Materials Science Forum Vols. 794-796

Abstract: The effects of pre-aging and natural aging on the bake hardening behavior of Al-0.62Mg-0.93Si (mass%) alloy with multi-step aging process were investigated by means of Vickers hardness test, tensile test, differential scanning calorimetry analysis (DSC) and transmission electron microscopy (TEM). The characteristics of nanoclusters (nano scale solute atom clusters) formed during pre-aging and natural aging were also investigated using the three dimensional atom probe (3DAP) analysis. The results revealed the occurrence of natural age hardening and that the bake hardening response was decreased after the extended natural aging even though the pre-aging was conducted before natural aging. Since the 3DAP results exhibited the Si-rich clusters were newly formed during extended natural aging, it was assumed that the Si-rich clusters caused the natural age hardening and the reduced bake hardening response corresponding to Cluster(1). The decrease of the bake hardening response was markedly higher in the later stage of bake hardening than in the early stage. The size of the β’’ precipitates were reduced with increasing the natural aging time. Exothermic peaks of Peak 2 and Peak 2’ were observed in the DSC curves for the alloys pre-aged at 363K. Peak 2’ became larger with the natural aging time. This is well understood by the following model. The transition from Cluster(2) to the β’’ phase occurs preferentially at the early stage of the bake hardening. Then the growth of the β’’ phase is inhibited by the presence of Cluster(1) at the later stage of bake hardening. The combined formation of Cluster(1) and Cluster(2) by the multi-step aging essentially affects the bake hardening response and the β’’ precipitates in the Al-Mg-Si alloys.

Abstract: The microstructures of an Al-Mg-Ga cast alloy have been investigated to fundamentally understand its hardening response during heat-treatment. In as-cast, solution heat-treated, and aged conditions, the alloy is characterized by having a variety of metallurgical phases with unique morphologies and chemical compositions. Micro-hardness indentations subsequent to artificial aging between 50°C and 300°C revealed that the investigated Al-Mg-Ga alloy is capable of Vickers hardness values in excess of 180, therefore demonstrating a remarkably elevated age hardening behavior relative to conventional aluminum alloys. SEM and TEM observations, coupled with DSC analysis with several aging conditions, revealed that hardening is induced by submicron platelet precipitates having the length, width, and density that depend largely on the alloy aging conditions.

Abstract: This study investigates the precipitation behavior of two similar cast Al-Mg-Sc-Zr alloys with different solidification and cooling rates. Microstructural analyses and hardness testing were performed after casting and ageing. In addition, the precipitation and hardness evolution after rapid quenching using electron beam re-solidification were studied. It was shown that the amount of Al₃(Sc,Zr) phases present after casting or re-solidification increases with decreasing solidification and cooling rate. Consequently, the degree of supersaturation in Sc and Zr at room temperature and thus the hardening potential in a subsequent ageing process increases with the solidification and cooling rate. Therefore, the electron beam re-solidified samples revealed the most pronounced hardening.

Abstract: The high temperature strength of aluminum can be improved by forming thermal stable precipitates of microalloying elements such as Er, Sc and Zr. Our previous research indicates that composite addition of Er and Zr can improve the amount of precipitations, but the aging time to approach the peak hardness is relatively long. In this paper, we will focus on the deformation behavior of the Al-Er-Zr alloy during hot deformation process and the corresponding microstructure evolution. The results show that the strain can induce rapid precipitation in Al-Er-Zr alloy during hot working conditions. The mechanism of the rapid precipitation and its effect on thermomechanical processing are discussed.

Abstract: In this work, a new ultra-high strength Al-Cu-Li alloy was investigated. The ultimate strength, yield strength and elongation of the newly designed alloy by artificial aging are 647.2MPa, 609.4MPa and 7.3% respectively. Among the main strengthening phases of T₁, θ′ and S′ in the experimental alloys, T₁ is the dominant one. The combined addition of Mg and Ag promoted the precipitation of T₁ and increased the strength of the new alloy greatly. Zn had a similar effect as Ag during the aging strengthening progress, when added with Mg. Among the three micro-alloying elements, Mg, Ag and Zn, Mg had the strongest influence on age strengthening. Compared with the combined additions of (Mg +Ag) and (Mg + Zn), (Ag + Zn) had the weakest influence on aging strengthening. Pre-deformation before aging promoted the precipitation of T1 phase which weakened the influence of micro-alloying elements (Mg, Ag and Zn) on strengthening the alloys and minished the strength difference between alloy containing (Mg + Ag + Zn) and alloys containing two of them.

Abstract: Microstructure evolution and mechanical properties of a 1469 alloy and a Sc-free1469 type alloy were examined. SEM observation indicates that AlCuSc ternary phases (W) are formed after homogenization annealing, and cannot be dissolved during the following heat treatments. These coarse particles consume abundant Cu atoms from the Al matrix that are available for solutioning, which results in the decrease of precipitation of the T1 phase during aging treatment. The W phase has negative effects on the examined alloy’s mechanical properties. The tensile strength of Sc-added alloy is 40MPa lower than that of the Sc-free alloy. The formation of the W phase has a close relationship with Cu/Sc ratio, which shows the importance of controlling the concentration of Cu and addition of Sc. Formation of W phase suppress the effect of precipitation hardening of the T1 phase in high strength Al-Cu-Li alloys

Abstract: The aluminium scrap market is undergoing rapid changes which will trigger off new recycling strategies. As the cast scrap market saturates it will become economically feasible to apply scrap also in aluminium wrought alloy production. As part of an EU project, Sustainable and efficient Production of Light weight solutions (SuPLight) a method for assessing life cycle environmental and economic consequences of applying aluminium scrap in high-end products has been developed. In this work, the method has been applied to assess life cycle environmental and economic impacts for six scenarios, embracing five various strategies for scrap treatment. This includes processes in material and component production, as well as fuel use during operation of vehicle and end-of-life treatment. The model for scrap strategies includes three grades of sorting and separation, plus simple refining by low-temp electrolysis and fluxing, and refining by Hoopes process. Not surprising, we find that sorting is beneficial compared to refining. More notable, perhaps, is the relative large difference between scenarios with regards to the environmental impacts considered. Finally, we discuss benefits from the life-cycle evaluation of scrap scenarios and use of the tool in business development.

Abstract: Aluminium dross is composed of metallic aluminium and non-metallic compounds such as oxides, nitrides and carbides. The dross created during primary and secondary aluminium production can amount up to 1% of the total tonnage and may contain up to 90% aluminium metal. The conventional procedure for dross treatment is to use salt for separating the metallic aluminium and the non-metallic compounds. The resulting salt cake has to be processed further. This paper describes a process using a plasma rotary furnace with a non-oxidizing atmosphere and heated by a graphite plasma torch to separate the aluminium from the non-metallic compounds. A pilot scale furnace (charge 500kg) has been designed and tested. In the experiment presented here a mixture of aluminium and aluminium dross were charged into the furnace. Good atmosphere control was maintained during the experiment. No wear on the refractory and only minor wear on the graphite electrodes was observed after the experiment. The result shows that 98.9% of the aluminium in the charged raw materials was recovered in tapped metal without using any salt anywhere in the process. The aluminium metal tapped from the process had only minor increase in the trace elements compared to the metal originally produced. The inclusion content in the metal was also not increased significantly. The residual non-metallic compounds should be possible to recycle in the electrolysis or used in the cement industry.

Abstract: The present paper introduces the microstructure and mechanical properties of the Al-Mg-Si-Mn alloy made by recycled materials, in which the impurity levels of iron are mainly concerned. It is found that the increased Fe content reduces the ductility and yield strength but slightly increases the UTS of the diecast alloy. The tolerable Fe content is 0.45wt.%, at which the recycled alloys are still able to produce castings with the mechanical properties of yield strength over 140MPa, UTS over 280MPa and elongation over 15%.The Fe content is steadily accumulated in the alloy with the increase of recycle times. However, after 13 cycles, the recycled alloys are still able to produce ductile alloys with satisfied mechanical properties.

Abstract: It is important for aluminum producers to predict metal quality and melt yield when scrap is remelted. Therefore, these parameters are studied in individual remelt experiments with (i) Turnings AA3005 and (ii) Coil AA3003 as scrap, as a function of percent added scrap up to 33 %. To ensure same melting conditions the scrap is melted in crucibles inside a Morgan resistance furnace, measuring temperature and hydrogen. Bifilm index is used as a measure of metal quality. The results indicate that the addition of scrap (up to 33 %) does not alter metal quality. Melt yield is however affected, with a metal recovery of around 90 % for the different types of scrap.