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

Transcription

1 CHARACTERIZING THE CRACKING BEHAVIOR OF HARD ALPHA DEFECTS IN ROTOR GRADE Ti-6-4 ALLOY P. C. McKeighan Southwest Research Institute P. O. Drawer San Antonio, TX L. C. Perocchi General Electric Company Corporate Research and Development P. O. Box 8, Schenectady, NY A. E. Nicholls R. C. McClung Southwest Research Institute P. O. Drawer San Antonio, TX Abstract A program sponsored by the FAA is currently underway to develop predictive tools utilizing state-of-the-art damage tolerance and probabilistic methodologies that can be used in the life management of high energy rotors. The program is focusing on fatigue crack nucleation and growth from anomalies in titanium alloys known as hard alpha, an inclusion-like feature that can occur during the melting process. In the work detailed in this paper, two sizes of synthetic hard alpha defects are created in Ti-6Al-4V and subjected to static and fatigue loading. In addition, two different geometry anomalies are considered: one intersecting the surface of the specimen and another embedded internally. A number of crack detection transducers are used and 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, typically on the order of 5-10 ksi. Although it appeared from the crack detection transducers that little or no cracking occurred in the interior anomaly specimens given an applied static stress of 100 ksi, subsequent metallographic sectioning demonstrated more extensive cracking and damage. The observed cracking behavior indicates that the diffusion zone may play an important role in the structural integrity of the hard alpha anomalies.

2 Introduction Aircraft gas turbine industry experience as well as other research [1] has shown that the occurrence of certain material and manufacturing anomalies can potentially degrade the structural integrity of high energy rotors. These infrequent anomalies represent a departure from assumed nominal conditions and are not typically addressed in conventional rotor life management methods or in the supporting materials databases. The Federal Aviation Administration (FAA) has requested that industry determine whether a damage tolerance (fracture mechanics) approach could be introduced as a supplement to conventional safe-life methods in order to produce a reduction in the rate of uncontained rotor events. Following the recommendations of an industry working group, Southwest Research Institute and a team of domestic gas turbine engine manufacturers are conducting an FAA-sponsored program [2] to develop enhanced predictive tools and supplementary material/anomaly behavior characterization in support of a new probabilistic damage tolerance life management process. The FAA program is currently focusing on fatigue crack nucleation and growth from anomalies in titanium alloys known as hard alpha (HA), small zones where the alpha phase has been stabilized by the presence of nitrogen introduced during the melting process. Large hard alpha anomalies 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 the size of the HA region is present at the beginning of life. This assumption could be overly conservative, however, for smaller HA anomalies and zones with lower nitrogen content, which may 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 may play a significant role in the probabilistic risk assessment especially considering that a relatively larger number of these smaller defects are postulated to exist in rotor material. Therefore, it is useful to assess the cracking tendencies of the HA anomalies under static and fatigue loading in order to potentially include some nonzero crack nucleation life in the fatigue life prediction methodology. The research presented in this paper was conducted on synthetic anomalies created artificially using 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) embedded in a Ti-6Al-4V plate specimen that is subjected to both static and fatigue loading to more fully understand how cracks initiate and grow. The anomalies, consisting of both interior and surface breaking geometries, were also characterized in terms of composition and hardness. Various crack detection transducers were used as well as extensive metallurgical sectioning to definitively characterize cracking following testing. The work presented herein was conducted as 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 relatively complex involving multiple steps: a) preparation of the seeds, b) creation of the artificial diffusion 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 metal sponge 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 the ingot 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.

3 The HIPped ingot was initially sectioned with wire electro-discharge machining (EDM) to yield two thin diametral slices each approximately inch thick that were used to insure microstructural homogeneity and verify chemistry. Both oxygen and nitrogen analyses were performed using a fusion technique and an acid dissolution and titration method, respectively. Once the metallographic and microstructural work insured that the ingot met specification, small cylindrical 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 and hydrofluoric acid to clean the surfaces. The pieces were then mounted in epoxy and cut on a diamond 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 and rinsing in propanol. Core diameters and lengths were measured and carefully documented. The various cores were then placed in a matrix pattern of cylindrical cavities created in a Ti-6-4 block with a cover plate mechanically clamped. The assembly was then electron beam welded and HIPped at 1400 C at 30 ksi for three hours. This creates a natural diffusion zone around the defect (nominally defined as the hard alpha core and the diffusion zone) that is subsequently EDMed out, along with the core. The specimen blanks, in the form of a rectangular block with embedded defects, were manufactured from Ti-6-4 rolled rings. The blocks are split through-the-thickness and the mating surfaces are ground to insure excellent dimensional agreement. The plane of the major axis of the cylindrical defect was perpendicular to the split surfaces and a flat bottom hole of the appropriate size to insert the defect is drilled into the block. The pieces of the block were then cleaned in hot phenol (88% in water), soaked in hot Oakite-90 solvent, rinsed in hot water and then etched in a nitric and hydrofluoric acid bath. Following etching, the parts were bathed in distilled 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 with mechanical clamps to maintain alignment during electron beam (EB) welding. The block was then welded in vacuum using typical conditions of 125kV, 10mA and a welding speed of 30 inch per minute. Following welding, the blocks were HIPped using the same pressure and time conditions as during defect manufacture but at a temperature of 900 C (below the beta transus temperature) to yield a select volume fraction of primary alpha. The resulting block was ground on all surfaces with equal amounts taken from opposite sides while carefully maintaining the orientation relative to the processing. Finally, the bond plane of the block was examined by ultrasonic 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 specimens suitable for both static and fatigue testing (Figure 2). The gage length and width of the specimens were 1 inch with a thickness of 0.5 inch. Four specimens were machined with a large diameter surface defect and two specimens with a large and small interior defect. The average hard alpha core and diffusion diameters (in inches) were (HA) and (DZ) for the small defects and (HA) and (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 test machine. All specimens were loaded statically although two were also subjected to cyclic fatigue loading at low stress ratio (typically R = ). Frequent visual inspections were performed during loading on specimens with surface defects. All specimens were also subjected to ultrasonic, acoustic emission and potential drop measurements during testing to aid in detecting crack nucleation and growth.

4 For the surface defects, ultrasonic surface wave and 45 shear wave techniques were used with a 10 MHz, 0.25-inch diameter transducer. Alternatively, unfocused ultrasonic techniques using 45 shear wave shoes (one in pulse echo and the other in pitch catch) were used to detect cracking for the interior defects. A 0.75-inch diameter, wide-band acoustic microphone with a LOCAN 320 system (Physical Acoustics, Lawrenceville, NJ) recording data was also used to detect acoustic emission in the samples during loading. Finally, a dual-probe, pulsed DCcurrent potential drop system was also used to assess voltage perturbations in the sample indicative of crack nucleation and growth. All three types of sensors were located as close to the 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 obtained from assessments of hardness and chemistry for the defects. A microprobe scan to determine chemical constituents was performed by Pratt and Whitney. The hard alpha core consisted of 5% nitrogen (weight percent), with the balance titanium. The natural diffusion that occurred as shown in the chemical profile in Figure 3 resulted in a high level of 4% nitrogen just outside the hard alpha core. This nitrogen composition then drops sharply to a constant level of 0.8-1% at a radial location inch deep into the DZ. Furthermore, the grading in mechanical properties is also apparent from the hardness profile shown in Figure 4. The defect core has a relatively constant hardness approximately 2.2 times greater (> R C 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 cracking allow assessing overall performance and sensitivity of the UT, AE and PD methods (which were relied on completely for the interior specimens). The data from one of the statically loaded specimens are indicated in Figure 5. As can be clearly observed, all three transducers provided 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 transducer output. A summary of the findings from the static and fatigue loaded specimens are shown in Table 1. Similar results from the three transducers are shown in Figure 6 for measurements made during growth 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 should also be noted that during the fatigue process, the acoustic emission tended not to indicate significant signal change until the crack was relatively long (hence, AE sensitivity to the growing fatigue crack was less than the other methods). However, the acoustic event frequency of defect ( khz), diffusion zone ( khz) and base metal (>300 khz) cracking varied 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 surface defect specimens tended to occur at quite low stress levels, on the order of 5-10 ksi. Low stress cracking was observed both visually and with the three crack detection transducers. Moreover, different behavior was noted for the interior defects, with no transducer indications observed with the small core and only minimal indications (at 90 ksi) for the large core.

5 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 Observations HA-SL-A1 surface large static (110 ksi) numerous HA-SL-B1 surface large static (50 ksi) numerous HA-SL-A2 surface large static (75 ksi) numerous HA-SL-C1 surface large static (40 ksi) numerous fatigue (34 ksi, rapid DZ cracking 24kcycles) HA-IS-D1 interior small static (100 ksi) none fatigue (80 ksi, none 20kcycles) static (100 ksi) none HA-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 the performance of the non-visual crack detection transducers, the distance of the transducers from the 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 in view 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 interior defect cases, the cracking in the surface defects at such low stress levels (under 10 ksi) and absence 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 performed by 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 light microscope 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 than expected based upon the crack detection transducers. All diffusion zone sections for both specimens exhibited microcracking. Complete cracking was observed across each of the whole diffusion 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 interface between the diffusion zone and base metal. Only one crack, in the section depicted in Figure 7(b), was observed in the core of the hard alpha. However, this crack was slightly curved and oriented parallel to the loading direction. The typical size of the cracks observed in the diffusion zone of the smaller defect was 5-10 mils, with only a few mil and only one 75 mil long (extended into the base metal presumably by the fatigue loading). The diffusion zone cracking in the smaller defect tended to occur over multiple sites and did not appear to be continuous. For the larger defect (ID No. HA-IL-F1, Figure 7), cracks were observed in both the core and diffusion zone for all the cross sections examined. For the first half of the HA core, the observed cracking was extensive, somewhat discontinuous and confined to parallel bands indicative of defect shattering. Furthermore, cracks in a wide variety of lengths from mils were

6 observed with DZ cracking tending to be continuous and generally planar (in contrast to the multiple 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 during testing of both of the interior defect specimens implies that the extent and type of cracking observed is at or below the threshold for detection. In fact, the excellent performance of the transducers for the surface defect specimens (e.g., Figure 5) might be somewhat misleading since all observations were surface derived. Prior to damage being manifested on the surface of the specimens, 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) the metallographic sectioning, they would be expected to be more random in orientation. The results of these investigations indicate several key points. First, the metallographic sectioning provides the most insight into the finest-scale damage and cracking. Second, the nonvisual 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, the diffusion zone regions of interior defects tend to exhibit a large amount of damage and could be extensively cracked with little or no apparent defect damage. This is a critical observation from the viewpoint of modeling the damage progression process in hard alpha anomalies since it implies 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 issues raised in this work, is currently under investigation in the second part of the phased testing strategy. Acknowledgements This work is funded by the FAA under Grant No. 95-G-041 administered by the FAA Technical Center in Atlantic City, NJ. The support of Messrs. B. Fenton, J. Wilson and T. Mouzakis, all from the FAA, is acknowledged, along with the assistance of Dr. G. Leverant (SwRI Program Manager) and P. Scherer (Pratt & Whitney). Furthermore, the efforts of SwRI colleagues Messrs. 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 Life by Melt Inclusions in Ti-6Al-4V, Proceedings of the Conference on Electron Beam Melting and Refining State of the Art 1992, Reno, Nevada, October 25-27, [2] G. R. Leverant, D. L. Littlefield, R. C. McClung, H. R. Millwater, and J. Y. Wu, A Probabilistic Approach to Aircraft Turbine Material Design, Paper 97-GT-22, ASME International Gas Turbine & Aeroengine Congress, June [3] M. F. X. Gigliotti, L. C. Perocchi, E. J. Nieters and R. S. Gilmore, Design and Fabrication of Forged Ti-6Al-4V Blocks with Synthetic Ti-N Inclusions for Estimation of Detectability by Ultrasonic Signal-to-Noise, Review of Progress in Quantitative Nondestructive Evaluation, Vol. 14, 1995, pp

7 2.5 inch (approx) Subsurface Defect 1/2 of ingot (with samples EDMed) thickness (a) defects* * prior to being cut to the appropriate length (b) 1.0 inch (approx) defects* * following cutting to the appropriate length large surface HA defect defect diffusion zone Surface Defect thickness Figure 1. The (a) ingot used and (b) artificial defect seeds. Figure 2. Photograph of a test specimen with a surface breaking defect and nominal subsurface dimensions. 4 HA-SL-B1 HA core: 5% Nitrogen 95% Titanium 80 HA-SL-C1 Weight Percent Nitrogen diffusion zone base metal HRC hardness base metal diffusion zone defect core Distance from HA core / DZ interface, mils Distance from HA core center, mil Figure 3. The composition of a defect diffusion zone determined by a microprobe scan. Figure 4. Hardness profile across the core and diffusion zone.

8 UT AE PD HA-SL-B1 crack length potential drop (PD) acoustic emission (AE) ultrasonic (UT) PD trend UT trend AE trend Applied Stress Level, ksi Visual Crack Length, inch Crack Length, inch Normalized Transducer Output HA-SL-C1 Test HA-SL-C1 DZ Max fully stress cracked 34 ksi at R=0.05 base metal cracking HA fully cracked HA partly cracked PD AE UT Fatigue Cycles Figure 5. Results from statically loaded surface defect specimen. Figure 6. Results from a fatigue loaded surface defect specimen. (a) inch loading axis (all views) core (HA) microcracking diffusion zone (DZ) Figs. 7(b) and 7(c) omitted due to space limitations. Please contact author for hard copy of manuscript. HA-SL-C1 crack base metal (BM) Figure 7. Cracking and damage manifested in a (a) large, surface defect, (b) small interior defect (40% deep) and (c) large interior defect (95% deep).