EP0419375B1 - High strength magnesium alloys and process for manufacturing by rapid solidification - Google Patents

Description

The present invention relates to magnesium alloys with high mechanical strength and their manufacturing process and constitutes a development of the invention described in French application 88-02885 (FR-A-2627 780).

• une composition pondérale située dans les limites suivantes:

  Aluminium

  2-11 %, de préférence 3 à 9%

  Zinc

  0-12 %, de préférence 0 à 3%

  Manganèse

  0-1 %, de préférence 0,1 à 0,2%

  Calcium

  0,5-7%, de préférence 1 à 7%

  Terres Rares (TR)

  0,1-4%, de préférence 0,5 à 2,5%

  avec les teneurs suivantes en impuretés principales:

  Silicium

  < 0,6 %

  Cuivre

  < 0,2 %

  Fer

  < 0,1 %

  Nickel

  < 0,01 %

  le reste étant du magnésium.
• une dimension moyenne de grains inférieure à 3 µm
• ils sont constitués d'une matrice homogène renforcée par des particules de composés intermétalliques précipitées aux joints de grains Mg₁₇Al₁₂ éventuellement Al₂Ca, selon la concentration en Ca, Mg₃₂(Al,Zn)₄₉, si Zn est présent dans l'alliage, Mg-TR et/ou Al-TR, selon la teneur et/ou la nature de la terre-rare, ces particules étant d'une taille moyenne inférieure à 1µm et de préférence inférieure à 0,5µm cette structure demeurant inchangée après maintien de 24h à 300°C. Quand Mn est présent, c'est un élément au moins quaternaire et sa teneur pondérale minimum est de préférence de 0,1 %.

These alloys have a breaking load at least equal to 290 MPa, but more particularly at least 400 MPa and an elongation at break generally of at least 5% and have, in combination, the following characteristics:

• a weight composition within the following limits:

  Aluminum

  2-11%, preferably 3 to 9%

  Zinc

  0-12%, preferably 0 to 3%

  Manganese

  0-1%, preferably 0.1 to 0.2%

  Calcium

  0.5-7%, preferably 1 to 7%

  Rare Earth (TR)

  0.1-4%, preferably 0.5-2.5%

  with the following contents of main impurities:

  Silicon

  <0.6%

  Copper

  <0.2%

  Iron

  <0.1%

  Nickel

  <0.01%

  the rest being magnesium.
• an average grain size of less than 3 µm
• they consist of a homogeneous matrix reinforced by particles of intermetallic compounds precipitated at the grain boundaries Mg₁₇Al₁₂ possibly Al₂Ca, depending on the concentration of Ca, Mg₃₂ (Al, Zn) ₄₉, if Zn is present in the alloy, Mg-TR and / or Al-TR, depending on the content and / or the nature of the rare earth, these particles being of an average size less than 1 μm and preferably less than 0.5 μm this structure remaining unchanged after 24 hours at 300 ° C. When Mn is present, it is an at least quaternary element and its minimum weight content is preferably 0.1%.

Such alloys also have improved corrosion resistance; in fact, unlike the alloys described in French applications 88-02885 and 89-01913 (FR-A-2642 439), which exhibit localized corrosions (for example pitting, corrosion according to the machining streaks, etc.) which can cause along the weak zones, they exhibit corrosion that is at least as weak but also more homogeneous. The alloys according to the invention therefore contain, in the required proportions, both calcium and rare earths, in particular Y (understood here as a TR), Nd, Ce, La, Pr or misch metal (MM). These additions make it possible to improve the mechanical properties of the magnesium-based alloys obtained after rapid quenching and consolidation by spinning, including for spinning temperatures which can, while retaining an attractive level of characteristics, reach or even exceed 350 ° C. Such a property makes it possible in particular to increase the spinning ratios and speeds, the alloy supporting the resulting heating without losing its characteristics, and thus makes it possible to improve the productivity. The mechanical properties obtained are thus superior to those of magnesium-aluminum-zinc alloys containing rare earths but no calcium, such as those described in EP-A-219 628.

In the final alloy, the calcium may be in the form of Al₂Ca dispersoids precipitated at the grain boundaries and / or in solid solution. The particles of the intermetallic compound Al₂Ca appear when the Ca concentration is sufficient; they are less than 1 µm in size and preferably less than 0.5 µm in size. The same is true for the TRs, the dispersoids appearing from certain concentrations specific to each of the TRs. The presence of Mn is not necessary.

Other intermetallic particles, for example based on Al and Mn, of very small size (of the order of 40 to 50 nanometers) can also be dispersed in the magnesium grains.

According to the invention, the alloys are obtained by the casting, rapid cooling and consolidation processes and their various modes of implementation described in the French application 88-02885 already cited which form an integral part of the description. We note, in summary, that the alloy in the liquid state is subjected to rapid solidification, at a speed at least equal to 10⁴K sec ⁻¹, generally less than 10⁶K sec⁻¹, so as to obtain a solidified product, of which at least one of the dimensions is less than 150 μm, said product then being consolidated directly by precompaction and compacting or by direct compacting, the compacting taking place at a temperature between 200 and 350 ° C. It is preferable that the solidified product does not undergo any other conditioning operation such as grinding before being consolidated by precompaction and / or compacting, this operation possibly being of a nature to alter the mechanical characteristics of the consolidated alloy obtained.

• soit par coulée sous forme de ruban sur un appareil dit "d'hypertrempe sur rouleau", constitué habituellement d'un tambour refroidi énergiquement sur lequel on coule le métal.
• soit par fusion d'une électrode ou par jet de métal liquide; le métal liquide est alors mécaniquement divisé ou atomisé et projeté sur une surface énergiquement refroidie et maintenue dégagée,
• soit par atomisation de l'alliage liquide dans un jet de gaz inerte.

Rapid cooling for solidification can be obtained:

• either by casting in the form of a ribbon on an apparatus known as "hyper-quenching on a roller", usually consisting of an energetically cooled drum onto which the metal is poured.
• either by fusion of an electrode or by jet of liquid metal; the liquid metal is then mechanically divided or atomized and projected onto an energetically cooled surface and kept clear,
• or by atomization of the liquid alloy in a jet of inert gas.

The first two modes of application make it possible to obtain a solid in the form of ribbons, scales or platelets, while the latter gives powder. These methods are described in detail in French application 88-02885 and do not form part of the invention as such.
The rapidly solidified product can be degassed under vacuum at a temperature less than or equal to 350 ° C. before consolidation.

Consolidation, also described in French application 88-02885, is carried out, according to the invention, directly on the solidified products, in particular directly on the scales or plates. To preserve the fine and original structure obtained by rapid solidification, it is essential to avoid long exposures to high temperatures. We chose to operate a warm spinning which minimizes the duration of passage at high temperature.

The spinning temperature is between 200 and 350 ° C; the spinning ratio is generally between 10 and 40, preferably between 10 and 20, and simultaneously the speed of advance of the pestle is preferably between 0.5 and 3 mm / sec, but it can be higher.

As described in the main application, the solid product before consolidation can be introduced directly into the press container, or after precompaction at a temperature of at most 350 ° C with introduction into a sheath of Mg or its alloys, or Al or its alloys, itself introduced into said container.

As a variant, it is possible to implement other compaction methods which do not produce a rise in temperature of the product beyond 350 ° C.: among these optional methods, mention may be made of hydrostatic spinning, forging, rolling and superplastic forming.

Thus the process according to the invention makes it possible to unexpectedly obtain a consolidated magnesium alloy which has, as already described, a fine structure (grains less than 3 μm) reinforced by intermetallic compounds, and high mechanical characteristics remaining unchanged, as well as the structure of said alloy, after prolonged maintenance at a temperature reaching, or even exceeding, 350 ° C.

Corrosion resistance is also improved in uniformity and in weight loss (which is reduced).

EXAMPLE

Several alloys were produced under rapid solidification conditions, identical to those used in the examples of the main application: casting on a wheel, peripheral speed of the wheel 10 to 40 m / s, cooling speed of between 10⁵ and 10⁶K s⁻ ¹. The ribbons obtained were then directly introduced into the container of a spinning press to obtain a consolidated alloy on which the characterization tests were carried out: microscopic examination, measurement of mechanical characteristics, resistance to corrosion (measured by soaking in a 5% NaCl solution for 3 days).

In Table 1, the operating conditions for spinning are given, and the characteristics of the alloys obtained:
Hv = Vickers hardness expressed in kg / mm2
TYS = elastic limit measured at 0.2% elongation, expressed in MPa
UTS = breaking load expressed in MPa
e = elongation at break expressed in%
corrosion = weight loss expressed in mg / cm² / day (mcd) aspect of corrosion

In this table appear tests 20-21-22 which illustrate the present invention, while tests 4-23-7-9-11-12 illustrate the prior art and are drawn in part from application FR 89-01913, already cited.

Tests 4 and 23 relate to alloys treated by rapid solidification and consolidation of composition identical to that of AZ91; tests 7-9-11-12 relate to alloys containing Ca also obtained by rapid solidification and consolidation. It is noted that all of these alloys exhibit corrosion results and / or mechanical characteristics that are lower than those of the alloys according to the invention. Samples 23, 4 and 7 undergo heterogeneous corrosion with relatively high weight losses; samples 4 and 7 also have mechanical characteristics much lower than those of the alloys according to the invention. Sample 11 exhibits uniform corrosion but a high loss of weight, comparable to that of alloy 20, and of mechanical properties much lower than those of the latter and also those of alloys 21 or 22. Finally, the sample 12 has excellent resistance to corrosion, on the other hand its mechanical characteristics are much lower than those of the alloys according to the invention.

It is seen, according to the invention, that the addition of rare earths allows a higher level of mechanical characteristics, improves the uniformity of corrosion (test 20-21-22) and reduces the weight loss (tests 21-22 ). It should be noted that the mechanical characteristics are obtained after consolidation spinning at 300 ° C., and that the difference with the prior art would increase if in the tests of said prior art the spinning had been done at such a high temperature.

Thus, the invention makes it possible to obtain alloys having an improved corrosion resistance (uniform corrosion, generally reduced weight loss) while having increased mechanical characteristics for a high spinning temperature. This last advantage is important since such temperatures make it possible to spin profiles of large dimensions and / or increase the spinning speeds while retaining good mechanical characteristics.

It should also be noted that this high spinning temperature makes it possible to improve the fatigue resistance of the alloys of the invention.

Claims (13)

1. Magnesium-based alloy with a breaking load of at least 290 mPa and an elongation at break of generally at least 5 % and having the combination of the following elements:

   - the weight composition falls within the following limits:

   Aluminium   2-11 %

   Zinc   0-12%

   Manganese   0-1 %

   Calcium   0.5-7%

   Rare Earths (RE)   0.1-4%

   with the following contents of the main impurities:

   Silicon   < 0.6 %

   Copper   < 0.2 %

   Iron   < 0.1 %

   Nickel   < 0.01%

   the remainder being magnesium;

   - there is a mean grain size below 3 µm and it is constituted by a homogeneous matrix reinforced by particles of the intermetallic compounds Mg₁₇Al₁₂ and optionally Al₂Ca as a function of the concentration of Ca,Mg₃₂(Al,Zn)₄₉, if Zn is present in the alloy, Mg- RE and/or Al-RE, as a function of the content and/or nature of the rare earth, said particles having a mean size below 1 µm and preferably below 0.5 µm, which are precipitated at the grain boundaries, said structure remaining unchanged if kept at 300°C for 24h.
2. Alloy according to claim 1, characterized in that its weight composition is within the following limits:

   Aluminium   3-9 %

   Zinc   0-3 %

   Manganese   0.1-0.2%

   Calcium   1 to 7%

   RE   0.5 to 2.5%

   with the following contents of the main impurities:

   Silicon   0.1-0.6 %

   Copper   < 0.2%

   Iron   < 0.1%

   Nickel   < 0.01%

   the remainder being magnesium.
3. Alloy according to either of the claims 1 and 2, characterized in that the rare earths are constituted by Y, Nd, Ce, La, Pr or Misch Metal.
4. Process for the production of an alloy according to claims 1 to 3, characterized in that said alloy, in the liquid state, undergoes rapid cooling at a speed of at least 10⁴ K sec⁻¹ , so as to obtain a solidified product, whereof at least one of the dimensions is below 150 µm and which is then directly compacted at a temperature between 200 and 350°C.
5. Process according to claim 4, characterized in that the rapid cooling is obtained by casting or pouring onto a highly cooled moving surface in the form of a continuous strip with a thickness below 150 µm.
6. Process according to claim 4, characterized in that the rapid cooling is obtained by spraying the liquid alloy into a highly cooled surface which is kept free.
7. Process according to claim 4, characterized in that the fast cooling is obtained by atomizing the liquid alloy by means of an inert gas jet.
8. Process according to one of the claims 4, 5, 6 and 7, characterized in that the rapidly solidified product is compacted by a means chosen from among press drawing, hydrostatic drawing, rolling, forging and superplastic deformation.
9. Process according to claim 8, characterized in that the rapidly solidified product is compacted by press drawing at a temperature between 200 and 350°C, with a drawing ratio between 10 and 40 and preferably between 10 and 20 and with a press ram advance speed between 0.5 and 3 mm/second.
10. Process according to claim 9, characterized in that the rapidly cooled product is introduced rapidly into the container of the drawing press.
11. Process according to claim 9, characterized in that the rapidly cooled product is previously introduced into a metal sheath made from aluminium, magnesium or an alloy based on one or other of these two metals.
12. Process according to any one of the claims 9 to 11, characterized in that the rapidly solidified product is firstly precompacted in the form of a billet at a temperature of at the most 350°C.
13. Process according to any one of the claims 9 to 11, characterized in that the rapidly cooled product is degassed in vacuo at a temperature equal to or below 350°C prior to consolidation.