Improving Global Quality of Life

Up-to-date, titanium alloys have been used primarily in high performance fighter aircraft. They are now also
being considered for structural material in new composite wrapped commercial aircraft (e.g A350). Titanium
could replace aluminium, which is known to have a compatibility problem with graphite composites. Many
new approaches to joining are likely to be employed, including riveting, arc and laser welding, and friction
stir welding. Friction stir welding of titanium, however, posses a unique problem regarding tool wear,
making it much less attractive than with aluminium. Due to its exceptional corrosion resistance, commercial
grade titanium has been used as a replacement for stainless steel piping in heat exchangers for power
plants. Similarly, beta alloys have been used as a drill pipe for sour oil wells due to their resistance to H 2
S
environments. Such pipe applications typically involve gas-tungsten arc welding. Titanium has also been
given serious consideration for use as armour on military tanks. This would require multi-pass arc welding
of thick sections using the gas-metal arc process. Such welding leads to large columnar grains in the weld
metal, continuous from one pass to another, that significantly reduces toughness. These difficulties which
are peculiar to titanium alloys have to be investigated more thoroughly in future. The extent to which Tialloys
will be used in the future, in the applications discussed above, depends to a large part upon the
availability of relatively inexpensive grades of titanium. One of the ways of reducing costs is to tolerate
higher impurity levels, such as iron. It has to be investigated, however, how this might affect weldability or
joint properties, including strength and corrosion resistance.
Mg-Alloys
Most magnesium alloys can be welded reasonably well, but present some unique problems with regard
to bead control, spatter and oxidation. Due to a lower density, magnesium weld pools react differently
to the arc, gravity and surface tensions acting on them, resulting in unconventional bead shapes. Due to
magnesium’s high vapour pressure and the small interval between melting and vaporising (600-1100°C),
wire transfer during gas-metal arc welding can result in explosive expulsion of material (i.e. spatter). Also,
like aluminium and titanium, magnesium has a high affinity for oxygen. For welds made with arc processes
using electrode negative polarity, surface oxides must be removed prior to welding in order to avoid thick
(crusty) oxides on the weld surface. Use of alternating current or variable polarity also helps in this regard.
For gas-metal arc welds made with electrode positive polarity, a thin surface oxide surface layer on the joint
may actually be beneficial for arc stability. Hot and cold cracking are not normally a problem, and porosity
originating from hydrogen contamination is seldom encountered.
Mg-alloys have typically been used in selected automotive and aerospace applications, primarily in the form
of die castings. With the recent development of rolled sheet, however, there exist many new possibilities
for welded constructions, with the higher strength wrought alloys feeding the need for weight reduction. In
addition, new power supply technology has allowed the gas-metal arc process to operate in the short circuit
mode with reduced spatter. This process shows promise for the welding of thin section magnesium, as does
the use of laser and friction stir welding processes. Laser beam welding of AZ31 sheets with and without
wire additions provides excellent hardness, ductility, fatigue (including crack propagation) and fracture
properties for the butt-joints.
Due to the difficulty in forming HCP magnesium, filler wire of good quality is only available from a few
suppliers, and is typically limited to one or two sizes (e.g. 1.2 and 1.6 mm dia.) suitable for gas-metal arc
welding. To solve this task for gas-tungsten arc or laser welding, finer wires have to be developed in the
near future. In addition, there are typically only a few alloys available in wire form: e.g. AZ31 and AZ61. The
question then becomes, what is the desired filler composition so as to optimise joint properties in terms of
corrosion resistance? Weld metal is often the weak link in magnesium welds, suggesting that improvements
are possible with filler alloy development. For use of welded components in automotive applications, fatigue
and in particular corrosion fatigue of welds becomes an important design criterion. When dealing with a
reactive metal like magnesium, corrosion must always be considered when evaluating lifetime behaviour.
While limited data is available for magnesium weld fatigue, little or no information is available for weld
24 Improving Global Quality of Life Through Optimum Use and Innovation of Welding and Joining Technologies