Microstructure And Properties Of Er And Zr Composite Modified Aluminium Alloy Fabricated By Selective Laser Melting

Selective laser melting（SLM）technology of metal materials has the characteristics of"high precision forming of complex structure"and"high performance material manufacturing",which has become an effective way for rapid manufacturing of high performance aluminum alloy complex structural parts.However,there are few kinds of high-strength aluminum alloy materials that can be used under this technology,which is mainly the special additive alloy developed with Sc modified element.The high price of Sc and the high content in additive alloy restrict its wide application in industrial field.Rare earth element Er has similar microalloying effect to Sc,but the price is only 1/80～1/100 of Sc,and it shows synergistic effect after compound addition with Zr.The effects of Er and Zr composite microalloying on the properties and microstructure of aluminum alloy fabricated by SLM process are studied in this paper.In this paper,the existence form and action mechanism of Er and Zr in pure Al under SLM process are discussed.On this basis,Er and Zr modified elements were added into Al-Mg and Al-Si-Mg alloys to study the mechanism of microstructure modification and its effect on properties.The formation mechanism of bi-modal grain structure of Al-Mg-Er-Zr alloy and the synergic precipitation effect of Er and Zr under rapid solidification process were studied.The directional regulation of microstructure and crack density by composition and process parameters was realized,and Al-Mg-Er-Zr alloy with strength over 500MPa was developed.In addition,the strength and elongation of Al-Si-Mg alloy were improved by the addition of Er and Zr,and the relationship between microstructure and tensile properties of Al-Si alloy were revealed by various heat treatment systems.The main research results are as follows:（1）When Er and Zr are added to pure Al,fine equiaxed grain structure is formed at the boundary of molten pool,and the grain size is refined.At the same time,SLM process expands the solid solubility of Er and Zr elements in pure Al.The homogenization and aging heat treatment in the casting process leads to the precipitation of Er and Zr elements around the in-situ secondary phase and grain boundary eutectic phase,resulting in a significant decrease in hardness,so it is no longer suitable for the SLM process.After direct aging treatment of Al-Er-Zr alloy at 375℃/3h,nano-sized secondary Al₃（Er,Zr）phase is precipitated,which greatly improves the hardness of the sample.At the same time,Al₃Er particles distributed at the grain boundary play the role of pinning the grain boundary and preventing grain growth,so the fine grain structure can be retained.（2）After solidification,Al-6.5Mg-0.4Er-1.2Zr（wt%）alloy forms bi-modal grain structure consisting of equiaxed grain at the edge of molten pool and columnar grain at the center of molten pool,which inhibits the solidification crack of Al-Mg alloy.The formation of bi-modal grain structure is mainly related to the existence form of Zr element.The cooling rate of the SLM molten pool boundary region is slow,Zr precipitates in the form of Al₃Zr primary phase,which plays the role of non-uniform nucleation and promotes the formation of equiaxed grain.However,the cooling rate in the molten pool center is fast,and the precipitation of Zr element is inhibited and exists in the matrix in the form of solid solution,which promotes the formation of columnar grain.In addition,Er is mainly distributed at grain boundaries in the form of Al₃Er,which hinders grain growth and plays a role in grain refinement during solidification.（3）During the direct aging process at 375℃in Al-6.5Mg-0.4Er-1.2Zr alloy,the Er element with faster diffusion rate first precipitated in the form of Al₃Er precipitates,providing nucleating particles for Zr,and then the Zr element with slower diffusion rate precipitated around Al₃Er to inhibit its rapid coarsening and form Al₃（Er,Zr）precipitation phase with core-shell structure.Er and Zr elements show synergistic precipitation effect,which greatly improved the mechanical properties of the alloy.The yield strength and tensile strength reached 440MPa and 520MPa,and the elongation exceeded 15%under the combined action of grain boundary strengthening,nano-sized Al₃（Er,Zr）precipitation strengthening and solid solution strengthening.（4）It is found that by adjusting the content of modified elements and the laser volumetric energy density,the fully equiaxed grain structure with uniform size can be obtained.The results show that the tensile strength of Al-Mg-Er-Zr alloy is positively correlated with the content of main alloying element Mg,and the yield strength is positively correlated with the content of Er and Zr.When Zr content exceeds 1.5wt%,discontinuous precipitation occurs in the aging process,which weakens the effect of precipitation strengthening.Intergranular corrosion performance is related to Mg content and heat treatment state.The peak aging state of 4.5Mg alloy shows excellent long-term intergranular corrosion resistance.（5）In the study of SLM forming Al-7Si-0.6Mg-0.2Er alloy,it is found that the defocusing distance parameter can regulate the energy input and affect the cellular size by influencing the morphology and volume of the molten pool.The optimized process parameters are△f=0,P=370W,v=1500mm/s,h=0.13mm.The energy density E is distributed between 59-97 J/mm³,and the relative density is more than 99%.（6）The（Al,Si）₃（Er,Zr）phase precipitated during solidification promotes the non-uniform nucleation and refines grain size in 0.4Er-0.4Zr modified Al-7Si-0.6Mg（A357）alloy,and the"skeleton"composed of eutectic Si network is distributed at the cellular boundary.This alloy has good comprehensive tensile properties,yield strength of330MPa and elongation of 12%.After stress annealing at 300℃/2h and solution treatment at 540℃/1h,eutectic Si network spheroidized and coarsened into large size Si particles.Although nano-sized Al₃（Er,Zr）strengthening phase is precipitated,it cannot make up for the strength loss caused by the coarsening of eutectic Si,resulting in a significant reduction in strength.The dispersed precipitated Mg₂Si precipitates after T6 solution aging heat treatment greatly improves the yield strength of the alloy,which has high strength and elongation.