Thixoforming : Semi-solid Metal Processing

116j 4 Design of Al and Al–Li Alloys for Thixoforming
Figure 4.8 Grain structure of thixo-cast step samples.
different raw materials were also retained after forming. Examination of the chemical
homogeneity over the length of the component indicated that for both raw materials
no significant mix separation had occurred.
4.3
Fundamentals of Aluminium–Lithium Alloys
4.3.1
State of the Art
Aluminium–lithium alloys have been in use since 1950. In particular, wrought alloys
such as AA8090 and AA2091 are used in the form of continuous-pressed profiles or
rolled sheets in aerospace constructions. Also, forged components such as Al–Li
brake callipers are used in motor racing [16]. Their low density, high specific solidity
and high modulus of elasticity make these materials highly suitable for a multitude of
high-load applications. Because of the high raw material cost of lithium and high
expenditure on processing, however, they have not been very widely used to date. The
value of 5D kg 1 of tolerable extra costs in transportation systems for reduction in
weight per kilogram indicates that their introduction in automotive mass production
would only occur if significant cost reductions were feasible.
However, aluminium–lithium alloys are subject to low toughness values, stresscracking
corrosion and heavy anisotropy of mechanical properties [18, 19].
Aluminium–lithium alloys can be produced by melt technologies and powder-based
metallurgical methods. The latter require more complex operating lines (cost issue)
but permit a higher lithium content to be applied. However powder-based metallurgy
can be subject to high contamination of the powder during the jet-spraying process
(e.g. oxide formation – unfavourable for ductility) and also the high reactivity of the
lithium itself. Melt-based metallurgical production is more common but the achievable
lithium content is limited to 10%. During continuous casting of Al–Li semifinished
products, an aluminium melt (99.5%) with the slightest Li content will