EP0886683B1 - Thixotropic aluminium-silicon-copper alloy suitable for semi-solid shaping - Google Patents

Abstract

The invention relates to an aluminum alloy for thixoforming with the composition (by weight): Si: 5%-7.2% Cu: 1%-5% Mg<1% Zn<3% Fe<1.5% other elements<1% each and<3% in total, with % Si<7.5-% Cu/3, which, when reheated to the semisolid state to the point at which a liquid fraction ratio between 35 and 55% is obtained, has an absence of non-remelted polyhedral silicon crystals.

Description

Field of the invention

The invention relates to the field of aluminum-silicon-copper alloys, which can possibly contain other additives such as magnesium, cast in the form of billets having a globular solidification structure conferring thixotropic properties and intended to be shaped, by forging or injection under pressure, after reheating in the semi-solid state. Such shaping is known as thixoforming

State of the art

Thixoforming developed from the discovery made in the early years 1970 by the team of Pr FLEMINGS at MIT that a metal, developed in some particular conditions, presents, when it is reheated in the semi-solid state, a apparent viscosity which strongly depends on the shear rate, so that it behaves like a solid during handling and like a liquid viscous when injected into a mold. This property leads, compared to traditional shaping processes, better metallurgical quality of parts produced, higher production rates, less wear of tools and molds and energy saving.

For this purpose, the solidification of the metal with thixoforming must lead to a structure globular, not dendritic, which can be obtained either by mechanical agitation of the solid-liquid mixture as in US Patent 3,948,650 of the MIT, either by mixing electromagnetic as in US patents 4434837 and US 4457355 of ITT-ALUMAX or the patents EP 0351327 and EP 0439981 of ALUMINUM PECHINEY. The billets thus cast are cut into pieces corresponding to the quantity of metal necessary for the manufacture of the part to be formed, these pieces being heated to the semi-solid state, usually by induction heating, and transferred to shaping equipment (forging press or pressure injection machine).

This process has been developed industrially mainly for alloys aluminum for the manufacture of parts for the automotive industry. In fact, the almost all deliveries related to alloys of the Al-Si7Mg type at 7% silicon and less than 1% magnesium, for example the alloys Al-Si7Mg0,3 and Al-Si7Mg0,6 (A356 and 357 according to the nomenclature of the Aluminum Association for molding alloys). These alloys have good thixoforming ability. In effect, when they are heated so as to obtain a liquid fraction rate of of the order of 50%, corresponding to an optimum of the rheological properties of the metal la eutectic phase is completely overhauled while the fusion of the primary phase silicon is not started.

The mechanical characteristics of the parts produced using these alloys are good and we have the possibility of adapting the resistance and / or the ductility by the use of different heat treatments. However, the maximum breaking strength, for an alloy of this type with 0.6% magnesium, remains limited to approximately 350 MPa at state T6.

The applicant's patent FR 2266749, published in 1975, describes a method of forming, for example by pressure casting, of aluminum alloys in the semi-solid state, with a liquid fraction rate higher than 35%. Examples mention AS9U3 alloys with 9% silicon and 3% copper, and AU4SG with 4% copper, 0.75% silicon and 0.5% magnesium.

Problem

To improve the mechanical resistance of alloys intended for thixoforming, to increase the resistance of the parts produced, or to facilitate machining, tried to use alloys containing 1 to 5% copper. With for example a alloy with 3% copper, no particular problem is encountered with the casting of billets, and the mechanical resistance at the level of the billet is effectively improved more than 25%. If the reheating temperature is adjusted to the semi-solid state, by lowering it by a few degrees C, to stay at a liquid fraction rate close to 50%, the thixoforming of this alloy is also easy. However, we can see a significant decrease, of the order of half, in the elongation on the part treated T6 compared to that measured on the billet in the same metallurgical state, whereas, for the copper-free alloy, the elongation of the treated billet and that of the treated part are practically identical.

The Applicant has attempted to elucidate the reason for this surprising behavior. A microstructural analysis of copper alloy plots reheated in the semi-solid state, then soaked in water, revealed the presence of embrittling clusters of silicon crystals polyhedral shape. These same clusters have also been highlighted on the surface of rupture of tensile test pieces drawn from thixoformed parts from these plots. One hypothesis to explain this microstructure is that the phase eutectic has not been completely recast, as in the case of Al-Si7Mg without copper, and the eutectic silicon coalesced to form clusters of crystals rude.

To avoid these clumps of silicon crystals detrimental to the elongation of the parts, the inventors tried to increase the reheating temperature to obtain a complete remelting of the eutectic phase. But this has led to a fraction rate liquid of the order of 60%, causing a collapse of the heated piece during handling, which no longer allows thixoforming under industrial conditions acceptable.

Purpose of the invention

The invention aims to find a field of composition of aluminum-silicon alloys more than 5% silicon and containing 1 to 5% copper allowing get out of the dilemma set out above, i.e. allow both thixoforming without problem and to obtain parts having a high mechanical resistance and a good elongation.

Subject of the invention

The subject of the invention is the use of an aluminum alloy of composition (% by weight): If: 5% - 7.2% Cu: 1% - 5% Mg <1% Zn <3% Fe <1.5% other elements <1% each and 3% in total, aluminum remains, such as:% Si <7.5 - (% Cu / 3), for forming in the semi-solid state, at a rate of liquid fraction included between 35% and 55%, and with, in this state, a structure free of polyhedral crystals not remelted silicon.

1) Si: 5% - 7%   Cu: 1%- 1,5%

2) Si: 5% - 6,3%   Cu: 2,5 - 3,5%

3) Si: 5% - 6%   Cu: 3,5% - 4,5%

It also relates to a method according to claim 7 for thixoforming parts having high mechanical strength and good elongation from this alloy. In this area, we can define 3 specific compositions such as:

1) If: 5% - 7% Cu: 1% - 1.5%

2) If: 5% - 6.3% Cu: 2.5 - 3.5%

3) If: 5% - 6% Cu: 3.5% - 4.5%

Description of the figure

The single figure represents, in a diagram having for abscissa the content of silicon and for ordinate the copper content, the lines of equal eutectic fraction and the composition domain according to the invention.

Description of the invention

The alloys according to the invention remain in the usual composition ranges of AlSiCu molding alloys. We do not go below 5% silicon because the alloy becomes difficult to flow. The addition of copper has a significant effect on the mechanical strength and the machinability only from a content of approximately 1% and, beyond 5%, there is a very unfavorable effect on the elongation . Magnesium, in a content of less than 1%, increases the response to heat treatment thanks to the formation of hardening particles Mg ₂ Si, but, beyond 1%, there is also an unfavorable effect on the elongation.

Relatively high levels can be observed for zinc and iron in case where we start from secondary metal from recycling. These contents are obviously much more reduced if we start from primary metal.

We can also add, as we usually do in AlSi alloys of foundry, a eutectic silicon modifier, such as sodium, strontium or antimony, which prevents the formation of coarse grains of silicon Sodium and strontium can be present alone or together, antimony being always alone. For strontium for example, the content is between 0.005 and 0.05%. Likewise, an addition of titanium up to 0.2% and / or boron up to 0.1%, allows a refinement of the grain and a better heat resistance.

In order to maintain, for copper alloys, the same rheological properties during thixoforming as for alloys of identical composition but without copper, while also obtaining a complete remelting of the eutectic silicon in the piece heated in the semi-solid state , pledge of a good elongation of the finished part, the inventors had the idea of modifying the silicon content as a function of the copper content. They thus found that one could obtain a thixoforming behavior identical to that of an Al-Si7 alloy for an Al-SiCu alloy if the Si and Cu contents of this alloy satisfy the relationship: (1)% Si = 7 -% Cu / 3.

The line representing this relation in the figure is therefore the line of the compositions corresponding to 50% of eutectic fraction. Thus, an alloy Ai-Si6Cu3Mg0,6 or a Al-Si6,5Cu1,5Mg0,6 alloy have a thixoforming behavior identical to that of an Al-Si7Mg0.6 alloy, that is to say that a rate of liquid fraction close to 50% with a complete fusion of the eutectic, and therefore a absence of polyhedral silicon crystals.

It was verified, for the 2 compositions mentioned, that the metal loss was 8 ± 2%, identical to that of the alloy Ai-Si7Mg0.6. The apparent viscosity of pieces heated at a temperature between 2 and 5 ° C. above the eutectic plateau was measured, using a penetration test consisting in measuring the resistance to deformation F of the heated piece compressed by a tool at constant speed at the end of a stroke of determined length. The ratio of this force F to a constant threshold force F _{s is established} , for a conventional value of metal loss by exudation of 8%, the metal loss being an indicator of the rate of liquid fraction for a given material.

For the two compositions mentioned, there is an F / Fs ratio of the order of 0.45, similar to that measured on a piece of Al-Si7Mg0.6 alloy.

As the rate of liquid fraction is controllable to within ± 5%, taking into account the usual silicon content intervals allowed by the standards and specifications for the alloys considered, it can be estimated that, in the figure, the composition of the alloy must be such that the Si and Cu contents satisfy the relation: (2) 6.5 -% Cu / 3 <% Si <7.5 -% Cu / 3 which corresponds to the fact that the rate of liquid fraction obtained with complete fusion of the eutectic is between 45 and 55%, or that the eutectic fraction of the alloy is between 45 and 55%.

In addition, it has been found that, for these copper alloys, it is possible to obtain good thixoforming behavior by heating the pieces up to a rate of liquid fraction clearly lower than 50%. Thus, for an alloy with 5% Si and 3% Cu, we can go down to 40% of liquid fraction and, for an alloy with 5% Si and 1.5% Cu, up to around 35 %. On the other hand, by trying an alloy with 4% of silicon and 3% of copper, it was noted initially that because of the large interval of solidification (625 - 560 ° C), the casting of thixotropic billets is done with difficulty , resulting in casting defects such as tears and breakthroughs. Furthermore, the thixoforming behavior is poor: as soon as the filling of the mold cavity begins, the heat loss by exchange with the mold wall leads to partial resolidification and an increase in the apparent viscosity which causes defects in the part. injected, such as folds, shrinkage or non-arrival.
Thus, with reference to the figure representing the silicon and copper contents, on which the lines of equal eutectic fraction have been shown, it can be seen that the range corresponding to the compositions according to the invention comprises not only the band between the straight lines representing the eutectic fractions of 55% and 45%, i.e. the fringe surrounding the straight line representing 50%, but also the area between 45% and 35% which, taking into account the lower limit of Cu to 1 %, practically corresponds to the adjacent triangle.

Claims (12)

1. Use of an alloy of aluminium of composition (by weight) :
   Si: 5% - 7.2%; Cu: 1% - 5%; Mg: < 1%; Zn: < 3%; Fe: < 1.5%; and possibly Sr: 0.005 - 0.05%; Ti: up to 0.2%; B: up to 0.1%, with other elements < 1% each and < 3% in total, the rest being aluminium with the condition that %Si < 7.5 -(%Cu/3),
   for forming, in the semi-solid state, with a liquid fraction of between 35 and 55% and with a structure free of polyhedral crystals of unremelted silicon.
2. Use of an alloy according to Claim 1 such that Si is between 5 and 7% and Cu between 1% and 1.5%.
3. Use of an alloy according to Claim 1 such that Si is between 5 and 6.3% and Cu between 2.5% and 3.5%.
4. Use of an alloy according to Claim 1 such that Si is between 5 and 6% and Cu between 3.5% and 4.5%.
5. Use of an alloy according to one of Claims 1 to 4 containing between 0.005 and 0.05% of strontium.
6. Use of an alloy according to one of Claims 1 to 5 containing up to 0.2% titanium and/or up to 0.1% boron.
7. A method of producing aluminium alloy articles by forming, in the semi-solid state and with a liquid fraction of between 35 and 55% and with a structure free of polyhedral crystals of unremelted silicon, an alloy of composition (by weight):
   Si: 5% - 7.2%; Cu: 1% - 5%; Mg: < 1%; Zn: < 3%; Fe: < 1.5%; and possibly Sr: 0.005 - 0.05%; Ti: up to 0.2%; B: up to 0.1%, with other elements < 1% each and < 3% in total, the rest being aluminium with the condition that %Si < 7.5 -(%Cu/3).
8. A method according to Claim 7 characterised in that Si is between 5 and 7% and Cu between 1% and 1.5%.
9. A method according to Claim 7 characterised in that Si is between 5 and 6.3% and Cu between 2.5% and 3.5%.
10. A method according to Claim 7 characterised in that Si is between 5 and 6% and Cu between 3.5% and 4.5%.
11. A method according to one of Claims 7 to 10 characterised in that the alloy contains between 0.005 and 0.05% of strontium.
12. A method according to one of Claims 7 to 11 characterised in that the alloy contains up to 0.2% titanium and/or up to 0.1% boron.

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