EP0167460B1 - Process for manufacturing titane-based alloys with small granular dimensions by means of powder metallurgy - Google Patents

Description

The subject of the invention is a process for the preparation of a titanium-based alloy having a small grain size, the term “alloy to be interpreted as extending to a product containing, in addition to titanium, only elements addition at a sufficiently low content so that the α-β phase transformation 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 prepare 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 phase p, 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 phase 13 to phase a during cooling to room temperature following the heat treatment.

However, the choice of a temperature lower than the transformation point leads to depriving ourselves of the advantages which the acicular type structure which the partial transformation of the β phase obtained at high temperature, in the α phase gives: gives among these advantages the better resistance to the brutal propagation of cracks (i.e. the toughness of the alloy), to the propagation of cracks in fatigue, to creep, and to corrosion under tension 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 (ex β) depends directly on that obtained at the end of heat treatment.

Document GB-A-928 407 also discloses a method according to the preamble of claim 1; but the process according to the patent includes a cold homogenization treatment step. It provides for the addition of a metal or alloy soluble in titanium; cerium and lanthanum do not meet this condition.

The invention aims to provide a process for the production of a titanium-based alloy which meets the requirements of the practice better than those previously known, 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 provides a process for the production of an alloy based on titanium of the above-defined type in accordance with claim 1.

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 β-grain joints by the fine particles of the species which remain stable or are transformed during the development, distributed around the periphery of the other powder particles. The metallurgical grains appearing during the transition to the β phase 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 β 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 it is envisaged the use of an adduct having a weakening effect) .

The two above phenomena have been observed, one or the other being the only one present or preponderant for some. Additives 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, because 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 close to 1000 ° C., that is to say near the point of transformation in the 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 C sufficient to give hope for the possibility of obtaining favorable results. Similarly, the examination of the properties of various elements frequently used in metallurgy show a solubility sufficiently low to meet the required conditions. Indeed, we have in particular or use various oxides, and in particular Y ₂ 0 ₃ and Dy ₂ 0 ₃ .

It was also possible to use metalloids in elementary form or of compounds, in particular boron, BN, B4C, B4Si, B6Si and LaB6. Sulfur, WS, MoS2, ZnS can also be considered, as well than phosphorus, which however requires 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 40 to 300 _! Lm will generally be acceptable.

It is possible in particular to use pre-alloyed titanium powders obtained by sputtering according to a rotary electrode method, currently well known and widely used for the production 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 wm. 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 ;

We can therefore adopt:

• the compounds having a high 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 l lutetium oxide, neodymium oxide, dysprosium oxide;

the 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):

• En supposant les particules sphériques, et si on désigne par ;
• 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,
• le revêtement de particules du produit d'addition sera efficace en général pour ;
• 
• Or, dans le cas de particules sphériques
  
• ce qui conduit, pour une valeur moyenne de D égale à 100 µm qu'on peut considérer comme représentative. à :
  
• Dans cette formule, C est un nombre sans dimension, d est en micromètres.

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 will generally have to be maintained between two values, which one corresponds 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 separation of the particles of the additive product on the surface of the metal powder grains.

• Assuming spherical particles, and if we denote by;
• d the average particle diameter of the adduct,
• 1 the distance from center to center of the particles of the adduct,
• D is the average diameter of the alloy powder particles,
• the particle coating of the adduct will generally be effective for;
• 
• Now, in the case of spherical particles
  
• which leads, for an average value of D equal to 100 μm which can be considered as representative. at :
  
• In this formula, C is a dimensionless number, d is in micrometers.

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

The particle size of the initial particles of the adduct acts on the content C to be adopted. It can therefore be seen 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 of the particles smaller by 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 ceramic-type particles, such as yttrin Y20 ₃ , for which the particle size is approximately 2 μm.

The heat treatment temperature has little influence on the results obtained. A temperature approximately 50 ° C above the point of transformation 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 one hour hold 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 p, 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 a fixed temperature phase a becomes phase j3. So the homogenization of an alloy containing particles enriched in alpha-element such as aluminum, oxygen, carbon, nitrogen will be accelerated if the treatment temperature exceeds that where the particle in question passes completely in phase p. Particles of this kind, as well as inclusions of elements foreign to the alloy, can be found in the products obtained by the processes of powder metallurgy.

Despite its advantage in this case, it is however not possible to envisage a homogenization treatment at a temperature above the transformation point in phase 13 in the absence of adduct, because 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 powder granules 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 p.

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 alloyed titanium 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, with the pulverized and then sieved shavings, powder with a lower interstitial content. 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 the grain p.

Synthesis of alloy by sintering a mixture of fine parts of titanium sponge and mother alloy powder; this solution, attractive from an economic point of view, has the drawback of leading, when produced by current techniques, to products containing chlorine originating from the 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. Elimination of chlorine from the fine parts of the sponge causes an exaggerated magnification of the β grain during. the homogenization of the alloy at a temperature higher than the transus 13. These products can be improved by implementing the method according to the invention, since the latter makes it possible to carry out diffusion and isostatic pressing treatments in the field 13 without exaggerated magnification of the metallurgical grain, hence homogenizing and closing the pores.

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 content of oxygen, nitrogen and carbon, 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 p-phase solubility 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, the solid lines showing the grain boundaries β and the possible lines in dashes showing the original particle limits, when they do not coincide with the grain boundaries p.

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 and 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, a heat treatment at 1050 ° C for one hour, followed by quenching with water, was carried out.

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 of 200 vpm and 100 vpm respectively. 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).

FIG. 4 shows that a content of 100 vpm still considerably slows down the growth of β grain, the size of which 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, 1,100 and 2,200 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 dysprosine Dy ₂ 0 ₃ . The obtained results appear in FIGS. 5 and 6. In FIG. 5, corresponding to a content of 550 vpm, there is a braking of the grain boundaries, limiting the growth of the β grains. In FIG. 6, for a content of 1,100 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 (FIG. 7), to a sample at 550 vpm of Y ₂ 0 ₃ (Figure 8), to a 550 vpm sample of D _Y2 O ₃ (Figure 9) and to a sample at 1100 vpm of D _Y2 O ₃ (Figure 10).

There was a blockage of the grain boundary p, for 1,100 vpm of Dy ₂ 0 ₃ and 550 vpm of Y ₂ 0 ₃ , a significant braking for 550 vpm of D _Y2 O 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 (2)

1. Process for forming a titanium base alloy comprising the steps of :

heat compacting a powder formed, on one hand, of particles of pre-allied titanium or of articles of titanium and mover alloy and, on the other hand, of dispersed fine particles of an addition product ;

heat treating at a temperature higher than the point of transformation into phase β, characterized in that the dispersed particles are of an addition product which curbs the growth of the grain size, selected among S, P, B, As, Se, Te, Y and lanthanides, in elementary condition or in combination and in a proportion per volume less than that which would lead to the formation of a continuous layer of fine particles about the particles of titanium powder, and in that said alloy is subjected to quenching after the heat treatment.

2. Process according to claim 1 for forming a TA6V alloy, characterized in that the heat treatment is carried out at a temperature higher than the point of transformation to (3 phase by about 50 °C and is followed by a quenching operation.

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