Correlation between porosity and processing parameters in TiAl6V4 produced by selective laser melting

This study presents the correlation between process parameters and porosity formation in a TiAl6V4 alloy produced by selective laser melting. The porosity is investigated by 2D and 3D methods aiming to identify the mechanisms of void formation, their morphology as well as volume fraction as a function of the energy density. An evident minimumvolume fraction is observed between process parameters with significant overheating and insufficient fusion. It is shown that these two marginal regions define two mechanisms of void formation. Two dominant types of voids morphology are identified and examined regarding pore orientation versus their elongation, which together with the curvature distribution analysis allow revealing critical defects

Two-stage 2D-to-3D reconstruction of realistic microstructures: Implementation and numerical validation by effective properties

Realistic microscale domains are an essential step towards making modern multiscale simulations more applicable to computational materials engineering. For this purpose, 3D computed tomography scans can be very expensive or technically impossible for certain materials, whereas 2D information can be easier obtained. Based on a single or three orthogonal 2D slices, the recently proposed differentiable microstructure characterization and reconstruction (DMCR) algorithm is able to reconstruct multiple plausible 3D realizations of the microstructure based on statistical descriptors, i.e., without the need for a training data set. Building upon DMCR, this work introduces a highly accurate two-stage reconstruction algorithm that refines the DMCR results under consideration of microstructure descriptors. Furthermore, the 2D-to-3D reconstruction is validated using a real computed tomography (CT) scan of a recently developed beta-Ti/TiFe alloy as well as anisotropic "bone-like" spinodoid structures. After a detailed discussion of systematic errors in the descriptor space, the reconstructed microstructures are compared to the reference in terms of the numerically obtained effective elastic and plastic properties. Together with the free accessibility of the presented algorithms in MCRpy, the excellent results in this study motivate interdisciplinary cooperation in applying numerical multiscale simulations for computational materials engineering

Interface-Mediated Twinning-Induced Plasticity in a Fine Hexagonal Microstructure Generated by Additive Manufacturing

The grain size is a determinant microstructural feature to enable the activation of deformation twinning in hexagonal close-packed (hcp) metals. Although deformation twinning is one of the most effective mechanisms for improving the strength–ductility trade-off of structural alloys, its activation is reduced with decreasing grain size. This work reports the discovery of the activation of deformation twinning in a fine-grained hcp microstructure by introducing ductile body-centered cubic (bcc) nano-layer interfaces. The fast solidification and cooling conditions of laser-based additive manufacturing are exploited to obtain a fine microstructure that, coupled with an intensified intrinsic heat treatment, permits to generate the bcc nano-layers. In situ high-energy synchrotron X-ray diffraction allows tracking the activation and evolution of mechanical twinning in real-time. The findings obtained show the potential of ductile nano-layering for the novel design of hcp damage tolerant materials with improved life spans.Fil: Barriobero Vila, Pere. German Aerospace Center.; AlemaniaFil: Vallejos, Juan Manuel. Universidad Nacional del Nordeste. Facultad de Ingeniería; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste; ArgentinaFil: Gussone, Joachim. German Aerospace Center.; AlemaniaFil: Haubrich, Jan. German Aerospace Center.; AlemaniaFil: Kelm, Klemens. German Aerospace Center.; AlemaniaFil: Stark, Andreas. German Aerospace Center.; AlemaniaFil: Schell, Norbert. German Aerospace Center.; AlemaniaFil: Requena, Guillermo. German Aerospace Center.; Alemani

Laser powder bed fusion of Ti-22Al-25Nb at low and high pre-heating temperatures

Titanium alloys based on the orthorhombic Ti2AlNb phase are being considered as potential structural lightweight alloys since the early 1990s due to their favourable mechanical performance, i.e., balanced strength and ductility at room and high temperatures as well as high oxidation and creep resistance. With the emergence of additive manufacturing these alloys become particularly interesting again as the microstructures and properties differ considerably from conventionally processed materials. In our work, we consider the whole process chain including the powder production and explain the microstructure formation of the orthorhombic alloy Ti-22Al-25Nb and the effects of in situ and intrinsic heating during laser powder bed fusion with differing energy densities at low and high pre-heating temperatures by means of state-of-the-art characterization techniques such as in situ high energy synchrotron X-ray diffraction and advanced electron microscopy. Fast cooling rates during low-temperature LPBF lead to metastable weakly ordered β phase. For high-temperature LPBF a Widmanstätten microstructure was observed with lenticular O phase precipitates within the β matrix

Electrolytic Deposition of Titanium on SiC-Fibres as First Step in Titanium Matrix Composite Production

The feasibility of titanium matrix composite production based on a molten salt fibre coating process is studied. In the presentation, it will be demonstrated that sufficiently thick and coherent titanium coatings can be deposited on SiC-fibres in LiCl-KCl-TiCl2 at 700 K. One important aspect of the study is the knowledge of the behaviour of the substrate (carbon coated SiC-fibre) during electrolysis. Therefore additional features related to SiC-fibres and carbonaceous substrates in general will be discussed. Morphology and microstructure of electrolytically coated SiC-fibres investigated by electron microscopy will be compared to fibres coated by PVD technique

Tensile Properties and Microstructure of SiC Fibre Reinforced Multi Metal Matrix Composites

The matrix coated fibre production route is established for titanium matrix composites of highest quality. However, the commonly used method for consolidation (hot isostatic pressing) leads to shrinkage, distortion or even fibre fracture. Some fibre arrangements require consolidation without pressure. The process proposed in this paper uses low melting alloys to consolidate the composite material by infiltrating the matrix coated fibres. Tensile tests at room temperature have shown that these metal matrix composites may have the potential to compete with conventionally processed titanium matrix composites. No significant decrease in strength is expected up to 400°C. One important goal of the paper is to emphasize important points that have to be considered for the development of this new type of composite material

Dendrite Formation during Electro-Deposition of Titanium on SiC fibres

To develop an electrolytic production route for titanium matrix composites, experimentswere performed to deposit pure titanium on carbon coated SiC-fibres using molten LiCl-KCl-K2TiF6 as the electrolyte. At least 4 different types of dendrites and particulate could be observed, instead of the expected coherent layers. Dendrite formation is well known in titanium electrolysis, especially in all chloride melts [1]. Fluoride addition, however, should help to improve the coherence of the deposit [2]. Besides the fact that the fluoride content was apparently too low to unfold its advantageous influence on crystal growth, the following reasons may be responsible for dendrite and powder formation: The concentration of approx. 0.1mol-% K2TiF6 was comparably low and the cathodic overvoltage was certainly higher than the critical values (ηi, ηc) for dendrite or even powder formation respectively (see e.g. [3]). Some of the i-t curves indicate that there may be an induction time before dendrite formation starts. The same reasons can be responsible for the fact that it was not possible to observe nucleation phenomena in the chronoamperogram as described by [4]. In further experiments, it is planned to improve the quality of the deposits by increasing the K2TiF6 concentration and to stabilize the trivalent titanium complexes (TiF6 3-) by raising the F/Ti ratio, e.g. by adding KF. The electrical parameters will have to be carefully adjusted to make sure that the critical value ηi will not be exceeded. In the poster, typical SEM pictures are depicted and the current vs. time curves of the corresponding electrolysis are discussed. Literature: [1] Ehrlich, P. and Kühnl, H., Über die schmelzelektrolytische Abscheidung von Titanmetall. Z. anorg. allg. Chem., 1959. 298(3-4): p. 176-192. [2] Ene, N. and Zuca, S., Role of free F- anions in the electrorefining of titanium in molten alkali halide mixtures. J. Appl. Electrochem., 1995. 25(7): p. 671-676. [3] Popov, K.I., et al., Dendritic electrocrystallization and the mechanism of powder formation in the potentiostatic electrodeposition of metals. J. Appl. Electrochem., 1981. 11(2): p. 239-246. [4] Haarberg, G.M., et al., Electrodeposition of titanium from chloride melts. J. Appl. Electrochem., 1993. 23(3): p. 217-224