| In this dissertation,7075 aluminium alloys with 0wt%-0.2wt%rare earth element Sc were achieved by melting and casting techniques. Best solid solution time and aging time were confirmed after forging and rolling. Alloys plates with different Sc content were treated by peak aging and various duplex aging. Mechanic property, conductivity and corrosion resistance of the alloys in different state were tested. Metallography, SEM and TEM were used to research the existing state of Sc and how Sc affect the structure and property of the alloy.Research result claims that Sc element can refine grains. When Sc content is 0.2% wt, the grain size is 20μm.Dendrite reduced and equiaxed grain increased. The microhardness is 138.6HV, about 2.5%higher than the alloy without Sc. Sc has three main functions in aluminium alloy:purification, metamorphism and micro-alloying, combined action to improve the comprehensive performance of the alloy. The 460℃24h homogenizing treatment can eliminate segregation and residual stress, reduce the hardness of alloy and improve the alloy shape. Determine the best peak-aged system, the craft is 470℃60min solution treatment and 120℃ for 24h aged treatment, which lead the strength of the alloy reach the maximum. The strength of 0.2%Sc content is 541.1MPa, which is higher than the alloy without Sc about 7.9%, but the conductivity of alloy is poorer. The conductivity of the alloy with 0.2%Sc content is 24.95%IACS, and corrosion resistance is poor. After peak-aged, dispersed GP zones and a small amount of Simi-coherent η’(MgZn2) transition phase evenly distribute in the grain, and the phase from grain boundary distribute continuously and like a chain. After the aluminum alloy plate with 0.2%Sc at 120℃8h+180℃20h duplex-aged treatment, the strength is slightly lower than the peak-aged, which decreased to 530.76 MPa, but the conductivity increased to 39.97%LACS, while corrosion resistance improved. The phase is Simi-coherent η’(MgZn2) in the grain, and the η(MgZn2) phase from grain boundary was intermittent, island-like distribution. Sc exists in the form of Al3Sc in aluminum alloy, which can effectively improve tensile strength, electrical conductivity and corrosion resistance. |
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