WO2013034851A1

WO2013034851A1 - Method for preparing test parts for the mechanical characterisation of a titanium alloy

Info

Publication number: WO2013034851A1
Application number: PCT/FR2012/051987
Authority: WO
Inventors: Adeline BENETEAU
Original assignee: Snecma
Priority date: 2011-09-05
Filing date: 2012-09-05
Publication date: 2013-03-14
Also published as: FR2979702A1; FR2979702B1

Abstract

The invention relates to a method for manufacturing one or more test parts for the mechanical characterisation of a forged titanium alloy β, having a forge-skin microstructure with thick, fragmented needles, said method including the following steps: a) manufacturing a billet using said alloy, and sampling a cylindrical slug in the billet; b) heating the slug in a kiln (1) at a dissolution temperature T1, which is higher than the temperature Τβ, for a period of time t1 for rendering the temperature uniform; c) transferring the slug into a kiln (2) at a temperature T2, which is lower than Τβ, for a period of time t2 for rendering the temperature uniform; d) forging a roller by means of the hot axial die-forging of the slug, the deformation rate ε being higher than 0.5 (ε>0.5), so as to obtain said microstructure, followed by cooling to ambient temperature; and e) heat-treating the resulting roller, and machining of test parts for mechanical characterisation in the portion of the roller having said microstructure.

Description

Process for the preparation of mechanical characterization specimens

of a titanium alloy

The present invention relates to the field of the forging of titanium alloy parts, the titanium alloy having a β-domain at a temperature higher than the β-transus temperature, and aims at a method of producing by forging a roll of alloy of titanium having, at least in part, a "skin forge" type microstructure comprising alpha needles, thick, numerous and fragmented, so as to allow the extraction of mechanical characterization specimens of the material thus produced.

Prior art

Titanium alloys because of their high strength / mass ratios are used in the aeronautical field, in particular for making turbomachine parts, subjected to high stresses and high temperatures of up to 600 ° C. Pure titanium exists in two crystallographic forms: The phase a compact hexagonal structure is stable at room temperature, and the β phase of cubic structure centered is stable above the temperature called β-transus or transus β, which is equal to 883 ° C for pure titanium. On the phase diagrams of titanium alloys combined with other elements, we find the β phase above the β-transus temperature, and below this temperature a balance between the β phase and the α phase whose proportions depend on the alloying elements. The α-β microstructure consists of a mixture of phase a and phase β. The alloy elements have the effect in particular of varying the β-transus temperature around 883 ° C. The development of a titanium alloy having the desired properties includes selecting alloying elements, and choosing the thermomechanical treatment undergone by the alloy. The present applicant has developed processes for forging titanium alloy mechanical parts, for example the processes described in patent applications WO2010031982, WO2010031985, FR2952559 and FR2899241. A process for manufacturing alloy compressor disks Til7, whose formula is ΤΊ-5Α1-2Sn-2Zr-4Mo-4Cr, comprises several forging steps, one of which is carried out in the β-domain, ie ie, whose stamping operation is performed at a temperature above the β-transus temperature. On the surface of these parts, areas are formed whose mechanical properties are lower and the microstructure of which has relatively thick needles or alpha phase shuttles. These areas constitute what is known as the expression of "Blacksmith skin" The thickness of the latter varies according to the shape of the forging and also according to the parameters of the forging process in the β domain thereof. It can go, for a compressor disk, from 0 to 10 mm. This microstructure appears if the process used is forging in the open air in hot matrices. The more the temperature of the dies approaches the temperature of the part to be forged (quasi-isothermal forging), the more the thickness of the microstructure of the "forge skin" type decreases.

It would be desirable to be able to characterize the material in this zone, that is to say to analyze the mechanical properties, in order to evaluate its acceptability on parts. Indeed, on the parts such as titanium alloy discs forged in the β domain, forge skin type structures are formed and if these structures extend into the pre-machined part, they cause a override and additional controls to decide whether the part should be discarded or not.

A means for characterizing a metallic material consists of extracting specimens of this material and subjecting them to mechanical tests to determine their mechanical characteristics at ambient temperature and at the temperatures of use: Young's modulus, elongation at break, resistance tensile strength, elastic limit, oligocyclic or vibratory fatigue, more particularly fatigue failure limit, etc. In order for the useful part of the mechanical test piece to contain a representative volume of material, it is preferable to use cylindrical test pieces rather than flat test pieces.

However, under the usual forging conditions, the thickness obtained is too small to extract cylindrical barrel mechanical characterization specimens. One solution would be to voluntarily degrade the forging conditions of a sacrificed part, a compressor disc in ΤΊ17, to obtain a microstructure of forge skin type in thick. But such a method would be expensive to implement both time and sacrificed parts because it would require, for each condition tested, to repeat the tests with modification of the forging parameters to obtain the desired thickness. Presentation of the invention

Thus, the Applicant has set itself the objective of reproducing the desired microstructure in sufficient quantity to allow the machining of at least one specimen with which it will be possible to carry out mechanical characterization, fatigue or other tests.

The method according to the invention, for manufacturing one or more test specimens for the mechanical characterization of a titanium alloy with a β-transus temperature, Τβ, having a microstructure of forge skin type with thick and fragmented needles, includes the following steps:

Producing a billet of said alloy,

Sampling of a cylindrical slug in the billet,

Heating the slug in an oven at a solution temperature Tl, higher than the temperature Τβ, for a time tl homogenization of the temperature. Transfer of the slug in an oven to a temperature T2 lower than

Ββ during a time t2 for homogenizing the temperature. Forging a roller by axial die stamping of the billet, the deformation rate ε being greater than 0.5, ε> 0.5, so as to obtain said microstructure, then cooling to room temperature;

- Heat treatment of the wheel obtained;

Extraction of mechanical characterization specimens in the part of the roller having said microstructure.

The method of the invention has the advantage of providing, by simple forging operations, any desired quantity of material for the analysis of the mechanical properties. In addition, the parameters being perfectly defined, the method makes it possible to extract the specimens reproducibly on a large number of rollers.

The method is directed in particular to the Til7 alloy because of the application thereof to the manufacture of parts such as DAM type compressor discs, which is the acronym for monobloc blisk, where the blades form a single blade. block with the disc being obtained by machining a forged disc blank. The present process is applicable to other equivalent titanium alloys such as the ΤΊ6242 alloy or any other titanium alloy used to obtain a final beta-shaped workpiece.

Preferably, the homogenization heating temperature is Tl = Τβ + 25 ° C, ie about 915 ° C for the Til7 for a period of 2 hours, Τβ being between 880 ° C and 900 ° C.

More preferably, the heating temperature T2 in the second oven is between 750 ° C and 850 ° C; it is chosen according to the family of microstructure that one wishes to obtain. The heating time is preferably 2 hours longer.

According to another characteristic, the forging dies are heated and are preferably maintained at a temperature between T2 minus 50 ° C (T2-50 ° C) and T2.

According to another characteristic, the overall deformation rate ε is less than 2, preferably between 1 and 1.5. The overall deformation rate ε is defined as the logarithm of the ratio of the height or thickness of the billet before deformation and its thickness after deformation: If the part is not deformed, the deformation ratio is equal to 0. It has been found that that the deformation of the slug should be sufficient, therefore> 0.5, to obtain the formation of a microstructure of "forge skin" type.

The present invention also relates to a method for controlling a beta-forged titanium alloy having a forge-skin type microstructure with thick and fragmented needles comprising the manufacture of a test piece according to the above method and then the submission. from the test piece to mechanical characterization tests.

Presentation of figures

The invention will be better understood, and other objects, details, features and advantages thereof will become more apparent in the following detailed explanatory description, embodiment of the invention given by way of example purely illustrative and not limiting, with reference to the accompanying drawings.

On these drawings: - Figure 1 shows schematically the various steps of preparation of a roller having the desired microstructure;

- Figure 2 is a photograph of a section of a roller obtained by the method illustrated in Figure 1;

FIGS. 3 and 4 are optical micrographs of the two distinct zones of the section of the roller of FIG. 2

Detailed description of an embodiment of the invention

The method comprises a step of producing a billet of titanium alloy such as Ti 17 in which cylindrical slugs L are taken. In the example in question, the slugs have a height of 160 mm for a diameter of 80 mm. They are then enamelled; enamel having a lubricating function. As can be seen in FIG. 1, a billet L is placed in a furnace 1. Stage A, the slug is heated to a temperature T1 higher than the β-transus temperature for this alloy, ie Tp + 25 ° C. = 915 ° C. about for the alloy ΤΊ17. The slug is maintained at this temperature for 2h30, long enough for the temperature of the slug to be substantially homogeneous and equal to T1. The billet is taken out of the oven 1 and then transferred to a second furnace 2 in which, step B, it is maintained at a temperature T2, lower than the β-transus temperature. As for the previous heating, the billet is maintained at this temperature T2 sufficiently long, about 2 hours to 2:30, so that its temperature is homogenized. During this step B, thick needles are formed in the β matrix.

In the following step C the billet is transferred rapidly, in less than 30s, to a press 3, for example a 1000 ton Cosmo press where it undergoes the forging operation, stage D, and is shaped into matrices which are heated to a temperature between T2-50 ° C and T2, here the matrices are flat. The forging is done at this temperature. The displacement of the upper slider supporting the upper die is determined by the desired strain rate ε for the slug. Rates ε of between 1 and 1.5 are suitable for obtaining the desired structure. The slug took the form of a pebble which after cooling with air is heat-treated T3, R2, step E. The treatment T3 comprises heating at 800 ° C for 4 h followed by quenching with water and the Treatment R2 comprises heating at 625 ° C for 8 h followed by quenching in air. The rolls thus obtained are ready for one or more cylindrical cylindrical test pieces to be taken there to undergo the mechanical characterization tests.

The roller is shown in axial section in FIG. 2. There is a first zone I in the shape of a coated hourglass and two zones II, one upper and the other lower, on either side of the central portion of the hourglass.

As can be seen in FIGS. 3 and 4, the first zone I has a microstructure, called a forge-skin type, which is characterized by the presence of α, or short-needle, apertures at the grain boundaries or generalized, formed during forging, by fragmentation of the thick needles appeared in the furnace T2. It was noted that different families of microstructures were obtained depending on the forging temperature between 750 ° C and 850 ° C. The second zone II is a dead zone in which the rate of local deformation is zero or very low.

Claims

claims

A method of manufacturing at least one specimen for the mechanical characterization of a forged β titanium alloy, having a forge-skin type microstructure with thick and fragmented needles, comprising the following steps:

at. Production of a billet made of said alloy, - sampling of a billet of cylindrical shape in the billet,

b. Heating the slug in an oven (1) at a solution temperature Tl, higher than the temperature Τβ, during a time tl homogenization of the temperature,,

vs. Transfer of the billet in an oven (2) to a temperature T2 less than Τβ, during a period t2 of homogenizing the temperature,

d. Forging a roller by axial die stamping of the billet, the deformation rate ε being greater than 0.5, ε> 0.5, so as to obtain said microstructure, then cooling to room temperature.

e. Heat treatment of the obtained roller, - machining of mechanical characterization specimens in the part of the roller having said microstructure,

2. Method according to claim 1, the titanium alloy being the Til7 alloy.

3. Method according to one of the preceding claims, with Tl = Tp + 25 ° C.

4. Method according to claim 2, T2 being between 750 and 850 ° C.

5. Method according to one of the preceding claims, the forging dies being at a temperature between T2-50 ° C and T2.

6. Method according to one of the preceding claims, the overall deformation rate ε being less than 2, preferably between 1 and 1.5.

7. The method of claim 2, the heat treatment comprises a heating step at 800 ° C for 4 hours and cooling and a heating step at 625 ° C for 8 hours followed by cooling in air.

8. A method for controlling a beta-forged titanium alloy, having a forging-skin type microstructure with thick and fragmented needles, comprising the manufacture of a test piece according to the method of one of the preceding claims and then the submission of Γ test specimen with mechanical characterization tests.