The Effect Of Impurity Element SI On The Microstructure And Properties Of Ultra-high Strength Aluminum Alloy

Considering the current status of research on impurity elements in 7xxx series ultra-high strength aluminum alloys, one of typical impurity elements - silicon is studied by designing alloys with different Si concentrations based on 7050 alloy. The effects of Si and heat treatment on the microstructure and properties of ultra high strength aluminum alloys were studied systematically by tensile test, fracture toughness test, scanning electron microscope (SEM), energy-dispersive spectrometry (EDS) and optical metallography (OM). The existing form, mechanism and allowable limit of these effects were discussed.There is a strong correlation among the existing form of Si, contents, heat treatment and processes. Coarse particles have not been found in alloys with Si content lower than 0.134 wt.%. When Si content is higher than 0.134%, AlCuMgSi phase with 28%Si is found on grain boundaries and heterogeneous phase containing 3%Si in grains. AlCuFeSi phase with 5%Si is observed on grain boundaries in homogeneous annealed alloys. After extrusion process, coarse particles break up and distribute along the extrusion direction together with splinter-liked AlZnMgCuSi precipitates in matrix. After aging treatment, coarse heterogeneous phase is everywhere and the quantity of particles is related to Si content. The existing form is AlCuMgSi with 20wt.%Si.The properties under different heat treatments, such as dual-stage aging, MIL and RRA, are compared. The results show that the highest properties are acquired by RRA treatment, with yield strength, tensile strength, elongation and fracture toughness of 566.04MPa, 590.86MPa, 11.18% and 41.15MPa.m1/2 respectively. Therefore, the RRA scheme is selected to treat all other samples. The results of tensile test under room temperature show that 0.134%Si Alloy possesses the best properties with yield strength, tensile strength and elongation of 584.6MPa, 614.8MPa and13.7% respectively. When Si content is higher than 0.344%, all properties, especially plasticity, drop sharply with the increase of Si content.The results of fracture toughness show that Si content has strong impact on alloy fracture toughness. The fracture toughness drops by a half as the Si content increases from 0.033% to 0.491%. Combined with Hahn-Rosenfield Model and considering the distribution, shapes and sizes of particles, the negative influence of coarse particles, such as AlCuMgSi phase, on the fracture toughness is discussed. The coarse particles may reduce local plastic deforming ability and resistence to crack propagation, with such an effect increasing with Si content.