Process Control,microstructure And Properties Of Selective Laser Melted Aluminium Alloy

Based on the principle of discrete and stacking,selective laser melting（SLM）technology can melt metal powders through using high-energy laser beam as heat source.The solidified tracks can be directly formed structural parts with complex spatial configuration and excellent performance through layer-by-layer stacking.At this stage,conformal cooling structure and thermostat covers are widely studied for lightweight AlSi10Mg alloy materials with excellent casting properties,as well excellent thermal and electrical conductivity.However,aluminum has high free electron density and absorbs only 9%of the laser incident energy.The thermal conductivity is as high as 237 W/（m·K）,which results in reduction of heat accumulation.There are problems of less energy absorption and incomplete melting,resulting in poor interlayer bonding.Meanwhile,the aluminum is sensitive to oxygen,which inclines to metallurgical defects,such as balling,inclusions and pores,deteriorating forming quality.This hinders the industrial application.In order to solve the problems of balling phenomena and pores,this paper makes a study on the formation mechanism and characteristic evolution of the micro melt pool,which are described in combination with the single molten track temperature field simulation method.The processing parameters are optimized.The formability and design principles of typical geometric components are discussed.The source of defects in SLM-processed AlSi10Mg alloy was analyzed.The balling phenomena and pores could be reduced by introducing different remelting scanning strategy.The mechanism of defects reduction is studied.The melting and solidification behavior of melt pool,microstructure and properties evolution in SLM-processed samples are investigated.The following main results are obtained.Finite element simulation method is adopted to investigate the temperature field distribution and variations of single molten track of SLM-processed AlSi10Mg alloy.The influence of different processing parameters on the melt pool characteristics is analyzed and discussed,including the temperature,shapes and sizes.Combined with practical fabricating results,the Marangoni convection phenomenon and the deterioration reasons of surface quality are revealed.The processing parameters are optimized and the influence of processing parameters on relative density is analyzed.The optimized parameters are P=200 W,v=1100 mm/s,s=70μm and t=20μm.The surface quality could be improved by adding remelting treatment to the surface layer.The forming ability of typical geometrical shapes of SLM-processed is analyzed and discussed,including the design principles of grid-shaped thin wall parts,bore diameter range of pipe channel structure and the inclined angle of overhanging structure.The sizes of forming thin wall components should not be less than 0.2 mm,the pipe channel structures range betweenΦ3 mm¹0 mm.If the inclined angle of overhanging structure is lower than 45o,the overhanging part would collapse and stick powders.The formation and process control methods of defects in SLM-processed AlSi10Mg alloy samples are explored.The essential reasons of reducing pores and balling phenomena by different remelting scanning process are analyzed.Adding remelting scanning process on solidified layer can enhance the fluidity of liquid molten and fill pores.The liquid molten has enough time to float out bubbles,leading to porosity being decreased.The wettability among the interlayers is reinforced and surface fluctuation is reduced.The defects are effectively controlled.The melting and solidification behavior of melt pool are studied from the perspective of thermodynamics and kinetics.OM,SEM,XRD,EBSD,TEM and tensile experiment are used to characterize and test the microstructure and properties of SLM-processed AlSi10Mg alloy.The results show that the solidification prefers to start from the location with fast dissipation at the border of melt pool.Grains grow into different microstructure morphology,which are effected by temperature gradient and growth rate.The cellular dendrites dominate at the border of melt pool,equiaxed grains tend to be formed in the center of melt pool.A network Al-Si eutectic microstructure distribute onα-Al substrate.Rapid heating and cooling result to increase of eutectic Si solubility intoα-Al,in consequence crystal lattice distortion occurs.The grains orientation appear to be anisotropy and no special grain boundaries are discovered.The microstructure and properties of SLM-processed AlSi10Mg alloy specimens under different remelting scanning strategies are characterized and tested by means of XRD,EBSD,Nanoindentation and microhardness tester.The results indicate that the grains in microstructure show more refined equaixed morphology due to the fast thermal conductivity of solidified layer,containing more low angle grain boundaries distribution.The grain growth along the building direction in longitudinal section is related to the heat flow direction.Changing remelting scanning strategy results in the difference of texture orientation.The refinement,Mg₂Si precipitation and lattice distortion can increase the microhardness and Young’s modulus up to 184.12 HV_0.2and 93.56 GPa,respectively.This paper discussed the influence of processing parameters on the melt pool characteristics and solidification behavior,which enriches the non-equilibrium solidification theory.The mechanism to reduce the defects of stacking forming are studied through adding remelting scanning process layer-by-layer.New ideas and methods are provided to solve the pores and balling phenomena,as well improve the microstructure morphology and performances.This lays an theoretical basis and experimental foundation for the industrial application of SLM-processed AlSi10Mg alloy complex configuration structural component.