Showing papers by "Robert F. Singer published in 2012"

Creep properties of different γ′-strengthened Co-base superalloys

Abstract: The influence of various alloying elements on the creep properties of polycrystalline Co-base superalloys hardened by a ternary L1 2 compound, Co 3 (Al,W) (γ′-phase), was investigated. A Ti containing quaternary alloy shows creep strength similar to Ni-base superalloys IN100 and IN713C at 850 °C and strongly superior to conventional Co-base superalloys as Haynes188. The activation energy for creep between 850 and 950 °C is similar to the polycrystalline Ni-base superalloy IN100 in the same temperature range. Strengthening of the grain boundaries by third phase precipitates was found to be crucial for the mechanical properties. This can be achieved either by precipitation of borides or by additional intermetallic phases which precipitate due to oversaturation. During compressive creep at 850 °C only a slight tendency for directional coarsening occurs, while at 950 °C distinct γ/γ′-rafts perpendicular to the external compressive stress axis are formed which indicate a positive lattice misfit even at 950 °C.

In situ flaw detection by IR‐imaging during electron beam melting

Abstract: Purpose – The purpose of this paper is to investigate the possibility of in situ flaw detection for powder bed, beam‐based additive manufacturing processes using a thermal imaging system.Design/methodology/approach – The authors compare infrared images (IR) which were taken during the generation of Ti‐6Al‐4V parts in a selective electron beam melting system (SEBM) with metallographic images taken from destructive material investigation.Findings – A good match is found between the IR images and the material flaws detected by metallographic techniques.Research limitations/implications – First results are presented here, mechanisms of flaw formation and transfer between build layers are not addressed in detail.Originality/value – This work has important implications for quality assurance in SEBM and rapid manufacturing in general.

Mechanical characterisation of a periodic auxetic structure produced by SEBM

Abstract: We present a thorough investigation of the mechanical behaviour of a non-stochastic cellular auxetic structure. A combination of experimental and numerical methods is used to gain a deeper understanding of the mechanical behaviour and its dependence on the geometric properties of the cellular structure. The experimental samples are built from Ti-6Al-4V using selective electron beam melting, an additive manufacturing process giving the possibility to vary the geometry of the structure in a highly controlled manner. The use of finite element simulations and mathematical homogenisation allows us also to investigate off-axis properties of the cellular material. This leads to a more comprehensive understanding of the mechanical behaviour of the auxetics. Ultimately, the gained knowledge can be used to tailor auxetic materials to specific applications.

Combination of Extrinsic and Intrinsic Pathways Significantly Accelerates Axial Vascularization of Bioartificial Tissues

Abstract: BACKGROUND In this study, the authors present a modification of the arteriovenous loop model that combines extrinsic and intrinsic vascularization modes to enhance vascularization of bioartificial matrices. METHODS An arteriovenous loop was created in the medial thighs of 24 rats. The loop was placed in a newly developed titanium chamber, which was fabricated with an electron beam melting facility, and was embedded in a hydroxyapatite/β-tricalcium phosphate/fibrin matrix. At the explantation time points (2, 4, 6, and 8 weeks), constructs were perfused by differently colored dyes to determine the amount of tissue vascularized by either the intrinsic or the extrinsic vascular pathway. Specimens were investigated by means of micro-computed tomography and histologic and morphometric analysis. RESULTS Although there was an equal number of blood vessels originating from the center and the periphery, 83 percent of all vessels displayed a connection to the arteriovenous loop already at 2 weeks. There was a continuous increase of the relative proportion of vessels connected to the arteriovenous loop over time detectable. At 8 weeks, communications between the newly formed vessels and the arteriovenous loop were visible in 97 percent of all vessels. CONCLUSIONS This study demonstrates for the first time the enhancement of angiogenesis in an axially vascularized tissue by an additional extrinsic vascular pathway. By 2 weeks, both pathways showed connections, allowing transplantation of the entire construct using the arteriovenous loop pedicle. This approach will allow for reduction of the time interval between arteriovenous loop implantation and transplantation into the defect site and limitation of operative interventions.

Effect of eutectics on plastic deformation and subsequent recrystallization in the single crystal nickel base superalloy CMSX-4

Abstract: The electron backscattered diffraction (EBSD) technique and transmission electron microscopy (TEM) were used to characterize the microstructure of a locally deformed single crystal (SX) nickel-base superalloy - CMSX-4. The effect of eutectics on the deformation and recrystallization (RX) behavior was investigated. It was found that the texture component map is a reliable method for the determination of the severity of deformation in locally deformed SX superalloys. Severe deformation was mainly created in interdendritic regions, especially around eutectics. The dislocation distribution and configuration was consistent with the nucleation and the growth behavior of recrystallizing grains. (C) 2011 Elsevier B.V. All rights reserved.

Discontinuous Precipitation and Phase Stability In Re- and Ru-Containing Nickel-Base Superalloys

Abstract: The phase stability of nickel-base superalloys has been investigated using a new in-house-designed alloy series with stepwise increased additions of Re and Ru at otherwise fixed atomic fractions of alloying elements. Results presented in this study are focused on the lesser-known topologically closed packed (TCP) formation of columnar colonies or so-called discontinuous precipitation. A detailed investigation of these colonies allowed for identifying compositional changes during the growth process and for providing a three-dimensional (3-D) illustration of the TCP phases within these colonies. The results were used to compare the colony growth process with existing growth models. Furthermore, the influence of Re and Ru on the appearance of discontinuous precipitation has been investigated by means of colony width and the effect on creep properties. Larson-Miller plots are given to illustrate the creep strength of directionally solidified samples with and without TCP colonies compared with single crystalline samples free from TCP colonies.

Influence of MWCNT dispersion on corrosion behaviour of their Mg composites

Abstract: Carbon nanotubes (CNTs) have been demonstrated as good reinforcements to improve mechanical properties of magnesium alloys. This study reports on the influence of CNT dispersion on corrosion behaviour of their Mg composites. Experiments revealed that the corrosion rate is drastically increased by adding CNTs to the magnesium alloy due to galvanic coupling. Moreover, a homogeneous CNT dispersion leads to reduced corrosion resistivity. Surface pre-treatment is also found to be critical for the corrosion performance for both 1 and 5 wt% CNT composites.

Influence of the fabrication process on the functionality of piezoceramic patch transducers embedded in aluminum die castings

Abstract: Piezoceramic patch transducers are integrated into aluminum components using high-pressure die casting. Expanded metal has proven suitable as a supporting structure for placing the patch transducers inside the die cavity and for stabilization during the injection of molten metal. However, difficulties arise when the transducers are positioned off the neutral axis within the wall of the casting. Numerical simulations of the die filling are performed to analyse the evolution of the integration process. The asymmetric infiltration of the supporting structure is identified as the major factor contributing to the formation of cracks and perforations inside the piezoceramic transducer. By means of measurements and numerical calculations of the electrical impedance of the transducer, a close relation is established between mechanical damage patterns observed in radiographs of the patch transducers and loss of performance.

Rhenium-free or rhenium-reduced nickel-base superalloy

Abstract: The present invention relates to a nickel-base superalloy comprising aluminium, cobalt, chromium, molybdenum, tantalum, titanium and tungsten, in addition to nickel, as alloy constituents, wherein rhenium can additionally be contained and the rhenium content is less than or equal to 2 wt.% and wherein the titanium content is greater than or equal to 1.5 wt.%. The invention further relates to components made of the nickel-base superalloy.

Thermomechanical Behaviour of Turbocharger Compressor Wheels

Abstract: Compressor wheels on exhaust turbochargers in car and truck applications are highly stressed components. During the development of new compressor wheels the main focus is to design reliable parts with a reasonable lifetime as well as good efficiencies and low inertia providing improved engine efficiency and better dynamic engine performance. In order to fulfill the exceptional requirements on the thermodynamic characteristics of the turbocharger the material of the compressor wheel underlies high mechanical and thermal loads. Centrifugal compressor wheels made of an Al-Cu-Mg precipitation hardened wrought alloy (2618-T6) experience low cycle fatigue loading which results from centrifugal forces and temperature loadings. The development of compressor wheels requires exact methods to predict the mechanical and thermal loads and their influence on the highly stressed regions of the product. The assessment of relevant loadings from static FEA calculations is deficient. Alternatively a constitutive material model for the used aluminum alloy is implemented in FEA simulations. The constitutive material model of Chaboche type with modifications proposed by Jiang makes it possible to describe the time and temperature dependent deformation behavior of the whole compressor wheel. Especially the effects of cyclic plasticity including relaxation and creep can be considered consistently. Boundary conditions on the compressor wheel including wall heat transfer coefficients and wall adjacent temperatures are provided by static heat transfer calculations. The boundary conditions are necessary for transient heat transfer calculations in FEA. In this paper the temperature distribution on the centrifugal compressor wheel for different operating points defined by rotational velocity and compressor inlet temperature is presented. The boundary conditions for transient heat transfer calculations in FEA are provided by conjugate heat transfer calculations for maximal power and idle speed of the turbocharger. The results of this method show time dependent temperature distribution on the compressor wheel under thermal shock conditions. The FEA calculations with boundary conditions from the transient heat transfer calculations describe the deformation behavior of the centrifugal compressor wheel during sequent thermal shock cycles. The thermomechanical behavior during different operating points and load cycles of the turbocharger is investigated. Furthermore relaxation and creep effects on highly stressed regions of the compressor wheel during full power application are presented.Copyright © 2012 by ASME