Control Of Grain Structure Research Articles

Scaling-up ultrasonic vibration assisted additive manufacturing to build 316 L 3 m3 waste container flange

Directed-energy deposition is a 3D printing method that uses a focused energy source, such as a plasma arc, laser, or electron beam to melt a material that is simultaneously deposited by a nozzle. As with other additive manufacturing processes, this technology is used to add material to existing components, for repairs, or to build new parts. Direct-energy deposition additive manufacturing techniques have gained much attention from the industry to build/repair in-service components. However, this process undergoes complex dynamics of melting and solidification raising challenges to the effective control of grain structure causing potential structural failure. This research study was conducted to investigate the potential of using high-intensity ultrasonic to control the solidification process and scaling up the system to manufacture large components. From the feasibility study, it was noted that ultrasonic can assist in the refinement of the grain structure and also reduce anomalies such as porosities. Under the feasibility study, a range of frequencies and power configurations were considered to ease the scale-up of the system. Based on the studied ultrasonic configurations, the 40 kHz 60 W configuration was finalized to use in the scale-up. It was also noted the reduction of hot cracks in the ultrasonic-assisted additive manufacturing due to the constitutional supercooling during solidification by lowering the temperature gradient in the bulk of the melt pool. Furthermore, it was also noted that the grain orientation is perpendicular to the direction of vibration which potentially can be used to control the orientation of the grains as required. This new finding provides new applications to exploit the ultrasonic-assisted additive manufacturing process.

Controlling microstructural and mechanical properties of direct laser deposited Inconel 718 via laser power

The control of grain structure, texture and micro-segregation during additive manufacturing is important due to their impact on the mechanical properties. In this work, multiple single walls were deposited using Inconel 718 powder by the Direct Laser Deposition (DLD) process. The influence of a wide range of laser power (150–1900 W), as well as the thermal gradient, were characterized using electron microscopy and hardness testing. The heat input effect on the microstructural, dendritic morphology, and microhardness was also examined. The results illustrated that a higher thermal gradient due to lower heat input tends to form equiaxed grains and more Nb-segregation, whilst a higher heat input develops columnar grains with low Nb-segregation. The segregations within the interdendritic boundaries increased in size when the heat input was increased, whilst the contribution from precipitation strengthening was not significantly evident in the as-fabricated condition. A wall with two different powers was also deposited to demonstrate the possibility of creating functionally graded structures through the control of laser power.

Grain refinement of stainless steel in ultrasound-assisted additive manufacturing

Metals and alloys fabricated by fusion-based additive manufacturing (AM), or 3D printing, undergo complex dynamics of melting and solidification, presenting challenges to the effective control of grain structure. Herein, we report on the use of high-intensity ultrasound that controls the process of solidification during AM of 316L stainless steel. We find that the use of ultrasound favours the columnar-to-equiaxed transition, promoting the formation of fine equiaxed grains with random crystallographic texture. Moreover, the use of ultrasound increases the number density of grains from 305 mm-2 to 2748 mm-2 despite an associated decrease in cooling rate and temperature gradient in the melt pool during AM. Our assessment of the relationship between grain size and cooling rate indicates that the formation of crystallites during AM is enhanced by ultrasound. Furthermore, the use of ultrasound increases the amount of constitutional supercooling during solidification by lowering the temperature gradient in the bulk of the melt pool, thus creating an environment that favours nucleation, growth, and survival of grains. This new understanding provides opportunities to better exploit ultrasound to control grain structure in AM-fabricated metal products.

Microstructural evolution in manganese cobaltite films grown on Crofer 22 APU substrates by pulsed laser deposition

Abstract The use of pulsed laser deposition to produce high-quality manganese cobaltite spinel coatings on Crofer 22 APU substrates has been investigated, and the microstructures of the coatings have been studied using a combination of X-ray diffraction and electron microscopy techniques. It is shown that deposition from ceramic Mn1.5Co1.5O4 targets at a chamber pressure of 2.8 × 10− 4 mTorr N2 with the substrate pre-heated to 400 °C resulted in smooth, dense polycrystalline coatings. These coatings contain a mixture of rock-salt (Mn,Co)O and FCC Co phases due to reduction of the target material in the plume during deposition. Post-deposition annealing at atmospheric pressure in laboratory air at 800 °C led to re-oxidation of the coatings to give spinel phases with a thin chromia layer at the interface with the substrate. An annealing time of 0.5 h resulted in a coating with a fine uniform mixture of tetragonal and cubic spinel phases. The presence of pores at the location of the prior Co grains and at the original coating surface was used to infer the mechanisms by which the re-oxidation occurs. An annealing time of 14 h led to phase segregation in the coatings with more Co-rich cubic spinel at the surface and at the coating/substrate interface, and more Mn-rich tetragonal spinel in the center of the coating. There is Cr in the cubic phase only, indicating that the phase segregation occurs due to outward diffusion of Cr from the substrate through the chromia layer. There are local variations in the porosity and surface morphology of the coating, which appear to be correlated with the orientation of the underlying substrate grains. These observations indicate that control of grain structure in the substrate may be necessary to promote microstructural stability in such coatings.

Investment casting of nozzle guide vanes from nickel-based superalloys: part II – grain structure prediction

AbstractThe control of grain structure, which develops during solidification processes in investment casting of nozzle guide vanes (NGVs), is a key issue for optimization of their mechanical properties. The main objective of this part of the work was to develop a simulation tool for predicting grain structure in the new generation NGVs made from MAR-M247 Ni-based superalloy. A cellular automata - finite element (CAFE) module is employed to predict the three-dimensional (3D) grain structure in the as-cast NGV. The grain structure in the critical sections of the experimentally cast NGV is carefully analyzed, the experimental results are compared with the modeling outcomes, and the model is calibrated via tuning parameters which govern grain nucleation and growth. The grain structures predicted by the calibrated model show a very good accordance with the real ones observed in the critical sections of the as-cast NGV. It is demonstrated that the calibrated CAFE model is a reliable tool for the foundry industry to predict grain structure of the as-cast NGVs with very high accuracy.

Crystallite Growth Kinetics of BaFe<sub>12</sub>O<sub>19</sub>/SrTiO<sub>3</sub> Based Composites Derived from Mechanical Alloying

Barium hexaferrite and strontium titanate are respectively well established permanent magnet and piezoelectric materials which are technologically and scientifically attractive due to their potential for various applications in the field of magnetic electronics functional materials. However, the material properties for both require a careful control of grain structure as well as microstructure design to meet specific applications. In this work, we report some results of materials characterization especially particles and crystallites in a BaFe12O19/SrTiO3 composite which were promoted during mechanical milling. The composite was synthesized using a planetary ball mill with a ball to powder ratio 10:1. Changing in the particle and crystallite-sizes at various milling time up to 60 hours are studied with the aid of particle-size analyzer and X-ray diffraction. It was found that the particle size of composite powders initially increased due to laminated layers formation of a composite and then decreased to an asymptotic value of ~8 μm as the milling time extended even to a relatively longer time. However, based on results of line broadening analysis the mean crystallite size of the particles was found in the nanometer scale. We thus believed that mechanical blending and milling of mixture components for the composite materials has promoted heterogeneous nucleation and only after successive sintering at 1100 °C the millled powder transformed into particles of nanograin. The crystallite growth kinetics at isothermal temperatures follow the relaxation equation with the activation energy value for BHF (QBHF) and STO (QSTO) are respectively 73.63 kJ/mol and 122.69 kJ/mol.

Control of Structures and Properties of Cold-Rolled Sheet Steels

Precise microstructural control is essential to obtain the required properties of sheet steels. Since excellent formability is required for mild steels, the control of grain structure and texture are of utmost importance. First, the roles of initial grain boundaries prior to cold rolling, shear bands, and the interaction between C and Mn during recovery are critically reviewed as the essential factors controlling recrystallization texture. A model for the grain size of hot bands in interstitial free (IF) steels is presented in conjunction with texture control. Moreover, the grain growth during annealing, which leads to the development of ND//⟨111⟩texture, is discussed from the viewpoint of the pinning effect due to fine precipitates. The texture memory effect, typically found in Mn-IF steels, is also discussed. Concerning high strength steel sheets, the history of the development is firstly introduced. The discussion is focused on transformation induced plasticity (TRIP) steel by placing an emphasis on the relationship between the mechanical properties and the plastic stability of retained austenite. Furthermore, the prediction model for structures in TRIP steels during the continuous annealing process is presented based on the competitive reaction between bainite transformation and cementite precipitation together with a prediction model for the stress-strain curve. Finally, the factors to be overcome for further strengthening of sheet steels are described together with the future direction of research.

局所加熱法による純銅の結晶粒組織制御

The present work deals with a preferential grain growth process in a localized region utilizing local heating method in order to fabricate some unique microstructures different from those fabricated in the homogeneous way of microstructure evolution. A Monte Carlo simulation of grain growth under a heterogeneous temperature gradient, i.e. spot heating, was performed. Steep temperature gradient brought about a preferential grain growth in the higher temperature region, showing that the local heating was effective for the control of grain structure of polycrystalline materials. Such type of preferential grain growth became less significant under the mild temperature gradient. Local heating of pure copper foil with 0.2mm in thickness utilizing laser beam was performed by changing the irradiation conditions. In the case of 200W for laser power and 18mm/s for sweep velocity, some grains were observed to have larger grain sizes than their surrounding grains, suggesting a possibility of preferential grain growth in the localized region.

Control of Grain Structure through Transition Elemental Additions in an Al-Li Base Alloy

Transition elements Zr, Sc and Ti were added to an Al-Li alloy equivalent to 8090 to study the influence on grain structure. Increasing the Zr content within the 8090 composition range (having 0.02% Ti) resulted in a change from coarse columnar to fine equiaxed as cast grain structure. When Sc was added to the 8090 alloy, containing the highest level of Zr (∼0.14%), further grain refinement was noticed for Sc contents in excess of ∼0.4%. Grain structure of high Zr + Sc containing alloys did not change if Ti was not added. Processing of ingots (4:1 reduction) and further heat treatments did not result in recrystallisation. Tensile properties (especially the % elongation) were superior in products originated from fine grain ingots.

Control of Grain Structure of Sputtering Lead-Zirconate-Titanate Thin Film Using Amorphous Lead-Titanate Buffer Layer

Crystalline lead-zirconate-titanate (PZT) films of perovskite structure with fine grains were successfully obtained on Ir substrates using an amorphous lead-titanate (a-PTO) buffer layer. The grain size of PZT ranged from 100 to 150 nm. The results of the temperature dependence of the X-ray diffraction patterns of PZT films prepared on a-PTO buffer layer revealed that PZT films with a single perovskite phase were obtained at 500°C. In addition, 250-nm-thick PZT films fabricated with a-PTO buffer layer and crystallized at 600°C showed excellent ferroelectric properties. The 2Pr values of Ir/PZT/Ir capacitors with a-PTO buffer layer satisfactorily saturated at the applied voltage of 3.5 V. No degradation in the polarization density was observed for switching cycles up to 1×109.

Control of Grain Structure of Laser-Deposited (Ba, Sr)TiO3 Films to Reduce Leakage Current

A new technique to realize a low leakage current of (Ba, Sr)TiO3 (BSTO) thin films by controlling the grain structure of BSTO has been proposed. This includes the deposition of amorphous BSTO at 500° C and annealing of the amorphous films at 650° C in O2 for crystallization. The leakage current of the film produced from the amorphous one was significantly lower than that of BSTO films deposited directly at the substrate temperature of 650° C. The crystalline structure of the film produced from the amorphous one was found to be circular. In contrast, as-grown crystalline film showed columnar grain structures. The reduction of the leakage current may be attributed to the differences in the grain structures.

Improved grid alloys for deep-cycling leadcalcium batteries

Leadcalcium standby batteries contain free acid, thick grids of low calcium content, and are discharged infrequently. Valve-regulated lead/acid batteries (VRLAs) are deeply discharged, contain immobilized electrolyte, and contain much thinner grids of leadcalciumtin alloys. Tin is added to enhance rechargeability from deep discharge and to enhance the mechanical properties of the alloys, particularly creep resistance. Until 1980, leadcalciumtin alloys suffered from wide variations in grain structure, caused mainly by poor control of the calcium content. The latter is due to calcium oxidation and the inclusion of suspended oxide into the grids. The introduction of aluminium into leadcalcium and leadcalciumtinaluminium alloys prevents calcium loss from the melt. Leadcalciumtinaluminium alloys can be produced to the desired specification without fear of either calcium loss or poor control of grain structure. Aluminium also serves both as a nucleant to significantly reduce the initial cast grain-structure and as a means to enhance the precipitation of calcium in leadcalciumtin alloys. Grain-structure control is now possible even in very high tin-content alloys. Leadcalciumtin alloys with aluminium are not susceptible to penetrating grain boundary corrosion and will become the favored alloys for deep-cycling VRLAs of the future.

Chromium addition and environmental embrittlement in Fe3Al

Iron aluminides based on Fe[sub 3]Al afford excellent oxidation properties at relatively low cost, making them candidates for use as structural material in corrosive environments. Recently, efforts have been devoted to understanding and improving their ductility through control of grain structure, alloy additions and material processing. Studies at this laboratory have shown that the ambient temperature ductility can be increased significantly by additions of up to 6% Cr. This increase in ductility was earlier attributed to increased cleavage strength, easier cross slip due to lower antiphase boundary (APB) energy, and solid softening. Very recent studies of FeAl and Fe[sub 3]Al in various tensile testing environments have indicated that both alloy systems are relatively more ductile at room temperature when tested in vacuum or dry oxygen. Ductilities of 12--18% were attained in both iron aluminide systems in an oxygen pressure of 6.7 [times] 10[sup 4] Pa, while only 2--4% ductility was achieved in normal laboratory air. It seems appropriate to reexamine the mechanism by which chromium produces improved ductility at room temperature in laboratory air and to correlate it with the environmental effects on mechanical properties. In the current investigation, the authors have evaluated room temperature tensile properties of the binary more » alloy (Fe-28Al, at.%) and ternary alloy containing chromium (Fe-28Al-4Cr) as a function of surface condition and heat treatment. The results indicate that, although chromium may affect cleavage strength and APB energies, its most significant effect on room temperature ductility is to modify the protective surface oxide, resulting in a minimization of environmental embrittlement. « less

ALUMINIUM-LITHIUM-COPPER-MAGNESIUM-ZIRCONIUM ALLOYS WITH HIGH STRENGTH AND HIGH TOUGHNESS - SOLVING THE PERCEIVED DICHOTOMY

Throughout the past decade extensive research and development has been carried out on aluminium-lithium base alloys because of the attractive combination of lower density and higher modulus that can be achieved in this system compared with conventional aluminium alloys. Much of this effort has been directed at understanding and overcoming their Achilles heel of low ductility and poor fracture toughness (particularly for crack planes perpendicular to the short transverse direction). This study reviews the metallurgical features which affect ductility and damage tolerance in the 8090 and 8091 alloys developed at Alcan and the Royal Aircraft Establishment. The paper shows that control of these metallurgical features can be used to markedly improve these properties. Three examples will be given. In the first case the production of material with a high damage tolerance will be discussed. It will be shown that by control of grain structure and ageing practice, 8090 sheet with a plane stress fracture toughness (Kc) of >130 MPa/[MATH] can be achieved at similar strength levels to 2024-T3 sheet. There is also a decrease in fatigue crack growth rate. This results in sheet with an overall damage tolerance capability comparable with that of 2024-T3. The other examples concern the processing of 8090 and 8091 plate to meet medium and high strength airframe requirements respectively. 8090 plate (<100 mm thick) can achieve the property requirements of 7010-T7651, with short transverse ductilities in excess of 3% and short transverse fracture toughness of 218 MPa/[MATH]. This is accomplished by controlled thermomechanical treatment. Similar treatments applied to the 8091 alloy result in a material competitive with 7150-T651 for specific apflications (e.g.upper wing skins, where short transverse performance is sacrificed in favour of in-plane properties).