EP1433863A1

EP1433863A1 - Method for producing homogeneous fine grain titanium alloys suitable for ultrasonic inspection

Info

Publication number: EP1433863A1
Application number: EP03258060A
Authority: EP
Inventors: Bernard Patrick Bewlay; Robert Snee Gilmore; Michael Francis Xavier Gigliotti; John Broddus Deaton Jr; Gennady A Salishchev
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2002-12-26
Filing date: 2003-12-19
Publication date: 2004-06-30
Anticipated expiration: 2023-12-19
Also published as: DE60314589D1; RU2321674C2; DE60314589T2; EP1433863B1

Abstract

A titanium material production method for producing homogeneous fine grain titanium material in which the titanium material has a grain size in a range from about 5µm to about 20µm. The method comprises providing a titanium material blank; conducting a first heat treatment on the titanium material blank to heat the titanium material blank to a β-range; quenching the titanium material blank from the β-region to the α+β-region; forging the titanium material blank; and conducting a second heat treatment on the titanium material blank. The titanium material production method subjects the titanium material blank to superplasticity conditions during part of the titanium material production method.

Description

The invention relates to titanium material production methods. In particular, the invention relates to titanium material production methods that can produce titanium materials suitable for inspection using ultrasonic energy inspection methods and systems.
The production of titanium material with titanium material grain sizes and nature of αTi particle colony structures may be important variables that influence titanium material applications. Further, size of titanium grains and a nature of αTi particle colony structures may influence the ultrasonic noise and ultrasonic inspection in single phase and two-phase titanium alloys and materials, in which the ultrasonic inspection can be used to determine suitability of the titanium material for various applications. The size of titanium grains and the nature of αTi particle colony structures may influence ultrasonic inspection techniques, methods, and results by creating undesirable noise during ultrasonic inspection. This noise may hide or mask critical flaws in titanium that may limit applications of the titanium.
Colony structures are formed during titanium material production methods, for example during cooling a titanium material from a high temperature. The colonies are believed to form as βTi transforms to αTi and may define a "textured" titanium material microstructure. A crystallographic relation exists between the αTi and the parent βTi grain, such that there are only three crystallographic orientations that αTi will take forming from a given βTi grain. If the cooling rate is high and there is uniform nucleation of αTi throughout the grain, neighboring αTi particles have different orientations, and each behave as a distinct acoustic scattering entity. However, if there are only a few sites of αTi nucleation within the βTi grain, then the αTi particles in a given area all grow with the same orientation, and a colony structure results. This colony becomes the acoustic entity. Since a colony is formed within a βTi grain, the colony size will be less than the βTi grain size. While thermomechanical processing techniques that rely on dynamic recrystallization in the α+β temperature range to achieve uniform fine grain (UFG) αTi particles and prevent colony formation have been developed to improve titanium microstructure, defects may remain in the titanium material. These defects may be undesirable for some titanium material applications. Thus, the defects should be discovered prior to use of the titanium material in various microstructurally sensitive applications.
Titanium material production methods are known and are varied. One such titanium material production method relies on dynamic recrystallization of titanium material in the α+β temperature range. This recrystallization is intended to achieve relatively uniform fine grain (UFG) αTi particles and prevent colony formation. While this type of recrystallization has been proposed to improve titanium material microstructure, defects may remain in the titanium material, and these defects may limit applications of the titanium material. Some of the defects in the titanium material may be difficult to detect using conventional ultrasonic inspection techniques and methods.
Nondestructive evaluation of articles and structures by ultrasonic inspection and ultrasonic inspection is a known testing and evaluation method. Ultrasonic inspection and testing typically requires that defects or flaws in the articles and structures possess different acoustic behaviors from bulk material articles and structures undergoing similar ultrasonic inspection to be detectable. This different behavior permits the ultrasonic inspection to detect flaws, grains, imperfections, and other related microstructural characteristics (defects) for a material. Materials forming articles and structures with large, elastically anisotropic grains, such as, but not limited to, cast ingots of steels, titanium alloys, and nickel alloys, are often difficult to evaluate by ultrasonic testing. The difficulties arise, at least in part to, because sound waves, which are used for ultrasonic inspection, are reflected from grains and grain arrays sharing common elastic behavior, and represent a background "noise." The generated background noise can mask flaws in the material, and is thus undesirable.
Ultrasonic inspection techniques have been developed that use focused ultrasonic beams to enhance a flaw fraction within any instantaneously insonified volume of material in articles and structures. These developed ultrasonic inspection techniques can identify indications based both on maximum signal, as well as signal to noise. A scattering of sound in a polycrystalline metallic material body, which is also known in the art as an attenuation of a propagating sound wave, can be described as a function of at least one of grain dimensions, intrinsic material characteristics, and ultrasound frequency. Typically, three different functional relationships among scattering, frequency, and grain dimensions have been described. These are:
for λ > 2πD, a equal to about Tν⁴Θ, termed "Rayleigh" scattering;
for λ < 2πD or λ ≅ D, a equal to about Dν²Σ, termed "stochastic" or "phase" scattering; and
for λ « D, a ∝ 1/D, termed "diffusion" scattering;

D

T

A titanium material microstructure can be determined by measuring the scattering of sound in a material. A titanium material microstructure's sound scattering sensitivity can be attributed to αTi particles that are arranged into colonies. These titanium material colonies typically have a common crystallographic (and elastic) orientation, and these colonies of αTi particles can behave as large grains in the titanium material. An individual αTi particle might be about 5µm in diameter, however, a colony of αTi particles could be greater than about 200µm in diameter. Thus, the size contribution attributed to sound scattering sensitivity from αTi particles could vary, for example vary over 40-fold, among differing microstructures. Additionally, the sound scattering sensitivity due to αTi particles could change between that from randomly crystallographically oriented αTi particles to that from αTi particles within crystallographically oriented colonies of αTi particles.
While ultrasonic inspection of most titanium material articles can be preformed with some degree of certainty, the shape, size, configuration, structure, and orientation of the articles, titanium material grains and microstructures formed during a titanium material production method undergoing ultrasonic inspection may impair the ultrasonic inspection. Thus, in order to have acceptable titanium material for certain applications, it is desirable to provide titanium material production methods that produce titanium articles and structures that can be subjected to an ultrasonic inspection that enhances the determination and differentiation of noise during ultrasonic inspection. Thus, the ultrasonic inspection method can determine if ultrasonic inspection noise is attributed to a defect in the titanium material, or is due to other factors.
Therefore, a need exists for a titanium material production method that is suitable for producing titanium material, articles, and structures for ultrasonic inspection methods that can be subjected to an ultrasonic inspection that enhances the determination and differentiation of noise during ultrasonic inspection.
A titanium material production method for producing homogeneous fine grain titanium material in which the titanium material has a grain size in a range from about 5µm to about 20µm is provided by the invention. The method comprises providing a titanium material blank; conducting a first heat treatment on the titanium material blank to heat the titanium material blank to a β-range; quenching the titanium material blank from the β-region to the α+β-region; forging the titanium material blank; and conducting a second treatment on the titanium material blank. The titanium material production method subjects the titanium material blank to superplasticity conditions during the titanium material production method.
A titanium material production method for producing homogeneous fine grain titanium material in which the titanium material has a grain size in a range from about 15µm to about 20µm, generally equiaxed titanium grains and generally equally sized titanium grains, and substantially even distribution of second phase particles and alloying elements is also provided by the invention. The method comprises steps of: providing a titanium material blank, the titanium material comprising a two-phase titanium material; conducting a first heat treatment on the titanium material blank to heat the titanium material blank to a β-range; quenching the titanium material blank from the β-region to the α+β-region; forging the titanium material blank; and conducting a second heat treatment on the titanium material blank. The titanium material production method subjects the titanium material blank to superplasticity conditions during the titanium material production method. The titanium material production method comprises heating the titanium material blank to a temperature in a range from about 600°C to about Tpt, wherein Tpt is a polymorphous transformation temperature for the titanium material.
Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Figure 1 illustrates light micrographs of ti6242 material that in the following conditions: (a) conventional billet; (b) conventional forging; (c) uniform fine grain (ufg) billet; (d) a forging of a ufg billet;
Figure 2 illustrates icosahedral images generated from EBSP analysis of a Ti6242 material in the following conditions: (a) conventional billet; (b) conventional forging; (c) UFG billet; (d) forged UFG billet;
Figure 3 illustrate [0001] pole figures generated from EBSP analysis of a Ti6242 material in the following conditions: (a) conventional billet; (b) conventional forging; (c) UFG billet; (d) forged UFG billet;
Figure 4 illustrate 5 MHz C-scan images of Ti6242 blocks containing arrays of 0.79mm (1/32 inch) diameter flat bottom holes, which are drilled 25mm below a surface, in which the top left is titanium UFG billet, the top right is a conventional titanium billet, the bottom left is a conventional titanium forging, and the bottom right is a titanium UFG forging, whereing the 5 MHz C-scan images are taken at 12dB attenuation noise scan;
Figure 5 illustrate 5 MHz C-scan images of Ti6242 blocks containing arrays of 0.79mm (1/32 inch) diameter flat bottom holes, which are drilled 25mm below a surface, in which the top left is titanium UFG billet, the top right is a conventional titanium billet, the bottom left is a conventional titanium forging, and the bottom right is a titanium UFG forging, wherein the 5 MHz C-scan images are taken at 34dB attenuation signal scan;
Figure 6 illustrates a graph of average signals from flat bottom holes with respect to those in the block machined from the conventional billet;
Figure 7 illustrates a graph of average noise from blocks referenced to that from the block machined from the conventional billet; and
Figure 8 illustrates a graph of signal to noise ratios of Ti6242 blocks as a function of frequency.
A titanium material production method, as embodied by the invention, comprises a plurality of metallurgical processing steps to provide a titanium material with a homogeneous, fine grain microstructure. The produced titanium material is suitable for various microstructurally sensitive applications, including but not limited to, turbine component applications. The produced titanium material can also be readily inspected by ultrasonic inspection methods and systems. The ultrasonic inspection of the titanium material that is prodiced by a titanium material production method, as embodied by the invention, will readily indicate titanium material characteristics, for example, but not limited to, scattering types, grain size, and other microstructure characteristics.
A titanium material production method, as embodied by the invention, comprises at least steps of providing a titanium material blank, in which the titanium material blank may be formed by suitable titanium material production methods, including but not limited to, powder metallurgy methods; heat treating the titanium material blank to a temperature in the β range for titanium; quenching of the thus heated titanium material blank; forging the quenched titanium material blank; and another heat treatment of the forged titanium material blank, in which superplasticity conditions are achieved in the titanium material production method. The resultant titanium material possesses a microstructure with a grain size in a range from about 5µm to about 20µm, for example in a range from about 15µm to about 20µm.
A homogeneous fine grain titanium material microstructure is created by recrystallization of the titanium material during the titanium material production method, as embodied by the invention. The recrystallization of the titanium material often occurs during plastic deformation of the titanium material, for example during annealing or deformation of the titanium material. Therefore, resultant microstructural grain size is in a range from about 5µm to about 20µm, for example in a range from about 15µm to about 20µm. The titanium material grain size lends to a decrease in defects in the titanium material.
Traditional titanium material plastic deformation processes are not able to form highly homogeneous titanium material microstructures. These known plastic deformation processes can result in different metallographic and crystallographic titanium microstructures, with differing inhomogeneous distributions of second phase particles and different shaped particles in the titanium material microstructure. This titanium material microstructure, even though it may possess small grain sizes, provides considerable noise levels during ultrasonic inspection, which of course is undesirable.
A homogeneous fine grain microstructure with a grain size in a range from about 5µm to about 20µm, for example in a range from about 15µm to about 20µm can be formed by the titanium material production method, as embodied by the invention. This homogeneous fine grain microstructure is formed by dynamic titanium material grain recrystallization, which is often accompanied by a creation of second phases. Temperature and rate conditions for the titanium material production method, as embodied by the invention, include a temperature range between about 600°C to about Tpt, in which Tpt is the polymorphous transformation temperature for the titanium material. The rate interval for the titanium material production method is provided in a range from about 10^-5 to about 10^-1s^-1. In the titanium material production method, as embodied by the invention, a lower deformation temperature and a higher strain rate, provides a smaller grain size. These temperature and deformation rate ranges include superplastic deformation conditions, which result in dynamic recrystallization of titanium material and formation of recrystallized titanium material grains with a size in a range from about 5µm to about 20µm.
Superplastic conditions occur under flow during one of the processing steps of the titanium material production method, as embodied by the invention. The titanium material microstructure can become homogeneous if the titanium material is subjected to superplastic deformation, in which the homogeneity provides substantially equiaxed grains with generally equal sized grains. Further, any titanium material secondary phase particles can be substantially uniformly distributed in the titanium material, and any alloying elements therein can be substantially distributed between the phases. In general, the titanium material, which is produced by a titanium material production method, as embodied by the invention, provides a generally textureless state, meaning that the titanium material does not contain colonies that would impair ultrasonic inspection. As the result, the noises during ultrasonic inspection can be decreased, and the sensitivity of the ultrasonic inspection enhances the detection of titanium material defects.
The titanium material blank that is provided by the titanium material production method, as embodied by the invention, can comprise a two-phase titanium material, for example a two-phase titanium alloy, which can be prepared by any suitable metallurgical process including but not limited to, powder metallurgy. The titanium alloy can comprise any suitable titanium material or titanium alloy, for example, but not limited to, Ti64 alloys, Ti6242 alloys, Ti-5AI-2.5Sn alloys, Ti-5Al-2.5Sn-Eli alloys, IMI550 titanium alloys, VT8-1 titanium alloys, VT6 titanium alloys, and other titanium materials. The titanium alloys discussed herein are exemplary of the titanium materials for titanium articles and structures within the scope of the invention. The description of titanium alloys is in no way intended to limit the scope of the invention.
The formation of homogeneous fine grain microstructure in titanium materials can be related to an initial titanium material microstructure before any deformation. For example, an initial titanium material microstructure before any deformation in the (α+β)-region tends to comprise grains that are coarse and lamellar. This grain size is in a range from about 300µm to about 500µm.
A smaller and more homogeneous titanium material grain size, which can be obtained during deformation in the β-region, can be provided during deformation in (α+β)-region. In order to obtain this titanium material microstructure, multiple forging steps in the titanium material production method are performed with a temperature around Tpt. Thus, recrystallization annealing or secondary deformation in the titanium material production method is conducted in the β-region to form a homogeneous microstructure with fine β-grains.
Dissimilar initial titanium material grain orientations, when under applied stress in the titanium material production method, can result in non-uniform recrystallization. This non-uniform or inhomogeneous recrystallization (textured microstructure) may lead to non-uniform deformation distribution in the titanium material. The titanium material production method, as embodied by the invention, can increase distribution deformation homogeneity and microstructure homogeneity in the titanium material and thus provides a desirable titanium material microstructure.
The titanium material production method, as embodied by the invention, will now be discussed with reference to examples for producing titanium materials. These examples are merely exemplary of the titanium material production methods within the scope of the invention, and are not intended to limit the invention in any manner. The scope of the invention comprises other titanium material production methods. Further, the values, ranges, and amounts set forth in the specification are approximate, unless otherwise indicated.

EXAMPLE 1

Titanium material blanks comprising a two-phase titanium alloy (Ti-6242) having a Tpt of about 1000°C were provided. The titanium material blanks were cut from a deformed β-region in a titanium material rod. The dimensions of the titanium material blanks were 100mm by 100mm by 200mm. The β-grain size was in from about 3mm to about 5mm. The titanium material microstructure was extended or elongated in a direction of deformation.
The titanium material blanks were initially heated to a temperature in the β-region (T equal to about 1020°C, dwelling time equal to about 1 hour). The titanium material blanks were then quenched from the temperature of the β-region to create a homogeneous fine grain microstructure in (α + β)―region. Disperse lamellar microstructure was formed and a layer of α-phase titanium was formed disposed around boundaries of the β-grains with a reduced thickness compared to conventional titanium material production methods. This titanium material production method increases grain and microstructure homogeneity during recrystallization.
Forging in the (α+β)-region was conducted at the temperature T equal to about 875°C and an average strain rate of 3x10^-3s^-1. An isothermal hydraulic press with the 1,600 ton capacity was used, in which the press comprised an isothermal die block. The block was manufactured from the heat resistant nickel alloy and was heated up to the same temperature as the blank. The deformation of the titanium material in the titanium material production method was conducted by forging with changing deformation axes. After two deformation steps (as above) were conducted one after the other, a homoheneous titanium material microstructure with a grain size of about 5µm was formed. The strain during each forging was 50% with respect to a height dimension of the titanium material blank. Sum relative strain, measured by a change in titanium material blank area during each stage was 12. Accordingly, the titanium material was determined to be in superplasticity conditions because the resultant grain size is about 5 µm, a deformation temperature was about 875°C, a strain rate ε equal to about 3x10^-3 s^-1, and a rate sensitivity coefficient m equal to about 0.39. In order to complete recrystallization of the titanium, the titanium material blanks were subjected to annealing at the deformation temperature for a period of about 1 hour.

EXAMPLE 2

Titanium material blanks comprising a two-phase titanium alloy (IMI550) were provided. The alloy had a Tpt of about 965°C for an ingot and a Tpt of about 980°C for a forging. The titanium material formed as an ingot (billet) with an approximate size of 634mm by 540mm was prepared by a titanium material production method that included subjecting the titanium material to a forging in the β-region. This step was followed by heat treatment at about 1200°C, and thereafter by forging and rollforming. This step included settling, forging on the square, and rollforming. A heat treatment step followed with heating at 1140°C and forging to 390mm. These steps were followed by cooling in air. Further, the titanium material production method, as embodied by the invention, included a step of heating at Tpt-30°C and forging to 310mm, heating at 1060 °C, forging to 280mm, and cooling by air were conducted. Further, the titanium material blank was subjected to heating at Tpt -30 °C and forging, which included settling, forging to a square, rollforming, and forging to 245mm. After heating the titanium material blank in the β-region, for example Tpt +20 °C, a homogeneous titanium microstructure with a grain size in a range from about 700µm to about 940µm was formed. Cooling of the titanium material blanks was conducted by water quenching.
The titanium material production method, as embodied by the invention, comprised forging in the (α+β)―region for titanium at a temperature of about 930 °C with an average strain rate of 10^-3s^-1. The dimensions of the titanium material blank were 230mm by 435mm. An isothermal hydraulic press having 1600 ton capacity was used for forging. The press comprised an isothermal die block, which was manufactured from a heat resistant nickel alloy that was heated up to a similar temperature as the titanium material blank. The deformation corresponded to forging with similar deformation axes. After two repeated deformation steps as decribed above, a highly homoheneous titanium material microstructure having a grain size in a range from about 4µm to about 8µm was formed. Strain during forging was about 50% in relation to the titanium material blank height dimension. Sum relative strain, measured by a change of the titanium material blank area on each stage, was about 12.
The titanium material blank was in superplasticity ranges during the titanium material production method, as a grain size in a range from about 4µm to about 8µm, a deformation temperature of about 930°C, strain rate ε equal to about10^-3 s^-1, and a sensitivity rate coefficient m of about 0.49. The titanium material blanks were subjected to annealing at the deformation temperature for about 1 hour for recrystallization. The final dimensions of the blank were about 250mm by about 300mm.

EXAMPLE 3

Titanium material blanks comprising a two-phase titanium alloy (VT8-1) were provided, in which the titanium material blanks possessed a Tpt of about 965°C as an ingot and a Tpt of about 1000°C as a forging. The ingot, which has a size of about 628mm by 535mm, was subjected to a forging in the β-region of titanium. The forging was followed by heat treatment at about 1200°C, and forging that included rollforming, settling, forging on a square, and rollforming. This step was followed by heat treatment at about 1140°C, forging to about 390mm, and a cooling by air. Further, a heat treatment at about Tpt-30°C and forging to 310mm, heating at about 1060 °C and forging to 280mm followed by cooling in air were also conducted on the titanium material blanks.
The titanium material blank can then be subjected to heating at about Tpt -30 °C followed by forging. The forging included settling, forging on a square, roll-forming, and forging to 245mm. After a heat treatment in the β-region (Tpt +20 °C), a homogeneous titanium material microstructure with a grain size in a range from about 810pm to about 850µm was formed. Cooling of the titanium material blanks was conducted by water quenching.
Forging in the (α+β)―region for titanium material was conducted at a temperature of about 930 °C and an average strain rate 10^-3s^-1. The dimensions of the titanium material blank were 230mm by 435mm. An isothermal hydraulic press with the 1600 ton capacity was used for forging. The press comprised an isothermal die block, which was manufactured from a heat resistant nickel alloy. The die block was heated up to the same temperature as the titanium material blank. The deformation corresponded to a forging deformation axis. After two deformation steps were conducted, a highly homoheneous titanium material microstructure with the fine grain size in a range from about 5µm to about 8µm was formed. Strain during forging was 50% in relation to the titanium material blank height dimension. Sum relative strain, measured by a change in titanium material blank area, was 12.
The titanium material was in superplasticity regions during the titanium material production method, because of the grain size in a range from about 4µm to about 8µm, a deformation temperature of about 930°C, a strain rate ε equal to about10 ^-3s^-1, and rate sensitivity coefficient m of about 0.49. The titanium material blanks were subjected to annealing at the deformation temperature for about 1 hour for recrystallization. The final dimensions of the blank were about 250mm by 300mm.

EXAMPLE 4

Titanium material blanks comprising a two-phase titanium alloy VT6 were used, in which the titanium material blanks had a Tpt of about 990°C in an ingot and Tpt of about 990°C in a forging. The titanium material ingot has a size of about 736mm by 1523mm and was subjected to forging in the β-region. The forging included heating to 1200°C, extention to 620mm, and heating to 1200°C and extention to 510mm. The titanium material blank was then cut in 2 pieces, and subjected to further heat treatment. The heat treatment included heating to 1100°C, extention to 410mm, and cooling by air. Further, the titanium material production method included heating at a temperature (Tpt-40°C), extention to 370mm, heating at 950°C, and forging to 320mm were conducted. Further, the titanium material production method included heating to 1060 °C, extention to 295mm and water cooling, and cutting into two pieces. Further, the titanium material blank was heated to Tpt-30 °C, deforming to a height of about 390mm, heating to 960 °C, deforming to a height 350mm, forging to a square 280mm, roll forming to 320mm were conducted. Further, a repeat of these steps operations were conducted and final titanium material blank had size of about 245mm.
Forging in the (α+β)―region was conducted at about T equal to about 940 °C and the average strain rate 10^-3s^-1. The dimensions of the blank were 230mm by 435mm. Isothermal hydraulic press with a 1600 ton capacity was used. The press comprised the isothermal die block that was manufactured from heat resistant nickel alloy and was heated to the temperature of the titanium material blank. The deformation corresponded to deformation axes. After two steps of deformation were conducted, a highly titanium material homoheneous microstructure with a fine grain size in a range from about 6µm to about 10 µm was formed. The strain during forging was about 50% with respect to titanium material blank dimensions. Sum relative strain measured by a change of the titanium material blank was about 12.
It was determined from the grain size in a range from about 6µm to about 10µm, a deformation temperature at about 930°C, strain rate ε equal to about10 ^-3s^-1, and a rate sensitivity coefficient m equal to about 0.35 that superplasticity conditions were provided in the titanium material production method, as embodied by the invention. To reach recrystallization, the titanium material blanks were subjected to annealing at the deformation temperature for about 1 hour. The final dimensions of the titanium material blank were 250mm by 300mm.

EXAMPLE 5

Titanium material blanks comprising a two-phase titanium alloy (VT6) was used, in which the titanium material had a Tpt of about 990°C in an ingot form and a Tpt of about 990°C as a forging. Ingot dimensions were 736 by 2500mm. Titanium material blanks were cut with dimensions 180 by 220mm. The sizes of the titanium material grain in a longtitudal direction were in a range from about 50mm to about 70mm, and in the lateral direction were in a range from about 15mm to about 20mm.
The titanium material blank was subjected to forging, which included heating at 1100°C, settling, deformation to 130mm, heating at 1050°C, settling, deformation to 130mm, and cooling by water. Further, the titanium material production method included heating at Tpt-40°C, settling, and deformation to 130mm. Further, heating at 1020 °C, deformation to 130mm and water cooling were included in the titanium material production method, as embodied by the invention.
The titanium material production method included forging in the (α+β)―region and with average strain rate 2x10^-2s^-1. The dimensions of the blank were 230mm by 435mm. Isothermal hydraulic press with a 1600 ton capacity was used. The press comprised the isothermal die block that was manufactured from heat resistant nickel alloy and was heated to the temperature of the titanium material blank, for example a temperature in a range from about 400°C to about 450°C. At T equal to about 980°C, the titanium material blank was subjected to the settling of 50%. Further, at the temperatures of 850 °C and 950 °C, the further settling was conducted followed by quenching. After three deformation steps were conducted with annealing at 900°C, the highly homoheneous microstructure with the grain size in a range from about 2µm to about 5µm was formed. Sum relative strain measured by a change of titanium material blank area was 16. The final dimensions of the titanium material blank were 110mm by 300mm.
The titanium material production methods, as embodied by the invention including those discussed above, can provide titanium articles and structures with suitable homogeneous fine-grain microstructures. The produced titanium material is intended to be suitable for various applications, such as, but not limited to, turbine component applications. Further, the produced titanium material possesses homogeneous fine-grain microstructures that can be readily evaluated by ultrasonic inspection methods and systems.
A general discussion of ultrasonic inspection will now be provided with reference to titanium materials, which can be produced by titanium material production methods, including titanium material production methods, as embodied by the invention. The following discussion will refer to titanium articles and structures, which include titanium materials that are prodiced by titanium material production methods, as embodied by the invention.
The titanium material produced by titanium material production methods, as embodied by the invention, can be inspected to determine if the titanium material microstructures comprise fine-scale granular αTi particles. Also, the titanium material can be used to form titanium material articles and structures that can be evaluated by ultrasonic inspection to result in enhanced determinations and indications of uniform-fine grain (UFG) billets and forgings made from UFG billets. Further, the produced titanium material can provide titanium material articles and structures. in which the titanium articles and structures generally generate predominantly Rayleigh scattering during ultrasonic inspection, which is indicative of uniform-fine grain microstructure in the titanium material. The functionality of scattering as a function of acoustic entity size and ultrasound wavelength varies in a smooth fashion from one regime ("Rayleigh" to "phase" to "diffusion") to another. For adequate inspection to find critical flaws, and to assure predominantly Rayleigh scattering, the acoustic entity size needs to be not greater than about 1/10 the wavelength of the sound used for inspection. The generated Rayleigh scattering from titanium articles and structures, as embodied by the invention, is typically indicative that the titanium articles and structures comprise uniform-fine grains (UFG). Thus, the produced titanium materials, as embodied by the invention, are suitable for various microstructurally sensitive applications, such as but not limited to turbine components.
Therefore, the titanium material that is produced by titanium material production methods, as embodied by the invention, can be inspected by ultrasonic inspection with enhanced results, because UFG titanium microstructures generate predominantly Rayleigh scattering. If the ultrasonic inspection determines scattering other than predominantly Rayleigh scattering, for example phase scattering alone or in combination with Rayleigh scattering, it is possible to characterize the titanium articles and structures as not comprising uniform-fine grain titanium.
αTi particles are generally less than about 5 µm in diameter, and are generally formed with an absence of crystallographic texture. The ultrasonic inspectability of these UFG titanium materials is characterized by a signal to noise ratio from machined flat bottom holes. The signal to noise ratio obtained by ultrasonic inspection is greater in UFG titanium materials than in the conventional titanium materials. It has been determined that there is less ultrasonic backscattered noise in the UFG titanium materials than in the conventional titanium materials. Further, it has been determined using ultrasonic inspection of titanium articles and structures that an ultrasonic signal from machined flat-bottomed holes is higher in the UFG titanium material.
Further, the ultrasonic inspection of titanium articles and structures indicates that the presence of a αTi particle colony structure is associated with ultrasonic noise. For titanium materials with αTi particles less than about 10 µm in size, differences in αTi particle size typically do not have a significant effect on generated ultrasonic noise. For example, UFG billets display chiefly Rayleigh scattering, while conventional billets, which are not be characterized by UFG properties, display Rayleigh scattering plus phase scattering when subjected to ultrasonic inspection. Therefore, the inspectability of titanium-containing materials is enhanced using titanium articles and structures that generate predominantly Rayleigh scattering.
The titanium articles and structures for ultrasonic inspection comprise UFG microstructural characteristics and features that can be determined using the titanium article's sound scattering sensitivity. The ultrasonic inspection method comprises providing a titanium articles and structures, for example a Ti6242 alloy. This Ti6242 alloy material is merely exemplary of the titanium materials for titanium articles and structures within the scope of the invention. The description of a Ti6242 alloy for the titanium articles and structures is in no way intended to limit the scope of the invention.
The titanium articles and structures (or "titanium material") is subjected to ultrasonic inspection by directing ultrasonic energy onto the titanium material. The ultrasonic energy directed into the material typically comprises a pulse of sound at a selected frequency. The sound pulse is scattered in a manner determined by the frequency of the sound pulse, the microstructural features of the titanium material, and by intrinsic physical characteristics, such as but not limited to, elastic constants and mass density, of the titanium material. The scattered energy is then analyzed and a determination of the characteristics of the scattered noise is made with regard to the nature of the scattering for the titanium articles and structures.
The titanium material for ultrasonic inspection comprises a uniform fine grain (UFG) material, which can be produced by forging a billet of conventional titanium material into various and appropriate structures, configurations, and shapes. For example, the UFG titanium articles and structures can be formed by steps of press forging, heat-treating, quenching, and subsequent cooling. The titanium that is actually subjected to the ultrasonic inspection may be further prepared by providing a titanium billet with at least one, for example a series, of flat bottom holes. These flat bottom holes will serve as pixel intensity standards, upon which the ultrasonic inspection can be gauged.
A signal to noise ratio for synthetic flaws machined in the Ti6242 blocks is strongly influenced by titanium microstructural condition, for example as the Ti6242 is defined by electron backscatter diffraction analysis. Ti6242 blocks having a microstructure comprising uniform, fine, texture-free αTi particles typically provided signal to noise ratios about 20 dB greater than similar titanium blocks with microstructures having colonies comprising crystallographically aligned αTi particles.
The ultrasonic inspection method and procedure will now be described with reference to titanium articles and structures and titanium materials, which are produced by titanium material production methods, as embodied by the invention. In the following discussion, the terms are used with their normal meanings as understood by person of ordinary skill in the art, unless discussed to the contrary. Further, the dimensions are approximate, unless stated to be exact.
The ultrasonic inspection provides titanium articles and structures, such as a Ti6242 alloy, for evaluation. The Ti6242 material is evaluated when the titanium material has been configured into four microstructural conditions: a conventional billet; a conventional forging from conventional billet; a uniform fine grain (UFG) billet; and a forging of the UFG billet. The individual billets will be referred to by the above names, and collectively as "billets".
The conventional billet is about 23 centimeters (cm) (9 inch) in diameter. The conventional forging is from the bore region of a disk forging, for example a compressor disk forging. The UFG billet is produced as two bars from about 10 cm x 10 cm x 20 cm sections taken from the commercial billet and having its grain refined under accepted titanium alloy grain refinement processes. The forging of UFG billet is produced by press forging at temperatures of about 900°C about a 7 cm tall, 6.35 cm diameter cylinder of the UFG billet to about a 2.80 cm final height at 2.5 cm/min pressing speed. The forging of UFG billet is given a heat treatment of about 970°C, for about 1 hour, followed by a helium quench, at about 705°C, for about 8 hours, followed by an air cool.
The microstructure of each billet is then evaluated by light microscopy. The crystallographic texture of each billet is then determined by electron backscatter diffraction pattern (EBSP) analysis. Light micrographs for each billet are displayed in Figure 1, where legend (a) is the conventional billet; legend (b) is the conventional forging; legend (c) is the UFG billet; and legend (d) is the forged UFG billet. Figure 2 shows EBSP "icosahedral" images, in which the [0001] pole inclination of a scanned microstructure is represented in colors. In Fig. 2, colors that are close to one another on an accepted "20-sided (icosahedral) color sphere" represent microstructure inclinations that are similar in pole inclination. Further, in Fig. 2, a black pixel is a pixel for which no crystallographic orientation can be determined. Further, Fig. 3 shows [0001] pole figures for the regions of the scanned images Fig. 2. The legends for Figs. 2 and 3 are similar to those of Fig. 1.
As illustrated, the conventional billet microstructure contains primary αTi particles, with a thickness of about 5µm, and lengths in a range from about 5µm to about 10µm, as illustrated in Figure 1, legend (a). The αTi particles are arranged in colonies, typically about 100µm wide and over about 300µm long, as illustrated in Fig. 2, legend (a). The αTi phase orientation of the sample scanned in Fig. 2, legend (a) indicate strong crystallographic texture, with most [0001] poles in the lower region of the pole, as illustrated in Fig. 3, legend (a).
The microstructure of the forging from the conventional billet contains primary αTi particles, with diameters in a range from about 5 to about 10µm, Figure 1, legend (b). As illustrated, there appears to be substantial breakup of the billet microstructure. αTi particles are arranged in large colonies comprising similar crystallographic orientations. For example, some αTi colonies are about 300µm wide and often greater than about 1000µm long, as illustrated in Fig. 2, legend (b). The αTi phase orientation of the sample scanned in Fig. 2, legend (b) has strong crystallographic texture, meaning that a majority of the [0001] poles are grouped within two regions of the pole figure, as illustrated in Fig. 3, legend (b). This strong grouping of the poles suggests that the scanned region comprises two colonies.
The ultrasonic inspection of the UFG billet indicates a microstructure comprising αTi particles. The particles comprise diameters about 5µm, as illustrated in Fig. 1, legend (c). These αTi particles do not appear to be provided in colonies, as illustrated in Fig. 2, legend (c). The αTi phase orientation of the sample scanned as illustrated in Fig. 2, legend (c) appears random, as illustrated in Fig. 3, legend (c).
The microstructure of the heat-treated forging of the UFG billet indicates that it comprises αTi particles. The αTi particles have diameters about 10µm, as illustrated in Fig. 1, legend (d). These αTi particles are larger than the billet from which the αTi particles are formed, and this suggests grain growth during at least one of forging or heat treatment of the UFG billet. The αTi particles are not provided in colonies, as illustrated in Fig. 2, legend (d). The αTi phase orientation appears random, as illustrated in Fig. 3, legend (d).
The ultrasonic characteristics of the billets formed different titanium articles and structures are determined by C-scans of blocks formed from billets of the titanium articles and structures. The titanium articles and structures are provided as blocks about 0.79mm (1/32 inch) diameter flat bottom holes. The titanium blocks are formed about 38mm thick with holes drilled to about 25mm below top surface of the block. Each of the conventional billet, conventional forging, and UFG billet have surface dimensions about 64 millimeters (mm) square, and each also has 9 flat bottom holes. The forging made from the UFG material had dimensions about 64mm by about 28mm, and is provided with 6 flat bottom holes. Each titanium block is machined with sufficient orientations so that an ultrasonic inspection direction is similar to that of a larger component formed from the titanium articles and structures. For example, the 38mm thickness of the titanium block is disposed in the radial direction of the billet or forging.

The ultrasonic transducers used for the ultrasonic inspection by C-scanning processes are listed in Table 1. Table 1 also provides characteristics of the ultrasonic transducers. The transducers comprise polyvinylidine fluoride (PVDF) as a piezoelectric element. Center frequencies for the ultrasonic transducers are measured from signals reflected off the backwall of a fused silica block.

Characteristics of Transducers
Transducer	Nominal Frequency	Actual Frequency	Diameter	Focal Length	Aperture
1	5 MHz	6.62 MHz	19mm	133mm	f/7
2	10 MHz	11.36 MHz	19mm	133mm	f/7
3	20 MHz	18.43 MHz	19mm	133mm	f/7

Two separate series of water immersion ultrasonic C-scans were performed on the titanium-containing blocks. The series of water immersion ultrasonic C-scans were performed at nominal frequencies of about 5 MHz, about 10 MHz, and 20 MHz. One scan at each of the above-frequencies is performed to measure a signal from the flat bottom holes. A second scan at each of the above-frequencies is performed at a higher amplification to get noise and sound scattering sensitivity statistics.
Each of the scans is made over a square region about 147.5mm in length and width, with about a 0.144mm scan and index increment. The sound is focused about 25mm below the top surface of the blocks, which is disposed in the approximate the plane of the flat bottom holes. The width of scan signal gate is about 4 microseconds. The obtained C-scan images are about 1024 pixels by about 1024 pixels.
Figure 4, legends (a)-(d), illustrate C-scan images made at about 5 MHz. With reference to Fig. 4, the UFG billet material is in the upper left, the conventional billet is on the upper right, the conventional forging is on the lower left, and the forging of the UFG material is on the lower right. The conventional billet and forging exhibit a higher background noise, as indicated by brighter pixels in those blocks as illustrated in Fig. 4, legend (a). A lower intensity is exhibited from the flat bottom holes, as indicated by a lower intensity of pixels from those regions as illustrated in Fig. 4, legend (b).

Quantitative measures of signal and noise can then be determined from the C-scans. The signal from each flat bottom hole is taken as the brightest pixel within the 3 x 3 array of the nine brightest pixels. Noise statistics and sound scattering sensitivity can then be determined from square pixel arrays that did not comprise the flat bottom holes. The quantitative data is presented in Table 2. In Table 2, a signal is an average signal from all flat bottom holes in the respective block. The signal to noise ratios are calculated both as: (Average Signal-Average Noise)÷(Maximum Noise-Average Noise) as well as: (Average Signal-Average Noise)÷(3 σNoise).

Ultrasonic Signals and Noise Measurements in Ti6242 Blocks
	FBH Signals	Noise
Material	MHz	Attenuation DB	S	Attenuation dB	N_ave	N_Max	σ_noise
Conventional billet	6.62	-34	94.4	-12	61.3	141	11.1
Conventional, forged	6.62	-34	53.4	-12	44.7	107.5	9.48
UFG billet	6.62	-34	216.1	-12	9.1	34.5	1.70
UFG, forged	6.62	-34	108.5	-12	4.3	12.5	0.973
Conventional billet	11.36	-49	75.4	-12.5	130.9	243.5	21.5
Conventional, forged	11.36	-49	42.7	-12.5	81.8	249.5	17.2
UFG billet	11.36	-49	214.5	-12.5	23.1	59.5	4.38
UFG, forged	11.36	-49	100.5	-12.5	5.9	11.5	1.11
Conventional billet	18.43	-48.5	51.3	-10	73.6	168.5	12.4
Conventional, forged	18.43	-48.5	20.4	-10	38.8	142.5	8.35
UFG billet	18.43	-48.5	212.2	-10	21.5	71.5	4.11
UFG, forged	18.43	-48.5	93.5	-10	11.9	20.5	1.40

The determined signal to noise ratio calculations for titanium materials are listed in Table 3. Both calculation methods, as described above, provide a measure of a signal's intensity in a selected block relative to noise spikes in the same block.

Signal to Noise Ratio in Ti6242 Blocks
		Signal to Noise Ratio
Material	MHz	(S_ave-N_ave)/(N_Max-N_ave)	(S_ave-N_ave)/3σ_noise
Conventional billet	6.62	14.2	33.9
Conventional, forged	6.62	10.0	22.0
UFG billet	6.62	106.7	531.3
UFG, forged	6.62	166.6	466.4
Conventional billet	11.36	43.6	76.1
Conventional, forged	11.36	16.5	53.7
UFG billet	11.36	393.7	1089.8
UFG, forged	11.36	1195.2	2015.7
Conventional billet	18.43	44.7	113.8
Conventional, forged	18.43	16.2	67.0
UFG billet	18.43	356.7	1445.5
UFG, forged	18.43	915.0	1873.2

Accordingly, if the determining a signal to noise ratio level is conducted by (Average Signal-Average Noise)÷(Maximum Noise-Average Noise), it can be generalized that the material comprises uniform fine grains at 6.62 MHz if the a signal to noise ratio - for a signal from 0.79mm (1/32 inch) diameter flat bottom holes 25mm below the inspected surface of the material - is at least about 20; at 11.36 MHz a signal to noise ratio level is at least about 50; and at 18.43 MHz a signal to noise ratio level is at least about 50. Further, if the determining a signal to noise ratio level is conducted by (Average Signal-Average Noise)÷(3 σ_Noise) for the subject flat bottom holes, it can be also generalized that the material comprises uniform fine grains at 6.62 MHz if the a signal to noise ratio level is at least about 50; at 11.36 MHz a signal to noise ratio level is at least about 100; and at 18.43 MHz a signal to noise ratio level is at least about 150. Each of these signal to noise ratio levels correspond to a preset noise level as determined by the pre-drilled holes in the material.
The highest signal from flat bottom holes is measured in the UFG billet, and the lowest signal from flat bottom holes is measured in a conventional forging, as illustrated in the graph of Fig. 6. The highest average noise, the largest maximum noise, and the largest standard deviation of noise are measured in a conventional billet. The lowest average noise, the smallest maximum noise, and the smallest standard deviation of noise are measured in forging of UFG material, as illustrated in the graph of Fig. 7. Accordingly, it can be determined that the forged UFG material possesses the highest signal to noise ratio, and that the conventional forging had the lowest signal to noise ratio, as illustrated in the graph of Fig. 8.
In the ultrasonic inspection of the titanium articles and structures, longitudinal sound velocities were measured in a Ti6242 extrusion. The Ti6242 extrusion was processed to create a strong [0001] texture in the direction of extrusion. For example, the extrusion of the Ti6242 is performed at about 1040°C and a ratio of about 8:1. The extrusion is then heat treated at about 593°C for about 8 hours. X-ray investigation and analysis determine the grain and microstructure orientation of the Ti6242. This investigation and analysis of the Ti6242 indicates a strong [0001] texture along the extrusion direction, with [0001] intensity along the extrusion direction. The intensity has been determined to be about 22 times random.
The ultrasonic behavior of small titanium articles and structures, for example a Ti6242 alloy, can be determined by ultrasonic inspection of the titanium articles and structures as a function of ultrasonic frequency and material microstructure. The speed of sound in αTi is about 6mm/µs. At an ultrasonic frequency of 5 MHz, the wavelength is about 1.2mm in the titanium articles and structures. Colony sizes greater than about 200 µm could change the scattering character from Rayleigh toward stochastic (phase). Sound velocities in the Ti6242 are measured on rectangular Ti6242 pieces that are formed from the respective Ti6242 billets. The rectangular Ti6242 pieces are about 16mm long in the extrusion direction and about 12mm in length in a direction normal to the extrusion direction. Longitudinal velocity is measured at about 10 MHz with a contact transducer, amplifier, and oscilloscope. The longitudinal velocity is determined by measuring a time for a sound pulse to travel down the selected direction and return. The sound velocity along the extrusion direction is about 6.28mm/µs; while the sound velocity in a direction normal to the extrusion direction is about 6.1 mm/µs.
The results from the ultrasonic inspection and the determination of the titanium articles and structures, along with microstructure characteristic of the titanium articles and structures are based on UFG billet blocks, which are formed from conventional billet material, as described above. The UFG process produces samples in which the original αTi colony structure in the conventional billet is eliminated. The steps of forging the UFG material at about 900°C and with a corresponding about a 60% height reduction did not re-create αTi colonies or develop strong texture and αTi microstructure.
With reference to Figs. 6 and 7, differences in sound scattering sensitivity and noise are illustrated to be generally dependent on frequency. This dependency suggests that a scattering entity size, such as the size of a colony, in the conventional material increases the contribution to scattering, sound scattering sensitivity, and attenuation from phase scattering. This change in contribution is not a complete shirt from one pure scattering mechanism to the other scattering mechanism, such as a Rayleigh scattering mechanism to a phase scattering mechanism, since such a shift would give a slope of about -2 in Fig. 5.
The αTi particle size is generally not significant in any determining of a signal to noise ratio, since the αTi particle sizes are similar in all materials and are generally smaller in size than the ultrasonic wavelength. A difference in the various materials, in the ultrasonic inspection comprises a presence of large colonies in conventional billets and forgings. Noting this difference, the speed of sound in Ti6242 extrusion samples is about 6mm/µs. This speed typically corresponds to ultrasonic inspection wavelengths of about 1.2mm at about 5 MHz, about 600µm at about 10 MHz, and about 300µm at about 20 MHz. Therefore, the colony dimensions in the conventional billet and forging are comparable to the ultrasonic wavelength.
The relative contributions of Rayleigh scattering and phase scattering are frequency dependent, for example in the ultrasonic frequency range. The frequency dependency is due, at least in part, to the 18.43 MHz wavelength of about 300µm being about the size of a αTi colony thickness. The 6.62 MHz wavelength of about 900µm is about 3 times a αTi colony size. Scattering at 6.62 MHz enters the phase scattering region for its contribution, while scattering at 18.43 MHz provides substantial phase scattering contributions.
The UFG forged material results in a slightly larger grain size than the original billet. However, UFG forged material possesses a lower noise and higher signal, as indicated in Table 2. This behavior may be due to a slightly lower volume fraction of αTi particles in the forged material, which is illustrated in Fig. 1, legends (c) and (d). The conventional forging possesses a lower noise than a conventional billet, however, has a lower signal to noise ratio, which may be due in part to low signals from the flat bottom holes. The conventional forging has a lower volume fraction of αTi particles than the billet. The lower signal in the conventional forging may be caused by attenuation due, at least in part, to sound traveling along highly textured regions. The dimensions of the reflecting entity αTi colonies up to about 1 mm in length and about 300µm in width in the conventional billet and forging may result in a stochastic (phase) component to the resultant scattering. It is also possible that a αTi colony structure above the flat bottom holes scatters the reflection from the flat bottom holes.
The microstructures of UFG billets and forgings made from UFG billets comprise fine-scale granular αTi particles. These αTi particles are generally less than about 5 µm in diameter, and are generally provided with an absence of crystallographic texture. Ultrasonic inspectability, which is characterized by signal to noise ratio from machined flat bottom holes, is greater in the UFG materials than in the conventional materials. There is less ultrasonic backscattered noise in the UFG materials than there is in the conventional materials. Further, the ultrasonic signal from machined flat bottomed holes is higher in the UFG material.
The presence of αTi colony structure is associated with ultrasonic noise generated by ultrasonic inspection of titanium articles and structures, as embodied by the invention. For materials with αTi particles less than about 10 µm in size, differences in αTi particle size typically do not have a significant effect on generated ultrasonic noise. For example, UFG billets, which may be formed by a titanium material production method, as embodied by the invention, can display chiefly Rayleigh scattering, while conventional billets, which can not be characterized by UFG properties, display Rayleigh scattering plus phase scattering. The inspectability of titanium-containing materials is enhanced with predominantly Rayleigh scattering.

Claims

A titanium material production method for producing homogeneous fine grain titanium material in which the titanium material has a grain size in a range from about 5µm to about 20µm, the method comprising the steps of:

providing a titanium material blank;

conducting a first heat treatment on the titanium material blank to heat the titanium material blank to a β-range;

quenching the titanium material blank from the β-region to the α+β-region;

forging the titanium material blank; and

conducting a second heat treatment;

wherein the titanium material production method subjects the titanium material blank to superplasticity conditions during the titanium material production method.
A titanium material production method according to claim 1, the titanium material production method comprising heating the titanium material blank to a temperature in a range from about 600°C to about Tpt, wherein Tpt is a polymorphous transformation temperature for the titanium material.
A titanium material production method according to claim 1, the step of providing a titanium material blank comprises providing a titanium material blank formed by powder metallurgy processes.
A titanium material production method according to claim 1, wherein the step of conducting a first heat treatment on the titanium material blank to heat the titanium material blank to a β-range comprises heating at about 1200°C for about 1 hour.
A titanium material production method according to claim 1, wherein the titanium material production method comprises heating the titanium material blank to a temperature in a range from about 875°C to about 1200°C.
A titanium material production method according to claim 1, wherein the step of forging comprises deforming the titanium material blank in a isothermal press.
A titanium material production method according to claim 1, wherein the titanium material production method produces titanium material with a grain size in a range from about 15µm to about 20µm.
A titanium material production method according to claim 1, wherein the step of forging comprises at least one of: roll-forming, deformation in a press, and forging in a square.
A titanium material production method according to claim 6 or claim 7, wherein the step of forging comprises multiple steps of deforming.
A titanium material production method for producing homogeneous fine grain titanium material in which the titanium material has a grain size in a range from about 15µm to about 20µm, generally equiaxed titanium grains and generally equally sized titanium grains, and substantially even distribution of second phase particles and alloying elements; the method comprising the steps of:

providing a titanium material blank, the titanium material comprising a two-phase titanium material;

conducting a first heat treatment on the titanium material blank to heat the titanium material blank to a β-range;

quenching the titanium material blank from the β-region to the α+β-region;

forging the titanium material blank; and

conducting a second heat treatment on the titanium material blank,

wherein the titanium material production method subjects the titanium material blank to superplasticity conditions during part of the titanium material production method, the titanium material production method comprising heating the titanium material blank to a temperature in a range from about 600°C to about Tpt, wherein Tpt is a polymorphous transformation temperature for the titanium material.