Microstructure and mechanical properties of an advanced Ag-microalloyed aluminum crossover alloy tailored for wire-arc directed energy deposition

The implementation of wire-arc directed energy deposition requires the development of novel, process-adapted, high-performance aluminum alloys. Conventional high-strength alloys are, however, difficult to process as they are prone to hot-cracking. Crossover alloys based on Al-Mg-Zn combine good processability with good mechanical properties following artificial aging. Here, we present an effort to further improve the mechanical properties of Al-Mg-Zn crossover alloys using Ag microalloying. No cracks and few porosities were observed in the samples. The microstructure is dominated by fine and globular grains with a grain size ˜ 26.6 µm. The grain structure is essentially free of texture and contains fine microsegregation zones with ˜ 3–5 µm thickness of segregation seams. Upon heat treatment these microsegregation zones are dissolved and T-phase precipitates are formed as clarified by diffraction experiments. This precipitation reaction results in a microhardness of ˜ 155 HV0.1, a yield strength of 391.3 MPa and 418.6 MPa, an ultimate tensile strength of 452.7 MPa and 529.4 MPa and a fracture strain of 3.4% and 4.4% in transversal and in longitudinal directions, respectively. The gained results suggest that highly loaded structures can be manufactured by wire-arc directed energy deposition using the newly developed aluminum crossover alloy.Peer ReviewedPostprint (published version

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

The Influence of Iron in Minimizing the Microstructural Anisotropy of Ti-6Al-4V Produced by Laser Powder-Bed Fusion

There remains a significant challenge in adapting alloys for metal based Additive Manufacturing (AM). Adjusting alloy composition to suit the process, particularly under regimes close to industrial practice, is therefore a potential solution. With the aim of designing new Ti-based alloys of superior mechanical properties for use in laser powder-bed fusion, this research investigates the influence of Fe on the microstructural development of Ti-6Al-4V. The operating mechanisms that govern the relationship between the alloy composition (and Fe in particular) and the grain size are explored using EBSD, TEM and in-situ high-energy synchrotron X-ray diffraction. It was found that Fe additions up to 3 wt% lead to a progressive refinement of the microstructure. By exploiting the cooling rates of AM and suitable amount of Fe additions, it was possible to obtain microstructures that can be optimized by heat treatment without obvious precipitation of detrimental brittle phases. The resulting microstructure consists of a desirable and well studied fully laminar α+ β structure in refined prior-β grains

Sequence of phase transformations in metastable ß Zr–12Nb alloy studied in situ by HEXRD and complementary techniques

Phase transformations in a metastable beta Zr–12Nb alloy were investigated by high-energy X-ray diffraction (HEXRD) measured simultaneously with thermal expansion in situ during linear heating from room temperature to 800 °C. Complementary in-situ methods of electrical resistance and differential scanning calorimetry, which were performed using the same heating conditions as in the HEXRD experiment, provided additional information on the transformation sequence occurring in the Zr–12Nb alloy. Two bcc phases with a different lattice parameter, ßZr and ßNb, were observed in the investigated temperature range and identified using the phase diagram of the Zr–Nb system. In the initial solution-treated condition, metastable ßZr phase and athermal ¿ particles are present in the material. At about 300 °C, Nb-rich ßNb phase starts to form in the material and the original ßZr phase gradually disappears. Ex-situ observations of the microstructure using transmission electron microscopy revealed a cuboidal shape of the ¿ particles, which is related to a relatively large misfit between the ¿ and ß phases. At 560 °C, ¿ solvus was observed, identified by an abrupt dissolution of ¿ particles which was followed by growth of the a phase.Peer ReviewedPostprint (published version

The influence of iron in minimizing the microstructural anisotropy of Ti-6Al-4V produced by laser powder-bed fusion

There remains a significant challenge in adapting alloys for metal-based additive manufacturing (AM). Adjusting alloy composition to suit the process, particularly under regimes close to industrial practice, is therefore a potential solution. With the aim of designing new Ti-based alloys of superior mechanical properties for use in laser powder-bed fusion, this research investigates the influence of Fe on the microstructural development of Ti-6Al-4V. The operating mechanisms that govern the relationship between the alloy composition (and Fe in particular) and the grain size are explored using EBSD, TEM, and in situ high-energy synchrotron X-ray diffraction. It was found that Fe additions up to 3 wt pct lead to a progressive refinement of the microstructure. By exploiting the cooling rates of AM and suitable amount of Fe additions, it was possible to obtain microstructures that can be optimized by heat treatment without obvious precipitation of detrimental brittle phases. The resulting microstructure consists of a desirable and well-studied fully laminar α + β structure in refined prior-β grains

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

Phase transformation kinetics during continuous heating of α\alpha+β\beta and metastable β titanium alloys

The progress of environmental as well as performance targets that must be guaranteed by the transportation sector is highly conditioned to the availability of materials that can offer structural weight savings and improve engine performance. Titanium alloys are key materials in this regard owing to their superior specific strength with respect to other structural alloys and excellent corrosion resistance up to ~ 500 °C. However, these alloys are still associated with high production costs, and therefore, advances in manufacturing optimization are necessary to further extent their implementation. Since the mechanical properties of titanium alloys are consequence of microstructural-based alloy design controlled by the phase transformation kinetics during heat treating, a correct understanding of these processes is required.This dissertation focuses in the continuous and univocal study of the phase transformation kinetics during linear heating of α+β and metastable β titanium alloys from room temperature to the β field. An initial bi-modal microstructure is used to analyse the evolution of stable phases, namely α and β for the first group of alloys. On the other hand, decomposition of the β-quenched condition leading to formation of metastable products such as αʺ, ω and βʹ+β is studied for the second group of alloys. The investigations are carried out combining laboratory characterization methods with advanced synchrotron-based techniques including in situ high energy X-ray diffraction and micro X-ray fluorescence. Variations in the phase transformation sequences are presented as a function of heating rate. Furthermore, the continuous evolution of the crystal structure of phases is analysed in terms of the physical mechanisms involved during phase transformation