Thermodynamics Of Molten Pool,Microstructure And Performance Control Of Selective Laser Melting Of Aluminum Based Materials

Selective laser melting(SLM),as one of the important technologies of the laser additive manufacturing,has the capability to manufacture the complicated structures,such as the irregular curved surface and the inner channel,with the superior mechanical properties and,this processing technology is especially suitable for the integration fabrication of the complex and functional components applied in the aerospace field.At present,most researches are fouced on the density,top/side surface roughness,microstructure and mechanical properties of the SLM-processed aluminium alloys.However,the study of the scientific mechanisms of thermodynamic behavior within the molten pool,the material stacking behavior of the melt,the rearrangement of the reinforcing particles and the microstructure evolution during the SLM of aluminum based alloys is relatively few.The high reflection and high oxygen affinity of aluminum alloys and their composites are selected as research objects and,further research and thorough understanding of the theories and key issues is conducted with the main studies as follows.The melting and solidification physical model describing the SLM of aluminium based alloys was established.The influence of the remelting process and the continuous two layers 90°rotate scan strategy on the molten pool boundary,dimensional accuracy,microstructure feature,tensile properties,microscopic sliding behavior and the fracture mechanism has been studied.The residual pores within the solidified part could be efficiently reduced using the remelting process and the defects,such as the part distortion,delamination and cracks,were significantly eliminated using the continuous two layers90°rotate scan strategy.It was shown that the defects,such as the part distortion,delamination and cracks,were significantly eliminated with the deformation rate less than 1%.The mechanical behavior of the SLM-processed Al Si12 parts can be significantly enhanced with the ultimate tensile strength,yield strength and elongation of 476.3 MPa,315.5 MPa and 6.7%,respectively.The influence of the heat treatment on the molten pool boundary evolution,microstructure evolution and the resultant mechanical properties has been elucidated.It showed that the heat treatment could homogenize the microstructure and soften the molten pool boundary of the SLM-processed parts.Therefore,the tensile force could be delivered efficiently in the homogeneous microstructure and softening boundary of the molten pool in the heat induced parts.The solubility of Si atoms was decreased and rejected into fine Si particles with the formation of the molten pool boundary softening and the homogeneous distribution in the heat induced part.The ultimate tensile strength decreased from 476.8 MPa for the as-fabricated part to 320.5 MPa while,the fracture ductility significantly increased from 7.33%to13.3%as the test specimens were heat treated at 573 K for 2 h.It indicated that the microstructure evolution and tensile properties of the as-fabricated Al Si10 Mg alloy could be tailored through molten pool softening and size and morphology of the Si phase at the suitable heat treatments.A physical model of selective laser melting(SLM)of Ti C/Al Si10Mg coupled with the radiation transfer and thermal diffusion is proposed,taking account of the transition of the heat conduction to the keyhole-mode in the melting and solidification process.The influence of the processing parameters and protective atmosphere on the thermodynamic behavior and surface morphology evolution within the molten pool.For a relatively high LEPUL of 750 J/m,the molten material in the center of the melt pool has a tendency to flow towards the rear part,resulting in the stack of molten material and the attendant formation of a poor surface quality.For an optimized processing condition,LEPUL=500 J/m,a complete spreading of the molten material driven by the surface tension is obtained,leading to the formation of a fine and flat melt pool surface.As Ar protective atmosphere was used,the vector direction of the evaporation material was typically upwards,leading to a uniform recoil pressure forced on the free surface and the formation of fine and flat melt pool surface.The effect of LEPUL on the temperature distribution,the surface tension,thermo-capillary convection,the cooling rate,the arrangement of the reinforcing particles,the surface quality and the resultant microstructure evolution during SLM of Al N/Al Si10Mg composites has been investigated.It showed that the thermo-capillary convection pattern changes from inward flow pattern to outward one,due to the appearance of the oxidation in molten pool.Therefore,the morphology of the top surface undergoes a continuous variation from the balling phenomenon,to the discontinuous tracks and finally to the formation of a flat and dense one.Meanwhile,both the clockwise and counterclockwise convection patterns are produced in the molten pool,caused by the interaction of reinforcing particles and the melt.An increase in LEPUL will significantly intensify the thermo-capillary convection whereas result in a decrease in the cooling rate of the molten pool.As LEPUL decreases from 1800J/m to 450 J/m,the distribution state of Al N particles changes from the severe aggregation,then to the formation of partial aggregation and finally to the homogeneous distribution in the solidified matrix.The influence of the processing parameters on the densification behavior,distribution of the residual pores,defects produced on the surface,distribution of the reinforcing particles and the wear properties has been studied.It showed that the appearance of the oxidation on the melt would promote the formation of defects due to the contrast variation of the surface tension gradient and the resultant radially inward flow,resulting in the formation of the continuous gaps between solidified tracks.However,the efficient melt spreading,the wetting behavior and the resultant high surface quality and the high densification rate was realized for the application of high scan speed(>200 mm/s),due to the surface tension gradient and the resultant radially outward convection.The Al N nanoparticles tended to distribute in the grain border and the center region while the aggregation into clusters at high scan speed due to the high viscosity and the insufficient rearrangement.At the optimized laser volume energy density of 420 J/mm³,the nanocomposite showed the strain-hardened adherent tribolayer with the lowest wear rate of 3.4×10^-4 mm³ N^-1 m^-1due to the high densification rate,nanoscale Al N particle and fine grain size of the matrix.