First industrial production of ZnAl15Cu1Mg
Since 2014, when the first hot chamber die casting tests were carried out with the hypereutectic ZnAl15Cu1Mg as part of the UNIBS & partner project with the Lombardy Region, numerous tests have been carried out to manage the various issues concerning the construction of a hot chamber die casting machine suitable for casting the alloy.
In 2018, after the resumption of the project for the hot chamber die casting of ZnAl15Cu1Mg, Studio Olafur Eliasson carried out the definitive feasibility test in an artistic project by MORI (WT), in Tokyo. The choice of a zinc alloy was born from the idea of recovering the main metal, zinc, from domestic incineration dust.
Since the artwork is a multi-volume object suspended from the ceiling and must be earthquake-proof, numerous mechanical simulations were carried out that highlighted the need for a higher performing alloy in terms of viscous flow (or creep) and mechanical resistance, so the choice fell on the use of the ZnAl15Cu1Mg alloy compared to the classic zinc alloy for die casting.
After verifying the stability of the alloy in the hot chamber die casting machine, a series of mechanical test samples were cast to verify their overall properties. The recyclability of the alloy was demonstrated in another series, so that normal production cycles could be achieved.
The casting of the cores of the artwork highlighted the possibility of expanding the range of applications of the zinc alloy cast in the hot chamber machine.
Introduction on industrial production ZnAl15Cu1Mg
Since 2018, after four years of intensive testing of the performance of the ZEP® 1510 alloy under different conditions in the hot chamber die casting machine, it was decided to identify the suitable project to start a full industrial series. This was achieved in conjunction with an artistic project by the Art & design studio SOE in Berlin, with the creation of a series of four die casting sculptures to be hung from the ceiling of a building in Tokyo, Japan. The sculptures consist of 7000 eleven-sided bodies. The individual bodies are die-cast and should be made of recycled zinc alloy, obtained by electrolysis of zinc recovered from dust from household waste incineration.
Result of hot chamber die casting of zinc and aluminum
Since the artwork is suspended from a ceiling, issues such as weight are extremely important in choosing the alloy to use for die casting. Classic ZL5 (zamak5) has a density of around 6.7 g/cm³, so the sculpture could experience creep under its own weight. For this reason, the new ZEP® 1510 alloy was chosen, which reduces the weight by around 20% and has a creep resistance 100 times higher at room temperature. The improvement in mechanical resistance (20% more than Zamak 5) would also allow the designers to expand the concept.
Before the series could be realized, however, it was necessary to carry out further tests in the foundry, including:
- The stability of the alloy during a longer casting period;
- The actual mechanical properties of the alloy to ensure its stability during aging;
- The recyclability of the alloy, which involves the reuse of the sprues and casting burrs.
These three series of castings were performed in the laboratory of Oscar Frech GmbH.
First, on a small Frech W20 hot chamber die casting machine with a 12-cavity mould to check the castability of the alloy and to identify the limit setting of the parameters. It soon became clear (5000 injections in total in one casting week) that the casting temperature had to be increased to 530°C to make the alloy fluid enough to allow good cavity filling.
At the same time, the recycling ability was tested using three weight ratios of virgin to recyclable material (25% /50% and 100% recycled content of the casting). Although the alloy remained stable, the Fe increased due to contamination of the crucible content when an already recycled alloy containing a higher amount of Fe was introduced. The analyses were performed by spectral analysis at Grillo Werke AG on samples taken from different parts of the crucible.
The second series was performed on a Frech W80 hot chamber die casting machine, in order to cast a sufficient number of mechanical samples (2000 injections) in order to start an accurate verification of the mechanical performances and compare them with the normal hot chamber die casting material Zamak 5.
The mechanical properties of 3 mm flat samples (UTS/R0.2%/modulus of elasticity/elongation) with the alloy in as-cast, artificially aged (2 hours at 105°C) and heat treated (4 hours at 150°C) conditions were verified against the same properties as Zamak5. Meanwhile, the verification of the properties during natural aging revealed a modest aging tendency compared to that known for Zamak5. The aging of ZEP® 1510 over the last 3.5 years appears linear.
The analysis therefore found that the alloy is stable enough to initiate the 10,000 injections required to obtain the volume needed for the SOE artwork bodies.
The series was cast on Frech W125 with a double cavity mold. The casting conditions were set at a temperature of 530°C for the crucible and 170°C for the mould, with an injection speed of up to 40m/s at the mouth of the cavities. This was done at the same time as the recycling of the sprues. In order to limit the temperature loss, it was necessary to cut the sprues as soon as possible to recover the residual heat. At the same time, the stability of the alloy was checked by taking three samples per day in order to ensure the quality of the molten product. The entire casting of the series would last more than six weeks and would also indicate the aggressiveness of the alloy with respect to the crucible, the gooseneck, the elastic bands and all the equipment immersed in the alloy during the period under study.
The study verified the stability of the alloying element Al present in the composition of the alloy itself and indicates that the latter can be used and is suitable for hot chamber equipment without particular risks of decomposition.
The first series of casting products on a hot chamber die casting machine has shown that the ZEP® 1510 alloy is suitable for industrial use. The alloy remains stable as long as all parts immersed in the crucible are protected with ceramic paint. Piston rings must be considered as consumables and have a lifespan of approximately 1000 injections. The high melting temperature of the alloy in the crucible requires a high-performance heating system in order to ensure the heat supply necessary to melt the alloy ingots and keep up with production.
Source: In Fonderia – ll magazine dell’industria fusoria italiana