| Most of aero-engines are manufactured by casting titanium alloy.At present,shielded metal-arc welding is the most popular method for repairing the casting defects in China,whose quality is difficult to control because of the large deformation and high potentiality of cracks.However,laser cladding has been used to repair the similar products in developed countries.Owing to the low heat input,small deformation and high degree of automation,laser cladding repair technique has a broad prospect in the manufacture of titanium alloy components.In this study,the TC4 powder was used as the added material to repair the round hole defects in the ZTC4 titanium alloy plate with the thickness of 5.5mm by the method of the coaxial powder feeding laser melting deposition.Through the combination of experiment and numerical simulation,the laser repair parameter and scanning path optimization were developed and the suitable repair and heat treatment parameters were achieved.At the same time,the test and evaluation of mechanical properties of the repair zone and the heat affected zone were finished,which brought experimental and theoretical support to the application of laser repair technology.In this paper,the laser repair parameters were optimized by single pass cladding and numerical simulation at first.In order to compare the effects of different heat inputs on the repair quality and thermal stress,a large spot diameter of ?2.5mm(defocusing amount-15mm)and a smaller spot diameter of ?1.0mm(defocusing amount-5mm)were developed by adjusting the defocusing amount.The influence of laser power,scanning speed and powder feeding speed on the quality of cladding forming was studied and the parameter ranges of two heat input modes were developed.Subsequently,it was found that when the small spot parameter was used in the laser repair process,the heat accumulation was higher due to more repair layers and tracks.Compared with the large spot parameter,the Von Mises stress increased by 2.3% and the final deformation increased by 30.0%.Through the numerical simulation of residual stress within different scanning paths,the optimization of scanning path was completed.When the defects were repaired by “edge banding”,the residual stress was lower.The average value reduced by 5.96% and the maximum value reduced by 4.36%.Based on the result of simulation,the scanning paths of “edge banding” by both big and small spot parameters and “rotation repair” were utilized to finish the laser cladding repair of the holes with the depth of 3mm and 5mm.The ZTC4 plates had less deformation and low porosity rate after repairing.However,there were still a small number of non-fusion defects between layers when using the big spot parameter.When the small spot parameter was utilized,no defects were detected in the specimens,but the repair efficiency was considerably lower.Subsequently,the analysis of microstructure and mechanical properties of the laser repair specimens and the effect of multiple laser repairs were carried out.The specimens repaired by the small spot parameter had lower tensile strength and elongation and they broke at the heat affected zone,while the ones repaired by the big spot parameter had higher tensile strength and elongation and broke at the base metal.This is due to when the small spot and “edge banding” are utilized in laser cladding repair,the large residual stress at the heat affected zone will contribute to microcracks.After three times of laser repair,the width and hardness of the heat affected zone increased by 127.3% and 4.0%.Finally,the repaired specimens were heat treated by both annealing and solid solution+aging.The comprehensive mechanical properties of the repaired samples before and after heat treatments wouldn't change obviously.However,the maximum value of Von Mises stress decreased by 39.4% and the average value decreased by 22.9%,which proved that annealing after laser repair had a significant influence to the residual stress. |
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