ALUMINIUM CASTING ALLOY

Abstract

 Aluminium casting alloy consisting of 8-9% by weight of silicon, 0.29-0.4% by weight of iron, less than 0.2% by weight of copper, 0.2-0.45% by weight of manganese, less than 0.3% by weight of zinc, less than 0.3% by weight of magnesium, less than 0.2% by weight of titanium, less than 0.05% by weight of chrome, less than 0.05% by weight of nickel, less than 0.05% by weight of strontium, less than 0.05% by weight of lead, less than 0.05% by weight of tin, and aluminium as the remainder.

Description

FIELD OF THE INVENTION

[0001] 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 fulfil premium mechanical requirements in as-cast condition.

BACKGROUND ART

[0002] 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).

[0003] 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.

[0004] Alloys of primary quality with a Fe/Mn ratio of ½ has been disclosed in the prior art, decreasing die soldering and reducing 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.

[0005] Unfortunately, heat treatment, which is mainly useful for the AlSiMg and AlCuTi aluminium alloys families 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 cast surface if no adequate mold filling and vacuum technique is performed what requires skilled technicians.

[0006] 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.

[0007] Document DE 19524564 discloses an aluminium-silicon alloy for casting cylinder heads. Minor variations in the composition of the alloys produce a change over the different proprieties of the alloys. By a minor addition of alloying elements or by varying the concentration of an alloying element, non-expected properties can be obtained. This document is silent about the obtained mechanical properties of the alloy and it doesn't mention the high pressure die casting (HPDC) process. This document discloses an alloy with a 5-11 wt % and 8-11 wt % of Si and 0.8-2 wt % of Cu.

[0008] Depending on the process employed to produce a part, the mechanical properties that can be achieved change completely, as shown in the DIN 1706 Standard, where mechanical properties change for sand, permanent mould casting, pressure die casting (HPDC) and investment casting.

[0009] Annex A of standard EN AC 43000 series discloses mechanical properties of pressure die cast alloys (Table A.1 - Mechanical properties of pressure die cast alloys).

[0010] Document EP 1978120A1 discloses an aluminium-silicon alloy for engine components. In this document there are no references to the HPDC process. This documents discloses very low elongation values of the obtained samples at room temperature in the as cast state (<0.7%). All the samples disclosed in this document have Si values with an eutectic or hypereutectic composition well above 9% by weight. This document also discloses an alloy with a 5-25% by weight of Si and 0.0007-0.1% by weight of C.

[0011] Secondary aluminium alloys disclosed in the prior art have limited elongation properties due to the presence of detrimental β-iron Al5FeSi needles. The prior art discloses different ways of suppressing the formation of β -Al5FeSi phase: addition of sufficient manganese and, in alloys without manganese, high cooling rates. Another way to avoid this problem is based on the development of primary aluminium alloys with small percentages of iron, as the Aural™ alloys with iron approximately less than 0.22% and 0.03% by weight of copper. It has also been disclosed alloys with high elongation with less than 0.2% by weight of iron content and others. It has also been disclosed limiting the silicon content to a maximum of 0.15% in weight in order to obtain high elongation alloys.

[0012] Document US 5573606 discloses addition of Mg and limiting the iron content to less than 0.6% by weight.

[0013] Document EP 2771493A2 discloses an AlSiMgCu casting alloy. This document discloses 0.5-2% by weight of copper and discloses the use of thermal treatments. This document discloses that an increasing Cu content can increase the strength due to higher amount of θ'-Al2Cu and Q' precipitates, but reducing the ductility. This document aims to optimize the alloy composition, the solution and aging heat treatments to minimize/eliminate un-dissolved Q-phase (AlSiMgSi) and maximize solid solution/precipitation strengthening.

[0014] Document JPH093610 (A) proposes a die-casting alloy having 5 to 13 wt % Si, up to 0.5 wt % Mg, 0.1 to 1.0 wt % Mn, 0.1 to 2.0 wt % Fe. In this document, Cu and Zn contaminants are not taken into consideration, as these usually occur in significant amounts in the case of secondary aluminium. The document discloses that thermal treatments are necessary to improve ductility because eutectic Si becomes roundish by heat treatment.

[0015] Document EP2657360 discloses a die casting alloy consisting of 6-12% by weight of Si, at least 0.3% by weight of iron, 0.25% by weight of Mn, 0.1 % by weight of Cu, 0.24 to 0.8% by weight of Mg and 0.4 to 1.5% by weight of Zn. This document discloses the use of eutectic modificators, as Sr, Na and Sb, alone or in combination, and grain refiners as Ti, Zr, V.

[0016] Document EP 1612286 discloses an aluminium die casting alloy having 8 to 11.5% by weight of Si, 0.3 to 0.8% by weight of Mn, 0.08 to 0.4% by weight of Mg, max. 0.4% by weight of Fe, max. 0.1% by weight of Cu, max. 0.1% by weight of Zn, max. 0.15% by weight of Ti and 0.05 to 0.5% by weight of Mo. Cu and Zn content have been limited and the content of secondary aluminium is very restricted, which leads the production of the alloy by electrolysis.

[0017] 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 130 MPa and ultimate tensile strength (Rm) equal to or more than 230 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 maintain other processability properties as the alloy fluidity, low soldering to the die, easy welding or high machinability, among others.

[0018] The problem to be solved can also be defined as providing an alloy with high elongation values obtained for HPDC processing with secondary melted aluminium alloys.

SUMMARY OF INVENTION

[0019] The present invention provides an aluminium casting alloy, wherein said alloy is consisting of:

8-9% by weight of silicon,

0.29-0.4% by weight of iron,

less than 0.2% by weight of copper,

0.2-0.45% by weight of manganese,

less than 0.3% by weight of zinc,

less than 0.3% by weight of magnesium,

less than 0.2% by weight of titanium,

less than 0.05% by weight of chrome,

less than 0.05% by weight of nickel,

less than 0.05% by weight of strontium,

less than 0.05% by weight of lead,

less than 0.05% by weight of tin,

and aluminium as the remainder.

[0020] In the invention, silicon content is restricted to the range 8-9% 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.

[0021] In the invention, copper content is restricted to less than 0.2% by weight to guarantee a minimum elastic yield and ultimate tensile strength.

[0022] In the invention, iron content is restricted to 0.29-0.4% 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.

[0023] In the invention, manganese content is restricted to 0.2-0.45% by weight to transform the Al5FeSi intermetallics into alpha-AI12(Mn,Fe)Si2 to reduce as much as possible the negative effect of those intermetallics, and to avoid the sludge problem that occurs with high percentages of Mn in combination with Fe and other alloying elements.

[0024] In the invention, magnesium content helps to increase the yield strength, but always with a minimum percentage of copper and iron to avoid elongation to be affected. For small increases of magnesium percentages if enough silicon is available Mg2Si intermetallics can be produced.

[0025] In the invention, zinc content helps to achieve larger elongation values at low magnesium contents taking advantage of its high solubility index, what means that for contents less than 0.3% by weight of zinc, larger elongation values can be reached since no matrix discontinuity appears.

[0026] The desired properties are obtained due to the formation of a very fine eutectic phase, the semi-globular shape of the dendrites and the absence of fragile β-iron needles in the HPDC samples due to the combination of the different elements with the iron in the new developed alloy. It can be observed in Figure 1 an example the described micro-structures with some porosity inherent to the standard HPDC process at x25 augmentations.

[0027] It can be observed in Figure 2 with x400 augmentation the absence of large β-iron needles.

[0028] The alloy according to the invention differs from the alloy of DE 19524564 in that it contains 8-9% by weight of silicon and less than 0.2% by weight of copper

[0029] In the present invention, the minimum elongation value obtained in the as cast state is 5,25%. However, in Annex A of standard EN AC 43400 the mechanical properties for the alloy are limited in the as foundry state to low mechanical values (less than 1 %).

[0030] The content of the alloying elements in the alloy according to the invention is related to the obtained mechanical properties of the alloy. These mechanical properties clearly vary with small changes in the composition. This can be seen in the alloys of the example, which shows changes of the properties with minor composition variations.

[0031] The alloy according to the invention differs from EP 1978120A1 in that it contains 8-9% by weight of silicon and that it does not contain C.

[0032] The alloy according to the invention differs from EP 2771493A2 in that it contains less than 0.2% by weight of copper. The concentration of copper in the alloy according to the invention lead to an increase in the elongation, in comparison with the values mentioned in EP 2771493A2, which discloses that an increasing Cu content can increase the strength due to higher amount of θ'-Al2Cu and Q' precipitates but reducing ductility.

[0033] A thermal treatment of the alloy according to the invention is not necessary, due to the appearance of a very fine eutectic and a quite globular dendrite structure in the alloy. The reduced content of Cu and Zn in comparison with the alloy of document JPH093610 (A) avoids the use of secondary aluminium as disclosed in JPH093610 (A).

[0034] The alloy according to the invention differs mainly from the alloy of document EP2657360 in that it contains less than 0.3% by weight of Zn. An increase in the Zn percentage leads to a lower corrosion resistance, and because of that, the Zn percentage has been limited in the alloy according to the invention, in order to obtain parts that don't need extra surface treatments. Also, the alloy according to the invention has high ductility.

[0035] The alloy according to the invention differs from document EP 1612286 in that it does not contain Mo. Also, the alloy according to the invention has a final elongation value higher than 5%.

BRIEF DESCRIPTION OF DRAWINGS

[0036] 

Figure 1. HPDC alloy microstructure at x25 augmentations.

1: Porosity

2: Semi-globular dendrites

3: Fine eutectic structure

Figure 2. HPDC alloy microstructure at x400 augmentations

4: Eutectic Al Si

5: Al

DESCRIPTION OF EMBODIMENTS

Example 1. Aluminium casting alloys (preparation, composition and mechanical properties)

[0037] Aluminium compositions have been prepared by melting a standard EN-AC 43000 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.

[0038] 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.

[0039] Several compositions were tested, the content of the content if specified in Table 1. The obtained results are also specified in Table 1.

Table 1

	Alloy 1	Alloy 2	Alloy 3
Si (% by weight)	8.99	8.02	8.66
Fe (% by weight)	0.384	0.291	0.296
Cu (% by weight)	0.03	0.016	0.019
Mn (% by weight)	0.335	0.209	0.21
Mg (% by weight)	0.299	0.188	0.142
Zn (% by weight)	0.01	0.0036	0.287
Ti (% by weight)	0.02	0.014	0.199
Cr (% by weight)	0.012	0.0092	0.0096
Ni (% by weight)	0.0001	0.001	0.001
Pb (% by weight)	0.001	0.001	0.27
Sn (% by weight)	0.001	0.001	0.04
Sr (% by weight)	0.021	0.003	0.014
Rp0.2 (MPa)	132.2	130.2	131.2
Rm (MPa)	261.2	232.1	245.3
A (%)	6.0	5.6	5.25

[0040] The lowest value of elongation (A) obtained was 5.25% (Alloy 3), the lowest value of yield strength (Rp0.2) obtained was 130.2 MPa (Alloy 2) and the lowest ultimate tensile strength (Rm) was 232.1 MPa (Alloy 2). All the alloys of the example have elongations (A) values equal or above 5%, yield strength (Rp0.2) values above 130 Mpa and ultimate tensile strength values (Rm) above 230 MPa.

[0041] Document EP2657360 (B1) discloses the use of eutectic modificators, as Sr, Na and Sb, alone or in combination, and grain refiners as Ti, Zr, V. The alloy according to the invention has less than 0.2% by weight of titanium and less than 0.05% by weight of strontium. The use of Sr in the alloys of the example don't shown a significant benefit over the elongation, with similar values. In the case of Ti, only alloy 2 has been grain refined with titanium, obtaining a 0.199% by weight of Ti in his final composition, and the alloys of the example don't show a significant benefit over the elongation, with similar values. This can be explained as much as the grain refining and a modification of the structure can be obtained by a rapid cooling (up to 100°c/s) of the injected part and a multiplication pressure (up to 120 Mpa) applied over the metal in the solidification in the high pressure die casting process (HPDC).

Claims

1. Aluminium casting alloy, characterized in that said alloy is consisting of:

8-9% by weight of silicon,

0.29-0.4% by weight of iron,

less than 0.2% by weight of copper,

0.2-0.45% by weight of manganese,

less than 0.3% by weight of zinc,

less than 0.3% by weight of magnesium,

less than 0.2% by weight of titanium,

less than 0.05% by weight of chrome,

less than 0.05% by weight of nickel,

less than 0.05% by weight of strontium,

less than 0.05% by weight of lead,

less than 0.05% by weight of tin,

and aluminium as the remainder.

Drawing