Rare Earth Nd on Casting Microstructure and Properties of Al-6.1Zn-2.3Mg-1.7Cu-0.15Zr Aluminum Alloy AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY22050047

Chinese Library Classification Number:TG146.21

Citation Information:

The microstructure and properties of Al-Zn-Mg-Cu-Zr aluminum alloy after deformation were mainly investigated，but the microstructure and properties of cast aluminum alloy were less reported. Appropriate microalloying could effectively enhance the microstructure and process properties of casting alloys，reduce casting defects and improve alloy properties，which can provide reference for the ingots of this series of deformed aluminum alloys. Rare earth elements were the most effective additive elements to optimize the comprehensive properties of ultra-high strength aluminum alloys. The interaction between rare earth (RE) elements and matrix elements to form and bulk compounds （AlZnMgCuRE）. These dispersed phases uniformly distributed in the matrix can strongly pin dislocations and subgrain boundaries，which could significantly inhibit recrystallization，thus enhancing the stress corrosion resistance，exfoliation corrosion resistance and fracture toughness of the alloy. Rare earth Nd was rarely applied in Al-Zn-Mg-Cu-Zr series aluminum alloys，most of be illustrated by application of 2xxx aluminum alloys and Al-Si series alloys. The addition of Nd could change the morphology and size of the as-cast primary phase simultaneously，improve the heat resistance of aluminum alloys，which could provide a basis for exploring and developing new Al-Zn-Mg-Cu-Zr high strength aluminum alloys. Due to the negative effect of excessive rare earth content on the structure and properties of the alloy，the usual addition amount was 0.2%~0.4%. Optical microscope （OM），scanning electron microscope （SEM），energy spectrum analysis （EDS），X-ray diffraction （XRD） and Brinell hardness test were used to investigate the effect of rare earth Nd on the microstructure of the as-cast alloy，the variety and morphology of the non-equilibrium eutectic phase，the element distribution and the mechanical properties. Then the optimal addition amount of rare earth was selected. The results showed that the microstructure of as-cast alloy was typical dendritic crystal，which was mainly composed of α-Al matrix，lamellar Mg（Al，Cu，Zn）₂，η（MgZn₂），a small amount of light gray θ（Al₂Cu） phase and irregular block Al₈Cu₄Nd phase；Nd element had a better effect on inhibiting dendrite growth and refining grains，and 0.2% （mass fraction） Nd had the optimum effect of refining and comprehensive performance. The eutectic structure of 0.2% Nd alloy became discontinuous and thin，the dendrite spacing and the non-equilibrium eutectic phase at the crystal boundary were also relatively reduced. However，with the continuous increase of the addition amount，the grains tended to grow again，and the refinement effect of the alloy was relatively weakened. This was mainly due to the high chemical activity of rare earth element Nd. During the process of alloy casting and solidification，the limitation of solidification diffusion kinetic conditions makes the diffusion rate larger，composition segregation occurred at the front of the solid/liquid interface，and the solute redistribution generated in the process of crystal nucleation and growth，which changed the composition undercooling zone，thus inhibiting the growth of dendrite during solidification of the alloy. When the addition amount of rare earth Nd exceeded a certain limit，a large number of Nd intermetallic compounds would be formed and coexist at the grain boundary，which reduced the solidification rate，coarsens the grain boundary，weakened the effect of Nd atoms on the supercooling of alloy composition，and the dendrite arm spacing got increased. Compared with the alloy without Nd，the average spacing and average grain size of the secondary dendrite spacing of the alloy with 0.2% Nd decreased by 48.1% （53.24 μm） and 50.5% （258.98 μm），respectively. The addition of rare earth Nd was beneficial to reduce the segregation of alloy elements，especially Cu element. This phenomenon was mainly that grain refinement，therefore the segregation distance was shortened，easily diffusion homogenization，thereby reducing intracrystalline segregation. With the continuous addition of Nd content，the increase of Brinell hardness remained in a relatively stable state，this phenomenon indicated that the alloy with Nd element had good uniformity of structure. Brinell hardness of the sample with 0.2% Nd reached the maximum value of HBW 105.2，the tensile strength reaches the maximum value of 163 MPa，and the elongation reaches 2.5%，which were 44.9%，34.3% and 44% higher respectively than alloy without Nd. With the addition amount of Nd further increased，Brinell hardness of different alloys showed a slow decreasing trend，the minimum Brinell hardness value of the alloy with 0.4% Nd reduced to HBW 99.8，the tensile strength reached the minimum value of 127 MPa，which were still 41.9% and 15.7% higher respectively than alloy without Nd. It was mainly that the precipitation of a small rare earth phase during the melt solidification，which could be used as the heterogeneous nucleation substrate of α-Al matrix，so as to improve the nucleation rate，refine the grain and improve Brinell hardness. In the subsequent studies，it should be noted that the rare earth phase with high melting point would be formed after the addition of rare earth Nd. The formation of high melting point phase could enhance the pinning effect on the grain boundary movement. At the same time，the strain field around the precipitated second phase and the precipitated second phase could effectively hinder the movement of dislocation. The hindrance effect depended on the type，size and dispersion of precipitates. The more obvious the hindrance effect of dislocation movement，the higher the strength of the alloy，which provided a direction for further research.