EP0362086B1 - Process for producing items made from an aluminium alloy retaining a good fatigue resistance after a prolonged stay at a high temperature - Google Patents

Abstract

The invention relates to a process for the production of aluminum alloy components retaining a good fatigue strength when used hot. This process consists of producing an alloy containing by weight 11 to 26% silicon, 2 to 5% iron, 0.5 to 5% copper, 0.1 to 2% magnesium, 0.1 to 0.4% zirconium and 0.5 to 1.5% manganese, subjecting the alloy in the molten state to a fast solidification means, bringing it into the form of parts or components and optionally subjecting the latter to a heat treatment at between 490 DEG and 520 DEG C., followed by water hardening and annealing at between 170 DEG and 210 DEG C. These components are used more particularly as rods, piston rods and pistons.

Description

The present invention relates to a process for manufacturing parts made of aluminum alloy retaining good resistance to fatigue after prolonged hot keeping.

We know that aluminum has the particular property of being three times lighter than steel and of having good corrosion resistance. By combining it with metals such as copper and magnesium, its mechanical resistance is greatly improved. Furthermore, the addition of silicon gives a product having good wear resistance. These alloys doped with other elements such as iron, nickel, cobalt, chromium and manganese, see their heat resistance improved. A compromise between these addition elements makes aluminum a material of choice for the manufacture of parts for automobiles such as engine block, piston, cylinder, etc.

Thus, EP 144898 teaches an aluminum alloy containing by weight 10 to 36% of silicon, 1 to 12% of copper, 0.1 to 3% of magnesium and 2 to 10% of at least one element chosen from the group Fe, Ni, Co, Cr and Mn.

This alloy is applicable to the manufacture of parts intended for both the aeronautical and automotive industries, said parts being obtained by the technique of powder metallurgy comprising, in addition to shaping by compacting and spinning, an intermediate processing step thermal between 250 and 550 ° C.

If these parts respond well to the various properties set out above, there is one that has not been taken into account, namely fatigue resistance.

Document JP-A-6342344 is also known, which teaches an aluminum alloy with excellent mechanical properties at high temperature by powder metallurgy, characterized in that it consists of Si, Fe, Cu, Mg and Mn in the weight proportions. 12 ≦ If ≦ 28%, 2.0 ≦ Fe ≦ 10%, 0.8 ≦ Cu ≦ 5.0%, 0.3 ≦ Mg ≦ 3.5% and 0.5 ≦ Mn ≦ 5%, of one or more elements chosen from Zr, Hf, Ni, Ti, V, Cr , Mo, Nb and Ta in a proportion by weight of 0.02 - 2.0% and, for the rest, of aluminum and inevitable impurities. But here, as in the previous document, no mention is made of the fatigue resistance properties after being kept for 1000 hours at 150 ° C. and the range of manganese content is relatively wide.

However, a person skilled in the art knows that fatigue corresponds to a permanent, localized and gradual change in the metallic structure which occurs in materials subjected to a succession of discontinuous stresses and which can cause cracks and even ruptures. parts after an application of said stresses according to a greater or lesser number of cycles and this while their intensity is most often significantly lower than that which must be applied to the material continuously to obtain a rupture by traction. This is why the values of modulus of elasticity, of tensile strength, of hardness, of the rate of shrinkage not creep stated in the cited documents can not account for the ability of the alloy to fatigue resistance .

However, it is important for parts such as the connecting rods or the piston pins, for example, which work in dynamics and which are subjected to periodic forces, to have good resistance to fatigue.

This is why the applicant, having looked into this problem, has certainly observed that the parts manufactured from alloys falling within the scope of the above-mentioned document exhibited a resistance to fatigue which could be suitable for certain applications but, that it was possible to significantly improve this property by modifying their composition. It is for this purpose that it has developed parts in aluminum alloys containing by weight 11 to 22% of silicon, 2 to 5% of iron, 0.5 to 4% of copper, 0.2 to 1.5% magnesium, characterized in that it also contains 0.4 to 1.5% zirconium.

This invention was also the subject of French patent application No. 87-17674.

However, the Applicant has noticed that if the zirconium brought a significant improvement from the point of view of the fatigue limit at 20 ° C, since it went from 150 to 185 MPa, on the other hand, after a maintenance of 1000 hours at 150 ° C (which roughly represents the working conditions of a connecting rod at mid-life of an engine), that limit dropped to 143 Mpa, a reduction of more than 22%.

Continuing her work, she found that this drawback could be remedied by combining the action of zirconium with that of manganese.

Hence the present invention which consists of a process for manufacturing aluminum alloy parts obtained from the alloy in the state melted by a means of rapid solidification, retaining a suitable resistance to fatigue after prolonged keeping hot which contain by weight 11 to 26% of silicon, 2 to 5% of iron, 0.5 to 5% of copper, 0, 1% to 2% magnesium, zirconium, at least 0.5% manganese, optionally minor additions of nickel and / or cobalt and the aluminum balance, characterized in that an alloy containing 0.1 to 0.4% of zirconium and up to 1.5% of manganese is used.

These ranges frame the values for adding zirconium and manganese below which the effect is not significant and above which either the addition of zirconium no longer has a decisive influence, or the addition of manganese leads to embrittlement of the part and to a fall in the fatigue limit of a notched part, that is to say having surface irregularities such as no screws, connecting radii, etc.

Thus, compared to the composition described in the aforementioned patent application, manganese has been substituted for part of the zirconium, which on the one hand allows savings on raw materials: manganese being cheaper than zirconium , on the other hand facilitates the melting conditions of the alloy since a binary alloy containing 1% of zirconium has a liquidus temperature of 875 ° C whereas if it is 1% of manganese this temperature remains close 660 ° C.

However, in addition to the particular composition of the alloy used, the invention is also characterized in that the alloy is subjected in the molten state to a rapid solidification means before putting it in the form of parts. Indeed, as elements such as iron, zirconium and manganese are very little soluble in the alloy, it is essential to obtain parts meeting the desired characteristics to avoid a rough and heterogeneous precipitation of these elements what we achieves by cooling them as quickly as possible. In addition, the alloy is preferably melted at a temperature above 700 ° C so as to avoid any phenomenon of premature precipitation.

• 1) On divise l'alliage fondu sous forme de fines gouttelettes
  • soit par atomisation du métal fondu à l'aide d'un gaz ou par atomisation mécanique suivie d'un refroidissement dans un gaz (air, hélium, argon).
  • soit par pulvérisation centrifuge ou autre procédé apparenté.
  
  Cela conduit à des poudres de granulométrie inférieure à 400 µm qui sont ensuite, suivant les techniques bien connues de la métallurgie des poudres, mises en forme par compactage à froid ou à chaud dans une presse uniaxiale ou isostatique puis filage et/ou forgeage ;
• 2) On projette l'alliage fondu contre une surface métallique refroidie, suivant par exemple les techniques désignées par les Anglo-Saxons sous l'expression "melt spinning" ou "planar flow casting" et dont on trouve des descriptions dans les brevets US 4389258 et EP 136508, ou encore "melt overflow" et les techniques apparentées. On génère ainsi des rubans d'épaisseur inférieure à 100 µm qui sont ensuite mis en forme comme ci-dessus ;
• 3) On projette l'alliage fondu atomisé dans un courant de gaz contre un substrat, suivant par exemple les techniques encore appelées "spray déposition" ou "spray casting" dont une description est donnée dans le brevet GB 1379261 et qui conduit à un dépôt cohérent suffisamment malléable pour être mis en forme par forgeage, filage ou matriçage.

There are several ways to operate this rapid solidification:

• 1) The molten alloy is divided into fine droplets
  • either by atomization of the molten metal using a gas or by mechanical atomization followed by cooling in a gas (air, helium, argon).
  • either by centrifugal spraying or other related process.
  
  This leads to powders with a particle size of less than 400 μm which are then, according to well known techniques in powder metallurgy, shaped by cold or hot compaction in a uniaxial or isostatic press then spinning and / or forging;
• 2) The molten alloy is projected against a cooled metal surface, according for example to the techniques designated by the Anglo-Saxons under the expression "melt spinning" or "planar flow casting" and of which descriptions are found in US patents 4,389,258 and EP 136508, or also "melt overflow" and related techniques. Ribbons with a thickness of less than 100 μm are thus generated which are then shaped as above;
• 3) The atomized molten alloy is projected into a stream of gas against a substrate, for example according to the techniques also called "spray deposition" or "spray casting", a description of which is given in patent GB 1379261 and which leads to deposition coherent enough malleable to be shaped by forging, spinning or stamping.

This list is of course not exhaustive.

In order to further refine the precipitation structure, the parts, after being possibly subjected to machining, are heat treated between 490 and 520 ° for 1 to 10 hours, then quenched in water before undergoing a tempering treatment between 170 to 210 ° C for 2 to 32 hours, which improves their mechanical characteristics.

The invention will be better understood with the aid of the following application examples: a mass of base alloy, containing by weight 18% of silicon, 3% of iron, 1% of copper, 1% of magnesium, aluminum balance was melted around 900 ° C and then divided into 8 lots numbered 0 to 7.

• la gamme métallurgie des poudres (PM) comprend une atomisation dans une atmosphère d'azote de particules de granulométrie inférieure à 200 µm, puis un compactage sous 300 MPa dans une presse isostatique, suivi d'un filage sous forme de barres de diamètre 40 mm
• la gamme spray déposition (SD) utilise la technique du GB 1379261 et permet d'obtenir un dépôt sous forme d'une billette cylindrique qui est ensuite transformée en barre de diamètre 40 mm par filage.

To lots 1 to 7 different quantities of zirconium and manganese were added, lot 0 serving as a control.
Then these batches were treated either by powder metallurgy or by deposition spray:

• the powder metallurgy (PM) range includes atomization in a nitrogen atmosphere of particles with a particle size of less than 200 µm, then compacting under 300 MPa in an isostatic press, followed by spinning in the form of bars with a diameter of 40 mm
• the spray deposition range (SD) uses the technique of GB 1379261 and makes it possible to obtain a deposit in the form of a cylindrical billet which is then transformed into a bar with a diameter of 40 mm by spinning.

• le module d'Young E en GPa
• la limite élastique conventionnelle à 0,2% : R0,2 en MPa, la charge de rupture Rm en MPa, l'allongement A en %, ces mesures étant faites à 20°C puis à 150°C après 100 heures de maintien
• la limite de fatigue à 20°C au bout de 10⁷ cycles, Lf en MPa, sur des éprouvettes lisses à l'état T6 suivant les normes de l'Aluminium Association et sollicitées par flexion rotative
• la même mesure que précédemment mais après un maintien de l'éprouvette pendant 1000 heures à 150°C
• le rapport d'endurance Lf/Rm à 20°C
• la limite de fatigue à 20°C comme ci-dessus mais sur éprouvette entaillée avec Kt = 2,2
• le coefficient de sensibilité à l'entaille $$\text{q =}\frac{\text{Kf-1}}{\text{Kt-1}}$$
  
  
     ou Kf est le rapport de la limite de fatigue mesurée sur éprouvette lisse à la limite de fatigue sur éprouvette entaillée (l'alliage est d'autant plus sensible à l'entaille que q est élevé).

These parts are then treated for 2 hours between 490 and 520 ° C then quenched with water and subjected for 8 hours at a temperature between 170 and 200 ° C.
The following characteristics were measured on test tubes of each of these parts according to techniques well known to those skilled in the art:

• the Young E module in GPa
• the conventional elastic limit at 0.2%: R0.2 in MPa, the breaking load Rm in MPa, the elongation A in%, these measurements being made at 20 ° C and then at 150 ° C after 100 hours of holding
• the fatigue limit at 20 ° C after 10⁷ cycles, Lf in MPa, on smooth specimens in the T6 state according to the standards of the Aluminum Association and stressed by rotary bending
• the same measurement as above but after holding the test piece for 1000 hours at 150 ° C.
• the endurance ratio Lf / Rm at 20 ° C
• the fatigue limit at 20 ° C as above but on a notched test piece with Kt = 2.2
• the cut sensitivity coefficient $$\text{q =}\frac{\text{Kf-1}}{\text{Kt-1}}$$
  
  
  where Kf is the ratio of the fatigue limit measured on a smooth specimen to the fatigue limit on a notched specimen (the alloy is all the more sensitive to the notch as q is high).

All the results of these measurements are shown in the following table.

From these measurements, we deduce that if the fatigue limit after maintaining 1000 hours at 150 ° C is 120 MPa for an alloy containing neither zirconium nor manganese (N ° = 0), the addition of 1% of zirconium (N ° = 1) increases this characteristic to 148 MPa and the simultaneous addition of zirconium and manganese with a lesser amount of zirconium (N ° = 5) allows to reach a value of 177 MPa.

In addition, the simultaneous presence of zirconium and manganese makes it possible to greatly attenuate the degradation of the fatigue limit which occurs after maintenance at 150 ° C. Indeed, with the alloy N ° = 1 without manganese, Lf goes from 185 to 143 MPa or a degradation of 42 MPa, while with the alloy N ° = 5 containing 1.2% of manganese, Lf goes from 193 to 177 MPa is a degradation of 16 MPa, a much lower value than the previous one.

These measurements also show that these elements improve the fatigue limit on notched parts but that their presence in too large quantities contributes to degrading this characteristic and to increasing the brittleness. Thus, the value of this limit goes from 100 MPa for the test piece N ° = 0 to 125 MPa for the test piece N ° = 3 (0.1% Zr - 0.6% Mn) but drops to 105 MPa for the 'test tube N ° = 7 more loaded with zirconium and manganese.

It can thus be seen that the simultaneous presence of zirconium and manganese in the proportions of the invention (alloys 5, 4, 3, 6) leads to a lower coefficient of sensitivity to the notch (0.51-0, 48-0.43-0.51) than for alloys of the prior art where the coefficient is around 0.6 apart from alloy n ° = 0 which, moreover, cannot be used because of its too low mechanical resistance.

Thus according to the invention, the combination of zirconium-manganese in limited quantities and the rapid solidification of the alloy obtained contribute to improving the resistance to fatigue, whether cold or hot, of parts liable to exhibit irregularities. surface like threads or connection curves and which find their application in the automobile industry, in particular in the confection of rods, axes of pistons and pistons.

Claims (5)

1. Process for the production of aluminium alloy components obtained from the alloy in the molten state by a fast solidification means, retaining an appropriate fatigue strength after keeping hot for a long time and containing by weight 11 to 26% silicon, 2 to 5% iron, 0.5 to 5% copper, 0.1 to 2% magnesium, zirconium, at least 0.5% manganese and optionally minor additions of nickel and/or cobalt and the remainder aluminium, characterized in that use is made of an alloy containing 0.1 to 0.4% zirconium and up to 1.5% manganese.
2. Process according to claim 1, charcterized in that the fast solidification means consists of dividing the molten alloy into the form of fine droplets.
3. Process according to claim 1, characterized in that the fast solidification means consists of projecting the molten alloy against a cooled metal surface.
4. Process according to claim 1, characterized in that the fast solidification means consists of projecting the atomized alloy in a gas flow against a substrate.
5. Process according to claim 1, characterized in that the parts undergo a heat treatment at a temperature between 490 and 520°C, water hardening and annealing at between 170 and 210°C.