| Aluminum alloys are increasingly attracted to be employed in aerospace,automotive,and military industries because of their excellent strength-to-weight ratio,corrosion resistance and light weight.However,traditional manufacturing technology can't satisfy the increasing requirements of fabricating complex structures of aluminum alloy parts.Selective laser melting(SLM)is an emerging additive manufacturing technology for metal parts with complex structure,which is expected to become an effective manufacturing technology for aluminum alloy parts with complex structures,especially for the thin-wall parts.Owing to the high reflection,high crack susceptibility,high activity of oxidation,very few researches focused on SLMed AlCu5MnCdVA,which seriously restricts their applications.This thesis studied the melting and solidification behavior of SLMed AlCu5MnCdVA alloy systematically,including the evolution of the molten pool shape,the formation mechanism of the defects,the relationship between the microstructure and the mechcanical properties,the role of atmospheric oxygen during the process and the heat treatment technology.The crack free SLMed AlCu5MnCdVA alloy parts with high density and mechanical properties were achieved.The main contents and results are as follows:(1)Based on the cross-sectional mpdel of the contact interface,the contact angle envolution of SLMed AlCu5MnCdVA alloy was throretically analyzed.It is found that the evolution of the contact angle is not determined by the equilibrium state but the evolution of the driving forces for fluid flow and the lifetime of the molten pool.With the increase of the scanning velocity and the hatch spacing,the contact angle increases.When the sample is subjected to the horizontal thermal cycles of SLM,the contact angle decreases dramatically first and then becomes stable as the tracks increase.When the sample is subjected to the complex thermal cycles of SLM,the contact angle increases first and then decreases and finally becomes stable as the layers increase.The experimental results agree with the predictive results.(2)The evolution of the molten pool dimension and the surface roughness of SLMed AlCu5MnCdVA fabricated with different process parameters during complex heat history was studied systematically.The results show that the width of the molten pool decreases with the increasing of the scanning velocity and the hatch spacing.As the scanning velocity increases,the suface roughness of the parts decreases first and then increases.As the hatch spacing increases,the surface roughness of the parts increases.When the sample is subjected to the horizontal thermal cycles of SLM,the width of the molten pool increases first and then becomes stable as the tracks increase.When the sample is subjected to the vertical thermal cycles of SLM,the width of the molten pool decreases first,then increases and finally becomes stable as the layers increase,and the melting penetration depth increases first and then becomes stable as the layers increase.When the sample is subjected to the complex thermal cycles of SLM,the width of the molten pool and the melting penetration depth increase first and then become stable as the layers increase.The surface roughness increases first,then decreases and finally becomes stable as the layers increase.The process parameters have no effect on the trend of the evolution but the values.The morphology of molten pool is influenced by the thermal accumulation,the fulid flow of the molten pool,the lifetime of the molten pool,the laser absorption of the powder bed and the magnitude and direction of the heat conduction.The roughness is determined by the smoothness along the laser scanning direciton and the stable contact angle perpendicular to the laser scanning direction.(3)The characteristic and the formation mechanism of the defects using different scanning velocity and hatch spacing were investigated.The results show that there is great relationship between the process parameters and the size as well as the shape of the defects.With the increasing of the scanning velocity and the hatch spacing,the size and the roundness of the defects become larger.The main defects during the process are solidification cracks.The insufficient backing filling feeding rate of the terminal liquid is the main reason for the solidification cracks.Finally,cracks will form under the local stress.Based on Feurer's criterion,threshold of the scanning velocity has been obtained by theoretically calculation.The results show that the crack susceptibility increases as the scanning velocity increases.By optimizing the process parameters,highly dense parts without crack can be achieved.The highest relative density is 99.9972%.(4)The microstructure and the mechanical properties of the SLMed AlCu5MnCdVA was studied.The results show that there is no great difference in mechanical properties when the scanning velocity changes.The anisotropy is not obvious.The microstructure consists of ?-Al and Al2 Cu.In addition,the supersaturated solid solution with fine grain,high dislocation density and weak texture has been obtained.The solid solution strengthening,the grain size strengthening,the Orowan strengthening and the dislocation strengthening are likely to contribute to strength of the parts.The calculation shows that the grain size strengthening makes the greatest contribution to the strength of the parts.(5)The influences of the atmospheric oxygen content on the formability,microstructures and the mechanical properties of the parts were investigated.The results show that the mechanical properties of the samples fabricated with higher atmospheric oxygen content(200 ppm,HOC samples)is much worse than that of the samples fabricated with lower atmospheric oxygen content(20 ppm,LOC samples).For the HOC samples and the LOC samples,there are little differences in formability,relative density,grain size and the phase composition.The reason for the earlier fracture of HOC samples is that the size of the oxygen containing particles of HOC samples is much larger than that of LOC samples.They are formed by the growing and colliding of the first particles formed during chemical reaction.The higher the atmospheric oxygen content is,the more the first particles will form,the larger the final particles will be,resulting in earlier fracture.Therefore,the atmospheric oxygen content should be rigidly controlled when aluminum alloys are being selective laser melted.(6)The heat treatment of the SLMed parts was also investigated.The results show that the solution time becomes shorter comparing to that of the parts fabricated by traditional methods because of the supersaturated solid solution formed during SLM process.After aging treatment,precipitate strengthening makes the greatest contribution to the strength of the parts.The mechanical properties are influenced by the type,the quantity,the size and the shape of the precipitates.The parts exhibit excellent mechanical properties after solution treatment.The yield strength,the ultimate tensile strength and the elongation are 217.2 MPa,412.7 MPa and 19.18%,respectively.After different aging treatment,there are great differences in precipitation behavior.The type and the shape of the precipitate will change while the aging temperature changes.As the aging temperature increases,the growth speed of the precipitate becomes fast,and the aging time to achieve maximum strength becomes shorter.As the aging time increases,the precipitates become larger,the strength increases first.When the precipitates become too coarse,the strength will decrease.The dispering ?'' is the main reason for the improvement in yield strength.In addition,?'' has an effect on the texture of the parts.After aging treatment,the mechanical properties of the parts improve obviously.The parts exhibit the highest yield strength and ultimate tensile strength: 383.3 MPa and 476.2 MPa,respectively.At this time,the elongation is 7.85%.The parts exhibit ductile and brittle mixed fracture. |
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