Process for recycling light metal parts

The invention relates to a process for recycling light metal parts with gas inclusions or non-metallic particles.

The invention further has as part of its objects the use of die casting scrap material as well as a use of chips of passivating metals.

An efficient utilisation of resources is propagated nowadays not only from the point of view of environment protection but is also increasingly offered from the operational-economic point of view. High raw stock prices and disposal costs compel product manufacturers in various fields to reuse or recycle waste created within the framework of the production. For example, manufacturers of light metal components find themselves confronted with the requirement of purposefully utilising large quantities of waste from casting processes and chip-forming shaping processes.

In an optimum manner, a raw stock-saving and energy-saving utilisation of wastes yields high-value products in a few process steps. This obviously holds good also for the preparation and re-utilisation or recycling of light metal wastes. Depending on the origin of the waste, light metal wastes can have gas inclusions and/or non-metallic particles, which makes. it difficult or even impossible to integrate such wastes into a production process without complexity.

Light metal parts or wastes having gas inclusions that occur in large quantities, for example, during casting process, particularly during die casting, are not suitable for direct production of high-class light metal components on account of the mentioned gas inclusions. Therefore, in order to enable re-use even for light metal components of high class, such light metal parts are melted and the melt is degassed under overheating, so that after solidification of the melt one obtains a dense light metal that can be subsequently used e. g. in a casting process.

Just as in the case of light metal parts having gas inclusions, also in case of light metal parts having non metallic particles like magnesium chips, a cumbersome cleaning/purification of the material is necessary before the light metal material can be used for producing high-class components. Here one proceeds in such a way that the impure material is melted, the disturbing particles are removed from the melt and the melt is subsequently allowed to solidify. At a high degree of purity, a preliminary material thus obtained can be subsequently used for production of light metal parts of high quality.

Although it is possible to produce high-class preliminary materials for light metal parts from light metal parts with gas inclusions or non-metallic impurities, complicated cleaning/purification operations are necessary for this.

Based on these factors, it is the problem of this invention to present a process for preparing light metal parts with gas inclusions or non-metallic particles, whereby such waste products can be transformed in a simple process-technical manner into high class products, without any requirement of cleaning/purifying the material used.

This problem is solved by a process of the type mentioned above, characterized therein that from at least one light metal part with gas inclusions and at least one rather dense light metal part with non-metallic particles, a gas-containing metal melt is produced and the metal melt is allowed to solidify at vacuum for at least sometime under formation of a light metal foam body.

The advantages targeted by the invention can mainly be seen therein, that impure or contaminated light metal parts can be directly transformed into light-weight metal foam bodies in a simple process, without any necessity of cleaning/purifying the light metal parts used. Advantageously, in the process according to the invention, light metal parts with gas inclusions as well as rather dense, non-metallic particles are used, so that different contaminated light metal parts or types of light metal wastes can be re-used or recycled at the same time. The obtained metal foam bodies are suitable as energy-absorbing and sound-absorbing components for thermal insulation or as reinforcing elements in the automobile industry. Thus low class waste products can be transformed directly into high class metal foam bodies with multiple uses according to the invention.

With regard to the mechanism of metal foam formation, it is assumed that gas inclusions or non-metallic particles introduced into the metal melt by the respective light metal parts act together: through the gas inclusions of a first light metal part, the gas required for metal foam formation is brought in, whereby during melting of the light metal part the gas inclusions are retained in their form; similarly, the non-metallic particles present in the metal melt deposit themselves on the surface of the gas inclusions or gas bubbles due to energetic reasons, whereby these get stabilised and a coalescence of the same is prevented. Moreover, the non-metallic particles increase the viscosity of the metal melt and thus reduce a mobility of the gas inclusions or gas bubbles in the metal melt. This also reduces the tendency of the gas inclusions to rise up to the surface of the melt and exit there. Applying vacuum ultimately causes foaming of the gas-containing metal melt under formation of the light metal foam body.

As far as a comparison to traditional processes for producing metal foam bodies can be drawn, the special feature of the process according to the invention is that, neither are special measures required to introduce a gas nor is it necessary to carry out a complicated separate production of expanding agent components and light metal powder components, as is the case in melt-metallurgical or powder-metallurgical processes.

Basically, in a process according to the invention, light metal parts from different light metals can be used. However, it is favourable if light metal parts from the same metal or the same alloy are used. These light metals then melt within a small temperature interval, which makes conducting and control of the process simpler.

Furthermore, it is advantageous if a die casting scrap is used as light metal part with gas inclusions. Die casting scrap, for example in the form of so-called overflows that occur during die casting process, can have a share of gas inclusions of more than approx. 10 vol. %, which has proved to be useful with respect to high introduction of gas in the metal melt.

It is particularly advantageous if the die casting scrap part consists of magnesium or magnesium alloy. Parts of these materials are rendered passive on the surface, which leads to formation of magnesium particles or magnesium films. If such parts are subsequently used in a process according to the invention, then stabilising magnesium oxide particles can be additionally fed. Besides, even gas inclusions in the interior portions are rendered passive at the same time; this contributes positively to the stability of gas inclusions or gas bubbles in the melt.

Similarly, a part made of magnesium or a magnesium alloy can be used as dense light metal part, so that stabilisation of gas bubbles in the metal melt is mainly caused by magnesium oxide particles.

It is advantageous if the non-metallic particles are essentially oxide particles, as these particles behave uniformly inert with respect to reactions with light metal melts. Contrary to carbides, for example, in case of presence of such particles in a material used, it can be assumed that these particles are present almost unchanged in the formed metal foam body. Thereby even properties of the formed light metal foam body can be reliably influenced. Thus, on this basis, the content of non-metallic particles in the produced metal foam body can be controlled. If a particle content in the used material is known, then one merely has to select a weight ratio of light metal parts with gas inclusions to light metal parts with oxide particles according to the desired particle content in the metal foam body.

In the context of non-metallic particles it has been seen that it is advantageous if these have an average size of lesser than 200 μm.

Ideally, a metal melt is produced with a volume share of non-metallic particles of 2 to 10%. Above a particle content of 2% a good stabilising of gas inclusions/gas bubbles in the melt is attained; up to a particle content of 10% the gas-containing metal melts can easily be foamed up by applying vacuum: besides, increasing viscosity would make foaming of the metal melts difficult.

The metal melt is foamed and transformed into a light metal foam body by allowing it to solidify at an vacuum of 10 to 400 mbar, particularly 50 to 200 mbar.

It is also possible to additionally feed gas into the metal melt over the melt surface by application of gas pressure, in order to support subsequent foaming. However, process-technically and apparatus-wise it is particularly simple if the metal melt is produced at atmospheric pressure. In this case, during the entire process the atmospheric pressure is not exceeded.

After creation, the metal melt should not be overheated by more than 20° C. The viscosity of metal melt decreases with increasing temperature, which basically favours a mobility of gas bubbles and hence degassing. It would therefore be ideal to keep overheating of the metal melt controlled and low.

It is preferable if the vacuum a applied on reaching a temperature in the range of 5° C. above to 5° C. below the solidification temperature or the solidification interval of the metal melt. In this temperature range the fluid phase of the metal has a high viscosity, which proves to be favourable with respect to the structural stability of the formed metal foam.