· web viewvia simulation the influence of the alu- minum microstructure on typical failure...

Institute of Materials Science and Mechanics of MaterialsExperimental and theoretical characterization of metallic materials

n The focus of the Institute of Materials Science and Mechanics of Materials (WKM) in 2015 was to develop further the field of processing microstructure properties relationships of load bearing materials concentrating on metallic materials such as high strength steels, titanium, nickel, aluminum and tungsten alloys. Research is performed employing theoretical, numericaland experimental techniques with equal importance. The associated State Material Testing Laboratory serves as an important interface to industry with respect to research oriented (offroutine) testing of materials.Research activities concentrate on plasticity and failure of high strength dual-phase steels, strain localization of multiaxially deformed high strength steel sheets and residual stress analysis and quantification of the time dependence of residual stress states via experimental and theoretical approaches.Significant advances were made in themodeling of microstructure evolution of the titanium alloy Ti-6Al-4V during selective laser melting, the integration of precipitation hardening into the process of densification of cast aluminum alloys by hot isostatic pressing and the solution of several inverse problems associated with the design of working cathodes as used in electrochemical machining.Work has begun on several new topicssuch as accumulative roll bonding of steelsheets or high entropy high temperature

X-ray diffractometer equipped with Eulerian cradle and area detector for in-situ measurement of phase transformations, texture and stress analysis(Photo: WKM)

alloys. One of the highlights of 2015 was the successful completion of the habilita- tion of Dr. C. Krempaszky for mechanics of materials.

Prof. Dr. mont. habil. Dr. rer. nat. h. c. Ewald Werner

Contact

www.wkm.mw.tum.de [email protected] Phone +49.89.289.15247

Plasticity and Failure of High-Strength Sheet Materials for Automotive ApplicationsThe desire to produce steel sheet material with high strength, excellent formability and a rather low content of alloying elements has led to the development of a multitude of microstructure-strengthened steels. Among these are the ferritic- martensitic dual-phase (DP) steel grades, which are produced in a multi-step heat treatment. Their microstructure consistsof a matrix of ferrite (soft), reinforced withisolated particles of martensite (hard).

2 Institute of Materials Science and Mechanics of Materials

Research activities at WKM related to DP steels focus on heat treatment- induced microstructural stresses and strains, their unusual plastic deformation behavior, and their microstructure governed failure behavior.

Phase averaged von Mises stresses in two different microstructures (bottom left and right) as a function of macroscopic strain in tension and compression. The martensite of the left microstructure exhibits an ‘unloading’ from heat treatment-induced stresses (i.e. the initial stress in the graph) up to roughly 0.5% of macroscopic strain. This effect causes higher macroscopic flow stress in compression than in tension. (Source: WKM)

For a systematic study of the influence of microstructure features on plastic flow and failure of DP steels, a simulation model based on micro-continuum mechanicsand tessellated microstructures wasdeveloped at WKM. Systematic simula- tions conducted with computer generated microstructures subjected to the same heat treatment and subsequent deformation routes as real DP steels served to explain several phenomena typical to this steel class.For the first time it became possibleto clearify the importance of heat treatment-induced micro-strains for the initial plastic flow behavior and that of heat treatment-induced local deviatoricstresses for the experimentally observedload type-dependent macroscopic flow stress (see figure).The macroscopic strain for microstructuralfailure initiation in DP steels can be corre-lated with their microstructural heteroge-

neity. The higher the heterogeneity caused by a high contrast of phase strength or a large martensite phase fraction, the lower is the macroscopic strain for local failure initiation.Investigations are also conducted onthe process of accumulated roll bonding (ARB) of dual-phase steels to evaluate the feasibility of this special production route for high strength steels. ARB currently is only applied to low strength steel grades and soft aluminum alloys, there revealing its potential to increase strength markedly by grain refinement induced be severe plastic deformation.

Projectsn Micromechanical modeling of the

formability and failure of DP steels (voestalpine Stahl GmbH)

n Accumulated roll bonding of dual phase

steels (voestalpine Stahl GmbH)

Strain Localization Analysis of MultiaxiallyDeformed High Strength Sheet SteelStrain localization is limiting the formability of sheets and occurs on different length-scales. Particularly in case of multiphase materials exhibiting a high contrast in phase-specific strength, it appears necessary to take into account the microscale.In the course of the research activities at WKM, the forming limits of dual-phase steels are investigated using a decoupled sequential multi-scale modeling approach. This approach adopts a shear band analysis employing the concept of the acoustic tensor and combines a mechanical analysis of unit cells representing the microstructure on the grain level and the evolution of macroscopic mechanical field quantities in Nakajima test specimens.It could be shown that the

morphology ofthe microstructure influences the position and ori


entation of the global shear band within the microstructure while the phase strength contrast triggers shear band formation.

Shear band analysis in a DP500 unit cell, loaded in equibiaxial tension (BAT). The effective stress-strain curve is depicted in blue. Furthermore contour plots of the distribution of the hardening, h, the shear band variable, Q

s,

and the maximum principal strain, I,are shown for two specific strains. The white areasin the contour plots of Q

s represent martensite,

that does not reach a critical state. (Source: http://dx.doi. org/10.1016/j.commatsci.2015.09.046)

Projectsn Prognose des mechanischen Ver-

haltens von Dualphasen-Stählen mit Hilfe eines mikromechanischen Finite Elemente Modells (voestalpine Stahl GmbH)

Residual Stress Relaxation and Residual Stress Analysis

Experimental method for monitoring the evolution of thin-walled bent profile distortions. (Photo: WKM)

The redistribution of macroscopic residual stresses may lead to component distortions, which, in the worst case, do not conform to the required dimensional accuracy of the component. Residual stress redistribution may result either from the removal of portions of the workpiece during machining or from stress relaxation of the material due to microscopic diffusion processes. For metallic materials, the latter play a substantial role only in case of elevated temperatures. However, in caseof very thin/slender components, evenslight stress redistributions due to stress

relaxation at ambient temperature can result in undesirable distortions increasing with time.In this context, research activities focus onthe investigation of stress redistributions in thin-walled bent profiles and the iden- tification of the underlying microscopic mechanisms. This includes the development and application of experimental methods for residual stress analysis and for monitoring the evolution of component distortions as well as the theoretical estimation of the evolution of the residual stress state using adequate semianalytical and numerical modeling approaches.

Projectsn Eigenspannungsbedingter

Bauteil- verzug in rollgebogenen Stahlprofilen (Dr. Johannes Heidenhain GmbH)

n Spannungsrelaxation in Abhängig-keit der Mikrostruktur über

mehrereLängenskalen in Haynes282 (DFG)

n Development of an advanced design and production process of high temperature Ni-based alloy forgings (HiTNiFo) (EU, Clean Sky)

Microstructure Evolution of Ti-6Al-4V During Selective Laser MeltingSelective Laser Melting (SLM) is an additive manufacturing process used to produce near net shape parts withcomplex geometries from 3D CAD data directly, thus being economical for small batch series and individual items. In SLM, components are produced layerwise from metal powder, which is locally fully melted by a laser and then solidifies by self-quen- ching, free convection of shielding gasand thermal radiation from the free surface. This thermomechanical treatment of the material results in microstructures that

differ significantly from those processed conventionally by forging or casting.WKM puts much effort into the development of multiphysics process models aiming at the prediction of SLM-generated microstructures. Main ingredients of these

models are – besides heat input and heatextraction modules – the consideration of kinetic aspects


of liquid-solid and solid-solid phase transformations which control the evolution of microstructures. Detailed microstructure investigationswere conducted for the important titanium

Polfigure of the [100] directionof the -phase. a) reconstructed from EBSD measurement of the‘-phase and b) predicted by the simulation. (Source: WKM)

alloy Ti-6Al-4V which after solidification undergoes a solid-solid transformation from the high-temperature -phaseto the ’-phase upon cooling to room temperature. Since there exists a distinct orientation relationship between these two phases, the crystallographic features of the -phase can be reconstructedfrom crystallographic data collected for the ’-phase. It could be shown that the-phase solidifies as elongated grainsoriented parallel to the build-direction of the part exhibiting a fibre texture.

Based on these findings a simulation tool was developed at WKM capturing interfacial motion, grain orientation and driving forces responsible for microstructure evolution.

Projectsn Light Weight-TCF –

Werkstoffentwick- lung und -charakterisierung im Rahmen der generativen Fertigungsverfahren (MTU Aero Engines AG, EOS GmbH – Electro Optical Systems)

Electrochemical Machining

Electrochemical machining of a cylindrical spring steel wire with a torus shaped cathode (not shown). Numerical simulation of the current density on the workpiece surface (left); experimental validation (right). (Source: WKM)

Anodic metal dissolution at high current densities (such as in electrochemical machining (ECM) and/or precise electrochemical machining (PECM)) is increas- ingly used when conventional machining reaches its economical and/or procedural limits, as e.g. for temperature sensitive, brittle or high strength metallic materials. In ECM the workpiece represents the anode of an electrolytic cell, whereas the tool is defined by the opposing cathodic- ally polarised electrode of a specific geometrical shape. To ensure that the desired workpiece geometry evolvesin a controlled machining process, the pre-determination of the necessary shape of the cathode is imperative. Thisdetermination poses a major challenge inECM.Therefore a procedure – based on the C++ library OpenFOAM – was developed at WKM to calculate cathode shapes for the manufacturing of complex 2- and3-dimensional workpiece surfaces. Dueto its improved accuracy compared to

existing shape-calculating methods, this approach reduces the number of expen- sive iterations in tool manufacturing. In addition, insights from simulation results (both the simulation of the dissolution process and the solution of the inverse problem to identify desired cathode shapes) are now used to identify process- and material-related effects that can be evaluated separately in an experimental test setup.Current experimental research efforts are directed towards the influence of varia- tions in mechanical material properties– such as residual stresses and disloca- tion densities, both of which can result from forming or heat treatment – on the macroscopic dissolution rate of selected materials.

Projectn Innovative, adaptive Prozessregelung

ECM/PECM (MTU Aero Engines AG)

Failure in Integrated Circuits


Growing demands on performance and durability of integrated circuits require an understanding of possible failure mechanisms like crack initiation within the interlayer dielectric and surfaceroughening of the metallization plate. One of the main causes for such failure arises

from the mismatch in thermal expansion between the materials involved (conductor paths are made of aluminum and the surrounding interlayer dielectric of silicon oxide) leading to thermo-mechanicalloads and, consequently, to various types of damage. Throughout their life,

electronic components undergo millions of thermo-mechanical load cycles, so that an experimental life cycle analysis during the development process is costly and maynot be feasible at all.Via simulation the influence of the aluminum microstructure on typical failure mechanisms was investigated utilizing a crystal plasticity material model taking into account the microstructure, the grain orientation, the grain size and the temperature dependency of relevant material properties. A number of experimentalobservations could be confirmed by thesecalculations like the stabilizing effect of a passivation layer on the surface roughening and the significant life extension by reducing the grain size of the conductor path grains. Grain size and orientationboth influence the magnitude of surfaceroughening (see figure). Compared tograin size the influence of grain orientation

Deformed simulation model representing a part of an integrated circuit after 10 load cycles. (left); Influence of the grain size and orientation on the magnitudeof surface roughening. (right) (Source: http://dx.doi. org/10.1007/s00161-015-0477-7)

seems to play a less important role. Para- meter studies are currently undertaken in order to find an optimum microstructure for aluminum leading to a minimum failure probability.

Failure in Tungsten under High Heat Flux Loads

Currently the International ThermonuclearExperimental Reactor (ITER) is beingbuilt at Cadarache, France. Engineers are facing challenging material selection problems especially when designing components which are in contact with the fusion plasma to extract thermal power from the plasma. Power densities of more the 10MW/m² are predicted to be withstood bythese materials. The thermo-cyclic loads imposed on plasma facing materials (PFMs) e.g. in the divertor of a fusion reactor show great similarities to the loadings of accelerator targets and X-ray anodes. For all these applications tungsten or tungsten alloys are the preferred materials to withstand short-duration high heat-flux loadings. While tungsten exhibits favorable short-term high temperature properties, it

is brittle at temperatures below the ductile to brittle-transition and shows undesired grain growth duringlong-term exposure. In collaboration with


Siemens AG and the Max-Planck-Institute for Plasma Physics, WKM successfully studied fracture behavior and microstructure evolution of relevant tun

gsten grades both experimentally and via micromechanical modeling. Finally, a new processing route was developed for the production of isotropic and fine grained tungsten grades via tape casting.

Projectsn The path of the heat flow through

a nuclear fusion power plant: Identi- fication of components with other applications outside the field of fusion (Siemens AG, Max-Planck-Institute for Plasma Physics)

n Reduced order modeling with applica-tion to microstructure engineering for process optimization (e. g. steel rolling) and component design (e. g. fusion reactor) (Siemens AG, Max-Planck- Institute for Plasma Physics)

Research Focusn Testing and modeling of metallic high

performance alloys (iron-, nickel-, titanium-, tungsten- and aluminum-based alloys)

n Residual stress determination viadiffraction methods (X-rays, neutrons)and incremental hole drilling

n Microstructure based numerical model-

ingn Electron and light microscopyn Mechanical testingn Electrochemical machining

Competencen High resolution scanning electron

microscopyn Diffraction techniquesn Material testing on demand

Infrastructuren Material testing equipmentn Light and electron microscopesn Hot isostatic pressn Dilatometers and annealing simulatorn Electrical and mechanical workshopsn Electrochemical machining workstationn X-ray diffractometers

Coursesn Materials Science I and IIn Engineering Materials Technologyn Engineering Mechanics for Business

Sciencesn Fracture Mechanics/Plasticity Theoryn Tensor Calculus for Engineersn Finite Elements in Mechanics of

Materialsn Electron Microscopyn Laboratory Courses on Materials

Science, Mechanics of Materials and Finite Element Methods

ManagementProf. Dr. mont. habil. Dr. rer. nat. h. c. Ewald Werner, DirectorDr.-Ing. Christian Krempaszky

Adjunct ProfessorsHon.-Prof. Dr.-Ing. Dr. Eng. (Univ. Nagoya/ Japan) Harald Bolt

Administrative StaffYvonne Jahn

Research ScientistsDr.-Ing. Alexander Fillafer Stephan Hafenstein, M.Sc. Dipl.-Ing. Peter Holfelder Dr.-Ing. Jinming LuDipl.-Ing. Felix Meier Marius Reiberg, M.Sc. Dipl.-Ing. Gerwin RiedlDr.-Ing. Cornelia Schwarz Johannes Seidl, M.Sc. Anneka Vogel, M.Sc. Helena vom Stein, M.Sc. Dr. mont. Zhonghua Wang Dipl.-Ing. Robert Wesenjak

Technical Staff Wolfgang Bauer Brigitte Hadler Alois HuberStefan HumplmairCarola ReiffJens Reuter, B.Sc.


Publications 2015

Journalsn M. Li, E. Werner, J.-H. You: Influence of heat flux

loading patterns on the surface cracking features of tungsten armor under ELM-like thermal shocks. J. Nuc. Mat. 457 (2015) 256-265. DOI: 10.1016/j. jnucmat.2014.11.026

n M. Li, E. Werner, J.-H. You: Low cycle fatigue beha-

vior of ITER-like divertor target under DEMO-relevant operation conditions. Fusion Engng. and Design 90 (2015) 88-96. DOI: 10.1016/j.fusengdes.2014.11.017

n M. Li, E. Werner, J.-H. You: Cracking behavior of tungsten armor under ELM-like thermal shock loads: A computational study. Nuc. Mat. Energy 2 (2015) 1-11. DOI: 10.1016/j.nme.2014.10.001

n M. Li, M. Sommerer, E. Werner, S. Lampenscherf,T. Steinkopff, P. Wolfrum, J.-H. You: Experimental and computational study of damage behaviorof tungsten under high energy electron beam irradiation. Engng. Frac. Mech. 135 (2015) 64-80. DOI: 10.1016/j.engfracmech.2015.01.017

n E. Werner, R. Wesenjak, A. Fillafer, F. Meier, C.Krempaszky: Microstructure-based modelling of multiphase materials and complex structures. Continuum Mech. and Thermodyn. 27 (2015) 22 p. DOI: 10.1007/s00161-015-0477-7

n S. Hafenstein, E. Werner, J. Wilzer, W. Theisen, S.

Weber, C. Suderkötter, M. Bachmann: Influence of temperature and tempering conditions on thermal conductivity of hot work tool steels for hot stamping applications. Steel Res. Int. 86 (2015) 1628-1635. DOI: 10.1002/srin.201400597

n R. Wesenjak, C. Krempaszky, E. Werner: Prediction

of forming-limit curves of dual-phase steels based on a multiple length scale modelling approach con- sidering materials instabilities. Comput. Mat. Sci. in press. DOI: 10.1016/j.commatsci. 2015.09.046

n F. Meier, C. Schwarz, E. Werner: Numerical calcu-

lation of the tangent stiffness tensor in materials modeling. Computer Methods in Applied Mecha- nics and Engineering. in press. DOI: 10.1016/j. cma.2015.11.034

Conferencesn E. Werner, C. Krempaszky, A. Fillafer: Werkstoff-

mechanik des Lochaufweitversuchs an hochfesten Stahlfeinblechen. 11. Tagung Gefüge und Bruch, Leoben, A, 2015.

n P. Holfelder, G. Hawranek, S. Primig, C. Krempas-

zky, E. Werner: Modellierung der Erstarrung von selektiv lasergeschmolzenem Ti-6Al-4V mit der Phasenfeld-Methode. 61. Metallkunde-Kolloquium, Lech am Arlberg, A, 2015.

n F. Meier, C. Schwarz, E. Werner: Influence ofmicro-structure of Al-components on the life time of integrated circuits. ICM12 – 12th Int. Conf. onthe Mechanical Behavior of Materials, Karlsruhe, D,2015

n F. Meier, C. Schwarz, E. Werner: Modeling and simulation of thin Al-film under cyclic thermal loading. IV Int. Conf. on Coupled Problems in Science and Engineering, Coupled Problems 2015, Venice, IT, 2015.n E. Werner, C. Krempaszky, A. Fillafer, R.

Wesenjak,F. Meier: Microstructure-based modelling of multi-phase materials and complex structures. ACE-X2015 – 9th Int. Conf. on Advanced ComputationalEngineering and Experimenting. Munich, D, 2015.

n P. Holfelder, C. Krempaszky, E. Werner: Selective laser melting of Ti-6Al-4V: Influence of process parameter on the microstructure. Numerical simulation of solification with a thermodynamically motivated nucleation and growth model. PM Titanium 2015, 3rd Conf. on Powder Processing, Consolidation and Metallurgy of Titanium. Lüne- burg, D, 2015.n A. Pichler, T. Hebesberger, D. Krizan, F.

Winkelhofer,K. Hausmann, E. Werner: Phase transformations, microstructures and mechanical properties of TBF/Q&P grades. MS&T 2015, Fundamentals, Characterization and Computational Modeling. Symposium: Phase Stability, Diffusion Kinetics, and their Applications (PSDK-X), Columbus, OH, USA,2015

Patentsn M. Sommerer, S. Walter: Electron Beam Welding

als Verbindungstechnik von W-Brennbahnen mit keramischen Drehanodenkörpern aus SiC. (DE102012210506)

n M. Sommerer, H. von Dewitz: Herstellung von Parti-

kel- und Faser verstärkten Refraktärmetallen mittelsFoliengießen oder Extrusion. (DE102012217182)

n M. Sommerer: Herstellung von (Thermoschock optimierter) Refraktärmetalle mit isotroper Werk- stoffstruktur mittels Extrusion. (DE102012217188)

n M. Sommerer, S. Lampenscherf, S. Walter, E.Werner, H. von Dewitz: Herstellung von (Ther- moschock optimierten) Refraktärmetallen mit isotroper Werkstoffstruktur mittels Foliengießen. (DE102012217191)

n M. Sommerer: Herstellung von isotropen Refrak-tärmetallschichten und -komponenten mittels Elektronenstrahlschmelzen oder Laserschmelzen für Anwendungen mit thermozyklischer und mechani- scher Belastung. (DE102012217194)n M. Sommerer: Verschleiß reduzierender

Betrieb vonim Thermoschock punktuell belasteten (Refrak- tärmetall-) Komponenten – insbesondere in den Anwendungen der Röntgenanode und dem Target in Beschleunigern. (DE102013203218)

· web viewvia simulation the influence of the alu- minum microstructure on typical failure...

Documents