Band 102, Heft 8

An overview is given of Plasma Generators developed with the focus on application in thermal spraying, some of which might even find use in other thermal plasma processes. The construction and operation principle of conventional plasma torches as well as of plasma torches with special features are presented. The particular properties and application potential of the innovative plasma generator types “Triplex”, “Delta” and “Large”, each operating with new physical principles, are discussed.

For high volume manufacturing of many stainless steel components, a continuous brazing process using a shielding gas furnace is usually preferred. Regarding the workability of the nickel-based brazes used here, furnace brazing in reducing or inert shielding gas is subject to considerable constraints: On the one hand, the maximum brazing temperatures are limited by the specific process and, on the other hand, limitations arise caused by the chemical reaction of the materials to be brazed with the process atmosphere. For this reason, the braze materials are an essential starting point for improving the process. In the present contribution, modified nickel-based brazes are introduced which meet the process conditions in continuous shielding gas furnaces and offer the prospect of improving the properties of stainless steel components joined in this way.

Reactive air brazing offers economically and technologically advantageous joining of ceramics to metals. Solid oxide fuel cells and membranes for oxyfuel combustion are recent fields of application. However, it remains a problem that strong metallurgical reactions between brazes and base materials occur. These reactions were analysed by differential scanning calorimetry tests to get a better understanding. Therefore, three braze alloys (Ag8Cu, Ag8Cu0.5Ti and Ag4Cu4Ni) and five base materials (alumina, 3YSZ partially stabilised zirconia, BSCF perovskite ceramic, X1CrTiLa22 and X15CrNiSi25-20) were investigated. The reaction peaks correlate with the formation of reaction layers, which were observed in metallographic analysis of brazed specimens. The results help to explain the reaction mechanisms and allow optimised selection of filler metals and brazing temperature.

An EN AW-6082 aluminium alloy was subjected to incremental deformation by a new process named gradation rolling. The process has been used to generate a grain size distribution in a rod-shaped billet. A grain size gradient ranging from coarse-grained in the billet core to ultrafine-grained at the surface of the billet can be obtained. The grain size distribution has been confirmed by electron backscatter diffraction showing the grain refinement related to the true strain. The fibre texture of the base material has been transformed to a rolling texture in the surface region. This results in strain-hardening and grain-boundary strengthening in the surface region (130 HV0.5) as compared to the unchanged core material (110 HV0.5).

The creep behaviour of an AW2124 Al-alloy unreinforced and reinforced with 25 vol.% of SiC particles of different sizes is investigated. The materials were produced via powder metallurgy using two powder blending techniques: ball milling and wet blending. The SiC particles are fractured during ball milling resulting in 2 – 3 vol.% of sub-μm SiC particles. The SiC particles in the wet blended composites are in the μm-range. The creep behaviour of the matrices and the ball milled composites is characterized by a low-stress and a high-stress region. In the low-stress region the creep deformation is governed by viscous drag of dislocations, while climb of dislocations dominates in the high-stress region. The high creep resistance exhibited by the ball milled composites is mainly due to the presence of oxide dispersoids introduced during blending of the powders.

In the present work some essential aspects related to the simulation of phase transformation during hot forming simulation are discussed. A new material model used for hot forming simulation in commercial finite element code LS-DYNA was applied. Experimental values and bibliographical data were used in order to find optimum input parameters. Based on these parameters a hypothetical hot forming process for a graded component part was simulated. Using tools at varied temperatures and with different thermal properties seems to be a feasible method to achieve a better control of the phase transitions during hot forming.

Hot crack assessment during production and processing of metallic materials is an essential prerequisite for the safety of welded structures. The hot cracking investigations presented here were carried out as part of a study aimed at the development of Cr/Ni low transformation temperature filler materials. Low transformation temperature alloys open up the possibility for welding high strength steels. The externally loaded Modified Varestraint Transvarestraint hot cracking test was employed in the experiments described. The hot cracking resistance was evaluated with the help of light microscopy applied at the specimen surface. The proportionality between hot cracking susceptibility and Cr/Ni alloy content was explained by the altered solidification kinetics and by the enlarged solidification interval. The internal crack paths and the three-dimensional structure of the crack net in the material volume were examined using X-ray computer tomography. The total crack lengths for different material depths and circumjacent rectangular volumes, respectively, were be determined. An increasing hot cracking susceptibility with increasing Cr/Ni alloy content was also be established for the specimen volume.

A hydrogen-free DLC (diamond-like carbon) coating was deposited with a bias voltage of 150 V on various high and low alloy tool steels to study the effect of the pre-treatment of the steel substrate on the wear behaviour of the DLC coating in sliding contact with uncoated counterparts. The morphology and mechanical properties of the DLC coating as well as the effect of plasma nitriding on the surface roughness and the hardness of the steels were studied in order to perform a correlation with the results of tribology tests. It could be concluded from the results that the plasma nitriding of the high alloy tool steel X210CrW12 leads to a significant decrease in the wear and friction coefficient of the DLC coating. Furthermore, it was found that plasma nitriding of the steel results in a decrease in the wear of uncoated counterparts as well. Finally, the wear mechanisms and failure of DLC coatings deposited on various steels were compared with each other and discussed analytically.

This paper contributes to the development of physical vapour deposition multilayer coatings to resist erosion in corrosive environments. The influence of the deposition parameters of the bonding layer on adhesion has been investigated. Furthermore, the influence of the layer design on the corrosion resistance and the thickness of the diamond-like carbon top layer on the erosion resistance of the investigated multilayer coating have been studied. Corrosion and erosion tests as well as scratch tests, metallographic and scanning electron microscope investigations were carried out to characterize the coatings and their degradation behaviour.

For the series production of adaptive fibre-reinforced thermoplastic structures, the development of process-adapted piezoceramic modules is gaining central importance. Thermoplastic-compatible piezoceramic modules are being developed which are suitable for a matrix-homogeneous adhesive-free integration of the modules in fibre-reinforced thermoplastic structures during a simultaneous welding process. Aiming at the continuous improvement of the novel module design and its manufacturing technology based on a hot-pressing technology, an adapted physical nonlinear simulation model was developed in order to analyze the material and process dependent function and consolidation behaviour. Special regard is given to residual stresses caused by different coefficients of thermal expansion of the component materials and their specific use to enhance the module performance.

The kinetics of the precipitation of Co from a supersaturated solid solution of Au-10.12 at.% Co was investigated by differential scanning calorimetry upon isothermal annealing at temperatures in the range from 567 K to 612 K and upon isochronal annealing at heating rates between 5 K min −1 and 40 K min −1 . The microstructural evolution during the course of precipitation was traced by (high-resolution) transmission electron microscopy, scanning electron microscopy, X-ray diffraction and hardness measurements. First, the exothermic formation of 0.5 – 1 nm thin plate-like bcc Co precipitates takes place parallel to {100} habit planes of the Au-matrix. Upon continued annealing, equiaxed fcc Co particles in epitaxial orientation with the Au lattice occur by a Bain-type transformation of the Co particles from the bcc to the fcc lattice structure associated with loss of coherency. Fitting of a modular model of transformation kinetics (Liu et al., Int. Mat. Rev. 52 (2007) 193) simultaneously to the first peak of all isothermal differential scanning calorimetry runs and simultaneously to the first peak of all isochronal differential scanning calorimetry runs demonstrated that the nucleation of bcc Co plates is governed by site saturation in the case of isothermal annealing and continuous nucleation in the case of isochronal annealing, and by linear growth for both isothermal and isochronal annealing. The activation energy for growth is remarkably small due to the presence of quenched-in vacancies. Recovery prior to precipitation leads to a change of the nucleation mechanism from site saturation to continuous nucleation as well as an increase in the activation energy for growth.

Phase equilibria in the Fe–Mg–Si system have been determined experimentally at 800 and 1 000 °C using alloys of fixed composition and diffusion couples. Besides the phases that occur in the binary systems no additional phases were observed. However, Fe 5 Si 3 , the only phase that shows a marked solid solubility for the third component, is stabilised by the addition of Mg to lower temperatures than in the binary Fe–Si system, thus forming a ternary composition at 800 °C. A complete isothermal section has been set up for 800 °C, while only selected phase equilibria were determined at 1 000 °C.

Single crystalline germanium nanorods have been prepared via a simple hydrothermal deposition process using Ge and GeO 2 as the starting materials under the hydrothermal conditions of 400 °C and 7.1 – 8 MPa. Scanning electron microscopy and transmission electron microscopy observations show the diameter of the nanorods ranging from 70 nm to 500 nm and the length greater than 10 μm. X-ray diffraction and high resolution transmission electron microscopy results indicate that the nanorods are mainly composed of single crystalline Ge with cubic diamond structure. The growth process has been proposed as the CuGeO 3 -assisted growth mechanism.

The Taguchi approach was employed to determine the softening kinetics of AISI 304 stainless steel during post-dynamic recrystallization. Analysis of variance was done to evaluate the effectiveness of parameters quantitatively. Using regression analysis, four equations for the time corresponding to softening of 50 % during pass intervals were obtained. The equations were related to the four strain regions ranging from 0.25 to 2 times the peak strain. With mathematical manipulation of some equations, relations for the transition strain and the corresponding time of 50 % softening were derived and explained by considering microstructures evolved during dynamic recrystallization.