This modern joining method satisfies increasingly stringent demands,. some of the most important of which are process stability, reproducibility and cost-effectiveness. The bundling together of specific material properties opens up a number of interesting possibilities. Material compounds impart to a component or product the desirable properties of the constituent materials. Previously, such results could only be achieved by mechanical means or by bonding. Of much greater interest, however, is the ability to use heat to join materials with differing properties. The main focus in this respect is on the joining of steel and aluminium, as this will be of particular interest to the automotive sector, where it could spawn a whole range of previously undreamt of innovations.
The joining of dissimilar materials requires precise knowledge of the properties of each material. Aluminium is highly regarded due to its low specific weight and its excellent usability and processing characteristics. On the other hand, its strength and low cost make steel indispensable in many areas of industry. Other requirements primarily address anti-corrosion features, thermal expansion coefficient, and atomic properties. When joining steel and aluminium under the influence of heat, what is known as an intermetallic phase is created at the interface between the two materials. The more heat that is applied, the more extensive the intermetallic phase and the poorer the mechanical properties of the join will be. However, the chemical and physical properties also require appropriate measures to be taken. The different thermal expansion coefficients of the two materials create a stress field around the join. There is also a marked tendency for corrosion to form as a result of the large electrochemical potential difference of steel compared with aluminium. All the technologies that have been used to join steel and aluminium in the past have only been able to deal with certain geometries or have required extensive control inputs. Although the perceived wisdom among many metallurgists was that steel and aluminium could not be welded together, extensive research in the field of MIG/MAG welding indicated that arc welding was indeed a potential way of joining the two materials. The CMT process evolved from the continuous adaptation of the MIG/MAG process to resolve the problems posed by the joining of steel and aluminium. CMT is a controlled process and allows the material transfer to take place with barely any flow of current. The aluminium base material melts together with the aluminium filler material, with the melt wetting the galvanised steel. The filler wire is constantly retracted at very short intervals. The precisely defined retraction of the wire facilitates controlled droplet detachment to give a clean, spatter-free material transfer. The wire movement takes place at a very high frequency and requires a quick-response, gearless wire drive directly on the torch. Obviously the main wirefeeder will not be able to keep up with these movements. The wirefeeding hose is therefore provided with a wire buffer that compensates for the additional forward and backward movement of the wire.
CMT welding is carried out exclusively using digital inverter power sources. The welding system basically uses the same latest state-of-the-art hardware as a MIG/MAG system, while at the same time taking certain specific requirements into account. Particularly noteworthy is the highly-dynamic wirefeeder mounted directly on the welding torch. The moment the power source detects a short circuit, the welding current drops and the filler wire starts to retract. Exactly one droplet is detached, with no spatter whatsoever. The filler wire then moves forwards again and the cycle is repeated. High frequency and extreme precision are the basic requirements for carefully controlled material transfer. The wire drive on the welding torch is designed for speed, not high tractive forces. The wire is therefore fed by a more powerful but, due to the above, slower main wirefeeder. A wire buffer on the wirefeeding hose is used to convert the superimposed, high-frequency wire movement into a linear wirefeed.
The highlight of the CMT process is without doubt its ability to join steel and aluminium. Although the steel base material is only wetted during this brazing process and does not melt, numerous trials always resulted in a break in the aluminium base material, not in the weld seam.
In addition to the joining of steel and aluminium, the CMT process is also highly suitable for a number of other applications. There is undoubtedly a great deal of interest in the almost spatter-free brazing of hot galvanised and electrolytically galvanised sheets using a filler wire made from an alloy of copper and silicon. Ongoing trials are examining the joining of galvanised sheet (0.8 mm) and black material (5 mm), with extremely low levels of distortion of the galvanised sheet.
Light-gauge welding (0.3 – 0.8 mm) of aluminium sheets is also perfectly feasible. The low level of heat insertion in the CMT process means that a weld pool is not required; even then there is no risk of weld drop-through. The results of welds performed on stainless steels and magnesium are also first class.
The CMT process provides a straightforward way of joining steel and aluminium. Over and above this, CMT has a number of more than satisfactory mechanical and technological characteristics. Interest is not just confined to the joining of steel and aluminium. There are a number of other extremely interesting potential applications, including the spatter-free brazing of coated sheets, the light-gauge welding of aluminium and the welding of magnesium. A large number of trials are currently in progress. Which other applications the CMT process is suitable for remains to be seen.