WO2003006698A1 - High-ductility aluminium alloy part cast under pressure - Google Patents

Abstract

The invention concerns a safety or structural part cast under pressure made of high-ductility aluminium alloy consisting of (in wt. %): Si 2-6; Mg < 0.40; Cu < 0.30; Zn < 0.30; Fe < 0.50; Ti < 0.30, at least an element for reducing adherence on the mould such as Mn (0.3-2%), Cr (0.1-0.3), Co (0.1-0.3), V (0.1-0.3) and Mo (0.1-0.4), and at least an element modifying eutectics, such as Sr (50-500 ppm), Na (20-100 ppm) and Ca (30-120 ppm); other elements < 0.05 each and < 0.10 in total, the balance being aluminium exhibiting, after T5 tempering at a temperature less than 220 DEG C, a yield strength Rp0.2 > 110 MPa and an elongation A > 10 %.

Description

 Die-cast aluminum alloy with high ductility

Field of the invention

The invention relates to the field of intended for the manufacture of aluminum-silicon alloy parts molded by die-casting, of relatively thin thickness, and in particular structural or safety parts for the automobile.

State of the art

The use of aluminum alloys for molding is developing rapidly in the automobile, especially for safety parts such as ground connections and structural elements, allowing the vehicle to be lightened. This reduction is all the more important as high mechanical strength can be obtained. Known alloys, such as the Al-Si9Cu3Mg alloy, make it possible to obtain, in the untreated state F, a breaking strength R _m of at least 300 MPa and an elastic limit R _p o _, d at least 230 MPa. On the other hand, the elongation at break A does not exceed 2%. However, structural or safety parts for the automobile require sufficient ductility to absorb energy and avoid rupture in the event of an impact, as well as to adapt to various methods of assembly. On the other hand, the high copper content, if it has a favorable influence on the mechanical strength, makes the alloy susceptible to corrosion. However, good resistance to corrosion, in particular to stress corrosion, is necessary to avoid deterioration of the part in a corrosive environment such as snow-removing salt. Various alloy formulations have been proposed to meet these requirements. For example, US Pat. No. 3,726,672, filed in 1970 by US Reduction, describes an alloy for a pressure casting wheel of composition (% by weight): Si: 11 - 13, 5 Mg: 0.25 - 0.6 Cu: 0.25 - 0.6 Mn <0.5 Zn <3 Fe: 0.5 - 1.5 Cr: 0.25 - 0.4 US patent 4104089, filed in 1976 by Nippon Light Métal relates to non-porous pressure-molded parts for the automobile with high mechanical resistance and impact resistance, of composition:

If: 7 - 12 Mg: 0.2 - 0.5 Mn: 0.55 - 1 Fe: 0.65 - 1.2 The parts are treated by dissolving between 450 and 530 ° C, quenched and subjected to a income of more than one hour between 150 and 230 ° C.

Patent EP 0687742, filed in 1994 by Aluminum Rheinfelden describes an alloy for die casting intended for safety molded parts, of composition

(% by weight): If: 9.5 - 11.5 Mg: 0.1 - 0.5 Mn: 0.5 - 0.8 Fe <0.15 Cu <0.03

Application WO 96/27686, filed in 1995 by Alcoa describes die-cast parts for automobile structures of the “space-frame” type made of an alloy of composition: 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 Aluminum Lend, relates to an alloy for pressure casting of composition:

Si: 9 - 12.5 Mg: 0.10 - 0.60 Mn: 0.30 - 0.45 Cr: 0.05 - 0.40 Fe <0.18 Cu <0.05 Zn <0.10 Ti : 0.01 - 0.20

Applications EP 0992601 and EP 0997550, filed in 1998 by Alusuisse, describe the manufacture by pressure casting of alloy parts of composition: Si: 9.5 - 11.5 Mg: 0.1 - 0.4 Mn: 0, 3 - 0.6 Fe: 0.15 - 0.35 Ti <0, l Sr: 90 - 180 ppm possibly Cr: 0, 1 - 0.3 Ni: 0, 1 - 0.3 Co: 0, 1 - 0.3 The parts are subjected to a partial dissolution between 400 and 490 ° C. These different alloys require, to reach the required mechanical resistance, a heat treatment with dissolution and quenching, which leads to significant deformations parts which must undergo straightening, leading to a significant increase in the cost price.

US patent 6132531, filed in 1997 by Alcoa, relates to an alloy for die casting, intended in particular for structure nodes of automobile bodywork of the “space-frame” type, of composition: 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 without heat treatment of the cast parts. The absence of silicon affects the flowability of the alloy. Purpose of the invention

The object of the invention is to provide aluminum alloys for the pressure casting of structural and safety parts for the automobile having both sufficient mechanical strength without necessarily requiring a complete heat treatment of the T6 or T7 type, high ductility, good corrosion resistance, good moldability and allowing the production of parts in large series under acceptable economic conditions.

Ob j and invention

The subject of the invention is safety or structural parts die-cast in a ductile aluminum alloy of composition (% by weight):

If: 2 - 6 Mg <0.40 Cu <0.30 Zn <0.30 Fe <0.50 Ti <0.30, at least one element intended to reduce sticking on the mold such as Mn (0.3 - 2%),

Cr (0.1 - 0.3), Co (0.1 - 0.3), V (0.1 - 0.3) and Mo (0.1 - 0.4), and at least one modifier eutectic, such as Sr (50 - 500 ppm), Na

(20 - 100 ppm) and Ca (30 - 120 ppm), other elements <0.05 each and <0.10 in total, remain aluminum, exhibiting after a T5 tempering at a temperature below 220 ° C, a limit of 'elasticity Rp _{0; 2} > 110 MPa and an elongation at break A> 10%.

Preferably, the following limit values are targeted for the composition:

If: 3.5 - 5 Mg: 0.05 - 0.20 Mn: 0.7 - 1.5 Ti: 0.05 - 0.15 Cu <0.10

Zn <0.10

Description of the figures

FIGS. 1a, 1b and 1c represent the respective variation of the breaking strength, the elastic limit and the elongation as a function of the silicon content, for different magnesium contents, on thick sample plates 2.5 mm pressure casting with vacuum assistance and not heat treated (state F). Figure 2 shows the mechanical characteristics of die cast parts

(SP) and in gravity shell (Coq) for alloys with 7% silicon according to their magnesium content.

Description of the invention

The invention is based in particular on the observation that by lowering the silicon content compared to the alloys of the prior art intended for the same applications, it is possible to obtain for the die-cast parts an interesting compromise between the mechanical strength, in particular at the simply returned state (T5) and ductility, while keeping an acceptable flowability, and an absence of cracks and shrinkage. The silicon content is at least 2% to maintain good flowability, and at most 6% to obtain high ductility in the T5 state. It is preferably between 3.5 and 5%. The influence of the silicon content on the elastic limit and the elongation is illustrated by Figures la and lb in the F state (untreated) and Figure 2 in the T5 state, which show in particular the rapid decrease in elongation as the silicon content increases.

Magnesium has the same effects as silicon by forming with it particles of Mg ₂ Si which have a hardening effect on heat treatment. However, the Applicant has surprisingly found that, at specific cooling rates for the pressure casting of thin parts, this hardening effect partially subsists in the absence of dissolution and quenching, probably by an effect of supersaturation of the solid aluminum solution. It is thus possible to limit the magnesium content to 0.4, or even 0.3 or 0.25%, which improves the elongation, while keeping a good elastic limit.

Iron, manganese, chromium, cobalt, vanadium, molybdenum and nickel form individually or in combination with aluminum weakening intermetallic compounds, and their content must be limited. However, when the rate of cooling on demolding is high, embrittlement is less, since these components have a reduced size and a more favorable morphology. Conversely, these same elements contribute to reducing the “sticking” in the mold, of casting, by reducing the chemical potential of the alloy compared to steel. As iron has an unfavorable influence on the elongation and must be limited to 0.5%, and preferably 0.2%, it is essential that in addition to the iron at least one of the other elements is present. In the case where this element is manganese, its content must be between 0.3 and 2%, and preferably between 0.7 and 1.5%. Titanium, combined with boron, is a refiner of the solid aluminum solution by reducing the grain size of the primary particles. In addition, it increases the supply capacity of the volumetric contraction during solidification, which contributes to improving the compactness of the parts.

Copper should be kept below 0.3%, and preferably 0.1% to avoid sensitivity to corrosion, and because it reduces elongation. Lead, tin and antimony inhibit the action of the modifying elements.

The modifying or refining elements of eutectics, such as strontium, sodium and calcium, modify the size and morphology of the silicon wafers by giving them a fiber structure. They can also act as refiners of certain intermetallic compounds. The die-cast parts, with or without vacuum assistance, from the alloys according to the invention, can be used without heat treatment (state F), or on the contrary be subjected to a complete heat treatment T6 or T7 comprising a setting solution, quenching and tempering, or T3 treatment with natural aging after quenching. They are particularly well suited to a simple tempering treatment (state T5), at a temperature below 220 ° C., lasting between 15 min and 1 h., This treatment being able to be carried out, for automobile parts, during of the painting firing operation, which is generally done at a temperature between 150 and 220 ° C.

The alloys according to the invention make it possible to produce parts having both good mechanical strength and high elongation, leading to good impact resistance and making possible assemblies requiring high ductility, for example crimping. They also have good suitability for TIG, MIG or laser welding, and good compatibility with the 6000 aluminum alloys used for bodywork. The alloys have a low oxidizability in the liquid state, a good ease of recycling and a low loss on ignition at the remelting of waste. Examples

Example 1 Effect of the Composition on the Static Mechanical Characteristics

Sample plates in 9 different alloys A to I were produced by pressure casting, with vacuum assistance (residual pressure in the mold of 80 hPa), the composition of which is given in Table 1. The plates were 120 × 220 mm and 2.5 mm thick. The casting was done on a 3200 kN press with a closing force, with a piston injection speed of 0.7 m / s. The metal temperature in the oven was 780 ° C.

Table 1

In these non-heat treated plates, tensile test specimens were machined, and the tensile strength R _m (in MPa) was measured, the conventional elastic limit at 0.2% elongation Rp0.2 (in MPa) and the elongation at break A (in%). The results (means of 10 test pieces) are shown in Table 2 and in Figures la, 1b and le. Table 2

It is noted that, for alloys between 3 and 7% of silicon and between 0.1 and 0.2% of magnesium, the breaking strength and the elastic limit increase with the silicon content, and the elongation decreases. The elastic limit also increases with the magnesium content, while the effect of magnesium on the elongation is not significant.

Example 2: effect of the cooling rate

Machined test pieces with a diameter of 13.8 mm according to standard NF A 57-102 were cast by gravity in alloys containing 7% silicon and 0, 0.13 and 0.20% magnesium, respectively, and compared the mechanical characteristics with those of Example 1 for alloys G, H and I. The results are given in Table 3 and in Figure 2. Table 3

It is found, for the parts cast in a shell with a much lower cooling rate, the breaking strength and the elastic limit are much lower, especially at low magnesium contents, and the higher elongation.

Claims

claims 1. Safety or structural part cast in ductile aluminum alloy of composition (% by weight): If: 2 - 6 Mg <0.40 Cu <0.30 Zn <0.30 Fe <0.50 Ti <0.30, at least one element intended to reduce sticking on the mold such as Mn (0.3 - 2%), Cr (0.1 - 0.3), Co (0.1 - 0.3), V (0.1 - 0.3) or Mo (0.1 - 0.4), and at least one eutectic modifier, such as Sr (50 - 500 ppm), Na (20 - 100 ppm) and Ca (30 - 120 ppm), other elements <0.05 each and <0.10 in total , remains aluminum, having, after tempering T5 at a temperature below 220 ° C, an elastic limit R _p0 , ₂ > 110 MPa and an elongation A> 10%. 2. Part according to claim 1, characterized in that the silicon content is between 3.5 and 5%. 3. Part according to one of claims 1 or 2, characterized in that the magnesium content is between 0.05 and 0.25%. 4. Part according to one of claims 1 to 3, characterized in that the titanium content is between 0.05 and 0.15%. 5. Piece according to one of claims 1 to 4, characterized in that the copper content is less than 0.10%. 6. Part according to one of claims 1 to 5, characterized in that the iron content is less than 0.20%. 7. Part according to one of claims 1 to 6, characterized in that the zinc content is less than 0.10%. Piece according to one of claims 1 to 7, characterized in that the manganese content is between 0.7 and 1.5%.