Microstructure And Properties Of High Strength Al-3.4Mg-1.08Sc-0.23Zr Alloy Fabricated By Selective Laser Melting

Currently,aluminum alloy materials fabricated by laser selective melting?SLM?present limited types and low strength generally.Al-Mg-Sc-Zr alloy fabricated by Sc and Zr alloying can significantly improve the strength and hardness of the alloy,and show a stable printing performance,which has become the focus of 3D printing research on high-strength aluminum alloy.But the study of Sc,Zr alloying of high strength aluminum alloy mainly concentrated in the range of Sc less than 0.66%,while the microstructure and properties of Al-Mg-Sc-Zr alloy with high scandium?>0.66%?alloying are rarely reported.This project developed a new light Al-3.40Mg-1.08Sc-0.23Zr alloy with high scandium?1.08%?alloying based on laser selective melting technology.By means of macroscopic properties such as its density,hardness,tensile test,and using microstructure analysis such as scanning electron microscopy?SEM?,electron backscattered diffraction analysis?EBSD?,energy spectrum analysis?EDS?,the study explored the evolution of alloy's microstructure and the composition with different scanning speed and platform temperature,revealed the mechanism of SLM kinetic and thermodynamic parameters on alloy supersaturated solid solubility,second phase precipitation mechanism,and mechanical behavior.Besides,the corrosion resistant properties of the alloy under different process parameters were studied by adopting intergranular corrosion,exfoliation corrosion and electrochemical corrosion testing method,the corrosion mechanism and the main affecting factors of corrosion were revealed.The main conclusions are as follows:The microstructure of Al-3.40Mg-1.08Sc-0.23Zr alloy is composed of ultrafine equiaxed grains and surrounding relatively thick columnar grains.The morphology characteristics of stacked molten pool can be observed under metallographic microscope.The second phase in the alloy is mainly Al₂Cu phase and the primary Al₃?Sc,Zr?phase with a shape of square distributed in the fine equiaxed grain zone at the bottom of the molten pool.Increase the scanning speed,the cooling speed of the molten pool is higher,the degree of supersaturated solid solution of Al matrix is higher,the columnar grains are longer and narrower and the equiaxed grains are smaller in the alloy are observed,the number and size of the second phase particles in the alloy are smaller due to the less effect of heat input.Decrease the scanning speed,the alloy is affected by more heat input,the equiaxed grains grow and the length ratio of columnar grain decreases.Under the influence of laser thermal cycle,the second phase particles in the alloy are more numerous and larger in size.Increase the platform temperature,the temperature gradient becomes smaller,the equiaxed grain size in the alloy becomes larger,and the columnar grain becomes coarser and shorter,the morphology difference of these two grains is smaller.The microstructure of the experimental alloy fabricated at the substrate temperature of200?and the scanning speed of 800mm/s shows almost uniform equiaxed grain microstructure.The temperature of platform at 200?generates more heat input,the Al₃?Sc,Zr?particles in the alloy coarse in size.The aging effect is caused by continuous heat input during the printing process,the supersaturated solid solubility of the alloy is decreased,and the number and size of the second phase is increased.Considering the density,hardness and tensile properties of the experimental alloy,the optimal scanning speed of Al-3.40Mg-1.08Sc-0.23Zr alloy fabricated at the platform temperature of 35?is 1600 mm/s.Under this processing parameter,the relative density is as high as 99.77%,hardness is 117.8 HV,yield strength is289.3±9.3 MPa and tensile strength is 321.6±7.9 MPa.At the scanning speed of 1600mm/s,the alloy samples printed at 200?platform temperature are aged due to the influence of continuous heat input.At the scanning speed of 1600 mm/s,compared with the sample fabricated at the platform temperature of 35?,the hardness of200?alloy samples is improved by 18.36%,and the yield strength and tensile strength are increased by 37.03%and 24.38%,respectively.The corrosion resistance of Al-3.40Mg-1.08Sc-0.23Zr alloy was affected by the size of the fine equiaxed grain region,the Al₂Cu phase and the coarse primary Al₃?Sc,Zr?particles.As the Al₂Cu formed in the equiaxed fine grain zone at the bottom of the molten pool is not coherent with the Al matrix,it is easy to form Al-Cu/Al₂Cu battery,leading to the preferential corrosion in the equiaxed fine grain zone.With the decrease of scanning speed,the fine equiaxed grain area in the alloy becomes larger,the number of Cu-rich phase precipitates from the bottom of the molten pool increases,the number of primary Al₃?Sc,Zr?particles increases,thus the corrosion resistance of the alloy decreases.At the platform temperature of 200?,the number of equiaxied grains in the alloy increases,the primary Al₃?Sc,Zr?particles are coarsed,and the number of Al₂Cu phase precipitated along grain boundary increased due to aging effect,these factors result in the decrease of corrosion resistance of the alloy.At the platform temperature of 200?,the fabricated alloy exhibits better intergranular,peeling and electrochemical corrosion resistance.