Traditional welding of aluminumleading to a mechanical degradation of the welded material
Traditionally aluminum has been welded using fusion welding (e.g. GMAW), a technology originally developed for joining of steel. Fusion welding does not reduce the strength of steel after welding, but for aluminum the situation is different. The microstructural changes occurring during welding lead to severe heat-affected zone (HAZ) softening as well as cracking and corrosion problems in the fusion zone (FZ).
Reheating of aluminum above 350-400 °C during welding reduces the strength in the HAZ by 50% compared to the base metal (BM) and consequently the load bearing capacity of the component.
However, metallic bonding can be obtained without fusion. It can even be achieved at room temperature if the bonding conditions are optimal.
Several attempts – like Friction Stir Welding (FSW) – have been made in order to obtain bonding at temperatures where the metal is still in the ‘solid state’ to reduce the mechanical degradation associated with fusion welding of aluminum. Although FSW only reaches a maximum process temperature of about 550 °C, it still leads to the formation of a wide HAZ and the same softening problems as traditional fusion welding. The heavy forces involved in FSW also require the use of rigid fixture and heavy clamping of the base plates, thereby reducing the flexibility of the technology significantly.
Temperature profile of welded aluminum
Strength profile of welded aluminum
The new HYB method a low temperature solid state method
The basic idea behind the new innovation, called the Hybrid Metal Extrusion & Bonding (HYB) process, has been to develop a low temperature solid state method. This new method eliminates HAZ softening and cracking and corrosion problems in the fusion zone. It also makes use of a filler metal with the same properties as the base metal.
Metallic bonding is a result of interatomic attraction between the atoms. If the oxide layer is removed and the atoms are brought sufficiently close to each other, full metallic bonding will be achieved.
The positively charged atom cores are bonded by mutual attraction to the negatively charged electrons.
The HYB technologya bonding revolution
Continuous extrusion is applied to plasticize the aluminum filler wire and subsequently squeeze the oxide-free filler metal into the groove between the two base plates to be joined. This, in turn, creates favorable conditions for achieving metallic bonding between the filler metal and the base metal in the groove.
The temperature of the process is below 300°C, and hence there will be no HAZ softening. The composition of the filler wire further provides favorable cracking and corrosion properties of the joint.
The compact extruder design and the low reaction forces involved make the technology well suited for flexible robotic automation, which is vital for widespread industrial use of the HYB technology.