Effect of trace Zr and V on 3D morphology of Fe-rich phase and mechanical properties in recycled Al-Mg-Si alloy

Abstract

Trace amounts of Zr and V can increase the recrystallization temperature of Al-Mg-Si wrought aluminum alloys, which is expected to regulate the recrystallization grain. In this paper, trace amounts of V and Zr were added to recycled Al-Mg-Si alloys, and their effects on the microstructure and mechanical properties of the cast alloys were studied by scanning electron microscopy (SEM) and synchrotron radiation X-ray tomography (SRXT). The results show that the addition of Zr significantly increases the grain sizes due to the “Zr poisoning”; V addition has no significant effect on the grain size. The morphology of Fe-rich phase gradually changes from the large Chinese-script shape to the fine short rod and curved long strip shape, and the distribution uniformity is improved with the combined addition of V and Zr. The three-dimensional (3D) morphology of Fe-rich phase includes granular, short rod-like, simple branch and multi-branch structures. The individual addition of V and Zr has no significant effect on the morphology of Fe-rich phase; but the combined addition of V and Zr significantly increases the number and volume fraction of Fe-rich phase with small size (diameter £15 μm), the number of branches in the largest Fe-rich phase is significantly reduced, resulting in the improvement of elongation. This work provides a theoretical basis for the development of new recycled Al-Mg-Si alloys in industrial application.

1. Introduction

Al-Mg-Si aluminum alloys are widely used in construction, automobiles, rail transit, and electronic and electrical industries due to their high specific strength, high specific stiffness, and good processing and forming performance [1, 2]. Limited by the poor casting performance, the Al-Mg-Si alloys are usually applied to products in the form of deformation, such as extrusion, rolling, and stamping [3]. After high-temperature and large plastic deformation, Al-Mg-Si is prone to recrystallization and abnormal grain growth, resulting in a coarse grain ring, which reduces the mechanical properties and stress corrosion resistance of the alloys. Studies [3 −6] have found that the use of transition elements, such as Mn and Cr, formed high-temperature dispersed phases (α-Al(MnCr)Si) during homogenization, which effectively suppress the growth of recrystallized grains by pinning or slowing down the movement of dislocations and grain boundaries. The higher the Mn and Cr content, the more significant the suppression effect, but Mn and Cr are easy to form coarse Fe-rich phases, significantly reducing the plasticity of the alloys [7, 8].

In recent years, it has been found [9, 10] that Zr has a low solubility and slow diffusion rate in aluminum alloys, and its solubility decreases with increasing temperature. Through heat treatment, a nano-scale L12 structured Al3Zr can be formed. This phase has a mismatch of only 0.8% with the Al matrix, which can be used as a strengthening phase to improve the strength of the alloy. At the same time, it has a strong pinning effect on the slippage and movement of dislocations and grain boundaries, inhibiting the abnormal growth of recrystallized grains and further improving the strength and toughness of the alloy [11, 12]. TSIVOULAS et al [12] found that the addition of 0.1% Zr in AA2198 alloy resulted in the formation of a nano-sized Al3Zr precipitates with an average particle size of 18.8 nm, which has a better effect on suppressing recrystallization than 0.4% Mn alloys. When kept at higher temperatures for a longer time, the L12 structured Al3Zr is prone to transforming into the equilibrium state of DO23-Al3Zr phase or DO22-(Al, Si)3Zr [11, 13], significantly reducing the strengthening effect. To enhance the stability of the