EP0676479A1 - Aluminium alloys containing bismuth, cadmium, indium and/or lead in a very finely dispersed state and process for manufacturing these alloys - Google Patents

Abstract

Aluminium alloys contg. at least one metal from the group Bi, Cd, In and Pb are produced by adding the metal in an quantity of above the max. solubility such that more than 80 wt.% of the added metal is finely dispersed in the A1 matrix in the form of globules or crystals of size below 35 mm. More than 50 wt% Pb as additive is finely dispersed in the Al matrix in the form of globules or crystals of size below 1mm. To produce the alloy, the liquid metal is mechanically or electromagnetically stirred during solidification. An alternating magnetic field is arranged coaxially to the axis of concasting material. <IMAGE>

Description

TECHNICAL AREA

The invention relates to aluminum alloys containing in the very finely dispersed state metals having very low solubility in solid aluminum such as Bi, Cd, In and / or Pb as well as their solidification process.

STATE OF THE ART

For many years, the evolution of knowledge and techniques has led to the development and marketing of aluminum alloys with increasingly high performance. This increase in performance has been achieved, in particular, by defining the ranges of composition of these alloys which are increasingly narrow and targeted, including by incorporating therein chemical elements at very low contents also in the range of very narrow composition.

As an extreme example of this development, mention may be made of refined aluminum intended for the manufacture of electrolytic capacitors, the performance of which has been very significantly improved by incorporating therein, in the form of "traces" (fractions of ppm , or ppm) certain elements such as Bismuth, Cadmium, Indium and / or Lead.

Examples of the very favorable action of such dopings, in traces of these metals, are described in numerous documents and in particular:
J 53 114059 (SHOWA AL)
J 54 043563 (SHOWA AL)
J 57 057856 (SHOWA AL)
J 57 110 646 (SUMITOMO AL and TOYO)
J 63 288008 (SUMITOMO LIGHT METALS), or
J 01 128419 (SUMITOMO LIGHT METALS).

These patents, although fairly broadly defining the desirable dopings, do not specify the practical way of carrying them out, nor even the preferred, practically very narrow ranges of contents.

The universal way of practicing such additions, very small and very precise, in elements favorable to the final use of the metal, consists in the addition, the fusion, and the dispersion of master alloys containing these favorable elements in the bath. of liquid aluminum alloy which it is desired to optimize, in quantities such that the final content of molten metal in favorable elements is within a range considered to be optimal.

PROBLEM

In the particular case of the addition to the aluminum of metals belonging to the group formed by Bismuth, Cadmium, Indium and Lead, in amounts not exceeding 10 parts per million in the final alloy, the applicant found that this universal way of proceeding, using the mother alloys available on the market, even very pure, led to erratic and extremely dispersed results, not compatible with an optimization of the final properties required of the product thus produced.

By examining the reasons which could explain such an excessive variability of the results, the Applicant has found that the major origin could be a very insufficient homogeneity of composition of the mother alloys used.

Generally, these commercially available master alloys are obtained by natural solidification in molds of the molten master alloy, so as to obtain molded parts which can be used for correcting the desired composition. These molded parts are most often presented in the form of molded plates with a thickness of a few centimeters, possibly separable, or ingots of a few hundred grams.

However, a careful examination of these products by the applicant has shown that heavy filler metals such as Bismuth, Cadmium, Indium and Lead with low melting point, very poorly soluble in solid aluminum, and very dense, they were abnormally distributed in a very heterogeneous way, and most often present in the form of globules or crystals of size greater than 20 micrometers and sometimes greater than 1 mm.

It was then reasonable to think that such large and very dense globules or crystals could have remained trapped by their density at the bottom of the processing furnace, and that the small specific surface of large globules or crystals of filler metal thus sedimented could cause a very low rate of dissolution and diffusion of these dense filler metals in the bath of much less dense liquid aluminum alloy, thereby leading to final contents of these very erratic and dispersed metals.

The problem to be solved was therefore to develop master alloys containing Bismuth, Cadmium, Indium and / or Lead in which these dense elements and poorly soluble in aluminum would be very finely dispersed in the matrix of Aluminum, and very homogeneously throughout the volume.

However, if we consider the phase diagrams of the binary alloys Al-Bi, Al-Cd, Al-In and Al-Pb, collected in Figures 1a, 1b, 1c, 1d, we see that these diagrams have very strong analogies, and that consequently Bi, Cd, In and Pb form a very particular and very homogeneous group of elements of alloy of Aluminum.

The essential point which would explain to a large extent the practical difficulties encountered is that the alloys of Aluminum with these metals which are very insoluble in the solid state have a miscibility gap in the liquid state (zone indicated (L1) + (L2 ) on these diagrams), which implies that the usual master alloys of aluminum with these metals are necessarily two-phase and heterogeneous in the solidified state, and necessarily comprising zones very rich in addition metals, therefore very poor in aluminum.

Aluminum poor in addition metals solidifies first, by "rejecting" a liquid very enriched in dense addition metals, which therefore tends to gather in large heterogeneous globules under the effect of surface tension forces. and gravity.

It therefore seemed unreasonable to seek to obtain master alloys, containing significant contents in additions of "heavy" metals belonging to the group of Bi, Cd, In and / or Pb, where these metals would be very finely dispersed in the aluminum matrix. Examination of the products available on the market supported this analysis.

OBJECT OF THE INVENTION

The invention nevertheless proposes new aluminum master alloys containing, in addition, heavy metals in quantities greater than the maximum solubility of these metals in aluminum and very finely dispersed to have a high speed of dissolution and a high dissolution yield. in liquid aluminum alloys.

More precisely, it relates to an aluminum alloy containing at least one metal belonging to the group formed by Bismuth, Cadmium, Indium and Lead in an amount greater than the maximum solubility of these metals in solid aluminum, characterized in that more than 80% by weight of these addition metals are finely dispersed in the solid aluminum matrix in the form of globules or crystals of size less than 5 micrometers.

It also consists in revealing a process for solidifying such alloys comprising mechanical or electromagnetic stirring of the metal being solidified which makes it possible to produce a homogeneous mixture of aluminum crystals poor in addition metal and the residual liquid enriched in metal d 'addition, suitable during final solidification to give an alloy where the addition metals belonging to the group formed by Bi, Cd, In and / or Pb are finely dispersed in the aluminum or alloy matrix.

DESCRIPTION OF THE INVENTION

In a first experiment, the applicant conventionally produced a molten aluminum alloy containing 0.20% of Lead, by melting 50 kg of refined aluminum and 100 g of lead, in an electric furnace with a graphite crucible . The alloy thus melted was homogenized by stirring the liquid using a graphite rotor.

A first part of the molten alloy was poured into small ingots of diameter 50 mm and height about 50 mm, with a unit weight of about 250 g, in small crucibles made of aluminous refractories. 100 ingots were thus produced.

After being rehomogenized by mixing with the rotor, the rest of the molten alloy was poured into a billet with a diameter of 100 mm and a length of about 1,150 mm, in a vertical continuous casting installation, the molding ring of which had been surrounded by a turn coaxial with this ring, traversed by an alternating current of low frequency (<100 Hertz) according to French patents FR 2530510 and FR 2530511. The object of this turn traversed by an alternating current was to cause a magneto-hydrodynamic mixing of the liquid metal during solidification, so as to maintain as complete a homogeneity as possible in the composition of this metal until complete solidification.

This billet was then cut with a band saw into slices approximately 15 mm thick. There were thus obtained 70 slices of average unit weight approximately 320 grams.

We then compared, by macrography and micrography, the distribution of lead in slices taken axially from 10 ingots and 10 slices of billets.

In the case of billet slices, an extremely fine dispersion of lead has been observed, in the form of small globules of size mainly between 0.1 μm and 1 μm, with exceptionally globules of size greater than 5 μm without exceeding 10 µm.

An example of the micrographs obtained, after polishing and anodic oxidation of the billet slices, is given in FIG. 2b.

It is noted that the small lead globules are distributed very homogeneously inside the aluminum grains, at the junction of the dendritic solidification cells which have constituted these grains.

On the other hand, in the case of ingot wafers, we see according to FIG. 2a representative of the prior art the presence of globules of size much greater than 20 micrometers, with sometimes millimeter segregations. In addition, the distribution of these globules is not homogeneous over the section of the ingots.

These master alloys, in the form of ingots or billet slices, were then used to make lead additions in refined aluminum intended for the manufacture of electrolytic capacitors.

9 castings of approximately 12 tonnes per unit were carried out, with the addition of lead in the form of ingots, and 8 flows of approximately 12 tonnes per unit, with the addition of lead in the form of billet slices.

The overall results were as follows:

EXAMPLE 1 Addition of Lead in the Form of Ingots

The balance of the 9 flows is established as follows: Weight of molten aluminum 109.275 kg Initial lead content of this aluminum 0.193 ppm Weight of longotins loaded 22,120 kg Aluminum lead content 0.435 ppm
Recovery efficiency of lead brought in by ingots: 26.38 grams actually recovered in cast aluminum, compared to 44.24 grams introduced via ingots, an average yield of 59%.

It is further noted that, casting by casting, the results of the recovery yield of the lead introduced are extremely variable, sometimes falling to 30%, and sometimes rising to almost 150%, which shows that poorly dissolved lead during a operation may resurface during a next pour.

On average, out of the 9 castings considered, the recovery yield of the lead introduced has a standard deviation of 27%.

Example 2: Addition of lead in the form of billet slices

The balance sheet of the 8 castings carried out is established overall as follows: Weight of molten aluminum 95.530 kg Initial lead content of this aluminum 0.175 ppm Weight of slices loaded 17.22 kg Aluminum lead content 0.473 ppm
Yield of recovery of the lead brought by the slices of billets: 28.66 g recovered for 34.40 g introduced via the slices, ie an average yield of 83%.

There is also a much lower dispersion of the yields calculated cast by cast: the standard deviation of the individual yields falls to 17%, the majority of which can be attributed to the analytical uncertainty on the analysis of lead to also low contents.

The comparison of Examples 1 and 2 therefore shows quite clearly that the more or less fine structure of the aluminum-lead master alloy has a very clear influence on the recovery yield, in the final product, of the lead introduced. , and on the reproducibility of the results.

This comparison further demonstrates that a master alloy structure such as the majority (by weight) of lead is very finely dispersed in the aluminum matrix, in the form of globules or crystals of size less than 1 micrometer, leads to much higher and much more reproducible recovery yields than a structure of mother alloy such that lead is mainly present in the form of globules of size greater than 20 micrometers.

The master alloy according to the invention therefore has a very clear advantage over the master alloys according to the prior art.

A particular mode of obtaining such a master alloy, with more than 80% by weight of the finely dispersed lead in the form of globules or crystals of size less than 5 μm and more than 50% by weight of the lead is finely dispersed under The shape of globules or crystals of size less than 1 μm has been described, which consists of an electromagnetic agitation of the liquid being solidified during a continuous vertical casting of the metal.

In a second step, we then investigated whether other equivalent methods of stirring the liquid during solidification gave equivalent dispersion results, not only for lead, but also for other dense metals sparingly soluble in solid aluminum, belonging to the group formed by Bismuth, Cadmium and Indium.

In a first step, molten alloys were produced comprising 0.15% by weight of lead, 0.50% by weight of Bismuth, 1% by weight of Cadmium and 1% by weight of Indium respectively. These contents are all higher than the maximum solubility of these metals in solid aluminum and lower than the monotectic content beyond which appears a phenomenon of demixion in liquid phase, even before any beginning of solidification.

• un 1er lot a été solidifié par coulée en continu, sous forme de billette cylindrique, avec agitation électromagnétique par une spire d'induction coaxiale à l'axe de coulée,
• un deuxième lot a été solidifié en petits moules de réfractaire alumineux, sans agitation,
• un troisième lot a été solidifié par coulée en continu, sous forme de billette cylindrique, avec un brassage du métal liquide en cours de solidification à l'aide d'une hélice en graphite de diamètre égal à 0,5 fois le diamètre de la billette et une vitesse de rotation de 250 tours par minute,
• un quatrième lot a été solidifié en moules de réfractaire alumineux, ces moules étant placés à l'intérieur d'une spire d'induction parcourue par un courant alternatif de 60 Hertz, permettant d'effectuer un brassage électromagnétique du métal en cours de solidification.

In each case, the liquid alloys were homogenized, in a crucible furnace, using a graphite rotor, then poured under the following conditions:

• a 1st batch was solidified by continuous casting, in the form of a cylindrical billet, with electromagnetic agitation by an induction coil coaxial with the casting axis,
• a second batch was solidified into small aluminous refractory molds, without stirring,
• a third batch was solidified by continuous casting, in the form of a cylindrical billet, with stirring of the liquid metal being solidified using a graphite propeller with a diameter equal to 0.5 times the diameter of the billet and a rotation speed of 250 revolutions per minute,
• a fourth batch was solidified into aluminous refractory molds, these molds being placed inside an induction coil traversed by an alternating current of 60 Hertz, allowing electromagnetic stirring of the metal being solidified.

• a) les billettes coulées en continu avec brassage du métal en cours de solidification donnaient dans tous les cas la dispersion la plus fine du métal d'apport quel qu'ait été le procédé d'agitation utilisé, électromagnétique (1er lot) ou mécanique (3° lot).
  Plus de 80% en poids du métal d'apport était dispersé dans la matrice d'Aluminium sous forme de globules ou cristaux de taille inférieure à 2 micromètres pour le Plomb, à 3 micromètres pour le Bismuth et à 5 micromètres pour le Cadmium et l'Indium.
• b) les lingotins solidifiés en moule de réfractaire alumineux sans agitation (2° lot) présentaient dans tous les cas la dispersion la moins bonne, avec très fréquemment la présence de fortes ségrégations des métaux d'apport, de taille supérieure à 100 micromètres et parfois même supérieure à 1 millimètre.
• c) les lingotins solidifiés en moule réfractaire alumineux (4° lot) avec agitation électromagnétique du métal en cours de solidification, présentaient des caractéristiques intermédiaires entre les cas (a) et (b), présentant une taille moyenne des globules ou cristaux de métal d'apport sensiblement plus faible que la structure observée dans le cas (b), mais avec parfois la présence de ségrégations de taille supérieure à 100 micromètres.

Micrographic examination of aluminum alloys containing additions of Bismuth, Cadmium, Indium or Lead gave the following results:

• a) the continuously cast billets with stirring of the metal in the course of solidification gave in all cases the finest dispersion of the filler metal whatever the stirring process used, electromagnetic (1st batch) or mechanical ( 3rd lot).
  More than 80% by weight of the filler metal was dispersed in the Aluminum matrix in the form of globules or crystals of size less than 2 micrometers for Lead, 3 micrometers for Bismuth and 5 micrometers for Cadmium and l 'Indium.
• b) the ingots solidified in an aluminous refractory mold without stirring (2nd batch) had in all cases the least good dispersion, with very frequently the presence of strong segregation of the filler metals, of size greater than 100 micrometers and sometimes even greater than 1 millimeter.
• c) the ingots solidified in an aluminous refractory mold (4 ° batch) with electromagnetic agitation of the metal being solidified, had intermediate characteristics between cases (a) and (b), having an average size of the globules or crystals of metal input significantly lower than the structure observed in case (b), but sometimes with the presence of segregations larger than 100 micrometers.

While presenting a clear improvement compared to ingots solidified without agitation of the liquid, the ingots thus poured did not present a structure quite as fine and regular as that of the continuous casting billets, solidified with mechanical or electromagnetic agitation of the marsh of liquid.

Claims (4)

Aluminum alloy containing at least one metal belonging to the group formed by Bismuth, Cadmium, Indium and Lead, in an amount greater than the maximum solubility of these metals in solid aluminum, characterized in that more than 80 % by weight of these addition metals are finely dispersed in the solid aluminum matrix in the form of globules or crystals of size less than 5 micrometers. Aluminum alloy according to claim 1, characterized in that more than 50% by weight of lead, as an addition metal, is finely dispersed in the solid aluminum matrix in the form of globules or crystals of size less than 1 micrometer. Method for solidifying an aluminum alloy comprising at least one metal belonging to the group formed by Bismuth, Cadmium, Indium and Lead, in an amount greater than the maximum solubility of these metals in solid aluminum, characterized in that the liquid metal being solidified is subjected to mechanical or electromagnetic stirring. Solidification method according to claim 3, characterized in that this method comprises a continuous casting of the liquid alloy with stirring by an alternating magnetic field coaxial with the axis of continuous casting.

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