| Recently, the study and use of Al-Mg-Si alloy in vehicles has been an irresistible trend with the rapid development of automobile industry. The micro-alloying is an effective means to improve the alloy properties. The addition of trace element produce two-side influences, on the one hand, the grain refining effects, on the other hand, the trace element affects the alloy aging precipitation and improves alloy properties. On the base of 6111 aluminum alloy, the Al-Mg-Si-Cu-Mn-Zr-XEr alloy with the composition of Al-0.8Mg-1.0Si-0.8Cu-0.2Mn-0.15Zr-(0.1、0.2、0.3) Er was designed in order to reduce the cost, improve the mechanical properties of Al-Mg-Si alloy. An Al-Mg-Si-Cu-Mn-Zr-Er alloy with high tensile strength and toughness was obtained through the micro structure, properties, and texture of Al-Mg-Si-Cu-Mn-Zr-XEr alloy, and heat treatment process, precipitation behavior and strengthening mechanism of Al-0.8Mg-1.0Si-0.8Cu-0.2Mn-0.15Zr-0.3Er alloy.The grain of Al-Mg-Si-Cu-Mn-Zr-XEr alloy casting ingot shows dendrites and equiaxed crystal, and the second phases mainly existing in casting alloy are Mg2Si, Al1.9CuMg4.1Si3.3, CuAl2, Al4Mn3Si2, Al3Er, Al3Zr and β-Si. After homogenization, the microstructure of alloy sheets more uniformly. The Mg2Si, Al1.9CuMg4.iSi3.3 and CuAl2 phases are not found during homogenizing, and the Al3(Er,Zr) composite phase with a core-shell structure is formed. The grain of Al-Mg-Si-Cu-Mn-Zr-XEr as-extruded alloy is extended in extrusion direction shows directional distribution, and lots of precipitates concentrate at the interior of the grains and the grain boundary. The thin and long grain of rolling alloy show small and blur filamentary microstructure, and is extended in rolling direction shows obvious directional distribution. Precipitates of rolling alloy are crushed during rolling. The grain of T6 heat treatment alloy shows equiaxed crystal, and low melting point solute atoms precipitate out of supersaturate solid solution.For casting, as-extruded, rolling and heat treatment Al-Mg-Si-Cu-Mn-Zr-XEr alloy, as the element Er content increases, the microstructure is refined gradually, and the strength and hardness are improved, while the elongation is not decreased greatly. For solid solution Al-Mg-Si-Cu-Mn-Zr-XEr alloy during the aging treatment, as the element Er content increases, the time of the peak hardness is accelerated. After the T6 heat treatment, Al-0.8Mg-1.0Si-0.8Cu-0.2Mn-0.15Zr-0.3Er alloy has good comprehensive mechanical properties. The recrystallized microstructure of alloy is quickly refined, precipitates concentrate at the grain boundary, and solute atoms are completely precipitated. The otherness of performance in different directions is reduced with the element Er content increases. The values of IPA for Al-0.8Mg-1.0Si-0.8Cu-0.2Mn-0.15Zr-0.3Er alloy are the lowest, and the values are 0.39%,1.59%,0.77%.The major textures of Al-Mg-Si-Cu-Mn-Zr-XEr alloy rolling sheets are rotated cube texture{100}<011>, and{lll}<110>>{111}<112> brass R texture, while conventional rolling texture B, S, C are weaker. As the Er content increases, the content of texture are decreased, random component are increased, and the earing rate is reduced. The addition of Er affects the matrix lattice, reduces the energy of the fault layer, hinders the cross slide, and changes the content of texture. After solution heat treatment, the major textures of Al-0.8Mg-1.0Si-0.8Cu-0.2Mn-0.15Zr-0.1Er alloy are Brass texture and Cube+ND25 texture, and the PSN is one of the recrystallization nucleation mechanism for alloy. As the Er element content increases to 0.2%,0.3%, the major texture of aluminum alloy is weaker Brass texture, and the PSN nucleation is receded.The study of T4, T4P and T6 heat treatment process for Al-0.8Mg-l.0Si-0.8Cu-0.2Mn-0.15Zr-0.3Er alloy show that at natural aging time 1.5d and 4d, the hardness and tensile mechanical properties of Al-0.8Mg-l.0Si-0.8Cu-0.2Mn-0.15Zr-0.3Er alloy appear the double peaks feature before and after paint-bake aging. During the paint-bake aging technology, for T4 temper Al-Mg-Si-Cu-Mn-Zr-Er alloy, the hardening rate is reduced, the precipitation of P" phase is delayed, the precipitates are mainly p" nuclei, and the paint-bake aging softening is appeared. After the paint-bake aging, the precipitates of T4P temper Al-0.8Mg-1.0Si-0.8Cu-0.2Mn-0.15Zr-0.3Er alloy are mainly P" phase, and the paint-bake aging hardening is realized. As pre-aging time increases, the paint-bake aging hardening is improved. The parameters of peak aging treatment of Al-0.8Mg-1.0Si-0.8Cu-0.2Mn-0.15Zr-0.3Er alloy sheets is solution-treated at 540℃ for 1h, and then aged at 180℃ for 5h. At that time, the alloy sheets mechanical properties data are 392.4MPa,321.8MPa,117HB and 24.32% for the tensile strength, yield strength, the hardness and the elongation. Considering the demand of stamping properties for auto body sheets, the appropriate pre-aging regime of alloy sheets is solution and water quenching, and then age at 170℃ for 5min.The addition of Er and Zr accelerates the dispersively precipitation of fine and uniform β" phase, the precipitation temperature of P" phase is reduced, and the time of the first natural aging peak and peak aging is accelerated. As the element Er content increases, precipitate activation energy of P" phase for alloy is decreased, and the values are 96.3,93.6 and 84.9 kJ/mol. The kinetics expressions are obtained as follows, YEr0.1=1-exp[-1.534×1013 1011t1.5exp(-15324.9/T)]. The TTT expressions are obtained as follows, TEr0.1=17382.7/[30.4 +1.5lnt-lnln(1-Y)-1], TEr0.2=16895.3/[29.4+1.5lnt-lnln(1-Y)-1] TEr0.3=15324.9/[26.4+1.5lnt-lnln(1-Y)-1].The addition of Er and Zr contributes a lot to the strengthening of Al-Mg-Si-Cu-Mn-Zr-Er alloy. The alloy is strengthening by work hardening, fine particles, solution strengthening and second-phase strengthening, and second-phase strengthening includes dispersion and precipitation strengthening. The reinforcing mechanism of alloy is Orowan mechanism. |
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