Hot Sheet Metal Forming Strategies for High-Strength Aluminum Alloys: A Review—Fundamentals and Applications

In the past decade, aluminum alloys have become important structural materials in the automotive industry, thanks to their low density, high strength, high fracture toughness, and good fatigue performance. However, an important limitation of aluminum alloys is their poor formability at room temperature; as a result, numerous studies have been conducted with the aim of developing forming techniques to overcome this and facilitate the forming of more complex-shaped components. Following an overview on the metallurgical background of aluminum alloys, this article reviews recent developments in forming processes for aluminum alloys. The focus is on process variants at room temperature and at higher temperatures and on a new hot forming technique promising considerable improvements in formability. This review summarizes the influence of different process parameters on microstructures and mechanical properties. Particular emphasis is given to process design and to the underlying microstructural phenomena governing the strengthening mechanisms.     

The effect of thermal exposure on the electrical conductivity and static mechanical behavior of several age hardenable aluminum alloys

Aluminum alloys 2014-T6, 2024-T3, 6061-T6, 7050-T7451, and 7075-T6 were thermally exposed at different times (1 min to 20 days) and temperatures 177–482 °C (350–900 F). This study was conducted to simulate the effects of heat damage on aluminum alloys and to determine the correlations existing between the static mechanical and electrical conductivity properties. Results indicate that at the temperatures below 260 °C (500 F) all five alloys showed clear correlations between the mechanical and physical properties.     

Improved fracture resistance of 7075 through thermomechanical processing

Aluminium alloys remain a popular choice of materials for use in subsonic aircraft systems. Their high strength to weight ratio, low cost and good formability make these alloys very attractive for use in lightweight structures that will not be exposed to severe temperature environments. Unfortunately though, the use of high strength aluminum alloys is limited by two factors—their relatively poor fatigue strength and their susceptibility to stress corrosion cracking. It is in the intent of this paper to review the improvements in these and other properties that have been achieved in 7075 at the Air Force Materials Laboratory through the use of relatively simple thermomechanical treatments.Two thermomechanical treatments have been considered, one a high strength condition equivalent to the conventional T-6 treatment and the other an over-aged condition equivalent to the T-73 treatment. However, because of the higher strength possible through thermomechanical processing the over-aged condition still maintains strength levels very close to that of conventional T-6. Data is presented showing the improvements obtained in fatigue strength, stress corrosion resistance and fracture toughness through the use of these processes.     

The Investigation of Thermal and Magnetic Properties and?Microstructure Analysis of?Cu�CAl�CMn Shape Memory Alloys

In this study, the transformation temperatures and shape memory effect of Cu?CAl?CMn shape memory alloys were investigated by calorimetric measurements. Cu?CAl?CMn alloys in the range of 10?C15 wt% of aluminum and 0?C10 wt% of manganese; exhibiting ??-phase at high temperatures, were prepared. With an increase in the aluminum and manganese concentrations of the alloy, the martensite morphology is modified and the transformation temperatures are decreased. The activation energies of the alloys were calculated according to the Ozawa and Kissinger methods. Magnetic properties of the alloys were studied and it was seen that the alloys show low magnetization. The microstructures of the alloys were investigated by optical micrographs where the martensite variants with different orientations can be observed from the micrographs.     

Verbund‐Gießschmieden hybrider Aluminiumbauteile

In compound‐cast‐forging of hybrid aluminum parts the positive characteristics of casting and forging processes as well as the different materials are combined. This makes it possible to produce components with complex geometries and different local characteristics. Component areas with high complexity (e. g. with an undercut) are cast, areas that are exposed to high mechanical stresses, are forged. In the conducted investigations a preformed, massive formed semi‐finished part out of a wrought aluminum alloy was joined with a die‐cast aluminum alloy by casting and forming from the casting temperature in one cast‐forging process. The results of the studies show a good joining quality of cast‐forged components made of different aluminum alloys.     

Interfacial effects between platinum alloys containing oxide dispersions

Oxide dispersion strengthened (ODS) platinum offers significant advantages over conventional wrought platinum alloys, particularly for aPfJications requiring very high strength and surface stability at temperatures up to 1650°C. In order to exploit these advantages in practical fabrications, it is frequently necessary to join ODS platinum to conventional platinum alloys by either solid-state or fusion welding techniques. Laboratory tests have shown that, when alloys of different composition are joined to an oxide-containing alloy, and when the weld is exposed for extended times at very high temperatures, a band of fine porosity can be formed in the oxide-containing material adjacent to the weld. The effect, which has not been previously reported, has been attributed to a Kirkendall diffusion mechanism; the enhanced porosity formation only occurs in the presence of the oxide dispersion. The conditions under which the phenomenon might occur in practice are established and a practical means of eliminating any potential problems in service is described.     

Characterization of electron beam welded AA2024

For aerospace manufacturing, the perseverance for improving performance (high strength to density ratio) and reducing weight and costs has motivated consideration of welding techniques applicable to aluminum alloys. During fusion welding of aluminum alloy (AA) 2024, the avoidance of defects (e.g., porosity, oxides, solidification cracking, undercutting) and the optimization of the microstructure-property characteristics are of critical concern. In this work, AA2024 was electron beam (EB) welded as part of a study to determine the influence of parametric conditions on the characteristics of the weldment to optimize the joining process. Specifically, the evolution in the weld geometry, microstructure and mechanical properties was examined as a function of the process conditions, including beam current, beam focus, beam oscillation, and welding speed. For optimized parametric conditions, microstructural examination of the joints revealed narrow fusion and heat-affected zones comprising of dendritic structures without the occurrence of defects that enabled a maximized joint efficiency.     

Water vapor effects on the cyclic oxidation resistance of alumina forming alloys

Abstract

Water vapor is present in most environments in which alloys are used at elevated temperatures and there are papers in the literature that show water vapor usually has adverse effects on the oxidation resistance of alloys. However, the exact effect of water vapor is dependent on the particular alloy under consideration. This paper is concerned with the oxidation of alloys that rely upon the development of α-Al₂O₃ scales for oxidation resistance. This paper describes two major deleterious effects of water vapor on the oxidation of such alloys. One effect involves increased spalling of α-Al₂O₃ which will be shown to be less significant in the case of alloys with extremely adherent α-Al₂O₃ scales. It is proposed that water vapor causes the α-Al₂O₃-alloy interfacial toughness to be decreased, however this effect is not sufficient to cause spalling of extremely adherent α-Al₂O₃ scales. Another effect of water vapor is that it causes more transient oxides to be formed during the selective oxidation of aluminum in alloys. This condition becomes more severe at lower temperatures. Possible mechanisms by which water vapor affects the selective oxidation of aluminum in alloys will be described.     

CASTING ALUMINUM-LITHIUM ALLOYS IN OPEN ATMOSPHERE

The increasing need for lightweight and cost-effective materials for structural applications has resulted in significant improvements and development of new aluminum alloys for structural applications. Lithium addition to aluminum has the potential for providing a class of high strength alloys with exceptional properties suitable for weight-critical and stiffness-critical applications. Casting of aluminum-lithium alloys presents a variety of problems and high-quality ingots are not readily obtainable through conventional procedures. The reactive nature of lithium necessitates the need for inert gas treatments in order to ensure melts of acceptable quality. Exploitation of the properties of these alloys is possible only if the alloys could be cast in an open atmosphere using conventional casting techniques. This paper describes a method for casting binary and ternary aluminum-3% lithium ingots. The alloys were cast using conventional techniques in an open atmosphere under salt flux. The melting procedure and casting technique are highlighted and the intricate characteristics of the castings are discussed. The mechanical properties of the heat treated and aged castings are also examined so as to provide a basis for understanding the quality of the cast ingots.     

Improved properties of A319 aluminum casting alloy modified with Zr

The beneficial effects of 0.15 wt.% Zr addition on mechanical properties and wear resistance of A319 aluminum casting alloy were investigated. The cast alloys were given a solutionizing treatment followed by artificial aging in the temperature range 175 to 235 °C for different period of times. Mechanical properties and wear behavior of the Zr-containing material were determined and compared to those of the base A319 alloy in both as-cast and age-hardened conditions. It is shown that minor addition of Zr results in the precipitation of Al₃Zr particles in the aluminum matrix. These particles are stable upon heating due to the low solubility of zirconium in aluminum matrix. The main effects of such particles are an increase in hardness, strength, quality index and wear resistance. This is very promising where these aluminum cast alloys are to be used at relatively high temperatures.     

Entwicklung von Diffusionsschichten auf Nickelbasiswerkstoffen für den Einsatz in chlorhaltigen Hochtemperaturprozessen

Development of diffusion coatings on nickel base alloys for the use in chlorine‐containing high temperature processes To open up the possibility of using sewage sludge ashes as fertilizers the removal of their heavy metal contents is obligatory. A process newly developed at the BAM Berlin executes this separation in highly chlorine‐containing atmospheres at temperatures of up to 1000 °C [1]. Unfortunately there are no materials available which can withstand such conditions over longer periods of time. This project deals with the development of materials that allow the operation in highly corrosive environments. The corrosion resistance of nickel base alloys against chlorine‐induced high‐temperature corrosion will be optimized by application of aluminum‐ and/or silicon‐containing diffusion coatings. As coating method the pack cementation process was selected. In this process, the metal to be coated is embedded in a powder, consisting of the coating metal, a halogen‐distributor (e.g. ammonium chloride) and aluminum oxide as filler material. During an annealing process of several hours at temperatures of 800 to 1000 °C, gaseous metal halides form. They diffuse through the powder pack and decompose at the substrate surface, thereby depositing the coating metal. Subsequent solid phase diffusion results in the formation of a protective diffusion layer. From the thermodynamic point of view, materials with a high content of aluminum and silicon show best prerequisites to build up slow‐growing, stable oxide layers with a high potential to protect the material against corrosive attacks. The actual performance of the materials will be examined in long‐time tests under simulated field conditions (high temperatures and chlorine‐containing atmospheres).     

Al-Cu-Mg-Ag合金与ZL205A合金性能与微观组织对比

采用半连续铸造法分别制备了Al-Cu-Mg-Ag合金与ZL205A合金。对两种铝合金的流动性能、室温及高温拉伸性能、拉伸断口形貌、晶间腐蚀性能和微观组织进行了对比分析。结果表明:浇铸温度为720℃和740℃时,Al-Cu-Mg-Ag合金的流动性能均优于ZL205A合金的。室温、150℃、200℃、250℃下,Al-Cu-Mg-Ag合金的强度均高于ZL205A合金的,且随着温度的升高,其强度降低的幅度要明显低于ZL205A合金的,两种材料的室温伸长率基本相当。Al-Cu-Mg-Ag合金的晶间腐蚀深度在182～246 μm之间,优于ZL205A合金的274～337 μm。Al-Cu-Mg-Ag合金无析出带的宽度要窄于ZL205A合金的。      

Precipitation in liquid-quenched Al-base Ag alloys

Al-base Ag alloys have been quenched from the liquid state and aged at temperatures within the metastable miscibility gap. It is found that in the liquid-quenched condition the alloys contain a solute cluster size-distribution which is considerably narrower than that obtained by normal bulk-quenching. Indeed, liquid-quenching is far more effective than fast bulk-quenching in minimizing clustering. Electrical resistivity, wide-angle and smallangle X-ray techniques and transmission electron microscopy have been used to study the evolution of the decomposition products on ageing the liquid-quenched alloys.     

Ca-modified Al–Mg–Sc alloy with high strength at elevated temperatures due to a hierarchical microstructure

Al-Mg alloys are normally prone to lose part of their yield and tensile strength at high temperatures due to insufficient thermal stability of the microstructure. Here, we present a Ca-modified Al–Mg–Sc alloy demonstrating high strength at elevated temperatures. The microstructure contains Al₄Ca phases distributed as a network along the grain boundary and Al₃(Sc,Zr) nano-particles dispersed within the grains. The microstructure evolution and age-hardening analysis indicate that the combination of an Al₄Ca network and Sc-rich nano-particles leads to excellent thermal stability even upon aging at 300 °C. The tensile strength of the alloy for temperatures up to 250 °C is significantly improved by an aging treatment and is comparable with the commercial heat-resistant aluminum alloys, i.e., A356 and A319. At a high temperature of 300 °C, the tensile strength is superior to the above-mentioned commercial alloys, even more so when expressed as the specific strength due to the low density of Ca-modified Al–Mg–Sc alloy. The excellent high-temperature strength results from a synergistic effect of solid solution strengthening, grain boundary strengthening and nanoparticle order strengthening.

     

Transformation in metal materials during hot isostatic pressing

The paper presents a review of transformations in a number of metal materials during hot isostatic pressing which result in changes in the shape, density, morphology, and composition of the structural components of some metals, including cast, powder, and composite titanium alloys, heat-resistant nickel alloys, hard alloys of the WC-Co system, and also some aluminum and aluminide materials. Methods for studying the compaction processes using an eddy current dilatometric cell and differential barothermal analysis to determine the baric shift of the characteristic temperatures for some heat-resistant nickel alloys are described. The data on the practical application of hot isostatic pressing are presented, which makes it possible to fabricate the most important parts of gas turbines. It is shown that hot isostatic pressing causing transformations of different physicochemical origin should be considered as a necessary step of the production of metals to ensure their limiting strength characteristics.     

从航空看轨道交通高强铝合金的发展趋势

本文简要介绍了高强铝合金在航空和轨道交通领域的应用,综述了航空高强铝合金微合金化发展历程以及微量合金元素对7系高强铝合金性能的影响。最后针对轨道交通铝合金的特点指出系统开展基于综合考虑可焊性和防腐性能的微量元素合金化是研发轨道交通高强铝合金的发展趋势。     

Tensile behaviour of squeeze cast AM100 magnesium alloy and its Al2O3 fibre reinforced composites

Magnesium alloys are increasingly used in automotive and aerospace applications mainly due to their light weight combined with reasonably high tensile properties. In addition to providing a large reduction in weight, magnesium alloys exhibit excellent machinability and good damping capacity. However, their low mechanical properties when exposed to elevated temperatures limit their usage. Making composites out of these magnesium alloys by reinforcing them with ceramic particles or fibres appears to be a viable alternative for improving their thermal stability. The work reported here involved experimental studies on the tensile behaviour of AM100 magnesium alloy and its composites at different temperatures. Fractographic studies justify the effect of temperature on the tensile behaviour.     

Hot-corrosion-resistant duplex coatings for a superalloy

The goal of this program was to evaluate the hot corrosion resistance of a variety of cobalt-, nickel- and iron-based coating alloys containing chromium and sometimes aluminum and/or yttrium. The coatings were deposited onto two nickel-based superalloys by physical vapor deposition from an electron-beam-heated source. As a group, the cobalt-based compositions were found to be much more hot corrosion resistant than the nickel-based alloys. The iron-based coatings had relatively poor hot corrosion resistance.The coefficients of thermal expansion for a number of Co-Cr-Al-Y alloys were measured. Compositions with low (0–6%) aluminum and high (26–35%) chromium contents were found to have a reasonably close thermal expansion match with IN-738. If a duplex overlayer of aluminum was diffused into the outer regions of these coatings, it greatly enhanced their hot corrosion resistance.Ductile-to-brittle transition temperatures in the range 760–871 °C were measured for several Co-Cr-based coatings on IN-738. Measurements made on similarly coated IN-738 test bars showed that stress-rupture properties were generally lower than the baseline data for IN-738 but were above the standard deviation (3σ) curve.     

超高强铝合金研究进展

超高强铝合金具有很高的强度和韧性,是航空航天领域极具应用前景的结构材料.评述了超高强铝合金的国内外发展情况,论述了铝合金的强化技术和方法,并就今后的研究开发提出了建议.