High - Strength steel hollow spheres

Metal hollow sphere structures (MHS) feature excellent properties for functional applications [1]. But, since their mechanical properties are still too weak for an use in lightweight constructions. Automotive industry demands a significant increase in strength and a decrease of density. In the present work, a study on the increase in the strength of carbon steel hollow spheres by additional heat treatment was conducted. Such additional heat treatments for improving the steel properties are quite usual. But, in contrast to the hardening of bulk material, metal hollow spheres show an extreme low thermal conductivity and high internal surfaces. Thus, the additional heat treatment of carbon steel hollow spheres gives rise to three main challenges, which are closely related. First the carbon content must be adjusted at an amount of 0.6 -0.8 wt.-percent and this amount must be held without a loss during austenitisation. This carbon content is optimal for high strength combined with high ductility. The second challenge is how to avoid the oxidation. Metal Hollow spheres show high internal surfaces. Hence, oxidation is promoted. Since massive oxidation of the shells of the MHS will harm the mechanical properties, it can hide the effect of the increasing strength of the residual shell due to hardening. Oxidation can occur during austenitisation by the atmosphere, during cooling by water or air and after the process, due to remaining water between the MHS. The third challenge is the fast cooling of MHS. Low alloyed and unalloyed carbon steel requires fast cooling in order to obtain phase changes from austenite to martensite. On the one hand water as cooling fluid allows the maximal cooling rate due to its high heat capacity. On the other hand, cooling in water is challenging owing to the corrosiveness of remaining water. Furthermore, packages and structures of MHS are closed-cell foams with low thermal conductivity. This lowers the cooling rale in the centre of structure or packages. Consequently, incomplete phase transition may occur, causing a gradient in the strength of such structures. Entnommen aus TEMA</a

Improving machine accuracy by using Cellular Metals: Poster at Cellular Materials, 27-29 Oct 2010, Dresden; CellMat 2010

Zellulare Metalle wie Aluminiumschäume und metallische Hohlkugeln eignen sich hervorragend für Leichbaukonstruktionen besonders im Werkzeugmaschinenbau. Die Werkstoffe werden bevorzugt als Kernwerkstoff in Sandwiches verwendet. Diese Halbzeuge sind vergleichsweise leicht und weniger schwingungsanfällig als massive Stahlhalbzeuge. Mit Verwendung dieser Werkstoffe in bewegten Baugruppen von Werkzeugmaschinen lässt sich eine deutlich verbesserte Dynamik im Bearbeitungsprozess bei gleicher Bearbeitungsqqualität erzielen. Im Rahmen eines FuE-Vorhabens wurde der Nachweis für diese These geführt

Den Schwingungen einen Dämpfer versetzt: Leichtgebaute Maschinenschlitten mit zellularer Hohlraumstruktur haben die Ruhe weg

Dass Maschinenbaugruppen leicht und trotzdem sehr steif sein können, ist nicht neu. Doch werden sie auch mit Schwingungen fertig? Sogar noch viel besser: das haben jetzt die Fraunhofer Institute IFAM und IWu am Beispiel von Maschinenschlitten mit zellularer Struktur nachgewiesen

Comparative assessment of Young's modulus measurements of metal-ceramic composites using mechanical and non-destructive tests and micro-CT based computational modeling

It is commonly known that the available non-destructive and mechanical methods of the Young modulus measurement yield different results. This paper presents comparison of the results of experimental determination and numerical modeling of the Young modulus of Cr-Al2O3-Re composites (MMC) processed by a powder metallurgical method (SPS). In the computational model a finite element analysis is combined with images of the real material microstructure obtained from micro-computed tomography (micro-CT). Experimental measurements were carried out by four testing methods: three-point bending, resonance frequency damping analysis (RFDA), ultrasonic pulse-echo technique, and scanning acoustic microscopy. The paper also addresses the issue which of the four experimental methods at hand gives results closest to the theoretical predictions of the micro-CT based FEM model

On the mechanical properties of sintered metallic fibre structures

The present study investigates mechanical properties of a novel sintered metallic fibre structure with different relative densities (i.e. 0.19, 0.27, and 0.46). The compressive mechanical properties Young's modulus, Poisson's ratio and 0.2% offset yield stress are determined. For this purpose, state of the art simulations are performed based on the real material structure using micro-computed tomography images. Computed results are compared with experimental uni-axial compression tests and good agreement between both methods is observed. Numerical analysis allows the investigation of directional dependence and mechanical anisotropy is observed to be governed by the fibre orientation. In addition, Young's modulus and 0.2% offset yield stress increase with rising relative density

Functionalized metallic hollow sphere structures

Metal hollow spheres (MHS) and metal hollow sphere structures are special types of cellular metals with an enormous application potential in structural and functional applications. A lot of effort has been made during the last two decades in order to develop the manufacturing process and to investigate the properties of this type of structure. Additional functionalizing of MHS is possible by filling the spheres with ceramic powders or phase change materials. Furthermore, by coating the spheres with different ceramic layers new properties and functions are added. Thus, the structures show excellent mechanical damping properties as well as high heat capacity for thermal storage and fast heat loading and unloading. Grinding or ceramic coating bring more functions to the surface of the spheres. The manufacture and properties of different types of functionalized MHS and their possible applications are highlighted here

Microstructure and Mechanical Properties of Welded Additively Manufactured Stainless Steels SS316L

Microstructure and mechanical properties of additively manufactured SS316L has been investigated. The samples produced by selective electron beam melting machine were then subjected to gas tungsten arc welding. Various examinations were performed including metallography and microscopy, hardness testing, and tensile testing coupled with digital image correlation software. Strain distribution was clearly evident on the samples during tensile testing with necking taking place at the heat affected zone on both sides of the weldments. From tensile testing, it was clear that the ductility and strengths of the samples were equal to those of conventionally produced samples such as rolled sheet. Hardness testing indicated the uniform distribution across the base metal and the weldments. Scanning electron microscopy identified the presence of Cr and Mo-rich precipitates on the grain boundaries, while the fracture surface was entirely covered with dimples (microvoid coalescence) indicating a ductile fracture mode

Experimentelle Ermittlung des Elastizitätsmoduls von Cr(Re)/Al2O3-Verbundwerkstoffen

Im Maschinenbau und der Werkstoffwissenschaft kommt dem Elastizitätsmodul eine grundlegende Bedeutung zu. Die Bestimmung des E-Moduls kann mittels Zugversuch, Drei-Punkt-Biegung und Ultraschall-Impuls-Echo-Verfahren erfolgen. Entwickelt wurden weitere experimentelle und numerische Verfahren, die alle Vor- und Nachteile haben und teilweise unterschiedliche Werte für den E-Modul eines Werkstoffs liefern. Es werden die Ergebnisse experimenteller Untersuchungen sowie numerischer Modellierungen am Verbundwerkstoff Cr(Re)/Al2O3 (MMC) vorgestellt und verglichen. Die erhaltenen Ergebnisse belegen die Schwierigkeit einer genauen Ermittlung des Elastizitätsmoduls, insbesondere von Verbundwerkstoffen. Mögliche Gefügeinhomogenitäten dieser Werkstoffe wirken sich offensichtlich verschieden intensiv auf die angewandten unterschiedlichen Messprinzipien des E-Modules aus. Alle vier Verfahren nutzen verschiedene physikalische Vorgänge, um Rückschlüsse auf die elastischen Konstanten des Werkstoffs zu ziehen. Dabei spielen nicht nur lokale Phänomene wie Spannungsverteilung, Verfomungsamplitude und -geschwindigkeit eine entscheidende Rolle, auch eine akkurate Probenvorbereitung ist für die Minimierung des Ausmaßes an Fehlmessungen sehr bedeutend. Da die Drei-Punkt-Biegung sehr empfindlich auf Gefüge- und Oberflächenfehler reagiert, sollte sie nur dann eingesetzt werden, wenn keines der zerstörungsfreien Prüfverfahren zur Verfügung steht oder ein Vergleich mit schon vorhandenen Messungen dieses Verfahren erfordert. Die beiden vorgestellten FEM-Modelle können hilfreiche Hinweise bei der Werkstoffentwicklung liefern. Sie erlauben eine gute Vorhersage der elastischen Konstanten eines noch nicht entwickelten Verbundwerkstoffs