Effect of rolling on fatigue crack growth rate of Wire and Arc Additive Manufacture (WAAM) processed Titanium

摘要

Titanium (Ti) alloys have been commonly used in the aerospace industry, notonly because they have a high strength-to-weight ratio (comparing to the steels)but also their satisfactory corrosion resistance. Furthermore, they can beassembled with the carbon fibre composite parts. However, conventionalmanufacturing methods cause high material scrap rate and require lots ofmachining to obtain the final shape and size, which increases both themanufacturing time and cost. In order to improve the efficiency and reduce thecost of Ti parts, Additive Manufacturing (AM) has been developed.Rolled Wire and Arc Additive Manufacturing (rolled WAAM) is one of the AMprocesses. The main characteristics of this technology is the reduced β grainsize to refine the alloy's microstructure. Both the ultimate tensile strength andyield strength of Ti alloy made by rolled WAAM are at least 10% higher thantraditional wrought Ti.This project is to investigate the fatigue crack growth rates of the Ti-6Al-4V builtby rolled WAAM process in both the longitudinal and transverse orientations tostudy the effect of rolling on fatigue crack growth rate of WAAM processed Ti.The project was carried out by testing the fatigue crack growth rates for 4compact tension specimens. The test results of different orientations werecompared with each other, and scatters in fatigue life and fatigue crack growthrate were found. Fatigue crack growth rate is lower in the longitudinalspecimens. The results are also compared with those of the unrolled WAAMspecimens tested in a previous project. It was found that rolling can significantlyimprove the fatigue crack growth behaviour in WAAM processed Ti, and canreduce the difference between the two orientations, i.e. achieving betterisotropic material properties. Recorded scatters may be caused by the processinduced residual stresses, error in measurement, and the test machine loadrange being much higher than the applied loads. More specimens can be testedto validate above observations further.