CHARACTERIZING THE CRACKING BEHAVIOR OF HARDALPHA DEFECTS IN ROTOR GRADE Ti-6-4 ALLOY

P. C. McKeighanSouthwest Research Institute

P. O. Drawer 28510San Antonio, TX 78228-0510

L. C. PerocchiGeneral Electric Company

Corporate Research and DevelopmentP. O. Box 8, Schenectady, NY 12301

A. E. NichollsR. C. McClung

Southwest Research InstituteP. O. Drawer 28510

San Antonio, TX 78228-0510

Abstract

A program sponsored by the FAA is currently underway to develop predictive tools utilizingstate-of-the-art damage tolerance and probabilistic methodologies that can be used in the lifemanagement of high energy rotors. The program is focusing on fatigue crack nucleation andgrowth from anomalies in titanium alloys known as hard alpha, an inclusion-like feature thatcan occur during the melting process. In the work detailed in this paper, two sizes of synthetichard alpha defects are created in Ti-6Al-4V and subjected to static and fatigue loading. Inaddition, two different geometry anomalies are considered: one intersecting the surface of thespecimen and another embedded internally. A number of crack detection transducers are usedand shown to compare well to results from visual inspections on the surface defect specimens.These surface specimens tend to exhibit defect cracking at relatively low stress levels, typicallyon the order of 5-10 ksi. Although it appeared from the crack detection transducers that littleor no cracking occurred in the interior anomaly specimens given an applied static stress of100 ksi, subsequent metallographic sectioning demonstrated more extensive cracking anddamage. The observed cracking behavior indicates that the diffusion zone may play animportant role in the structural integrity of the hard alpha anomalies.

Introduction

Aircraft gas turbine industry experience as well as other research [1] has shown that theoccurrence of certain material and manufacturing anomalies can potentially degrade thestructural integrity of high energy rotors. These infrequent anomalies represent a departure fromassumed nominal conditions and are not typically addressed in conventional rotor lifemanagement methods or in the supporting materials databases. The Federal AviationAdministration (FAA) has requested that industry determine whether a damage tolerance(fracture mechanics) approach could be introduced as a supplement to conventional safe-lifemethods in order to produce a reduction in the rate of uncontained rotor events. Following therecommendations of an industry working group, Southwest Research Institute and a team ofdomestic gas turbine engine manufacturers are conducting an FAA-sponsored program [2] todevelop enhanced predictive tools and supplementary material/anomaly behaviorcharacterization in support of a new probabilistic damage tolerance life management process.

The FAA program is currently focusing on fatigue crack nucleation and growth from anomaliesin titanium alloys known as hard alpha (HA), small zones where the alpha phase has beenstabilized by the presence of nitrogen introduced during the melting process. Large hard alphaanomalies are often extensively voided and cracked in the final forged rotor configuration.Current life prediction methodologies commonly assume that an initial crack size equal to thesize of the HA region is present at the beginning of life. This assumption could be overlyconservative, however, for smaller HA anomalies and zones with lower nitrogen content, whichmay be less likely to be extensively cracked and voided after the forging process and, hence,more difficult to detect using nondestructive evaluation. Consequently, these anomalies mayplay a significant role in the probabilistic risk assessment especially considering that a relativelylarger number of these smaller defects are postulated to exist in rotor material. Therefore, it isuseful to assess the cracking tendencies of the HA anomalies under static and fatigue loading inorder to potentially include some nonzero crack nucleation life in the fatigue life predictionmethodology.

The research presented in this paper was conducted on synthetic anomalies created artificiallyusing techniques pioneered to develop inspection standards used for refining NDE methods [3].These anomalies consist of a hard alpha defect and surrounding diffusion zone (DZ) embeddedin a Ti-6Al-4V plate specimen that is subjected to both static and fatigue loading to more fullyunderstand how cracks initiate and grow. The anomalies, consisting of both interior and surfacebreaking geometries, were also characterized in terms of composition and hardness. Variouscrack detection transducers were used as well as extensive metallurgical sectioning todefinitively characterize cracking following testing. The work presented herein was conductedas a preliminary assessment for a more extensive and detailed effort that is currently underway.

Specimen Preparation and Test Procedures

Preparation of the Embedded-Anomaly Specimens

The procedures used to prepare the artificially seeded blocks and specimens were relativelycomplex involving multiple steps: a) preparation of the seeds, b) creation of the artificialdiffusion zones, c) machining of the blocks, d) assembly of the combined seed and blocks,e) joining and finishing of the block, and f) machining of the specimens. First, titanium metalsponge and titanium nitride powder were added to the cold copper hearth of a non-consumable,arc-melting furnace. The material was melted three times, measuring the weight and flipping theingot 180 degrees between each melt. The arc-melted Ti-N was hot isostatically pressed(HIPped) at 1200°C and 15 ksi for 3 to 4 hours to close any solidification porosity.

The HIPped ingot was initially sectioned with wire electro-discharge machining (EDM) to yieldtwo thin diametral slices each approximately 0.100 inch thick that were used to insuremicrostructural homogeneity and verify chemistry. Both oxygen and nitrogen analyses wereperformed using a fusion technique and an acid dissolution and titration method, respectively.Once the metallographic and microstructural work insured that the ingot met specification, smallcylindrical pieces approximately 1-inch long were wire EDMed as indicated in Figure 1(a).These were then hand polished to eliminate machining burrs and etched in a nitric andhydrofluoric acid to clean the surfaces. The pieces were then mounted in epoxy and cut on adiamond saw to the required length to create the artificial hard-α core (Figure 1(b)).

The defect cores were then removed from the epoxy by soaking in dimethylformamide andrinsing in propanol. Core diameters and lengths were measured and carefully documented. Thevarious cores were then placed in a matrix pattern of cylindrical cavities created in a Ti-6-4 blockwith a cover plate mechanically clamped. The assembly was then electron beam welded andHIPped at 1400°C at 30 ksi for three hours. This creates a natural diffusion zone around thedefect (nominally defined as the hard alpha core and the diffusion zone) that is subsequentlyEDMed out, along with the core.

The specimen blanks, in the form of a rectangular block with embedded defects, weremanufactured from Ti-6-4 rolled rings. The blocks are split through-the-thickness and themating surfaces are ground to insure excellent dimensional agreement. The plane of the majoraxis of the cylindrical defect was perpendicular to the split surfaces and a flat bottom hole of theappropriate size to insert the defect is drilled into the block. The pieces of the block were thencleaned in hot phenol (88% in water), soaked in hot Oakite-90 solvent, rinsed in hot water andthen etched in a nitric and hydrofluoric acid bath. Following etching, the parts were bathed indistilled water, rinsed in propanol and dried with oil-free air.

The defects were then placed in the block and the cover plate positioned in place withmechanical clamps to maintain alignment during electron beam (EB) welding. The block wasthen welded in vacuum using typical conditions of 125kV, 10mA and a welding speed of 30 inchper minute. Following welding, the blocks were HIPped using the same pressure and timeconditions as during defect manufacture but at a temperature of 900°C (below the beta transustemperature) to yield a select volume fraction of primary alpha. The resulting block was groundon all surfaces with equal amounts taken from opposite sides while carefully maintaining theorientation relative to the processing. Finally, the bond plane of the block was examined byultrasonic NDE to assure a metallurgical bond and an absence of voids or disbonds.

The blocks were subsequently carefully machined into typical 6-inch long “dogbone” specimenssuitable for both static and fatigue testing (Figure 2). The gage length and width of thespecimens were 1 inch with a thickness of 0.5 inch. Four specimens were machined with a largediameter surface defect and two specimens with a large and small interior defect. The averagehard alpha core and diffusion diameters (in inches) were 0.013 (HA) and 0.059 (DZ) for thesmall defects and 0.062 (HA) and 0.119 (DZ) for the large defects.

Test Procedures and Crack Detection

The specimens were loaded in a set of hydraulic wedge grips in a 200 kip servohydraulic testmachine. All specimens were loaded statically although two were also subjected to cyclicfatigue loading at low stress ratio (typically R = 0.05 - 0.1). Frequent visual inspections wereperformed during loading on specimens with surface defects. All specimens were alsosubjected to ultrasonic, acoustic emission and potential drop measurements during testing to aidin detecting crack nucleation and growth.

For the surface defects, ultrasonic surface wave and 45° shear wave techniques were used with a10 MHz, 0.25-inch diameter transducer. Alternatively, unfocused ultrasonic techniques using45° shear wave shoes (one in pulse echo and the other in pitch catch) were used to detectcracking for the interior defects. A 0.75-inch diameter, wide-band acoustic microphone with aLOCAN 320 system (Physical Acoustics, Lawrenceville, NJ) recording data was also used todetect acoustic emission in the samples during loading. Finally, a dual-probe, pulsed DC-current potential drop system was also used to assess voltage perturbations in the sampleindicative of crack nucleation and growth. All three types of sensors were located as close tothe defects as possible to maximize measurement sensitivity.

Results

HA Core and Diffusion Zone Chemical and Hardness Characterization

Prior to discussing the observed cracking behavior, it is useful to summarize the results obtainedfrom assessments of hardness and chemistry for the defects. A microprobe scan to determinechemical constituents was performed by Pratt and Whitney. The hard alpha core consisted of5% nitrogen (weight percent), with the balance titanium. The natural diffusion that occurred asshown in the chemical profile in Figure 3 resulted in a high level of 4% nitrogen just outside thehard alpha core. This nitrogen composition then drops sharply to a constant level of 0.8-1% at aradial location 0.002 inch deep into the DZ. Furthermore, the grading in mechanical propertiesis also apparent from the hardness profile shown in Figure 4. The defect core has a relativelyconstant hardness approximately 2.2 times greater (> RC 64) than the base metal level.

Observed Crack Initiation and Growth

One of the advantages of testing the surface defects is that the visual observations of crackingallow assessing overall performance and sensitivity of the UT, AE and PD methods (whichwere relied on completely for the interior specimens). The data from one of the staticallyloaded specimens are indicated in Figure 5. As can be clearly observed, all three transducersprovided a clear indication of change at the same time the visual inspection revealed a crack.Furthermore, as the crack grew, all three non-visual methods yielded an increase in transduceroutput. A summary of the findings from the static and fatigue loaded specimens are shown inTable 1.

Similar results from the three transducers are shown in Figure 6 for measurements made duringgrowth of a fatigue crack. However, the ultrasonic (UT) output sometimes tended to be erratic,probably as a consequence of the method used to attach the sensor to the specimen. It shouldalso be noted that during the fatigue process, the acoustic emission tended not to indicatesignificant signal change until the crack was relatively long (hence, AE sensitivity to thegrowing fatigue crack was less than the other methods). However, the acoustic event frequencyof defect (155-170 kHz), diffusion zone (200-205 kHz) and base metal (>300 kHz) crackingvaried sufficiently to sometimes differentiate the physical location of AE events.

The data included in Table 1 illustrate that crack initiation in the hard alpha core of the surfacedefect specimens tended to occur at quite low stress levels, on the order of 5-10 ksi. Low stresscracking was observed both visually and with the three crack detection transducers. Moreover,different behavior was noted for the interior defects, with no transducer indications observedwith the small core and only minimal indications (at 90 ksi) for the large core.

Table 1. Results from static and fatigue loaded samples with artificial HA anomalies.

Defect Stress (ksi) applied to..Specimen Description Description of Initiate Initiate Full DZ Sensor

ID No. Type Size Loading Type in HA in DZ Cracked ObservationsHA-SL-A1 surface large static (110 ksi) 6 32-36 ≈100 numerousHA-SL-B1 surface large static (50 ksi) 8 22 ≈50 numerousHA-SL-A2 surface large static (75 ksi) 10-18 42 ≈70 numerousHA-SL-C1 surface large static (40 ksi) 5-10 - - numerous

fatigue (34 ksi,24kcycles)

- - - rapid DZ cracking

HA-IS-D1 interior small static (100 ksi) - - - nonefatigue (80 ksi,20kcycles)

- - - none

static (100 ksi) - - - noneHA-IL-F1 interior large static (100 ksi) - - - minimal (90 ksi)

Discussion and Summary

Although one of the primary benefits of testing surface defects was the ability to verify theperformance of the non-visual crack detection transducers, the distance of the transducers fromthe defect was larger for the subsurface defects (hence, implying a possible loss in sensitivity).Furthermore, the virtual absence of apparent cracking in the interior defects was surprising inview of the extent of cracking observed in the surface defect cases (e.g., note Figure 7(a)).Although from a mechanic’s viewpoint the constraint states do differ for the surface and interiordefect cases, the cracking in the surface defects at such low stress levels (under 10 ksi) andabsence of apparent cracking for the interior defects is unusual and somewhat unexpected.Consequently, a more definitive assessment of the integrity of the interior defects was performedby examining metallographic cross-sections of both defects following the testing.

Serial Sectioning of the Interior Defect Specimens

Numerous (six to seven) sections of each of the interior defects were examined in a lightmicroscope following polishing and etching. Two sample sections are indicated in Figure 7(b)and (c) for the small and large defects, respectively. The extent of cracking was greater thanexpected based upon the crack detection transducers. All diffusion zone sections for bothspecimens exhibited microcracking. Complete cracking was observed across each of the wholediffusion zones but not for the full length of the defect.

For the smaller defect in Figure 7(b), cracks were observed primarily confined to the interfacebetween the diffusion zone and base metal. Only one crack, in the section depicted inFigure 7(b), was observed in the core of the hard alpha. However, this crack was slightly curvedand oriented parallel to the loading direction. The typical size of the cracks observed in thediffusion zone of the smaller defect was 5-10 mils, with only a few 25-50 mil and only one75 mil long (extended into the base metal presumably by the fatigue loading). The diffusionzone cracking in the smaller defect tended to occur over multiple sites and did not appear to becontinuous.

For the larger defect (ID No. HA-IL-F1, Figure 7), cracks were observed in both the core anddiffusion zone for all the cross sections examined. For the first half of the HA core, the observedcracking was extensive, somewhat discontinuous and confined to parallel bands indicative ofdefect shattering. Furthermore, cracks in a wide variety of lengths from 10-125 mils were

observed with DZ cracking tending to be continuous and generally planar (in contrast to themultiple site, interfacial cracking observed in the smaller defect).

Implications of the Extent of Damage and Cracking

The fact that the non-visual crack detection transducers indicated little or no cracking duringtesting of both of the interior defect specimens implies that the extent and type of crackingobserved is at or below the threshold for detection. In fact, the excellent performance of thetransducers for the surface defect specimens (e.g., Figure 5) might be somewhat misleading sinceall observations were surface derived. Prior to damage being manifested on the surface of thespecimens, more might have been present sub-surface and beyond visual inspection capability.

Furthermore, it is possible that the defect damage and cracking may have occurred before testing.However, this is doubtful since the cracks were predominantly perpendicular to the loading axis.Had the cracks been a consequence of either (a) the defect processing or the (b) themetallographic sectioning, they would be expected to be more random in orientation.

The results of these investigations indicate several key points. First, the metallographicsectioning provides the most insight into the finest-scale damage and cracking. Second, the non-visual crack detection transducers yielded the most meaningful data for the less constrained,more heavily damaged surface defects that exhibited first cracking at or below 10 ksi. Third, thediffusion zone regions of interior defects tend to exhibit a large amount of damage and could beextensively cracked with little or no apparent defect damage. This is a critical observation fromthe viewpoint of modeling the damage progression process in hard alpha anomalies since itimplies that the diffusion zone (which is more difficult to detect during routine NDE inspections)may play a key role in the nucleation of fatigue cracks. This observation, as well as other issuesraised in this work, is currently under investigation in the second part of the phased testingstrategy.

Acknowledgements

This work is funded by the FAA under Grant No. 95-G-041 administered by the FAA TechnicalCenter in Atlantic City, NJ. The support of Messrs. B. Fenton, J. Wilson and T. Mouzakis, allfrom the FAA, is acknowledged, along with the assistance of Dr. G. Leverant (SwRI ProgramManager) and P. Scherer (Pratt & Whitney). Furthermore, the efforts of SwRI colleaguesMessrs. H. Saldana, I. Rodriguez and S. Salazar (metallurgical) and Drs. G. Light and A.Minachi (NDE) are gratefully acknowledged.

References

[1] B. Dillard, K. R. Clark, T. Denda, B. C. Hendrix and J. K. Tien, “Reduction of Fatigue Lifeby Melt Inclusions in Ti-6Al-4V,” Proceedings of the Conference on Electron BeamMelting and Refining – State of the Art 1992, Reno, Nevada, October 25-27, 1992.

[2] G. R. Leverant, D. L. Littlefield, R. C. McClung, H. R. Millwater, and J. Y. Wu, “AProbabilistic Approach to Aircraft Turbine Material Design,” Paper 97-GT-22, ASMEInternational Gas Turbine & Aeroengine Congress, June 1997.

[3] M. F. X. Gigliotti, L. C. Perocchi, E. J. Nieters and R. S. Gilmore, “Design and Fabricationof Forged Ti-6Al-4V Blocks with Synthetic Ti-N Inclusions for Estimation of Detectabilityby Ultrasonic Signal-to-Noise,” Review of Progress in Quantitative NondestructiveEvaluation, Vol. 14, 1995, pp. 2089-2096.

Figure 1. The (a) ingot used and Figure 2. Photograph of a test specimen with a surface(b) artificial defect seeds. breaking defect and nominal subsurface dimensions.

Figure 3. The composition of a defect diffusion Figure 4. Hardness profile across the corezone determined by a microprobe scan. and diffusion zone.

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Please contact author for hard copy of manuscript.