23
Through Optimum Use and Innovation of Welding and Joining Technologies
Improving Global Quality of Life
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.
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%
4
Needs and challenges in welding and joining technologies
Figure 4.4
Laser beam welding of skin-stringer joints for aircraft fuselage applications
(
Reproduced courtesy: GKSS, M. Koçak)
