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Improved Properties of Lower-cost Titanium-alloy Materials for Aerospace Fastener Applications

Dr. Steven G. Keener Technical Fellow

Boeing Research & Technology The Boeing Company, Huntington Beach, CA

Copyright © 2007 ITA

ABSTRACT

A potentially lower-cost approach for the production of advanced titanium and titanium-alloy materials has been demonstrated using cryogenic technology. Near-nano and nano-grained materials with high-angle boundaries and finely-dispersed particulates have been produced through severe plastic deformation achieved with high-energy ball-milling and subsequent consolidation techniques. These materials offer the potential for significant improvements in material properties since the Hall-Petch relationship predicts the strengthening of materials by reducing the average crystallite (grain) size. These features combine to impart excellent strength levels, good ductility, and excellent microstructural thermal stability. Furthermore, the starting powders are macroscopically in the micrometer range having pre-alloying capabilities that allow for easy handling, cleaner surfaces, and no environmental dangers.

This paper summarizes the preliminary results of the macrostructures, microstructures, chemistries, and mechanical properties achieved via this cryogenic processing approach. Initial results for commercially-pure titanium material will be presented, that show near-nanometer size nitride particles formed in-situ during processing along with the resulting elevated temperature stability of the alloy. A comparison is made showing the increases achieved in certain mechanical properties of the cryomilled near-nano and nanocrystalline materials over those of conventionally-produced materials. INTRODUCTION

Titanium products are relatively expensive today primarily because the current production methodologies exclusively involve the Kroll process for refining the ore to raw material. As a result, the overall production approach is a multi-step, high-temperature batch process, which is extremely labor- and capital-intensive. Consequently, a lower-cost processing approach, such as the cryomilling methodology discussed herein, will indeed have far reaching implications for the use of titanium and its alloys in the production of certain suitable product forms for the fabrication of aerospace

fastening systems and related components. These improvements are expected to reduce the associated cost and time to produce such high-use components. Specifically, with the cryomilling methodology, the processing time to produce raw material needed to fabricate aerospace fasteners and similar components can be reduced to a few days instead of a week or more that is currently required with the Kroll process⎯a feature which in turn can offer substantial savings in titanium component fabrication costs. Furthermore, there are currently on-going, parallel research and development efforts dedicated to generating processes capable of producing titanium powder material itself that may be suitable for more efficient powder processing⎯a feature, which, in turn, can offer additional cost savings when coupled with the processing methodology described herein that focuses on low-cost titanium raw material fabrication.

The Boeing Company and its suppliers are actively engaged in collaborative efforts involving development and processing technologies associated with the production and fabrication of titanium and titanium-alloy powders, including cryogenic milling, to achieve long-term reductions in raw material costs. Near-term objectives of this project are focused on aircraft fastener applications, where the goal is to replace certain conventional Ti-6Al-4V titanium-alloy fastener components. Further work remains to optimize the process through internal Boeing and government-funded efforts and ready it for commercialization.

In cryogenic milling, nanocrystalline materials are produced by milling conventional feedstock material powder in a cryogenic environment. Cryogenic milling, or cryomilling, a metallic powder in liquid nitrogen provides both grain refinement as well as the potential, beneficial formation of nitrides. Past efforts by Boeing and others with Al-Mg-based-alloy materials have shown an increase in nitrogen content as a function of the milling time.

Near-nano and nano-grained materials exhibit great potential for applications in the aerospace industry. Producing nanocrystalline materials by cryomilling and subsequent consolidation has been successfully reported by several organizations including The Boeing

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Company and others involved in advanced structural materials development.

CURRENT PROCESS

For all intents and purposes, current production of titanium exclusively uses the Kroll process (see Figure 1), which produces a titanium sponge mass that must be further broken down via laborious processing steps to produce quality titanium mill products[1]. The broken-down sponge constituents are then charged along with other scrap metal and certain additives suitable for compaction into electrodes for either multiple-arc melting/re-melting steps in a vacuum, or electron-beam melting processing into ingots of either commercially pure (CP) or alloyed titanium ingots. These ingots are often consolidated through various processes to eliminate voids and are then fabricated into raw material billets suitable for further multiple-step thermo-mechanical processing.

Basically, the current, state-of-the-art titanium extraction process is an ingot-based, labor-intensive and very costly method of thermo-mechanically processing ingot-based preforms. As delineated in the flow chart shown in Figure 1, this methodology represents a multiple-step batch process, which begins with the chemical conversion of titanium dioxide (TiO2) into titanium tetrachloride (TiCl4), which is then reduced with magnesium, to obtain titanium ‘sponge’ in a batch process that is very labor- and capital-intensive. This pyrometallurgical method for the production of titanium sponge was invented by William J. Kroll in 1940 and is still the processing method used, by and large, throughout the industry today.

These steps of the Kroll process have been essentially the same over the years and, apart from retort size enlargements by certain major suppliers, nothing has really changed since this 1948 technology emerged. Any further refinements have dealt with the reduction of interstitial content, e.g. Extra-Low Interstitial (“ELI”) titanium-alloy grades, and optimization of thermo-mechanical processing of ingot billets and preforms into plate, sheet, bar, etc. With each of these additional processing steps, though, the price of titanium per pound continues to climb.

PROPOSED PROCESS

In an attempt to eliminate a certain number of expensive and sometimes repetitive processing steps utilized in the current process, i.e., those steps following the ‘Comminution’ step shown in the process flow diagram of Figure 1, a different approach was undertaken using cryogenic milling and subsequent processing steps, which had been successfully demonstrated during prior research and development projects. In internal Boeing programs conducted from 2002 to 2006, a fundamentally similar, advanced processing technology approach was developed, refined, and successfully demonstrated with aluminum and aluminum-alloy powder materials. Further, the need

for certain additional thermal-treatment steps employed in the current process, which will be discussed in more detail below, may also be eliminated as a result of the beneficial cryogenic-milling processing step based upon previous data.

Figure 1. Flow diagram of ‘Kroll’ process showing steps that innovative cryomilling approach will replace.

Specifically, the Boeing-patented, cryogenic-milling process presented by this paper begins with either commercially-pure (CP) titanium powder or even extended to the use of blended, pre-alloyed titanium-alloy powders obtained from the initial steps of the current process or other related developmental program. Once the titanium powder is obtained, this proposed process takes the powder and produces a lower-cost, robust extruded titanium raw material feed stock for

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subsequent component fabrication. This is accomplished by basically eliminating most of the labor- and capital-intensive processing steps that follow the ‘Comminution’ process step involving the titanium sponge. In addition, in some cases there no longer may be the need for added costs associated with alloying process steps or associated requirements and costs of the alloying materials, which would save in not only material costs but also time associated with elimination of unnecessary alloy conditioning steps.

The introduction of the proposed cryomilling and consolidation technology in producing the extruded feed stock form for subsequent production processing and/or providing near-net shape raw material will enable the end user to bypass or eliminate a number of costly processing steps as illustrated schematically in Figure 1. It is estimated that nearly 50% of the cost of rolled product form is due to reduction of the ingot billet by its rolling reduction to plate form (see Figure 2)[2]. Added to that is approximately 15% of the overall cost that is associated with the vacuum-arc melt/re-melt (VAR) step of the material process, which can be performed several times. Together, the combined costs associated with these process segments represent almost two thirds of the current, overall cost of producing raw titanium plate stock material needed for subsequent component fabrication.

Figure 2. Breakdown summary of cost elements associated with various ingot processing steps in preparing product forms for rolled titanium plate from titanium ingot.

The sharp rise seen from 2004 to 2006 in the cost of titanium ingots per pound has softened and fallen slightly. However, it is anticipated that these costs will increase again in the future as the result of increasing use and resulting demand for titanium raw material and will cause further and continued escalations in the overall cost of titanium procurement, continuing to force major customers, such as Boeing, to move in the direction of reducing their buy-to-fly ratios on all product forms. This, in turn, will require a serious consideration of the more cost-efficient and scrap-reducing thermo-mechanical processing approaches.

With the introduction of this new, Boeing-patented cryomilling and consolidation process technology, the company will potentially be able to realize substantial savings through reduced costs of aerospace fastener components produced from more affordable titanium material, while possessing enhanced corrosion resistance as well as improved compatibility with lightweight composite materials (such as those to be used on the new 787 commercial aircraft program).

PROCESSING CONSIDERATIONS

Titanium usage in aircraft structures has always been on the rise especially in Boeing commercial aircraft structures. New Boeing aircraft programs, e.g. 787, will use more than 20% of their total material weight of titanium and titanium-alloy materials. Currently, titanium is relatively expensive. As noted earlier, this is due to a large extent as a result of the current processing steps used for reducing the naturally-occurring ore to titanium raw material, which are extremely labor- as well as capital-intensive propositions. Furthermore, lead times on some titanium order deliveries are upwards of one-year or more for major domestic U.S. suppliers from the time of ingot order placement to product form delivery of aircraft-usable, ingot-based product forms (sheet, plate, forgings, extrusions, bars, etc.). This is especially true for aerospace fastener and component orders as well.

In the near term, there is expected to be somewhat of a shortage in the supply-versus-demand of titanium sponge produced world-wide for making ingots. In order to support commercial aircraft delivery rates over the next decade or so, The Boeing Company must be proactive and seek ways of accelerating the insertion of powder-based, continuous-extraction processing, direct-consolidated titanium-alloy product forms.

Presently, there are a number of new processes, including powder-based methods, being developed for titanium extraction from refined ore(s), which are actively competing to replace the existing state-of-the-art technology with the goal to significantly-reduce costs in a long-term, continuous-factory, powder production process. The criterion by which these new and developing processes will be evaluated to determine which process becomes the industry standard include, but are not limited to:

1. final product cost/process affordability (i.e., $/lb),

2. process scale-ability to meet high-volume demands for a broad range of applications,

3. conformance of final product to existing chemical, mechanical, and physical property requirements, and,

4. process robustness to control homogeneity and interstitial content, e.g., “ELI.”

As these new powder-based technology processes are developed and refined against all of the requirements listed above, the technical responsibility and associated financial investments for these new titanium product technology development efforts ultimately lies with each of the major customer(s), e.g. Boeing, for addressing and ensuring that their respective requirements are established, monitored, and maintained. Consequently, it is imperative that an aggressive technical plan and strategy be pursued for accelerated insertion of low-cost titanium technologies for near-term and new, long-term commercial aircraft products.

Based upon current industry usage, one of the more widely used and, therefore, higher priority needs in the

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development of titanium processing technology is that of extruded raw material product forms. With a relatively large amount of the total metallic structural weight of an airframe being typically produced from extruded product forms, approaching 25% by some estimates, this project plans to focus on advanced, lower-cost processing for titanium and titanium-alloy material extruded material, exploring the feasibility of leveraging the low-cost titanium technology processing development effort of this project for use in the production of small aerospace fastener components that typically have high part-counts per aircraft, such as threaded pins and lockbolts.

RESULTS AND DISCUSSION

Material Processing. The titanium fabrication process evaluated utilized commercially-pure titanium (CP Ti) powder, e.g., Grades 2 thru 4, provided to the program by Titanium Metals (TIMET) Corporation with an example shown in Figure 3. This powder supply is generated during sponge fine comminution process and subsequently collected as a by-product of the normal titanium ingot production.

Figure 3. Titanium powder sample used in cryomilling process.

Commercially-pure (CP) titanium powder materials were cryogenically-milled using a Szegvari (Union Process, Akron, Ohio) Model No. S1 high-energy, ball-mill attritor similar to the basic unit shown in Figure 4, but modified for liquid nitrogen use. Stainless steel balls are used as milling media at a particular ball-to-charge ratio for a given material. A 30:1 ratio of ball diameter-to-charge size is common for milling metallic powders. The milling media and ball diameter are established by the powder to be milled. Table 1 presents the representative variables that are utilized for a given cryomilling run.

Table 1: Typical Cryogenic-milling Parameters

Element Parametric Value Powder weight (kg) 1 kg Milling time (hrs.) 8 hrs. or greater

Ball-to-weight ratio 30:1 (varies with material) Speed (rpm) 180 rpm or greater

Figure 4: Cryogenic high-energy, ball-mill attritor

Nitrogen Content in Milled Titanium Powder: During the cryomilling, powder samples were periodically taken out of the ball-mill attritor at selected intervals. The powder samples were used to study the evolution of the nitrogen content as a function of milling time.

The nitrogen content was measured and found to be 0.77wt% in the final material. The resulting data are displayed in Figure 5. This data is in reasonably good agreement with the content level that was found in the milled powder at 0.71wt%. Note that this is a very important result as it is not always the case that the nitrogen present in the cryomilled powder remains present after the degassing and/or consolidation steps.

Figure 5. Nitrogen content versus milling time in cryomilled CP Ti powder

During the initial milling run, the level of hydrogen was also measured and was determined to be 360 ppm. This level is relatively high for a titanium-alloy material. For a conventional titanium-alloy material, this high

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amount of hydrogen impurity could lead to a material condition susceptible to hydrogen embrittlement resulting in low to no ductility at all.

Similarly, oxygen content was also determined via infrared detection and found to be 0.47wt%. This again is a somewhat high level of oxygen for a titanium-alloy material.

It should be noted that, as a result of both of the relatively high hydrogen and oxygen contents, adjustments in the processing parameters have been made in subsequent processing of the cryogenically-milled, commercially-pure (CP) titanium, CP Ti, materials to achieve improved, more desirable levels in each of these elements. Continued optimization efforts involving the processing parameters are being conducted.

Overall, the data show typical results seen in other nitride-forming alloys, such as:

The nitrogen content increases with milling time. (Note the data point taken at 6 hours is considered to be an experimental anomaly or artifact);

The increase in nitrogen content is almost linear with milling time;

The rate of nitrogen pick-up was approximately higher than that experienced in the milling of aluminum alloys, ~0.150wt%/hr versus ~0.050wt%/hr, respectively, depending upon the exact composition of the alloy.

Morphology of the Milled Titanium Powders. From SEM photomicrographs taken at the Boeing Metallography Laboratory, the morphology of the powders at various periods in the milling process is presented as a function of milling time (see Figure 6). The images in Figure 6 show the plate-like morphology of the powder developing as early as 1 hour into the cryomilling process. The growth of the plates as a function of milling time is also illustrated after various milling periods.

Grain refinement along with the formation of nitrides occurs in CP titanium powder material similar to that which was demonstrated earlier with aluminum-alloy powder materials. A layered structure is formed from the agglomeration (e.g., micro-welding) of the milled material with nano- and near-nano grains produced in these agglomerated layers. The agglomerated-powder particles are micron size in scale (e.g., 10-100 microns) while the grains are nano- and near-nano in size. Microphotographs of the cryomilled and consolidated CP titanium material are shown below.

Degassing: After being placed in sealed stainless-steel canisters, the milled powders are subjected to a very important degassing process. This degassing step removes contaminants and impurities that can detrimentally alter the outcome of subsequent processing steps. The degas process uses a turbo molecular pump and furnace system. The complete degassing cycle consumes a little over 24 hours when including cooling time.

Consolidation: Once degassed, the milled powders were subsequently processed using hot isostatic

pressing to consolidate the powders into a bulk product. Hot isostatic pressing (HIP) is the simultaneous application of temperature and isostatic pressure. It differs from hot pressing in that the pressure is applied simultaneously and uniformly in all three directions rather than uniaxially. This process also allows simultaneous densification and bonding of powders or porous bodies. The processing parameters for HIP-processed CP titanium are provided in Table 2.

Prior to milling After 1-hr of milling After 2-hrs of milling After 3-hrs of milling After 5-hrs of milling After 6-hrs of milling After 8-hrs of milling

Figure 6. Morphology of CP Ti powder versus milling time Table 2: Typical Hot Isostatic Pressing (HIP) Parameters for CP

Titanium

Element Parametric Values Consolidation temperature (°C) 815 Consolidation pressure (MPa) 69.0

Resulting consolidation was very good and the cans appeared visually to be in excellent condition. However, it is felt that the use of adjusted consolidation parameters in future processing efforts may well further reduce the resulting grain size similar to those levels achieved in the processing of aluminum-alloy materials. The material property results of the cryomilled CP Ti powder, which was hot isostatically pressed (HIP), are provided in Table 3.

Commercially pure (CP) titanium, Grade 4, material has a specified minimum tensile strength value of 550 MPa (80.0 ksi). The average ultimate shear strength of cryomilled CP titanium was found to be 648.1 MPa (94.0 ksi), which exceeds the industry-specified tensile

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strength value. As the shear value of titanium has been referenced as 80% of the tensile strength, it is reasonably fair to expect that the tensile strength of the cryomilled CP Ti material should be considerably higher than that of the conventionally-produced CP Ti, Grade 4, material.

Table 3: HIP Cryomilled CP Titanium

Element Hardness (HVN1000g)

Ultimate Shear Strength (MPa)

Cryomilled CP Ti 393.2 Transverse 404.2 Longitudinal 648.3

Microscopy Observations. The cryomilled, consolidated CP Ti material was cut, polished, and observed in a JEOL 7000 Field Emission Scanning Electron Microscope (FESEM). The microscope is equipped with an EDAX Energy Dispersive Spectroscopy (EDS) system to analyze sample chemistry.

The grain structure can be seen clearly in SEM Backscatter Electron Imaging (BEI). Figures 7 to 11 show a grain size of approximately 1 to 10 micrometers. The grain size is larger than initially anticipated. However, it is important to point out that no impurities and no porosity are seen in the material.

Figure 7: SEM BEI photomicrograph of consolidated cryomilled

CP Ti material, 2,500X, shows the grain structure.

Figure 8: SEM photomicrograph of consolidated cryomilled CP

Ti material, 2,500X, showing well recovered grains about 1 to 10 um in size.

Figure 9. SEM photomicrograph of consolidated cryomilled CP Ti material, 2,500X, shows the some deformed grain areas.

Figure 10: SEM photomicrograph of consolidated cryomilled CP Ti material, 5,000X, showing recovered grains.

Figure 11. SEM photomicrograph of consolidated cryomilled CP Ti material, 7,000X, showing grains as large as several micrometers.

For ductile polycrystalline materials, the empirical Hall-Petch equation[3,4] has been found to express the grain-size dependence of flow stress at any plastic strain out to ductile fracture. This relationship is used to predict the grain size-yield strength relationship for large-grained materials (d >100 nm) and is limited to relatively strain-free materials, predicting that the yield point will increase linearly with the inverse square root of the grain size. In terms of yield stress, the expression for this relationship is shown below. Similar results have been obtained for material hardness, with a similar relationship being also shown below. In both relationships, the Hall-Petch effect is due to dislocation motion/generation in materials that exhibit plastic deformation.

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σo = σi + kd -½ Ho = Hi + kd -½

where σo is yield stress, σi is friction stress opposing dislocation motion, k is a constant, and d is the mean grain size. Most of the mechanical property data on nanocrystalline materials have pertained to hardness, although some tensile test data has become available, with these recent efforts summarizing the material behaviors.[5-8] It is clear that as grain size is reduced through the nanoscale regime (<100 nm), hardness typically increases with decreasing grain size and can be several times (i.e., ~2 to 7) greater for pure nanocrystalline metallic materials (~10 nm grain size) than for large-grained (>1 μm) metallic materials.

When comparing the mechanical properties of the near-nano and nanocrystalline materials with the properties of conventional materials above, the increases shown by the cryomilled powder materials corroborate the Hall-Petch relationship, which predicts the strengthening of materials by reducing the average crystalline (grain) size.

Extrusion. Round specimens were removed from bulk material for subsequent extrusion step. (See Figure 12) The consolidated, cryomilled CP Ti material was cold extruded at high processing rate in a cold forging machine. The material did deform at the relatively low temperature. As-extruded feed stock can be seen in Figure 13. Note that such processing is extremely aggressive on the material since:

There is no external heating applied to the sample;

The deformation rate is extremely fast (>1 s-1); After the extrusion process, the sample is

struck in the reverse direction to remove it from the extrusion die.

Figure 12. Round specimens from cryomilled, consolidated CP Ti powder material.

The extrusion process was very successful and was repeated on several samples. The resulting strength levels of the extrudate were also very high (see Table 4 below). Attempts were made to improve or optimize the lubrication for this particular material. To date, improved results have not been attempted due to time and budgetary constraints.

Figure 13. Optical photograph showing an extruded sample of

cryomilled, consolidated CP Ti material. Table 4. Shear Strength Values

Cryomilled CP Ti Material (Avg)

Current Ti-6Al-4V Fastener Reqmnt

As HIPed As Extruded

As Heat-treated

Ult Shear Strength, ksi 95.0 min 94.0 108.0 109.5

Material Properties. Ultimate Shear Strength Levels. Extruded samples were machined to achieve a representative diameter associated with that used for nominal 3/16”-diameter fastener standards. Once machined to the representative diameter, the specimens were then tested using industry-standard, undriven shear test procedure to determine the shear strength of the consolidated material. The ‘as-HIPed’ material exhibited a shear strength level of 93.0 and 95.0 Ksi on two separate samples. Furthermore, the beneficial affects of cold-working achieved by the extruding process were demonstrated in the resulting ‘as-extruded’ average material shear strength level of 108.0 ksi.

In addition, several extruded material specimens were subjected to subsequent thermal treatment of 925°F for 12 hours. The resulting shear strength level of the ‘as-heat-treated’ material was 109.0 and 110.0 Ksi on the specimen samples. The various values are summarized in Table 4. Based upon these results, it is shown that thermal treatments of cryogenic-milled materials are not required to meet existing fastener performance specification minimums, thus contributing further to the overall cost savings.

To demonstrate repeatability of the process, additional CP Ti Grade 2 powder batches from TIMET were cryomilled and consolidated. The images in Figure 14 show formed 3/16”-diameter CP Ti material specimen ‘blanks’ produced from the additional runs. Undriven

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shear testing of CP Ti powder material specimens produced with the cryomilling process exhibited ultimate shear strength levels in excess of 100 ksi following thermal treatment but tested prior to forming. Again, as shown in prior material batches, the thermal stability of the material was demonstrated by unchanged strength levels after being subjected to subsequent thermal treatment.

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Figure 14. Optical photographs showing formed consolidated, cryomilled CP Ti material specimens.

Forming. As mentioned, several cryomill-processed material specimens were subsequently subjected to different types of rudimentary forming. The forming efforts conducted on these specimens were done rather hurriedly and to serve primarily as initial attempts or trials to ascertain the formability of the material. In-depth and detailed metallurgical evaluations of these formed specimens are currently being conducted. The specimen shown Figure 14b was processed under normal thread-rolling procedures, while the two specimens shown in Figures 14a and 14c were subjected to extremely high strain-rate compressive forces, similar to what a rivet might undergo during the upset process. Macrophotographs in Figure 15 show the cross sections of the thread-rolled (Figure 15a) and extremely deformed (Figure 15b) specimens. The grain microstructure of the threaded area of the thread-rolled specimen can be seen in the SEM microphotograhs contained in Figures 16a, 17, and 18 under increasing levels of magnification.

Comparative Costs. As mentioned earlier, costs associated with the current processing methods used to produce titanium metal, which are essentially batch processes that involve a multiplicity of production steps (see Figure 1) are increasing. The additional processing steps following the ‘Comminution’ step account for a considerable portion of the cost of titanium ingots along with energy requirements. For instance, the cost per pound of one vacuum arc (VA) melting cycle for titanium correlates approximately to the selling price per pound of typical low-carbon steel. The power required to produce one pound of titanium metal ingot is between 2.0–2.5 kilowatt hours [9]. Adding further to the overall costs and energy requirements, the sponge is often compacted with scrap metal and any alloying metals that are necessary to produce a certain alloyed material. In addition, the yield of the wrought product may be as low as 50% of the starting ingot size, adding a further

cumulative step in the high selling price for titanium wrought products.

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Figure 15. Macrophotographs showing cross sections of (a) thread rolled and (b) formed consolidated, cryomilled CP Ti material specimens.

With the cryomilling approach presented in this paper, the need for multiple processing steps, such as VA melting or remelting and other thermal treatment steps, are eliminated. Further, by producing CP Ti that meets the current material performance requirements, alloying materials and associated processing steps are also eliminated. By the elimination of these particular steps, it is projected that the overall costs of obtaining extruded round stock raw material would likely be reduced by approximately 15–25% of the cost to produce similar alloyed titanium raw material.

‘Buy-to-Fly’ Comments. A final consideration in the cost associated with titanium material is the “buy-to-fly” (B/F) ratio. Airframers speak of material efficiency in terms of this "buy-to-fly" ratio—how much metal they have to buy versus how much makes it into the aircraft.

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With conventional forging technologies that produce complex die forgings, by press or hammer techniques, average "buy-to-fly" (B/F) ratios can be reduced to about 7:1. Also, another consideration in this analysis is the subsequent machining processes, which are usually required to reach a component’s desired, final configuration, can be very expensive and add to the final cost of the component, especially in the case of titanium, where forging stock can be very expensive.

This machining of titanium can also generate a great deal of material waste. In the aerospace industry, for example, the typical 'buy-to-fly' ratio for machined titanium (the amount of metal that must be purchased versus the amount of metal actually needed to make the aircraft) is about 15:1. This translates to about 90% of this costly material being scrapped.

In efforts to reduce or at least maintain the current cost of titanium, the scrap from the part/form manufacturing steps is collected, cleaned, and recycled to the sponge and ingot alloying process. Because of current market conditions, the supply and pricing of scrap is therefore a major factor in total availability and pricing of titanium. The aerospace industry is working hard on technologies to reduce their currently high “buy-to-fly” ratios. With less scrap to recycle, though, the price of titanium ingot would thereby increase.

On the other hand, the buy-to-fly ratios for parts made from more appropriately-shaped raw material utilizing more efficient processes might be more on the order of 3:1. With the process presented in this paper, not only can the resulting B/F ratios be relatively low, approaching this order of magnitude or possibly even better as a result of less wasteful use of material when applied to fastener applications, but the lead time for producing the fasteners could be reduced as well by the elimination of certain existing processing steps.

These and other problems combine to considerably increase the lead-time for producing titanium for the military and aerospace industries. These challenges, in turn, indirectly affect the cost per pound of available titanium metal making it even scarcer and more likely to have added market demand-versus-supply price hikes. Even today, it is not unusual for aerospace airframers to face long lead times, in some cases varying from 80 to 102 weeks, upon placing orders until final deliveries.

In terms of B/F ratios, raw wire material produced by such a process as presented in this paper, provides another argument in favor of pushing cryomilled raw material processing as a viable source for titanium, particularly in aerospace fastening system components.

Future Plans. Regarding powder source progress, efforts have been made in collecting other sponge fine specimen types (specifically, four different types that possess varying sizes of mesh fine) from the supplier, TIMET. The specimen types range from fine-grain to coarse-grain sponge fine powder material specimens with varying degrees of contaminants.

An assessment methodology is being outlined that is intended to evaluate the various powder types to identify the optimum powder supply for the cryomilling process. This investigation includes a rather inexpensive cleaning

process that has been suggested by TIMET personnel. One of these fine-grain mesh powder specimen types appear to offer encouraging potential as an inexpensive source of basic raw material for the cryomilling process. Processed specimens from these raw material types along with evaluations of the various processing parameter details and other processing techniques are being undertaken and will be assessed in upcoming efforts. These assessments include detailed metallurgical evaluations of the trial run specimens along with formed parts (similar to the ones below) from additional batch runs.

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Figure 16. Macrophotograph (a) and SEM microphotograph (b),150X, of thread form area of cryomilled, consolidated CP Ti material specimen.

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Figure 17. SEM photomicrograph of thread form area of

cryomilled, consolidated CP Ti material specimen, 1,000X, shows the grain structure.

Figure 18. SEM photomicrograph of thread form area of

consolidated, cryomilled CP Ti material specimen, 2,500X, shows the grain structure.

In addition, some cryomilled and consolidated material will be cold-extruded, as was done previously, for limited evaluations of different coating types and their resulting effect on fastener raw material rod production. Furthermore, continued work on refining and documenting the processing parameter details involving the degassing and consolidation steps (including Spark Plasma Sintering, or SPS, as an alternative consolidation process approach) is on-going as well. Eventually, of course, additional assessments that have been previously identified and planned, such as overall ductility characteristic of the material in order to address such issues as notch sensitivity and fracture toughness among other properties, are expected in upcoming efforts.

SUMMARY

The high extraction cost of titanium metal is but one of the current barriers to broad applications. To make parts, the purified ingots are forged into mill forms, such as bars, rods, wire, plate, and sheet. These are product are formed and machined to final part shapes, expensive processes given the hardness of the metal itself. These processes also generate considerable quantities of resulting scrap, including impurities from tool wear and other related handling. This overall scrap generated from sponge to final product can exceed ten pounds of scrap per every pound of final net product in the aerospace industry.

By incorporating the new processing approach delineated in this study, a number of expensive and time-consuming steps can be eliminated offering a lower cost alternative to aerospace fastener production, with the added advantage of much lower scrap generation. The current downside of the new processing approach is the reliance upon current, relatively high costs of generating powered titanium.

CONCLUSION

Cryogenic milling of CP titanium powder was successfully performed using Boeing-patented processing technology. The average ultimate shear strength value of cryomilled CP titanium material was found to be 648.1 MPa (94.0 ksi). This is greater than the minimum ultimate tensile strength specified for conventional CP Ti, Grade 4, material. Further increases in shear strength values in excess of 105 ksi were achieved through cold-working of the material imparted by subsequent extrusion processing.

Bulk powder consolidation techniques using hot isostatic pressing (HIP) processing provide valuable processing techniques for consolidating near-nano and nanocrystalline powders. The properties of HIP-consolidated forms of near-nano and nano-grained powders are stronger and harder than consolidated forms consisting of conventionally-sized (micron-size) materials.

In addition, the following points can be drawn from the different sections of the report.

Microscopic observations illustrate grain structure approaches nanocrystalline, in agreement with previous work on cryogenically-milled aluminum-alloy material compositions;

No porosity was present in the processed CP Ti material;

Room temperature ultimate shear strength level of the processed CP Ti material demonstrated consistently in excess of 100 ksi

Processed CP Ti material repeatedly illustrated good, high-temperature stability;

Adjustments in processing parameters appear to have direct impacts on the resulting material content compositions and relative grain sizes;

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Cost comparisons indicate potential savings based upon reduced processing including elimination of thermal treatment steps. ACKNOWLEDGMENTS

The author would like to thank California Nanotechnologies, Inc. and TIMET Corporation for their active, collaborative involvement and technical support, which contributed significantly to not only the work presented in this paper, but to expanding the overall body of existing data in the field.

REFERENCES

1. EHK Technologies, 2004. “Summary of Emerging Titanium Cost Reduction Technologies. A Study Performed for US Department of Energy & Oak Ridge National Laboratory.” Subcontract No. 4000023694, January 2004.

2. Peter, W. H., et al., 2007. “Solid State Processing of New Low Cost Titanium Powders Enabling Affordable Automotive Components,“ presented at 13th Diesel Engine-Efficiency & Emissions Research Conference (DEER), 16 August 2007.

3. Hall, E. O., 1951. “The Deformation and Ageing of Mild Steel: III. Discussion of Results,” in Proceedings of Physical Society, Vol. B64, pp. 747-753.

4. Petch, N. J., 1953. “The Cleavage Strength of Polycrystals,” in Journal of Iron and Steel Institute, Vol. 174, pp. 25-28.

5. Fougere, G. E. and Siegel, R. W., 1995. “Mechanical Properties of Nanophase Metals,” in Nanostructured Materials, Vol. 6, pp. 205-216.

6. Siegel, R. W., 1997. "Mechanical Properties of Nanophase Materials," in Materials Science Forum, Vol. 235-238: pp. 851-860.

7. Morris, D. G. and Morris, M. A., 1997. “Hardness, Strength, Ductility, and Toughness of Nanocrystalline Materials,” in Materials Science Forum, Vol. 235-238: pp. 861-872.

8. Averback, R. S. and Weertman, J. R., 1996. “Mechanical Properties,” in Nanomaterials: Synthesis, Properties, and Applications, A. S. Edlestein and R. C. Cammarata, eds., Bristol: Institute of Physics Publishing, pp. 323-345.

9. Marsh, E., 1996. “A Technological and Market Study on the Future Prospects for Titanium to the Year 2000,” European Commission, Joint Research Centre, November, 1996.

Improved Properties of Lower-cost Titanium alloy Materials forTitanium-alloy Materials for

Aerospace Fastener Applications

Dr. Steven G. KeenerTechnical Fellow

Boeing Research & Technology

Overview

Contents

OverviewMaterial processingMaterial fabricationMaterial propertiesPerformanceStatusSummary and conclusions

2

Overview: titanium-alloy usage100

um,

ure

wei

ght

80

60

F-18AAV-8B

F-18E/FC 17A

V-22B-2

Tita

ni%

of s

truc

tu

40

20 C

787

F-22

F-35

MD-11

F-15F-16

B-757B-767 777MD-

F-15F-14F-16

B-757B-767 A380

C-17AB-1B A320

1970 1980 1990 2000 20100

Year of First Flight2020

MD-11

F-1514F-16

B-757B-767

C-B-1B

Business aircraft and rotorcraftMilitary rotorcraftFighter and attack aircraftLarge commercial and military aircraftBomber

3

Overview: current titanium costs

Converting of TiO2 to sponge is expensive, ~$2-5/lb.Reactivity and need for high processing temperatures.Multiple processing steps needed to obtain millMultiple processing steps needed to obtain mill

products, ~$20-50/lb.Limited number of processors, i.e., ATI, Osaka, RTI,

TIMET, Toho, & VSMPO.Aerospace demands have reinforced cyclic market.

4

Overview: current titanium process

Steps replaced by cryomilling/consolidation

process

5

Overview: advantages of cryomilled titaniumOverview: advantages of cryomilled titanium

Projected cost reduction of ~15-25% of cryomilled titanium alloy fasteners versus current Ti 6Al 4V fastenerstitanium-alloy fasteners versus current Ti-6Al-4V fasteners due to lower raw material costs and manufacturing process improvements.

Equivalent strength without the need for alloyingEquivalent strength without the need for alloying.No thermal treatment required to achieve required

strength.Superior microstructure stability allows for potentialSuperior microstructure stability allows for potential

increase in operating temperatures.Reliability improvements associated with improved

material formability and corrosion resistance throughmaterial formability and corrosion resistance through reduced grain size.

6

Produced by ‘cryomilling’ in liquid N

Overview: cryomilling process

Produced by cryomilling in liquid N2.Utilizes pre-alloyed powder tailored for specific

applicationsSevere mechanical deformation techniqueSevere mechanical deformation technique.Can add particulates and break them during

processing.H tit i it id ti l t f d i itHave titanium-nitride particulates formed in-situ.Consolidation via hot isostatic pressing (HIPing)

or Ceracon forging.E t i f i t i lExtrusion of raw wire material.

7

Material processing: overview of stepsMaterial processing: overview of steps

Powders Cryomill ConsolidateHIP or Ceracon Forge

ExtrudeDegas

8

Material processing: cryomilling operation

Very strong vortex created at 180 rpm

Speed differential SEM photomicrograph,

2kx magnification,pbetween balls is small

The periphery sees the highest speeds but things are neatly ordered

after 2 hrs of milling

are neatly orderedThe center shows a more

disordered motion but at low speeds

Some “crushing” in the zone between the arms and the balls

Mostly a “grinding” effectMostly a grinding effect in most places

Neat rows of stainless balls at the periphery

Disordered motion in the center

9

at the periphery center

Material processing: chemistry vs. milling timep g y gTi Powder Chemistry vs. Milling Time

1.40

1.60

t%

0.60

0.80

1.00

1.20

tal C

ompo

sitio

n, w

gt

SEM prior to milling SEM after 8 hrs of milling

0.00

0.20

0.40

0 2 4 6 8 10Milling Time, hrs

Elem

ent

Nitrogen

SEM prior to milling g

SEM after 1 hr of milling SEM after 6 hrs of millingSEM after 1 hr of milling

SEM after 2 hrs of milling SEM after 5 hrs of milling

SEM after 6 hrs of milling

10

SEM after 3 hrs of milling

Material processing: resulting microstructureGrains can be seen in this SEM Backscatter

Electron Imaging (BEI) photomicrograph.

g g

11

Material processing: resulting microstructure

Note: No impurities or porosity are seen in the material.

10 nm 1 nm 0 1 nm1000 μm 10 μm 1 μm 0 1 μm100 μmμm

Typical lattice parameter,~ 0.25 nm

Cryomilled Ti powder size, ~ 40 μm Cryomilled aluminum grain size and

reinforcements size

10 nm 1 nm 0.1 nm1000 μm(1 mm)

10 μm 1 μm(1000 nm)

0.1 μm(100 nm)

100 μm

2 4 6 8 10 120 14ASTM E 112 Grain Size

C t Al Ri t Si 50 75

12

Current Al Rivet Size, ~ 50-75 μm

Material processing: resulting microstructureHigher magnification views

13

Material processing: scale-upCooled

Stationary TankGas Seal

~ 7 ftRotatin

gImpeller

High-energy, attritor-type b ll illi d i d f

Steel Balls

ball-milling device used for mechanical alloying

14

Material processing: inert gas welding

Welding in Argon inert gas is important for safety and material quality. All oxides and adsorbed moisture or process control agent must be driven off before consolidation.

15

Material processing: canning and degassing

16

Material processing: HIPing consolidation

Consolidated Al-Mg-NConsolidated Al-Mg-N and Al-Ti-Ni-N

Up to 30ksi and 1500°C

17

Material processing: consolidated material

18

Material fabrication: extrudingExtruded cryomilled titanium raw material.

19

Material properties: ultimate shear strengthExtruded samples of consolidated cryomilled CP Ti.

Material properties: ultimate shear strength

Ulti t Sh St th V l k iUltimate Shear Strength Values, ksi

CurrentTi-6Al-4V

Fastener, Min

Cryomilled CP Ti, Avg

As HIPed As Extruded As Heat-treated

Ultimate Shear Strength, ksi 95.0 94.0 108.0 109.5

20

Material forming: thread rollingMaterial forming: thread rollingFormed samples of cryomilled, consolidated CP Ti.

21

Material forming: thread rollingMaterial forming: thread rollingThreaded samples of consolidated cryomilled CP Ti.

Cross section ofCross section of thread form area of cryomilled, consolidated CP Ti material specimen shown inshown in macrophotograph (far left), and SEM microphotograph,150X (left).( )

SEM photomicrographs of grain structure in thread form area of

cryomilled consolidatedcryomilled, consolidated CP Ti material

specimen, 1,000X (right) & 2,500X (far right).

22

Concl sions and s mmaConclusions and summarySuccessfully produced titanium bulk material with cryomilling

process resulting in material with improved strength and elevated temperature stabilitytemperature stability

Estimated raw material costs of cryomilling process similar to or less than current cost in bulk quantities.

Estimated lower manufacturing costs, i.e., no heat treatment, g , , ,equates to lower overall fastener costs. (Note: In the past, some raw material costs have additional $20/lb surcharges if allotted supplies are exceeded.)

Projected cost savings at least 15 25% over production ofProjected cost savings at least 15-25% over production of current Ti-6Al-4V fasteners, not to mention reduced delivery times.

This translates into significant cost savings. For example, on the C-17 program, conservative cost savings of ~$80,000/aircraft is achieved considering a $0.20 average cost savings/fastener X 400,000 titanium fasteners/aircraft.

23

Acknowledgements

Messrs. Aditya Agarwal & Max Runyan,Boeing-BR&T, Huntington Beach, CA

Dr. Cliff BamptonUTC, Pratt & Whitney-Rocketdyne, Canoga Park, CA

California Nanotechnologies Corp., Cerritos, CA

TIMET, Henderson, NV

24

Questions & CommentsQ

25