| The laser melting deposition manufacturing technology has high efficiency and good versatility.Complex structural parts can be prepared directly without mold.The printed CoCrNi alloy has high tensile strength and plasticity without heat treatment.High temperature gradient and cooling rate are formed due to the rapid melting and solidification of the printed powder by the high energy laser beam in the printing process.In the process of track-by-track deposition and layer-by-layer printing,the deposited part can undergo one or two remelting and be affected by multiple thermal cycles.The temperature gradient and cooling rate of different positions in the deposited layer can be controlled by comprehensive control of process parameters,so as to control the micro-morphology,grain size and the performance of deposited samples.Therefore,multiple groups of process parameters were selected to prepare single-track and single-layer samples,and the influences of printing process parameters,thermal cycling and thermal accumulation on the molten pool morphology,defects and microstructure of the deposition were comprehensively analyzed.The main work is as follows:(1)The effects of laser power,scanning speed,powder feeding rate and laser beam diameter on the single-track morphology of deposition were investigated.With the increase of laser power,the track width,height,depth,dilution rate and construction rate all increase,and the thickness to width ratio is basically unchanged.With the increase of scanning speed,the track height,width,depth and thick-width ratio decreased,and the dilution rate gradually increased,but the construction rate had little effect.With the increase of powder feeding rate,the change of track width and depth is not obvious,the track height and construction rate increase gradually,and the dilution rate decreases gradually.With the increase of laser beam diameter,the track height and width are increased,the track depth is basically unchanged,the dilution rate decreases gradually,and the construction rate decreases slightly.(2)The influence of technological parameters on the microstructure of the deposited layer was analyzed.It is found that planar crystals,epitaxial columnar crystals and equiaxed crystals are distributed successively from the bottom of the molten pool.The solidification microstructure subsequently changes from equiaxed grains to elongated and columnar dendrites in the overlapping region when the overlapping rate increases.The distance between primary dendrite arms increases with the increase of laser power and decreases with the increase of scanning speed.At50%overlapping rate,the primary dendrite arm spacing is smaller and the cooling rate is higher.The stable and evenly distributed primary dendrite arm spacing can be obtained.(3)The influence of process parameters on metallurgical defects of deposited tracks is revealed.When the scanning velocity is small,the porosity of deposited layer increases first and then decreases with the increase of laser power,while the effect of scanning velocity is opposite.With the increase of powder feeding rate,the porosity gradually increases,and the faster the scanning rate,the faster the porosity growth rate.The increase of laser beam diameter makes the pore first decrease and then increase.When the laser energy density is 100 J/mm~2,that is,the laser power is 2200 W,the scanning speed is 11 mm/s,the powder feeding rate is 19.0 g/min,and the laser beam diameter is 2.5 mm,the minimum porosity of about 0.0133%can be obtained,and the forming quality is better under this parameter combination.This work provides an important basis for obtaining the best process window,improving the printing efficiency and preparing high quality multi-component alloys. |
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