US20220258234A1

US20220258234A1 - Method for manufacturing a metal part made from titanium, by rapid sintering, and sintered metal part made from titanium

Info

Publication number: US20220258234A1
Application number: US17/618,769
Authority: US
Inventors: Bogdan CHETROIU; Nicolas PAILLONCY
Original assignee: Metal Additive Technologies
Current assignee: Metal Additive Technologies
Priority date: 2019-06-14
Filing date: 2020-06-13
Publication date: 2022-08-18
Also published as: EP3983569A1; WO2020249808A1

Abstract

Disclosed is a method for manufacturing a metal part made from titanium, by sintering a powder, the method comprising a step of mixing a spherical titanium powder and a dendritic titanium powder in order to form a mixture, a step of agglomerating the mixture of titanium powders by compaction with a ram moving at a speed of more than 2 m·s⁻¹, the mixture of titanium powders being free of binders, in particular organic binders, forming a green body or agglomerate suitable for sintering having a density above 78%, and preferably 80%, of the density of the solid metal, and a step of sintering.

Description

FIELD OF THE INVENTION

The invention relates to the field of titanium sintering.

BACKGROUND

In the aerospace and automotive industries, titanium is of interest due to the density thereof, the excellent behavior thereof in corrosive environments and the good mechanical properties thereof.
In technological terms, the manufacture of titanium alloys from melting into ingots requires very good know-how in order to obtain homogeneous and high-quality ingots to subsequently convert them into rolling products. In practice, defects such as high interstitial defects, solute segregation, high-density inclusions, beta phases or pores occur during solidification and cannot be avoided. The difficulty in controlling solidification also depends on the chemical composition of the alloy. The high reactivity of titanium results in the formation of high oxygen, carbon or nitrogen concentration type interstitial defects. High-density inclusions result from the contamination of the titanium alloy with raw materials (molybdenum, tantalum, tungsten or tungsten carbide) used in the production of the alloy, cf. G. Lütjering, J. C. Williams “Titanium”, second edition, 3-technological aspects Springer, 2007, pages 53 to 73, ISBN 978-3-540-71397-5. For this reason, the presence of TiN in the microstructure lowers the fatigue performance of the material. As mentioned in the article mentioned above, alloys containing eutectoid beta elements such as Fe, Ni, Mn, Cr and Cu have low solidification temperatures. This situation can lead to solute segregation during ingot solidification. The segregated zones can have a length of up to a few millimeters. Once formed, these defects are difficult to remove during subsequent treatment steps.
The presence of these defects in the microstructure is detrimental to mechanical behavior, cf. R. Boyer, E. W. Collings, and G. Welsch: “Materials Properties Handbook: Titanium Alloys”, 1994, ISBN 978-0-87170-481-8.
One of the methods used for converting factory products—slugs, ingots, etc.—into finished parts is forging. Forging makes it possible to obtain the required microstructure via several thermomechanical treatment steps. Table 1 highlights the steps for producing an entirely lamellar microstructure for Alpha-Beta titanium alloys such as Ti-6Al-4V (grade 5), Ti-6Al-2Zn-4Zr-2Mo-—Si (Ti-6242) or Ti-6AI-6V-2Sn (Ti-662), cf. G. Lütjering, J. C. Williams “Titanium” second edition, 5 -Alpha+Beta alloys Springer, 2007, pages 203 to 258, ISBN 978-3-540-71397-5.

TABLE 1

Alpha - beta titanium alloy treatment steps.

Steps	Homogenisation	Deformation	Recrystallisation	Annealing

Alpha-beta alloy	Performed above the beta	Forging or rolling of	Performed from 30° C. to 50° C.	Performed in alpha + beta
lamellar microstructure	transus for several hours	material in beta phase	above the beta transus for	phase field to harden
		field and in alpha	several hours in order to control	microstructure by presence
		phase field	the grain size of (≈600 10⁻⁶m).	of Ti3Al precipitates.

The most important parameter in the recrystallisation step is the cooling rate which will influence the grain size, the alpha lamellae, the thickness of the alpha layers at the beta grain boundaries.
As mentioned in G. Lutjering, J. C. Williams “Titanium” second edition, 5-Alpha+Beta alloys Springer, 2007, pages 203 to 258, ISBN 978-3-540-71397-5, in the final annealing heat treatment step, the temperature is greater than the holding time as the temperature determines whether the age hardening of the alpha phase thanks to Ti3Al particles occurs or not. In the case of the titanium alloy, Ti-6Al-4V (grade 5), an ageing at 500° C. will favor the appearance of Ti3Al particles. Critical parameters determine the final microstructure, for example: the deformation time, the deformation mode, the degree of deformation and the cooling rate, cf. N. Gey, M. Humbert, M. J. Philippe, Y. Combres, Investigation of the alpha- and beta-texture evolution of hot rolled Ti-64 products, Materials Science and Engineering: A, Volume 219, Issues 1-2, 1996, Pages 80 to 88, ISSN 0921-5093. These parameters are costly to control and influence the shape of the beta grains, the recrystallized structure, the size of the alpha colonies as well as the width of the alpha lamellae.
Moreover, these shapes created by forging are then finished by material removal (machining). In economic terms, forging is costly if it is sought to create high-quality components. The cost increases due to the intensive machining of the forced parts in order to create a lightweight component of complex shape. The intensive machining requirement is the consequence of two main reasons. On one hand, the inability to create a homogeneous microstructure, and on the other, the need to create a rectilinear shape known as the sonic shape for ultrasound investigation, cf. G. Lutjering, J. C. Williams “Titanium”, second edition, 3—technological aspects Springer, 2007, pages 53 to 73, ISBN 978-3-540-71397-5. It is common to obtain a finished part wherein the weight represents merely 10% of the initial forging block. Forging thus has definite environmental, economic and practical drawbacks. The teachings mentioned above are not transposable to sintered alloyed titanium.
The aim of the invention is that of providing a manufacturing method and resulting parts of high chemical purity and high mechanical strength.

SUMMARY

The present invention relates to a method making it possible to approach, or even reach the theoretical maximum density of a hybrid titanium powder alloy by high compression/compaction speed lamination without using an internal organic binder and by sintering in high vacuum. Hybrid powder denotes here a mixture of spherical powder and powder of dendritic or coral-like morphology. A grain of powder of dendritic or coral-like morphology comprises convex zones and concave zones. A grain of spherical powder comprises convex zones and is substantially devoid of concave zones. Spherical is understood here in the very broad industrial sense and encompasses ovoid shapes.
In an embodiment, the method for manufacturing a titanium-based metal part comprises rapid sintering of a powder. The method comprises a step of mixing a spherical titanium powder and a dendritic titanium powder to form a mixture, a step of agglomerating the titanium powder mixture by compaction with a ram moving at a speed greater than 2 m·s−1, the titanium powder mixture being devoid of binder, particularly organic, forming a green body suitable for sintering having a density greater than 78% of the density of the solid metal of the same composition. The green body is an agglomerate of pure compacted hybrid powder. The green body is devoid of solid or viscous volatile binder. Volatile means here: changing to the gaseous state at a temperature less than or equal to the sintering temperature.
In an embodiment, the density of the powder, before agglomeration, is between 60 and 65% of the density of the same solid metal.
Preferably, the green body suitable for sintering has a density greater than 80% of the density of the solid metal of the same composition.
In an embodiment, the method comprises before the mixing step, a step of providing from 60 to 90% by mass of spherical titanium powder and from 10 to 40% by mass of dendritic titanium powder.
In an embodiment, the green body tested as per ASTM B312-14, “Standard Test Method for Green Strength” has a green strength greater than 3 MPa (Charpy test).
The green body can be handled in a production line.
In an embodiment, the pressure exerted by the ram is between 600 and 1500 MPa.
In an embodiment, the ram moves at a speed greater than 4 m·5⁻¹. The speed can reach 10, 20 or 30 m·s⁻¹. The ram has a shape adapted to the part to be produced.
In an embodiment, the method comprises a step of sintering the green body in a neutral to reducing atmosphere, at a pressure less than 0.13 Pa (10-4 Torr) and at a temperature between 1200° C. and 1350° C. to obtain a sintered body. The sintered body essentially comprises titanium, alloy metals and few impurities. The neutral to reducing atmosphere and the very low pressure reduce the formation of metal oxides, particularly TiO2.
In an embodiment, the sintering pressure is greater than 0.13 mPa (10⁻⁷Torr).
In an embodiment, the step of sintering the green body is conducted until a density greater than 97% of the density of the solid metal is obtained, particularly for over 4 hours.
In an embodiment, the step of sintering the green body is conducted until a density greater than 98.5%, more preferably greater than 99%, or 99.9%, of the density of the solid metal is obtained, particularly for over 4 hours, or over 6 hours.
In an embodiment, the sintered body comprises by mass less than 0.20% Fe, less than 0.04% C, less than 0.03% N, less than 0.005% H, and/or less than 0.02% O. The formation of titanium carbides, nitrides or oxides is reduced and the probability of the presence of localized zones rich in titanium carbides, nitrides or oxides is furthermore reduced.
In an embodiment, the sintered body comprises by mass at least 88% Ti.
In an embodiment, the sintered body has a Young's modulus greater than 820 N·mm−2, and an elongation greater than 10%, preferably 12%.
In an embodiment, the sintered body comprises by mass less than 0.002% Fe, less than 0.01% C, less than 0.02% N, less than 0.01% H, less than 0.1% O, the remainder being Ti and unavoidable impurities.
In an embodiment, the sintered body has a Young's modulus greater than 748 N·mm⁻²and an elongation greater than 18%.
In an embodiment, the sintered body comprises by mass less than 0.3% Fe, less than 0.08% C, less than 0.03% N, less than 0.015% H, less than 0.25% O, the remainder being Ti and unavoidable impurities.
In an embodiment, the method comprises, after the sintering step, a cooling step at a pressure less than 0.13 Pa (10⁻⁴Torr) of a duration between 12 and 48 hours. Slow cooling at a very low pressure is favorable, typically 1° C./minute, for enabling alpha phase formation which improves ductility, in a neutral to reducing atmosphere. The atmosphere can be the same as during sintering.
In an embodiment, the spherical titanium powder is obtained by plasma atomization, by gas atomization or by the plasma rotating electrode process.
In an embodiment, the spherical titanium powder is Ti-6Al-4V or Ti grade 2.
In an embodiment, the spherical titanium powder has a grain size less than 0.150 mm. Above this, the sintering would be slower and the porosity rate would be higher. However, a porosity less than 0.220 mm is possible as an alternative. The spherical powder can comprise one part of grain size less than or equal to 0.100 mm and one part of grain size greater than 0.100 mm.
In an embodiment, the spherical titanium powder has a grain size between 0.070 and 0.150 mm.
In an embodiment, the dendritic titanium powder is obtained by the sodium metal iodide reduction process.
In an embodiment, the dendritic titanium powder is Ti-6Al-4V or Ti grade 2.
In an embodiment, the dendritic titanium powder has a grain size less than 0.100 mm, preferably 0.050 mm. It is interesting that the maximum grain size of the dendritic powder is less than the maximum grain size of the spherical powder.
In an embodiment, the method comprises before the mixing step, a step of providing from 40 to less than 90% by mass of spherical titanium powder and from 60 to more than 10% by mass of dendritic titanium powder.
In an embodiment, the method comprises before the mixing step, a step of providing from 40 to 87% by mass of spherical titanium powder and from 60 to 13% by mass of dendritic titanium powder.
In an embodiment, the method comprises before the mixing step, a step of providing from 50 to 82.5% by mass of spherical titanium powder and from 50 to 17.5% by mass of dendritic titanium powder.
The invention also proposes a titanium-based sintered metal part, of density greater than 97%, preferably greater than 98.5%, more preferably greater than 99%, or 99.9%, of the density of the solid metal.
In an embodiment, the sintered metal part comprises by mass, from 5.50 to 6.75% Al, from 3.50 to 4.50% V, less than 0.20% Fe, less than 0.04% C, less than 0.03% N, less than 0.005% H, less than 0.02% O, the remainder being Ti and unavoidable impurities.
In an embodiment, the sintered metal part comprises by mass less than 0.002% Fe, less than 0.01% C, less than 0.02% N, less than 0.01% H, less than 0.1% O, the remainder being Ti and unavoidable impurities.
In an embodiment, the sintered metal part has a Young's modulus greater than 780 N·mm−2
In an embodiment, the sintered metal part has an elongation greater than 10%, preferably 12%.
In an embodiment, the sintered metal part comprises by mass less than 0.3% Fe, less than 0.08% C, less than 0.03% N, less than 0.015% H, less than 0.25% O, the remainder being Ti and unavoidable impurities.
In an embodiment, the sintered metal part has a Young's modulus greater than 748 N·mm−2.
In an embodiment, the sintered metal part has an elongation greater than 12%.
The spherical titanium powder can be manufactured by a gas atomization process GA, by a Plasma Rotating Electrode Process PREP, or by plasma atomization PA.
The dendritic powder can be manufactured with the Armstrong Process, cf. K. Araci, D. Mangabhai, K. Akhtar, 9—Production of titanium by the Armstrong process, Author(s): Ma Qian, Francis H. (Sam) Froes, Titanium Powder Metallurgy, Butterworth-Heinemann, 2015, pages 149 to 162, ISBN 9780128000540.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will emerge on studying the detailed description hereinafter, and the appended drawings:

FIG. 1: a microscope view of a PREP powder,

FIG. 2: a microscope view of a dendritic powder,

FIG. 3: a microscope view of a sample from batch 1 showing the porosity

FIG. 4: a microscope view of a sample from batch 1 showing the porosity

FIG. 5: a microscope view of the microstructure of a sample from batch 1

FIG. 6: an enlarged microscope view of the microstructure of a sample from batch 1.

The appended drawings may not only serve to supplement the invention, but also contribute to the definition thereof, if necessary.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Powder metallurgy has been increasingly developed in recent decades, essentially due to the homogeneity of the chemical composition obtained thanks to powder particles. Powder metallurgy has an advantage in terms of profitability, as it produces clear shapes with a material yield of 80%. This is essentially due to the fact that the final design requires little material removal. In order to produce complex parts, the powder is used by different technologies such as additive manufacturing, metal injection molding, pressing and sintering or hot isostatic pressing.
Of these technologies, additive manufacturing technologies have a drawback for industrial-scale complex part production. Indeed, they are limited on one hand by the slow rate of production and, on the other, by the material cost due to the requirement of a very fine spherical powder of grain size less than 100 μm, cf. JP2018145467, US2018021854 and US2018112293.
Known sintering technologies provide parts with high porosity rates.
The Applicant explored the process of producing components similar to the final shape by pressing and sintering pre-alloyed powder. The technological processes for producing titanium powder such as PREP processes or plasma atomization and gas atomization processes make it possible to obtain very pure chemistry with a very regular morphology. Grade 5 spherical powder titanium alloys obtained from the PREP process, gas atomization or plasma sputtering process have a very pure chemistry with a low oxygen content that can be less than 1500 ppm. Oxygen has a significant interstitial solubility in titanium. On one hand, interstitial oxygen offers a reinforcing effect. On the other, it degrades ductility. This is explained by the precipitation of alpha from beta resulting in alpha grain boundary precipitates that are detrimental to ductility. Another reason, as mentioned by M. Yan, H. P. Tang, M. Qian, 15—Scavenging of oxygen and chlorine from powder metallurgy (PM) titanium and titanium alloys, Authors: Ma Qian, Francis H. (Sam) Froes, Titanium Powder Metallurgy, Butterworth-Heinemann, 2015, Pages 253 to 276, ISBN 9780128000540, is that oxygen can induce an enriched or orderly oxygen pole and that such heterogeneities of the microstructure can block plastic deformation and hence reduce ductility. For this reason, the oxygen content of the metallurgy of Ti-6Al-4V ingots is limited to 0.2% maximum.
Spherical titanium powder having a low oxygen content as well as a pure chemistry with few defects makes it possible to obtain good mechanical properties if the density of the sintered part is close to the theoretical density, i.e. that of the solid alloy of the same composition. This is the reason why parts manufactured from spherical powder are of particular interest for aerospace applications. However, spherical powder has poor behavior in relation to a compression force, particularly implemented during the preparation of the green part before the sintering step. Indeed, the resulting green part has a low relative density and it is very difficult to reach a high density after sintering. Furthermore, a significant volume reduction rate between the green part and the sintered part is not sought.
To be compressed, the spherical powder can be mixed with a hydrocolloidal binder before compression. However, the green part has a low green strength, as per the standard ASTM B312-14 Standard Test Method for Green Strength of Specimens Compacted from Metal Powders, ASTM International, West Conshohocken, Pa., 2014, and requires precautions to be handled. Removing the organic binder from the green part requires a debinding step before sintering. Debinding can be performed for 4 to 7 hours at a temperature ranging from 300° C. to 500° C. However, the organic binder is not completely evaporated in the temperature range mentioned above. Amines (CH—NH) or amides (CONH2) which represent 10% to 20% of the total weight of the molecule have a higher debinding temperature (700° C. to 900° C.). These molecules are very reactive with titanium and can result in the formation of carbides, nitrides and oxides in the matrix during the final densification. The presence of these carbides can be harmful for mechanical behavior and does not meet the requirements of aerospace applications.
High-velocity compaction (HVC) technology is a powder compression method which makes it possible to obtain high-density parts. As mentioned in N. Gey, M. Humbert, M. J. Philippe, Y. Combres, Investigation of the alpha- and beta-texture evolution of hot rolled Ti-64 products, Materials Science and Engineering: A, Volume 219, Issues 1-2, 1996, Pages 80 to 88, ISSN 0921-5093, the compaction phase is 500 to 1000 times faster than with a conventional method. HVC densification is obtained by intense shock waves created by a hydraulically controlled hammer which transfers the compaction energy to the powder, cf. P. Skoglund, M. Kejzelman, I. Hauer “High density components PM by high velocity compaction” PM2 TEC, Orlando, USA, 2002. According to US2015239045, high-velocity compaction technology is used to reach densities greater than 99.5% by the compaction of a spherical powder agglomerated with an organic binder. The binder fulfils the three-fold function of lubricant, dispersant, and temporary adhesive. However, in the case of titanium or a titanium alloy, the Applicant realized that the presence of the organic binder in the composition of the green part can be harmful for mechanical properties due to the high reactivity of titanium with carbon, nitrogen or oxygen from the binder.
In industrial terms, high-velocity compaction is of particular interest due to the high densities obtained and enhanced mechanical properties with regard to products manufactured by conventional methods (pressing, forging).
The geometry of the parts that can be produced by the high-velocity compaction method is varied.
The present invention has focused on overcoming the drawbacks of the organic binder previously deemed necessary and on removing the debinding process from the sintering cycle. Furthermore, the Applicant has focused on enhancing the mechanical properties of the compressed and sintered part. Pressing was performed by high-velocity compaction and by high-vacuum sintering. The mechanical behavior meets the standard requirements for aerospace applications and is very competitive with respect to conventional metallurgy parts.
The aim of the invention is that of manufacturing and rendering industrially available sintered titanium parts of precise chemical composition, low porosity, and high mechanical strength.
As a general rule, the method for manufacturing alloyed titanium metal parts, by rapid sintering of a powder, comprises mixing a spherical titanium powder and a dendritic titanium powder to form a mixture, agglomerating the mixture of titanium powders by compaction with a ram moving at a speed greater than 2 m·5⁻¹, the mixture of titanium powders being devoid of organic binder, forming a green body suitable for sintering and having a density greater than 78%, preferably 80%, of the density of the solid metal. Before mixing, it is possible to provide from 60 to 90% by mass of spherical titanium powder and from 10 to 40% by mass of dendritic titanium powder. The green body has a green strength greater than 3 MPa. The pressure applied by the ram can be between 600 and 1500 MPa. The ram can move at a speed greater than 4 m·s⁻¹. The sintering of the green body can take place in a reducing atmosphere, at a pressure less than 0.13 Pa (10⁻⁴Torr) and at a temperature between 1200° C. and 1350° C. to obtain a sintered body. The pressure in the sintering enclosure can be greater than 0.13 mPa (10⁻⁷Torr). The sintering of the green body can be conducted until a density greater than 97%, preferably greater than 98.5%, more preferably greater than 99%, or even 99.9% of the density of the solid metal is obtained, particularly for more than 4 hours. The sintered body can comprise by mass less than 0.20% Fe, less than 0.04% C, less than 0.03% N, less than 0.005% H, and/or less than 0.02% O, and have a Young's modulus greater than 820 N·mm⁻², and an elongation greater than 10%, preferably 12%. After sintering, cooling at a pressure less than 0.13 Pa (10⁻⁴Torr), of a duration between 12 and 48 hours can be provided. The spherical titanium powder can be obtained by plasma atomization, by gas atomization or by the plasma rotating electrode process, of Ti-6Al-4V and have a grain size less than 0.150 mm, and the dendritic titanium powder can be obtained by a metal iodide reduction process by sodium from Ti-6Al-4V and have a grain size less than 0.050 mm. Unexpectedly, the dendritic powder seems to provide certain functions of the binder and makes it possible to do away with other functions.

Spherical Powder

The spherical powder can be obtained by gas atomization GA. A gas atomization method for titanium was developed by Crucible Research Division de Crucible Materials Corporation in 1988, C. F. Yolton, Francis H. Froes, 2—Conventional titanium powder production, Editor(s): Ma Qian, Francis H. (Sam) Froes, Titanium Powder Metallurgy, Butterworth-Heinemann, 2015, Pages 21 to 32, ISBN 9780128000540. The initial load is heated by induction in a crucible in a vacuum or inert gas. The molten load is cast down into a nozzle heated by induction. The resulting metal flux is atomized with high-pressure argon gas. The resulting droplets rapidly solidify and enable the formation of spherical powder particles with small satellites. The powder particles produced by the gas atomization process range from 0.010 mm to 0.500 mm and have a very good castability. The oxygen content is dependent on the size of the powder particles. A mean oxygen content of 0.06% can be obtained with a coarse particle powder (0.090 mm to 0.500 mm). However, a mean content of 0.1% can be reached with the finest particle powder (0.010 mm to 0.090 mm). The carbon, nitrogen and hydrogen contents are not dependent on the particle size. The plasma rotating electrode process PREP is a centrifugal atomization process for manufacturing pre-alloyed titanium powder developed by Metals/Starmet, cf. C. F. Yolton, Francis H. Froes, 2—Conventional titanium powder production, Editor(s): Ma Qian, Francis H. (Sam) Froes, Titanium Powder Metallurgy, Butterworth-Heinemann, 2015, Pages 21 to 32, ISBN 9780128000540. A helium plasma is used to melt the end of a rapidly rotating bar. The molten droplets are spun and solidify in flight in a helium atmosphere. PREP powder is spherical and is shown in FIG. 1. PREP powder has a very good castability. The particle size is between 0.050 and 0.350 mm, as per M. Yan, H. P. Tang, M. Qian, 15—Scavenging of oxygen and chlorine from powder metallurgy (PM) titanium and titanium alloys, Author(s): Ma Qian, Francis H. (Sam) Froes, Titanium Powder Metallurgy, Butterworth-Heinemann, 2015, Pages 253 to 276, ISBN 9780128000540. The oxygen content of PREP powder is between 0.06% and 0.15% and is also dependent on the particle size. A higher oxygen content is reached with particles having a diameter less than 0.100 mm whereas a lower oxygen content is reached with powder in which the diameter is greater than 0.100 mm.
The spherical powder can be obtained by plasma atomization PA. A titanium cord is subjected to a non-transferred arc plasma torch. The high-velocity plasma melts the cord and divides the liquid into fine droplets which solidify in flight. The powders produced by this method are spherical and have a grain size distribution of 0.025 mm to 0.250 mm. The oxygen content is less than 0.15% and is also dependent on the particle size.

Dendritic Powder

The dendritic powder can be obtained by a process frequently referred to as the Armstrong process. Reference can be made to K. Araci, D. Mangabhai, K. Akhtar, 9—Production of titanium by the Armstrong Process®, Authors(s): Ma Qian, Francis H. (Sam) Froes, Titanium Powder Metallurgy, Butterworth-Heinemann, 2015, pages 149 to 162, ISBN 9780128000540. Metal iodides are reduced which enables the formation of a titanium powder from a gaseous titanium tetrachloride solution introduced into the reaction with a liquid Na solution. The reaction takes place as follows: TiCl4(g)+4Na(I) Ti(s)+4NaCl(s)
Similarly, other metal chlorides such as aluminum trichloride and vanadium tetrachloride can be introduced into the flux to produce a homogeneous pre-alloyed Ti-6Al-4V. The powder obtained has a dendritic particle or coral morphology and is illustrated in FIG. 2. The size of the powder particles varies from a few microns to 250.10-6 m. Powders obtained from the Armstrong Process® have an oxygen content between 0.12% and 0.2%, which meets the standard requirements for a grade 5 titanium and also for a grade 2 titanium.
The powder has a high area-to-volume ratio and hence offers very good compressibility and very good green strength behavior.

Tests

A spherical powder from the PA process was tested in tests. The dendritic powder is obtained from the Armstrong process. The chemical composition and the particle size of all the powders are shown in Table 2.

TABLE 2

Chemical composition of powders tested.

										Particle size
Powder grade	Type	Ti	Al	V	Fe	C	N	H	O	10⁻⁶m

Ti-6Al-4V	Spherical	Rem. %	6.29	3.94	0.15	0.01	0.02	0.01	0.07	<150
Ti-6Al-4V	Dendritic	Rem. %	5.31	3.85	0.01	0.01	0.01	0.003	0.14	<44
Ti-6Al-4V		Rem. %	5.50-6.75	3.50-4.50	0.4	0.08	0.05	0.015	0.3	N/A
ASTM B988-13					max	max	max	max	max

The agglomerated powder represents a mixture of spherical powder and dendritic powder. A total of 4 batches (mixtures) was produced with different weight percentage compositions (60 to 90% for the spherical powder and 10 to 40% for the dendritic powder). Here, the batch represents the agglomeration between the spherical and dendritic powder. The powders were mixed in a mixer for 2 hours to homogenize the distribution of dendritic powder and spherical powder. No organic binder was added to the mixture.
The green strength was measured on each batch as per the ASTM B312-14 standard and the results are shown in Table 3.

TABLE 3

Green strength results.

Batch	Mixture [% of weight]	Green strength [MPa]

1	70% Spherical	4
	30% Dendritic
2	90% Spherical	Modelled 3.5
	10% Dendritic
3	80% Spherical	Modelled 4
	20% Dendritic
4	60% Spherical	Modelled 8
	40% Dendritic

This result can be explained by the good distribution of dendritic powder and spherical powder, which enables the spherical particles to have a good adherence and hence a good green strength. Moreover, the positive effect of adding dendritic powder is observed on the mixture. The results show that the green strength changes from 4 MPa to 8 MPa as the provision of dendritic powder in the mixture increases (10% to 40% of the weight). Below 10% dendritic powder, the green strength is insufficient for quality sintering. At 100% dendritic powder, the green body has a strength of 10 MPa. At 100% spherical powder, the green body has an insufficient strength less than 0.5 MPa. The green strength was measured separately on an agglomerate of spherical powder. The green strength on the spherical powder was measured in two configurations: without organic binder and with an organic binder (5% by weight). The green strength results obtained with spherical powder without organic binder are 0 MPa. While the green strength obtained on the spherical powder with an organic binder was 2 MPa. The use of a dendritic powder enhances the green strength of the green part considerably.

Pressing by High-Velocity Compaction (HVC)

Cylindrical samples of a diameter of 82 mm and a thickness of 13 mm were pressed by high-velocity compaction (HVC). The HVC machine used for this experiment is equipped with a hydraulic hammer or ram and has a maximum capacity of 18 kJ. The speed of the ram used is greater than 2 m·s−1. Two pressing conditions were tested, i.e. 2 m·s−1 and 4 m·s−1. The samples pressed with ram compression speed of 2 m≠s−1 (=600 MPa) reach a relative density of 78%. Whereas the samples pressed with ram compression speed equal to 4 m·s−1 (1500 MPa) reach a relative density of 82%. Furthermore, this presentation shows the samples pressed with a ram speed equal to 4 m·s−1.
Four cylindrical samples from batch 1 (70% spherical powder—30% dendritic powder by mass) were pressed by HVC with a ram speed equal to 4 m·s−1. The mean relative density results obtained with the cylinders compressed by HVC are shown in Table 3.

TABLE 4

Mean results on samples compacted by high-velocity compaction.

Batch	Mixture [% of weight]	Relative density [%]

1	70% Spherical	82
	30% Dendritic

Sintering

Sintering can be performed immediately after compaction, particularly in a vacuum furnace. The lack of degassing of the agglomerate enables a rapid temperature rise and a preservation of the furnace properties. The sintering cycle was performed in a temperature range from 1200° C. to 1350° C. at a high vacuum pressure ranging from 0.13 Pa (10-4 Torr) to 0.13 mPa (10-7 Torr) and the holding time whereof is 7 hours in order to obtain high densification. Slow cooling in high vacuum was carried out for 24 hours in order to reach ambient temperature. Here, the results obtained on samples sintered at 1300° C. for 7 hours followed by slow cooling at high vacuum at 0.0013 Pa (10-6 Torr) for 24 hours to reach ambient temperature are shown. At a temperature of 1350° C., the pressure can be higher while remaining at most 0.13 Pa (10-4 Torr).

Porosity, Microstructure and Chemistry

The results obtained on batch 1 are reproduced in FIG. 3 et seq. Once the cylindrical samples were sintered and cooled, the density, internal porosity and microstructure were measured. The apparent density was measured by the Archimedes technique as per the standard ASTM B-311-17, Standard Test Method for Density of Powder Metallurgy (PM) Materials Containing Less Than Two Percent Porosity, ASTM International, West Conshohocken, Pa., 2017. The cylindrical samples reached a density greater than 98.5%. A higher density can be obtained by increasing the sintering time. The mean porosity was measured by transverse cross-section image analysis. FIGS. 3 and 4 show the porosity obtained on the sintered cylinder of batch 1. The samples are close to the theoretical total density and confirm the results obtained by the Archimedes technique. The mean porosity measured on batch 1 is 1.2%±0.3.
The microstructure of the two samples, illustrated in FIGS. 5 and 6, is a typical Widman-Stätten structure with acicular alpha grains and a beta intergranular porosity. On the samples from batch 1, the average size of the alpha lamellae is 5.10-6 m and the alpha colonies have a size of about 100. 10-6 m. This is consistent with the slow cooling rate which makes it possible to prevent oxygen capture during cooling. No carbide is observed on the sintered microstructures.
The chemical analysis was performed on sintered samples using a Leco ONH836 type analyzer for the oxygen and nitrogen measurements and a Leco CS444 type analyzer for the carbon measurements. The analyses were performed as per the standard ASTM E1409-13, Standard Test Method for Determination of Oxygen and Nitrogen in Titanium and Titanium Alloys by Inert Gas Fusion, ASTM International, West Conshohocken, Pa., 2013, to identify the oxygen and nitrogen content. The standard ASTM E1447-09(2016), Standard Test Method for Determination of Hydrogen in Titanium and Titanium Alloys by Inert Gas Fusion Thermal Conductivity/Infrared Detection Method, ASTM International, West Conshohocken, Pa., 2016 was used to determine the hydrogen content. Finally, the standard ASTM E1941-10(2016), Standard Test Method for Determination of Carbon in Refractory and Reactive Metals and Their Alloys by Combustion Analysis, ASTM International, West Conshohocken, Pa., 2016, was used to measure the carbon content. The results are shown in Table 5 and compared to the standards ASTM B988-13 Standard specification for powder metallurgy titanium and titanium alloy structural component, ASTM International, West Conshohocken, Pa., 2013, and ASTM B-348-13 Standard specification for titanium and titanium alloys bar and billets, ASTM International, West Conshohocken, Pa., 2013:

TABLE 5

Chemical analysis as mass percentage obtained on sintered samples with respect to standards.

Elements	Ti	Al	V	Fe	O	N	C	H

70% spherical/30% dendritic	Remainder	6.3	4.1	0.01	0.18	0.016	0.02	0.0018
After sintering
ASTM B-988 13 max	Remainder	5.5-6.75	3.5-4.5	0.4	0.3	0.05	0.08	0.015
ASTM B-348-13 max	Remainder	5.5-6.75	3.5-4.5	0.4	0.2	0.05	0.08	0.015

The analysis of Table 5 shows that the low oxygen during sintering was preserved, much as the powders initially used had a low oxygen content, see Table 2. Furthermore, the oxygen content was less than 0.2% (value set as maximum permissible value by the standard ASTM B-348-13). This can be explained by the high vacuum at 0.0013 Pa (10-6 torr) which prevented oxidation during sintering. A test conducted at 0.00013 Pa (10-7 torr) with 100% dendritic powder gave similar results in terms of density and mechanical properties. An oxygen trap can allow a higher pressure up to 0.13 Pa (10-4 torr). In conclusion, the chemical composition of the sintered parts meets all the requirements of the standards ASTM B-348-13 and ASTM B988-13.

Mechanical Tests

From the sintered cylinders of batch 1, two traction test specimens were machined in order to test the mechanical behavior. The traction test machine used for these experiments is Zwick/Roell Z250. The traction test was performed as per the standard SS EN 6892. The traction test specimen used is 6B30 type. The traction tests were performed at ambient temperature. The results of the mechanical properties are shown in Table 5 and compared to the standards ASTM B-988-13. The standard B-988-13 is required for titanium in powder metallurgy and for titanium structural components.

TABLE 6

Mechanical results obtained on sintered PA/Armstrong samples
compared to ASTM B-348 and ASTM 988-13 grade 5

		Stress/Rupture
Titanium	Yield strength	strength	Elongation A
Grade 5	Rp_0.2[N/mm²]	Rm [N/mm²]	[%]

spherical/	749	839	13
dendritic
Grade 5	745	810	9
ASTM B 988-13
Minimum
required

The mechanical properties stated in Table 6 show that the yield strength, rupture strength and elongation at break obtained on the sintered samples are greater than the requirements of the standard ASTM B-988-13 which is the standard applied for parts manufactured using alloyed titanium powder.
The present invention makes it possible to remove debinding. No organic binder was used for agglomerating the powders. In terms of environmental impact, the material yield—i.e. the deviation between the quantity of material used in the method and the quantity of material obtained—is 97% in the operating sequence thus described and estimated at 80% minimum during the final machining.
Indeed, the method according to the invention makes it possible to obtain part dimensions very close to the final dimension. Overall, the material yield is therefore 78% to obtain dense parts from powder. The material yield of similar parts via the forging process is at best of the order of 40%. This difference in material yield also results advantageously into a reduction in energy consumption.
Additional tests were conducted with the same powders as above. Three mixtures were measured by mass:
A: 50% spherical powder, 50% dendritic powder,
B: 70% spherical powder, 30% dendritic powder,
C: 82.5% spherical powder, 17.5% dendritic powder.
The green strength is satisfactory. The density after compaction and before sintering is between 81 and 83%. After sintering, the mean porosity is 1.07% in a part produced with mixture A; 2.18% for the part produced with mixture B; 2.24% for the part produced with mixture C. The density after sintering is greater than 97% measured by the Archimedes method. The Vickers hardness is at least 370. The tests were conducted with a dendritic powder stored in ambient atmosphere and a spherical powder leaving production. An improvement of the density and a decrease in porosity are expected with powders leaving production or a storage of powders in a protected atmosphere preventing moisture uptake.
The invention is not restricted to the process and apparatus examples described above, merely by way of example, but it encompasses any alternative embodiments that may be envisaged by a person skilled in the art within the scope of the claims hereinafter.

Claims

1. A method for manufacturing a titanium-based metal part, by sintering a powder, the method comprising:

mixing a spherical titanium powder and a dendritic titanium powder to form a mixture,

agglomerating the titanium powder mixture by compaction with a ram moving at a speed greater than 2 m·s⁻¹, the titanium powder mixture being devoid of binder,

forming a green body or agglomerate suitable for sintering having a density greater than 78% of the density of the solid metal, and a

sintering the green body.

2. The method according to claim 1, comprising before mixing:

providing from 60 to 90% by mass of spherical titanium powder and from 10 to 40% by mass of dendritic titanium powder, wherein the green body has a green strength greater than 3 MPa.

3. The method according to claim 1, wherein the ram exerts a pressure between 600 and 1500 MPa.

4. The method according to claim 1, wherein the ram moves at a speed greater than 4 m·s⁻¹.

5. The method according to claim 1, wherein sintering the green body is performed in a neutral to reducing atmosphere, at a pressure less than 0.13 Pa (10⁻⁴Torr) and at a temperature between 1200° C. and 1350° C. to obtain a sintered body.

6. The method according to claim 5, wherein the sintering pressure is greater than 0.13 mPa (10⁻⁷Torr).

7. The method according to claim 5, wherein sintering the green body is conducted until a density greater than 97% of the density of the solid metal is obtained, particularly for over 4 hours.

8. The method according to claim 1, further comprising:

cooling at a pressure less than 0.13 Pa (10⁻⁴Torr) of a duration between 12 and 48 hours.

9. The method according to claim 1, wherein the spherical titanium powder is obtained by plasma atomization, by gas atomization or by a plasma rotating electrode process, and has a grain size less than 150.10⁻⁶m, and the dendritic titanium powder is obtained by the sodium metal iodide reduction process and has a grain size less than 100.10⁻⁶m.

10. The method according to claim 1, further comprising, before mixing,

providing from 40 to less than 90% by mass of spherical titanium powder and from 60 to more than 10% by mass of dendritic titanium powder.

11. Titanium-based sintered metal part, of density greater than 97%, of the density of the solid metal, wherein the sintered metal part is produced by the method according to claim 1.

12. The sintered metal part according to claim 11, comprising by mass, from 5.50 to 6.75% Al, from 3.50 to 4.50% V, less than 0.20% Fe, less than 0.04% C, less than 0.03% N, less than 0.005% H, less than 0.02% O, the remainder being Ti and unavoidable impurities and having a Young's modulus greater than 820 N·mm⁻², and an elongation greater than 10%.

13. The sintered metal part according to claim 11, comprising by mass, less than 0.002% Fe, less than 0.01% C, less than 0.02% N, less than 0.01% H, less than 0.1% O, the remainder being Ti and unavoidable impurities and having a Young's modulus greater than 748 N·mm⁻²and an elongation greater than 18%.

14. The sintered metal part according to claim 11, comprising by mass less than 0.02% O.