Dirk Lehmhus

Properties of heat-treated aluminium foams

Abstract Aluminium foams based on the wrought alloy AA6061 were subjected to different heat treatments in order to evaluate possibilities to tailor their mechanical properties. Quenching after foaming was carried out either in air or in water. Solution heat treatment conditions were varied from ‘no treatment’ to treatments combined with different air and water quenching methods. This gave rise to different cooling rates that were measured in-situ. Full age-hardening cycles were compared to heat treatments in which a simplified programme was applied. In total, the effect of nine different heat treatments on micro-hardness and the compression properties of foams were investigated. Foams that were fully heat-treated performed best with an increase of strength by 60–75% over the untreated samples. However, age-hardening directly after foaming also caused significant improvements of strength, thus allowing to omit water quenching steps which carry the danger of damaging the foamed structure.

Influence of heat treatment on compression fatigue of aluminium foams

Metal foams were produced by means of the powder compact melting technique. Specimens were made of a wrought aluminium alloy similar in composition to AA 6061. A part of the samples was subjected to a precipitation hardening treatment after foaming, others were left in the state “as foamed”. Cyclic tests were then carried out under compressive stresses. S-N curves of untreated and heat-treated foams are compared. Values for fatigue strength were estimated and compared to the static strength found for comparable specimens. As a reference system with a brittle failure mode foams based on the aluminium casting alloy AlSi7 are examined.

Cloud-Based Automated Design and Additive Manufacturing: A Usage Data-Enabled Paradigm Shift

Integration of sensors into various kinds of products and machines provides access to in-depth usage information as basis for product optimization. Presently, this large potential for more user-friendly and efficient products is not being realized because (a) sensor integration and thus usage information is not available on a large scale and (b) product optimization requires considerable efforts in terms of manpower and adaptation of production equipment. However, with the advent of cloud-based services and highly flexible additive manufacturing techniques, these obstacles are currently crumbling away at rapid pace. The present study explores the state of the art in gathering and evaluating product usage and life cycle data, additive manufacturing and sensor integration, automated design and cloud-based services in manufacturing. By joining and extrapolating development trends in these areas, it delimits the foundations of a manufacturing concept that will allow continuous and economically viable product optimization on a general, user group or individual user level. This projection is checked against three different application scenarios, each of which stresses different aspects of the underlying holistic concept. The following discussion identifies critical issues and research needs by adopting the relevant stakeholder perspectives.

Carbonates as Foaming Agent in Chip-based Aluminium Foam Precursor

Replacement of TiH2 as foaming agent by CaCO3 (lime) and CaMg(CO3)2 (dolomite) for AlMg4.5- and AlSi9Cu3-foams was investigated considering influences on foaming capability and cellular structure. Precursor materials were produced from alloy chip and powder mixtures by means of the thixocasting process. AlSi9Cu3 variants showed expansion levels sufficient for commercial use. Variations in expansion observed when CaCO3 and CaMg(CO3)2 were compared as foaming agent are explained based on the course of decomposition. Improved performance of dolomite-based foams relies on formation of stabilizing MgO phases, which do not develop during decomposition of CaCO3 in Al-Si-Cu alloys

AlSi7 metallic foams – aspects of material modelling for crash analysis

Metallic foam samples of matrix alloy AlSi7 have been produced and mechanically tested under quasi-static and dynamic load. Model parameters for the Deshpande–Fleck and the ABAQUS ‘crushable foam’ material model were determined covering a density of 0.3–0.8 g/cm3. Yield surface determination uses uniaxial hydrostatic compression test results, extended by tensile test results for the latter model. Strain hardening was described on the basis of uniaxial compression by fitting a Rusch model to the experimental data, deriving its parameters as function of density. The predictive capabilities of the parameterised models were evaluated using experimental data gathered for load cases characterised by superimposed uniaxial and hydrostatic compression. Analyses show good agreement between simulation and experiment. Further uniaxial compression tests performed at varying strain rates over 4 orders of magnitude revealed no significant strain rate dependency of material properties and thus qualify the material model parameters determined for crash simulation.

Adaptation of aluminium foam properties by means of precipitation hardening

Abstract Aluminium foam samples based on four aluminium alloys were investigated with respect to their reaction to heat treatments, namely precipitation hardening treatments. Foam samples were produced according to the powder compact foaming or Fraunhofer process. 6000 and 7000 series alloys containing significant amounts of copper (6061, 7075) were compared to members of the same group with lower copper content (6082, 7020) as matrix alloys. Comparison was based on strength values and failure modes as reflected in the stress–strain curves obtained in quasi-static compression tests. Measurements were performed on samples without heat treatment and samples subjected to different precipitation hardening treatments. To evaluate the influence of quench sensitivity, the quench rate was varied for the alloys 6082 and 7020 by using air and water as quenchants.

Structure and Compressive Properties of Invar-Cenosphere Syntactic Foams

The present study investigates the mechanical performance of syntactic foams produced by means of the metal powder injection molding process having an Invar (FeNi36) matrix and including cenospheres as hollow particles at weight fractions (wt.%) of 5 and 10, respectively, corresponding to approximately 41.6 and 60.0 vol.% in relation to the metal content and at 0.6 g/cm3 hollow particle density. The synthesis process results in survival of cenospheres and provides low density syntactic foams. The microstructure of the materials is investigated as well as the mechanical performance under quasi-static and high strain rate compressive loads. The compressive stress-strain curves of syntactic foams reveal a continuous strain hardening behavior in the plastic region, followed by a densification region. The results reveal a strain rate sensitivity in cenosphere-based Invar matrix syntactic foams. Differences in properties between cenosphere- and glass microsphere-based materials are discussed in relation to the findings of microstructural investigations. Cenospheres present a viable choice as filler material in iron-based syntactic foams due to their higher thermal stability compared to glass microspheres.

From Stochastic Foam to Designed Structure: Balancing Cost and Performance of Cellular Metals

Over the past two decades, a large number of metallic foams have been developed. In recent years research on this multi-functional material class has further intensified. However, despite their unique properties only a limited number of large-scale applications have emerged. One important reason for this sluggish uptake is their high cost. Many cellular metals require expensive raw materials, complex manufacturing procedures, or a combination thereof. Some attempts have been made to decrease costs by introducing novel foams based on cheaper components and new manufacturing procedures. However, this has often yielded materials with unreliable properties that inhibit utilization of their full potential. The resulting balance between cost and performance of cellular metals is probed in this editorial, which attempts to consider cost not in absolute figures, but in relation to performance. To approach such a distinction, an alternative classification of cellular metals is suggested which centers on structural aspects and the effort of realizing them. The range thus covered extends from fully stochastic foams to cellular structures designed-to-purpose.

When nothing is constant but change: Adaptive and sensorial materials and their impact on product design

This article is the preface to the Special Issue of the Journal of Intelligent Material Systems and Structures on occasion of the Symposium A53 ‘MEMS/NEMS for Sensorial and Actorial Materials’ held at the Euromat 2011 Conference, Montpellier, France, September 12-15, 2011. The authors outline the concept of material-integrated sensing and intelligence, which is summarized in the term sensorial materials. Such materials are understood to incorporate sensing, signal and data processing as well as communication facilities to autonomously evaluate their own condition and/or their environment. To achieve these capabilities, bottom-up as well as top-down approaches are currently being discussed. The latter is highlighted in this work. Research efforts towards it range from materials science to sensor and microelectromechanical system (MEMS)/nanoelectromechanical system (NEMS) technology, microelectronics and computer science and were covered in the underlying Euromat symposium. As a specific aspect linked to the envisaged autonomy and a resulting adaptivity, for example, of internal self-representation and data interpretation, potential consequences for engineering design are sketched.

Tool chain for harvesting, simulation and management of energy in Sensorial Materials

The continuing decrease in size and energy demand of electronic sensor circuits allows endowing engineering structures and, to an increasing degree, materials with integrated sensing and data processing capabilities. Materials that adhere to this description are designated as Sensorial Materials. Their development is multidisciplinary and requires knowledge beyond materials science in fields like sensor science, computer science, energy harvesting, microsystems technology, low-power electronics, energy management, and communication. Development of such materials will benefit from systematic support for bridging research area boundaries. The present article introduces the backbone of an easy-to-use toolbox for layout of the energy supply of smart sensor nodes within a sensorial material. The fundamental approach is transferred from rapid control development, where a comparable MATLAB/Simulink tool chain is already in use. The main goal is to manage limited power resources without unacceptably compromising functionality in a given application scenario. The toolbox allows analysis of the modeled system in terms of energy and power and allows analyzing factors such as energy harvesting, use of predictive power estimation, power saving (e.g. sleep modes), model-based cognitive data reduction methods, and energy aware algorithm switching. It is linked to a simulation environment allowing analysis of energy demand and production in a specific application scenario. Its initial version presented here supports single self-powered sensor nodes. A broad set of application cases is used to develop scenario-dependent solutions with minimum energy needs and thus demonstrate the use of the toolbox and the associated development process. The initial test case is a large-scale sensor network with optical fiber–based data and energy transmission, for which optimization of energy consumption is attempted. The toolbox can be used to improve the power-aware design of sensor nodes on digital hardware level using advanced high-level synthesis approaches and provides input for sensor node and sensor network level.