Zhihong Jia

The Influence of Composition on the Clustering and Precipitation Behavior of Al-Mg-Si-Cu Alloys

The natural aging (NA) and artificial aging (AA) behavior of Al-Mg-Si-Cu alloys with different Mg/Si ratios and Cu additions were systematically investigated by means of hardness test, atom probe tomography, transmission electron microscopy, and Monte Carlo simulation. The Si-rich low-Cu alloys displayed higher hardness compared to the Mg-rich equivalents because Si atoms play a dominant role in clustering of solute atoms during both natural and artificial aging. In the high-Cu alloys, Cu did not obviously change the cluster distribution during NA, but significantly refines the clusters and precipitates due to the strong interaction of Cu atoms with Mg atoms during AA. In contrast to the low-Cu alloys, the Mg-rich high-Cu alloys exhibit higher hardness in the early and over-aged stages of artificial aging, with similar or slightly higher hardness in the peak aging condition compared to their Si-rich counterparts. Three types of precipitates (β″, Q′, and L) are favored in the high-Cu alloys. The Mg-rich high-Cu alloy has more L phase, while the Si-rich variant is abundant in Q′ phase. The negative effect of NA on subsequent AA behavior is less dependent on Mg/Si ratio in the high-Cu alloys due to a synergistic action of the residual Si and Cu atoms, but is closely related to Mg/Si ratio in low-Cu alloys.

Effects of high temperature pre-straining on natural aging and bake hardening response of Al–Mg–Si alloys

Abstract The influences of high temperature pre-straining (HT-PS) on the natural aging and bake hardening of Al–Mg–Si alloys were investigated by Vickers microhardness measurements, differential scanning calorimetry (DSC) analysis and transmission electron microscopy (TEM) characterization. The results show that pre-straining at 170 °C immediately after quenching can effectively resolve the rather high T4 temper hardness caused by the conventional room temperature (RT) pre-straining treatment, and give a better bake hardening response (BHR) after paint-bake cycle. HT-PS 7% at 170 °C for 10 min is chosen as the optimum process as it provides lower T4 temper hardness and better BHR. The simultaneous introduction of dislocations and Cluster (2) can significantly suppress the natural aging and promote the precipitation of β″ phase, and reduce the effects of deformation hardening by dynamic recovery.

Microstructure of Al–Si–Mg alloy with Zr/Hf additions during solidification and solution treatment

In this study, the morphology, composition and evolution of intermetallics in Al–7Si–0.3Mg, Al–7Si–0.3Mg–0.47Hf, Al–7Si–0.3Mg–0.16Zr and Al–7Si–0.3Mg–0.14Zr–0.44Hf alloys were characterized by optical microscope (OM), scanning electron microscope (SEM) with energy-dispersive spectrometer (EDS) and transmission electron microscope (TEM). The AlSiZr, AlSiHf and AlSiZrHf phases are formed with additions of Zr and/or Hf in base alloy during solidification, especially the AlZrHf primary particles. The three-dimensional morphology of the AlZrHf particle evolves from 10-polyhedron-like to compressed cube-like during solidification. The new AlZrHf phase experiences partial dissolution after solution treatment, while the others remain in the initial morphology. High Si contents lead to form thermally stable Zr/Hf-rich precipitates in Zr/Hf-containing alloys during solution treatment, especially the nanobelt-like Si2Zr, Si2Hf and Si2(Zr,Hf) precipitates. Particular orientation relationships and the growth mechanism are identified. The finding of the work broadens the elevated temperature application of Al–Si–Mg in modern automotive aluminum engine.

Effects of Sn/In additions on natural and artificial ageing of Al–Mg–Si alloys

ABSTRACT The effect of Sn or In addition on natural ageing and subsequent artificial ageing of Al–Mg–Si alloys was investigated. In addition not only retarded the natural ageing but also improved the T6 hardness. Although Sn addition retarded the natural ageing as well, precipitation kinetics was delayed. In addition, slightly refined the β′′ phase distribution. But β′′ phase was coarse with addition of Sn. During artificial ageing, the vacancies released from binding complexes promote the formation of β′′. The detrimental effects of Sn on artificial ageing could be attributed to the reduction of the amount of Sn solutes and of available Mg solutes forming β′′.