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2.4.1 Introduction
Typically using an aluminium casting to replace an iron casting will result in cutting the
component weight by half (ref. A1) which means that automobile manufacturers are investigating
potential applications in areas including engine, drive train and suspension components. Most
applications make use of the A 300 series, aluminium - silicon series alloys, particularly
the hypoeutectic alloys:
A356 - nominally 7% silicon, 0.35% magnesium and
A357 - nominally 7%, silicon 0.55% magnesium.
The silicon gives good fluidity
when casting, enabling thin sections to be successfully cast. The magnesium provides strength
(through heat treatment) while maintaining reasonable ductility. The table below, adapted for
ref. A3, compares A357 in the T61 condition with ductile iron D4512.
| Alloy |
Property Typ./Min. |
UTS, MPa |
Yield Strength, MPa |
Elongation, % |
| D4512 |
Typical |
464 |
332 |
15 |
| A357-T61 |
Typical |
360 |
290 |
8 |
| A357-T61 |
Minimum |
310 |
248 |
3 |
The lowest cost general purpose alloy is 356-T6, alloy A356-T6 is frequently used in the
aerospace industry. The ductility of A444-T4 is better than that of many wrought alloys.
These 3 are among the most weldable of the casting alloys. The cast alloys incorporating copper
(2xx.x) generally have the highest strengths at elevated temperatures.
In hypoeutectic alloys the silicon varies between 5.5 and 10.5%, and primary aluminium
is the first phase to solidify when cooling. The micro-structure consists of primary aluminium
dendrites within a eutectic matrix.
Eutectic alloys contain 10.5 to 13.5% silicon and have micro - structures consisting mainly of
aluminium - silicon eutectic.
Hypereutectic alloys contain 16 to 23% silicon. In these the first phase to solidify,
the primary phase, is silicon. These alloys tend to have a distribution of coarse silicon
particles which provide excellent wear resistance. Aluminium alloy 390 containing 17% silicon (ref. A2) was
originally developed for engine block applications, but is currently used in applications
requiring excellent wear resistance such as pistons and pulleys.
Many of the components manufactured in this alloy are die cast, but porosity is a frequent problem.
A further problem is that this alloy has a high latent heat and a wide melting range,
and requires a high melt temperature 700 - 760oC. This leads to short die lives
and the long solidification time makes it difficult to avoid segregation of the silicon
particles. To overcome these problem experiments have been carried out die casting this alloy
in the semi - solid condition, approximately 50% solid and 50% liquid.
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2.4.2 Fatigue Strength
An important feature of aluminium and its alloys (and other non - ferrous alloys)
is that unlike ferrous alloys that exhibit a finite fatigue endurance strength, the fatigue
strength aluminium alloys continues to fall with increasing stress cycles and this must be
accounted for in design process. Prior to stress analysis the required number of stress cycles
must be specified, for critical applications in automotive applications a figure of 109
may be used as a starting point (ref. A2). Experience may however permit the requirements to be more
accurately defined. Porosity of cast components can have a significantly deleterious effect on
the fatigue strength of aluminium castings and care must be taken to minimise the entrapment of
gas during casting.
2.4.3 Processing
To ensure high quality castings a range of procedures are commonly used. When casting into
permanent moulds, extensive simulation will often be carried out and cooling passages provided
for air or water cooling to target directional cooling towards the molten metal feed points.
Vacuum may be applied to assist flow and to reduce porosity.
One particular process is believed to involve pumping in the molten aluminium from the bottom
of the mould rather than using gravity fill from the top. This reduces air entrapment, reducing
porosity and giving cleaner, better quality castings. By using such techniques, parts can be cast
with a wall thickness of only 2.3 mm.
2.4.4 Aluminium Engine Blocks
Aluminium alloy engine blocks - often in grade 319 (7% Si, 1% Fe, 3.5% Cu, 0.5% Mn, 0.35% Ni,
1% Zn, 0.25% Ti) - normally use either 'wet' or 'dry' cast iron sleeves, the latter either
cast in or pressed in. Using aluminium alloy blocks without liners has some important advantages,
including reduced cylinder spacing, improved heat transfer, but blocks in 319 require Nikasil
coating and other alloys such as 390 require special coatings which are expensive. There is
significant research being carried out in an effort to develop coatings that can be applied
directly to the aluminium bore without incurring an excessive cost penalty. Plasma and thermal spray
technology are being investigated and plasma spray has the flexibility of being able to include
virtually any material, including solid lubricants. A major portion of engine oil consumption
arises from bore and piston ring wear and the bore / piston / ring combination accounts for
significant losses due to friction. It is thought that incorporating solid lubricants into a
suitable plasma spray will provide significant benefits in these areas. Developments have also taken
place in plasma application equipment so large scale operation is now believed to be feasible.
It is likely that there will be some important developments in these areas in the future.
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