Study On The Process Of Gradient Microstructure In A Nickel-based Alloy By Laser Additive Manufacturing

Functionally graded component realizes its graded properties by spatial arrangement of microstructure/grain morphology with the same material,and it has important application prospects in many fields,such as aerospace,defense,military and biological medicine.Laser additive manufacturing(LAM)technology has unique advantages on fabricating of functional component due to its layer-by-layer processing style.However,it is very difficult to fabricate a component with a fully controlled microstructure and property,because LAM involves the complicated physical process and a lot of processing parameters.At present,there are many problems for LAM fabricated components,such as metallurgical defects and uncontrolled microstructures,which seriously restrict the wide application of such functionally gradient components.In this thesis,two processing modes,continuous-wave(CW)LAM and pulsed-wave(PW)LAM,were used to obtain the gradient microstructure.Inconel 718 powder was selected as the forming material.Effects of process parameters on the morphology,microstructure and hardness of deposits were studied.The relationship among process,microstructure and property was established.This work aims to explore the processing method for fabrication of functionally graded component and to provide the experimental basis for fabrication of functionally graded component using LAM.The detailed research contents and results are as follows:The effects of parameters such as laser power,scanning speed,feed rate,pulse frequency,and duty cycle on the macroscopic morphology,microstructure and mechanical properties of components were studied using orthogonal tests and single-factor tests.The results show that the scanning speed and the laser power are the main factors affecting the macroscopic morphology of the deposit.The effect of the amount of uplift on the macroscopic morphology of the cladding is relatively small.This is mainly related to the appearance of the molten pool and the mass of the powder entering the bath in unit time.In the pulsed laser mode,the pulse frequency shows a great influence on the morphology of the microstructure,which is closely related to the thermal process of the molten pool.The high-speed camera was used to photograph the pulsed laser cladding process in real time.The effects of pulsed laser power,scanning speed,pulse frequency,duty cycle and other parameters on the melting pool overlap rate were analyzed.The results show that the pulse frequency and scanning speed have the greatest influence on the melting rate of the molten pool,while the laser power has little effect on the melting pool overlap rate.This is because the scanning speed and the pulse frequency greatly affect the movement distance and the dispersion of the molten pool,and the laser power affects the size of the individual molten pool,and therefore has little effect on the lap rate of the molten pool.In order to make the cladding layer continuous?smooth,and obtain a good cladding morphology,the lap rate of the molten pool needs to be between 60%and 85%through the pulse process parameters.Different microstructure was prepared using two laser processing modes with different process parameters.The component with a gradient microstructure was fabricated.The results show that both laser processing mode and process parameter have a great influence on the solidification microstructure.The different morphology was obtained and the component with a gradient microstructure was fabricated by the combined CW-PW processing mode.The graded bulk component and the graded circle component were fabricated by CW-PW processing mode.The results show that the functionally graded component can be fabricated using CW-PW processing modes by adjusting process parameters.