What are the key stages of the aluminum casting process?
Aluminum casting involves pouring molten aluminum alloy into a mold, where the metal solidifies and takes the shape of the finished part. The process consists of several stages: alloy preparation and melting, mold making, pouring, shakeout and cleaning of the casting, machining, and quality control. Below we describe each of them and, for selected stages, how we work in our foundry.
1. Material preparation
Alloy selection and preparation
The first step is selecting an aluminum alloy that meets the technical and mechanical requirements of the part. Aluminum casting alloys contain alloying elements – primarily silicon, as well as magnesium and copper – which improve castability and strength. The composition of casting alloys is specified in PN-EN 1706; we cast alloys including EN AC-42000, 46200, 46500, 47100 and 51300. The charge consists of ingots of ready-made casting alloy. You can read more about material selection on the page aluminum casting alloys.
Melting aluminum and cleaning the alloy
Pure aluminum melts at 660 °C, while aluminum casting alloys melt at lower temperatures, because silicon lowers the melting point (the eutectic alloy AlSi12 solidifies at approx. 577 °C). For pouring into molds, the metal is usually superheated to approx. 680–760 °C, and slightly less in die casting. In our foundry, we melt the charge in electric melting furnaces with digital melt control, and we refine the molten metal – removing hydrogen and oxides – and modify it, which improves the structure and properties of the casting.
2. Making the casting mold
Mold design
The mold is designed on the basis of a drawing or 3D model of the part: the parting plane, gating system and risers are determined, and machining allowances and casting shrinkage are added to the dimensions. We design die casting molds in CAD/CAM systems and verify them with metal flow simulation.
Mold making
The mold material depends on the casting method. A sand mold is made from molding sand – sand with a binder – using a pattern; after pouring, the mold is broken up to remove the casting. A pattern is much cheaper than a permanent mold, which is why this method works well for prototypes, short runs and large castings. Permanent mold is a permanent metal mold for repeated use, intended for repeatable production runs, while a die (high-pressure die casting) is a tool steel mold for high-pressure casting. We make patterns, core boxes and permanent molds in our own pattern shop (for some projects, with trusted partners), and die casting molds in our own tooling department. We compare the types of molds in the article casting molds.
3. The casting process
Pouring metal into the mold
Molten metal is poured into the mold through the gating system – smoothly, without interruption and at the right temperature. Overly turbulent pouring introduces oxides into the casting, and metal that is too cold will not fill thin walls. Sand molds and permanent molds are filled by gravity, usually with a foundry ladle, whereas in high-pressure die casting the machine forces the metal into the mold under high pressure.
Cooling and crystallization
After pouring, the metal solidifies and cools. In a metal permanent mold or die, heat is extracted faster than in sand, so the castings have a finer structure, and the smooth mold surface produces a smoother casting surface. Solidification is directional, so that the risers feed the casting with molten metal as it shrinks – otherwise cavities and shrinkage porosity form.
4. Removing the casting from the mold
Shakeout or opening the permanent mold
A sand mold is broken up (shaken out) once the casting has cooled sufficiently, while a permanent mold or die is opened immediately after the casting solidifies and is reused.
Casting Cleaning
The gating system and risers are separated from the casting, and residual molding sand and flash are removed. Depending on requirements, the casting surface is shot blasted, ground or polished.
5. Finishing
Machining
Functional surfaces – holes, threads, mating faces – are usually machined after casting: by milling, turning, drilling and tapping. We machine castings on CNC machines in the same facility, and on request we powder coat them.
Heat treatment
Some castings undergo heat treatment – for example annealing, which relieves stresses and stabilizes dimensions, or solution treatment and aging, which increase strength and hardness. We perform heat treatment at the customer's request; the types are described in the article heat treatment of castings.
6. Quality control
Visual and dimensional inspection
Castings are inspected for surface defects such as cracks, misruns or porosity, and measured to confirm that dimensions are within the tolerances on the drawing – often specified according to the standard PN-EN ISO 8062-3.
Non-destructive testing
If necessary, internal defects such as gas pores or shrinkage cavities are detected by radiographic testing, and discontinuities open to the surface by penetrant testing. The type and scope of testing and the acceptance criteria are agreed with the customer in technical acceptance conditions for castings.
Strength testing
At the customer's request, we perform mechanical testing and material structure analysis. These confirm that the casting has the required mechanical properties.
Summary
Aluminum casting comprises alloy preparation and melting, mold making, pouring and solidification, shakeout and cleaning of the casting, machining, and quality control – each of these stages affects the quality of the finished part. In our foundry, we cast aluminum alloys in permanent molds (0.1 to 50 kg), in sand molds (1 to 100 kg) and by high-pressure die casting (in runs of up to 100,000 pcs per year), and we machine and coat the castings in the same facility.
See also: most common defects in aluminum castings and their causes, aluminum casting alloys i how much an aluminum casting costs.
Need aluminum castings? Send a drawing or 3D model to odlewnia@adjatech.pl or call: +48 61 662 43 37 (Mon–Fri 8:30 a.m.–4:30 p.m.).
Scope of services: foundry production capabilities.