Low cycle fatigue property of laser forming repaired TC4 forgings

Surface-repaired samples of TC4 forgings were prepared by Laser Forming Repair (LFR), and the microstructure, tensile property and low cycle fatigue property of the repaired samples were investigated. Microstructure of laser-repaired zone (LRZ) is typical Widmanstaten structure consisting of initial coarse columnar β grains, within which are fine acicular α and α+β basket weave structure, and there is a good metallurgical bonding between LRZ and the forging substrate. The heat affected zone (HAZ) presents a continuous change from the forging substrate microstructure to the repaired zone microstructure. The repaired samples were divided into two groups, and then they were treated by relief annealing (RA) and RA + shot peening (SP) respectively and their low cycle fatigue curves were obtained. It can be found that the fatigue life of the two groups is higher than that of the die forging in the area of low strain, but lower than that in the area of high strain. This is because the strength of the repaired sample is higher than that of forging, while the plasticity is poorer than the forging's. The strength play a major role in fatigue life when samples are suffering a low strain, and the plasticity decides the fatigue life at high strain. It is shown that the fatigue life is improved by SP both at low strain and high strain. Moreover, the fatigue life of the LFRed samples with SP is close to the level of the forging at high strain, and longer than that of the forging at low strain by 1 order of magnitude.