R.J. Hellmig

University of Siegen · Department of Mechanical Engineering

Design of Experiments (DoE) is essential for data-driven models. It is applied to create uniformly distributed designs for the model’s input space. For good uniformity, the point density of the generated design shall be kept roughly constant, i.e., too close points and too big data holes shall be avoided. The generation of space-filling designs, ev...

The quality of data used for data-driven modeling affects the model performance significantly. Thus, design of experiments (DoE) is an important part during model development. The design space is constrained in many applications. In this work, the constrained case is investigated. An Latin hypercube based approach is applied and analyzed for strong...

The quality of data-driven models depends significantly on the data distribution in the input space. In this work, design of experiments (DoE) methods for constrained input spaces are discussed. An approach based on an Latin hypercube (LH) design is introduced to deal with strongly constrained input spaces. For the unconstrained case, where the inp...

The quality of data-driven models depends significantly on the data distribution in the input space. In this work, design of experiments (DoE) methods for constrained input spaces are discussed. An approach based on an Latin hypercube (LH) design is introduced to deal with strongly constrained input spaces. For the unconstrained case, where the inp...

We report first results on producing a metal – metal composite by forcing solid aluminium to fill a porous steel preform. This process is akin to forcefill used for filling of vias when producing metallic interconnects in microelectronics. Using scanning electron microscopy with energy-dispersive X-ray spectroscopy, it was shown that for nominal po...

The deformation behavior of copper during equal channel angular pressing (ECAP) was calculated using a three-dimensional version of a constitutive model based on the dislocation density evolution. Finite element simulations of the variation of the dislocation density and the dislocation cell size with the number of ECAP passes are reported. The cal...

For the production of aluminum screws, an effective thermomechanical treatment is necessary for enabling high strength combined with good formability. In this study, the influence of pre-aging as initial heat treatment prior to following processing steps was investigated for the precipitation hardenable 6056 aluminum alloy. The short-term low tempe...

In the present study, the influence of the initial heat-treatment conditions on the artificial aging behavior after conventional linear extrusion at room temperature was investigated for the precipitation hardening of a 6056 aluminum alloy. A solution-annealed condition was systematically compared to naturally-aged and pre-aged conditions. Differen...

Motivated by the improvement of the cost and resource efficiency of the manufacturing process of age-hardening aluminum components, the influence of an initial heat-treatment, prior to plastic deformation, on the artificial aging behavior and the hardness is investigated. Systematic work is done by comparing three initial heat-treatment conditions...

Die Direktverschraubung ist bei vielen Anwendungen das Maß der Dinge für eine sichere und kostengünstige Verbindung von Bauteilen. Bei speziellen Werkstoffen, wie höchstfesten Blechen oder Gussmaterialien, stieß diese Technik in der Vergangenheit bisweilen an ihre Grenzen. Grund dafür war ein nicht zu unterschätzendes Restrisiko, dass die eingesetz...

In this work, we consider a phase-field framework for crack propagation problems in elasticity and elasto-plasticity. We propose a rate-dependent formulation for solving the elasto-plastic problem. An irreversibility constraint for crack evolution avoids non-physical healing of the crack. The resulting coupled two-field problem is solved in a decou...

Enhanced use of lightweight materials, namely aluminium and magnesium, requires the application of appropriate production techniques. Self-tapping screws represent an innovative joining technology. High-strength aluminium alloys can be used as screw material. Stringent requirements regarding the wear resistance of screw surfaces result from the thr...

Die Umsetzung der Mischbauweise in der Automobilindustrie und in der Luft- und Raumfahrt stellt das Fügen mittels Schraube vor neue Problemstellungen. Anodisierte hochfeste Aluminiumschrauben sollen gewindefurchende Stahlschrauben ersetzen und die Schädigung der Werkstoffverbunde durch elektrochemische Korrosion verhindern. Untersucht wurden die Ko...

Im Sinne des Leichtbaus und damit einhergehender ökonomischer und ökologischer Aspekte ist die Verwendung hochfester Al-Schrauben besonders interessant. Gegenstand des Projektes T1 ist der Transfer unterschiedlicher, in Grundlagenversuchen als erfolgversprechend eingestufter Oberflächenbehandlungsverfahren in die industrielle Bewertung und Nutzung....

Cu–4.5 wt % Cr and Cu–4.5 wt % Cr–3 wt % Ag alloys, with and without nanocrystalline Al2O3 dispersions (particle size <10 nm), were synthesized by mechanical alloying/milling and consolidated by equal-channel angular pressing (ECAP) at ambient temperature. Microstructural characterization and phase analysis by X-ray diffraction, as well as scanning...

Copper is one of the most important materials used for electrical connectors with a high thermal and electricial conductivity, and there is an ongoing demand to improve its mechanical properties without sacrificing other beneficial properties. A possible approach to achieving this aim 14 is to use the method of severe plastic deformation to obtain...

To investigate the influence of equal channel angular pressing on the microstructure and texture of the magnesium alloy AZ31, electron backscattering diffraction and well as neutron diffraction experiments were carried out. Through these experiments it was possible to trace the microstructure and texture evolution with strain accumulated with the i...

Pure Cu, CuZr and an Al-alloy were processed by Equal Channel Angular Pressing (ECAP) at room temperature applying route Bc. Microstructure evolution during ECAP and subsequent annealing was investigated. The deformed and annealed states were characterized by EBSD, TEM and microhardness tests. The microstructure variation was recorded and compared...

Microstructure and texture evolution of pure copper (99.95%) after equal-channel angular pressing (ECAP) with route Bc up to 12 passes and subsequent heat treatment were investigated. After deformation the samples were annealed at different temperatures. The deformed and annealed states were characterized by X-ray pole figures, electron back scatte...

A squeeze cast magnesium–aluminium–zinc alloy AZ31 was subjected to equal-channel angular pressing (ECAP) at 200 °C up to four passes following route Bc. The grain size was reduced by a factor of about 100–200 through ECAP processing. The mechanical properties of as cast (non-pressed) and the ECAP-processed material were examined using compression...

To investigate the effect of severe plastic deformation on the acoustic emission behaviour of the magnesium alloy AZ31, equal channel angular pressing (ECAP) was carried out at 200°C up to four passes following route BC. Acoustic emission was measured during compression tests at various temperatures from room temperature up to 300°C. The compressio...

Deformation behaviour of magnesium alloy AZ31 under uniaxial compression was investigated. AZ31 samples produced by squeeze casting, hot rolling and hot rolling with subsequent equal channel angular pressing (ECAP) were tested. It was shown that ECAP processing produces a fine-grained microstructure that gives rise to improved mechanical properties...

Samples of pure Cu and Cu with 0.17 wt.% Zr were processed by equal-channel angular pressing up to 12 passes with a 90° die angle using route B c. Microstructure evolution after deformation and subsequent heat treatment of this severely deformed material was investigated. The deformed and annealed states were characterized by electron back scatter...

Microstructure and texture evolution of an AlMg1Mn0.14 alloy after Equal-Channel Angular Pressing (ECAP) and subsequent heat treatment were investigated. Initially the material was subjected to up to 8 passes with a 90° die angle using routes A and Bc. The deformed and annealed states were characterised by crystallographic texture analysis, Electro...

The objective of the present study is to find out how much micrograin boundary carbides contribute to the creep strength of tempered martensite ferritic steels. For this purpose we compare the creep behaviour and the microstructural stability of a tempered martensite ferritic steel (German grade: X20) with that of a strongly deformed binary Fe10Cr...

This article reports on the influence of zirconium (Zr) addition (0.17 wt pct) on the cyclic stability of ultrafine-grained (UFG) oxygen-free high-conductivity (OFHC) copper (Cu) of originally high (99.995wt pct) purity processed via equal-channel angular extrusion (ECAE). Systematic low-cycle fatigue (LCF) tests accompanied by microstructural inve...

Using equal channel angular pressing (ECAP), ultrafine grained copper can be produced by severe plastic deformation (SPD) via structural refinement of coarse grained polycrystalline copper. This leads to a new class of materials exhibiting superior mechanical properties, especially a significantly enhanced yield strength at reasonable ductility. To...

The mechanical properties of polycrystalline Cu (purity 99.95%) prepared by severe plastic deformation were studied at low homologous temperatures from 0.5 K to room temperature. Material with three different microstructures was prepared by annealing of ultrafine-grained Cu. At cryogenic temperatures (0.5 and 4.2 K) the material exhibited an invers...

Some aspects of grain refinement in the process of equal channel angular pressing (ECAP) are considered. Characteristics of strength, fatigue, and superplasticity of magnesium alloys AZ31, ZK60, and AS21X are studied. The possibilities of increasing the rate of hydrogen absorption by alloy ZK60 after ECAP treatment are discussed.

We report the results of measurements of the grain size distribution function and the thermal conductivity of ultrafine-grained copper produced by equal channel angular pressing (ECAP), with special attention to the evolution of these quantities with the number of pressing cycles. To explain the experimental findings, the equilibrium grain size dis...

ECAP (equal channel angular pressing) is a well-known severe plastic deformation method used to produce ultra-fine grained materials. The dimensions of ECAP specimens are usually in the centimeter range. For producing high strength wires or fibres with diameter in the micrometer/millimeter range, downscaling of the ECAP process may be a viable opti...

Copper specimens were deformed by equal channel angular pressing (ECAP) up to 8 passes. The microstructure was studied by X-ray line profile analysis. The crystallite size is reduced to a few tens of nanometers even after the first ECAP pass and it does not change significantly during further deformation. At the same time, the dislocation density i...

Microstracture evolution and corrosion behaviour of ultrafine-grained copper processed by equal channel angular pressing (route Bc) were studied. The results of TEM investigation of the microstructure evolution are presented along with the measurements of the corrosion potential, the corrosion current density and the anodic current density for two...

Typically, magnesium alloys with conventional grain size exhibit microplastic behaviour already at low stresses. This behaviour restricts the technological utilization of these materials. The aim of this study was to investigate whether ECAP can be applied to enlarge the elastic range of AS21X. Cyclic tensile tests at room temperature were carried...

Extreme grain refinement by severe plastic deformation (SPD) is an established processing approach broadly applied to bulk materials. The dimensions of specimens or workpieces used are typically in the centimeter range. We propose to adopt an analogue of equal channel angular pressing in the context of microforming. It is suggested that using sub-m...

Microstructure evolution and corrosion behaviour of ultrafine-grained copper processed by equal channel angular pressing (route B c) were studied. The results of TEM investigation of the microstructure evolution are presented along with the measurements of the corrosion potential, the corrosion current density and the anodic current density for two...

Microstructure evolution and corrosion behaviour of ultrafine-grained copper processed by equal channel angular pressing (route B c) were studied. The results of TEM investigation of the microstructure evolution are presented along with the measurements of the corrosion potential, the corrosion current density and the anodic current density for two...

We present a selection of own results demonstrating the effect of equal channel angular pressing (ECAP) on several wrought and cast Mg alloys, including AZ31, AS21X, Mg-Ni and ZK60. The enhancement of the yield strength and changes in ductility, as well as the extension of fatigue life are discussed, along with the microstructures produced. In addi...

Crystallite size determined by X-ray line profile analysis is often smaller than the grain or subgrain size obtained by transmission electron microscopy, especially when the material has been produced by plastic deformation. It is shown that besides differences in orientation between grains or subgrains, dipolar dislocation walls without difference...

A three-dimensional version of a dislocation density based constitutive model that allows calculating the evolution of microstructure, mechanical properties and texture was used to simulate the equal channel angular pressing (ECAP) of copper. The present paper focuses on the concomitant variation of the misorientation angle with increasing strain....

We propose to use porous steel ‘dies’ for solid state ‘infiltration’ with a metallic material by a variant of the known forcefill process. Severe plastic deformation (SPD) that occurs during the material's plastic flow through a tortuous channel is believed to lead to grain refinement and an improvement of mechanical properties referred to as micro...

We report first results on producing a metal-metal composite by forcing solid aluminium to fill a porous steel preform. This process is akin to forceful used for filling of vias when producing metallic interconnects in microelectronics. Using scanning electron microscopy with energy-dispersive X-ray spectroscopy, it was shown that for nominal pore...

Copper specimens were severely deformed by equal channel angular pressing (ECAP) up to eight passes. The microstructure was studied by X-ray diffraction peak profile analysis as a function of strain (ɛ). It was found that the crystallite size is reduced to a few tens of nanometers already at ɛ=0.7 and it does not change significantly during further...

Over the last few years [1], equal channel angular pressing (ECAP) has become a popular method for producing bulk ultra-fine structured materials. It allows repetitive pressing of a billet through a die having two intersecting channels without changing the billet's cross-sectional dimensions. Therefore, very high shear strains can be introduced int...

The mechanisms of this extreme grain refinement is far from being understood. There are reasons to believe, however, that it is related to a decrease of the average dislocation cell size, possibly accompanied with transformation of cell walls into large angle grain boundaries due to accumulation of misorientations between cells with strain [6]. A m...

Submicrocrystalline and/or nanocrystalline materials prepared by severe plastic deformation possess improved properties as compared to their coarse grained (CG) counter-parts. Ultra-fine grained (UFG) copper polycrystals prepared by equal channel angular pressing (ECAP) to various strain (1, 2, 4 and 8 passes) were investigated. ECAP processing res...

Nanoscaled Al2O3 powder is used as additive in coarse-grained Al2O3 green bodies. Its influence on the green density is investigated. The potential effect of powder mixing on the green density is described by a model for multimodal powder mixtures that predicts the powder composition for maximum green body density. The model is applied to bimodal a...

Two essentially different metals, fcc copper and hexagonal titanium, were deformed by equal channel angular pressing (ECAP) up to eight passes. The microstructure developed as a result of severe plastic deformation (SPD) was studied by X-ray peak profile analysis. The formation of submicron grain sized structures was studied as a function of the nu...

Copper and aluminum were processed by equal channel angular pressing (ECAP). The effect of the ECAP route and the number of passes on strength and ductility as well as on the dislocation cell structure and texture was studied for both materials. To investigate the stress distribution in ECAP deformed workpieces, hardness maps with small imprint spa...

Texture evolution was analyzed with the full-constraint Taylor model for an idealized perfectly plastic face-centered cubic material as well as for real, strain-hardening copper subjected to equal channel angular pressing (ECAP). For the idealized material, the stress in the plastically deformed part of the billet was shown to be uniform leading to...

Texture evolution was analyzed with the full-constraint Taylor model for an idealized perfectly plastic face-centered cubic material as well as for real, strain-hardening copper subjected to equal channel angular pressing (ECAP). For the idealized material, the stress in the plastically deformed part of the billet was shown to be uniform leading to...

In this study, the deformation behavior of aluminium during equal channel angular pressing (ECAP) was calculated on the basis of a dislocation density-based model. The behavior of the material under ECAP, including the dislocation density and cell size evolution as well as texture development, was simulated using the finite element method (FEM). Th...

The deformation behavior of copper during equal channel angular pressing (ECAP) was calculated using a three-dimensional version of a constitutive model based on the dislocation density evolution. Finite element simulations of the variation of the dislocation density and the dislocation cell size with the number of ECAP passes are reported. The cal...

A phase mixture model is employed to describe the deformation behaviour of ultra fine grained materials (down to the nanometer grain size scale). The main feature of the model is that the grain boundaries are treated as a separate phase. Dislocation glide in combination with diffusional creep mechanisms are considered as the mechanisms of plasticit...

A two-dimensional model that describes the strain hardening behavior of dislocation cell-forming crystalline materials at large strains was recently extended to the three-dimensional case In the present study, the deformation behavior of IF steel during equal channel angular pressing (ECAP) was calculated on the basis of the 3D version of the model...

Alumina-zirconia nanopowder mixtures of different composition were produced by laser ablation from micropowder pellets followed by condensation of the induced vapour in a controlled aggregation gas. The employed laser was a pulsed 1000 W Nd:YAG-laser. The median particle diameter of the produced nanopowder was about 13 nm. Nanopowders of different...

Alumina-zirconia nanopowder mixtures of different composition were produced by laser ablation from micropowder pellets followed by condensation of the induced vapour in a controlled aggregation gas. The employed laser was a pulsed 1000 W Nd:YAG-laser. The median particle diameter of the produced nanopowder was about 13 nm. Nanopowders of different...

The high sinter activity of alumina nanoparticles was used to bond coarse-grained corundum ceramics. The employed nanopowder was generated by the evaporation of solid corundum rods using the radiation of a Nd:YAG-laser followed by condensation of the vapour in a controlled atmosphere. Some nanopowder was sandwiched between corundum disks followed b...

The agglomeration of nanoscaled powders in general decreases the density of green bodies pressed from those nanopowders. Depending on the strength and morphology of the agglomerates different densities can be achieved. It is shown that in the case of ceramic nanopowders as alumina and yttria-stabilised zirconia which are produced by laser ablation...

Nanoscaled alumina particles were employed for bonding of commercial coarse-grained corundum ceramics. The nanopowder was generated by ablation of solid corundum samples with the pulsed radiation of a Nd:YAG-laser and subsequent condensation of the vapor in a controlled atmosphere. Some of the nanopowder was sandwiched between corundum discs follow...

N-type Hg1−xCdxTe layers with x values of 0.3 and 0.7 have been grown by molecular beam epitaxy using iodine in the form of CdI2 as a dopant. Carrier concentrations up to 1.1 1018 cm−3 have been achieved for x = 0.7 and up to 7.6 1017 cm−3 for x=0.3. The best low temperature mobilities are 460 cm2/(Vs) and 1.2 105 cm2/(Vs) for x=0.7 and x=0.3, resp...

Zugl.: Clausthal, Techn. Universiẗat, Diss., 2000.