seam of high mechanical strength is not achieved. Therefore, in order to improve the strength of the seam, an additive, a welding wire, is additionally used.
Arc welding technology for profile pipes
Profile pipes allow them to be welded to the structure at any angle, as well as butt, overlap, T-joint. This gives some difficulties in the work, but arc welding will help in any situation and will be selected even to the most difficult and uncomfortable places. However, if the metal is thick enough - more than 4 mm, you will first need to prepare the edges. 
It is important to know what electrodes to weld the profile pipe. The choice of electrodes depends on the thickness of the metal. For example, for pipes with walls of 1.5 mm, an electrode of up to 2 mm is required. In addition, the welding machine sets such a current to not burn through the metal. The modes depend on the electrode diameter, current strength, voltage, polarity, type of current( variable or constant), the speed at which the electrode is moved. Weld the profile pipe uniformly. It is necessary that the edges have the same melting and at the same time, not allowing the metal to sag. 
Contact welding of profile pipes
This method is subject only to highly qualified specialists and on special equipment. It is performed in difficult climatic and installation conditions. In the case of contact welding, the principle of heating metal by electric current under pressure acts. Additives are not used, and strong compression in the weld region produces plastic deformation, which ensures the connection of metal elements to each other.