Study On Microstructure,Mechanical Properties And Strengthening Mechanism Of TiN/AlSi10Mg Composites Fabricated By Selective Laser Melting

Selective laser melting(SLM)is a powder-bed-based rapid prototyping technique.It uses high-energy laser beams as a heat source to melt metal powder in a layer-by-layer manner.It not only breaks through the restrictions of traditional processing techniques on the geometry of the parts but also improves the material utilization rate.The high-frequency thermal cycle during the processing makes the material in an unbalanced state for a long time,which greatly improves the strength of the material.The rapid development of SLM technology has brought a broad space for the preparation and application of high-performance lightweight materials.Therefore,the research on SLM-produced aluminum-based high-performance materials has become very important.This research firstly discusses the production of Al Si10Mg powder using double-nozzle atomization technique;secondly,Ti N nanoparticle modified Al Si10Mg composite powder are produced by ultrasonic vibration dispersion process for SLM.The effects of processing parameters,Ti N addition,heat treatment process on the SLM processability,microstructure,phase transition,crystal orientation,mechanical properties of SLM fabricated TiN/AlSi10Mg composites have been investigated;Finally,lattice structures with different shapes have been designed and formed by SLM,and finite element analysis are performed.The compressive behavior and energy absorption capacity of porous lattice structures are discussed.The main conclusions are as follows:(1)The optimal atomization parameters for Al Si10Mg powder prepared by a double-nozzle gas atomization technology are:gas pressure 3.0 MPa,delivery tube diameter 4.2 mm,melt superheat 350 K,and the fine powder yield reaches 72.14%.Atomization parameters have an important influence on the characteristics of the produced powder.When keeping the atomization pressure and melt superheat unchanged,the increase in delivery tube diameter from4.0 mm to 5.0 mm increases the powder size,and the fine powder(<50?m)yield rate is reduced from 69.8%to 65.2%.The average value of the sphericity reduce from 77.2%to 65.15%,and the powder satellite index is also reduced;When keeping the delivery tube diameter and the melt superheat unchanged,the increase in gas atomization pressure from 2.0 MPa to 4.0 MPa,gradually decreases the powder particle size,and the fine powder yield rate increases from 60.5%to 65.5%.The powder sphericity and satellite index tends to increase;When keeping the delivery tube diameter and gas pressure unchanged,the increase in melt superheat from 200 K to 350 K,the median particle size D50 of the powder presents a trend of slightly increasing first and then rapidly decreasing.The powder sphericity and satellite index have no obvious changes.The main microstructure of Al Si10Mg powder is composed of a supersaturated?-Al matrix and a network of eutectic Si network distributed in the matrix.(2)The TiN/AlSi10Mg composite powder is successfully prepared by the ultrasonic vibration dispersion method.Ti N nanoparticles are evenly distributed on the surface of Al Si10Mg powder,and the composite powder remains spherical.With the increase of Ti N content,the laser reflectivity of the composite powder is greatly reduced,indicating a better SLM processability.The microstructures of Al Si10Mg and TiN/AlSi10Mg composites both contain cellular primary?-Al matrix and fibrous eutectic Si network.As the scanning speed increases,the grain size of the composite material is significantly refined.Nano-sized Ti N particles are uniformly dispersed in the matrix,while a small amount of agglomerated Ti N particles grow into micron-sized clusters and are embedded in the Al matrix.The interdiffusion and in-situ reaction between the Ti N clusters and the matrix formed a gradient interface layer.Increasing the Ti N addition can gradually transform the crystallographic orientation from a strong(001)direction to a relatively random distribution that vanishes the preferred(001)texture.The continuous heterogeneous nucleation,recrystallization of?-Al grains,and Zener pinning along the grain boundaries occur during the SLM process due to the presence of the Ti N nanoparticles,resulting in a significantly refined microstructure.The Ti N nanoparticles are dominantly distributed along the Al grain boundaries,while rod-like nano-Si precipitates are well-dispersed inside the grains.Both Ti N and nano-sized Si have well-bonded interfaces with the Al matrix.The optimal Ti N content is found to be 4 wt.%,at which the additively manufactured specimens exhibit high tensile strength(492±5.5 MPa),high ductility(7.5%±0.29),and microhardness(157±4.9 HV)simultaneously,comparable or higher than previously reported SLM produced Al-Si alloys,and other Al-based composites.(3)The microstructure and mechanical properties reveal inconspicuous changes after AA where the eutectic fibrous Si networks remain.However,SHT vanishes the Si networks,precipitates and coarsens Si particles,and eventually softens the Al matrix,leading to a significant decrease in tensile strength and hardness of heat-treated composites and an increase in ductility.The addition of Ti N nanoparticles plays a significant role in microstructural evolution during heat treatment.By increasing solution temperature from 460?°C to 540?°C,Mg₂Si phase precipitates out of the Al matrix,followed by Al Fe Si intermetallic at 500?°C and Al Si Ti intermetallic at 540?°C,respectively.Interestingly,unexpected enhancements of hardness(from 91.5?±?2.3 HV to 105.9?±?2.1 HV)and tensile strength(from 268.7?±?2.5?MPa to336.8?±?1.5?MPa)are achieved as the solution temperature increases.This abnormal phenomenon is attributed to the precipitation hardening by the Mg₂Si precipitate and needle-like intermetallics,which overcomes the effects of microstructural coarsening and matrix softening.(4)A novel plate-like lattice structure called Isomax is designed and then prduced by SLM.A suitable material constitutive model is established to simulate the compression deformation behavior of the lattice structure.It is found that the maximum compressive stress and energy absorption capacity of Isomax structure are about 3 times and 5 times higher than that of high-strength truss structures(Octet truss,BCC,Tetrakaidecahedron)with equal mass.Different topological shapes have an important influence on the surface quality of lattice materials formed by SLM.Due to the staircase effect,irregularly sized bumps and powder adhesion will appear on the beam/rod surface which has a certain inclination angle to the processing direction,and the forming quality is relatively rough.The Octet truss,Tetrakaidecahedron and BCC structures have relatively similar behavior in the linear elastic stage of compression deformation,but in the yield and fracture stage,the Octet truss and BCC structures have shear bands along the 45°direction,while the Isomax and Tetrakaidecahedron structure are uniformly deformed layer by layer.