| As the proportion of China's nuclear power,ensuring the safe operation of nuclear power is crucial to the further development of China's nuclear power industry.Nuclear power emergency diesel engine is one of the key units to guarantee the safe operation of nuclear power.The traditional processing methods of nuclear power emergency diesel engine crankshaft are forging and casting.It's need long manufacturing cycle and dependence on foreign imports.It is urgent to find a more efficient and precise manufacturing method.As a new manufacturing technology,laser melting deposition technology is widely used in nuclear power,aerospace and other fields with high precision,high efficiency and high utilization rate.The repeated heat input in the laser melting deposition process will produce large thermal stress in the deposition layer,which will cause deformation of the substrate.How to control the large deformation in the laser melting deposition process is a big challenge at present.In this paper,12 CrNi2 alloy steel was used as experimental material to study the deformation of the substrate during laser melting deposition by using laser displacement sensor.The process parameters were optimized to obtain specimens with small amount of deformation and high relarive density.The microstructure,relative density and mechanical properties of the deposited specimens were analyzed.In order to evaluate whether the laser deposition sample met the requirements of emergency diesel engine camshaft,the fatigue properties of the deposited samples were evaluated by rotating bending fatigue test and fatigue crack growth test.The main research contents and conclusions are as follows:(1)The deformation measurement system was designed and built.The influence of process parameters on the substrate deformation were studied.In order to obtain the samples with small deformation and high relative density,the process parameters were optimized.The scanning speed,powder feed speed and scanning strategy had great influence on the substrate deformation,while the laser power had little influence on the substrate deformation.The optimized process parameters were laser power 2700 W,scanning speed 280mm/s,powder feed rate 13.0g/min,and overlap rate 40%.(2)The influences of different scanning strategies on the microstructure and mechanical properties of laser melting deposited specimens were studied.The microstructure of the asdeposited sample was mainly bainite.The tensile strength and elongation of single direction scanning were 1011 MPa and 9.64%.The tensile strength and elongation of cross direction scanning were 1038 MPa and 7.42%.The sample's relative density of single direction scanning and cross direction scanning were 99.0% and 98.5%,respectively.(3)The influences of different scanning strategies on fatigue strength of laser melting deposited specimens were investigated.The fatigue strength of single direction scanning and cross direction scanning samples were 320 MPa and 200 MPa,respectively.The main reason for the decrease of fatigue strength of cross direction scanning specimens was that the higher porosity makes the cracks easier to crack initiation and propagation.Generally,the fatigue cracks originated near the surface and internal defects of the specimen,and then the crack spread radially to the surroundings until fracture.(4)The influences of different scanning strategies and notch orientation on the crack growth rate of laser deposited specimens were investigated.The results showed that the crack propagation rate of the vertically oriented sample was higher than that of the horizontally oriented sample with the same scanning strategy.When the orientation of the samples were the same,the complexity of the internal dendrite orientation of the cross direction scanning sample made it have a lower crack growth rate in the low stress intensity factor(?K)region,and more internal defects made it have a higher crack growth rate in the high stress intensity factor(?K)region. |
PDF Full Text Request