Aluminium Castings

Overview

Fundamental reasons why Al castings:

low melting point
lovely Al-Si eutectic
not reactive
- to sand
- (strongly to) die steels

Castable alloys:

Near to (Al-Si) eutectic composition -> narrow freezing range
- limited segregation
- fewer feeding problems (must feed 7% loss of volume on solidification)

Reasons to watch casting technology

Computational casting simulation

These physical features are very well-ordered:

thermal conductivity of metal and mould (fn T, composition)
latent heat of solidification of metal (fn composition)
temperatures, masses of metal, mould, coolant-line fluids, environmental "boundaries"

Less well-ordered:

fluid-flow on mould-filling
surface heat transfer coefficients
- between metal and mould - however, becomes quite well-behaved when successfully squeeze-cast
- die to coolant-line fluids

On balance, can get very accurate prediction of casting performance. Geometric complexity and other conditions no problem. Can see what is (would be) wrong with proposed designs. Can home-in on designs which will fulfil the product objectives and cast well, cheaply.

Computational simulation of casting gets you:

cheap to get to good castings
very complex castings can become feasible

Generally you can get much closer to a designer's ideal component form.

Computer-control of processing

Makes possible

squeeze-casting
semi-solid processing

Competition watch! - these are very good processes!

Squeeze-casting

Smooth non-turbulent filling then pressurised feeding during solidification. Get:

fully sound (controlled filling and feeding)
fine structure (rapid cooling as casting's solidifying shell pushed against die walls)
v.good dimensional accuracy (solidifying shell pushed against die walls)
heat-treatable, as no entrapped air bubbles to expand and rupture the casting

When "squeezing" - surf.ht.trans.coeff metal<->mould becomes large, constant and known -> high quality and accurate simulations.

Semi-solid processing

Computing process control makes many semi-solid processing options viable.

Tight control and sensing of

temperature
shear rate and torque
mass
material feed rate

could open up these possibilities for "wrought only" properties to appear in cast components

What you get with semi-solid processing:

segregation limited and short range -> not restricted to near-eutectic alloys
latent heat of solidification - the "thermal loading" - on dies is much less
- cheaper familiar castings or
- fuller realisation of desired casting form
viscous - less problem of turbulent flow including surface oxides
oxide-dispersion-strengthening? Stir-in "nano-rods" of insoluble refractory to defeat problem of low S.F.E.?
volatile alloying elements introduced "at last moment"? eg. wire-feed Li?

Direct squeeze casting of semi-solid metal

Direct squeeze-casting: computer-control -> bye-bye previous problem to its widespread use:

Computer-control -> semi-solid processing unit can certainly supply precise metered metal-mass control
Direct squeeze casting -> mass of melt supplied controls "height" dimension of casting

so first point eliminates as a problem the second point.

Advantages obtained: (combining direct squeeze casting and semi-solid metal processing)

100% metal use - can use expensive and/or volatile alloying elements no problem (no runners, risers, feeders, etc)
exert huge closure pressures - can "brute-force" close shrinkage porosity in "thick" parts if desired
very little metal movement -> less tendency to "include" surface oxides

Suggest don't assume Al-Li not accessible by cast route! (???)

Dispersion-strengthened or particulate-reinforced squeeze-casting - as Al has no solubility for C, can use diamond tools to finish-machine (only machining necessary).

"Other" process routes