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Die casting is the most common method of fabricating magnesium alloy parts. Die casting foundries can manage the amount of process scrap in three different ways. Scrap can be sold on the open market however the metal is often times downgraded for use in the desulphurization of steel or other markets. Due to the high dependence on alternate material supply, the prices paid for scrap vary considerably. Foundries can also recycle magnesium scrap either internally or externally. Often times, this is the most cost effective method of controlling scrap quantities. The process scrap that is generated in die casting is kept within a closed loop system reducing demand of primary material by up to 50%.
There are several factors that dictate the amount of processed scrap that is produced and recycled and the amount of primary metal that is utilized. The ratio of scrap to product for a single shot, the amount of material lost in the melting cycle, the quantity of different components that are cast, the percentage of cast parts that must be rejected during production, the end quality of process scrap, and the recycling operation efficiency all affect the amount of process scrap and primary magnesium utilized.
Methods for Recycling
Magnesium alloys, even those used for structural applications, can be recycled back into products displaying the same chemical, physical, and mechanical characteristics as primary metal. Recycling magnesium alloys only requires 5% of the energy required to produce primary magnesium alloys.
Recycling is actively encouraged within the industry given its positive impact on the environment and life cycle analysis studies looking at greenhouse gas emissions for example, Prentice (CSIRO) and Ehrenberger (DLR) take into account the end of life recovery of material to obtain a full perspective on the lifetime environmental benefits from using magnesium.
Currently the major source of magnesium alloy scrap is from the magnesium die-casting industry. The die-cast parts normally include the alloy designation stamped on the part by the die, therefore aiding good segregation not only of process scrap but also at end of life by scrap dismantlers.
For both in-house (at die casting plant) and external recycling, a range of methods can be used involving flux refining after melting, fluxless melting and settling, or fluxless melting and gas sparging to produce high quality recycled ingot for reuse in die casting. Larger scale recycling operations permit economies of scale in processing and greater sophistication in control of melt quality and composition which is equivalent to standard primary ingot.
Re-melting of magnesium chips from machining of die castings is another common recycling process however due to magnesium’s susceptibility to oxidation this process suffers higher melt losses than for solid scrap. Generally the chips require some aqueous washing treatment to remove machining lubricants. Alternatives to feeding the material into molten magnesium involve the pressing into briquettes which allow the chips to be used as an alloying material for aluminium as an alternative to primary metal. It has been demonstrated that these pucks may be directly hot extruded as a solid state recycling method; since the metal is not melted, a special protective environment is not required. A study conducted at the Harbin University of Science and Technology in China showed that solid-state recycling of magnesium alloy chips is an efficient method of recycling.
The Brunel Centre for Advanced Solidification Technologies (BCAST) developed the Melt Conditioned High Pressure Die Casting (MC-HPDC) process for recycling high quality magnesium cast components. This process consists of imposing intensive shearing directly to the alloy melt before it is poured into the die. The MC-HPDC process was shown to be an excellent candidate for physical recycling of high grade magnesium alloy scrap. The castings produced had consistent ultimate tensile strength and elongation properties that were comparable to those of the primary material tested.
The same quality criteria in regards to chemical composition and oxide content must be met for both recycled alloy ingots and primary metal. Processing of End of Life Vehicles is most commonly done through shredding for economic reasons. This method results in the mixing of different magnesium alloys as well as their integrated elements. Shredded magnesium can be contaminated with iron, nickel and copper from coatings and fasteners all of which are detrimental to the corrosion resistance of the metal. Through the addition of manganese, the levels of iron can be reduced. Nickel and copper on the other hand may only be controlled through distillation or dilution. Re-melting alloys consumes at the most, 50% of the energy that is required for distillation.
Secondary alloys are being developed for creation of new components from re-melted scrap metal with a minimum amount of primary metal while still achieving the desired and needed composition of the alloy. A recent study completed in Germany shows that it is possible to address the problems associated with mixed alloying elements and impurities in recycling within a single alloy system. These results promote recycling of ELVs through re-melting which uses minimal primary metal and energy.
Despite the existence of various current methods of recycling magnesium, there is still room for further improvement. The further development of recycling methods is becoming a challenge in the technical, economic and environmental fields. See the Magnesium Recycling Study in the EU published in 2017 for further discussion on this topic.