Gesenkschmieden hochfester Magnesium‐Knetlegierungen für Bauteile der Automobil‐ und Luftfahrtindustrie

Die‐forging of high strength magnesium wrought alloys for the automotive and aerospace industry Forging is an attractive method to produce complex components from magnesium alloys. In this context it is important to choose adequate processing parameters. Magnesium forgings provide superior quality, they are pressure sealed, free of pores and shrinkage and have excellent mechanical properties. The potential of magnesium forgings is very promising, but usage of magnesium forgings is still limited mainly due to the present cost situation.     

Bruchzähigkeitsverhalten von Magnesium‐Druckgusslegierungen

Fracture Toughness of Pressure Die Cast Magnesium Alloys The increasing use of pressure die cast alloys for the manufacturing of automotive components requires materials values, which describe the mechanical behaviour comprehensively. There is especially a lack of data for the design of safety relevant components. In this paper fracture mechanics investigations are presented to assess the crack resistance of the alloys AZ91, AM50 and AE42 on the basis of the multispecimen test method. The investigations occurred on SENB and CT‐specimens taken from pressure die cast plates of the same dimensions. The results reveal a clear correlation between the microstructure, especially the concentration and the distribution of intermetallic phases, and the crack resistance. Because of the cast‐induced defects like microshrinkage and gas inclusions the good intrinsic properties of the alloys can not be completely exploited.     

Verhalten von Magnesiumlegierungen für Anwendungen im Fahrzeugbau bei mechanisch‐elektrochemischer Komplexbeanspruchung: Einfluss passivierender Deckschichten und Mechanismen der lokalen Schädigung

Behavior of Magnesium‐Alloys for Automotive Applications under Mechanical and Environmental Loading: Influence of Passivating Films and Mechanisms of Local Breakdown To assure an efficient design of components under cyclic loading, all available data concerning fatigue have to be observed. Therefore the influences of manufacturing on the material condition, the mechanical loads and environmental effects have to be analysed. Magnesium‐alloys are of special interest for lightweight applications because of their excellent strength‐density ratio. The corrosion resistance of magnesium‐alloys depends on the same factors that are critical to other metals. The alloys have a good stability to atmospheric exposure and a good resistance to attack by alkali, chromic and hydrofluoric acids. However, because of the electrochemical activity of magnesium, the relative importance of some factors is greatly amplified. The nature and composition of passive films formed on magnesium‐alloys depend on the prevailing conditions, viz. alloy‐composition, passivation potential, pH, electrolyte composition and temperature. Passive films may be damaged by local breakdown. Because of this, magnesium‐alloys suffer a degradation of their properties when exposed to an aqueous environment. The main topic of the present investigations is the verification of mechanisms of the local breakdown of the protecting film. At least two mechanisms are possible for this localization: mechanical breakdown by slip steps and electrochemical breakdown (for e.g. by the effects of chloride ions). Corrosion and passivation of different high purity alloys have been studied in different solutions (neutral, alkaline with specific anions and cations) using electrochemical techniques. The diecasted alloys were tested as produced and machined. The results clarified that depending on alloy/material and surface condition/corrosion environment different mechanisms for electrochemical breakdown of the protecting films are possible. Hence fatigue life under environmental loading is influenced by surface and testing conditions.     

Fatigue of rare‐earth containing magnesium alloys: a review

Lightweighting in ground vehicles is today considered as one of the most effective strategies to improve fuel economy and reduce anthropogenic environment‐damaging and climate‐changing emissions. Magnesium (Mg) alloy, as a strategic ultra‐lightweight metallic material, has recently drawn a considerable interest in the transportation industry to reduce the weight of vehicles due to their high strength‐to‐weight ratio, dimensional stability, good machinability and recyclability. However, the hexagonal close‐packed crystal structure of Mg alloys gives only limited slip systems and develops sharp deformation textures associated with strong mechanical anisotropy and tension–compression yield asymmetry. For the vehicle components subjected to dynamic loading, such asymmetry could exert an unfavourable influence on the material performance. This problem could be conquered through weakening the texture via addition of rare‐earth (RE) elements. Thus, a number of RE‐containing Mg alloys have recently been developed. To guarantee the structural integrity, durability and safety of highly loaded structural components, understanding the characteristics and mechanisms of cyclic deformation and fatigue fracture of such RE‐Mg alloys is of vital importance. In this review, the available fatigue properties including stress‐controlled fatigue strength, strain‐controlled cyclic deformation characteristics and fatigue crack propagation behaviour are summarized, along with the microstructural change and crystallographic texture weakening in the RE‐containing Mg alloys in different forms (cast, extruded and heat‐treated states), in comparison with those of RE‐free Mg alloys.     

Magnesium – der Zukunftswerkstoff für die Automobilindustrie?

Magnesium – future material for automotive industry? Magnesium alloys show a very high potential in automotive applications as constructive metal, whereas the main focus lies on die cast parts. Electronic industry is the major commercial consumer for die castings besides the automobile industry. Room temperature applications like steering wheels and frame components in cars as well as mobile phone‐ or notebook housings are well established. These castings are produced with AZ‐ or AM‐magnesium alloys, which show good room temperature properties and a good castability. The great alloy development challenge in extending the use of magnesium cast alloys are application for higher temperatures. The application in powertrain components is considered to be the benchmark here. Besides alloy development there are also further research activities in development of casting processes. Semi‐solid processes like New‐Rheocasting (NRC), Thoxomolding ? or Thixocasting (TC) are adapted to the requirements of newly developed alloys. Not only cast alloys but also magnesium wrought alloys have moved to the centre of interest in the last decade. Alloy development for improving the formability on the one hand as well as process development in extrusion or rolling has to be done in order to find optimum parameters for deforming magnesium alloys properly.     

Superplastische Eigenschaften einer konventionellen AM20‐Magnesiumlegierung

Superplastic Characteristics of a Conventional AM20 Magnesium Alloy Despite the increasing interest of the industry in lightweight materials during the last years, an intensive industrial use of magnesium based alloys due to the their restricted cold‐workability caused by the hexagonal lattice is still very limited. Considering this limitation a solution is provided by the superplastic forming of magnesium based alloys which, in contrast to other types of materials, is neither metallurgically developed nor process optimized. A very promising step is the extrusion of the conventional AM20 magnesium alloy followed by controlled cooling of the extruded material in order to reduce the grain growth caused by secondary recrystallisation are suitable means to produce a fine grained microstructure. After a short presentation of the theoretical background and methods to determine the superplastic characteristics, the strongly improved material properties of the AM20 magnesium alloy are revealed by increased m‐values and higher elongation‐to‐fracture‐values (ε_max = 550%) determined by tensile tests at constant strain rates.     

Utilizing Low‐Cost Eggshell Particles to Enhance the Mechanical Response of Mg–2.5Zn Magnesium Alloy Matrix

The search for lightweight high‐performance materials is growing exponentially primarily due to ever‐increasing stricter environmental regulations and stringent service conditions. To cater to these requirements, the use of low‐cost reinforcements has been explored in the Mg matrix to develop Economically Conscious Magnesium (ECo–Mg) composites. In this study, eggshell particles (3, 5, and 7 wt%) reinforced Mg–Zn composites are synthesized using blend‐press‐sinter powder metallurgy technique. The results reveal that the addition of eggshell particles enhances microhardness, thermal stability, damping, and yield strength with an inappreciable change in the density. In particular, Mg2.5Zn7ES composite do not ignite till ≈750 °C. The overall combination of properties exhibited by Mg–Zn–ES composites exceeds many of currently used commercial alloys in the transportation sector. An attempt is made, in this study, to interrelate microstructure and properties and to study the viability of compression and ignition properties with a comparison to commercially used Mg alloys.

     

Innovative magnesiumgerechte Gießtechnik

Innovative Magnesium adepted Casting Technology A main reason for the relatively small use of magnesium alloys as construction material is the demanding casting technology. The reactivity with ambient atmosphere and several other materials, the low specific heat values like heat capacity and heat of fusion, the small melting range and comparatively high liquidus temperatures of alloys with improved properties demand the optimizing of excisting and development new casting processes. In the scope of the Thrust Research Program SFB 390 “Magnesium Technology” founded by the German Research Council (DFG) the project B10 “Casting Technology” is engaged in the development and optimisation of casting techniques for Mg‐alloys. The emphasis of the developments in the Institute of Materials Science (IW) of the Universtity of Hanover is set on reliable, low contaminated processes of melting, melt handling and mould filling for the production of low defect structural components, even dealing with alloys difficult to handle.     

Fügen von Magnesium und Magnesiumlegierungen

Welding of Magnesium and Magnesium alloys Magnesium is mainly connected by screws. In this paper the results of experiments with different welding processes will be presented. The following methods have been applied: TIG, MIG, Nd: YAG‐Laser and CO₂‐Laser welding, electron beam welding and High Power Diode Laser welding.     

Veränderungen der mechanischen Eigenschaften von Magnesium‐Druckgusslegierungen nach langzeitiger thermischer Beanspruchung

Changes of the Mechanical Properties of Pressure Die Cast Magnesium Alloys Subjected to Long‐Term Thermal Exposure The thermal resistance of pressure die cast magnesium alloys is yet not investigated sufficiently. In order to assess the effect of a thermal exposure on the microstructural stability and on the strength and deformation behaviour, the alloys AZ91, AM50 and AE42 are subjected to a long‐term annealing for 1000 h at 150 °C and 200 °C. After the annealing, tensile tests, in situ tensile tests and microhardness measurements are carried out and the results are discussed on the basis of the microstructural changes.     

Mikrostrukturelle Veränderungen von Magnesium‐Druckgusslegierungen nach langzeitiger thermischer Beanspruchung

Microstructural Changes of Pressure Die Cast Magnesium Alloys after Long‐Term Thermic Loading The expansion of the application of pressure die cast magnesium alloys for automobiles requires the development of new alloys and the comprehensive assessment of available alloys on aggravated conditions, too. Such conditions are also given at higher temperatures, which can cause the creep of the material and lead to the component failure. Because the microstructural stability decisively depends on the thermic loading, this paper deals with the change of the microstructure and the hardness of the alloys AZ91, AM50 and AE42 after a long‐term annealing at 150 °C and 200 °C in comparison to the pressure die as‐cast condition. The results reveal clear differences of the microstructural stability of the alloys AZ91 and AM50 on the one hand and the alloy AE42 on the other hand. Due to the long‐term annealing at 150 °C the alloys AZ91 and AM50 show chiefly an intense precipitation of Mg₁₇Al₁₂ from the Al‐rich eutectic α‐phase. Furthermore at 200 °C, it is observed the coagulation and coarsening of these precipitates, too. The last appearances are connected with a weakening of the material. Regarding the alloy AE42, the changes of the precipitation state are not so intensely and do yet not lead to a microstructural weakening.     

Untersuchungen an magnesiumbasierten Legierungen als neue Materialien in der Implantologie

Exploration of Magnesium Alloys as New Material for Implantation Magnesium‐based alloys show as well a fitting Youngs's Modulus (45 GPa) referring to the corticalis as high physico‐chemical biocompatibility due to its essential character. The biodegradation of Magnesium‐alloys could be used to obtain temporary implants. The corrosion resistance of commercial alloys as AM20 and AZ31 and testing materials with higher biocompatibility, basing on cph Mg‐Li, is tested in a body ullage.     

Mo‐Si‐B Alloys for Ultrahigh‐Temperature Structural Applications

A continuing quest in science is the development of materials capable of operating structurally at ever‐increasing temperatures. Indeed, the development of gas‐turbine engines for aircraft/aerospace, which has had a seminal impact on our ability to travel, has been controlled by the availability of materials capable of withstanding the higher‐temperature hostile environments encountered in these engines. Nickel‐base superalloys, particularly as single crystals, represent a crowning achievement here as they can operate in the combustors at ～1100 °C, with hot spots of ～1200 °C. As this represents ～90% of their melting temperature, if higher‐temperature engines are ever to be a reality, alternative materials must be utilized. One such class of materials is Mo‐Si‐B alloys; they have higher density but could operate several hundred degrees hotter. Here we describe the processing and structure versus mechanical properties of Mo‐Si‐B alloys and further document ways to optimize their nano/microstructures to achieve an appropriate balance of properties to realistically compete with Ni‐alloys for elevated‐temperature structural applications.     

Nanoporous Al‐Ni‐Co‐Ir‐Mo High‐Entropy Alloy for Record‐High Water Splitting Activity in Acidic Environments

Ir‐based binary and ternary alloys are effective catalysts for the electrochemical oxygen evolution reaction (OER) in acidic solutions. Nevertheless, decreasing the Ir content to less than 50 at% while maintaining or even enhancing the overall electrocatalytic activity and durability remains a grand challenge. Herein, by dealloying predesigned Al‐based precursor alloys, it is possible to controllably incorporate Ir with another four metal elements into one single nanostructured phase with merely ≈20 at% Ir. The obtained nanoporous quinary alloys, i.e., nanoporous high‐entropy alloys (np‐HEAs) provide infinite possibilities for tuning alloy's electronic properties and maximizing catalytic activities owing to the endless element combinations. Particularly, a record‐high OER activity is found for a quinary AlNiCoIrMo np‐HEA. Forming HEAs also greatly enhances the structural and catalytic durability regardless of the alloy compositions. With the advantages of low Ir loading and high activity, these np‐HEA catalysts are very promising and suitable for activity tailoring/maximization.     

Magnetic Interactions in Ni‐Mn‐Based Magnetic Shape‐Memory Heusler Alloys

Ni‐Mn‐based Heusler alloys exhibit a variety of features related to martensitic transformations and are materials that are sought to be employed in actuation applications. To be able to exploit their properties, it is necessary to understand the rich variety and subtle magnetic coupling mechanisms occurring in these alloys. We review complementary neutron polarization analysis and ferromagnetic resonance experiments and give an account on the complex magnetism of these alloys in the austenite and martensite states.     

Magnesium — an old material with new applications

Magnesium alloys have been made more competitive for engineering applications through developments in alloys and casting techniques. The properties of magnesium alloys and the advantages of die casting are described. High strength-to-weight ratios allow thin-section designs which retain good fatigue and creep resistance and are suitable for EMI shielding applications. Machining, joining, surface treatment and corrosion resistance of alloys are considered.     

High‐Entropy Alloys: Potential Candidates for High‐Temperature Applications – An Overview

Multi‐principal elemental alloys, commonly referred to as high‐entropy alloys (HEAs), are a new class of emerging advanced materials with novel alloy design concept. Unlike the design of conventional alloys, which is based on one or at most two principal elements, the design of HEA is based on multi‐principal elements in equal or near‐equal atomic ratio. The advent of HEA has revived the alloy design perception and paved the way to produce an ample number of compositions with different combinations of promising properties for a variety of structural applications. Among the properties possessed by HEAs, sluggish diffusion and strength retention at elevated temperature have caught wide attention. The need to develop new materials for high‐temperature applications with superior high‐temperature properties over superalloys has been one of the prime concerns of the high‐temperature materials research community. The current article shows that HEAs have the potential to replace Ni‐base superalloys as the next generation high‐temperature materials. This review focuses on the phase stability, microstructural stability, and high‐temperature mechanical properties of HEAs. This article will be highly beneficial for materials engineering and science community whose interest is in the development and understanding of HEAs for high‐temperature applications.     

Semi‐Solid Processing of Tailored Aluminium‐Lithium Alloys for Automotive Applications

This paper describes the development and evaluation of thixoformable Al‐Li‐Mg‐based alloys performed at the collaborative research center SFB 289, RWTH Aachen. Scandium and zirconium were added to AlLi2.1Mg5.5 (A1420) with the aid of DoE (Design of Experiments), and precursor billets were manufactured by pressure induction melting (PIM). To evaluate the thixoformability of the synthesized alloys semi‐solid processed connecting rods were manufactured by the rheo container process (RCP). Subsequent heat treatment raised the mechanical properties to maximum values of tensile strength, 430 MPa, yield strength of 250 MPa, and an elongation to fracture of 13 %. The RCP process was designed for the special requirements of highly reactive alloys. The paper presents the remarkable property and process benefits of the semi‐solid processing of Al‐Li alloys.     

The influence of defect and temperature on the fatigue behaviours of Al‐Si‐Cu‐Mg‐Ni alloy

High‐cycle fatigue (HCF) properties of two Al‐Si‐Cu‐Mg‐Ni alloys with different defect sizes named as alloys A (smaller ones) and B (bigger ones) were investigated at 350°C and 425°C, respectively. The results indicate that fatigue strengths of both alloys decrease as the temperature increases. Fatigue cracks originated from pores and oxide films at both temperatures. They propagated preferentially through cracked matrix at 350°C and debonded interface and grain boundary at 425°C. Alloy A exhibits higher fatigue life and fatigue strength than alloy B at 350°C due to its smaller pore sizes. However, it has slightly worse fatigue properties than alloy B at 425°C because the fatigue crack initiation is controlled by oxide film at this temperature and is not affected by its size. This indicates that there is a transition of predominant initiation site from pores to oxide films when the temperature increases. The fatigue strength estimated through defect size is consistent with the experimental results at 350°C, while unsuitable at 425°C.     

Oxidation‐Resistant Coatings for Application on High‐Temperature Titanium Alloys in Aeroengines

High‐temperature application of titanium alloys in aeroengines is often limited by their insufficient resistance to the aggressive environment. Magnetron‐sputtered Ti–Al based coatings were developed in order to increase the maximum service temperature of conventional titanium alloys from the present 520–600 °C, the temperature limit set by the mechanical capabilities of most advanced alloys. The coatings not only demonstrated excellent oxidation resistance but also demonstrated beneficial effects on mechanical properties. Most importantly, the fatigue behavior of the substrate alloys was not degraded, a major hurdle for coating application on titanium alloys so far. Initial results on Cr‐containing Ti–Al based coatings indicated significant potential for application on titanium aluminides.