Aluminium castings are made by several distinct processes with very different tooling costs and very different results. The choice is driven by volume first, then by the wall thickness and the integrity the part needs. Getting it wrong shows up as either a tooling bill that never pays back or a part that leaks under pressure test.
Sand casting
A pattern is used to form a cavity in bonded sand, which is destroyed after each pour. Tooling is cheap and quick, so it suits low volumes, large parts and prototypes. The surface is rough, walls must be thick, and dimensional tolerances are loose, so machining allowance is generous. It remains the cheapest route to a first casting by a wide margin.
Gravity die casting
Metal is poured into a permanent steel die under gravity. Tooling costs considerably more than sand and lasts for many thousands of parts. The surface is better, walls can be thinner and tolerances are tighter, so less machining is needed. It is the usual middle ground for volumes that do not justify pressure die casting.
Low pressure casting
Metal is pushed up into the die from a sealed furnace under low pressure, filling the cavity smoothly from the bottom. That controlled fill reduces turbulence and therefore reduces the entrained gas that causes porosity, so it suits parts that must be pressure tight or heat treated. Wheels and structural castings are typical, and tooling is a serious investment.
Porosity, heat treatment and machining
Porosity is the recurring problem in aluminium castings and it comes from turbulent filling and from dissolved gas. It matters when a part is pressure tight, when it is heat treated, or when machining will open a subsurface void. Specify the integrity requirement and the test rather than assuming, and leave sensible machining allowance on every surface that must be accurate.
Questions people ask about aluminum casting
Which aluminium casting process should I choose?
Volume decides first: sand for low volumes and large parts, gravity die for the middle, low pressure or pressure die casting for high volume and better integrity.
Why do castings have porosity?
Turbulent filling entrains gas and solidification leaves shrinkage voids. Controlled filling, good gating and degassing the melt all reduce it, and the requirement should be specified.
How much machining allowance is needed?
More on sand than on die castings. Ask the foundry for their recommended allowance for the process and the feature size rather than guessing.
Can castings be heat treated?
Many alloys can, but porosity can cause blistering during solution treatment, which is one reason low pressure and controlled fill processes are chosen for heat treated parts.