Abstract
The Zr element is one of the important grain refiners for 7xxx series Al-Zn-Mg-Cu alloys, but the effect of Zr content more than 0.15 wt.% needs to be deeply investigated under the action of ultrasonic vibration. In this study, the effects of Zr contents (0.1 to 0.25 wt.%) on microstructure and mechanical properties of Al-Zn-Mg-Cu alloy were studied. The results showed that Zr element could refine grains, but when the Zr content was greater than 0.15 wt.%, the grain size was not uniform, the number of second phase particles increased, and the segregation of components became more serious. It was found that after ultrasonic treatment, the grain-size inhomogeneity was greatly improved, and the Zr content could be added up to 0.2 wt.%. When the Zr content is equal or lower than 0.2 wt.%, ultrasonic treatment can effectively improve the mechanical properties of materials by refining grains and weakening segregation. However, when the Zr content is up to 0.25 wt.%, the effect is getting worse.
Highlights
The Zr element is one of the important grain refiners for 7xxx series Al-Zn-Mg-Cu alloys, but the effect of Zr content more than 0.15 wt.% needs to be deeply investigated under the action of ultrasonic vibration
With the continuous increase of Zr content, part of the grains become coarse, and the grain inhomogeneity of alloy D (Figure 2g) is serious, minimum grain size is less than 30 μm, and maximum grain size is more than 120 μm
At high magnification (Figure 2b,d,f,h), some black substances with the shape of dots were observed in the crystals, and the amount of black spots increased with the increase of Zr content
Summary
The Zr element is one of the important grain refiners for 7xxx series Al-Zn-Mg-Cu alloys, but the effect of Zr content more than 0.15 wt.% needs to be deeply investigated under the action of ultrasonic vibration. When the Zr content is equal or lower than 0.2 wt.%, ultrasonic treatment can effectively improve the mechanical properties of materials by refining grains and weakening segregation. Squeeze casting is a process to make the melt solidify under high pressure and produce a small amount of plastic deformation, which can effectively reduce the shrinkage cavity and porosity in the casting process, increase the density of the casting significantly, and improve the mechanical properties of the casting. Xu [7] found that squeeze casting can effectively improve the shrinkage cavity and porosity caused by solidification shrinkage, it cannot eliminate the thermal cracking and segregation defects caused by the high alloying characteristics of Al-Zn-Mg-Cu series alloys
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