The fatigue crack growth resistance of α–β titanium alloys can be altered by microstructural modification. During welding, the fusion zone microstructure depends on cooling rate. In the present work, the alloy Ti-6Al-4V was welded over a range of heat inputs, using electron beam and gas tungsten arc welding. The weld microstructure varied from predominantly martensitic under rapid cooling conditions to a mixture of martensite and diffusional products on slower cooling. Post-weld heat treatment resulted in a basketweave α–β aggregate that coarsened with temperature and time. In all welded and heat treated conditions, the fusion zone exhibited a fatigue crack growth resistance superior to that of the base material, which was in part attributed to the lamellar microstructure of the fusion zone. Welding residual stresses also played a beneficial role in the as welded condition. Post-weld heat treatment eliminated the advantage resulting from the welding stresses but not that as a result of microstructure.

Fusion zone microstructure and fatigue crack growth behaviour in Ti-6Al-4V alloy weldments

Fatigue crack growth in an tl + titanium alloy (TA6V) with three microstructural conditions is studied in high vacuum at 300°C. The faster growth rates are observed in the material with a low content of primary c( phase and a fine equiaxed microstructure. Near the threshold regime, a slow crystallographic Stage I-like propagation is observed in the three microstructural types where slip occurs along a single-plane system which develops within the CL grains. These crystallographic facets are identified using an electron backscattering pattern (EBSP) technique and were found to lie on basal planes.

Electron backscattering pattern identification of surface morphology of fatigue cracks in TA6V

Fatigue Crack Thresholds

{beta}-21S titanium alloy sheet fatigue crack growth behavior was investigated at 25 C and 175 C under constant amplitude (R = 0.1 and 0.5) and miniTWIST flight spectra. Based upon nominal {Delta}K values, constant amplitude fatigue crack growth behavior at 175 C was either similar to (R = 0.1), or slightly better than (R = 0.5) 25 C. With crack closure taken into account, the fatigue crack growth curves at 175 C, plotted as a function of K{sub eff}, were shifted to the left of the fatigue crack growth curves at 25 C at near threshold values. Under flight spectra conditions, fatigue crack growth life at 175 C was 40 to 90% longer than at 25 C. Flight spectra life calculations using NASA/FLAGRO based upon constant amplitude fatigue crack growth data, were primarily conservative but in good agreement with experimental data. Fatigue crack growth was transgranular with crystalline facets and striations that were evident at higher constant amplitude fatigue crack growth rates and with the miniTWIST spectra. Striations were observed to a limited extent at threshold and near threshold conditions at 25 C, but not at 175 C. Based upon desirable constant and variable amplitude fatigue crack growth and fatigue/fracturemore » crack morphology, this {beta}-21S sheet alloy appears to be an acceptable material for damage tolerant aerospace situations between 25 C and 175 C.« less

Fatigue Crack Growth of β-21S Titanium Alloy Under Constant Amplitude and miniTWIST Flight Spectra at 25°C and 175°C

Fatigue crack growth of β-21S and Ti-62222 in sheet form was investigated under constant and miniTWIST flight spectra loading conditions at 25 and 175 °C Variable amplitude results were compared with life calculations performed using NASA/FLAGRO software and constant amplitude fatigue crack growth results Single tensile overloads under constant ΔK were performed to evaluate load interaction effects Constant amplitude results showed that fatigue crack growth resistance was slightly better for Ti-62222 than β-21S at 25 and 175 °C The presence of crack closure under various conditions caused moderate shifts in the fatigue crack growth data Under miniTWIST flight spectra loading, Ti-62222 exhibited a greater extension in life in comparison to the β-21S at elevated temperature, consistent with the NASA/FLAGRO calculations This was also consistent with the single tensile overloads where 25 °C tests were comparable for both materials, while at 175 °C, delay cycles were greater by a factor of almost three for Ti-62222 Extensive secondary cracking in Ti-62222 at elevated temperature accounted for the extended fatigue lives

W. Berata

Papers

Fatigue crack growth behaviour of Ti6Al4V at elevated temperature in high vacuum