IIW White Paper

4 Needs and challenges in welding and joining technologies

Alloy extrusions containing magnesium and silicon (6xxx type) are among the most common form of wrought aluminium, and are widely used in welded constructions. These alloys can be welded, but only when using the correct filler alloy (e.g. Al-Si 4xxx or Al-Mg 5xxx alloys). When welded autogenously, these alloys are highly prone to solidification cracking. Work is ongoing to define how much filler is needed to avoid cracking, and under what conditions of restraint. These alloys are also highly susceptible to liquation cracking, producing very fine micro-cracks in the heat affected zone. As another research gap, the effect of these micro-cracks on mechanical behaviour is not well known and, because they are hard to detect without metallography, their presence often goes undetected. Use of extruded aluminium alloys in modular space frame constructions has become commonplace in the automotive industry. Box frames can be constructed by piecing together extrusions joined at nodes, where they are typically gas-metal arc welded. Aluminiumbody panels are commonly attached using resistance spot welding. Future developments have to match the growing desire to replace this process with more effective methods, including friction stir spot welding and laser welding. Adhesives and adhesive-weld combinations are also becoming attractive alternatives. Joints made with these new processes require considerable development to characterise their performance, meanwhile aluminium designs face stiff competition from new pre-coated high strength-low alloy steels. In the skin-to-stringer fabrication ( see Figure 4.4 ) of an aircraft fuselage, the use of mechanical riveting has partially been replaced by laser beam welding. New weldable 2xxx (e.g. AA2139 and Al-Li 2198) and 6xxx ( e.g. AA6013, AA6056, AA6156 etc.) alloys have been successfully welded using 12%Si containing filler wires (4047) by using CO2 and Nd:YAG lasers for airframe applications. Furthermore, the riveting process may also be partially replaced by the friction stir welding process even for the non-weldable Al-alloys since this process does not involve melting and hence there cannot be any solidification cracks. FSW may introduce new types of flaws which need to be taken into the consideration, however. Aerospace fuel tank construction typically involves arc welding, although FSW is also making inroads into this area. Because of safety concerns and the need for high quality welds, variable polarity arc welding is often employed, using either plasma or gas-tungsten arc processes. New high strength lithium-containing alloys have been used on the Space Shuttle for fuel tanks (e.g. alloy 2195), specifically developed for both strength and weldability.

Gluing 37%

Figure 4.4 Laser beam welding of skin-stringer joints for aircraft fuselage applications ( Reproduced courtesy: GKSS, M. Koçak)

Ti-alloys Most titanium alloys can be welded achieving quite good quality of the joints, provided that special procedures are followed to avoid contamination with oxygen and hydrogen. Under normal conditions, problems with hot cracking are seldom encountered. With contamination, however, serious problems with cold cracking and porosity can be experienced. Both oxygen and hydrogen readily dissolve in titanium as interstitials at high temperature (above 500°C), reducing the ductility of HCP alpha-titanium. In addition, hydrogen can form titanium-hydrides that further embrittle the material. Future research is required to determine how much hydrogen can be tolerated in different alloy weldments (e.g. a, α/ß, and ß), before mechanical properties are compromised.

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Through Optimum Use and Innovation of Welding and Joining Technologies

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