Quantitative Characterization Of Microstructural Evolution During Solidification And Ageing Of Al-Mg-Si Alloys With Cu Additions

Abstract:6XXX series Al-Mg-Si alloys have been widely applied in automobile, aircraft and building industries due to their outstanding corrosion resistance, excellent formability and high strength/weight ratio obtained by ageing treatments. Their low recycling cost as well as their unique contribution in improving fuel economy have attached more importance to them in an era of depleting energy and resources. Recent study has found that the addition of Cu to Al-Mg-Si alloys can increase the age-hardening response, in a way that is currently unknown. As is known to all, the mechanical properties of materials are determined by the microstructures. Hence in this study, the role of Cu in the precipitation hardening processes of Al-Mg-Si alloys has been studied from the aspect of microstructure.Transmission electron microscopy, combined with other techniques such as scanning electron microscopy,3-dimensional atom probe (3DAP), hardness test and X-ray diffraction, has been applied in this work to investigate the microstructures of the materials. In the first step, a comparative study on the mechanical properties and microstructures of two6005alloys with no Cu or0.10wt.%Cu during age-hardening process has been systematically done. It has been found that minor Cu addition to the alloy has a notable effect on the precipitation sequence during ageing. Further, a careful study has been performed on the evolution of mechanical properties and microstructures during solidification, solution heat treatment and all stages of the ageing heat treatment of a6111Al-Mg-Si-Cu alloy. The role of Cu in the whole fabrication process has thus been systematically described. The conclusions of the study are given as below:(1) By comparing the two Al-Mg-Si6005alloys (containing no Cu or0.10wt.%Cu) aged at150℃, it has been found that minor Cu addition has a notable effect on the precipitation sequence of the Al-Mg-Si alloy. In detail, the precipitation-hardening process has been accelerated in the under-aged stage due to the addition of minor Cu. And a type of platelet-like G.P. zones along{111}Al planes has been found in the Cu-containing alloy. Moreover, the extra Cu content increases the hardening effect at the over-aged stage due to the formation of Q’ and Q precipitates, and the corresponding reduction in the amount of β and Si precipitates due to the reduced amount of solutes.(2) During the solidification process of the6111alloy,1.14±0.06vol.%elongated Q particles with dendrite-like inner structures and0.65±0.33vol.%leaf-like Si particles were formed. It has been found that the uniformly distributed Q particles tend to grow along the solidification direction of the columnar cast ingot, in a new orientation in the matrix that is different with those found in aged alloys. In contrast, the Si particles are found to mainly aggregate along the grain boundaries. Twins are observed in the Si particles. These structural features decrease the nucleation and growth energies of the Si particles in the melt. The Q particles can be almost fully absorbed during solution heat treatment and thus help to form a uniform intra-granular microstructure during ageing. In contrast, the aggregation of Si particles along the grain boundaries provides sufficient Si atoms for the regeneration of plate-like Si particles along the grain boundaries in the over-aged microstructure and thus decreases the width of the precipitate-free zones. Furthermore, via thermodynamic simulation of the solidification process based on the Scheil model, the experimentally determined volume fractions of the phases were well predicted in a reasonable range of error, and their solidification behaviors were well accounted.(3) The precipitation sequence during the ageing of the6111alloy at175℃has been observed as SSSâ†'atomic clustersâ†'G.P. zonesâ†'βP"+Câ†'Q+Si. Cu enrichment has been found in the whole precipitation sequence except in Si precipitates. For the first time, the G.P. zones in the under-aged alloy have been found to have a body-centered tetragonal unit cell with a=b=0.296nm, c=0.405nm, as well as a slight lattice mismatch of3.09%with a-Al. In the peak-aged stage or shortly later, mainly β" precipitates and some minor C precipitates were observed in the microstructures. By high-angle annular dark-field scanning transmission electron microscopy, it has been observed that Cu exists in the structures of β" and C. Cu atoms partially substituted for atoms in the Si3column pairs and the Mg1columns of the Cu-free β"-Mg5Si6atomic model, and the substitution was preferential in one column of each Si3column pair. An average β" composition of28.6at.%Al-38.7at.%Mg-26.5at.%Si-5.17at.%Cu was measured by3DAP, revealing that the Mg1and Si3sites in the β" are mainly substituted by Al and Cu. This finding provides significant guidance in designing the compositions of Al-Mg-Si-Cu alloys. It has also been found that the two C/a-Al interfaces at the two sides of a C lath are asymmetric, one interface is f.c.c.-type with a small lattice misfit, while the other one is similar to the b.c.c.-type, with a larger lattice misfit. This interfacial structure can hopefully improve the precipitation-hardening effect.(4) The following innovations in the aspect of investigation method has been made in this study:an accurate method of simulating the complicated electron diffraction patterns from the precipitates in12equivalent orientations in the α-Al matrix has been established and successfully applied in other systems such as Ni-Ti shape memory alloys; the volume fraction of the atomic clusters and G.P. zones in the under-aged6111alloy has been determined for the first time by the combination of3DAP and HRTEM; the volume fraction of the lath/rod-like Q precipitates in the aged6111alloy has been exactly determined by combining the techniques of convergent beam electron diffraction and dark field imaging; a systematical method of determining the volume fractions of particles/precipitates in all size-scales has been established, and hence the volume fractions of the precipitates at different size-scales during the whole fabrication process of the6111alloy have been quantitatively characterized, which are important references for the design of Al alloys and the numerical simulation of the microstructure evolution.