EP0167460A1 - Process for manufacturing titane-based alloys with small granular dimensions by means of powder metallurgy - Google Patents

Abstract

A titanium-based alloy is produced by compacting a powder consisting of particles of pre-alloyed titanium or particles of titanium and master alloy, heat treatment at high temperature and quenching. The heat treatment is carried out at least in part at a temperature above the point of transformation in the β phase. The powder also contains a dispersion of fine particles of a product slowing the growth of the grain size, product constituted by a chemical species stable with respect to titanium or having at least one element very little soluble in titanium at temperatures reached during heat treatment. The magnification of the β grain is thus blocked.

Description

The subject of the invention is a process for the production of a titanium-based alloy having a small grain size, the term "alloy" having to be interpreted as extending to a product containing, in addition only to titanium, only addition elements with sufficiently low content so that the transformation of phase α → β remains.

Titanium, pure and above all in the form of an alloy of which it is the main constituent, has properties of lightness, mechanical strength and temperature which are leading to the use of it more and more in many fields, and in particular in aeronautics. As a general rule, in order to produce titanium parts by powder metallurgy, one starts with a titanium powder containing the addition elements, densifies by spinning or isostatic compression and performs a heat treatment. It has hitherto generally been considered advantageous to carry out this treatment at a temperature below the point of transformation in the β phase, because beyond this point there is an excessive magnification of the metallurgical grains, magnification which is detrimental to certain mechanical characteristics, for example of the ductility and the tensile strength of the product obtained, even after partial return of the single-phase p phase to phase a during cooling to room temperature following the heat treatment.

However, the choice of a temperature below the transformation point leads to depriving ourselves of the advantages which the acicular type structure which the partial transformation of the p phase obtained at high temperature, in α phase gives: among these advantages we can cite the better resistance to sudden crack propagation (i.e. the toughness of alloy), the propagation of fatigue cracks, creep, and stress corrosion in salt water.

The invention starts from the observation that the harmful influence of passing through a temperature above the transformation point is not inherent in the acicular structure, but is linked to the size of the β-grain, which increases very rapidly during maintaining the temperature until it reaches and exceeds 0.5 mm, and leads after the final quenching operation to a structure whose grain size (eg 0) depends directly on that obtained at the end of the heat treatment.

This problem also exists for certain titanium alloys called "almost alpha" for which the treatment in β phase is commonly applied and leads to an excessive magnification of the metallurgical grain.

The invention therefore aims to provide a process for developing a titanium-based alloy which better meets those previously known than the requirements of the practice, in particular in that it provides the advantages associated with a heat treatment at high temperature, while avoiding exaggerated magnification of grain in β phase, magnification detrimental to mechanical properties.

To this end, the invention proposes a process for the preparation of a titanium-based alloy comprising the steps of compacting (or densifying) a powder consisting of particles of pre-alloyed titanium or particles of titanium and of master alloy, high temperature heat treatment and quenching, characterized in that the powder also contains a dispersion of fine particles of a product slowing the growth of grain size at a volume content lower than that which would lead to the formation of a continuous layer of fine particles around the particles of titanium powder, the particles of product appearing on the surface of the titanium grains in the alloy produced.

The product will consist of a simple body, a compound or a mixture of a species chemically stable with respect to titanium or having at least one element very sparingly soluble in titanium. This adduct may be stable during processing, or may be transformed by reaction with the elements of the alloy.

One can think, without the completeness and rigor of this explanation having to be considered as an imperative for the validity of the present patent, that two phenomena intervene to limit the grain size.

A first phenomenon is constituted by a blocking of the grain boundaries P by the fine particles of the species which remain stable or are transformed during production, distributed around the periphery of the other powder particles. The metallurgical grains appearing during the transition to phase P are then confused with the particles of the original powder.

A second phenomenon consists of braking the migration of the grain seal β during the heat treatment. In this case, the particles of the adduct, stable or transformed, are then inside the metallurgical grain P which is larger than the initial powder grains, but remains smaller than the metallurgical grains in free control samples. of adduct and this up to very low contents of adduct (which constitutes a very favorable factor insofar as the use of an additive having a weakening effect is envisaged) .

The two above phenomena have been observed, one or the other being the only one present or preponderant for certain adducts or certain content ranges.

A priori one could think that it would be impossible to find a product which can be used in fine particles and fulfilling the required function, made of the very high reactivity of titanium vis-à-vis chemical species however reputedly difficult to decompose. For example, silica, tantalum carbide, alumina are rapidly dissolved at temperatures of 1000 ^° C, that is to say close to the transformation point in β phase.

However, examination of the free enthalpy of formation of certain oxides, and in particular of rare earth oxides, reveals values at 1000 ° C sufficient to give hope for the possibility of obtaining favorable results. Similarly, examination of the properties of various elements frequently used in metallurgy show a sufficiently low solubility to meet the required conditions. Indeed, it has been possible in particular to use various oxides, and in particular Y ₂ O ₃ and Dy ₂ O ₃ .

It has also been possible to use metalloids in elementary form or of compounds, in particular boron, BN. B4C, 84Si, B6Si and LaB6. Sulfur, WS, MoS2, ZnS can also be considered, as well as phosphorus, the latter however requiring special precautions due to its high flammability. We can also use elements from columns 5 and 6 of the periodic table which are related to S and P (Se, Te, As). All of these have a low solubility in titanium.

Satisfactory results are obtained by respecting various conditions relating to the starting powder (pre-alloyed particles or mixture), adduct and heat treatment.

These conditions will now be considered, with particular reference to the particularly interesting case of the titanium alloy called TA6V, at 6% by weight of aluminum and 4% of vanadium.

The particles of pre-alloyed titanium (or of titanium and of addition elements) are advantageously produced by a process giving rise to approximately spherical particles, with a regular and fine particle size, the size of the particles conditioning that of the grains. The particle size range from 4Q to 300 µm will generally be acceptable.

It is possible in particular to use powders of pre-alloyed titanium obtained by spraying according to a process with a rotating electrode, currently well known and widely used for the preparation of powders of titanium alloys.

The median value of the diameter of TA6V alloy particles obtained by this process is of the order of 160 μm. It is possible by sieving to limit as much as desired the fraction retained.

The essential criterion for the choice of the initial particles of the additive intended to slow down the growth of the grain is dimensional stability. This dimensional stability can be obtained if the adduct used is chemically stable in the presence of the titanium alloy. However, this condition is not essential. The particles can react and chemically transform without dissolving in the metal matrix. One of the factors which oppose this dissolution is the very low solubility in titanium of an element entering into the composition of the initial particle and experience has shown that this factor is preponderant.

• - les composés présentant une grande stabilité chimique vis-à-vis du titane, c'est-à-dire surtout les oxydes possédant une enthalpie libre de formation inférieure à celle de l'alumine, en particulier les composés de la famille des lanthanides comme l'oxyde de lutétium, l'oxyde de néodyme, l'oxyde de dysprosium ;
• - les composés contenant au moins un élément très peu soluble dans le titane, c'est-à-dire notamment ceux déjà évoqués ci-dessus et dont la solubilité dans le titane est (en % en poids) :
  

We can therefore adopt:

• - the compounds having a great chemical stability with respect to titanium, that is to say especially the oxides having a free enthalpy of formation lower than that of alumina, in particular the compounds of the family of lanthanides like lutetium oxide, neodymium oxide, dysprosium oxide;
• - compounds containing at least one element very poorly soluble in titanium, that is to say in particular those already mentioned above and whose solubility in titanium is (in% by weight):
  

The most favorable adduct concentration range depends on many factors, including the particle size and that of the starting powder. It can be determined by experience for each particular product used. However, the volume concentration C of the particles of the adduct must as a general rule be maintained between two values, which correspond one to the formation of a continuous layer of particles on the grains of alloyed titanium powder (layer which could lead to embrittlement of the alloy by decohesion at the limit of the primitive powder grains), the other to an excessive spacing of the particles of the adduct on the surface of the metal powder grains.

• - d le diamètre moyen des particules du produit d'addition,
• - 1 la distance de centre à centre des particules du produit d'addition,
• - D le diamètre moyen des particules de poudre d'alliage,

Assuming spherical particles, and if we denote by:

• - d the average diameter of the particles of the adduct,
• - 1 the distance from center to center of the particles of the adduct,
• - D the average diameter of the alloy powder particles,

• d < 1 ≤ 5d

the coating of particles of the adduct will generally be effective for:

• d <1 ≤ 5d

ce qui conduit, pour une valeur moyenne de D égale à 100 µm qu'on peut considérer comme représentative, à :Now, in the case of spherical particles

which leads, for an average value of D equal to 100 µm which can be considered representative, to:

In this formula, C is a dimensionless number, d is in micrometers.

• B avec d = 0,2 µm : 63 vpm ≤ C < 1570 vpm
• Y₂O₃ avec d = 2 µm : 630 vpm ≤ C < 15.700 vpm

This criterion will lead for example to the following ranges, expressed in vpm (volume per million)

• B with d = 0.2 µm: 63 vpm ≤ C <1570 vpm
• Y ₂ O ₃ with d = 2 µm: 630 vpm ≤ C <15,700 vpm

The particle size of the initial particles of the adduct acts on the content C to be adopted. Qn therefore sees that the use of excessively coarse initial particles leads to the introduction of a high content, detrimental to the mechanical properties of the alloy, in particular to its ductility. In practice, a particle size smaller than two orders of magnitude or more than that of the titanium powder generally gives good results.

A reduction in the particle size of the initial particles of the adduct, at constant concentration C, results in a reduction in the distance between particles 1 and therefore greater efficiency in limiting the size of the β grain. This result was notably observed by comparing the grain sizes obtained with boron with a particle size of approximately 0.2 μm and with particles of ceramic type, such as yttrin Y ₂ O ₃ , for which the particle size is about 2 µm.

The heat treatment temperature has little influence on the results obtained. A temperature around 50 ° C higher at the processing point is generally satisfactory. When, for example, we want to develop the TA6V alloy for which the transformation point α + β / β is located between 995 ^* C and 1000 ° C. a holding one hour at a temperature between 1050 ° C and 1100 ^° C before quenching is satisfactory.

The method according to the invention can be implemented with a view to obtaining various results, but all linked to the possibility of retaining a reduced grain size despite exceeding the transformation temperature in the β phase.

First of all, as already indicated, the method according to the invention makes it possible to control the grain size β, and, consequently, that of the alloy grains after quenching. This size only depends practically on the particle size of the original powder. We can therefore obtain combinations of mechanical properties which take advantage of the beneficial effects of the acicular structures, while avoiding the handicap of a grain that is too large.

The process according to the invention also makes it possible to accelerate the homogenization of an alloy, without any adverse effect on its final mechanical properties. The diffusion coefficient of an element in an alloy is in fact an increasing function of the temperature, which shows the advantage of working at a temperature as high as possible to obtain rapid homogenization. In addition, in the case of a titanium matrix, the diffusion coefficient of many elements undergoes a notable discontinuity, in the direction of an increase, if at fixed temperature the phase a is transformed into β phase. So the homogenization of an alloy containing particles enriched in alphagene element like aluminum, oxygen, carbon, nitrogen will be accelerated if the treatment temperature exceeds that where the particle in question passes completely in β phase. Such particles, as well as inclusions of elements foreign to the alloy, can be found in products obtained by powder metallurgy processes.

Despite its advantage in this case, it is however not possible to envisage a homogenization treatment at a temperature higher than the transformation point in β phase in the absence of adduct, since it leads to an unacceptable magnification of the β grain. . The invention eliminates this drawback. In addition, compared to annealing the powder at high temperature before densification, it offers the advantage of allowing an exchange of elements between different granules of powder by diffusion in the solid state.

• - Amélioration de l'homogénéité chimique des alliages : cet effet trouve une application importante dans le processus de récupération des copeaux d'usinage de titane allié lorsque cette récupération comporte une transformation en poudre. Les copeaux d'usinage sont contaminés par l'oxygène, l'azote, le carbone et des particules étrangères à l'alliage. Le niveau moyen de contamination peut être abaissé en mélangeant, aux copeaux pulvérisés puis tamisés, de la poudre à teneur plus basse en interstitiels. Cette opération diminue au surplus la taille des particules les plus nocives. Dans la pratique le broyage de copeaux hydrurés permet d'obtenir des poudres particulièrement fines, jusqu'à environ 40 µm. Après mélange de ces poudres avec des particules de produit d'addition et densification, un traitement d'homogénéisation à haute température permet d'améliorer la qualité des produits finis, sans grossissement exagéré du grain β.
• - Synthèse d'alliage par frittage d'un mélange de parties fines d'éponge de titane et de poudre d'alliage-mère : cette solution, séduisante du point de vue économique, a l'inconvénient de conduire, lorsqu'elle est réalisée par les techniques actuelles à des produits contenant du chlore provenant de l'éponge de titane. Il en résulte une structure métallurgique qui présente une grande stabilité au cours des traitements thermiques mais dont la porosité ne peut être complètement fermée par pressage isostatique. La résistance à la fatigue qe ces produits en est considérablement dégradée. L'élimination du chlore des parties fines de l'éponge entraine un grossissement exagéré du grain β au cours de l'homogénéisation de l'alliage à une température supérieure au transus β. Ces produits peuvent être améliorés par mise en oeuvre du procédé suivant l'invention, car ce dernier permet d'effectuer les traitements de diffusion et de pressage isostatique dans le domaine P sans grossissement exagéré du grain métallurgique d'où une homogénéisation et la fermeture des pores.

Among the cases where the acceleration of the homogenization process is of great interest, the following may be cited, without limitation:

• - Improvement of the chemical homogeneity of the alloys: this effect finds an important application in the process of recovery of the machining chips of titanium alloy when this recovery involves a transformation into powder. The machining chips are contaminated with oxygen, nitrogen, carbon and particles foreign to the alloy. The average level of contamination can be lowered by mixing powder with a lower interstitial content with the pulverized and then sieved shavings. This operation also reduces the size of the most harmful particles. In practice, the grinding of hydrated chips makes it possible to obtain particularly fine powders, up to approximately 40 μm. After mixing these powders with particles of adduct and densification, a high temperature homogenization treatment makes it possible to improve the quality of the finished products, without exaggerating magnification of the β grain.
• - Alloy synthesis by sintering a mixture fine parts of titanium sponge and mother alloy powder: this solution, attractive from an economic point of view, has the disadvantage of leading, when produced by current techniques, to products containing chlorine from titanium sponge. The result is a metallurgical structure which exhibits great stability during thermal treatments but whose porosity cannot be completely closed by isostatic pressing. The fatigue resistance of these products is considerably degraded. The elimination of chlorine from the fine parts of the sponge leads to an exaggerated magnification of the β grain during the homogenization of the alloy at a temperature higher than the β transus. These products can be improved by implementing the method according to the invention, since the latter makes it possible to carry out the diffusion and isostatic pressing treatments in the P domain without exaggerated magnification of the metallurgical grain, hence homogenization and closure of the pores.

• - Mise en solution d'éléments tels que Si ou Ge dont la solubilité en phase β croit rapidement avec la température.

This technique can be combined with the previous one: it makes it possible to use a starting mixture comprising powders produced from shavings with a high oxygen, nitrogen and carbon content, fine parts of titanium sponge and powder of master alloy, the latter components being lightly loaded with interstitial elements.

• - Dissolution of elements such as Si or Ge whose solubility in β phase increases rapidly with temperature.

We will now describe various specific examples of implementation of the invention, corresponding to the development of TA6V titanium alloy by powder metallurgy. The description refers to the accompanying drawings and in which FIGS. 1 to 10 are simplified reproductions of micrographs of alloys obtained after heat treatment and quenching, solid lines showing the β grain boundaries and possible dashed lines showing the original particle boundaries, when they do not coincide with the β grain boundaries.

In all cases, the starting powder was obtained by spraying with TA6V alloy according to the method with a rotating arc electrode under neutral gas qt sieving at a particle size less than 160 μm.

Several contents, equal to or less than 2200 vpm of initial particles, have been tested. The same metallurgical treatments were applied to the mixture containing the adduct and to a control sample of alloy powder. The treatments carried out included densification by hot isostatic compression under representative conditions, that is to say at a temperature below the point of transformation of TA6V, or by spinning, then a treatment of one hour above the processing temperature, followed by quenching.

Example 1

0.16 g of boron powder having a particle size of approximately 0.2 μm was added to 1 kg of TA6V alloy powder sieved to a particle size of less than 160 μm. The content thus obtained was 300 vpm. The mixing was carried out in a rotary mixer in the presence of glass beads, of the "TURBULA" type, for 4 hours. The mixture was degassed hot under secondary vacuum and in a thin bed, according to the known process in powder metallurgy. It was then placed in a mild steel envelope with a shape corresponding to that of the part to be obtained. The envelope was sealed in a secondary vacuum by welding with an electron beam. The alloy was densified by hot isostatic pressing at 950 ° C. under 1000 bars for three hours. The envelope was removed by machining or chemical attack. Finally, heat treatment at 1050 ° C for one hour, followed by water quenching, was performed.

The same operations were also carried out on a control containing only TA6V alloy powder.

Figure 1 shows only a fraction of three adjacent grains in the control sample having undergone all of the treatments. A comparison with FIG. 2, which corresponds to the case of the sample containing 300 vpm of boron, shows that there has been complete blocking of the grain boundaries β and maintenance of a very fine grain size. It is essential to note that the optical magnification is not the same in Figures 1 and 2.

Examples 2 and 3

The same treatments as in Example 1 were carried out with boron contents respectively qe 200 vpm and 100 vpm. Figures 3 and 4 show the grain sizes obtained. It can be seen in FIG. 3, which corresponds to a content of 200 vpm, that a quasi-blocking of the grain boundaries is obtained. The limits of the β grains are, in most cases, confused with the limits of the old particles (indicated in dashes where there is no coincidence).

Figure 4 shows that a content of 100 vpm still significantly slows the growth of β grain. whose size remains much smaller than that of the control sample.

Examples 4. 5. 6 and 7

The same metallurgical treatment was applied to samples containing respectively 400, 550, 1100 and 2200 vpm of boron. In all cases, a complete blockage of the grain boundaries was obtained.

Examples 8 and 9

The same metallurgical treatment as in the previous examples was carried out on samples containing respectively 550 vpm and 1100 vpm of dysprosin Dy ₂ O ₃ . The results obtained appear on the Figures 5 and 6. In Figure 5, corresponding to a content of 550 vpm, there is a braking of the grain qe joints, limiting the growth of β grains. In FIG. 6, for a content of 1100 vpm, a complete blockage appears, the limit of the β grains being confused with the limits of the old particles.

Examples 10. 11 and 12

The same heat treatment as in the previous case, except that the temperature maintenance was at 1100 ° C instead of 1050 ° C, was applied to a control sample (Figure 7), to a sample at 550 ^vp m of Y ₂ O ₃ (Figure 8), to a 550 vpm sample of Dy ₂ O ₃ (Figure 9) and to a sample at 1100 v ^p m of Dy ₂ O ₃ (Figure 10).

There was a blockage of the grain boundary β, for 1100 vpm of Dy ₂ O ₃ and 550 vpm of Y ₂ O ₃ , a significant braking for 550 vpm of ^Dy ₂ ⁰ ₃ .

The above results and additional results are summarized in the table below, where it should be noted that the less favorable results obtained with B ₆ Si appear to be attributable to an insufficiently homogeneous distribution of the adduct in the mixture.

Claims (5)

1. Process for the preparation of a titanium-based alloy comprising the steps of compacting a powder consisting of particles of pre-alloyed titanium or particles of titanium and of master alloy, of heat treatment at a temperature above the point of transformation in β phase, and quenching, characterized in that the powder also contains a dispersion of fine particles of a product slowing the growth of the grain size at a volume content lower than that which would lead to the formation of a layer continuous fine particles around the particles of the titanium powder, the product particles being present on the surface of the titanium grains in the alloy produced. 2. Method according to claim 1, characterized in that said product consists of a chemical species stable with respect to titanium or having at least one element very sparingly soluble in titanium at the temperatures reached during the heat treatment. 3. Method according to claim 2, characterized in that said product is chosen from S, P, B, As, Se, Te, Y and lanthanides, in the elementary state or in combination. 4. Method according to any one of the preceding claims, characterized in that the fine particles of the product have a particle size smaller than a µ minus two orders of magnitude than that of the powder of titanium particles. 5. Method according to any one of the preceding claims for the production of TA6V alloy, characterized in that the heat treatment is carried out at a temperature approximately 50 ° C. above the point of transformation in the β phase, followed by a quenching.

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