Materials Science Forum Vols. 519-521

Abstract: Al-Fe compounds are usually present in the as-cast microstructure of Al-alloys as large needles or plates. As such, they have a detrimental effect on the mechanical properties of Al-alloys containing Fe, either as an impurity element or as an alloying addition. However, Fe-containing Al-alloys also offer attractive physical properties, such as improved stiffness, wear resistance and thermal resistance. If the needle and plate morphology of the Al-Fe compounds can be modified to a more compact morphology, with refined particle size and uniform distribution, the mechanical properties of Al-Fe based Al-alloys can be substantially improved, and therefore, they will find wider applications in many engineering sectors. A new semisolid metal processing technology, rheodiecasting (RDC), has been developed for production of Al-alloy components with high integrity. The RDC process innovatively combines the dispersive mixing power of the twin-screw mechanism, for the creation of high quality semisolid slurry, with the high efficiency, low cost nature of the high-pressure diecasting (HPDC) process for component shaping. In this paper, we present our experimental results on the effects of intensive melt shearing on the size and morphology of Al-Fe compounds in A380 alloys, with different levels of Fe additions. The experimental results have shown that intensive melt shearing during solidification can effectively change the particle shape from the usual needles and plates, to an equiaxed morphology. Samples which have undergone with melt shearing, exhibit much improved strength and ductility compared to those with the same level of Fe addition, but without exposure to melt shearing.

Abstract: The role of bismuth (50 to 9000 ppm) and calcium (50 to 200 ppm) additions on the microstructural characteristics in Sr-modified 319 alloys (with/without 0.4 wt% Mg addition) were investigated using optical and electron microscopy, and image analysis. It was found that the modification effect of Sr continuously diminished with Bi addition up to ~3000 ppm Bi; further Bi addition led to the modification of the Si particles due to the presence of Bi. In the Ca-containing alloys, a coarse eutectic Si structure resulted with Ca additions of 50 ppm, due to the formation of Alx(Ca,Sr)Siy compounds. Increased Ca additions (up to 200 ppm) did not alter the Si particle size. The Alx(Ca,Sr)Siy phase particles appeared in rod-like form in the Sr-modified alloys and in plate-like form in the 319+0.4 wt% Mg alloys. MgO, Al2O3, and AlP particles appear to act as nucleants for the precipitation of the plate-like Alx(Ca,Sr)Siy phase.

Abstract: Aluminum alloy is one of the candidates for the liners of compressed hydrogen tank mounted to fuel cell vehicles. It is crucial to elucidate the behavior of hydrogen in the alloy sheet with one side being exposed to hydrogen gas. In the present work, using the hydrogen microprint technique, in 6061 and 7075 aluminum alloy sheets, relationship between hydrogen pressure and the molar quantity of hydrogen emitted from the inside has been investigated. Under any pressure, the quantity of emitted hydrogen is about 10 times smaller in the 7075 alloy than in the 6061 alloy. This indicates that the amount of hydrogen atoms accumulating in the 7075 alloy may be much larger than that in the 6061 alloy.

Abstract: Two principal approaches are available to materials’ engineers to improve the overall cost-weight balance of metallic airframe structures: improving alloy performance and optimising materials’ utilisation. Although both approaches have been successful in the past, they are most effective when applied concomitantly. The Aluminium industry has a long record of improving aerospace alloys’ performance. Nevertheless, even in apparently well-explored alloy systems such as the 7xxx family, products with improved damage tolerance-strength balances have recently been developed, thanks to an improved understanding of the optimum Zn-Mg-Cu combinations for the required property balances but also to developments in casting capability. Novel dispersoids and dispersoid combinations have enabled further improvements of the performance of existing alloy families. For example, appropriate Sc and Zr additions have a significant impact on the grain structure of 2xxx alloys and thus on performance. Another high potential approach for alloy performance improvements is the optimisation of Al-Cu-Li-(Mg-Ag-Zn) alloys. These so-called “third generation Al-Li alloys” were principally developed for military and space applications; in order to meet the demands of future commercial airframes, more damage tolerant variants are being developed. AA2198 and AA2050 are used to illustrate the potential of these higher damage tolerance Al-Cu-Li alloys. However, materials performance improvements are only part of the potential developments of metallic solutions for airframes. Further gains of a similar magnitude in component weight and cost can be achieved by applying new technologies and new design solutions to metallic structures. The future of metallic airframes will depend on the concomitant application of both these approaches.

Abstract: Interpenetrating composites allow a completely 3-dimensional matrix of two phases, in this case an alumina (ceramic) and aluminium-magnesium alloy (metal), to be developed. This 3-dimensionality yields a material with mechanical and physical properties that are superior to either the metal or ceramic individually. The composites were produced by heating an alumina foam and aluminium-magnesium alloys together in flowing nitrogen to in excess of 900°C. At these temperatures the alloy is drawn into the ceramic foam by capillary action. The infiltration process is dependent on the interaction of the alloy with the nitrogen atmosphere in the furnaces. This complex interaction and its affect on the microstructural development has been studied using Electron Backscatter Diffraction (EBSD) coupled with Energy Dispersive x-ray Spectroscopy (EDS).

Abstract: Superplasticity refers to a high temperature deformation process involving a marked sensitivity of the flow stress to the imposed strain rate, with resulting enhanced ductility. Although conventionally associated with fine-grained materials, superplasticity has recently been observed in coarse-grained alloys. The present research involves the deformation behavior of Al-Mg base alloys, where superpure Al-3%Mg and Al-5%Mg, and commercial Al 5056 were selected for study. The results for the Al-5%Mg and Al 5056 alloys are presented in this article. Flat sheet-type samples were tensile tested to 10% strain at increasing temperatures and at prescribed strain rates (0.001/s, 0.01/s, and 0.1/s). The dependence of flow stress on temperature was found to display some unusual characteristics. This behavior is interpreted as resulting from the occurrence of dynamic strain ageing (DSA). The aim of the overall study is to determine the relation between DSA and superplasticity in coarse-grained Al-Mg alloys. This will, in turn, lead to the control of the strain ageing behavior so as to produce the largest possible values of strain rate sensitivity (and, hence, elongation).

Abstract: The electron beam freeform fabrication (EBF3) layer-additive manufacturing process has been developed to directly fabricate complex geometry components. EBF3 introduces metal wire into a molten pool created on the surface of a substrate by a focused electron beam. Part geometry is achieved by translating the substrate with respect to the beam to build the part one layer at a time. Tensile properties have been demonstrated for electron beam deposited aluminum and titanium alloys that are comparable to wrought products, although the microstructures of the deposits exhibit features more typical of cast material. Understanding the metallurgical mechanisms controlling mechanical properties is essential to maximizing application of the EBF3 process. In the current study, mechanical properties and resulting microstructures were examined for aluminum alloy 2219 fabricated over a range of EBF3 process variables. Material performance was evaluated based on tensile properties and results were compared with properties of Al 2219 wrought products. Unique microstructures were observed within the deposited layers and at interlayer boundaries, which varied within the deposit height due to microstructural evolution associated with the complex thermal history experienced during subsequent layer deposition. Microstructures exhibited irregularly shaped grains, typically with interior dendritic structures, which were described based on overall grain size, morphology, distribution, and dendrite spacing, and were correlated with deposition parameters. Fracture features were compared with microstructural elements to define fracture paths and aid in definition of basic processingmicrostructure- property correlations.

Abstract: Electron beam freeform fabrication (EBF3) is a new layer-additive process that has been developed for near-net shape fabrication of complex structures. EBF3 uses an electron beam to create a molten pool on the surface of a substrate. Wire is fed into the molten pool and the part translated with respect to the beam to build up a 3-dimensional structure one layer at a time. Unlike many other freeform fabrication processes, the energy coupling of the electron beam is extremely well suited to processing of aluminum alloys. The layer-additive nature of the EBF3 process results in a tortuous thermal path producing complex microstructures including: small homogeneous equiaxed grains; dendritic growth contained within larger grains; and/or pervasive dendritic formation in the interpass regions of the deposits. Several process control variables contribute to the formation of these different microstructures, including translation speed, wire feed rate, beam current and accelerating voltage. In electron beam processing, higher accelerating voltages embed the energy deeper below the surface of the substrate. Two EBF3 systems have been established at NASA Langley, one with a low-voltage (10-30kV) and the other a high-voltage (30-60 kV) electron beam gun. Aluminum alloy 2219 was processed over a range of different variables to explore the design space and correlate the resultant microstructures with the processing parameters. This report is specifically exploring the impact of accelerating voltage. Of particular interest is correlating energy to the resultant material characteristics to determine the potential of achieving microstructural control through precise management of the heat flux and cooling rates during deposition.

Abstract: The structure and phase composition of dispersion-strengthened composite materials based on multicomponent aluminium alloys of the Al–Cu–Mg and Al–Si–Cu–Fe systems at various stages of mechanical alloying were studied by the methods of optical, scanning electron and ion microscopies, electron probe microanalysis and X-ray diffraction analysis. A possibility of the efficient use of commercial scrap of Al alloy chips and initially large strengthening ceramic particles as the base of composite materials was shown, as well as a possibility of synthesis of strengthening particles during the treatment. After mechanical alloying, these materials possessed a homogeneous and disperse structure, high hardness at room and elevated temperatures and a low linear thermal expansion coefficient.

Abstract: Pure aluminum ingot (99.8 wt%) was melted to prepared chilled samples in this study. These samples were then removed to polish their surfaces and put in an ultrasonic cleaner filled with tap water. The polished surface would gradually show foggy marks after being subjected to a period of treating time. Oxide films, if entrapped, would crack, erode and detach from the chilled sample forming foggy marks on the polished surface. The sample then removed to measure oxygen and aluminum concentrations varied along the transition layer between the oxide film and aluminum matrix. Part of chilled samples was melted in a muffle furnace and subjected to different holding time. As the holding time increased, the transition layer between the oxide film and the matrix was increased and composed of different constituents varying from the Al matrix to the oxide film (mainly γ-Al2O3). This transition layer also showed different hardness measured by a nano-hardness tester. The morphologies of cracked oxide film and the eroded oxide particles were affected by the holding time after melted, and small amounts of silicon in the pure aluminum.