Additive Manufacturing of Ti-6AI-4V Alloy Components for Spacecraft Applications

Suraj Rawal and James Brantley Lockheed Martin Space Systems Company

Denver, CO 80201 USA suraj .p.rawal@lmco.co james.d.brantley@lmco.com

Abstract-In the last two decades, there have been significant

advancements in the field of additive manufacturing of Ti-6AI-4V

alloy and other metallic components for aerospace applications.

Generally, metallic brackets used in spacecraft applications are

machined from the bulk-rolled or extruded products of the

specific alloy. Using Arcam's electron beam melting-based

(EBM) additive manufacturing process, several Ti-6AI-4V alloy

brackets were produced, finish machined, and tested to

determine their mechanical properties compared to the bulk

alloy. This paper describes the use of several EBM-processed

Ti-6AI-4V brackets on the Juno spacecraft.

Initially, the three-dimensional (3-D) CAD model of each of

the bracket drawings was developed to include slightly excess

surface layers for machining to a smooth surface finish. Multiple

parts were built simultaneously in the available chamber size.

Preliminary cost and benefit analysis was also performed to

assess the merit of using additive manufacturing for future space­

craft components. Under the same EBM processing conditions,

multiple Ti-6AI-4V rod specimens were built to prepare the

subscale test specimens for mechanical property measurements.

Results of these tests indicated that the average material

properties were quite comparable to the wrought alloy. Each of

the manufactured parts was inspected using a nondestructive test

method to ensure the quality of the finished product. A few

component-level destructive tests validated that the component

had adequate margin of safety to survive operational load

conditions. Based on these results, several Ti-6AI-4V brackets

were used on the Juno spacecraft which was launched in August

2011. Successful insertion of EBM-processed parts should pave

the way for the use of Additive manufacturing in future

spacecraft components.

Keywords-metal additive manufacturing; electron beam melting; titanium alloy brackets; mechanical properties; spacecraft components.

1. INTRODUCTION

Titanium alloy, Ti-6AI-4V, is an attractive, lightweight material for spacecraft structures, as it provides an excellent combination of high strength, low density, high modulus, low coefficient of thermal expansion, and higher operational temperature than aluminum alloys. While spacecraft structures are mostly constructed from carbon/polymer matrix compo­sites, titanium alloys are used for several brackets, fittings, propulsion tubing lines, and support tubes. Typically, to fabricate the complex-shaped brackets and fittings, a solid billet of titanium alloy is machined down to the final configuration, fully consistent with the drawings. While

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machining such parts, scrap rates can be as high as 80% to 90%. In contrast to the conventional subtractive process, additive manufacturing (AM) technology offers a unique approach to design and build near-net-shape components without the need for tooling and with minimal machining at low scrap rates «10%).

Metal AM, or direct digital manufacturing (DDM), is a layer-by-Iayer technique of producing 3-D parts directly from its computer-aid-design (3-D) models. At the outset, it offers the designers a unique tool to envision innovative and integrated designs, eliminating the iterative cycle of generating several versions of the drawings. Specific details of several metal-based AM are well documented in the literature [1-7]. The AM technology has matured significantly from its development years in the late 1990s, now as a viable and affordable process to manufacture small- to large-scale complex parts for aerospace, medical, and automotive industries.

Fig. 1 shows that metal AM processes include two key inputs: Type of raw material and the energy source to form the part. For example, the raw material could be in the powder or wire form, and energy sources are generally either a laser or electron beam. Metal AM systems can be grouped into three broad categories: 1) powder bed, 2) laser powder injection, and 3) free-form fabrication. Tn a typical powder bed system, the laser or electron beam energy is directed on the powder bed (held in a vacuum or inert environment) to melt the powder to form the desired shape consistent with the 3-D CAD model. Whereas in laser powder injection system, the powder is fed through a nozzle and laser beam melts the powder. The free­form fabrication AM processes include e-beam deposition of metal wire, ultrasonic consolidation of metal layers, and arc deposition of powder and wire.

Powder Bed Systems • Laser Beam • Electron Beam

laser Powder Injection • Powder nozzle, laser beam

Fig. 1. Metal Additive Manufacturing Processes.

Free Form Fabrication • Electron beam Deposition ofWre, • Arc Dep of powder and wire • Ultrasonic Consolida�on of metal layers

In recent years, the American Society of Test Methods (ASTM) has taken the initiative to standardize AM terminol­ogy and to develop industry standards. According to the ASTM F-42 committee, the first standard, ASTM F2792-10, defines AM as, "The process of joining materials to make objects from 3-D model data, usually layer upon layer, as opposed to subtractive manufacturing technologies." Both the laser and EB-based metal AM technologies offer a few unique attributes such as ability to produce parts that are otherwise un­manufacturable, ability to produce multiple small parts in a single setup, high speed of manufacturing with minimal oversight, and low cost.

Several investigators [8-12] are conducting trade studies to assess the build rate, surface quality, overall deposition accuracy, level of impurities, post-processing treatment such as hot isostatic processing, heat treatment, extent of machining, and cost, compared to conventional subtractive processes. In this development project, several Ti-6Al-4V parts were manu­factured using Arcam-based electron beam melting (EBM) process [8]. More specifically, these parts were manufactured using Arcam Model EBM SI2 machine at the Boeing Aero­space, St. Louis, MO, facility under a separate collaborative project sponsored by the Air Force Research Laboratory, OR.

Without listing any specifics of the processing parameters, this paper discusses the specific details of evaluation and tests of as-processed Ti-6AI-4V parts, leading to their subsequent insertion on to the Juno spacecraft under an independent research and development (IR&D) effort. Successful insertion of the EBM-processed components on a spacecraft has been a pathfinder to assess the affordable processing of multiple complex parts and broader applicability of AM technology for future spacecraft structures.

II. EBM-PROCESSED TI-6AL-4V COMPONENTS

A. Component Selectionfor EBM Processing

Typical composite structure of the spacecraft bus includes several aluminum and titanium alloy-based brackets used for primary and secondary load bearing applications. Overall, quantities of the Ti-6AI-4V components with similar geometry are significantly lower than other aerospace applications. However, a few representative components were selected to assess the feasibility of fabricating thin-walled brackets and fittings. The Ti-6AI-4V powder used in the EBM process was 1001+325mesh (-1501+45 /lffi) per AMS4998 specification Table I lists the different components processed and evaluated in the project.

Fig. 2 shows the most of the as-processed and machined EBM Ti-6Al-4V components. Fig. 2b includes the isometric views and the Juno part number for each of the brackets. Alongside a few of the brackets, a few of the tensile bars were also processed, both in the X-Y and Z orientation. Each of these as-deposited brackets had about 2-mm build up on all surfaces to allow for finish machining, consistent with the tolerances indicated in the drawings of the bulk-machined units. Driven by the severe mass constraints, most of the brackets are designed to minimum thickness to satisty the worst-case load conditions. Fig. 3 shows two of the brackets in

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TABLE I. EBM-PROCESSED TI-6AL-4V COMPONENTS

Ti-6AI-4V Component Quantity Comments

Strut End Fitting 8 Primary Structure

Wave Guide (WG) Brackets-l 2 WG Bracket-2 5

Secondary WG Bracket -3 6 WG Bracket-4 7

Structure

Dog House Bracket 2 XY Tensile Bars 12 (Tests and

Z Tensile Bars 12 Microstructure)

WG Brackel-1

aj EBM-processed Ti-6AI-4V brackets and tensile bars

JUNA0624127·SGt I I JUN.A0624127001S I I JUNA0624127.sosl I JUN.A0624127.S061

���� --_. ----. --_.

WG Bracket-2 WG Bracket-3

b) Isometric view of the brackets with Juno part number

Fig. 2. EBM-processed components for Juno spacecraft.

aj Wave Guide Bracket JUNA0624124-505

bj Wave Guide Bracket-4: JUNA0624124-506

WG Bracket-4

Fig. 3. Two of the wave guide brackets before and after machining.

the as-deposited and fully-machined conditions. As-deposited components exhibited surface fmish (Ra) of �0.05 mm. A few exploratory experiments were conducted to evaluate the use of electropolishing to obtain an acceptable smooth surface fmish with very limited success.

B. Material Properties of EBM Ti-6AI-4 V

Using the XY and Z Tensile bars (�19 mm dia x 69 mm long), a few sub scale specimens per ASTM E8 standard were prepared to determine the yield strength, elastic modulus, ultimate strength, and percentage elongation, both at room temperature (RT) and at 121°C (250°F). Table IT lists the measured properties at RT, and Table III lists the tensile test data at 121°C, and Table TV gives the mean tensile properties of XY and Z-orientation-processed EBM bars and compares the measured properties with the typical properties of wrought (annealed), and cast and annealed Ti-6Al-4V alloy.

Results of these tests indicated the following:

• Z-orientation-processed tensile specimens do exhibit slightly higher tensile strength than the XV-orientation specimens

• Measured properties of EBM-processed Ti-6Al-4V at 121°C indicate � 10% reduction in strength, compared to RT.

• Tn Fig. 4, stress-strain plots show that EBM-processed specimens exhibited about�14% elongation.

• Each set of tensile property data exhibited quite low coefficient of variation suggesting uniform quality of as-deposited specimens. These results are quite consistent with the data available in the literature from similar tests [8-12].

• As-fractured surfaces (Fig. 5) seem to indicate a localized region of incomplete melt or impurity in two of the tested specimens (XY -1 and Z-I). Presence of localized incomplete melt can be attributed to insufficient EB energy to create a consistent melt pool in the thick powder-bed layer (> 1 00-150 microns). Prior to processing the parts, a few initial trials were done to optimize the layer thickness (thinner the better), flatness, and number of passes; so as to minimize any incidence of 'incomplete melt'.

TABLE II. TENSILE PROPERTIES OF EBM TI-6AL-4V XY BARS AT RT

Specimen ID 0.2% YS E UTS (MPa) (GPa) (MPa)

XY-l 978.4 121.27 1039.7

XY-2 966,7 116.44 1030.1

XY-3 963.9 112.31 1027.3

XY-4 9S0.1 112.31 1014.2

XY-S 972,2 116.44 1032.1

XY-6 971,S 116.44 1029.4

XY-7 967.4 114.37 1041,1

Mean 967.4 114.37 1030.7

Std. Dev. 8.9 2.82 8.9

Coef. Of Var (%) 0.9 2.4 0.9

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TABLE III. TENSILE PROPERTIES OF EBM TI-6AL-4V XY BARS AT 121°C

Specimen ID 0.2% YS E tiTS (MPa) (GPa) (MPa)

XY-8-2S0 814.4 113.7 890.2

XY-9-2S0 81S.1 114.4 897.1

XY- IO-2S0 807.S 109.6 893.6

Mean 812.3 112.3 893.6

Std. Dev. 4.13 2.S 3.S1

Coef. Of Var (%) O.S 2.2 0.4

TABLE IV. COMPARISON OF MEAN TENSILE PROPERTIES OF EBM TJ-6AL-4V AND WROUGHT AND CAST PRODUCTS

Specimen 0.2% YS E lJTS

(MPa) (GPa) (MPa)

MeanXY-@ RT 967.4 114.4 1030.7

Mean Z@ RT 986.7 119.2 1049.4

Mean XY @121°C 812.3 112.3 893.6

Mean Z @ 121°C 84S.4 109.6 923.3

Ti-6AI-4V Wrought @RT 903 110 923

(annealed) Ti-6AI-4V (Cast and

88S 110 930 Annealed)

'4" v v v ---I { { (I ' I I �� �

I 10(1

40

:0

" , , o � .. I! I. I.

-� :\.,;-� I

a) Stress-strain curves for XY-orientationspecimens.

I 1, If V

Ii Ii I: Ii 1\ I'.

rff1 l ;r�� I I \ � . �' 1." � . ' " I� I.� '! . " -� j i I j �. ' ; I Ii Ii _ :;: .iii ... t

b) Fractured XY-orientation specimens.

IB

Fig. 4. Stress vs. strain curves for tensile tests perfonned on EBM-processed Ti-6AI-4V in the XY-orientation specimens at Room Temperature.

'"

Fig. 5. Fracture surface of XY -\ tensile test specimen indicating a localized region of apparent incomplete melt.

C. Microstructure

Fig. 6 shows the representative photomicrographs from the examination of several specimens derived from different tensile bars and the components. These photomicrographs show the fine grain structure with lamellar a (white) phase in the � matrix (dark region). This microstructural morphology is consistent with such typical Widmanstatten structure observed in Ti-6AI-4V alloys. Presence of such a fme-grain micro­structure is attributed to the rapid cooling of the large, single­melt pool (�100 /lffi). A few regions in a few of the photomicrographs apparently reveal a fine boundary at the adjacent melt pools.

Fig. 7 shows the fractographs of test specimens indicating mostly dimples on the fracture surface, representative of ductile failure.

a) Photomicrograph of an EBM Ti-6Al-4V component.

b) Photomicrograph of EBM Ti-6Al-4V tensile bar.

Fig. 6. Photomicrographs of EBM Ti-6AI-4V alloy showing fine-grain structure with lamellar IT (white) phase and dark region of P matrix between them.

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Fig. 7. Photomicrographs of the fractured surfaces of EBM Ti-6AI-4V specimens showing primarily ductile failure.

Ill. MECHANICAL TESTS OF COMPONENTS

Each of the as-processed components was fully machined to the final dimensions, so as to generate confidence in viability of using metal AM for the spacecraft structure. Fig. 8 shows the different steps involved in the EBM-processed Ti-6AI-4V strut end fittings. Using the 3-D CAD models of strut end fittings, it was established that eight fitting could be processed simultaneously in the EBM chamber. Each component had about 2 mm of extra stock built up, and subsequently, the as­processed component was fully machined within the tolerances defined in the drawing.

Component-level mechanical tests were performed to detennine that machined EBM-processed parts do satisfY the operational loading conditions. This section describes the mechanical testing of machined EBM parts such as a pair of strut end fittings and a wave guide bracket.

A. Component-Level Tests: Strut End Fittings

A pair of end fittings was bonded onto a composite strut. Compression tests were performed to compare the perfonnance of composite struts with EBM Ti-6AI-4V end fittings and struts with the state-of-the-art machined T-6AI-4V parts. Both the strut! end fitting assemblies exhibited nearly similar mechanical (load vs. deflection, Fig. 9) response. Each of the assemblies, including the one with EBM-processed end fittings, exhibited failure loads 25% above the proof-load value. Results of tube compression tests indicated that the EBM-processedlmachined end fittings (specimen #502) exhibited failure load (�16,600 Ibf) equivalent to the SOA-machined (from wrought) fittings. The overall testing of these components was very successful. These results have given the spacecraft designers enough confidence to use the EBM-processed fittings and other brackets that can be manufactured using the AM process. Typical end fittings are a critical part of the primary structural strut members. A significant number of end fittings shall have to be processed and tested with reliable non-destructive inspection method to build a level of confidence for use on the aerospace structures.

B. Component-Level Tests: Wave Guide Brackets

Fig. 10 shows the schematic of the setup for static and fatigue testing of the wave guide brackets (labeled as JUNA0624127-506 Brackets).

FWsh Machl'llng Mac:hnId Inside surface

Fig. 8. EBM process to machining steps for Ti-6AI-4V components.

a) Compression test setup of composite strutlEBM Ti-6AI-4V end fitting assembly.

18000,----,-----,--------:-----,-----,----,-----,-----,---,

'-" 12000 . -g .Q 10000 . Q)

.� � IJ} 8000 · Q)

6. E 6000 · 8

4000 .

2000 .

O������-1�-�-������� -0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

Compressive extension (in) b) Compressive load vs. displacement plot of composite struts.

Fig. 9. Compression testing of composite specimen (#502) with EBM­processed Ti-6AI-4V end fittings, with composite struts with SOA Ti-6AI-4V fittings, showing nearly-identical mechanical behavior.

Fig. 10. Test setup for static and fatigue testing on EBM-processed/machined Ti-6AI-4V JUNA0624127-506 brackets.

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Two JUNA0624127-506 brackets were instrumented with strain gages. They both were loaded with the same fixture configuration (Fig. 11). The first specimen (discussed here) was loaded statically to determine an appropriate displacement range for fatigue testing. Results of the static test are shown in Fig. 12. It was decided that the maximum strain range be 8,000 micro-strain (i.e., ± 4000 micro-strain centered on zero) because this was expected to produce failure in a reasonable number of cycles. Based on Fig.12, the test machine was set to cycle between �.96 to 1.52 mm (-0.038 and 0.060 inches) in order to produce a range of 8,000 micro-strain. Fig. 13 shows the minimum and maximum applied forces for the duration of the fatigue test. The first leg failed at approximately 120,000 cycles, as indicated by the first set of sharp changes in forces.

Actuating hal of f ixlure

Cross· bar

Load Cell below (F1�edJ

Fig. ll. Test setup image for testing EBM-prototype Ti-6AI-4V JUNA0624127-506 brackets. The crossbar was fixed from rotating.

I D !

-ID

-lIl

L." � �I�

" � � � �� � � �

�

\\ l \ " .��. � \, .� � ... � '" � ""

--'ODD ....aJO ...-.0 -:!DOO -WOO 0 1000 !ooo JOI)Io 04000 »1]10 ---SGI SG2 SG3 = 005 8061

Fig. 12. Force vs. strain plots for static testing on EBM-prototype Ti-6AI-4V JUNA0624127-506 bracket #1. The strain range of 8,000 micro-strain (±4000 micro-strain centered on zero) was achieved on strain gages 3 and 4 by displacing between -0.038 and 0.060 inches. The non-zero starting positions of each strain gage are due to the preload applied when tightening down all the fasteners in the test setup.

.. f',., ..

/ 1 ..

/ tD�"'''d lalls kg

/ � Firsll�g lails � V

'\ ,--r-

At'. crk 0""1' "gi"" ...

Fig. 13. Minimum and maximum applied force during constant displacement range fatigue test on EBM-prototype Ti-6AI-4V JUNA0624127-506 bracket #1. Failures in the legs are indicated by sharp drops in force. Additionally, the steady change in minimum applied force indicated a visible crack had formed and was growing.

The second leg failed after approximately 174,000 cycles, as indicated by the second set of sharp drops in forces. Additionally, it was observed that a visible crack was present prior to failure, and the crack growth was indicated by a gradual, constant decrease in forces. Fig. 14 shows the photograph of fatigue failure in both the legs of wave guide bracket JUNA0624127-506-1.

C. Preliminary Assessment of Benefits

Results of material property tests and mechanical testing of EBM Ti-6Al-4V components gave confidence in the quality and robustness of parts produced by additive manufacturing. Several designers labeled these EBM-processed components as "sintered titanium alloy," while recognizing the differences between powder metallurgy-sintering and electron beam melting, and rapid quenching of a melt pool.

Prior to inserting EBM-processed components into the spacecraft structure, a thorough trade study shall be performed to determine the quantifiable benefits. Even processing a small batch of components in this project, the following benefits were recognized:

• Multiple complex-shaped (small to moderate size) components could be EBM processed to a near net shape in a very short time, saving several days in the schedule, thus significantly reducing lead time.

Fig. 14. Failure images for fatigue testing on EBM-prototype Ti-6AI-4V JUNA0624127-506 bracket # I. The image on the left shows the first leg failed while the second leg is still intact. The image on the right shows the bracket out of the test setup after both legs failed.

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• Eliminate or mlmmze the need of hard copy of drawings .

• Average cost of EBM processed/machined was about half of the SOA wrought and machined parts. The cost advantages could be maximized, if the number of "similar geometry part" was» 12 or so.

• EBM processing offered a unique manufacturing technique to produce integrated designs and/or complex shapes.

• Part quality was nearly equivalent to wrought and machined part, as compared to a cast part.

IV. COMPONENT INSERTION ON JUNO SPACECRFAT

Results of material property tests, mechanical testing, and the quality control documentation of each EBM processing run gave the designers the confidence to insert the technology for the secondary support structure applications. Consequently, four sets of wave guide brackets, as shown in Fig. 15, were selected for use on the Juno spacecraft structure. Fig. 16 shows the machined EBM Ti-6Al-4V wave guide brackets installed on the Juno spacecraft. These brackets successfully endured the system-level tests, including vibration and thermal cycling.

JUNA0624127·506

JUNA0624127·505 (HGAWG Brick •• As,y)

JUNA0624127·504

Fig. 15. Schematic showing the location of four sets of wave guide brackets on J uno spacecraft.

Fig. 16. A view showing a few of the installed wave guide brackets on Juno spacecraft.

V. CONCLUDING REMARKS

Initially, electron beam melting manufacturing process was used to produce Ti-6Al-4V cylindrical bars to evaluate the microstructure and mechanical properties. Results of these tests indicated that the properties of the EBM-processed specimens were nearly equivalent to the wrought and machined Ti-6AI-4V specimens. Subsequently, using the 3-D CAD files of a few Ti-6Al-4V components, such as strut end fittings and wave guide brackets (of four different configurations), were manufactured. As-processed components were near net shape with about �2-mm excess deposit than the final machined thickness. Outer surface fmish of as-processed components was somewhat rough for our first usage, therefore, each component was finish machined to its fmal dimensions.

Component-level mechanical performance tests, such as compression tests (of end fittings), and static and fatigue testing of different wave guide brackets, were performed. Results of the mechanical tests verified that the performance was satisfactory and equivalent to the performance of conventional bulk-machined Ti-6Al-4V component. A preliminary assessment of EBM-type additive manufacturing benefits suggested that multiple complex-shaped parts were processed at reduced lead time and cost with nearly equivalent properties. Metal EBM is still a new technology for spacecraft components; therefore, all due precautions were taken in assessing the quality, material properties, structural perform­ance, and potential risk. Based on the results of this development effort, spacecraft designers provided an opportunity to install EBMlmachined wave guide brackets on the Juno spacecraft which was launched successfully on August 5, 2011.

ACKNOWLEDGMENT

The EBM-processed components reported in this paper were manufactured and machined at Boeing-Phantom Works, St. Louis, MO, in an AFRL-sponsored collaborative project, "Metals Affordability Initiative-EB Free Form Fabrication," under an Agreement No. FA8650-07-2-5206. Authors thank Mary Kinsella of USAF WPAFB, OH, and Drs. Kevin Slattery, Blake Slaughter, and Eric Stern of Boeing-Phantom Works, st. Louis, MO, and Charles Pokross of Materion Inc., OH, for the technical support.

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The work reported in this paper was performed in a concurrent Lockheed Martin Space System Company's Independent Research and Development Project, D-90D, under the task entitled, "Innovative Materials and Structures." Authors express sincere thanks to colleagues Dr. David Chellman, Henry Phelps, Elliot Goldman, Sean Ley, and Geoff Niggeler who were extremely helpful in plarming, testing, and insertion of EBM-processed components on Juno spacecraft.

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