Patent Application
20050155676
The invention relates to the field of aluminium alloys intended for the manufacture of relatively thin aluminium-silicon alloy parts cast by means of die-casting and particularly automobile structural or safety parts.
The use of casting aluminium alloys is growing rapidly in the automobile sector, particularly for safety parts such as ground connections and structural components, enabling the vehicle to be lightened. This lightening is all the more important if a high mechanical resistance can be obtained. Known alloys, such as the alloy Al—Si9Cu3Mg make it possible to obtain, in the non-treated temper F, an ultimate tensile strength Rm of at least 300 MPa and a yield strength Rp0.2 of at least 230 MPa. On the other hand, the elongation A does not exceed 2%. However, automobile structural or safety parts require sufficient ductility to absorb energy and prevent rupture in the event of an impact, and also to adapt to various assembly methods.
In addition, the high copper content, while it has a favourable influence on mechanical resistance, renders the alloy susceptible to corrosion. However, a good corrosion resistance, particularly stress corrosion resistance, is required to prevent damage to the part in a corrosive environment such as snow removal salt.
Various alloy formulations have been proposed to meet these requirements. For example, the patent U.S. Pat. No. 3,726,672, filed in 1970 by US Reduction, discloses an alloy for die-cast wheels consisting of (% by weight):
The patent U.S. Pat. No. 4,104,089, filed in 1976 by Nippon Light Metal relates to porosity-free die-cast parts for the automobile sector with a high mechanical resistance and shock resistance, consisting of:
The parts are solution heat-treated between 450 and 530° C., quenched and subjected to artificial ageing for over one hour between 150 and 230° C.
The patent EP 0687742, filed in 1994 by Aluminium Rheinfelden, discloses an alloy for die-casting intended for cast safety parts, consisting of (% by weight):
The application WO 96/27686, filed in 1995 by Alcoa, discloses die-cast parts for alloy “space-frame” type automobile structures consisting of: Si: 8.5-11 Mg: 0.10-0.35 Mn: 0.4-0.8 Fe<0.50.
The patent AT 404844, filed in 1997 by Aluminium Lend, relates to an alloy for die-casting consisting of:
The applications EP 0992601 and EP 0997550, filed in 1998 by Alusuisse, disclose the die-casting manufacture of alloy parts consisting of:
The parts are subjected to a partial solution heat treatment between 400 and 490° C.
These various alloys require, to achieve the required mechanical resistance, a heat treatment with a solution heat treatment and quenching, which induces significant deformations of the parts which need to undergo rectification, resulting in a significant increase in the cost price.
The patient U.S. Pat. No. 6,132,531, filed in 1997 by Alcoa, relates to an alloy for die-casting, particularly intended for “space-frame” type automobile bodywork structural nodes consisting of: Si<0.20 Fe<0.20 Mg: 2.80-3.60 Mn: 1.10-1.40 Ti<0.15 Be: 0.0005-0.0015. Good mechanical properties are obtained with no heat treatment of the cast parts. The lack of silicon affects the castability of the alloy.
The purpose of the invention is to provide aluminium alloys for the die-casting of automobile structural and safety parts offering a sufficient mechanical resistance without necessarily requiring a complete T6 or T7 type heat treatment, a high ductility, a good corrosion resistance, a good castability and enabling the manufacture of mass-produced parts under economically acceptable conditions.
The invention relates to a die-cast safety or structural parts made of ductile aluminium alloy consisting of (% by weight):
Preferentially, the following limit values are targeted for the composition:
a, 1b and 1c represent the respective variation of the ultimate tensile strength, yield strength and elongation as a function of the silicon contents, for different magnesium contents, on 2.5 mm thick sample plates die-cast with vacuum assistance and no heat treatment (F temper).
The invention is particularly based on the observation that, by lowering the silicon content with respect to the alloys of the prior art intended for the same applications, it is possible to obtain for die-cast parts an interesting compromise between mechanical strength, particularly in the single aged temper T5 and ductility, while retaining an acceptable castability, and an absence of hot cracks and contraction cavities. The silicon content is at least 2% to retain a good castability, and not more than 6% to obtain a high ductility in the T5 yemper. It is preferentially between 3.5 and 5%. The influence of the silicon content on the yield strength and the elongation is illustrated by
Magnesium has the same effects as silicon by forming Mg2Si particles with it, which have an hardening effect during the heat treatment. However, the applicants surprisingly observed that at cooling rates specific to the die-casting of thin parts, this hardening partially remains in the absence of solution heat treatment and quenching, probably due to a supersaturation effect of the solid aluminium solution. It is thus possible to limit the magnesium content to 0.4, or even to 0.3 or 0.25%, which improves the elongation, while retaining a good yield strength.
Iron, manganese, chromium, cobalt, vanadium, molybdenum and nickel form, individually or in combination with aluminium, embrittling intermetallic compounds, and the content thereof must be limited. However, in the event of a high die release cooling rate, the embrittlement is lower, since these compounds have a smaller size and a more favourable shape, Conversely, the same elements help reduce the “adherence” in the die, by reducing the chemical potential of the alloy with respect to the steel. As iron has an unfavourable influence on elongation and must be limited to 0.5%, and preferentially 0.2%, it is essential that, in addition to iron, at least one of the other elements is present. If this element is manganese, its content must be between 0.3 and 2%, and preferentially between 0.7 and 1.5%. Titanium, combined with boron, is a refining agent of the solid aluminium solution by reducing the grain size of the primary particles. In addition, it increases the volumetric contraction supply capacity during solidification, which helps improve the compactness of the parts.
Copper must be kept below 0.3%, and preferentially 0.1%, to prevent susceptibility to corrosion, and because it reduces elongation. Lead, tin and antimony inhibit the action of the modifying elements.
Eutectic modifying or refining elements, such as strontium, sodium and calcium, modify the size and shape of silicon inserts by giving them a fibred structure. They may also act as refining agents for some intermetallic compounds.
Die-cast parts, with or without vacuum assistance, using the alloys according to the invention, may be used without a heat treatment (F temper), or, on the other hand, be subjected to a complete heat treatment T6 or T7 comprising a solution heat treatment, quenching and artificial ageing, or a treatment T3 with natural ageing after quenching. They are particularly well-suited to a single artificial ageing treatment (T5 temper), at a temperature below 220° C., lasting between 15 min and 1 hour, it being possible to carry out said treatment, for automobile parts, during the paintwork baking operation, which is generally carried out at a temperature between 150 and 220° C.
The alloys according to the invention can be used to produce parts with a good mechanical resistance and a high elongation, resulting in a good shock resistance and rendering assemblies requiring a high ductility, for example crimping, possible. They also show a good TIG, MIG or laser weldability and a good compatibility with aluminium 6000 alloys used for bodywork. The alloys have a low oxidisability in the liquid state, a good reyclability and a low melting loss during waste remelting.
Sample plates were produced by die-casting, with vacuum assistance (residual die pressure of 80 hPa) made of 9 different alloys A to I, wherein the composition is given in table 1. The format of the plates was 120×120 mm and they are 2.5 mm thick. Casting was carried out on a press with a locking force of 3200 kN, with a piston injection rate of 0.7 m/s. The temperature of the metal in the furnace was 780° C.
In these non-heat-treated plates, tensile test pieces were machined and the ultimate tensile strength Rm (in MPa), the yield strength at 0.2% elongation Rp0.2 (in MPa) and the elongation A (in %). The results (means of 10 test pieces) are given in table 2 and in
It is observed that, for alloys between 3 and 7% silicon and between 0.1 and 0.2% magnesium, the ultimate tensile strength and the yield strength increase with the silicon content, and the elongation decreases. The yield strength also increases with the magnesium content, while the effect of the magnesium on elongation is not significant.
Test pieces machined to a diameter of 13.8 mm in accordance with the standard NF A 57-102 were cast in gravitational casting dies in alloys containing 7% silicon and 0.13 and 0.20% magnesium, respectively and the mechanical characteristics were compared to those of example 1 for alloys G, H and I. The results are given in table 3 and in
It is observed, for gravitational die cast parts, with a much lower cooling rate, that the ultimate tensile strength and the yield strength are much lower, particularly in the event of low magnesium contents, and the elongation much higher.
| Number | Date | Country | Kind |
|---|---|---|---|
| 01/09142 | Jul 2001 | FR | national |
| 01/15700 | Dec 2001 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR02/02394 | 7/9/2002 | WO |
