Materials Science Forum Vols. 638-642

Abstract: The texture, the formability and the correlation between formability indices of Al-0.9Mg- 1.0Si-0.7Cu-0.6Mn alloy for automotive body sheets subjected to solid solution, T4, annealing treatment and artificial aging at 443k for different time were investigated by orientation distribution functions(ODF) analysis, tensile and cupping test, FLD measurement and regression analysis method. The results showed that the textures of cold rolled alloy sheets consist mainly of copper and brass orientations, which are transformed into the texture mainly containing {001}<310> orientation after recrystallization, and aging treatment has little influence on the recrystallization texture. The formability of alloy sheets subjected to solid solution, T4 and annealing treatment is similar, however, the formability was observably deteriorated after aging at 443k. The correlation between uniform elongation δu and FLD0 is the most remarkable in all the given formability indices, the correlation between strain-hardening exponent n and the FLD0 take second place, while there is no correlation between plastic strain ratio r and FLD0. The correlation between reduction of area ψ and cupping value IE is distinct, while ψ and IE have little correlation with FLD0.

Abstract: Ultrasonic vibration has been applied to various molten metal processes owing to the functions of (a) improvement in wettability, (b) liquid adhesion at a vibrating end surface and (c) sono-solidification such as grain refinement. The present study is focused on the sono-solidification with acoustic cavitaion in hypereutectic Al-18mass%Si alloy. There appears an equilibrium microstructure composed of primary silicon and coupled eutectic -Al/Si phases in Al-18mass%Si alloy, however, non-equilibrium -Al grains develop along with the equilibrium phases through the sono-solidification. During the sono-solidification of Al-18mass%Si alloy, non-equilibrium -Al grains are recognized in the molten metal close to the ultrasonic radiator just before reaching the eutectic temperature of 577 oC in addition to the refined primary silicon particles. The appearance of -Al grains is understood through acoustic cavitation: ultrasound in molten Al-Si alloys exhibits two outstanding behaviors of cavitation bubbling and acoustic streaming. Firstly the de-coupled eutectic reaction, which is recognized in the solidified eutectic Al-Si alloy with severe stirring, causes divorced -Al grains by the acoustic streaming with cavitation. Secondly it is expected that high pressure of over 1 GPa generated by the collapse of cavitaion babbles leads to not only an increase in the eutectic temperature, but also higher silicon content at the eutectic point in Al-Si alloy. Consequently, non-equilibrium -Al grains are nucleated at collapsed cavitaion bubble sites, and they are characterized by higher silicon content compared with that of primary -Al grains in hypoeutectic Al-7masst%Si alloy.

Abstract: In cutting aluminum alloy 6061, continuous chips have a negative influence on the machining operation. Usually, Pb is added in order to break continuous chips. However, from the standpoint of environmental protection, it is necessary to improve chip breakability without adding Pb. One effective measure is improving chip breakability by adding Si in aluminum alloy 6061. However, the influence of Si content on tool wear has not been fully examined. In this study, in order to clarify the influence of a diamond-like carbon (DLC) coating layer on cutting performance, aluminum alloys having different Si contents were turned. The substrate of the tool material was high speed steel (1.4%C). The chip configuration, cutting force and tool wear were experimentally investigated. The following results were obtained: (1) The DLC coating layer was effective for decreasing the cutting force. (2) In cutting Al-2mass%Si alloy, the wear progress of the DLC-coated tool was slower than that of the un-coated tool. The length of a chip with the DLC-coated tool was shorter than that with the un-coated tool.

Abstract: In this study, a high strength Al-Zn-Mg-Cu alloy was prepared by the spray deposition technique. The microstructures of spray-deposited and homogenized Al-Zn-Mg-Cu alloys were studied using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results indicate that the microstructure of the spray-deposited alloy mainly is composed of the Al matrix and the Mg(ZnCu)2 compounds. The spray-deposited process has an obvious modification in size, morphology and distribution of the secondary phases in the microstructure as well as reduce of segregation. After homogenization, the coarsening of the grains in the microstructure is not obvious, a phase transformation of primary Mg(ZnCu)2 particles to Al2CuMg phase was found.

Abstract: Synchrotron X-ray microtomography was used to investigate the three-dimensional fatigue crack propagation behavior of an aluminum alloy. Local crack growth rates along specimen thickness were calculated. Crack observation revealed that crack propagation was retarded in three regions where there exist overlapping crack segments. Crack growth retardation was found to occur in these crack overlapping regions due to stress shielding. Observation of crack propagation process indicated that one of the overlapping crack segments initiated from the twisted crack. Crack tip opening displacement was measured and it was found that crack opening was larger in single planar crack region than that in crack overlapping region.

Abstract: The in-situ investigation on morphology, chemical contents and crystal structure of non-equilibrium eutectics in semicontinuous casting ingot of Al-6.2Zn-2.3Mg-2.3Cu super-high strength aluminum alloy have been carried out by transmission electron microscopy equipped with energy dispersive X-ray spectroscopy (TEM-EDX). The results show that a large amount of lamellar non-equilibrium eutectics exists in semicontinuous casting ingot of this alloy; the non-matrix phase of this eutectics is a secondary phase which contains Al, Zn, Mg and Cu elements, with the atom ratio of approximately 1:1:1:1, and this non-matrix phase of non-equilibrium eutectics owns the same crystal structure as that of η phase (MgZn2).

Abstract: Precipitates in Al-Mg-Si-(Cu) alloys all contain a similar hexagonal arrangement of Si-atoms. Precipitates come and go but their inner Si ordering appears to vary little throughout the precipitation process. In order to improve understanding of precipitation and the related material properties, it is becoming increasingly clear that this includes a good understanding of the hexagonal Si-network, its relation to the precipitates and the Al matrix. Previous studies have revealed that adding Cu atoms to the ternary system, causes the Si network to twist slightly in the matrix about its hexagonal axis, favoring different precipitates. Here we investigate these two rotations. It is shown they can be viewed as a mirror of the network itself about a {310} Al plane. Since precipitates are coherent, the Si-network with its triangular arrangements of Si must also match a fourfold arrangement of Al on the {100} planes. Sets of Al lattice positions exist which can approximate the tree-fold Si symmetry, according to the experimentally observed orientations, and one or more large super-cells can be found having near fit in both lattices. The mirror plane is a main plane in one such super-cell. We show that the mirror leaves every seventh node of the network unchanged, thus defining a smaller hexagonal super-cell in the network, similar to the B’ or Q’/Q phase, where corners are invariant, but where the Si contents is flipped.

Abstract: Roping was investigated in two 6016 aluminium alloys that exhibit different levels of susceptibility to its occurrence. The level of roping is lower, as manifested by the less pronounced (roping) lines on the surface, in the GR material compared to the BR case. Through-process characterization of GR and BR materials by means of electron backscatter diffraction (EBSD) reveals similarities in the grain size, (grain) orientation texture and the spatial distribution of {100} <001> Cube grains up to, but not including the T4 state. Cube grains in the T4 state are spatially banded in the BR material but more uniformly distributed in the GR case. It was found that the thermo-mechanical treatments prior to the T4 state account for the difference in spatial distributions of Cube grains and hence, the different roping behaviours exhibited by these materials.

Abstract: Beta solution heat treatment is used to increase the fatigue crack growth resistance of Ti-6AL-4V. Unfortunately, the beta solution heat treatment is very sensitive to maximum temperature, time at temperature and cooling rate. In order to determine the effect of these parameters on mechanical properties, several different titanium billets and forgings were heat treated at various times and temperatures. The forgings had differing amounts of work, reflecting the potential for thermomechanical processing differences seen in a die forged component. Fracture toughness and tensile tests were conducted on the billets and forgings. In addition, sections of each work piece were excised and examined microscopically. The results from the study indicate there is a significant effect of heat treatment on thicker section components. In these cases, grains near the surface may grow large, while being barely transformed near the center. The change in microstructure has an effect on mechanical properties. Material with the larger grains tends to have worse ductility, while the fracture toughness properties of the material tend to decrease with grain size.

Abstract: Titanium alloys are usually used as structural parts in forged and subsequent heat treated state. Up to now, there have been rare researches appeared on their shaping or forming by multi-stand tandem hot rolling. In this article, we choose Ti-6Al-4V as experimental material and a simulated research has been carried out in a thermo-mechanical simulator MMS-300 to model its hot-rolling process. The plastic flow behavior for Ti-6Al-4V alloy during hot-working has been determined at various deformation temperatures and deformation rates by considering its hot-rolling in single or two phase region. The deformation mechanisms at elevated temperatures are also analyzed in correlation with its deformation activation energy. This research can provide guidance in designing processing parameters for hot-rolling of Ti-6Al-4V alloy.