Showing papers on "Equiaxed crystals published in 2020"

Grain structure control during metal 3D printing by high-intensity ultrasound.

Abstract: Additive manufacturing (AM) of metals, also known as metal 3D printing, typically leads to the formation of columnar grain structures along the build direction in most as-built metals and alloys. These long columnar grains can cause property anisotropy, which is usually detrimental to component qualification or targeted applications. Here, without changing alloy chemistry, we demonstrate an AM solidification-control solution to printing metallic alloys with an equiaxed grain structure and improved mechanical properties. Using the titanium alloy Ti-6Al-4V as a model alloy, we employ high-intensity ultrasound to achieve full transition from columnar grains to fine (~100 µm) equiaxed grains in AM Ti-6Al-4V samples by laser powder deposition. This results in a 12% improvement in both the yield stress and tensile strength compared with the conventional AM columnar Ti-6Al-4V. We further demonstrate the generality of our technique by achieving similar grain structure control results in the nickel-based superalloy Inconel 625, and expect that this method may be applicable to other metallic materials that exhibit columnar grain structures during AM. 3D printing of metals produces elongated columnar grains which are usually detrimental to component performance. Here, the authors combine ultrasound and 3D printing to promote equiaxed and refined microstructures in a titanium alloy and a nickel-based superalloy resulting in improved mechanical properties.

The role of texturing and microstructure evolution on the tensile behavior of heat-treated Inconel 625 produced via laser powder bed fusion

Abstract: Inconel 625 (IN625) alloy has high-temperature strength coupled with high oxidation and corrosion resistance. Additionally, due to its excellent weldability, IN625 can be processed by laser powder bed fusion (LPBF) additive manufacturing (AM) process allowing the production of complex shapes. However, post-AM heat treatment is necessary to develop the desired microstructure and mechanical properties to meet industrial needs. This work is focused on the influence of different heat treatment processes, namely stress relieving, recrystallization annealing and solution annealing on the microstructure and tensile properties of LPBF IN625 alloy. Investigation of the crystallographic texture by electron backscattered diffraction indicated that heat treatments at 1080 °C and 1150 °C tend to eliminate anisotropy in the material by the recrystallization and grain growth resulting in the formation of equiaxed grains. Tensile properties of heat-treated LPBF IN625 alloy built along different orientations revealed higher tensile properties than the minimum recommended values of wrought IN625 alloy in the annealed and solution annealed states.

Strength-ductility synergy of selective laser melted Al-Mg-Sc-Zr alloy with a heterogeneous grain structure

Abstract: In this work, a Sc/Zr modified Al-Mg alloy was processed by both selective laser melting (SLM) and directed energy deposition (DED). Due to different precipitation behavior of primary Al3(Sc,Zr)-L12 nucleation sites, a heterogeneous grain structure was formed in SLMed sample, which consisted of ultrafine equiaxed grains bands and columnar grains domains, while a fully equiaxed grain structure was obtained in DEDed sample. Tensile results showed that the as built SLMed sample had a good combination of strength and ductility. The yield strength of SLMed sample (335 ± 4 MPa) was about 2.8 times that of DEDed sample (118 ± 3 MPa), however, the ductility in uniform elongation (23.6 ± 1.9%) was still comparable to that of DEDed sample (23.8 ± 2.6%). Based on the relationship between the heterogeneous grain structure and strain hardening behavior, the strength-ductility synergy mechanism of the SLMed Al-Mg-Sc-Zr alloy was discussed. Stress partitioning tests showed that the contribution of back stress hardening to flow stress was higher in SLMed sample than DEDed sample, while effective stress hardening showed an opposite trend. Despite the overall strain hardening ability of SLMed sample was limited by the high dynamic recovery rate of ultrafine equiaxed grains, additional back stress hardening, which was caused by strain partitioning between equiaxed grains bands and columnar grains domains, improved its strain hardening ability and resulted in the good combination of strength and ductility.

Microstructure and mechanical properties of wire and arc additive manufactured AZ31 magnesium alloy using cold metal transfer process

Abstract: Wire and arc additive manufacturing (WAAM), using cold metal transfer (CMT) as heat source, exhibits a great potential for additive manufacturing of magnesium alloys due to low heat input. With the purpose of revealing the relationship between the microstructure and mechanical properties of WAAMed AZ31 material, the present study has been carried out. The manufactured AZ31 thin-walled deposit is mainly composed of columnar dendrite arrays, including dendritic α-Mg matrix, interdendritic eutectics (α-Mg and β-Mg17Al12) and some dispersive η-Al8Mn5 phases. The average primary dendrite arm spacing increases from 17 μm at the bottom to 39 μm at the top of the deposit, and the volume fraction of the interdendritic eutectic decreases from 52.1% to 39.3%. The microstructure of each layer except the top layer consists of vertical columnar dendrites and direction-changed columnar dendrites in sequence. The top layer appears equiaxed dendrites due to columnar to equiaxed transition (CET). The tensile properties present obvious anisotropic characteristics because of the epitaxial columnar dendritic growth along the building direction. The tensile properties also show obvious variation from the bottom to the top of the deposit because of the differing microstructures in different regions. The results are further analyzed in detail through the microstructure evolution resulted from the new manufacturing method.

Revealing the Mechanisms of Grain Nucleation and Formation During Additive Manufacturing

Abstract: The Interdependence model is now widely used to analyze the results of grain refinement studies. Although the model was developed to predict the grain size of an alloy cast under the assumptions of near equilibrium solidification and the presence of potent nucleant particles, it has been found to be applicable to a wide variety of alloys, casting methods, and cooling conditions. However, the strength of the Interdependence model is when it is used as a diagnostic tool that can reveal the mechanisms influencing the refinement of alloys under particular solidification conditions. This paper presents an introduction to the Interdependence model, its recent validation by experiment, and examples of how it can be applied to the solidification of alloys during additive manufacturing. For example, the model explains the difficulties in promoting a transition from columnar to equiaxed grains during additive manufacturing while also providing insights into how a fully equiaxed grain structure can be achieved.

Pure copper components fabricated by cold spray (CS) and selective laser melting (SLM) technology

Abstract: To study effect of the different metal additive manufacturing techniques on microstructural evolution, phase constitution and the relationship between the microstructure and tensile behavior of pure Cu parts, components were manufactured by selective laser melting (SLM) technology and cold spraying (CS) technology, respectively. The microstructure of Cu parts was detected using an optical microscope (OM) and scanning electron microscopy (SEM). The XRD spectrum revealed that only Cu phase is formed in the SLM Cu and CS Cu samples. The microstructure of the SLM Cu samples is constituted by the polycrystalline grains with substructures including the columnar dendrites and the equiaxed structures. As for the CS Cu samples, only the equiaxed grains were detected. In terms of the main physical properties, the averaged electrical conductivity of SLM Cu sample is 41% IACS, while that of CS Cu sample is 73% IACS. For the microhardness, the mean microhardness value of the CS Cu and the SLM Cu samples is 144.2 ± 4.3 HV0.05 and 83.6 ± 5.2 HV0.05, respectively. With regard to the statistic mechanical properties, the yield strength (YS) and ultimate tensile strength (UTS) of the SLM Cu part is approximately 185.8 ± 6.1 MPa and 242.2 ± 8.2 MPa, respectively.

Effect of heat treatment on the anisotropic microstructural and mechanical properties of Co–Cr–Mo alloys produced by selective laser melting

Abstract: The purpose of this study was to evaluate the effect of heat treatment on the anisotropy of the microstructure and mechanical properties of cobalt–chromium–molybdenum (Co–Cr–Mo) alloys fabricated by selective laser melting (SLM). Dumbbell samples were fabricated with the axes deviating from the build direction by 0° (0°-sample), 45° (45°-sample), or 90° (90°-sample) and were subjected to heat treatment at various temperatures (750, 900, 1050, or 1150 °C) for 6 h. In samples heat-treated at 750, 900, and 1050 °C, the microstructures exhibited columnar grains with a fiber texture along the build direction, the same as in the as-built state. The mechanical properties showed anisotropy; the 0.2% offset yield strengths (YS) of the 0°-samples were lower than those of the 90°-samples, and the elongations of the 0°-samples were significantly higher than those of the 45°- and 90°-samples. By contrast, in samples heated to 1150 °C for 6 h, the anisotropic columnar grains completely disappeared, and equiaxed grains with random orientations were found in all samples, indicating that recrystallization had occurred. Moreover, the specific microstructures and texture generated during SLM disappeared. Regarding tensile properties, the initially strong anisotropy exhibited by the as-SLM samples was significantly reduced. Thus, heat treatment at the recrystallization temperature produced uniform equiaxed grains with random texture, which contributed to reducing the mechanical anisotropy of the SLMed Co–Cr–Mo alloys.

Effects of laser additive manufacturing on microstructure and crystallographic texture of austenitic and martensitic stainless steels

Abstract: Powder-fed laser additive manufacturing (LAM) based on directed energy deposition (DED) technology is used to produce S316-L austenitic, and S410-L martensitic stainless steel structures by 3D-printing through a layer-upon-layer fashion. The microstructural features and crystallographic textural components are studied via electron backscattering diffraction (EBSD) analysis, hardness indentation and tensile testing. The results are compared with commercial rolled sheets of austenitic and martensitic stainless steels. A well-developed 200 > direction solidification texture (with a J-index of ∼11.5) is observed for the austenitic structure produced by the LAM process, compared to a J-index of ∼2.0 for the commercial austenitic rolled sheet. Such a texture in the LAM process is caused by equiaxed grain formation in the middle of each layer followed by columnar growth during layer-upon-layer deposition. A quite strong preferred orientation (J-index of 17.5) is noticed for martensitic steel developed by LAM. Large laths of martensite exhibit a dominant textural component of { 011 } 111 > in the α-phase, which is mainly controlled by transformation during layer-by-layer deposition. On the other hand, the martensitic commercial sheet consists of equiaxed grains without any preferred orientation or completely random orientations. In the case of the austenitic steel, mechanical properties such as tensile strength, hardness and ductility were severely deteriorated during the LAM deposition. A ductility loss of about 50% is recorded compared to the commercially rolled sheets that is attributed to the cast/solidified structure. However, LAM manufacturing of martensitic stainless steel structures leads to a considerably enhanced mechanical strength (more than double) at the expense of reduced ductility, because of martensitic phase transformations under higher cooling rates.

Microstructure and mechanical properties of 2219 aluminum alloy fabricated by double-electrode gas metal arc additive manufacturing

Abstract: The present study focuses on the fabrication of 2219 aluminum alloy components using double-electrode gas metal arc (DE-GMA) based additive manufacturing (AM) capable of reducing deposition heat input while maintaining high deposition rates, and how the microstructure and mechanical properties of as-built parts are affected by the bypass current ratio which is a key factor that determines the heat input to deposited layers in DE-GMA based AM. The bypass current ratios with 0%, 50 %, 70 %, and 90 % were employed to deposit thin-walled parts. As the bypass current ratio increases, the equiaxed dendrite size in the top region decreases, cellular grains in the strip area disappear gradually, and equiaxed grains in the inner-layer area are slowly transformed into columnar grains. The bypass current exhibits little effect on microhardness and a significant influence on tensile properties. By changing the bypass current ratio from 0% to 70 %, the ultimate tensile strengths and the elongations in the vertical as well as the horizontal directions increase by 9.71 Mpa, 18.89 Mpa, 4.17 %, and 5.74 %, respectively. No orientation dependence on the ultimate tensile strength can be observed. Extensive dimples at the fractures show typical ductile fracture characteristics.

Microstructural investigation and mechanical behavior of a two-material component fabricated through selective laser melting of AlSi10Mg on an Al-Cu-Ni-Fe-Mg cast alloy substrate

Abstract: A hybrid-part made of two materials was fabricated by selective laser melting (SLM) of AlSi10Mg on an Al-Cu-Ni-Fe-Mg cast alloy substrate. The microstructure of the two-material component and the interface is investigated using multi-scale characterization techniques including optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The microstructure of SLM-AlSi10Mg consists of fine cellular dendrites and columnar grains, developed along the building direction, where the substrate cast alloy is featured by large equiaxed grains. OM and SEM studies of the interface show a sound metallurgical bonding as a result of the melting of AlSi10Mg powder and partial melting of the cast substrate assisted by the circulate flows and Marangoni convection. The circulate flows cause complex phenomena at the interface, which lead to the dilution of alloying elements and a variation in the microstructure of the first consolidated layer of SLM-AlSi10Mg (as a result of variation in thermal gradient and solidification rate). TEM investigations of the interface reveal segregation of alloying elements at the interdendritic regions after solidification. Moreover, no precipitate is formed on top of the interface, due to the rapid solidification and dilution of the alloying elements. EBSD analysis of the interface shows substantial differences in the grain structure of SLM-AlSi10Mg and the cast substrate, in terms of size and morphology. Mechanical properties of the hybrid material are studied afterwards using Vickers microhardness measurements, nanoindentation and quasi-static uniaxial tensile tests. The SLM-AlSi10Mg side of the hybrid-part possesses better performance, mainly due to its finer and hierarchical microstructure.

Spheroidization and dynamic recrystallization mechanisms of Ti-55511 alloy with bimodal microstructures during hot compression in α+β region

Abstract: Spheroidization and dynamic recrystallization (DRX) mechanisms of a Ti-55511 alloy during hot compression in α+β region is investigated. It is found that the spheroidization of lamellar α phases and DRX of β phases, which are obviously influenced by deformation conditions, are the main softening mechanisms. The spheroidization fraction of lamellar α phases first increases with the raised temperature or strain rate. However, the spheroidization fraction of lamellar α phases begins to drop at about 700 °C as the strain rate is higher than 0.01 s−1, and the maximum spheroidization fraction of lamellar α phase is about 2.3%. This phenomenon is attributed to the Implanting-Mechanism, which is induced by the common or close orientation relationships of the spheroidized and equiaxed α phases, as well as few dislocations between the spheroidized and equiaxed α phases. Contrarily, the lamellar α phases are obstructed from equiaxed α phases by a mass of dislocations. Additionally, the DRX degree of β phases increases with the reduced temperature or the raised strain rate.