Thixoforming : Semi-solid Metal Processing

118j 4 Design of Al and Al–Li Alloys for Thixoforming
alloys by conventional melt-based metallurgical means. In the aluminium industry,
these may also contribute to their resistance to the growing trend toward the
application of non-metallic composite materials as structural materials in the
aerospace industry [22].
Hardened binary Al–Li alloys primarily suffer from low elasticity and toughness
under the massive stress peaks that can occur, because coherent d 0 precipitations are
sheared by moving displacements. This can give rise to fractures along the grain
boundaries. Correspondingly, this work has focused very heavily on the study of
elements forming suitable precipitations or dispersoids allowing the displacements
to be dispersed more homogeneously. The alloy systems developed to date are
compared in Table 4.2.
4.3.2
The System Al–Li–Cu
The first commercially applied alloy of this system was A2020. Copper on the one
hand reduces the solubility of lithium in its solid state, whereby the precipitation of d 0
is promoted, and on the other hand gives rise to the precipitation of phases, such as
the GP zones and q 0 , as familiar from binary Al–Cu alloys. The alloy A2020 combines
properties of high strength at room temperature (R p0.2 520 MPa) with good creep
behaviour at temperatures up to 175 C and has a modulus of elasticity 10%
higher than that of other aluminium alloys. Newer Al–Li–Cu alloys have a lower
Cu content and higher Li:Cu ratios, of which the best-known example is the alloy
A2090. In addition to the d 0 phase, this alloy is hardened by means of the hexagonal T1
phase (Al2CuLi). Zirconium additions ( 0.12%) are also applied in order to form
fine dispersoids of the type Al3Zr, which control recrystallization and grain size.
Another effect of this element is to promote grain formation of the d 0 precipitations
on these dispersoids. Overall, zirconium increases the properties of strength and
toughness [13].
Table 4.2 Selection of Al–Li alloys developed to date for the aerospace industry.
Alloy Li Cu Mg Zr Others
A2020 1.3 4.5 — — 0.5 Mn; 0.25 Cd
A2090 1.9–2.6 2.4–3.0