Research On Nano-TiN Modified 7075 Aluminum Alloy Fabricated By Selective Laser Melting

Selective laser melting is capable of fabricating high-quality metal parts with complex geometries,which exhibits high degree of design freedom and manufacturing flexibility.However,only a part of usable alloys have been reliably fabricated by SLM,therefore it is necessary to expanding a wider range of alloy systems available for SLM.7075 aluminum alloy is an advanced engineering material with high specific strength,which can combine with topology optimization through SLM technique to realize the production of lightweight and high-strength structural components.Nonetheless,this kind of alloy shows a severe thermal cracking tendency during the SLM process,and all SLM-prepared parts lose their application value due to the existence of cracks.At present,the cracking problem is mainly solved by introducing nucleating agents or alloying elements,therefore,the powder characteristics of7075 aluminum alloy and its SLM process parameters are analyzed at the first stage in this paper.Secondly,TiN nanoparticles are added to the Al7075 alloy powder,and the uniformly mixed TiN/Al7075 composite powders are produced by an ultrasonic vibration dispersion technique.The powder characteristics are analyzed subsequently.Finally,the effects of different additions of TiN and heat treatment process on the microstructure and properties of SLM-built TiN/Al7075 composites are investigated,and the effect of TiN nanoparticles as a new nucleation agent on the elimination of thermal cracking in Al7075 alloy is discussed.The conclusions of this paper are as follows:The power characteristics of 7075 aluminum alloy are analyzed,and the results show that:D₁₀=22.3?m,D₅₀=33.1?m and D₉₀=52.3?m.The average values of powder circularity and Occhio bluntness are 87.2%and 74.1%,respectively,and the proportion of powder without satellite particles is 33.6%,which indicated that the powder possesses a higher degree of satellitalization,and it also contains a part of irregular shaped particles.The powder is mainly composed of?-Al phase,and the powder surface mainly shows a dendritic and cellular crystal structure.After the SLM formation of the above powder,it can be found that the maximum relative density of the sample is 97.65%.When the scan speed is relatively high,the cracks in samples become shallower,the crack numbers decrease,and the average crack length increases as the increase of laser power.When the laser power is relatively low,the cracks in samples become deeper,the crack numbers increase,and the average crack length decreases as the increase of scan speed.In the range of the laser power of 310 W?325 W and the scan speed of1000 mm/s?1200 mm/s,the surface quality of the samples is superior,and the average top surface roughness is 8.52?m.In the range of the laser power of 295 W?310 W and the scan speed of 800 mm/s?1000 mm/s,the defects in the samples are less while the microhardness is higher.The morphology of the second phases of the sample are mainly round,needle-like and irregular,wherein the round phase is identified as Al₂Cu Mg phase.TiN/Al7075 composite powders are prepared by an ultrasonic vibration dispersion technique,and the results of the powders characteristics are as follows:TiN nanoparticles are uniformly dispersed on the surface of the Al7075 powder.The particle size distribution of all composite powders with different TiN contents is met with the requirement of SLM process,and the powder sphericity is consistent with the original Al7075 powder.With the gradually increase of TiN content,the laser reflectivity of the composite powders are decreased,resulting the SLM processability improved continuously.The SLM-fabricated TiN/Al7075 composites are analyzed subsequently,where the results are listed as followed:with the increase of TiN addition,the bulk energy density needed to obtain the optimal density of the samples decreases gradually.Appropriate TiN content(1 wt.%and 2 wt.%)can induce a columnar-to-equiaxed transition for microstructure,thus reducing the tendency of thermal cracking and suppressing or even eliminating the cracks formed during the SLM process.However,excessive amount of TiN(4 wt.%and 6 wt.%)can produce cavities and other defects in the sample.TiN nanoparticles are well combined with the Al matrix.What's more,these nanoparticles can inhibit the movement of grain boundaries,suppress the grain growth as well as promote the recrystallization process,therefore refining the microstructure and eliminating the texture orientation.2 wt.%TiN/Al7075 composite shows the superior mechanical performance,the compressive strength is 932±15 MPa,the yield strength is 369±10 MPa,and the compressive strain is 36.0±1.5%,which are much higher than the unmodified Al7075 sample.The heat treatment process of 2 wt.%TiN/Al7075 composite is optimized.The results reveal that the optimal heat treatment parameters are solid solution at 470?for 2 h and aging at 120?for 18 h.The microstructure after heat treatment consists of needle-like Al₂Cu phase and granular Al₂Cu Mg phase,and precipitate the GP zone,?'phase as well as?phase.In addition,the mechanical properties of heat treated composite are further improved,the tensile strength is 355±12 MPa,the yield strength is 324±9 MPa,and the elongation at breack is 6.5±0.4%.However,the existence of round cavities and brittle TiN clusters can easily cause the initiation of cracks,which then reduces the strength and ductility of the composite.