*1.2. Aluminum Alloys Weldability*

The welding of aluminum alloys is considered a slightly difficult process due to its high thermal and electrical conductivity, high thermal expansion coefficient, refractory aluminum oxide (Al2O3) formation tendency, and low stiffness. These characteristics, in general, make these alloys sensitive

to defect formation that may lead to the loss of chemical, metallurgical, and mechanical properties. Typical welding defects in aluminum alloys are gas porosity, oxide inclusions and/or oxide filming, solidification (hot) cracking or hot tearing, reduced strength in the weld and heat affected zone (HAZ), lack of fusion, and reduced corrosion and electrical resistance. These defects determine reduced strength and corrosion in the fusion zone (FZ) and HAZ, with a general decrease in mechanical properties. These defects are generally reduced, providing efficient protection from the contamination of atmospheric gases to the weld pool or/and decreasing the influence of the weld thermal load by using welding processes with higher energy density [11,12].

These problems have to be faced, mainly due to the increasing employment of Al in complex vehicle parts, which implies a higher number of potential applications of Al hybrid structures. For example, during the last few years, both cast and wrought Al parts have been introduced in automotive complex shapes, which entails the need to join them into the final structure. Indeed, the dissimilar aluminum alloy joint can combine good strength and corrosion resistance, which is typical of this material, with exceptional castability used where complex sub-sections are needed, and excellent mechanical properties, achieved in other specific areas made of extruded parts.
