Showing papers on "Equiaxed crystals published in 2009"

Prediction of Macrosegregation in Steel Ingots: Influence of the Motion and the Morphology of Equiaxed Grains

Abstract: Although a significant amount of work has already been devoted to the prediction of macrosegregation in steel ingots, most models considered the solid phase as fixed. As a result, it was not possible to correctly predict the macrosegregation in the center of the product. It is generally suspected that the motion of the equiaxed grains is responsible for this macrosegregation. A multiphase and multiscale model that describes the evolution of the morphology of the equiaxed crystals and their motion is presented. The model was used to simulate the solidification of a 3.3-ton steel ingot. Computations that take into account the motion of dendritic and globular grains and computations with a fixed solid phase were performed, and the solidification and macrosegregation formation due to the grain motion and flow of interdendritic liquid were analyzed. The predicted macrosegregation patterns are compared to the experimental results. Most important, it is demonstrated that it is essential to consider the grain morphology, in order to properly model the influence of grain motion on macrosegregation. Further, due to increased computing power, the presented computations could be performed using finer computational grids than was possible in previous studies; this made possible the prediction of mesosegregations, notably A segregates.

Effects of process variables and size-scale on solidification microstructure in beam-based fabrication of bulky 3D structures

Abstract: A number of laser and electron beam-based fabrication processes are under consideration for aerospace components, where the ability to obtain a consistent and desirable microstructure and resulting mechanical properties is of critical concern. To this end, this work employs a combination of analytical and numerical modeling approaches to investigate the effects of process variables and size-scale on solidification microstructure (grain size and morphology) in beam-based fabrication of bulky 3D structures. Thermal process maps are developed for predicting solidification microstructure in any material system, and results are plotted on solidification maps to investigate trends in grain size and morphology in Ti–6Al–4V. The results of this work suggest that changes in process variables (beam power and velocity) can result in a grading of the microstructure throughout the depth of the deposit, with a transition from columnar to mixed or equiaxed microstructure at higher powers.

Microstructure and mechanical properties of friction stir welded AA6063–B4C metal matrix composites

Abstract: The weldability, microstructure evolution and mechanical properties of an AA6063 aluminium alloy and of two composites with AA6063 matrix reinforced with 6 and 10.5 vol.% B4C during friction stir welding are investigated. A joint efficiency higher than 60% was obtained and increased to over 80% after artificial ageing. The B4C particles size and shape were not affected by the welding process and the particle distribution in the matrix was kept uniform in the weld zone. Evolution of the aluminium grain structure from coarse grains in the base material to the refined, equiaxed grains in the weld centre is described. The microhardness profiles of various B4C concentration materials were measured in both as-weld conditions and after post-weld heat treatments. Other microstructure changes of Al–B4C MMCs after welding and heat treatment are also reported.

Microstructure evolution associated with adiabatic shear bands and shear band failure in ballistic plug formation in Ti―6Al―4V targets

Abstract: The microstructures and microstructure evolution associated with adiabatic shear band (ASB) formation in ballistic plugging in thick (2.5 cm) Ti–6Al–4V targets impacted by cylindrical, 4340 steel projectiles (2.0 cm in height) at impact velocities ranging from 633 m/s to 1027 m/s (just above the ballistic limit) were investigated by optical and transmission electron microscopy. ASB width increased from 10 μm to 21 μm as the velocity increased. ASB evolution was accompanied by the evolution of dark deformation bands composed of α′ martensite platelets which increased in density with increasing impact velocity. The corresponding Vickers microindentation hardness also increased from HV 619 to HV 632 in contrast to the surrounding matrix microindentation hardness of HV 555. These deformation bands were not necessarily precursors to ASB formation. The ASB average Vickers microindentation hardness was essentially constant at HV 645, a 16% increase over the matrix. This constant microindentation hardness was characterized by a consistent DRX grain structure which varied from equiaxed, defect-free grains (∼2 μm diameter) to heavily dislocated, equiaxed grains. Cracks nucleating and propagating within the ABSs were observed to increase from 8% to 87% of the ASB length with increasing impact velocity.

Microstructure and densification of ZrB2–SiC composites prepared by spark plasma sintering

Abstract: ZrB2–SiC composites were prepared by spark plasma sintering (SPS) at temperatures of 1800–2100 °C for 180–300 s under a pressure of 20 MPa and at higher temperatures of above 2100 °C without a holding time under 10 MPa. Densification, microstructure and mechanical properties of ZrB2–SiC composites were investigated. Fully dense ZrB2–SiC composites containing 20–60 mass% SiC with a relative density of more than 99% were obtained at 2000 and 2100 °C for 180 s. Below 2120 °C, microstructures consisted of equiaxed ZrB2 grains with a size of 2–5 μm and α-SiC grains with a size of 2–4 μm. Morphological change from equiaxed to elongated α-SiC grains was observed at higher temperatures. Vickers hardness of ZrB2–SiC composites increased with increasing sintering temperature and SiC content up to 60 mass%, and ZrB2–SiC composite containing 60 mass% SiC sintered at 2100 °C for 180 s had the highest value of 26.8 GPa. The highest fracture toughness was observed for ZrB2–SiC composites containing 50 mass% SiC independent of sintering temperatures.

Development of methods for the quantification of microstructural features in α + β-processed α/β titanium alloys

Abstract: A set of stereological procedures has been developed for the rigorous quantification of microstructural features resolvable using scanning electron microscopy in α + β-processed α/β titanium alloys. This paper identifies the four microstructural features that most likely influence the mechanical properties in α + β-processed titanium alloy, including: the size of the equiaxed alpha, the volume fraction of the equiaxed alpha, the volume fraction of total alpha, and the thickness of the Widmanstatten alpha laths. The details regarding the quantification methodologies are provided, as are the origins of the associated uncertainties.

Observations on the ductile-to-brittle transition in ultrafine-grained tungsten of commercial purity

Abstract: Rods of commercial-purity tungsten were subjected to severe plastic deformation by means of equal-channel angular pressing (ECAP) An ultrafine-grained (UFG) equiaxed microstructure with an average grain size of about 09 μm was developed Microhardness measurements in a wide range of temperatures suggest a ductile-to-brittle transition at temperatures below 350 °C in the UFG samples after ECAP In contrast, the results on the as-received material suggest a ductile-to-brittle transition temperature above 483 °C

Characteristics of hot compression behavior of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy with an equiaxed microstructure

Abstract: The hot compression behavior of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy with an equiaxed microstructure was examined in the temperature range of 900–1060 °C and strain rate range of 0.001–10 s −1 . The dynamic recrystallization (DRX), flow instability and texture of compressed Ti alloy are characterized by the processing maps and EBSD. The stress–strain curves of Ti alloys show that the continuous flow softening occurs, while the broad oscillations induced by adiabatic shear bands exist above strain rates of 1.0 s −1 . The processing maps exhibit two DRX domains: α phase DRX domain in the range of 900–980 °C and 0.001–0.1 s −1 , and the β phase DRX domain in the range of 1000–1060 °C and 0.01–1.0 s −1 . It is also found that the instable deformation region increases with strain increasing. In addition, EBSD results exhibit that DRX can cause the low angle boundaries (LABs) to decrease and the high angle boundaries (HABs) to increase in the α + β region.

Microstructures and mechanical properties of AZ91D alloys with Y addition

Abstract: As-cast AZ91D + x Y ( x = 0, 0.5, 1, 2 mass%) magnesium alloys were prepared by a simple addition of Y-rich interalloy to AZ91D alloy. Influences of Y on the microstructures and tensile properties of AZ91D alloy were investigated. Influences of extrusion temperature on the microstructure and tensile properties of AZ91D + 2Y alloy were investigated. Moreover, microstructures and the resulting tensile of extruded AZ91D + 2Y alloy were also investigated under different heat-treatment conditions. The results show that the addition of Y to AZ91D alloy refines primary α-Mg matrix and β-Mg 17 Al 12 phase. The addition of 2 mass%Y forms rod-shaped Al 2 Y phase. After hot extrusion, the grain sizes of AZ91D and AZ91D + 2Y alloys are greatly refined through dynamic recrystallisation. At extrusion temperature below 300 °C, the microstructure of AZ91D + 2Y alloy is characterized by fine and equiaxed grains, which leads to a high tensile strength but relatively low elongation. With increasing extrusion temperature up to 400 °C, more equiaxed grains with larger size are obtained. A low strength and large elongation are achieved compared with those extruded at low temperature. The best combination of both high strength and large elongation takes place at 325 °C. As for extruded AZ91D + 2Y alloy, solution treatment at 413 °C (T4) causes parts of broken β-Mg 17 Al 12 phase to be dissolved into α-Mg matrix and the deterioration of tensile properties. During aging at 216 °C (T5), discontinuous precipitates appear at grain boundaries. Solid solution followed by aging (T6) treatment increases ultimate tensile strength but decreases yield strength and elongation.

Microstructure and mechanical properties of TA15 titanium alloy under multi-step local loading forming

Abstract: In this paper six different local loading processes were proposed to study the effects of local loading conditions (temperature, deformation degree, loading pass, heats, cooling modes and heat treatment) on the microstructure and mechanical properties of TA15 titanium alloy workpieces including room and high temperature tensile properties, impact property, fracture toughness and high temperature duration property. It is found that it is better to finish the local loading forming in one heating time, if multi-fire forging needed the optimal forging technique as follows: adopting conventional forging (950 °C) at first and then following near-beta forging to control the proportion of the equiaxed primary α phase and the transformed β phase, allocating deformation degree of each loading pass rationally and using WQ cooling mode. Thus the workpiece with good compositive mechanical properties can be obtained.

Enhanced Mechanical Properties of a Novel High-Nitrogen Cr-Mn-Ni-Si Austenitic Stainless Steel via TWIP/TRIP Effects

Abstract: Temperature- and strain-rate-dependent mechanical properties of a high-nitrogen austenitic stainless steel containing smaller amounts of nickel than conventional austenitic nickel-chromium stainless steels were investigated with special attention to the formation of martensite or mechanical twins during plastic deformation (TWIP/TRIP effect). After recrystallization treatment at 1050 °C for 0.5 hour, an equiaxed fully austenitic microstructure possessing annealing twins was observed. Tensile tests were carried out at strain rates ranging from 10−5 to 10−2 s−1 in the temperature range from −196 °C to 400 °C. Deformation-induced austenite-to-martensite transformation occurred at temperatures below 0 °C. From room temperature up to 200 °C, plastic deformation is controlled by dislocation glide and mechanical twinning. At temperatures above 200 °C, no deformation-induced structural changes were observed. The formations of bcc α′-martensite and hcp e-martensite, or twins during plastic deformation, were analyzed by optical microscopy, transmission electron microscopy (TEM), and X-ray diffraction.

Recrystallization of Cold Spray-Fabricated CP Titanium Structures

Abstract: Cold gas dynamic spray (cold spray) is a promising rapid deposition technology in which particles deposit at supersonic velocities. The effect of isothermal annealing on recrystallization and mechanical properties of commercial purity (CP) titanium structures that were directly fabricated through cold spray deposition is studied. The optimized cold spray parameters led to a dense cold spray structure. Results show that annealing improves ductility of the cold-sprayed CP titanium structure. The mechanism for softening is the nucleation and growth of equiaxed grains, which include an ultrafine grain structure. A physical-based model for recrystallization of the cold spray CP titanium is proposed. The results show that recrystallization does not eliminate preferred orientation inherited from the cold spray material.

Effects of copper addition on microstructure and strength of the hybrid laser-TIG welded joints between magnesium alloy and mild steel

Abstract: Lap joint of magnesium alloy AZ31B to mild steel Q235 with the addition of copper interlayer by hybrid laser-TIG welding technique was investigated. The microstructure, element distribution at interfaces, and intermediate phases of joints were examined by scanning electron microscopy (SEM), electron probe micro-analyzer (EPMA), and X-ray diffraction (XRD), respectively. The results showed that intermetallic compounds Mg2Cu with rod-like structure in the joint and equiaxed structure at interface were found, and the bonding between copper and steel was realized by mixing of copper and steel at upper margins of molten pool and a little solid solution of copper in iron at the bottom and side of molten pool. Besides, comparing with that without any interlayer, the wettability of molten magnesium alloy on steel was enhanced, which led to an intimate connection. In the end, the joining mechanism of magnesium–steel joints with copper interlayer was discussed.

A Comparison of Columnar-to-Equiaxed Transition Prediction Methods Using Simulation of the Growing Columnar Front

Abstract: In this article, the columnar-to-equiaxed transition (CET) in directionally solidified castings is investigated. Three CET prediction methods from the literature that use a simulation of the growing columnar front are compared to the experimental results, for a range of Al-Si alloys: Al-3 wt pct Si, Al-7 wt pct Si, and Al-11 wt pct Si. The three CET prediction methods are the constrained-to-unconstrained criterion, the critical cooling rate criterion, and the equiaxed index criterion. These methods are termed indirect methods, because no information is required for modeling the equiaxed nucleation and growth; only the columnar solidification is modeled. A two-dimensional (2-D) front-tracking model of columnar growth is used to compare each criterion applied to each alloy. The constrained-to-unconstrained criterion and a peak equiaxed index criterion agree well with each other and some agreement is found with the experimental findings. For the critical cooling rate criterion, a minimum value for the cooling rate (between 0.07 and 0.11 K/s) is found to occur close to the CET position. However, this range of values differs from those cited in the literature (0.15 to 0.16 K/s), leading to a considerable difference in the prediction of the CET positions. A reason for this discrepancy is suggested, based on the fundamental differences in the modeling approaches.

Friction stir processing of 316L stainless steel plate

Abstract: 316L stainless steel plates were friction stir processed using polycrystalline cubic boron nitride tools. Mechanical properties and microstructure evolution of the friction stir processing (FSP) zone were investigated. Tensile test results showed that the tensile strengths of the defect free FSP zones were equal to those of base metals. All the samples fractured at the base metal side. Microstructural observation results showed that the grains in the FSP zone were refined by the tool. The FSP zone was a roughly equiaxed grain structure and the grain size was in the range of 10–30 μm, which was smaller than that in the base metal (30–80 μm). Moreover, transmission electron microscopy observation revealed that sigma phases formed in this zone during FSP.