Titanium alloys under stress - more than just a specification

Failure of critical titanium parts, including some offshore components, has drawn attention to delayed cracking in Ti-6Al-4V alloys, but, given good design and alloy variant selection, such failures are avoidable.

Tasos Kostrivas graduated with an MSc and PhD from two of the world's leading academic centres for welding metallurgy, Cranfield University and Ohio State University. Since joining TWI's CRA, Surfacing and Analysis section he has worked on various aspects of non-ferrous metallurgy, leading failure investigations and research programmes. His recent work on titanium includes high temperature creep-fatigue interactions, sustained load cracking and MIG welding.

Mike Gittos is TWI's leading consultant for non-ferrous metallurgy. With more than thirty years service at TWI following graduation from Manchester University, Mike has accumulated significant experience of a wide range of industrial metallurgical issues. Mike's work covers all industrial sectors from aerospace superalloys to microelectronic solders and his successes have been acknowledged internationally with honours from the American Welding Society and the Institute of Materials, Minerals and Mining.

Lee Smith manages TWI's CRA, Surfacing and Analysis section, covering all aspects of stainless steel and non-ferrous metal, metallurgy and corrosion, surfacing and material analysis. Lee has ten years experience at TWI, following graduation as a metallurgist from Birmingham University and post doctorate work on non-ferrous alloys. In addition to managing the section, Lee leads some of TWI's more high profile consulting projects.

Delayed cracking, or sustained load cracking (SLC), can occur at low to moderate temperature (approximately -50 to 200°C, depending on the titanium alloy and condition. Appropriate testing methods are required to generate stress intensity threshold values (K _ISLC ) that can be incorporated into the design of titanium structures and recommendations are needed on the optimum chemistry and microstructure for greatest resistance. As Tasos Kostrivas, Lee Smith and Mike Gittos report in the present work threshold stress intensity factor (K _ISLC ) data were generated for Ti-6Al-4V alloy sheet, forgings, pipe and weldments using two different rising stress intensity factor test methods.

It is concluded that material with a beta-annealed microstructure and low oxygen content ( ie extra-low interstitial material such as ASTM Grades 23 and 29), has high resistance to SLC and that weld metal and transformed heat-affected zone also perform well, before and after postweld heat treatment, provided interstitial element pick-up during welding is prevented. Purchasing material in a general 'mill annealed' condition is not recommended for high stress service without specifying acceptable microstructures.

Titanium alloys are susceptible to time-dependent failure (sustained load cracking, SLC) if stresses or stress intensity factors exceed certain threshold values. Failures in critical titanium alloy components such as aeroengine fan blades and certain offshore components have drawn attention to sustained load phenomena in these materials, be they 'cold creep', sustained load cracking, dwell fatigue or ripple fatigue. Experience indicates that, for Ti-6Al-4V alloys, susceptibility to the most damaging sustained load phenomena is likely to be greatest for the following situations:

• Temperatures between 0 and 25°C
• Microstructures typical of mill-annealed products
• Materials with high aluminium and/or oxygen contents
• Products with some degree of crystallographic texture
• Materials with high hydrogen content (typically >150ppm), ie outside most material specifications

Data is scarce and is most needed to facilitate design for the avoidance of SLC in structures made from the most common alloy, Ti-6Al-4V. Any study must consider the variability inherent in Ti-6Al-4V that can result from differences in chemistry, heat treatment and processing route. Weldments are also clearly a concern, due to the potential for greater stress intensities at weld flaws and the stress concentrations presented by the weld toes. Thus, for the present work, material and weldments from selected forgings, plate and pipe were investigated. Standard (ASTM Grade 5) and extra-low interstitial (ELI, ASTM Grade 23) materials were examined with examples of typical mill-annealed and beta-annealed batches being included.

One of the difficulties in determining the threshold stress intensity factor, below which SLC will not occur, K _ISLC , is in the choice of test method. Two different strategies of testing are available, based on either rising or falling K. In a rising K test, a fracture mechanics specimen is held under a constant load. If the crack grows, then the K increases, resulting in accelerated crack growth. This is representative of components that experience a given load as opposed to a fixed displacement. Falling K tests are based on fracture mechanics specimens held under a fixed displacement. This type of test suffers from several problems, including that relaxation as a consequence of sustained load strain, will result in decreased K over time without necessarily resulting in cracking. Thus, only rising K testing was performed in the present work, although it is acknowledged that falling K testing might be appropriate for displacement-controlled situations. Two types of test were performed, one based on applying incrementally increasing K to a single specimen until failure. The other used a single load application per specimen. Whilst the former requires fewer tests, it may result in crack blunting due to sustained load strain before the specimen K exceeds K _ISLC , thereby giving misleading results. On this basis, it was considered appropriate to examine both methods.

The present article therefore, examines the methods used in a programme of work aimed at generating threshold fracture stress intensity factors (K _ISLC ) for parent metal, metal inert gas (MIG) and keyhole plasma arc weldments in Ti-6Al-4V alloys, using two different rising K test methods.

Experimental procedure

Materials

Various batches of Ti-6Al-4V material were used as shown in Table 1. These included forgings (M1 and M2), pipe (M3) and sheet (M4 and M5) parent materials, and a MIG weld (M4-MIG) in sheet M4 and a keyhole plasma weld (M5-PAW) in sheet M5. Welding procedures are not shown, but conformed with conventional practice for titanium welds. Some samples of the MIG weldment were postweld heat treated at 650°C for one hour.

Table 1 Material composition compared with relevant standards

Nomenclature

Test method

Full thickness, Bx2B (where B corresponds to through-wall thickness) single edge notched-bend (SENB) fracture mechanics specimens were manufactured and prepared, according to BS 7448:Part 1:1991 for parent metal studies and BS 7448:Part 2:1997, for weld metal and HAZ studies. Prior to notching in either the HAZ or at the weld metal centreline, each specimen was etched in an aqueous solution of 10%HNO3 + 2%HF to reveal the different regions of the weldment, and enable accurate notch positioning in the through-wall thickness direction. Fracture toughness tests were performed at 4°C as shown in Table 2.

Table 2 Matrix of tests

Two types of SLC testing were performed. Both were based on conventional pre-cracked SENB specimens, prepared as above. Estimates were made of the crack depth, from the measured crack length on the sides of the specimens and multiplying the average of these two values by 1.08, to allow for the effect of crack bowing. Loads were calculated that would achieve appropriate approximate stress intensity factors for the tests.

One test method (step-loading) involved loading the samples in a servo-hydraulic test machine and raising the load in incremental steps with dwells at set load points. Tests started by loading samples to a load resulting in approximately 40% of K at maximum load (as measured from the fracture toughness tests) and continued with step-load increments corresponding to 5% of this value. The displacement of a clip gauge attached across the crack mouth was monitored and the test was terminated once tearing ensued. These tests were performed at 4°C. The test temperature was selected on the basis that the risk of SLC is greatest close to this temperature. Post-test assessments included fractographic examination of the specimens to determine the depth of any SLC, and examination of the clip gauge displacement-time trace, for evidence of the onset of SLC. These data were used to calculate a value for the stress intensity factor at which SLC cracking initiated, K _init , and an estimated value for K _max , determined from the load at the onset of tearing and the combined length of the fatigue pre-crack and the SLC crack. Initiation of SLC in step-loaded samples was determined by observation of the slope of the clip gauge displacement versus time plots, being careful to differentiate between initial primary sustained load strain and SLC (an example is given in Part II in the results section). Crack initiation was often noted in the loading step immediately prior to that which resulted in failure. A threshold K value, Kth, for which no SLC was observed, was calculated from the load prior to that at which SLC was first noted.

The second test method (single-load testing) involved loading a series of specimens to set loads in static loading frames at 4°C. Single-load testing was carried out in cantilever arm, dead weight loading frames with a calibrated loading ratio. Each specimen was loaded under bending to a specific value of K, chosen after consideration of the results of the step-loaded tests. Discrepancies between target and actual K values occurred because of the uncertainty in the shape of the fatigue pre-crack. The specimens were carefully loaded over a period of one minute in an attempt to achieve smooth load transition and avoid dynamic loading effects that might blunt the crack-tip or result in premature sample failure. Testing was continued either until failure, noting the time to failure, or until a period of 720 hours had elapsed. On completion of testing, all specimens were broken open and the fracture faces examined using light and scanning electron microscopy, allowing post-fracture measurements of the fatigue pre-crack dimensions and the depth of any SLC, to permit calculation of the appropriate K values. These data were used to estimate a value for the stress intensity factor below which no SLC was observed, K _ISLC .

The matrix of tests is shown in Table 2. In most cases, duplicate tests were performed and an average taken, except for the single load SLC tests, for which nine tests were performed.

End of Part I - in Part II Kostrivas, Smith and Gittos reveal the results of their studies and discuss the implications.