Titanium alloys under stress - the results

Part 1

Bulletin recently examined the hot topic of failure within critical titanium parts as a result of delayed or sustained load cracking in Ti-6AI-4V alloys. The scene is set...

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.

Having established the experimental procedure and test method in the first episode of their work, the same authors, Tasos Kostrivas, Mike Gittos and Lee Smith reveal their results and findings in part two.

Results

Microstructure, composition and toughness

The microstructures of the parent materials and weldments are shown in Fig.1 and 2 and the chemical compositions of the alloys were given in Table 1 in Part 1 of this article published in the July/August issue of Bulletin. The materials used were within the desired range of compositions and microstructures, which enabled assessment of SLC phenomena for different material conditions. The aluminium content of parent material M1 (7.42%wt) exceeded the maximum permitted (6.75%wt) for ASTM Grade 5, but this was deemed acceptable for the purpose of establishing the influence of high Al content. A summary of parent material product form, microstructure and toughness is given in Table 3. Toughness is shown as K _Jmax , calculated from J at maximum load.

Table 3 Summary of microstructure

Materials M1 and M2 were mill-annealed forged parts, nominally to ASTM Grade 5. M1 exhibited a coarse microstructure containing a mixture of large, equiaxed and elongated alpha grains and isolated grain boundary beta ( Fig.1a). M2, however, exhibited a mixture of fine equiaxed alpha grains and alpha/beta colonies ( Fig.1b). The oxygen content was quite high (0.18%), but still within specification.

Material M3 was a rotary-pierced, beta-annealed pipe, to ASTM Grade 23. The prior beta grain size was quite coarse and contained a fully transformed, Widmanstätten alpha/beta microstructure ( Fig.1c). The aluminium and oxygen contents were low (6.03 and 0.095%, respectively). Not surprisingly this material exhibited the greatest toughness.

Materials M4 and M5 were both mill-annealed sheet, to ASTM Grade 5. The microstructure of M4 comprised a mixture of equiaxed alpha grains and globular grain boundary beta (Fig.1d). The oxygen content was quite high (0.17%), but still within specification. This combination of microstructure and chemistry gave the worst toughness. The microstructure of M5 comprised a mixture of equiaxed, alpha grains and alpha/beta colonies (Fig.1e). The oxygen content was quite high (0.16%), but still within specification. The microstructure of the weld metals of the MIG weld, M4-MIG, and the plasma arc weld, M5-PAW, were similar, comprising transformed-beta products in coarse, prior beta grains ( Fig.2).

The oxygen content of the MIG weld metal was slightly lower than that of the parent metal, whilst that of the plasma arc weld was significantly greater, but, as expected for titanium alloy weldments, this had little visible influence on microstructure. The microstructures of the HAZs of MIG weld M4-MIG and plasma arc weld M5-PAW were also similar, comprising transformed-beta products, with increasing prior beta grain size closest to the fusion boundary ( Fig.2).

The form of the clip gauge displacement versus time traces is shown in Fig.3, for a specimen from the MIG weld metal. This graph shows, schematically, the stages of the test and the difference between plastic deformation due to primary sustained load strain ( Fig.3a) and crack growth due to SLC ( Fig.3b). Micrographs of the fracture faces of one of the specimens are shown in Fig.4.

The deduced values of K _init and K _th are listed in Table 4 for each specimen (both shown as ratios against K _max , but K _th is also shown as an absolute value). Summaries of K _init /K _max , K _th /K _max and K _th are presented in Table 5, for the parent metals.

Table 4 Summary of step-loading testing of MIG and PAW weld specimens in Ti-6Al-4V plate

It can be seen that mill-annealed, forged Ti-6Al-4V alloys with high aluminium (M1) or oxygen (M2) content exhibited values of K _init that were 34-37% lower than K _max during step-loading tests ( Table 5). The sustained load performance of the beta-annealed ELI Ti-6Al-4V pipe (M3) was better, showing a value of K _init that was 27% lower than K _max and a K _th value of 67.1 MPa √m.

Table 5 Summary of step-loading test data for parent materials

The mill-annealed Ti-6Al-4V plate with globular grain boundary beta phase (material M4) exhibited the worst SLC performance in both as-welded and PWHT conditions. A K _th value of 25.6 MPa √m was calculated, which corresponds to a 30% decrease compared with K _max .

The fusion and heat affected zones of the MIG weld (in material M4) exhibited better SLC performance than the parent metal, both in as-welded and PWHT conditions. The minimum K _th values for the weld and HAZ were 50.8MPa √m and 57.3MPa √m., in the as-welded condition, and 60.2MPa √m. and 65.0MPa √m. in the PWHT condition, respectively. The increase in SLC threshold values in the HAZ and weld metal after PWHT (650°C, 1h) signifies the positive effect of heat treatment at a typical stress-relief temperature.

The SLC performance of the plasma weld metal was lower than that of the parent material (material M5) in the as-welded condition. The minimum K _th value for the weld metal was 40.0MPa √m., primarily as a consequence of higher weld metal oxygen content.

Sustained load testing (single loading)

Samples from the plasma arc weld, notched at the weld metal centreline, were loaded to a single load in static loading rigs, either failed soon after loading or remained intact with no crack extension for a period exceeding 720 hours. Both the initial K, K _test , and a value of K, K _max , at which SLC stopped and tearing began, were calculated. K _max was determined in two ways, depending on whether or not the specimens had failed. For the specimens which failed, an extended crack length, measured on broken samples using SEM fractography to determine the amount of SLC, was used. One test specimen, failed at K=36.7MPa √m., and three others failed at K values of 40.7, 44.4 and 47.4MPa √m., respectively. The other five specimens did not fail, with K values equal to or greater than 41.2MPa √m. Fractographic examination of the specimens, which failed shortly after loading, showed evidence of mixed brittle and ductile fracture morphology, within a few hundred microns of the fatigue pre-crack tip. There were some regions of fluted fracture features that have been associated with either SLC or slow strain rate failure.

Discussion

Parent metals

The SLC test results from the parent materials followed the trends predicted by the present authors previously with estimated threshold stress intensity factors for SLC being greatest for beta-annealed parent metal microstructures with low interstitial element contents. In absolute terms, the mill-annealed parent metal performed worse than the beta-annealed pipe, but of the mill-annealed parent metals, the two forgings appeared to perform better than sheet material, despite a chemical composition and microstructure that might be anticipated to increase susceptibility. This trend may be due to greater basal plane texture in the sheet, resulting in greater susceptibility to SLC and reduced toughness. The parent metals exhibited K _th values of between 25.6 and 67.1MPa √m., with increasing values in the sequence material M4, M5, M1, M2, M3.

It is clear that a wide range of microstructure, toughness and SLC performance is possible for material supplied in the 'mill-annealed' condition. Unfortunately, whilst many aerospace Ti-6Al-4V specifications stipulate microstructure, such guidance is not used typically for more general engineering applications. Thus, end users buying 'ASTM Grade 5' titanium alloy could be supplied with material that could readily have both an unfavourable microstructure and disadvantageous chemistry.

Weldments

The results from the present study suggest that, for poor toughness parent metal, the weld zone (HAZ and weld metal) tend to exhibit better SLC performance than the parent metal as a consequence of their fully-transformed microstructure. A caveat should be placed on this assessment, namely that no contamination of the weld metal occurs during welding. The as-welded MIG weld metal and HAZ exhibited K _th >45MPa √m. compared with the parent metals K _th =25.6MPa √m.

Postweld heat treatment (stress-relief) showed a consistent trend of increasing K _th to a value greater than the equivalent as-welded values. After PWHT, the minimum K _th in the weld zone was 56.3MPa √m. It is believed that this increase in K _th may be due to the effect of microstructural effects, but the exact mechanism is not clear. It is speculated that partial transformation of grain boundary beta to alpha, as well as tempering of pre-existing alpha, results in better SLC performance. Even so, this is an important result, since it indicates that stress-relief should not prove harmful to SLC performance (provided that no surface oxidation or alpha case results from poorly-controlled heat treatment).

The plasma arc weld exhibited lower K _th than the parent metal, but it is noted that the oxygen content of the keyhole plasma weld (0.19%) was close to the maximum permissible for ASTM Grade 5 (0.2%) and significantly greater than that in the neighbouring parent metal (0.16%). The weld metal exhibited a beta-transformed microstructure that partially offset the influence of the oxygen pick-up, but the higher oxygen content was the main factor in the degraded weld metal performance and highlights the need to ensure good gas shielding during welding.

Test method

The lower value of K _ISLC estimated for plasma arc weld metal by single-load testing compared to step-load testing, could be taken to indicate that the single-loading test is more conservative. However, insufficient tests have been made to determine the relative degrees of scatter inherent in the two test methods. Regardless, it is clear that more than two specimens should be tested for the step loading method. Important differences between the test methods include the overall rates of loading and the possibility of crack blunting in the step-loading test. However, the experimental observations made in this work suggested that blunting was not apparent in either fracture toughness or SLC tests, carried out on a range of Ti-6Al-4V materials.

More testing is required in order to state definitive test methods confidently, including the number of step-loading tests that should be conducted to define KISLC. From the current work, a summary of preliminary K _ISLC values is presented in Table 6 for the materials investigated.

Table 6 K _ISLC, based on step-loading tests

Practical implications

If toughness and SLC performance are important in a particular design, supplementary specification requirements should state that parent material should not exhibit a microstructure comprising equiaxed alpha grains and globular grain boundary beta phase. Aluminium content should also be close to 6% and oxygen content less than or equal to 0.17%. This compares with maxima of 6.75 and 0.2%, respectively, for ASTM grade 5. In the present investigation, material that met these requirements (material M2, 3 and 5) exhibited K _th greater than 45MPa √m.

When SLC testing is carried out for a particular application, the thickness of the test specimens should be comparable to the thickness employed. During fabrication of Ti-6Al-4V, low oxygen content consumables should be used and care should be taken to avoid oxygen pick-up. In order to minimise oxygen pick-up, procedure qualification and welder qualification test coupons should be analysed for oxygen in addition to applying more conventional weld colour assessments. Stress-relief heat treatment appears not to degrade SLC performance, provided no oxygen contamination occurs as a result. Indeed, lowered residual stresses should make SLC crack initiation or growth less likely.

Conclusions

For applications with high static stresses, supplementary material specifications should be employed to ensure that microstructure and chemistry are sufficiently controlled to achieve good resistance to sustained load cracking. Al and O contents should be close to 6% and lower than 0.17%, respectively. Microstructure should be either beta transformed, or mixed equiaxed alpha and transformed beta for mill annealed material.

Weldments (weld metal and HAZ) in mill annealed material perform as well as the parent metal provided oxygen entrainment is avoided during welding.

Post weld heat treatment, simulating typical stress relief, at 650°C for one hour proved slightly beneficial for resistance to sustained load cracking in the present work. In practice, the reduction of residual stress will also prove beneficial to the prevention of SLC.

Parent material with a beta-annealed (fully-transformed) microstructure and low aluminium and oxygen contents proved to exhibit the greatest KISLC value, 67.1MPa √m. Parent material with an equiaxed alpha and globular grain boundary beta microstructure and 0.17% oxygen had the worst SLC performance, KISLC =25.6MPa √m.

Step-loading has proved to be a useful way of studying SLC. However, the technique needs further development for the generation of quantitative data, before it can be recommended as an alternative to single load testing.

Acknowledgements

The work was funded by Industrial Members of TWI, as part of the Core Research Programme. Particular thanks go to Permascand AB for contributing the Ti-6Al-4V plasma weldment.