A flexible technique

In the foundry industries, a molten metal or alloy is poured into a mould and solidifies to form a casting of which the shape matches that of the mould.

Because they shape liquid metal, the foundry industries differ from the other metal transformation industries in two essential characteristics:

• metallurgy (metals and alloys);

• geometry (shapes and volumes).

In effect:

• The use of metal in liquid form entails the existence of melting equipment that is supplied on demand. The foundryman is therefore free to use alloys having extremely varied compositions.

• When moulds are filled with a liquid metal, the most complex shapes can be obtained (solid, hollow, etc.) provided that the moulds corresponding to the castings can themselves be designed and made. The molten metal occupies a larger volume than the casting after solidification and cooling, so the tooling must be oversized in proportions that vary according to the alloy. The technique of computer aided design and simulation of the filling of the moulds and the cooling of the metal can be combined to engineer the most complex castings. The result is parts without mechanical joints or welds that range in size from less than a gram to a few hundred tons.

A specialized industry

Specializations

- Metallurgy

Most foundries work with only one family of alloys, and it is rare that a foundry works with more than two. The two major categories of foundries are those that work with ferrous alloys and those that work with non-ferrous alloys. One of the essential properties of castings is that their materials respond to mechanical stresses the same way in all directions (isotropism).
The ferrous alloys are cast irons and steels; they are alloys of iron and carbon associated with other elements.

Cast irons are alloys having a high carbon content (2.5 to 4%). They have very good functional properties, but little elasticity in tension. That is because of the presence of carbon, in the form of flakes of free graphite that crystallize in the metallic matrix, creating weak spots that may initiate fractures and making the material somewhat brittle: these cast irons are called Flake graphite cast irons . To improve their properties, use is made of addition elements (nickel, chromium, etc.). This produces "alloy" cast irons in which the carbon content is generally less than 3%.

A new type of cast iron has since been introduced, spheroidal graphite cast iron, the properties of which are superior to those of some wrought steels, especially their fatigue strength: this is because, in these cast irons, the graphite crystallizes in the form of small spheres, not flakes. The automobile industry uses spheroidal graphite cast irons to mass-produce safety parts (crankshafts, steering knuckles, etc.). Spheroidal graphite cast iron pipes are used in water supplies for their strength and elasticity, which enable them to withstand terrain movements.

Steel, for its part, is an alloy of iron and carbon in which the carbon content is less than 0.5%. There are three families of casting steels: carbon steels (0.15 to 0.45% carbon), low alloy steels (less than 6% of such elements as chromium, nickel, molybdenum, etc.), and high alloy steels such as stainless and refractory steels. The application areas are essentially equipment for use in rail transport, public works, steel-making, energy production (turbines), and the nuclear industry.
Non-ferrous alloy foundries cast mainly copper alloys, lightweight alloys, ultra-light alloys, and zinc alloys.

Copper and copper alloy castings are used in particular for their ease of machining, their friction properties, their corrosion resistance, and their conductivity, but also for their attractive appearance and pleasant sound. These alloys are used in particular in valvegear, shipbuilding, the mechanical and electrical engineering industries, sculpture, and for bells. The alloys include bronzes (7 to 20% tin), brass (20 to 40% zinc), and cupro-aluminiums (9 to 12% aluminium).

Lightweight alloys are made with aluminium and ultra-light alloys with magnesium. Aluminium alloys are broken down into three main classes: aluminium-silicon alloys (5 to 25% silicon), aluminium-copper alloys (5 to 8% copper), and, aluminium-magnesium alloys (3 to 6% magnesium). They are used for their light weight (density: 2.7 g/cm3), their conductivity, and their resistance to corrosion. The main areas of use are the automobile industry, electrical construction, household appliances, weapons systems, and the aerospace industry.

Foundries making zinc alloy castings use alloys based on zinc and aluminium. These alloys are easy to work and have good mechanical properties, they are also well suited to the mass production of precise castings. The products range from zippers to the frames of office machines and include toys, electrical equipment, and automobile equipment.

- Other specializations

Foundries are also specialized with respect to:

• The size of the castings, which determines the volume of liquid metal to be prepared, the handling equipment needed, and the dimensions of the moulds.

• The number of castings per run, which leads to mechanized moulding processes, possibly automated, and influences the size of the company.

• The dimensional precision and the desired quality level, which may lead to the use of special moulding processes: shell and investment casting, etc., and suitable gauging equipment.

The contradiction between flexibility and specialization is only apparent. In effect, flexibility enables a foundry to satisfy many, many users with extremely varied products. On the other hand, this variety imposes specialization of the production equipment.

Despite the great diversity of foundries, all follow the same pattern, which reflects the definition of the industry.

Moulding is at the heart of all foundries

There are two types of mould :

Destructible moulds are made of sand or ceramics. After the metal is poured and solidifies, they must be broken up to release the castings. The moulding shop is organized around the production and handling of the moulds, which requires the existence of a sand shop and large transfers of materials.

Permanent moulds are metallic. They can be used several thousands of times, because they are simply opened to release the casting. The moulding shop is simplified by the elimination of the sand shop and reduced handling of the moulds.

The tooling includes, in the one case, the patterns or pattern plates used to make the imprints in the sand and, in the other, the permanent moulds. There are also "core boxes" used to make volumes of sand - called " cores " - used to produce hollows and surfaces having complex shapes: core making is essential in most foundries.

Melting transforms the alloys to the liquid state. The energy used may be provided by electricity, gas, coke, or fuel oil.

Pouring is the operation by which the liquid metal is put into the moulds. The temperatures necessary to pour the different metals and alloys are extremely varied: 1700°C for cast steels; 1500°C for cast irons; 1200°C for copper alloys; 700°C for aluminium alloys; 400°C for zinc alloys.

Finishing includes a series of operations, among them elimination of the sand from the mould and cores. According to the case, this may be followed by shot peening, heat treatments, machining, painting, or other surface treatments.

Every stage of production includes many checking operations (3D gauging, ultrasound, magnetic particle examinations, gamma radiography, etc.). Quality management is one of the keys to industrial success, especially in the foundry industries, where it is necessary to control a large number of parameters in the course of the production process.

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