Pouring molten metal is a precision process during which pouring speed (to achieve high throughput) must be balanced by effective flow control (to guarantee reliable mold filling.) And both of these must be adjusted according to the specific metal alloy, to ensure the desired metallurgical quality in the finished casting. Typically, for aluminum alloys, low-pressure casting is used to shield the molten metal from impurities that may be picked up during the transfer from the furnace chamber to the mold. The furnace is pressurized so that molten aluminum moves slowly through a riser to the mold or die, at a constant rate, avoiding oxide formation, entrapped air bubbles, or "cold currents."
Fill Machine Engineering's new contribution to the low-pressure casting process involves a magnetic field and short-circuit current at the end of the riser, just before the molten metal enters the mold or die, to ensure precision low-turbulence aluminum casting.
Fill, an Austrian company, is offering the process as an aid to productivity for high-volume casting of aluminum, and particularly for automotive series production. The new concept originated from a project seeking to improve control and regulation for low-pressure casting machines. The development also studied effective ways of slowing ("braking") the mold-filling speed, depending on the shape of the cavity.
In the process, pouring pressure is adjusted according to the level of the furnace bath. Lowering the bath level must be compensated for by the control system, so that the filling level and the rate of pressure increase relative to the volumetric flow rate of molten metal.
Expanding the mold cavity will cause aluminum to rise through the riser tube. If the molten metal level remains constant, the flow velocity increases. Expanding the cavity, especially after the sprue bush, results in a considerable increase in flow velocity in the riser. This increase may lead to turbulence in the molten metal.
In order to achieve low-turbulence mold filling, the molten metal must be actively decelerated, and experience has shown that casting filters or sieves that hold back oxides also help to slow the metal's flow. However, sieves introduce consumable cost as well as process and maintenance costs to pouring operations that incorporate them. Fill's development is an active flow decelerator that can be integrated into the regulation and control technology, meaning that casting sieves can be omitted if slowing the molten metal is their only purpose
Moving an electrical conductor (aluminum) through a magnetic field causes eddy currents to be generated in the conductor. This slows the flow of molten metal by forming an opposing field. The effect of this eddy-current brake depends on the speed: If the flow velocity in the riser increases, so does the braking effect. When the cross-section of furnace and mold cavities changes significantly normal to the filling level, the magnet box decelerates the molten metal flow and low-turbulence mold-filling begins. This is a self-regulating process that allows low-turbulence mold filling, regardless of complicated pressure curves that may have to be adjusted according to the mold cavity.
The magnet box also compensates for errors resulting from shifts in the pressure curve, depending on the filling level.
The magnetic field is controllable too, meaning that its field strength can be activated and changed from 0 to 500 mT. This makes it possible to set targets for the casting process. For a defined braking effect and casting process, it allows permanent magnets to be used — an important option for high-volume and series production.
Figure 2 shows the reduction in flow velocity in the riser tube over time at a constant rate of pressure increase. The entry to the die is indicated on the time axis. The riser tube with an inner diameter of 60 mm expands into a cavity with an inner diameter of 200 mm. The magenta line (B=0) indicates an overshoot and an increase in average flow velocity in the riser to over 500 mm/s at a constant rate of pressure increase of 7 mbar/s. With an activated field, the braking effect results in a reduction compared to the maximum to 270 mm/s without any overshoot.
As well as the eddy-current brake caused by the magnetic field, Fill Machine Engineering's magnet box is designed so that a short-circuit current can be sent through the molten metal, transverse to the magnetic field. And, the current density caused by the short-circuit current, perpendicular to the magnetic field, increases the braking effect on the molten metal to a significant extent.
The ideal braking effect under short-circuit current can be seen in Figure 3. A permanently applied short-circuit current induces a time lag in the braking effect due to the field (blue line.) If, on the other hand, the short-circuit current is ramped up linearly from the die ingress to 200 amps, as shown in figure 3 (orange line), this yields an even greater braking effect that can be activated by the magnet box.
The technical design permits magnetic flux densities of up to 500 mT and short-circuit currents up to 400 A.
Thanks to the new process parameters of field strength and short-circuit current, and their regulation, in addition to the pressure increase in the new magnetic induction casting process, a simulation of the magnet box has been developed. The Helmholtz Center in Dresden-Rossendorf (HZDR) used OpenFOAM computational fluid dynamics software to simulate an implementation of the magnetic field and short-circuit current.
The CFD graphic illustrates a simulated casting process in a cylindrical cavity: on the left with braking effect, on the right without braking effect. Simulation makes it possible to optimize the process parameters, for example by optimizing the details that indicate oxide inclusions or critical areas during filling. The purpose of the magnet box is to improve the performance of the process and quality of the products being cast. In cases involving new products, simulating the casting process makes it possible to improve settings through virtual testing and to determine the optimal casting parameters. Potential problems can be identified during production planning thanks to the options that may be evaluated during simulation.
In addition to benefits to low-pressure casting process control, the developers propose there will be cost advantages from the reduced total of scrap castings, as well as lower operating expenses due to more efficient use of resources. Fill also forecasts OpEx savings due to lower material consumption (specifically, the omission of casting sieves) and more "optimizable control technology.
Currently, the magnet box is being further developed to enhance its use as a sensor to supply data on actual filling speeds. This means, according to Fill, that it should be possible to gain more detailed insights to the new process, using production data together with conventional machine data.
A fully equipped Fill low-pressure casting machine with magnet box was supplied to SRI Livarski Inženiring d.o.o., a foundry in Slovenia, that will conduct equipment tests and small-scale casting operations.Also, an independent research center, Access e.V. at RWTH Aachen University in Germany, is evaluating the new technology for low-pressure precision diecasting — an effort that Fill develop anticipates will have potential for further integration of digital simulation tools to the pouring process.