brass

Special features

When materials with extreme mechanical and chemical properties are required and these merge with the advantages of HPDC, then brass die-cast is the material of choice. In particular, its high strength and excellent corrosion resistance are ideal for highly stressed cast parts that nevertheless need to be produced economically and precisely using the die casting process.
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The suitability of brass as a material for the HPDC process is often a source of surprise - triggered by the particularly high melting or casting temperature of the alloy in combination with the use of a permanent metal mould.
However, this should not be forgotten: Copper is one of the oldest materials on earth.
People recognised early on that alloys can be produced by skilfully mixing different metals, which then have better properties than the respective base materials. The first copper-zinc alloys were created in Palestine around 1400 BC by adding zinc carbonate during the smelting of copper. The origin of brass therefore dates back over 3000 years.
Around 150 AD, copper-zinc alloys were also produced in Germany using the same process. The industrial production of this alloy in Germany can be traced back to the 15th and 16th centuries.
Along with the development of different alloys, various casting processes have also emerged over the course of time. In the field of non-ferrous metals, the HPDC process is the most important today. Alloys based on the elements aluminium or zinc are traditionally used.
There is nothing wrong with this in principle, except that this statement is unfortunately incomplete:
Brass alloys are of great importance in die-casting and serve as materials with significant and unique properties.

brass

Casting temperature

In order for alloys to be processed at all, it is essential to liquefy them before their intended use. This takes place in a specially equipped crucible furnace with the aid of high heat.
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The alloy ingots, which are initially in the form of ingots, usually blocks with a trapezoidal cross-section, are charged into the crucible furnace and heated to such an extent that a melt is produced. The temperatures required to melt the respective alloy depend largely on the base material (or the main component of the alloy). Once the melting process is complete, the melt is kept at a defined casting temperature so that it can now be processed in the casting process.
While the casting temperature in the HPDC process for aluminium alloys is in a range of 640° C - 710° C and for zinc alloys between 410° C - 430° C, brass alloys can only be cast in HPDC from a temperature range of 960° C - 1050° C. The high temperatures involved in processing brass alloys place special demands on the process.
The permanent metal moulds used for the HPDC process, in whose cavities the cast parts are formed, are made of a special steel that is highly resistant and tough. This guarantees a long service life, which is important for the typical series production process that characterises the high-pressure die casting process.
In addition, the strength of alloys often depends on the melting temperature. In such cases, the strength of the material increases with increasing temperature.
This is also the case with the brass alloys used in HPDC. They have excellent strength properties and can therefore be used for cast parts that are subject to special and high strength requirements.

brass

Strength and corrosion resistance

If one assumes the achievable values for tensile strength without heat treatment measures in the HPDC process, an average of approx. 240 N/mm² is achieved for common aluminium alloys and approx. 300 N/mm² for zinc alloys. In comparison, brass alloys can easily achieve strengths of up to 350 N/mm² and special brass alloys up to 500 N/mm². It is therefore possible to combine and ultimately realise the thin wall thicknesses of the component, which are advantageous in HPDC, with increased strength requirements. Coupled with the aforementioned good corrosion resistance of brass alloys in drinking and industrial water and very good resistance to salt water in special brass alloys, the areas of application for these alloys are no longer limited to the sanitary sector. In fact, HPDC parts made of brass alloys are used in offshore installations and in the maritime sector, as complex thin-walled components that do not require expensive surface finishing lead to enormous potential savings. Isn't that a real benefit?
Definitely. And that's not all.
They can even be used under water without further coating. Even after years, the cast parts look almost unchanged from their original state. Comparable materials with similarly properties achieve these almost exclusively through enormous additional expenditure.
Either the material is very favourable in terms of strength, in which case it is often necessary to apply an appropriate surface coating or passivation. Or the corrosion resistance of the material is as desired, but the strength properties are inadequate.

Brass alloys processed by die casting close this gap and combine both properties in a good ratio.

brass

Surface roughness

As already mentioned, a permanent metal mould is at the heart of the HPDC process, as this is the only way to produce castings from the liquid metal. Today's established manufacturing processes make it possible to produce the cavities (contour-forming mould cavities) fully automatically. Either with milling machines or, in special cases, with special spark erosion systems.
The surfaces of the manufactured contours are very smooth and free of any traces that would indicate the manufacturing process. Due to the high casting temperatures of brass alloys, wear of the mould engraving surface occurs earlier than with other alloys in HPDC. This results in an increase in the average surface roughness to be expected, which is in the range of Rz 40 μm - 63 μm for brass castings.
Over time, the roughness of the mould engraving changes bit by bit. This also sounds logical, as the mould cavity is filled with liquid metal during each cycle, which solidifies under high pressure at the end of the casting process. The normal result is the onset of erosion of the previously smooth surface. To be clear, however, the increase takes place in very small steps, so that the production of series parts is not further impaired.
However, there is a second reason for the increased roughness: the melt itself solidifies into a coarser structure than with aluminium or zinc alloys, but still with a finer structure compared to other casting processes such as sand casting (Rz 100 μm - 150 μm).
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However, if brass HPDC is chosen, this small limitation in surface quality is usually not the sensitive issue. In this case, it is important to produce series parts that are as close to the final contour as possible and can be used without a great deal of mechanical machining or reworking. A clear advantage for die-cast brass!