July 2014 

NASA/TM-2014-218508 

Electron Beam Freeform Fabrication of 

Titanium Alloy Gradient Structures 

Craig A. Brice, John A. Newman, and R. Keith Bird 

Langley Research Center, Hampton, Virginia 

Ravi N. Shenoy 

Northrop Grumman, Hampton, Virginia 

James M. Baughman 

Analytical Mechanics Associates, Hampton, Virginia 

Vipul K. Gupta 

National Institute of Aerospace, Hampton, Virginia 
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 July 2014 

NASA/TM-2014-218508 

Electron Beam Freeform Fabrication of 

Titanium Alloy Gradient Structures 

Craig A. Brice, John A. Newman, and R. Keith Bird 

Langley Research Center, Hampton, Virginia 

Ravi N. Shenoy 

Northrop Grumman, Hampton, Virginia 

James M. Baughman 

Analytical Mechanics Associates, Hampton, Virginia 

Vipul K. Gupta 

National Institute of Aerospace, Hampton, Virginia 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 
1 

Abstract 

Historically, the structural optimization of aerospace components has been 

done through geometric methods. A monolithic material is chosen based on the 

best compromise between the competing design limiting criteria. Then the 

structure is geometrically optimized to give the best overall performance using 

the single material chosen. Functionally graded materials offer the potential to 

further improve structural efficiency by allowing the material composition and/or 

microstructural features to spatially vary within a single structure. Thus, local 

properties could be tailored to the local design limiting criteria. Additive 

manufacturing techniques enable the fabrication of such graded materials and 

structures. This paper presents the results of a graded material study using two 

titanium alloys processed using electron beam freeform fabrication, an additive 

manufacturing process. The results show that the two alloys uniformly mix at 

various ratios and the resultant static tensile properties of the mixed alloys 

behave according to rule-of-mixtures. Additionally, the crack growth behavior 

across an abrupt change from one alloy to the other shows no discontinuity and 

the crack smoothly transitions from one crack growth regime into another. 

Introduction

Electron beam freeform fabrication (EBF3) is an emerging additive manufacturing process 

that utilizes an electron beam heat source with alloy wire feedstock to create a fully dense, 

three-dimensional metal structure directly from a computer model [1]. A schematic of the 

process is shown in Figure 1. The computer model of the part to be fabricated is first sliced into 

discrete layers and rows and a tool path is generated that can recreate the part from these 

individual layers and rows. The electron beam is then used to form a molten pool on a 

substrate plate into which a feedstock wire is fed, thus creating a bead of deposited material. 

By following the tool path of the deconstructed 3-D object, the EBF3

 machine can deposit 

sequential beads and layers in order to create the part in a freeform manner. 

A particular advantage of the EBF3

 process is the ability to create functional gradient 

structures through the use of multiple feedstock wires. Thus, within a single structure the 

chemical composition can be locally changed. This could be advantageous for optimizing 

properties within a given structure based on the local design limiting criteria [2]. For example, 

areas of high toughness could be smoothly transitioned into areas of high strength with the 

transition determined by the loading conditions within the part. This could greatly increase the 

structural efficiency of complex loaded aerospace components. 

Two titanium alloys were chosen for demonstration of this gradient approach: commerciallypure (CP) grade 2 titanium and titanium with 8% aluminum and 1% erbium (by weight; wt%). 

Grade 2 titanium is a commonly-used CP variant characterized by low yield strength (275-410 

MPa), but with excellent weldability and corrosion resistance. For titanium alloys, aluminum is 

used as a solid-solution strengthener up to about 8 wt%. For aluminum content greater than 8 

wt% strength begins to suffer as an intermetallic compound is formed. The addition of erbium 

forms an dispersion of oxide nano-particles that also acts as a strengthener. Erbium is not 

typically alloyed with titanium because the formation of strengthening nano-dispersoids (Er2O3) 
2 

requires some level of rapid solidification, which standard ingot production does not offer. 

Alloying titanium with erbium is well suited for EBF3 processing as it involves high-cooling rates 

and rapid solidification. 

These materials (CP titanium and Ti-8Al-1Er) were chosen for a number of reasons. First, 

they are chemically compatible in that there are no brittle intermetallic phases expected over the 

full range of possible mixtures and processing temperatures. Second, they have vastly 

different static mechanical properties. Finally, the mixtures are easy to quantify by measuring 

aluminum concentration (in solid solution) and also by measuring particle density of Er2O3

dispersoids. This will give an indication of the diffusion-based mixing effects (aluminum in solid 

solution) and convection-based mixing effects (Er2O3 in liquid titanium). 

Experimental Results and Discussion 

Sample Preparation 

Electron beam freeform fabrication structures were produced using titanium and titanium 

alloy wire feedstock deposited on grade 2 commercially-pure (CP) titanium plate substrates. 

The deposited material was created using grade 2 CP-Ti wire, Ti-8Al-1Er wire, or a combination 

of the two wires. Constant ratio mixtures were created using fixed feed rates for each of the two 

wires. A graded composition deposit was created by changing feedstock wire halfway through 

each layer (i.e., five rows of CP-Ti followed by five rows of Ti-8Al-1Er). As there is about 33% 

overlap between successive rows, there is some degree of mixing between the two alloys as the 

wire feedstock is changed. This creates a gradient in composition near the interface between 

the deposited materials. Mechanical testing was conducted on EBF3

 titanium monolithic and 

graded materials, including tensile testing, micro-hardness testing, and fatigue crack growth 

testing. 

Tensile Testing 

A total of 10 single wall deposits were made for tensile testing. Each deposited wall was 

approximately 250 mm long and 14 mm high. A 973 K, 2 hour vacuum anneal heat treatment 

was performed on all the deposited walls prior to machining. From each wall, two flat dog-bone 

tensile test specimens were extracted using wire electro-discharge machining (EDM). The 

specimens had gage section dimensions of 32 mm long by 6.4 mm wide by 3.2 mm thick. Five 

different compositions were tested: 100% of each starting wire composition and mixed 

volumetric ratios of 25:75, 50:50, and 75:25. Four specimens were fabricated with each 

composition. Tensile testing was performed according to the ASTM Standard E8 [3]. 

The mechanical properties from tensile testing of the EBF3

 products are shown in Figures 2 

and 3 as a function of composition. The results shown in Figure 2 indicate a slight increase in 

elastic modulus (12%) as the composition varies from CP-Ti to Ti-8Al-1Er. This composition 

change also results in a reduction of elongation at failure (ductility) from approximately 50% to 

10%. The data in Figure 3 show an increase in yield stress from approximately 240 MPa to 740 

MPa due to the addition of the alloying elements (Al and Er). The strength increase can mainly 

be attributed to solid solution effects for aluminum and dispersoid effects for erbium. The trend 

is linear with respect to composition implying a rule-of-mixtures effect. The variation in ultimate 
3 

tensile strength is similar in that it increases nearly linearly with increasing alloying elements, 

and increased from approximately 340 MPa (CP-Ti) to 810 MPa (Ti-8Al-1Er). 

Microhardness Testing 

A single layer deposited sample containing five rows of 100% CP-Ti with a step change to 

five rows of 100% Ti-8Al-1Er was used to evaluate the interface transition. A cross-section cut 

was made in this deposit and micro-hardness measurements (Vickers indenter with 300g force) 

were made at constant intervals across the entire deposit. The micro-hardness data are plotted 

in Figure 4. The hardness values in the CP-Ti deposit are approximately 140 VHN300gf. A 

sudden increase in micro-hardness occurs at the interface, increasing from approximately 140 

VHN300gf to 240 VHN300gf over a distance of about 1 mm. In the Ti-8Al-1Er deposit, the hardness 

increased slightly with distance from the interface (increasing from about 240 VHN300gf to 290 

VHN300gf over a distance of about 8mm). This is attributable to the diminishing effects of dilution 

that occurs as successive rows are deposited further from the interface. 

Fatigue Crack Growth Testing 

Deposited structures of approximately 150 mm by 63 mm dimensions, as shown in Figure 5, 

were fabricated for fatigue crack growth testing. Within each structure, the deposited rows were 

approximately 6 mm wide and were oriented parallel to the loading axis of the crack growth 

specimens (see Figure 6, right side). The structures contained a total of 20 rows in each layer 

with five total layers per sample. A 973 K, 2 hour vacuum anneal heat treatment was performed 

on all samples prior to machining. Eccentrically-loaded single-edge notch tension (ESE(T)) 

fatigue crack growth specimens were machined from the EBF3

 product forms. A schematic of 

the ESE(T) specimen configuration is shown in Figure 6 (left side) [4]. As seen in the 

photograph in Figure 6 (right side), the loading direction is parallel to the direction EBF3

 material 

was deposited and the crack is expected to grow across (perpendicular to) the EBF3

 deposited 

rows. 

Fatigue crack growth testing was performed in accordance with the ASTM Standard E647 

[5]. During testing, crack length was monitored using a crack-mouth-opening displacement 

(CMOD) gauge and applied loads were continuously changed such that pre-determined values 

of the crack-tip stress intensity range (ΔK) were achieved [6]. Testing was conducted in room 

temperature laboratory air at a cyclic loading rate of 5 Hz and an R value of 0.1. Constant-ΔK 

testing was performed in this study to look for subtle changes in crack growth behavior as the 

crack propagated through the EBF3

 deposited material [7, 8]. Maintaining constant ΔK is useful 

should the crack growth resistance vary as crack propagation occurs across boundaries 

between the EBF3

 deposited rows. 

Constant-ΔK test data is best plotted as crack length (a) versus cycle count (N). This data is 

presented in Figure 7 for a monolithic CP titanium specimen and a monolithic Ti-8Al-1Er 

specimen. Note that the slope of these data is the fatigue crack growth rate (da/dN). A fitted 

line through these data is used to determine the crack growth rate for ΔK = 7.9 MPa√m (R = 

0.1), for CP-Ti (blue triangular symbols) and for Ti-8Al-1Er (red square symbols). Testing was 

conducted at room temperature in laboratory air at a cyclic loading rate of 5 Hz. From these 
4 

data, it can be seen that for a cyclic crack-tip stress intensity factor range of ΔK = 7.9 MPa√m 

that the CP-Ti material exhibits crack growth rates approximately twice that of the Ti-8Al-1Er 

alloy (da/dN = 3.55 x 10-9 m/cycle compared with da/dN = 1.75 x 10-9 m/cycle, respectively). 

Further, the crack growth path did not exhibit any deviation that would have been expected if the 

boundaries between EBF3

 deposition passes were relatively weak in fatigue crack growth 

resistance. This observation suggests that the material produced is rather homogenous in 

terms of fatigue crack growth performance. 

The fatigue crack growth data for the graded specimen are plotted in Figure 8. This 

specimen was tested at a constant ΔK value of 9.9 MPa√m. Steady state crack growth data in 

the Ti-8Al-1Er material (da/dN = 4.15 x 10-7 m/cycle), indicated as red square symbols, was 

shown to occur between crack lengths of approximately 6 mm to 12 mm. Around a crack length 

of 12 mm, the crack began to propagate into the transition region between the two materials; 

these transient data are indicated as open circular symbols. Upon propagating into the CP-Ti 

region, steady state crack growth was re-established (blue triangular symbols; da/dN = 8.65 x 

10-7 m/cycle) over a range in crack length between approximately 14 mm to 20 mm. Note that 

the earlier observation that the crack growth rates in CP-Ti were nearly a factor of 2 greater than 

for the Ti-8Al-1Er alloy (recall Figure 6; ΔK = 7.9 MPa√m) also is true at higher ΔK (= 9.9 

MPa√m). 

Photographs of the crack profiles are shown in Figure 9. Fatigue cracks can be seen 

originating at the crack starter notches on the left side of each photograph. The crack profile of 

the CP-Ti (Figure 9a) exhibited a few “jogs” where the crack briefly deviated from the direction 

perpendicular to the applied load. Two such jogs are shown in the red circle of Figure 9a. This 

seemed to occur along the boundaries between e-beam deposition rows, however, this did not 

produce any anomalous fatigue crack growth behavior. In comparison, the crack path of the Ti8Al-1Er alloy, shown in Figure 9b, seems to be somewhat less tortuous and did not exhibit any 

jogs in the crack path for the specimen tested. A photograph of the crack path in a specimen 

with a gradient from Ti-8Al-1Er to CP-Ti is shown in Figure 9c; note that the Ti-8Al-1Er region 

was better polished than the CP-Ti region despite being subjected to the same polishing 

procedure. Presumably this is due to the differences in the mechanical properties (e.g., 

hardness) between the materials in the two regions. The crack path in this specimen was 

straight and did not exhibit any of the jogs that were seen in the monolithic CP-titanium 

specimen. 

The crack growth testing presented in this document was done at a load ratio (ratio of 

minimum to maximum load in a fatigue load cycle) of R = 0.1. At such low values of R crack 

closure is expected to significantly affect fatigue crack growth rates, potentially more so for the 

relatively ductile CP-Ti. Further work will explore this area. 

Implications 

As with other additive manufacturing methods, EBF3 allows components to be fabricated 

near net shape, which significantly improves cost and lead time while reducing waste from 

machining operations. Further, by changing the wire product or processing parameters, EBF3

allows for development of materials with gradients in composition, microstructure, and 
5 

mechanical properties. While creating such gradients is not a new idea, further work is needed 

to develop a physics-based understanding of the interrelations of (1) the composition and 

processing parameters, with the resulting microstructure and (2) the microstructure with the 

resulting mechanical performance. The work presented in this document is a first step towards 

establishing this knowledge. Ultimately, such efforts will permit the design of advanced 

materials with specific mechanical performance (e.g., crack growth resistance) based on alloy 

composition and processing parameters. This would reduce the dependence on empirical 

experimental results, which would decrease the time and costs associated with qualification of 

new materials for use in aerospace structures. 

Summary and Future Work 

Electron beam freeform fabrication (EBF3) Ti products were produced to show the feasibility 

of fabricating metallic materials with gradients in composition and mechanical properties. 

Materials were made with a gradient between CP-Ti and Ti-8Al-1Er. Experimental data 

presented in this document show that chemical gradients can be created in EBF3

 deposited 

material that result in mechanical gradients in yield stress, ultimate tensile stress, elastic 

modulus, elongation at failure (ductility), hardness, and fatigue crack growth rates. 

There remains a good deal of experimental testing that should be conducted on the 1-D 

material produced for this study. To ensure that the fatigue crack growth trends observed in 

Figure 8 hold in the absence of crack closure, additional fatigue crack growth testing of gradient 

materials should be done at a higher value of R where crack closure is expected to have a 

negligible role (R = 0.5). 

The EBF3

 method deposits material in rows of a finite width; in this study, the rows were 

approximately 6 mm wide. It is possible that periodic variations in some material properties may 

occur with respect to the direction normal to the plane of deposition. Although tensile testing 

has been done on these materials, it was done using extensometers that might fail to capture 

such periodic variation in strain fields. Therefore, additional testing is planned to perform tensile 

testing using digital image correlation method, which will provide full-field strain data rather than 

a single scalar value from an extensometer. 

Finally, in order to use a computational approach for science-based design of structurally 

graded materials, an analytical model will be generated of the mechanical test response of 

these materials. Although the materials produced for this study had a gradient in only one 

direction (1-D), ultimately, the models will be generated for materials with gradients in multiple 

directions (2-D and fully 3-D gradients). 

References 

[1] K.M.B. Taminger and R.A. Hafley, “Electron Beam Freeform Fabrication: A Rapid Metal 

Deposition Process”, In: Proceedings of the 3rd Annual Composites Conference, Troy, MI, 

USA, 2003. 
6 

[2] J.Y. Hascoet, P. Muller, and P. Mognol, “Manufacturing of Complex Parts with Continuous 

Functionally Graded Materials (FGM)”, In: Proceedings of the Solid Freeform Fabrication 

Symposium. Austin, TX, USA, 2011. 

[3] ASTM Standard E8/E8M, 2009, “Standard Test Method for Tension Testing of Metallic 

Materials,” ASTM International, West Conshohocken, PA, 2009. 

[4] R.S. Piascik, J.C. Newman, Jr., and J.H. Underwood, “The Extended Compact Tension 

Specimen,” Fatigue and Fracture of Engineeing Materials and Structures, vol. 20, 1997, pp. 

559-563. 

[5] ASTM Standard E647, 2011, Standard Test Method for Measurement of Fatigue Crack 

Growth Rates,” ASTM International, West Conshohocken, PA, 2011. 

[6] A. Saxena and S.J. Hudak, Jr., “Review and Extention of Compliance Information for 

Common Crack Growth Specimens,” International Journal of Fracture, vol. 14, 1978, pp. 

453-468. 

[7] W.T. Riddell and R.S. Piascik, “Stress Ratio Effects on Crack Opening Loads and Crack 

Growth Rates in Alumium Alloy 2024,” ASTM STP 1332, T.L. Panontin and S.D. Sheppard, 

Eds., American Society for Testing and Materials, West Conshohocken, Pennsylvania, 

1999.

[8] J.A. Newman, “The Effects of Load Ratio on Threshold Fatigue Crack Growth in Aluminum 

Alloys,” Ph.D. dissertation, Virginia Polytechnic Institute and State University, Blacksburg, 

Virginia, 2000.
7 

Figures 

Figure 1: Schematic of the EBF3

 additive manufacturing process. 

. 
8 

Figure 2: Tensile properties (elastic modulus and elongation) as a function of alloy content for EBF3

deposited samples. 
9 

Figure 3: Tensile properties (ultimate and yield stress) as a function of alloy content for EBF3

 deposited 

samples. 
10 

Figure 4: Micro-hardness test data across the interface of CP-Ti to Ti-8Al-1Er alloy of EBF3

 deposited 

sample. 
11 

Figure 5: Photograph of EBF3

 titanium-alloy deposit on a CP-Ti substrate. 
12 

Figure 6: Fatigue crack growth specimen details; ESE(T) specimen configuration on the left and specimen 

orientation with respect to EBF3

 deposit on the right. 
13 

Figure 7: Fatigue crack growth results from CP-Ti and Ti-8Al-1Er monolithic ESE(T) specimens fabricated 

by EBF3
14 

Figure 8: Fatigue crack growth results for gradient CP-Ti/Ti-8Al-1Er ESE(T) specimen fabricated by EBF3

. 
15 

Figure 9: Photographs of crack profiles for monolithic (a) and (b), and gradient (c) ESE(T) specimens 

fabricated by EBF3

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Electron Beam Freeform Fabrication of Titanium Alloy Gradient Structures 

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Brice, Craig A.; Newman, John A.; Bird, R. Keith; Shenoy, Ravi N.; 

Baughman, James M.; Gupta, Vipul K.

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NASA Langley Research Center

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14. ABSTRACT

Historically, the structural optimization of aerospace components has been done through geometric methods. A monolithic material is chosen based on the best 

compromise between the competing design limiting criteria. Then the structure is geometrically optimized to give the best overall performance using the single 

material chosen. Functionally graded materials offer the potential to further improve structural efficiency by allowing the material composition and/or 

microstructural features to spatially vary within a single structure. Thus, local properties could be tailored to the local design limiting criteria. Additive 

manufacturing techniques enable the fabrication of such graded materials and structures. This paper presents the results of a graded material study using two 

titanium alloys processed using electron beam freeform fabrication, an additive manufacturing process. The results show that the two alloys uniformly mix at 

various ratios and the resultant static tensile properties of the mixed alloys behave according to rule-of-mixtures. Additionally, the crack growth behavior 

across an abrupt change from one alloy to the other shows no discontinuity and the crack smoothly transitions from one crack growth regime into another. 

15. SUBJECT TERMS

Additive manufacturing; Crack growth testing; Titanium

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