| Traditional manufacturing processes of three-dimensional unshrouded impeller have many disadvantages,such as high difficulties,low material efficiency,and long processing cycle.In addition,in the situation of fabricating the blade with high torsion angle and narrow channels,great challenge and difficulty could be faced for fabricating the impeller.As a new additive manufacturing process,laser materials deposition(LMD),in which the complex three-dimensional structures can be simplified to two-dimensional cases to fabricate near fully-dense complex part with superior properties,high material efficiency,and short process cycle,has great advantages and promising development prospect in the fabrication,development,and repair process of three-dimensional unshrouded impeller.Owing to the large overhang angle of the three-dimensional blade and parallel deposition way of LMD process,stair effect and collision would happen.Furthermore,the inconsistency of surface height and overhang angles at different regions of the blade,which contributes to the discrepancy of scanning pattern,energy input,and cooling rate of molten pool,could lead to the different microstructure characteristics and properties and increase the difficulty of deposition process control.However,there are few researches about that.In this study,based on the blade’s features of three-dimensional flow unshrouded impeller,the research work started from the investigation of effects of process parameters on geometrical characteristics of clad and analyzed the influencing factors of depositon of the vertical thin-walled structure,oblique thin-walled structure,and overhang structure.The effects of LMD process on microstructure were evaluated by combining the interaction principle between laser and material,the hardness and tensile property were tested as well.Evaluating the effects of deposition process on macro-and micro-characteristics of deposited feature structures syntheticlly provides the foundation of fabrication of blade by LMD.The main research contents and results are as follows:(1)Based on the laws of energy and mass conservation,the model was developed to predict the clad width,depth,and height.Based on the mass energy(ratio of laser energy to powder feed rate)and line mass(ratio powder feed rate to scanning speed),the prediction accuracy of model was analyzed.Based on the theory of Marangoni convection,the effects of process parameters(laser energy,powder feed rate,and scanning speed)on the geometrical characteristics(width,depth,and height)were analyzed.The results indicated that the effects of laser energy and powder feed rate were more obvious during the paramaters were low and scanning speed played a dominated role at high parameter conditions.(2)The effects of parameters(laser power,powder feed rate,scanning speed,energy input)on the microstructures and properties of vertical thin-walled part were investigated.The results indicated that grain size increased initially and then decreased and the hardness of top region decreased with the increase of parametersd during the mass energy was high.However,grain size increased and hardness increased during the mass energy was low,Higher ultimate tensile strength(UTS)values and reduced ductility values were obtained in comparison with that produced by the traditional processes.The specimens,which were paralleled to scanning direction,presented higher UTS values than the specimens,which were perpendicular to scanning direction.The UTS values of the specimens paralleled to scanning speed increase gradually from the bottom to the external surface of the thin-walled part.Higher values were obtained for the specimens from the zone near the central region of the thin-walled part.The UTS increased with the increase of parameters due to the refined grain.Compared with the part fabricated by continuous laser,the part fabricated by pulse laser exhibited more fine grain,higher hardness,higher UTS,and lower elongation because of lower energy input.In the situation of pulse laser deposition,during the pause time was constant,the grain size increased and hardness decreased with the increase of pulse time.During the duty cycle was constant,the hardness increased with the increase of pulse time.The residuals stress distributions of parts fabricated by different pulse laser parameters were investigated.The residual stress of part fabricated by pulse laser on Z-component presented tensile stress at the edge region and compress stress at the central region but opposite trend happens to the residual stress of part fabricated by continuous laser.The stress of part fabricated by pulse laser on Y-component presents a periodic variation between tensile stress and compress stress in the Y-direction of the part fabricated by pulse laser.However,the stress of part fabricated by continuous laser on Y-component presented tensile stress at the edge region and compress stress at the central region.(3)The influence mechanisms of defocusing distance and z-increment,as well as the effects of deposition strategies on vertical thin-walled part fabricated by LMD were investigated.The deposition height decreased with the increase of absolutely value of defocusing distance.In the LMD process,the negative feedback mechanism,in which the deposition height can be equal to the z-increment with the increase of layer number during the z-increment is appropriate,contributed to the fabrication of part.Based on the negative feedback mechanism,the 3D deposition strategy,which means the different z-increment at different position,was used to fabricate the oblique thin-walled part.The hardness distribution of oblique thin-walled part fabricated by parallel or 3D deposition strategy was obviously different,but the average hardness was similar.The ultimate tensile strength(UTS)and elongation of part fabricated by parallel deposition strategy were higher than the one fabricated by 3D deposition strategy.The UTS increased and the elongation decreased with the increase of oblique angle of part.The angle between the direction of tensile loading and the direction of slipping surface,which affected the shear stress along the slipping surface,had a significant effect on the tensile strength.With the parallel and 3D deposition way,tensile loading was perpendicular to slipping surface and the srear stress was minimum when the angle between the tensile loading and horizontal direction was 45° and 25°,respectively.Therefore,higher UTS was presented.The angle between tensile loading and slipping surface was minimum and the stear stress along slipping surface was maximum when the angle between the tensile loading and horizontal direction was 90.Therefore,lower UTS was presented(4)The effects of scanning pattern and z-increment on overhang thin-walled part,and the influencing factors of the maximum overhang angle were investigated.The scanning pattern(reciprocating and unidirectional deposition way)and z-increment had significant influence on the quality of deposited overhang.Compared with the unidirectional way,the overhang fabricated by reciprocating way presented high quality with smooth surface and symmetrical sides.By optimizing z-increment,the discrepancy between the designed and the experimental inclined angles of overhangs greatly decreased.However,the maximum inclined angle of deposited overhang was about 32° and the optimizing of z-increment is useless to improve the capability of overhang deposition.The maximum inclined angle of overhang,which was restricted by the relationship between surface tensile,gravity of molten pool,and the impulsion force of gas and powder steam,was affected by the geometry material characteristics.With the help of orthogonal experiments,the different optimal z-increments under different process parameters(laser power,powder feed rate,and scanning speed)were obtained.The optimal z-increment is proportional to(P/Qm)1/4(Qm/V). |
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