Additive Manufacturing Laser Deposition of Ti-6Al-4V forAerospace Repair Applications

Dustin CrouseDepartment of Mechanical Engineering,

Missouri University of Science & Technology

August 1, 2012

1 Abstract

Aerospace industries suffer from great main-tenance costs trying to find and repair defectsfound on their vehicles. It is important to finda process which can not only repair aerospacegrade parts deemed unusable but also do so in afast and efficient method. Additive manufactur-ing laser deposition processes shows promise inrepairing material defects. A titanium alloy, Ti-6Al-4V, is the workhorse of the titanium alloysand donated by Boeing as the testing material.The specimens were dented, repaired using laserdeposition processes. The tests for yield strengthand ultimate tensile strength show promisingvalues relative to standard titanium specifica-tions. Further work, such as microcharacteri-zation, needs to be conducted and presented toBoeing and the DOD.

2 Introduction

Laser Metal Deposition is an additive manu-facturing method in which metal powder is fedinto a melt pool created by a high powered laser.The laser is rastered across the sample and suc-cessively builds the sample layer by layer (Fig.1). The solidification speed and deposited layerthickness affect the specimen’s final microstruc-ture and mechanical properties. The aerospaceindustry is particularly interested in this fabri-cation method’s promise to mitigate aerospace

vehicle maintenance costs.

Figure 1: Laser deposition schematic

A titanium alloy, Ti-6Al-4V, is the buildingmaterial of choice for many aerospace applica-tions due to its light weight, high strength, highmelting temperature, and corrosion resistance[1]. Ti-6Al-4V Grade 5 also accounts for nearly50% of the world wide titanium usage, with theaerospace industry utilizing 80% of this usage.Furthermore, Ti-6Al-4V is expensive to fabri-cate, and its cost is quoted at $59.53 per kg of thesolid alloy [2]. It is the work horse of the alloysand used in jet engine parts, aerospace supportstructures, and corrosive processing plants.Defects can form on the part’s surface through-

out any aerospace vehicle’s service or duringthe manufacturing process. Fatigue cracks and

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micro-dents can cause subsequent reduced ma-terial strength and service life. Industry safetystandards regulate the size to which these de-fects can attain before the part of interest mustbe replaced. It seems reasonable, therefore, todevelop a restoring process in which the defectsare not only repaired, but also they maintain ifnot exceed most original Ti-6Al-4V mechanicalproperties.This work was advised by Dr. Frank Liou and

led by Todd Sparks and Day. Boeing graciouslyfunded the project as well as supplied the tita-nium alloy powder used in the laser depositingprocess. The goal at hand is meant to opti-mize laser deposition procedures and fabricaterepaired specimens with satisfactory propertiesto meet industry safety standards. The studypresents the repairing process and mechanicalprocessing of dented Ti-6Al-4V alloy specimens.Wherein, the final tensile and yield strength werecorrelated back to the laser deposited specimen’smicrostructure. The group was primarily inter-ested in attaining repaired specimens with ulti-mate tensile strengths, yield strengths, percent-ages of elongation, and microstructural proper-ties similar if not better than wrought Ti-6Al-4Valloys provided by Boeing.

3 Materials & Processes

3.1 Sample Preparation

Titanium alloy plates measuring 2” x 2” x0.25” were dented using a ball bearing com-pressed with 38 tons of force using hydraulicpress . The plate’s face opposite to the dent wasdeformed and protruded outward due to the de-fect. That face was ground and leveled. Thedefect was then filled using the laser depositionprocess. All titanium alloy powder was fed di-rectly into a diode laser having a beam wave-length of 808 nm. The laser’s power was set to1 kW and rastered across the sample a few min-utes to burn off oils that may have been depositedon the plate; thereafter, the laser was set at 600Watts. An x-y gantry table regulated the laserdeposition rate to 36 g/min whereas the injec-

tion head directed the thickness by which eachlayer was deposited to 500 mm. As the laser cre-ated a melt pool in the deformed area, Ti-6Al-4Vpowder was fed into the melt pool and allowedto solidify. This process was performed until thedent was completely filled with powder (Fig. 2).

Figure 2: Repaired defect

A water jet then cut both the plate’s lengthand width dimensions down to one inch. ACNC machine cut the testing sample’s outline0.125 inches into the sample. The plate wasthen flipped over and face milled until the samplewas approximately 0.05 inches thick. The testingsample was removed from the plate with remain-ing residue and burrs polished away. The testingsample’s dimensions are given below (Fig. 3).

Figure 3: Test specimen specifications

3.2 Mechanical Properties

Three testing samples were used to test eachmechanical property. Also, two parameters were

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tested and compared to one control group. Platesthat were dented but not repaired as well asplates that were dented and repaired were com-pared to a standard. The standard consisted ofa defect free wrought Ti-6Al-4V alloy plate fromthe same set donated by Boeing. The first step inmechanical testing was to calculate the sample’spercentage of elongation (Eq. 1).

%EL =Lf − L0

L0

(1)

Where,

L0=initial lengthLf=final length

It is important to have an optimum percentof elongation comparative with the wrought ma-terial. If a material is too brittle, the percentof elongation will be small. A brittle alloy maymean the sample is subject to catastrophic fail-ure due to fracturing with little plastic deforma-tion. Ductile materials, in contrast with brit-tle material, are found with large percentages ofelongation. These materials can plastically de-form much more than their original length with-out fracturing. Ideally it is better to maintainductile properties rather than brittle properties.Ductile properties ensure slow crack propagation,which results in a greater chance of the defectbeing caught and repaired before failure. Exper-imental data is provided below (Fig. 4).

Figure 4: Percent elongation of test specimens

The desired percent of elongation is averagedat about 12% [2]. The reader should note, how-ever, that elastic backlash and shrinkage afterbreakage as well as a tensile tester with an un-stable head mount may attribute to the smallpercentages seen in the repaired material. Forexample, an optical laser found that supposedlystationary head block moved down a few millime-ters as the tensile test was in progress. The re-sults have been skewed and may be deemed un-reliable.The repaired samples’ strength unexpect-

ingly exceeded exceeded the standard’s strength.The highest calculated ultimate tensile strength(UTS) and yield strength (YS) for the repairedspecimens is 1274 MPa and 1209 MPa respec-tively (Fig. 5). At minimum this is a 20% UTSincrease and 25% YS compared to reported val-ues [2][3]. As well, The deposition parameters,specifically the cooling and deposition rate, maydirectly influence the testing sample’s high struc-tural properties.

Figure 5: Strength of test specimens in MPa

3.3 Microcharacterization

Microstructural data was not obtained beforethe end of the program. Therefore, an introduc-tion pertaining to the grain structure’s affect onoverall mechanical properties is provided. Ti-6Al-4V is an α-β alloy whose room temperature

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microstructure varies based on thermal history.As solidification occurs, the liquid first crystal-lizes into a body-centered β phase. The priorβ phase greatly affects grain size, morphology,and crystallographic texture [4]. These proper-ties can play a significant role in determining theproperties of wrought Ti-6Al-4V. Laser additivemanufacturing has many variables that can in-fluence the soundness and mechanical propertiesof the resulting part by affecting characteristicssuch as porosity, residual stresses, fatigue crack-ing, microstructure, and surface texture.Past work with Todd Sparks has proven that

fast cooling rates during the laser deposition pro-cess results in a completely different grain struc-ture than the wrought material. The standardtitanium plates contain circular grains similarto those found in most metals. Upon cooling,the grains take on long and slender striations ina cross-hatched pattern named a Widmanstttengrain structures (Fig. 6).

Figure 6: Widmanstatten grain structure

The grains are at least twice as long as it iswide. The important granular properties whichallow the repaired specimens to attain a higherUTS and YS lie in how the grains distribute ap-plied forces. As a force is applied, the force vec-tors act along the grain boundaries. Impedingthese vectors’ movement by making it constantly

switch directions increase a specimens overallstrength. The Widmansttten cross hatches forcethe vectors to switch in perpendicular direc-tions many times, which greatly impedes fracturemovement and improves the material’s fractureresistance. Further testing, however, needs to becompleted before any conclusion can be made.

4 Conclusion

Laser deposition additive manufacturing mayvery well become the key to saving the aerospaceindustry tens of thousands of dollars each yeardue to manufacturing errors. The laser deposi-tion variables set by past projects have made itpossible to work on industrially applicable exper-iments. The rate at which titanium powder wasfed into a melt pool was kept constant while fab-ricating all test samples; though, these param-eters may have a significant affect on the sam-ple’s structural properties. The final UTS andYS measured show promising results well aboveexpected values. For Ti-6Al-4V, a UTS and YSnear 1200 MPa is an exceptionally sound mate-rial well within serviceable limits. Calculationsalso showed the repaired specimens have a muchlower percent of elongation and may be morebrittle than desired. The tensile testing machineslipping is the most probable cause for these er-roneous values and further tests with a differenttensile tester will conclude this theory. The mi-crostructural properties still need to be analyzebut were held back by faulty mounting epoxy.The group is working on finding a replacementfor the cold mounting. The current project willconclude in mid-November and shall test fourdifferent manufacturing defects: denting, cuttergouge, undercut, tooling pull out. The focus dur-ing this REU program was directed at testing therepairing process for a dented sample.

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References

[1] A. A. S. M. Inc., “Titanium ti-6al-4v (grade5), annealed,” 2012.

[2] A. G. H. H. S. S. T. S. E. V. P. G. H. F.J. O. J. D. J. K. K. M. T. Alcisto J., En-riquez and O. S. Es-Said, “Tensile propertiesand microstructures of laser-formed ti-6al-4v,” Journal of Materials Engineering andPerformance, vol. 20, no. 2, pp. 203–212,2011.

[3] Z. F. L. X. H. W. Tan Hua, Chen Jing,“Microstructure and mechanical properties oflaser solid formed ti-6al-4v from blended el-emental powders,” Rare Metal Materials andEngineering, vol. 38, p. 574578, April 2009.

[4] S. S. P.A. Kobryn, “The laser additive man-ufacture of ti-6al-4v,” JOM, vol. 53, no. 9,pp. 40–42, 2001.

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