Thixoforming : Semi-solid Metal Processing

120j 4 Design of Al and Al–Li Alloys for Thixoforming
600 MPa in the T6 state, which is double that of the conventional aluminium alloy
2219-T6 [22].
The original objective for the development of Al–Li–Cu–(Mg) alloys was as a
substitute for the conventional aluminium airframe alloys, with an expected density
reduction of about 10% and increase in rigidity of 10–15%. These alloys are classified
as high-strength (A2090, A8091), medium-strength (A8090) and damage-tolerant
(A2091, A8090) in respect of their mechanical properties. Their densities range from
2.53 to 2.60 g cm 3 and modulus of elasticity from 78 to 82 GPa. Nevertheless,
problems do exist in respect of their low lateral toughness in continuously cast
profiles. These problems appear to relate to their microstructure, primarily in the
grain boundary areas. Under continual loading of these alloys at increased temperatures
(>50 C), the incidence of creep fracture can also increase to a relatively
high degree [24].
4.3.6
Effects of Alkali Metal Contaminants
One known factor that reduces the ductility and fracture toughness of alloys containing
lithium is, along with hydrogen, the presence of alkali metal contaminants (in
particular sodium and potassium) and particularly at the grain boundaries. In
conventional aluminium alloys, these elements are combined with elements such
as silicon in harmless, permanent compounds. However, they react readily with
lithium. Commercial alloys containing lithium are produced by conventional melting
and casting techniques and usually contain 3–10 ppm of alkali metal contaminants.
This level can be reduced to below 1 ppm by means of vacuum melting and
refining [25]. It can be demonstrated that this leads to a considerable improvement in
fracture toughness at room temperature (Figure 4.9).
Figure 4.9 Effect of alkali metal contaminants on the crack
toughness on A2090 extruded profiles [26].