New machine replaces complete casting line

The four semisolid metal casting processes thixomolding®, thixocasting, rheocasting and stress induced melt activation, as well as the cold chamber die casting, hot chamber die casting and other processes such as vacuum die casting, did not fulfill end user expectations for quality, simplicity, energy savings, people safety, economy and environmental cleanliness. This is where we step into the development of a new semisolid molding/casting machine.

Cogwheels — The new semisolid casting machine can also process any specialty alloys with the preferred globular chips.

Illustration of the energy input and the shot weight for a 430-gram-engine support bracket as a function of the process.

A recent study from Neue Materialien Fürth GmbH, Germany, comparing a number of currently used processes for energy consumption per part or per gram weight of the finished part, shows the great advantage of the thixomolding®. The new semisolid vertical casting machine without extruder but with fast-moving injection piston and with its mold heat recycling is promising much better results, showing reduced estimated direct machine energy consumption of 30 percent compared to a 220-tons-thixomolding® machine.

Sebastian Müller, a professor at the Institute of Casting Technology (FAU-LGT) in Fürth of the Friedrich Alexander University Erlangen-Nuremberg, produced a study on 2019 energy consumption of 190 light alloy foundries in Germany. The energy efficiency was 6,2 percent only. MAXImolding® technology can reduce energy consumption by 94 percent and material consumption by 50 percent at zero harmful emissions.

Magnesium processor for particulate scrubbing and conditioning

The magnesium levitation processor should take magnesium feedstock pellets and de-contaminate, degrease, dry and pre-heat them uniformly to a maximum of approximately 200 Degree Celsius. Feedstock temperature uniformity over conditioner volume must stay within plus/minus 5 percent from set point temperature as measured by 12 thermocouples placed strategically throughout the working volume of the conditioner.

The conditioning process should be under an inert gas – like argon, nitrogen or carbon dioxide – cover from room temperature to set point temperature. Precise feedstock metering should allow for material dosing within 2 percent from set point with repeatability plus/minus 5 grams.

3D rendering of a new MAXImolding Metal Injection Molding Machine — 3D rendering of a new MAXImolding® Metal Injection Molding Machine.

In the second stage of a new semisolid casting machine pre-heated magnesium particulates are fed into main casting machine that by adding heat converts the solid magnesium into semisolid slash of material. This material structure flows like honey and yet is at temperatures often 100 Degree Celsius below current decanting processing temperature resulting in significant energy advantage.

The new semisolid casting machine single step process simplifies the semisolid die casting process even further by not using complex extruder as seen in Thixomolding® machines, but globular semisolid slurry is produced by melting mechanically stressed magnesium chips under the sole influence of external heat. No shear of chips in extruder is necessary. The cold deformed structure of mechanically comminuted chips is of the same nature as that reported for a Thixoforming® billet, produced by the stress induced melt activation (SIMA).

Ensuring high structural integrity and temperature uniformity

A clear step forward in the creation of high integrity castings is achieved by using extruded, cold rolled or mechanically stressed magnesium chips, instead of billets as seen in thixocasting, to create spherical micro structured semisolid slurry for injection into a permanent mold. Ordinary when chipped can be processed directly into semisolid slurry only by reheating chips to semi solidus temperature due to mechanical residual stress in the chip caused by the chipping process.

The fractured magnesium chips are made from standard die casting alloy AZ91D, which is readily available on the market. The oval shape and size of the chips improve feedstock flow and heat energy absorption due to high surface area. The new machine feeds the chips under a stuffer rods. The stuffer rods are pounding chips into the stuffer cylinders used for heating chips.

The stuffer rods with stuffer cylinders are placed co-axially around the injection piston to ensure high structural integrity and temperature uniformity, critical for slurry generation and magnesium processing. Material is heated to semisolid temperature very quickly by resistive or preferably inductive heaters, and slurry transfer under the injection piston is done in the center of the same high thermal mass block to ensure stable process temperature.

Slurry, in the form of ice-water slash, is transferred under a fast-moving piston designed to inject the semisolid slurry into a permanent mold to form the part.

Mold cooling results in process savings

Maintenance of the new semisolid casting machine is simple. No material hang-ups are possible. Semisolid slurry is fully contained within the machine. The mold is now capable of casting millions of parts due to reduced slurry temperatures. A significant advantage over any other die casting process is achieved because a separate and pot for slurry generation is no longer required. All is done in the same machine, within a fast cycle of 15 to 60 seconds.

Mold cooling is accomplished by water mist system (80 percent air, 20 percent water) circulating just below mold surface with very high cooling rates. Water air generation in closed cooling channels absorbs a large quantity of heat from the solidifying part and is in turn used to pre-heat the incoming magnesium chips. This results in significant process savings as well.

State of the art today is, that all the heat removed from the parts is dumped into the environment. Mold heat in all current processes is also dumped into the atmosphere and no recovery of heat is possible. That is due to liquid cooling with a very small differential in temperature, which is inefficient to the heat recovery process because a small differential in temperature requires in turn, a large volume of cooling medium.

Operating at temperatures just above solidus temperature

The part is then subjected to evaporative cooling with a very high rate of heat removal by water vapor and air mixture circulating just below mold surface. This high rate of cooling supports freezing the globular structure of the part. This cold-to-cold and solid-to-solid (S2S) semisolid magnesium-processing casting machine features several key advantages compared to other die casting or magnesium forming machines.

The major advantage is that the new semisolid magnesium machine operates at temperatures just above solidus temperature, for example 480 Degrees Celsius (896 Degrees Fahrenheit) to 580 Degree Celsius (1.076 Degrees Fahrenheit). This results in great energy savings when compared to molten material temperatures at 600 Degree Celsius (1.112 Degrees Fahrenheit) to 700 Degree Celsius (1.292 Degrees Fahrenheit) in processes today.

In this process, the heating of the magnesium begins at room temperature and rises to the processing semisolid temperature. The new semisolid casting machine can also process any specialty alloys with the preferred globular chips.

Sketch and MEGApress with multiple injectors for mega castings — MEGApress™ with multiple injectors for mega castings: used to produce large area, thin wall castings.

New level of casting

The cylindrical structure and coaxial arrangements of the piston and the plunger, surrounded by the processing stuffers, allow for processing at very narrow temperature windows and with tight temperature controls. Computer-controlled heating and high-speed injection at pressures of 15.000 pound per square inch (1.034 bar) allow a new level of casting, resulting in parts with properties that meet the requirements for use in high integrity applications. Energy savings are significant when compared to previously described casting processes.

The process is fully enclosed solid to solid part and machine could be operating in ordinary manufacturing plant. In order to produce large area, thin wall castings (less than 1 millimetre) multiple injectors above huge pulldown press with mold are used.

Illustration of the MAXImolding reactor — MAXImolding® reactor (vertical) on a cold chamber die casting machine.

Thixocasting 2.0: from chips to parts

The authors propose an innovative Thixocasting 2.0 process for casting lightweight metal parts. Thixocasting 1.0, as is known, involves reheating pre-cast billets to a semi-solid temperature, loading them into the shooting pot of a cold chamber die casting machine, and injecting them into the mold under high pressure. While Thixocasting provides advantages such as lower forming temperatures, longer die life, high part precision, efficient production, and improved mechanical properties, it requires the use of special, expensive feedstock (billets) which restricts the production of larger parts.

The newly proposed Thixocasting 2.0 process circumvents the need for costly feedstock (billets), enabling on-demand production of semi-solid slurry directly on the machine. The process commences with standard magnesium, aluminum, zinc-alloy ingots, or those suitable for semi-solid processing, which are fed into a chipping machine to produce metal particles of specific sizes. These chips are then transported via a vacuum-sealed pipe system to a hopper where inert gas is added to scrub contaminants.

Next, the chips are placed in a MAXImolding® reactor, heated from top to bottom, and according to specific thermal profile, to generate the semi-solid slurry. The slurry is then transferred to the shooting pot of a heated (now hot chamber) die casting machine and injected into the mold horizontally. Now, we get thixo-casted parts on a standard and well-known machine. This is a simplified casting for high integrity light alloy parts with machinery in place.

After forty years of experience and applied research, it can be said that the die casting industry is ready for a fully automated self-learning digital 21st century casting foundry, using fully automated data feedback from the x-ray machine.

You want to know more?

This is the fourth part of a series of articles by Ashley Stone and Edo Meyer that will take you through the development of the digital foundry and give you an overview of the state of the magnesium die casting industry. In the fifth part, you will learn all about unified and patented process control, which together with the semisolid casting machine and fully automated real-time inline x-ray inspection machine results in high integrity parts with minimal casting defects.

Did you miss the part before? Find the previous article here.

Or go on and read the next part of this series!