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At least 289 records · Page 16

Design and Application of CSC-Roll for Heavy Reduction of the Bloom Continuous Casting Process

Center quality control is a key issue affecting large-section bloom production and product quality. The central convex roll could avoid the bloom side part and yield a high reduction amount, which benefit the implement of heavy reduction (HR) for bloom continuous casting process without upgrading the mechanical and hydraulic system of withdrawal and straightening machine. In this work, a curving surface convex roll (CSC-Roll), which could significantly improve bloom center quality, was designed by finite element simulation for a 360 mm × 450 mm #45 steel continuous casting bloom. The optimal roll surface shape of the CSC-Roll was determined based on an analysis of the distribution of the reduction amount in the bloom center and the strain of the bloom centerline and inner surface. The calculation results showed that the optimal size of the CSC-Roll was 200 mm in convex platform region length, 30 mm in convex platform region height, and 80 mm in gradient curvature region length. The bloom heat transfer and deformation behavior in the CSC-Roll reduction process were then analyzed. The results showed that in the CSC-Roll HR process, 61.71 pct of the reduction amount could be transmitted to the inner side of the porosity zone. Macrographs of the 360 mm × 450 mm #45 steel continuous casting bloom proved that the bloom center quality was significantly improved after CSC-Roll HR was applied.

36 MATERIALS SCIENCE↗

CoCrFeNi Multi-principal Element Alloy Prepared Via Self-propagating High-Temperature Synthesis Plus Investment Casting Method

A CoCrFeNi multi-principal element alloy was successfully prepared via the combination of self-propagating high-temperature synthesis and investment casting. The phase identification, the as-cast microstructure, and the solidification phenomena were investigated. Unlike the dendrite morphology obtained by arc melting, cellular growth in the as-cast alloy was observed as a consequence of the rapid solidification. The solidification process is discussed with regard to the rate of advance of the liquid–solid interface, the equilibrium melting range, and the interface temperature gradient.

36 MATERIALS SCIENCE↗

A Comprehensive Analysis of Macrosegregation Formation During Twin-Roll Casting

A two-phase Eulerian–Eulerian volume-averaged model is used to predict the formation of macrosegregation during the twin-roll casting of inoculated Al-4 wt pct Cu alloys. For low solid fractions, the equiaxed crystals are modeled according to the submerged object approach. However, above a given transition limit, they are assumed to behave like a viscoplastic material. This means that the solid phase behaves as a coherent structure that can influence the motion of the liquid. Such approach allows one to take into account the flow dynamics arising from the occurrence of both solidification and hot rolling simultaneously, which usually occurs in twin-roll casting. Therefore, it is possible to explain the origin of the macrosegregation patterns obtained in the simulations based on the relative motion between the phases. Compression-induced expulsion of segregated melt is observed as a result of the deformation of the solidifying shells. Such occurrence leads to a negative macrosegregation region in the outer part of the as-cast strip. Then, because the solute-enriched melt is squeezed out toward adjacent regions, two positively segregated bands can be found near the center of the domain. Furthermore, it is shown how solidification-induced feeding weakens the absolute value of the negative and positive segregation bands.

36 MATERIALS SCIENCE↗

Inhibiting Brittle Intermetallic Layer in Magnesium/Aluminum Bimetallic Castings via In Situ Formation of Mg{sub 2}Si Phase

A quasi-continuous Mg{sub 2}Si layer is formed in situ at the magnesium/aluminum interface of bimetallic castings, by selecting a Si-containing aluminum alloy and overcasting process control. This effectively inhibits the formation of brittle intermetallic compounds (Al{sub 3}Mg{sub 2}, Al{sub 12}Mg{sub 17}) during the casting process, and the mechanisms are discussed based on phase diagram calculations and experimental validation. It is, thus, recommended to select Si-containing Al alloys as substrate for Mg/Al bimetallic castings.

36 MATERIALS SCIENCE↗

Closure of Internal Porosity in Continuous Casting Bloom During Heavy Reduction Process

To investigate the closure behavior of internal porosity (also referred to as internal void) in continuous casting bloom during heavy reduction (HR) and thus provide theoretical guidance for minimizing this kind of internal defect more effectively with HR, a three-dimensional (3D) mechanical model was developed based on the predicted temperature field by a 2D heat transfer model. With this 3D mechanical model, closure behaviors of internal porosity in continuous casting bloom during HR at and after the strand solidification end under different process conditions were numerically studied. It was found that the void axis length decreased significantly along the bloom thickness direction and increased slightly along the casting and bloom width directions after HR, and the influence of the initial void size on the void closure was not obvious. With a decrease of temperature difference between the bloom surface and center, HR efficiency for minimizing internal void decreased, while the required reduction force significantly increased. Compared with blooms with a uniform temperature distribution of 1100 °C, the void closure index after HR implemented at the strand solidification end was increased by ~ 25 pct. Compared with a conventional flat roll, the application of a convex roll during HR could contribute to minimizing the internal porosity more effectively and significantly enhance the reduction capacity of the withdrawal and straightening units. The void closure index of η{sub s} and η{sub v} (where η{sub s} and η{sub v} were defined based on the variation of the void aspect ratio and the void volume, respectively) was closely related to the equivalent strain (ε{sub eq}) and the hydrostatic integration parameter (Q), respectively, and two mathematical equations were derived to quantitatively describe the relationship of η{sub s} − ε{sub eq} and η{sub v} − Q.

36 MATERIALS SCIENCE↗

Effect of Al on Grain Refinement and Mechanical Properties of Mg-3Nd Casting Alloy

The effect of Al on the grain refinement and mechanical properties of as-cast Mg-3Nd alloy was investigated systematically by a series of microstructural analysis, solidification analysis and tensile tests. The results show that Al has an obvious refining effect on the as-cast Mg-3Nd alloy. With increasing Al content, the grain size of the as-cast Mg-3Nd alloy decreases firstly, then increases slightly after the Al content reaching 3 wt.%, and the minimum grain size of the Mg-3Nd alloy is 48 ± 4.0 μm. The refining mechanism can be attributed to the formation of Al{sub 2}Nd particles, which play an important role in the heterogeneous nucleation. The strength and elongation of the Mg-3Nd alloy refined by Al also increase with increasing Al content and slightly decrease when the Al content is more than 3 wt.%, and the strengthening mechanism is attributed to the grain refinement as well as dispersed intermetallic particles. Furthermore, the microstructural thermal stability of the Mg-3Nd-3Al alloy is higher than that of the Mg-3Nd-0.5Zr alloy. Overall, the Mg-3Nd alloy with Al addition is a novel alloy with wide and potential application prospects.

36 MATERIALS SCIENCE↗

Role of Centrifugal Casting on Electrochemical Corrosion Behavior of A356-SiCp Composite in 3.5 wt.% NaCl

Role of graded microstructure in a centrifugally cast A356-SiC particle (SiCp) composite alloy disk on passivity and localized corrosion behavior in 3.5 wt.% NaCl environment has been investigated. The area fraction of SiCp has been found to decrease from the outer periphery to the core of the disk, while the apparent size and area fraction of the intermetallics and Mg-rich phase increase from the periphery to the core. As a consequence, the corrosion resistance of the as-cast disk varies with the distance from the periphery in the order 45 < 65 < 25 < 5 mm. The corrosion resistance of the centrifugally cast disk can be significantly improved through solution annealing at 540 °C for 2 h and solution annealing + aging at 175 °C for 2 h. However, between the two, the former has shown a better corrosion resistance and passivity than the latter. In the latter case, the aging treatment resulted in the precipitation of Mg{sub 2}Si phase which is prone to the corrosion attack.

36 MATERIALS SCIENCE↗

Microstructure and Properties of A356 Alloy Wheels Fabricated by Low-Pressure Die Casting with Local Squeeze

The low-pressure die-casting process (LPDC) is widely used in fabricating aluminum alloy automotive wheels. With good castability and mechanical properties, A356 alloys are important and common materials for LPDC wheels. However, in practical casting process, defects often appear, especially porosities in the wheel thick-walled parts (i.e., spoke) due to inadequate interdendritic feeding, which greatly decreases the mechanical properties. In this investigation, to deal with this problem, local squeeze (LS) was added in conventional LPDC to enhance the interdendritic feeding. The practical A356 alloy wheel castings were fabricated with different LS velocities and times. The quantitative characteristics of α-Al cells, Si particles, and porosities with and without the LS analyzed by optical microscope, scanning electron microscope, and 3D x-ray computed tomography were compared. Their effects on the tensile and impact properties were discussed. The results have shown that the LS can increase the cooling rate and feed shrinkage of spoke parts, hence resulting in finer grains and denser microstructure. In result, the properties of spoke parts were improved with the LS.

36 MATERIALS SCIENCE↗

Quality Assessment of Al Castings Produced in Sand Molds Using Image and CT Analyses

Nowadays, the economic production of aluminum casts is becoming increasingly important. Therefore, the decision of whether to use alloys based on the primary aluminum (low Fe content, but high costs) or the secondary aluminum (high Fe content, but reasonable costs) for a cast product is a common problem. The main expected influence of higher iron content is to increase porosity and decrease mechanical and fatigue properties. Therefore, this work is focused on assessment of the higher Fe content influence on porosity and iron-rich intermetallic phases formation and, hence, changes in the mechanical properties, with simultaneously keeping the production economical. The quality of castings was measured by application of the 2D technology (image analysis) and the 3D technology (CT analysis), as well. Results show that technologies of assessment are comparable and length of the Fe-rich phases is not critical for mechanical properties, but it is important for porosity formation in materials with the higher amount of Fe.

36 MATERIALS SCIENCE↗

High-Resolution Electron Microscopy and Kinetic Studies of Precipitation Hardening Reactions in Cast Al-5.8Zn-2.2Mg-2.5Cu

Precipitation age hardening (T6) response of Al-5.8Zn-2.2Mg-2.5Cu alloy in near-net-shaped cast condition was investigated. Cast samples were manufactured using the controlled diffusion solidification (CDS) technology combined with a metallic mold tilt pour gravity casting process. This study was conducted using (1) non-isothermal aging during differential scanning calorimetry (DSC) experiments and (2) isothermal aging during which transient bulk and micro-hardness measurements taken at four aging temperatures of 343, 393, 433 and 473 K. A detailed microstructural investigation combined with a quantitative image analysis was carried out on both DSC and hardness specimens using high-resolution electron microscopy (HREM) and energy-dispersive spectroscopy (EDS) elemental mappings. Hardness measurements were analyzed using a kinetic modeling approach (single state variable combined with a kinetic equation that is valid for the Al-Zn-Mg-Cu alloys). The results confirm the formation of hardening phases of GP zone, intermediate hardening metastable phase η′ and equilibrium phase η. The results of calorimetric measurements, electron microscopy and selected area diffraction patterns are in good agreement and confirm transient precipitation/dissolution sequences. A mean value of activation energy Q{sub A} of 75 kJ/mol was evaluated for combined bulk diffusion rate of alloying elements during precipitation of strengthening phases within Al matrix.

36 MATERIALS SCIENCE↗

Interaction Between Ceramic Molds and High-Speed Steel during Gravity and Vacuum Investment Casting

Ceramic molds containing the ZrSiO{sub 4}/SiO{sub 2} primary face coat with and without addition of cobalt aluminate were produced. After firing, the surface and the cross sections of the face coats were subjected to detailed characterization using XRD, SEM and EDS. The interaction between the mold systems and 0.9C-6W-5Mo-4Cr-2V high-speed steel was studied during the gravity and vacuum investment casting. During casting, the isostructural transformation of the initial ZrSiO{sub 4} filler occurred. For the mold without addition of the cobalt aluminate, two different types of interaction products [multicomponent phase containing O, Al, Zr, Fe, W and V and (Fe, W,Mo,Cr){sub 2}Al{sub 2}O{sub 6})] were revealed. An interaction layer of a depth in the range of 5-30 μm was thoroughly studied after casting into the mold containing the cobalt aluminate. Addition of the cobalt aluminate into the ceramic system provided significant refinement of the high-speed steel microstructure.

36 MATERIALS SCIENCE↗

Bilayer hybrid graphite anodes via freeze tape casting for extreme fast charging applications

Extreme fast charging (XFC) of lithium-ion batteries is critical for continuous market adoption of electric vehicles. However, mass transport limitations and sluggish kinetics lead to lithium plating in graphite anodes under fast charging conditions. One approach to address the mass transport limitation in graphite is to design low-tortuosity electrodes to enhance ionic transport. In this study, a bilayer hybrid structured electrode with directionally aligned channels was developed via freeze tape casting, which enables faster lithium ion diffusion through the graphite electrode. A scalable roll-to-roll process was designed in which slurry can be cast on any substrates for simple processing. Additionally, electrochemical impedance spectroscopy measurements indicated that the bilayer hybrid coating had both low tortuosity and short diffusion pathways, enabling XFC charging. Rate testing for the bilayer hybrid electrode indicated superior performance over the other coatings, exhibiting a ~20% improvement in the charge capacity compared with the conventional coating at 5C and 10 min total charging time. The bilayer hybrid electrode also demonstrated a 10% improvement in capacity retention compared with the conventional electrode after 1000 cycles at XFC conditions. This study demonstrates freeze tape casting as a scalable way to fabricate low-tortuosity electrodes for XFC applications.

25 ENERGY STORAGE↗

A Monte Carlo volumetric-ray-casting estimator for global fluence tallies on GPUs

A Monte Carlo fluence estimator has been designed to take advantage of the computational power of graphical processing units (GPUs). This new estimator, termed the volumetric-ray-casting estimator, is an extension of the expectation estimator. It can be used as a replacement of the track-length estimator for the estimation of global fluence. Calculations for this estimator are performed on the GPU while the Monte Carlo random walk is performed on the central processing unit (CPU). This method lowers the implementation cost for GPU acceleration of existing Monte Carlo particle transport codes as there is little modification of the particle history logic flow. Three test problems have been evaluated to assess the performance of the volumetric-ray-casting estimator for neutron transport on GPU hardware in comparison to the standard track-length estimator on CPU hardware. Evaluation of neutron transport through air in a criticality accident scenario showed that the volumetric-ray-casting estimator achieved 23 times the performance of the track-length estimator using a single core CPU paired with a GPU and 15 times the performance of the track-length estimator using an eight core CPU paired with a GPU. Finally, simulation of a pressurized water reactor fuel assembly showed that the performance improvement was 6 times within the fuel and 7 times within the control rods using an eight core CPU paired with a single GPU.

74 ATOMIC AND MOLECULAR PHYSICS↗

Enhancing fatigue life of aluminum alloy castings through cavitation water jet peening: Experiments and simulations

This study presents an investigation into the enhancement of the fatigue life of aluminum castings through the application of cavitation water-jet peening (CWJP). CWJP harnesses the impacts of water cavitation to induce surface compressive residual stress within metallic materials. In this work, CWJP was applied to a high pressure die-cast (HPDC) Al–Si alloy A380 with three different water-jet traverse velocities. The fatigue-life improvement, evaluated in a 4-point bending configuration (stress ratio R = 0.1), was found to vary with applied stress level and ranges from 1.6 to 10 times that of the parent alloy. The data also shows that decreasing the traverse velocity results in greater compressive residual stresses within the surface layer and a concurrent increase in surface roughness. This residual stress layer extends to a depth of 400 μm below the surface, as confirmed by through-thickness residual stress and microhardness measurements. CWJP treatment effectively slows down fatigue crack propagation, as evidenced by microstructural observations of narrower striation spacing. Simulations reveal that compressive residual stresses, in addition to surface hardening during CWJP, are key to improving fatigue life. A 20% increase in surface hardness and 150 MPa compressive residual stress imposed by CWJP process provides an average 5-fold enhancement of fatigue life across different stress levels. This study demonstrates the potential of CWJP as an effective surface treatment to enhance the fatigue life of aluminum castings, such as HPDC components for automotive applications.

Al casting↗

Degradation of impact toughness in cast stainless steels during long-term thermal aging

Cast austenitic stainless steels (CASSs) have been extensively used for the large components of nuclear reactor primary coolant systems. Since the cast steels inevitably contain degradable metastable phases and replacement of the large coolant system components is impractical, the thermal embrittlement of CASS components has been a serious concern in the extended-term operation of nuclear power plants. This study aimed to systematically measure and analyze the effect of long-term thermal aging on the Charpy impact toughness to provide a comprehensive understanding of thermal degradation behavior and a practical aging model to predict the degree of thermal degradation in the cast stainless steels. Additionally, the materials tested in the research include eight CASS alloys (two CF3s, one CF3M, three CF8s, and two CF8Ms) and two reference wrought materials (304L and 316L), in which the nominal δ-ferrite content ranges from ~2% to 33%. These stainless steels have been thermally aged at two light water reactor (LWR) temperatures (290 and 330 °C) and at two accelerated-aging temperatures (360 and 400 °C) for up to 30,000 h; these include both under-aged and over-aged conditions relative to the extended service lifetime (80 years). Charpy impact testing was performed for aged and non-aged specimens, and the impact (absorbed) energy parameters were correlated with a new aging parameter (A). Both the reduction of impact fracture toughness and the shift of ductile-brittle transition temperature were strongly dependent on the δ-ferrite content and degree of thermal aging. A linear relationship was found between the increasing rate of the index transition temperature T 41J and aging parameter A; base on which an empirical model was proposed for prediction of the transition temperature as a simple function of the aging parameter (A) and δ-ferrite content (F). Finally, the critical aging parameter for embrittlement (AC) was evaluated and compared with the existing δ-ferrite content criteria.

36 MATERIALS SCIENCE↗

Effects of W on microstructure of as-cast 28 wt.%Cr–2.6 wt.%C–(0–10)wt.%W irons

Microstructures of as-cast 28 wt.%Cr–2.6 wt.%C irons containing (0–10)wt.%W with the Cr/C ratio about 10 were studied and related to their hardness. The experimental irons were cast into dry sand molds. Microstructural investigation was performed by light microscopy, X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectrometry. It was found that the irons with 1 to 10 wt.%W addition was hypereutectic containing large primary M{sub 7}C{sub 3}, whereas the reference iron without W addition was hypoeutectic. The matrix in all irons was austenite, partly transformed to martensite during cooling. The volume fractions of primary M{sub 7}C{sub 3} and the total carbides increased, but that of eutectic carbides decreased with increasing the W content of the irons. W addition promoted the formation of W-rich M{sub 7}C{sub 3}, M{sub 6}C and M{sub 23}C{sub 6}. At about 4 wt.%W, two eutectic carbides including M{sub 7}C{sub 3} and M{sub 6}C were observed together with primary M{sub 7}C{sub 3}. At 10 wt.%W, multiple carbides including primary M{sub 7}C{sub 3}, fish-bone M{sub 23}C{sub 6}, and M{sub 6}C were observed. M{sub x}C where x = 3 or less has not been found due possibly to the high M/C ratio in the studied irons. W distribution to all carbides has been determined increasing from ca. 0.3 to 0.8 in mass fraction as the W content in the irons was increased. W addition led to an increase in Vickers macro-hardness of the irons up to 671 kgf/(mm){sup 2} (HV30/15) obtained from the iron with 10 wt.%W. The formation of primary M{sub 7}C{sub 3} and aggregates of M{sub 6}C and M{sub 23}C{sub 6} were the main reasons for hardness increase, indicating potentially improved wear performance of the as-cast irons with W addition. - Highlights: • W addition at 1 up to 10 wt.%W to Fe–28Cr–2.6C produced “hypereutectic” structure. • W addition promoted the formation of W-rich M{sub 7}C{sub 3}, M{sub 6}C and M{sub 23}C{sub 6}. • M{sub 23}C{sub 6} has higher Fe/Cr and W/Cr atm% ratios than those in M{sub 7}C{sub 3}. • Si content and high W content over about 11 atm% are characteristics of M{sub 6}C. • Primary M{sub 7}C{sub 3} and aggregates of M{sub 6}C and M{sub 23}C{sub 6} increased the alloy macro-hardness.

36 MATERIALS SCIENCE↗

Evolution of microstructure, texture and inhibitor along the processing route for grain-oriented electrical steels using strip casting

In the present work, a regular grade GO sheet was produced successively by strip casting, hot rolling, normalizing annealing, two-stage cold rolling with intermediate annealing, primary recrystallization annealing, secondary recrystallization annealing and purification. The aim of this paper was to characterize the evolution of microstructure, texture and inhibitor along the new processing route by comprehensive utilization of optical microscopy, X-ray diffraction and transmission electron microscopy. It was found that a fine microstructure with the ferrite grain size range of 7–12 μm could be obtained in the primary recrystallization annealed sheet though a very coarse microstructure was produced in the initial as-cast strip. The main finding was that the “texture memory” effect on Goss texture started on the through-thickness intermediate annealed strip after first cold rolling, which was not similar to the “texture memory” effect on Goss texture starting on the surface layers of the hot rolled strip in the conventional production route. As a result, the origin of Goss nuclei capable of secondary recrystallization lied in the grains already presented in Goss orientation in the intermediate annealed strip after first cold rolling. Another finding was that fine and dispersive inhibitors (mainly AlN) were easy to be produced in the primary recrystallization microstructure due to the initial rapid solidification during strip casting and the subsequent rapid cooling, and the very high temperature reheating usually used before hot rolling in the conventional production route could be avoided. - Highlights: • A regular grade grain-oriented electrical steel was produced. • Evolution of microstructure, texture and inhibitor was characterized. • Origin of Goss nuclei lied in the intermediate annealed strip. • A fine primary recrystallization microstructure could be produced. • Effective inhibitors were easy to be obtained in the new processing route.

36 MATERIALS SCIENCE↗

Precipitation behavior near shrinkage porosity in a large sand casting of Haynes 282 alloy

The precipitation behavior of L1 2 structure Ni 3 (Al,Ti)-type γ' precipitates near shrinkage porosity in a large sand casting of Haynes 282 alloy has been investigated by scanning/transmission electron microscopy in conjunction with energy dispersive x-ray spectroscopy. The experimental results revealed that in the carbonitride-rich region close to the shrinkage porosity, γ' precipitates could still form with comparable sizes though at lower volume density than those in the general cast Ni matrix. The resulting γ' precipitates had a very high Al/Ti ratio. Close to oxide-rich pore surfaces, an absence of γ' precipitates was observed due to Al and Ti depletion by forming Al oxides and Ti carbonitrides, respectively. This work suggests that Al rather than Ti plays a dominant role in the formation of γ' precipitates in porous regions in cast Haynes 282 alloys.

36 MATERIALS SCIENCE↗