Compression testing of periodic cellular sandwich cores

Periodic cellular metal (PCM) hybrid sandwich cores with 95 pet open porosity have been constructed from perforated 3003 aluminum alloy (AA3003) sheets using perforation-stretching methods. Two compressive collapse mechanisms (i.e., plastic hinging and plastic buckling) were studied using two limiting test conditions: first, where the PCM nodes were restricted only by interfacial friction (i.e., free compression) and compressive forces were resisted primarily through strut bending and plastic hinging mechanisms; and, second, where the PCM nodes were laterally confined (i.e., confined compression) and compressive forces were resisted primarily through strutbuckling mechanisms. The contribution of each collapse mechanism to the overall truss core performance was studied. The strut bending during free compression was tracked by a nodal displacement mapping (NDM) technique, while the progression of confined compression strut buckling was correlated to the truss core stress-strain profile. The present data can be used to illustrate the different strengths between strut bending (free-PCM) and strut buckling (confined-PCM) collapse mechanisms.     

Development and Processing of Novel Aluminum Powder Metallurgy Materials for Heat Sink Applications

The objective of this research was to design aluminum powder metallurgy (PM) alloys and processing strategies that yielded sintered products with thermal properties that rivaled those of the cast and wrought aluminum alloys traditionally employed in heat sink manufacturing. Research has emphasized PM alloys within the Al-Mg-Sn system. In one sub-theme of research, the general processing response of each PM alloy was investigated through a combination of sintering trials, sintered density measurements, and microstructural assessments. In the second, the thermal properties of sintered products were studied in detail. Thermal conductivity was first determined using a calculated approach through discrete measurements of specific heat capacity, thermal diffusivity, and density and subsequently verified using a transient plane source technique on larger specimens. Experimental PM alloys achieved >99 pct theoretical density and exhibited thermal conductivity that ranged from 179 to 225 W/m K. Thermal performance was largely dominated by the amount of magnesium present within the aluminum grains and, in turn, bulk alloy chemistry. Data confirmed that the novel PM alloys were highly competitive with even the most advanced heat sink materials such as wrought 6063 and 6061.     

The effects of molybdenum and aluminum on the thermal expansion coefficients of nickel-base alloys

The effects of molybdenum and aluminum on the mean linear thermal expansion coefficients from room temperature to 1050°C were determined for two types of nickel-base alloys. The Solid Solution Alloys were cast and homogenized Ni-Co-Cr-Mo alloys with 0, 312, and 612 nominal wt pct molybdenum concentrations. The Gamma Prime Alloys were wrought and heat-treated Ni-Cr-Mo-Al(Ti) alloys with 0, 2, 5, and 8 nominal wt pct molybdenum in each of four aluminum plus titanium levels (3 pct Al, 412 pct Al, 6 pct Al, or 1 pct Al + 312 pct Ti nominal wt pct). Thermal expansion coefficients were determined on at least two specimens from each alloy. It was found that molybdenum lowers the thermal expansion coefficients of both the cast Ni-Co-Cr solid solutions and the wrought Ni-Cr-Al(Ti) two-phase alloys. Both aluminum and titanium were also observed to decrease expansion coefficients in the two-phase, γ + γ, alloys. Results are discussed in terms of relative melting point effects between solute and solvent elements, and in terms of the volume fraction of the γ phase present.     

The effects of molybdenum and aluminum on the thermal expansion coefficients of nickel-base alloys

The effects of molybdenum and aluminum on the mean linear thermal expansion coefficients from room temperature to 1050°C were determined for two types of nickel-base alloys. The Solid Solution Alloys were cast and homogenized Ni-Co-Cr-Mo alloys with 0, 312, and 612 nominal wt pct molybdenum concentrations. The Gamma Prime Alloys were wrought and heat-treated Ni-Cr-Mo-Al(Ti) alloys with 0, 2, 5, and 8 nominal wt pct molybdenum in each of four aluminum plus titanium levels (3 pct Al, 412 pct Al, 6 pct Al, or 1 pct Al + 312 pct Ti nominal wt pct). Thermal expansion coefficients were determined on at least two specimens from each alloy. It was found that molybdenum lowers the thermal expansion coefficients of both the cast Ni-Co-Cr solid solutions and the wrought Ni-Cr-Al(Ti) two-phase alloys. Both aluminum and titanium were also observed to decrease expansion coefficients in the two-phase, γ + γ, alloys. Results are discussed in terms of relative melting point effects between solute and solvent elements, and in terms of the volume fraction of the γ phase present.     

A Methodology to Predict the Effects of Quench Rates on Mechanical Properties of Cast Aluminum Alloys

The mechanical properties of age-hardenable Al-Si-Mg alloys depend on the rate at which the alloys are cooled after the solutionizing heat treatment. Quench factor analysis, developed by Evancho and Staley, was able to quantify the effects of quenching rates on the as-aged properties of an aluminum alloy. This method has been previously used to successfully predict yield strength and hardness of wrought aluminum alloys. However, the quench factor data for aluminum castings is still rare in the literature. In this study, the time-temperature during cooling and hardness were used as the inputs for quench factor modeling. The experimental data were collected using the Jominy end quench method. Multiple linear regression analysis was performed on the experimental data to estimate the kinetic parameters during quenching. Time-temperature-property curves of cast aluminum alloy A356 were generated using the estimated kinetic parameters. Experimental verification was performed on a cast engine head. The predicted hardness agreed well with that experimentally measured. The methodology described in this article requires little experimental effort and can also be used to experimentally estimate the kinetic parameters during quenching for other aluminum alloys. This article is based on a presentation made in the symposium “Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties” which occurred March 12–16, 2006, during the TMS Spring meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.     

铝锂合金材料研究应用现状与展望

铝锂合金是一类具有广阔应用前景的航空航天结构材料，已经历经90余年的发展历程.从铝锂合金的研发、生产和应用角度看，我国与国外仍存有不小差距.按成型方式铝锂合金可分为变形和铸造2大类, 目前研究较多和广泛应用的是变形铝锂合金.文中从析出相、热处理、腐蚀性和焊接性等方面介绍了变形铝锂合金的研究现状.同时从发展应用以及合金开发等方面, 总结了铸造铝锂合金的研究现状.结合当前国内外铝锂合金的研究现状对铝锂合金的未来发展做出了展望.      

Integrity of Sandwich Panels and Beams with Truss-Reinforced Cores

This paper outlines several issues related to the design of sandwich structures with a truss-reinforced polymeric or hollow core. The first part of the paper employs a simple analytical model that illustrates the contribution of such a core to the structure’s facing stability. While the enhancement of the facing stability due to truss reinforcement is shown to be considerable, the problems of local strength and truss-element (pin) stability have to be analyzed prior to the acceptance of the design concept discussed in the paper. Accordingly, the second part of the paper presents the results of finite-element analysis used to pinpoint potentially weak elements in the considered design. The paper results in a number of practical conclusions and recommendations that may be useful for the development and implementation of sandwich structures with truss-reinforced cores.     

Relationships between process parameters and temperature field of roll casting for wide sheet made of AZ61 magnesium alloy: Finite element method

In recent years, wide sheet made of AZ61 wrought magnesium alloys has been widely studied and applied in industry. Thin roll-casting technology for the new wrought magnesium alloy can provide acceptable quality wide and thin sheet made of AZ61 magnesium alloy. To study the influences of roll-casting process parameters on temperature field for wide and thin sheet made of AZ61 magnesium alloy plates, some simplification and assumptions have been done by characteristics of magnesium alloy. Two-dimensional FEM model for roll-casting has been established along casting direction. Simulations of temperature fields of the plates have been done by using finite element analysis ANSYS software. A series of researches on the temperature distributions under different process parameters (pouring temperature, heat-transfer coefficients and casting speeds) have been done. The simulation results and the literature about the casting process of the relevant theory are the same. The simulation results show that the process parameters of rapid-casting process for AZ61 magnesium alloy are mutual influenced on the temperature fields of wide sheet made of AZ61 magnesium alloy.     

Mechanics of Composite Sinusoidal Honeycomb Cores

Lightweight and heavy-duty fiber-reinforced polymer (FRP) composite honeycomb sandwich structures have been increasingly used in civil infrastructure. Unique cellular core configurations, such as sinusoidal core, have been applied in sandwich construction. Due to specific core geometry, the solutions for core effective stiffness properties are not readily available. This paper presents a mechanics of materials approach to evaluate the effective stiffness properties of sinusoidal cores. In particular, the internal forces of a curved wall in a unit cell are expressed in terms of resultant forces, and based on the energy method and principle of equivalence analysis, the in-plane stiffness properties of sinusoidal cores are derived. Both finite-element modeling and experimental testing are carried out to verify the accuracy of the proposed analytical formulation. To illustrate the present analytical approach as an efficient tool in optimal analysis and size selection of sinusoidal cores, several design plots are provided and discussed. The simplified analysis and formulation presented for sinusoidal cores can be used in design application of FRP honeycomb sandwich and optimization of efficient cellular core structures.     

欧盟对航空航天器变形镁合金应用的评估

回顾了变形镁合金的生产与在航空器方面的应用。2005年～2008年欧盟在PF6框架内进行了一个名为"AEROMAG"的研究计划,对现行变形镁合金的加工性能、成形工艺、表面处理、燃烧性能、连接工艺与结构性能等作了精心的评价,没有一个镁合金的各项性能可全面与5083及2024-T3铝合金的相抗衡,镁合金现在还没有取代航空铝合金的可能性。     

Comparison of thermodynamic databases for 3xx and 6xxx aluminum alloys

Computational thermodynamics, or Calculation of Phase Diagram (CALPHAD) methods have proven useful in applications to modeling a variety of alloy properties. However, the methods are only as accurate as the thermodynamic databases they use, and two commercial thermodynamic databases exist for aluminum alloys: Thermotech and Computherm. In order to provide a critical comparison of these databases, we used both the databases to calculate equilibrium solid-state phase fractions and phase diagram isothermal sections of several industrial aluminum alloys: a 319-type and 356 cast alloys, as well as the wrought alloys 6022 and 6111. All of these alloys may be generically described as being based on the Al-Mg-Si-Cu quaternary with other additions such as Fe, Mn, and Zn. Although many of the results are consistent between the two databases, several qualitative and quantitative differences were observed. Many of these differences are found to be due to the intermetallic compounds involving Fe, Mn, Cr, and Zn. On the other hand, thermodynamics involving only phases from the Al-Mg-Si-Cu quaternary show good agreement between the databases, although some small differences still exist, particularly involving the quaternary Q phase. To understand and assess these differences, formation enthalpies and reaction energies from the databases were compared against density functional first-principles energetics. These comparisons indicate possible avenues for future improvements of Al-alloy thermodynamic databases. Finally, we demonstrate an interesting correlation between the calculated phase fractions and the measured yield strengths across this wide family of 3xx cast and 6xxx wrought alloys.     

On the advantages of using powder metallurgy in new light metal alloy design

Historically, the production of metallic components for the automotive and aerospace industries has been dominated by wrought and ingot metallurgy metal forming practices. These technologies offer considerable design flexibility to engineers and are readily amenable to ferrous and nonferrous alloys alike. However, in applications that require precise dimensional control, the tolerances attainable are generally inadequate. This represents a formidable limitation and mandates the incorporation of expensive secondary machining. Furthermore, because these processes are carried out under conditions that approach those of equilibrium, these technologies are also faced with rather strict limitations on the range of alloy chemistries that can be employed. As the demands for improved material performance and process economics increase, the aforementioned shortcomings become increasingly important. Consequently, considerable attention has and continues to be focused on alternate metal forming techniques such as powder metallurgy (P/M). Using the P/M approach, dimensional tolerances are commonly improved by one to two orders of magnitude and alloy chemistry limitations are essentially eliminated. The work described herein provides an overview of select P/M techniques developed by the authors, initially to enhance the hardness and tensile properties of aluminum-based P/M alloys through a mineral dissociation/diffusion process. This is expanded through alloy development research wherein a P/M processing route designed to simulate industrial practices is used in the most recent work based on the effects of rare earth additions on selected mechanical properties of aluminum P/M alloys. These results include a compilation of theoretical calculations (thermodynamics and diffusion rates) that are supported by experimental data using techniques that include electron microprobe analyses, X-ray diffraction, tensile testing, and wear testing. Specific findings are that minerals/compounds such as wollastonite and silver nitrate can be successfully reacted to enhance selected mechanical properties of aluminum P/M alloys and that wear resistance may be improved through a P/M approach applied to AA2014. This article is based on a presentation made in the symposium entitled “Fourth International Alloy Conference,” which occurred in Kos. Greece, from June 26 to July 1, 2005, and was sponsored by Engineering Conferences International (ECI) and co-sponsored by Lawrence Livermore National Laboratory and Naval Research Laboratory, United Kingdom.     

Fatigue crack propagation in rapidly solidified aluminum alloys at 25 °C and 300 °C

The fatigue crack propagation performance of two rapidly solidified aluminum alloys was investigated in air at 25°C and 300°C. The results show that the crack propagation rates for continuous cycling tests of Al-8Fe-4Ce and Al-4.7-Fe-4.7Ni-0.2Cr alloys were similar at 25°C. Although the crack propagation rates of both alloys were increased at 300°C, the Al-Fe-Ce alloy exhibited the greater resistance to crack propagation. The inclusion of a tensile hold time in the fatigue loading cycle at 300°C produced an increase in the crack propagation rates for both alloys over the rates for continuous cycling. The fatigue crack propagation performance of the rapidly solidified alloys was not found to be superior when compared with the fatigue crack propagation performance of a wrought aluminum alloy tested under the same conditions. Transmission and scanning electron microscopy study of the tested specimens revealed that the crack propagation mode was primarily transgranular, with the metastable dispersoid particles providing impenetrable barriers to dislocation motion.     

Physical metallurgy of recycling wrought aluminum alloys

It is quite characteristic of aluminum alloys that if the amount of any element addition is gradually increased, then an upper limit (a tolerance limit) is eventually reached where some important aspect of the alloy’s overall behavior is affected harmfully. Although alloy compositional registers and specifications commonly list upper limits for only the main alloying additions and usual impurities, in principle a practical “compositional tolerance limit” exists for essentially every element in the Periodic Table. Fairly complete knowledge of these tolerance limits for all elements is especially needed in recycling operations where unexpected and unusual impurities can creep in inadvertently, and even normal impurities may tend to accumulate and build up to a disastrous degree. This paper reviews and summarizes what is presently known about the various compositional tolerance limits—and the consequences of exceeding them—in wrought aluminum alloys. Because processes are not available either to remove or to reduce significantly the amounts of various elements that occur in unsegregated scrap, the nearly universal alternative for controlling such elements in recycled aluminum alloys is to dilute them with purer alloy grades or virgin pig. To reduce the dilution requirements, scrap recycling programs must be carefully planned and executed to assure a high degree of scrap segregation into selected grades during collection.     

高强韧铝合金非等温时效工艺的研究进展

非等温时效工艺作为一种新兴的时效处理方法,能够有效地提高高强韧铝合金的综合性能。通过简要归纳近些年来应用于高强韧铝合金的非等温时效工艺,总结出经不同非等温时效处理后高强韧铝合金析出相的特征、合金力学性能和腐蚀性能的变化情况。非等温时效工艺的效率相较于传统时效工艺有很大提高,并且能够同时调控高强韧铝合金内基体析出相和晶界析出相的种类、尺寸和分布情况,使高强韧铝合金兼具与T6峰值时效态相差不多的力学性能和近T7x过时效态的腐蚀性能。最后,对未来高强韧铝合金非等温时效工艺的研究和应用进行了展望。      

Thermomechanical Behavior of Multifunctional GFRP Sandwich Structures with Encapsulated Photovoltaic Cells

The feasibility of encapsulating solar cells into the glass fiber-reinforced polymer (GFRP) skins of load-bearing and thermally insulating sandwich elements with foam cores has been evaluated. Exposure of the encapsulated cells to artificial sunlight led to a significant temperature increase on the top sandwich surface, which almost reached the glass transition temperature of the resin. Mechanical loading up to serviceability limit loads did not cause any damage to the solar cells. Stresses of less than 20% of the material strength arose in the face sheets due to thermal and mechanical loading up to failure. Composite action through the face sheets with encapsulated cells was maintained and no debonding between face sheets and foam core was observed. Thanks to the superior mechanical and thermal sandwich behavior, thin-film silicon cells are more appropriate than polycrystalline silicon cells for use in multifunctional GFRP sandwich structures, although they are less efficient.     

Evaluation of Precipitation Hardening Characteristics of Rheology-Forged Al 7075 Aluminum Alloy Using Nano- or Microindentation and Atomic Force Microscopy

The mechanical and tribological properties of rheo-formed Al 7075 wrought alloys are investigated using nano- or microindentation and nanoscratch techniques, incorporating optical microscopy and atomic force microscopy (AFM). The results are compared to results from a Vickers hardness test. The peak hardness and surface roughness of specimens aged for 24 hours are obtained for Al 7075 alloy. The tribological characteristics of rheologically formed materials are investigated using the constant load scratch (CLS) method. Using this technique, the heat treatment condition for rheologically formed wrought Al 7075 alloys is optimized.     

Influence of carbon content and ferrite morphology on the sensitization of duplex stainless steel

The influence of ferrite morphology and carbon content on the intergranular corrosion behavior of 308 stainless steel was investigated using four wrought alloys and six weld deposited alloys. The four wrought alloys were heat treated at four different annealing temperatures to introduce four different amounts of ferrite. The annealed samples along with the weld deposited alloys were aged at temperatures ranging from 480 to 700°C for times varying between 15 min and 1000 h and then tested for intergranular corrosion susceptibility in acidified copper-copper sulfate solution. For a given carbon content there exists a critical amount and distribution of α-γ boundary area above which the alloy is immune and below which it is susceptible to intergranular corrosion. For amounts and distributions of α-γ boundary area less than the critical value two types of sensitization behavior are possible. First, there may be a sufficient amount and distribution of α-γ boundary area to insure rapid healing of the sensitized microstructure. Second, there may be an inadequate amount or distribution of α-γ boundary area to produce either immunity or rapid healing and the alloy behaves as a fully austenitic alloy regardless of the amount of ferrite present. A model is presented which describes as a function of carbon content the critical amounts and distributions of α-γ boundary area required for rapid healing and immunity to sensitization.     

Advances in Microstructural Understanding of Wrought Aluminum Alloys

Metallurgical and Materials Transactions A - Wrought aluminum alloys are an attractive option in the quest for lightweight, recyclable, structural materials. Modern wrought aluminum alloys depend...     

The structure of rapidly solidified Al- Fe- Cr alloys

Four aluminum alloys, designed for use at elevated temperatures, were studied. The alloys were supersaturated with iron and chromium, and one of them contained small amounts of Ti, V, and Zr. The starting materials were alloy powders made by the RSR (Rapid Solidification Rate) centrifugal atomization process. Extrusion bars were made from the four powders. The as-extruded microstructure and the microstructure of the alloys after annealing at 482 °C were investigated by optical and transmission electron microscopy and by X-ray diffraction. The microstructure consists of equiaxed grains of aluminum matrix and two types of precipitates, namely, Al₃(Fe ,Cr) and a metastable phase, Al6(Fe,Cr). The precipitates were different in their shape, size, distribution, and location within the grains.