Aluminium casting- alloy comprising 6-9% by weight of silicon, 0.6-1% by weight of iron, 0.5-1.5% by weight of copper, 0.01-0.5% by weight of manganese, 3-5% by weight of zinc, 0.05-0.5% by weight of magnesium, 0.01-0.15% by weight of titanium, 0.01-0.1% by weight of chrome, 0.01-0.1 % by weight of nickel, 0.01-0.1% by weight of lead and 0.01-0.1% by weight of tin.
The field of the invention refers to aluminium casting alloys. Specifically, the present invention relates to a secondary aluminium alloy, useful to produce, by high pressure die casting, components which have to fulfill premium mechanical requirements in as-cast condition.
High pressure die castings have been traditionally limited to transport applications in which its structural functionality was of low responsibility, whereas the components with key structural responsibility have been traditionally manufactured with steel or aluminium alternative production processes, i.e, low pressure die casting (LPDC) or gravity die casting (GC).
However, high pressure die casting (HPDC) process has been dramatically expanded to new applications in the last twenty years due to its low cost per produced component ratio, high components reproducibility and reliability. To expand the HPDC process, besides the HPDC technological development (vacuum casting, improved mold materials and thermal management, etc.), new alloys with new metallurgical and microstructural properties have been developed, which must present high fluidity to fill the whole mold conveniently, low die soldering, easy weldability, high machinability and above all, high elongation and mechanical properties.
Alloys of primary quality with a Fe/Mn ratio of ½ has been disclosed in the prior art, and decrease die soldering and reduce as much as possible the negative effect of Al5FeSi intermetallics on the elongation values. Primary quality means mainly iron content below 0.15% by weight, copper content below 0.03% by weight and zinc content below 0.1% by weight, being those contents only achievable if aluminium is directly produced by smelting electrolysis from raw alumina. All refined aluminium produced from scraps, drosses and swarfs coming from post-processing operations and end of life products is hence limited to low mechanical responsibility applications what is a large limitation for the industry sustainability and aluminium recycling sector. Finally, the casted component made of primary aluminium can be thermally treated if desired, in order to reach mechanical properties similar to those produced in alternative manufacturing processes as the LPDC or the GC.
Unfortunately, heat treatment, which is mainly useful for the AlSiMg and AlCuTi aluminium alloys family implies costs increase and a new heat treatment facility in addition to the already existing holding furnace and injection machine. Thin walls distortion and stresses appearance is more than probable for complex castings hindering the manufacturing. Blistering can take place as well on the casts surface if no adequate mold filling and vacuum technique is performed what requires skilled technicians.
Some other alloys of the AlMg family have been later developed to eliminate the thermal treatments, but always with a common characteristic, i.e. keeping very low percentages of impurity elements as iron, copper and zinc among others, only achievable by primary alloys.
The problem to be solved is the provision of a novel alloy of secondary quality produced for HPDC which can be used in as-cast condition and that presents the following values of elongation and mechanical properties: elongation (A) equal to or more than 5%, yield strength (Rp0.2) equal to or more than 120 MPa and ultimate tensile strength (Rm) equal to or more than 200 MPa. Said values of elongation and mechanical properties are required for safety components when they are designed to support crash impacts (high energy absorption, i.e large deformation) or/and large static bending loads (high strength). The alloys of the invention also maintains other processability properties as the alloy fluidity, low soldering to the die, easy welding or high machinability, among others.
A preferred embodiment of the present invention is an aluminium casting alloy, wherein said alloy comprises:
Silicon content has been restricted into a low range between 6-9% by weight to reduce as much as possible the eutectic fraction what helps to maximize the elongation but maintaining the fluidity at minimal values that allow an adequate mold filling.
Copper content has been restricted into a minimum at 0.5% by weight to guarantee a minimum elastic yield and ultimate tensile strength.
Iron content has been limited to 0.6-1% by weight to guarantee both low mold soldering and small volume fraction of Al5FeSi intermetallics, which at the same time are minimized by the manganese content.
Manganese content helps to transform the Al5FeSi intermetallics into alpha-Al12(Mn,Fe)Si2 to reduce as much as possible the negative effect of those intermetallics. Values of manganese above 0.3% by weight were not found to be useful in terms of Al5FeSi intermetallics transformation.
Magnesium content helps to increase the yield strength, but always having a minimum percentage of copper because otherwise elongation can be affected.
A further embodiment of the invention is the aluminium casting alloy of the invention, wherein said alloy comprises 7-8% by weight of silicon.
A further embodiment of the invention is the aluminium casting alloy of the invention, wherein said alloy comprises 0.6-0.8% by weight of iron.
A further embodiment of the invention is the aluminium casting alloy of the invention, wherein said alloy comprises 0.7-1.4% by weight of copper.
A further embodiment of the invention is the aluminium casting alloy of the invention, wherein said alloy comprises 1-1.4% by weight of copper.
A further embodiment of the invention is the aluminium casting alloy of the invention, wherein said alloy comprises 0.05-0.3% by weight of manganese.
A further embodiment of the invention is the aluminium casting alloy of the invention, wherein said alloy comprises 3.5-5% by weight of zinc.
A further embodiment of the invention is the aluminium casting alloy of the invention, wherein said alloy comprises 0.1-0.3% by weight of magnesium.
Aluminium compositions have been prepared by melting a standard EN-AC 46500 alloy in a holding furnace at 690°C and later poured into the injection vessel, being injected into the mold cavity of a 950 tonnes closing force HPDC machine at 685°C. No vacuum conditions were applied.
A serial of 30 specimens were produced, for each composition. Casted specimens were cooled down in air. Specimens dimensions and later mechanical characterization were set and carried out following, respectively, UNE-EN ISO 6892-1 B:2010 standards.
Several compositions were tested, the content of the content if specified in Table 1. The obtained results are also specified in Table 1.
The values obtained after one month of natural aging is shown in Table 2.