Papers by Keyword: Microstructure Refinement

Abstract: To pertinently balance growth kinetics, solidification thermodynamics and dendrite expitaxy of multicomponent nickel-based single-crystal supralloy during laser processing, effect of thermometallurgy determinant factors, including laser power, welding speed and welding configuration, on solidification behavior, such as nonequilibrium solidification temperature range, and dendrite growth, such as dendrite trunk spacing, are progressively advanced to forestall solidification cracking phenomena. Symmetric developments of dendrite trunk spacing and solidification temperature range alongside solid/liquid interface are crystallographically driven by useful (001)/[100] welding configuration to auspiciously bring about crack-insusceptible and well-oriented dendrite growth. Dissimilarly, unsymmetrical developments of dendrite trunk spacing and solidification temperature range alongside solid/liquid interface are crystallographically driven by (001)/[110] welding configuration to insidiously favor crack-unresistant and disoriented dendrite growth. Higher heat input thermodynamically and kinetically boosts wide solidification temperature range, appalling stray grain growth with excess of solute ahead of dendrite tip and large size of crack-unresistant region to thermometallurgically disintegrate epitaxial growth for untoward solidification cracking, and therefore should be strictly withstood. Although geometry of symmetrical weld pool both sides is the same in infelicitous (001)/[110] welding configuration, [100] region of dendrite growth is more liable to ruinous stray grain growth and extensive solidification temperature range than [010] region of dendrite growth to complicate dendrite growth and exacerbate weld integrity. The determinant mechanism of crystallography-aided amelioration of solidification cracking resistance as result of kinetics-and thermodynamics-driven dendrite growth is propitiously proposed. Furthermore, the credible and understandable theoretical predictions are in conformity with the experiment results.

Abstract: The influence of various factors on the efficiency of microstructure refinement in two-phase titanium alloys with respect to a well-known Ti-6Al-4V alloy was discussed. The kinetics of microstructure evolution in titanium alloys with a lamellar type α/β microstructure during large plastic deformation depends mainly on temperature and strain rate, type of the initial microstructure, thickness of the α lamellae, path of deformation and chemical composition. Each parameter should be controlled to provide the most efficient microstructure refinement during conventional metalforming methods.

Abstract: The use of plasma MIG welding which consists of plasma flow in surrounding of the MIG wire causes the divergence of MIG current from MIG wire to outside. The divergence then disperses the heat and force to the upper side of MIG wire, thus reduces the droplet temperature and affects the droplet size as well as metal transfer frequency. In this study, the effects of plasma flow in plasma MIG welding to the refinement of microstructure at heat affected zone is investigated. The microstructure evaluation is carried out by means of electron backscattered diffraction (EBSD). As a result, we found that the microstructure refined at HAZ area especially at coarse grain HAZ. The difference in grain size at coarse grain HAZ area and fine grain HAZ area was almost eliminated. This result was compared with the one with conventional MIG welding, which shows the clear difference in grain size at coarse grain HAZ and fine grain HAZ area. It is suggested that, the presence of plasma flow in plasma MIG welding process refines the microstructure at HAZ area.

Abstract: The effect of various factors on the kinetics of microstructure evolution in commercial-purity titanium and two-phase Ti-6Al-4V alloy during deformation was studied. The kinetics of microstructure refinement can be raised via intensification of deformation twinning. In two-phase titanium alloys the kinetics of spheroidization can be increased considerably by decrease in the thickness of initial α lamellae. The influence of interphase boundaries energy on deformation behavior was discussed with respect to Ti-6Al-4V and Ti/TiB alloys.

Abstract: The effect of plastic deformation under various conditions of the equiatomic CoCrFeNiMn alloy with single face-centered cubic phase structure was studied. The alloy was rolled at room and cryogenic temperatures, and uniaxially compressed at room temperature and temperatures of 600-1100°C with different height reductions. In addition, multiaxial forging at 900-1000°C was performed. Scanning and transmission electron microscopy, including EBSD analysis, was widely employed to characterize microstructure of the deformed alloy. At room and cryogenic temperatures, mechanical twinning and shear banding plays play dominant role in microstructure evolution. Extensive refinement of the microstructure occurs as the result of rolling with reduction of 80%. During deformation at 600-1100°C, discontinuous dynamic recrystallization takes place. The recrystallized grains size and their volume fraction increases with increase of deformation temperature. Multiaxial forging at 900-1000°C was used to produce fully recrystallized structure with average grain size of 6.7 μm. The alloy in the initial condition had low yield strength of 160 Mpa but remarkable tensile ductility of 68%. Rolling substantial increases yield strength to 1120-1290 MPa, but results in loss of ductility. After multiaxial forging the alloy has balanced combination of properties – yield strength of 280 MPa and elongation of 56%.

Abstract: A process of finish rolling was researched to enhance lamellar tearing resistant of the high-rise building steel EH36. Experimental results indicate that gain size decreased with the decrease of finish temperature. When the finishing temperature on 840/810/780/750°C respectively, microstructure is finest with the size of 11.4μm. Moreover, the size of TiC precipitation turned ideal as the coiling temperature decreased. When coiling temperature is 550°C, the average size of TiC precipitation is only 0.09μm, refined by 40% relative to the conventional process. The microstructure and precipitation phase size of test steel are both effectively controlled because of the improved process. Therefore, high-rise building steel EH36 is finally prepared, and the lamellar tearing resistance reaches up to 78.2%.

Abstract: In this paper, the type and morphology of defects in heavy ingots were detected mainly on the basis of experimental results, which were obtained by metallographic analysis of the materials dissected from one 380 ton 30Cr2Ni4MoV ingot. Then the evolution of typical defects in the forging process was investigated at temperatures ranging from 900°C to 1200°C and reduction ratios ranging from 0% to 50%. It was shown that coarse grains and inclusions were the main types of defects. In the forging process, the refinement of coarse grains and crack initiation at inclusions were summarized respectively with respect to temperature, reduction ratio and multi-pass forging. The microstructure evolution of 30Cr2Ni4MoV steel is accompanied by the formation of fine grains on condition that multi-pass forging or an optimal reduction ratio of 10% is adopted at temperatures lower than 1000°C, while temperatures higher than 1100°C can lead up to coarse grains. Besides, cracks originate from CaO and FeS inclusions at 900°C and 1000°C related to intense shear stress and melting of the FeS-Fe eutectics distributing along grain boundaries.

Abstract: The microstructure of sodium or potassium added Mg-23.5 mass%Ni eutectic alloy was observed and the effect of sodium and potassium addition on the eutectic structure was investigated. All alloys showed the eutectic structure which composed of Mg phase and Mg₂Ni phase, and no evidence of the sodium or potassium precipitation occurred. For the sodium added and furnace cooled alloy, the lamellar spacing in the eutectic structure became a little narrow and the Mg₂Ni phase tends to become fragmentary as the amount of sodium increases. The sodium addition has a little effect for the refinement of the eutectic structure. For the potassium added and furnace cooled alloy, the lamellar spacing of the eutectic structure became clearly narrow even by the 0.1 mass% potassium additions. The morphology drastically varied in more than 0.5 mass% potassium added specimens, that is, the refinement and fragmentation of the Mg₂Ni phase occurred. The potassium addition has a large effect for the refinement and the fragmentation of the Mg₂Ni phase in the eutectic structure. For the water quenched specimens, the eutectic structure was extremely fine and globular shape with and without the additive element. The refinement effect by the water quenching is remarkably high even as the non-added specimen. The effect of sodium and potassium addition on the refinement of eutectic structure is not clear in the case of the rapid cooling speed during solidification.

Abstract: By taking combustion synthesis in enhanced high gravity field with increasing acceleration from 500 g to 2500 g at intervals of 500 g, a series of TiC-66.7 mol%TiB₂ composites were prepared through liquid forming and rapid solidification. The ceramics were composed of TiB₂ primary phases, TiC secondary phases and a few of Al₂O₃ inclusion and Cr metallic phases. Increasing high gravity acceleration not only promoted the combustion mode transfer from steady SHS mode to explosive one through enhancing the deposition of liquid product toward unreacted blend, but also accelerated liquid-liquid separation of TiC-TiB₂ / Al₂O₃ droplets and subsequent formation of layered melt through enhancing Stokes flow in mixed liquid products, thereby reducing sharply Al₂O₃ inclusions and shrinkage cavities in solidified TiC-TiB₂ composite. Meanwhile, the enhanced Stokes flow brought about constitutional uniformity through enhancing atomic diffusion in TiC-TiB₂ liquid, not only refining the microstructure of the solidified ceramic, but also improving the homogeneity of the ceramic by inducing TiB₂ primary phases to grow at similar velocity. As a result, hardness, flexural strength and fracture toughness of the composite increased greatly in enhanced high-gravity field.

Abstract: The aim of this study is to investigate efficiency of stainless steel powder inoculation into pure aluminum for microstructure refinement. Refiners consisting of pure aluminum powder (powder size: 106~180m) and stainless steel powder (powder size: 25~53m) have been fabricated through spark plasma sintering (SPS). The stainless steels used in the study include SUS304L, SUS316L and SUS434L. SUS 304L powder has achieved a great grain refinement in cast aluminum, for which fading phenomenon has been considerably avoided. SUS316L and SUS434L powders develop fine dendrite structures, which can lead to high hardness of cast aluminum.