EP1488021B1 - Method for the thermal treatment of a foundry piece made from an alloy based on aluminium and foundry piece - Google Patents

Description

The present invention generally relates to the heat treatment of aluminum-based and silicon-containing casting alloys, as well as the resulting molded parts.

Aluminum-based casting alloys have different families of compositions, most of which are capable of structural hardening by heat treatment. In particular, mention may be made of the aluminum / silicon / magnesium family typically represented by alloys of the AlSi7% Mg0.3%, AlSi7Mg0.6% or AlSi10% Mg0.3% type, and the aluminum / silicon / copper / magnesium family represented typically AlSi (5 to 10%) Cu (2 to 3.5%) Mg (0.2 to 0.3%) alloys.

All these alloys are widely used for the mass production of automotive components, for example cylinder heads subjected to very high loads in service. In order to maximize the mechanical properties of these alloys, it is customary at least for the most severe stress cases, to perform a heat treatment having a dissolution and quenching, followed by a structural hardening income.

Such a heat treatment is disclosed in WO01 / 16387 .

A disadvantage of this type of treatment is that it can make the alloy extremely difficult to machine, and in particular in the case of alloys with structural hardening having no or little copper (typically at levels less than or equal to 1 %).

In particular, the machining of very thin threads (eg cylinder head injector threads for diesel engines), long, small diameter holes or deburring of machined surfaces can be problematic (non-fragmentable burrs difficult to remove). by brushing for example).

We know from the document EP-A-1,065,292 a process for producing, as a result of the dissolution of a piece made of light alloy, which can be assimilated to a steep quench, by immersion in a salt bath so as to rapidly bring the temperature of the coin has a value between 350 and 450 ° C. Such a known method has the effect of increasing the tensile strength of the workpiece material at high temperature, but does not solve the machiningability problems. In addition, this process leads to losses of mechanical characteristics at room temperature which are unacceptable for combustion engine cylinder type applications.

Thus, there is currently no solution for facilitating the machining of alloy parts of the structural hardening type or the like.

The present invention aims to overcome these drawbacks and to make it possible to obtain foundry pieces having a good compromise between the intrinsic performances which are required of them, in particular as regards resistance to different types of stresses, and their aptitude for machining and deburring after machining.

To this end, it proposes, according to a first aspect, a method of heat treatment of a casting part made of aluminum alloy, according to claim 1.

• la deuxième gamme de températures est choisie de manière à ce que l'alliage traité présente une résistance à la traction diminuée d'environ 10% à 40%, et de préférence d'environ 15% à 35%, par rapport à la résistance à la traction qui serait obtenue avec une mise en solution unique dans la première gamme de températures et pendant une durée égale aux première et deuxième durées cumulées.
• la deuxième gamme de températures est inférieure à la première gamme de températures d'environ 8 à 14 %.
• la première durée est alors comprise entre environ 1h et 4h, et de préférence entre environ 1h et 2h.
• la deuxième durée est alors comprise entre environ 1h et 5h, et de préférence entre environ 1h et 3h.
• la durée de l'étape (b) est alors comprise entre environ 0h30 et 3h30, et de préférence entre environ 1h et 2h30.

Some preferred but non-limiting aspects of this method are the following:

• the second temperature range is chosen so that the treated alloy has a tensile strength decreased from about 10% to 40%, and preferably from about 15% to 35%, with respect to the traction that would be obtained with a single dissolution in the first temperature range and for a duration equal to the first and second cumulative times.
• the second temperature range is lower than the first temperature range of about 8 to 14%.
• the first duration is then between about 1h and 4h, and preferably between about 1h and 2h.
• the second duration is then between about 1h and 5h, and preferably between about 1h and 3h.
• the duration of step (b) is then between about 0:30 and 3:30, and preferably between about 1:00 and 2:30.

According to a second aspect, the present invention provides an aluminum-based alloy casting according to claim 7.

• la figure1 illustre la microstructure d'un premier type d'alliage traité selon l'art antérieur,
• la figure 2 illustre la microstructure de ce même type d'alliage traité selon la présente invention,
• la figure 3 illustre la microstructure d'un deuxième type d'alliage traité selon la présente invention,
• la figure 4 illustre la microstructure de ce même type d'alliage traité d'une façon différente de la présente invention, et
• la figure 5 est un diagramme de propriétés mécaniques résistance/allongement illustrant les propriétés pouvant être obtenues selon l'invention.

Other aspects, objects and advantages of the present invention will appear better on reading the following detailed description of a preferred embodiment thereof, given by way of nonlimiting example and with reference to the appended drawings. on which ones :

• the figure 1 illustrates the microstructure of a first type of alloy treated according to the prior art,
• the figure 2 illustrates the microstructure of this same type of alloy treated according to the present invention,
• the figure 3 illustrates the microstructure of a second type of alloy treated according to the present invention,
• the figure 4 illustrates the microstructure of this same type of alloy treated in a different way from the present invention, and
• the figure 5 is a resistance / elongation mechanical properties diagram illustrating the properties obtainable according to the invention.

The invention will now be described in detail.

A heat treatment according to the invention is carried out by a two-stage solution solution. A first step is performed in a range of high temperatures, that is to say in the usual temperature range of dissolution of the alloys considered that the skilled person will define according to well known references. Typically, for alloys containing less than 1% by weight of copper, this first temperature is between about 510 ° C and 550 ° C, preferably between about 520 ° C and 540 ° C, and more particularly around 530 ° C . For alloys having a higher copper content, this temperature will be lower, for example between about 475 ° C and 515 ° C, and preferably around 495 ° C for copper contents of 2 to 3% by weight. For reasons including economic, this first step is limited to durations of the order of 1h to 4h, preferably 1h to 2h, knowing that an extension of this plateau does not lead to significant improvements in the final properties of the material.

This first stage is followed by a second level of dissolution in a second lower temperature range. Still for an alloy containing at most 1% by weight of copper, this second temperature range is between about 455 ° C. and 485 ° C., preferably between about 460 ° C. and 480 ° C., and more preferentially around 465 ° C. C (it being specified that for an alloy with a copper content of 2 to 3% by weight, this second temperature range will advantageously be between 425 ° C. and 455 ° C., and more preferably around 450 ° C.).

More generally, the second temperature range is about 8 to 14% lower than the first temperature range.

The duration of this second step is of the order of 1h to 5h, preferably 1h to 3h. Indeed, it appears that extending the residence time at this second level does not bring significant changes to the final properties, and so again it is economically more interesting to reduce this time.

The cooling between the two dissolution stages is carried out so as to progressively move from the highest temperature to the lowest temperature in a time of between 30 minutes and 3 hours 30 minutes. Preferably, and again for mainly economic reasons, this time is between 1 hour and 2 hours 30 minutes.

After this second lower temperature dissolution stage, quenching is applied according to the usual conditions, for example quenching with water.

Finally, an income is made to develop the hardening precipitation of the alloy. This income can be chosen in the usual ranges of temperature and duration; Depending on the desired properties, it may be a sub-income, an income at the peak of resistance or an over-income.

The temperature of the second dissolution stage, chosen as determined above, makes it possible to ensure that the tensile properties of the alloy thus treated decrease by about 10 to 40%, and preferably by approximately 15 to 35%, with respect to the properties that would be obtained with a single dissolution at the first temperature and for a duration equal to the cumulative durations of the two stages (includes the cooling phase of the first stage to the second stage), and keeping the same conditions of quenching and income.

In practice, it is intended to achieve, compared to a conventional single-stage heat treatment, an improved compromise between the properties of tensile strength, elongation and quality index. More particularly, it has been found that it is preferable to place in the 15 to 35% reduction zone of properties if one wishes to optimize the resistance / elongation compromise while benefiting from an improved machinability.

Moreover, it has also been found that this two-stage solution dissolution very significantly reduces the residual stresses present in the part after the end of the heat treatment. This can have advantages significant on the holding of parts, including cylinder heads of combustion engines, very heavily solicited.

The invention will now be illustrated by the following examples:

1) Comparison between the invention and the prior art as regards the impact on the resistance / elongation compromise and on the microstructure

A cylinder head made of AlSi7% Mg0.4% secondary melting low pressure casting, whose weight is about 18kg, and which undergoes heat treatment of the prior art (dissolution at maximum temperature, tempering and tempering) present Difficulty in machining: large-sized drill chips tend to wrap around cutting tools (or remain in oil circuits, for example). Their evacuation is therefore difficult. These machining difficulties are related to the characteristics of the alloy obtained after this known heat treatment.

More precisely, after a traditional solution dissolution of 5 hours at 530 ° C. followed by quenching with water of 90 ° C. and a 5 hour tempering at 200 ° C., the mechanical characteristics obtained on the cylinder head on its tumbler face are as follows: Tear resistant 341 MPa Yield strength at 0.2% deformation 298 MPa Plastic stretching 2.17% Brinell hardness 112 Quality index 391 MPa

• un silicium globulisé par le séjour à haute température, comme le montre la figure 1 des dessins, et
• une mise en solution optimale, c'est-à-dire qu'aucun composé Mg2Si n'est observé hors solution.

The typical microstructure after the usual heat treatment reveals:

• a silicon globulized by the stay at high temperature, as shown by figure 1 drawings, and
• optimal dissolution, that is to say that no compound Mg2Si is observed out of solution.

A heat treatment is now carried out on an identical cylinder head, instead of the known heat treatment, comprising dissolving at a first temperature level of 530 ° C. for 2 hours and then at a second stage of 465 ° C. for 2 hours. , leaving 1 hour to the load to go from the first temperature to the second, then quenching with water at 90 ° C and a 5 hour income at 200 ° C.

The mechanical characteristics of the material become the following: Tear resistant 231 MPa (-32%) Yield strength at 0.2% deformation 207 MPa (-30 %) Plastic stretching 4.64% (+114%) Brinell hardness 90 (-20%) Quality index 331 MPa (-15%)

The breaking strength, the yield strength and the hardness, with the treatment according to the invention, decrease from 20 to 32%. This decrease is in favor of a very strong increase in plastic elongation (that is to say elongation at break) (+ 114%).

In terms of microstructure, the heat treatment according to the present invention reveals the presence of silicon globulized by high temperature dissolution, as illustrated by FIG. figure 2 drawings, while obtaining the reduction of resistances or hardness compared to the setting in conventional solution.

2) Impact of the temperature of the second tier

A dissolution in two stages with a second temperature lowered compared to the previous example, namely 450 ° C and 400 ° C respectively, was carried out in the context of the search for a second optimum temperature. In both cases, the dissolution at 530 ° C was carried out for 2 hours, and the dissolution at 450 or 400 ° C as the case required was for 3 hours, with a duration of 90 or 120 minutes to reach the second temperature.

Here again, the dissolution was followed by quenching with water at 90 ° C. and a 5 hour recovery at 200 ° C.

But such lower temperatures at the second level have been shown to cause too great a decline in mechanical characteristics, as illustrated in the table below: Dissolution in two stages, 530 ° C then 450 ° C 400 ° C Resistance to fracture (MPa) 207 (-39%) 169 (-50%) 0.2% yield stress (MPa) 151 (-49%) 97 (-67%) Plastic elongation (%) 4.22 (+ 94%) 8.41 (+ 288%) Brinell hardness 70 (-37%) 61 (-45%)

3) Impact of the first step at high temperature on the microstructure

Using a casting molded with an alloy of the above-mentioned type, modified with strontium, the two-stage solution heat treatment according to the invention was carried out, as well as a solution with a single bearing at the temperature of 465 ° C. ° C. In the first case, the stay at high temperature causes the silicon to globulate, as shown by the figure 3 , whereas in the second case there is no globulization, as shown in figure 4 .

4) Impact of the existence of two bearings on the mechanical characteristics

The mechanical characteristics are also affected by the type of dissolution performed, being better after a two-stage solution dissolution.

So the diagram shown on the figure 5 the drawings represent the trade-offs between the mechanical strengths (Rm tensile strength and yield strength Rp at 0.2% deformation, in MPa) and the elongation at break (in%) after single or two bearings, depending on the temperature of the single bearing or the second bearing. Squares (case of the invention) and triangles (case of a single stage heat treatment) correspond to different temperatures, as indicated.

On the figure 5 , the continuous line trajectory T1 shows the evolution of the torque (breaking strength, elongation) as a function of the second temperature of a two-stage solution solution, while the T2 line shows the evolution of the same characteristics as a function of the temperature of a single-stage solution solution. The dotted line trajectory T3 for its part shows the evolution of the torque (elastic limit, elongation) as a function of the second temperature of a two-stage solution solution, whereas the dotted line trajectory T4 shows the evolution of the same characteristics as a function of the temperature of a single-stage solution solution.

We also represented on the figure 5 in conjunction with the T1 and T3 curves obtained according to the invention, the decreases of 10% and 40% of the resistance corresponding to a field of the invention, as well as the decreases of 15% and 35% corresponding to a particularly preferred field (hatched domain) of the invention.

In addition, the figure 5 shows, by the relative positions of the curves T1 and T3 with respect to the curves T2 and T4, respectively, that a treatment of dissolution in solution with two stages has a compromise resistance / elongation with rupture superior to the same material subjected to a heat treatment at a single level, regardless of the temperature level of this single stage.

5) Machinability tests

To study the impact of the invention on machinability, two tests were carried out, one on a reference cylinder made according to the prior art (single-stage treatment at 530 ° C. for 5 hours) as described in FIG. 1) above, and with the same alloy, and the other on a cylinder head always made of the same alloy and heat-treated with two temperature levels according to the invention, namely for 2 hours at 530 ° C and during 2 h at 465 ° C, with intermediate cooling for 1 h.

In both cases, the dissolution in solution was followed by quenching with water at 90 ° C and then an income of 5 hours at 190 ° C.

It is found that the machinability of the cylinder heads treated according to the invention is facilitated. As an indication, the greater ease of machining has been characterized by a reduction in the average chip length and a large increase in the average density - which reflect a better ability to fragment the chip - according to the table below. below: average chip length average density Reference: 2.6 cm 0.11 g / cm ³ According to the invention: 2 populations: 0.7 cm * and 2.2 cm (measure on unsorted population)
0.64 g / cm ³ * majority population

Of course, the present invention is not limited to the embodiments described and shown, but the skilled person will be able to make many variations and modifications.

In particular, those skilled in the art will be able to vary the exact profile of the evolution of the temperature during the heat treatment of dissolution, with in particular more than two steps, or even steps in which the temperature can evolve in a certain range.

• du type d'alliage utilisé,
• du poids et/ou du volume de la pièce,
• de l'application envisagée,
• du type d'usinage à effectuer, etc.

In addition, those skilled in the art will be able to adapt the various parameters mainly but not exclusively:

• the type of alloy used,
• the weight and / or volume of the piece,
• the intended application,
• the type of machining to be performed, etc.

Claims (7)

1. Method of heat-treating a casting made of an alloy of aluminium, silicon and magnesium, with a low copper content of no more than 1% by weight, characterised in that it comprises the following steps:

   (a) solution heat treatment the casting at a first temperature range lying between 510°C and 555°C, and preferably between approximately 520°C and 540°C, for a first duration;

   (b) gradually cooling the casting to a second temperature lying within a second temperature range lower than the first range;

   (c) continuing solution heat treatment of the casting by maintaining it at the second temperature range lying between 455°C and 485°C, and preferably between approximately 460°C and 480°C, for a second duration;

   (d) quenching the casting; and

   (e) tempering the casting.
2. Method according to claim 1, characterised in that the second temperature range is selected in such a manner that the treated alloy has its tensile strength reduced by an amount in the range about 10% to about 40%, and preferably in the range about 15% to about 35%, relative to the tensile strength which would be obtained with single solution heat treatment at the first temperature range and for a duration equal to the sum of the first and second durations.
3. Method according to claim 1 or 2, characterised in that the second temperature range is lower than the first temperature range by about 8% to 14%.
4. Method according to one of claims 1 to 3, characterised in that the first duration lies in the range about 1 h to about 4 h, and preferably in the range about 1 h to about 2 h.
5. Method according to one of claims 1 to 4, characterised in that the second duration lies in the range about 1 h to about 5 h, and preferably in the range about 1 h to 3 h.
6. Method according to one of claims 1 to 5, characterised in that the duration of step (b) is greater than or equal to about 30 min, and preferably lies in the range about 1 h to 2 h and 30 min.
7. Casting made of an alloy based on aluminium, silicon and magnesium, and optionally copper, with in particular a copper,content of no more than 1% by weight, and presenting improved machinability, characterised in that it presents:

   - a tensile strength lying in the range 220 MPa to 300 MPa;

   - a 0.2% elastic limit lying in the range 170 MPa to 270 MPa;

   - a Brinell hardness number lying in the range 75 to 110.

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