Research Of Forming Quality And Microstructure Of 12CrNi2 Alloy Steel By Laser Melting Deposition

As one of the new manufacturing methods,laser melting deposition technology,which is contrary to traditional manufacturing methods,can improve the utilization rate of material and manufacture complex-shaped parts,and shorten the product development cycle.However,manufactured parts by laser melting deposition often have defects such as pore,poor fusion,and crack,which greatly limit the application of manufactured part.12CrNi2 alloy steel is used as the material for camshaft of nuclear emergency diesel generators,and it is very meaningful to use a suitable process method to ensure quality of the camshaft by laser melting deposition.This work focuses on the study of the influence of process parameters on quality and performance of 12CrNi2 alloy steel by laser melting deposition.Contents and conclusions of the study are as follows:(1)A single variable method was used to investigate the forming behavior of the single track.The effect of laser power,scanning speed and powder feeding rate on geometry of the single track were investigated separately.With the increase of laser power,the height of the single track didn't change much and the width of the single track increased.With the increase of scanning speed,the height and width of the single track decreased significantly.With the increase of powder feeding rate,the width of the single track increased slightly and the height of the single track increased.(2)Based on the geometry of the single track fabricated by different process parameters,the finite element models were established by ABAQUS software to analyze the thermodynamic characteristics of the melting pool.The accuracy of the numerical simulation result is verified by comparing the numerical simulation result with the experimental melting pool morphology.And the heat source parameters are provided for the numerical simulation of manufactured parts.This work investigates the effect of process parameters on the temperature field of manufactured parts.Because of the heat accumulation effect,the peak temperature and lifetime of melting pool increase,and the cooling rate decrease with the increasing number of track and layer.(3)Effect of process parameters on the surface quality,metallurgical defects,densification behavior,microstructures and microhardness of manufactured parts were investigated in detail,and the reasons were also analyzed with the help of numerical simulation results.The results of the study shows that with the increase of laser power,the number of pore decreases,the density increases and then decreases,and the average of microhardness increases and then decreases.At lower laser power,the manufactured parts are more likely to have fusion defects but finer grains.With the increase of scanning speed,the number of pore increases,the density increases and then decreases,and the average of microhardness increases.At higher scanning speed,the manufactured parts are more likely to have fusion defects,hole defects on the surface but finer grains.With the increase of powder feeding rate,the number of pore decreases and then increases,the density increases and then decreases,and the average of microhardness increases.At lower powder feeding rate,the surface of the manufactured parts has hole defects,and when the powder feeding rate is higher,it has poor fusion defects,but the grains are finer.The manufactured part by laser melting deposition usually experiences a very complex thermal history.This will lead to very poor uniformity of microstructure.For example,coarse,fine,and columnar crystal will appear in a particular region.The matrix of the manufactured parts by different process parameters is ferrite and granular carburite is distributed on the ferrite matrix.Ferrite near the upper region is mainly distributed along the columnar grain boundaries.Ferrite in the middle and near the lower region is distributed in the form of blocks,and the microstructure is mostly granular bainite or pearlite.