Search

The present study investigates the structure-property relationship of gravity die-cast and rheocast in situ processed 1wt%TiB2/Al-3.5Mg-0.15Sc-0.075Zr composites. The reinforcement was synthesized via the reaction of fluoride salts (K2TiF6 and KBF4) in the melt at 750 °C for an hour. The cooling slope technique was used for rheocasting, wherein the melt (pouring temperature of 700 °C) was allowed to flow over the cooling slope at an angle of 60°. Thereafter, all samples (gravity die cast and rheocast) were aged at 300 °C for 5 h. Microstructural characterization, hardness measurements, tensile tests, and fractography were necessary supplements to this investigation. The results obtained from this investigation infer: (a) The presence of nano-TiB2 particles increases the ductility (15%) of the gravity die-cast specimens; (b) rheocasting increases the strength of the specimen due to transformation from dendritic to equiaxed dendritic morphology and uniform particle distribution (reduced agglomeration). (c) Rheocast and aged specimens show maximum strength and adequate ductility due to the combined effects of structure refinement, reduced agglomeration, and the formation of fine precipitates. [ABSTRACT FROM AUTHOR]

Permanent mold (PM) and semi-solid (rheo) casting in ductile iron castings (DIC) were studied, and the microstructure was aimed to be chill free in as-cast condition. Denitrification was conducted understanding solubility properties of nitrogen in base molten iron, and re-nitrification was strictly avoided during molten treatment by Mg alloy. As the results, chill-free knuckle castings were possible with both casting methods in as-cast condition. The knuckle castings had no shrinkage cavity also. The solidification time by rheocasting was shorter than that of PM casting. It was approximately one in two and a half. The castings had ultrafine spheroidal graphite which were mostly under 10 µm. The graphite structure by rheocasing method was finer and more uniform than those of PM casting. The tensile strength in castings with both casting methods was approximately 1.3 times keeping elongation over 10% comparing with conventional sand knuckle castings. It was concluded that free nitrogen (NF) promoted chill formation, and chill could be avoided by promoting denitrification and minimum re-nitrification. [ABSTRACT FROM AUTHOR]

Nowadays it is common to see the production of complex and critical automotive and aeronautical components reduced in weight for energy efficiency using light alloys with improved microstructural and mechanical properties. The casting processes involved in this trend are strong; in this study, an optimized design of a vertical serpentine channel and a novel design of a horizontal serpentine channel to produce semi-solid slurry (S2S) with thixotropic behavior by self-stirring for rheocasting of A380 and A356 alloys are tested. Simultaneously, chilling during solidification, flow development, and shearing on the alloys to improve the performance of solid fractions and self-stirring at high shear rate are applied. The effects of these conditions on the modification of the morphology transition of the α(Al) phase from dendrite to equiaxed grain are discussed. The results suggest the ability of the mentioned processes to promote the morphological transition of the primary solid due to the produced equiaxed grains of α(Al) phase having sizes between 25–50 µm from A380 alloy processed by vertical self-stirring. On the other hand, the treatment of the A356 alloy using the new horizontal serpentine channel produces equiaxed grains with an average size of 39 µm. Unexpected Si crystals, trapped in the α(Al) phase using both methods with both alloys, are detected. The applied operation parameters were aided by gravity-pouring close to the liquidus temperature, and the obtained microstructural results show the ability for S2S to form alongside thixotropic behavior and non-dendritic solidification by mean of self-stirring in the serpentine channels, suggesting the potential for further experiments under die-casting conditions. [ABSTRACT FROM AUTHOR]

The current study aims to assess the impact of rheocast's input variables (cooling slope techniques) on its mechanical properties. The rheocast experiment was carried out by varying the input variables of slope angle (30º, 45º, and 60º), slope length (400, 500, and 600 mm), and pouring temperature (580, 585, and 590 °C). The effect of input variables on responses (ultimate tensile strength and elongation) and the factor interaction graph of responses have been evaluated by analysis of variance (ANOVA) and response surface methodology. The experimental findings showed that optimum response values (ultimate tensile strength, 181 MPa; elongation, 6.19%) were obtained at the following series of input variables: 585 °C pouring temperature, 500 mm slope length, and 45º slope angles. Analysis of variance results infers that pouring temperature is the most significant input variable, which highly influences the response factors. A confirmation experiment was performed at a 95% confidence level to ensure that the obtained values of response factors fall within permissible limits. [ABSTRACT FROM AUTHOR]

Aluminium can be essential in reducing climate impacts as weight reduction is critical. Rheocasting is getting more and more attention from the electronics and automotive industries. The solid fraction in Rheocasting determines the processing outcome. The RheoMetal process is one of the leading processes with the most significant deviation from equilibrium, making presetting the slurry-making parameters difficult. A deeper analysis of the physics of the solid fraction deviation from equilibrium is made based on literature data using a simplistic mathematical model. The developed model confirms that the process is far from equilibrium and that the growth conditions of the freeze-on layer on the cooling agent used in the process determine the slurry temperature and cause the formation of excess solid fraction. [ABSTRACT FROM AUTHOR]

Nowadays it is common to see the production of complex and critical automotive and aeronautical components reduced in weight for energy efficiency using light alloys with improved microstructural and mechanical properties. The casting processes involved in this trend are strong; in this study, an optimized design of a vertical serpentine channel and a novel design of a horizontal serpentine channel to produce semi-solid slurry (S2S) with thixotropic behavior by self-stirring for rheocasting of A380 and A356 alloys are tested. Simultaneously, chilling during solidification, flow development, and shearing on the alloys to improve the performance of solid fractions and self-stirring at high shear rate are applied. The effects of these conditions on the modification of the morphology transition of the α(Al) phase from dendrite to equiaxed grain are discussed. The results suggest the ability of the mentioned processes to promote the morphological transition of the primary solid due to the produced equiaxed grains of α(Al) phase having sizes between 25–50 µm from A380 alloy processed by vertical self-stirring. On the other hand, the treatment of the A356 alloy using the new horizontal serpentine channel produces equiaxed grains with an average size of 39 µm. Unexpected Si crystals, trapped in the α(Al) phase using both methods with both alloys, are detected. The applied operation parameters were aided by gravity-pouring close to the liquidus temperature, and the obtained microstructural results show the ability for S2S to form alongside thixotropic behavior and non-dendritic solidification by mean of self-stirring in the serpentine channels, suggesting the potential for further experiments under die-casting conditions.

Aluminium can be essential in reducing climate impacts as weight reduction is critical. Rheocasting is getting more and more attention from the electronics and automotive industries. The solid fraction in Rheocasting determines the processing outcome. The RheoMetal process is one of the leading processes with the most significant deviation from equilibrium, making presetting the slurry-making parameters difficult. A deeper analysis of the physics of the solid fraction deviation from equilibrium is made based on literature data using a simplistic mathematical model. The developed model confirms that the process is far from equilibrium and that the growth conditions of the freeze-on layer on the cooling agent used in the process determine the slurry temperature and cause the formation of excess solid fraction.

Four-point bending fatigue tests were performed on semi-solid Al–7Si–Mg castings with varying magnesium contents and heat treatment conditions. Additionally, the effect of anodising on the fatigue resistance of semi-solid Al–7Si–Mg castings was evaluated. Fracture surface and microstructure analysis showed that fatigue crack initiation occurred mainly at the periphery of regions of positive macrosegregation at the casting surface, resulting most likely from exudation. The microstructure of these macrosegregation regions was mostly eutectic and was frequently found surrounded by a layer of oxides. This layer of oxides promoted weak bonding between the macrosegregation region and the surrounding material and acted as a crack initiation site. In this study, primary α-Al globule agglomerates at the casting surface and surrounded by a layer of oxides also promoted fatigue crack initiation. Fatigue resistance of semi-solid Al–7Si–Mg castings in the T5 and T6 conditions increased with the increase in the magnesium content of the alloy from 0.3 to 0.45 wt.% due to the higher precipitation hardening response. However, the increase in the magnesium content from 0.45 to 0.6 wt.% resulted in a slight decrease in the fatigue resistance. The oxide layer formed during anodising had no significant effect on the fatigue resistance of the semi-solid Al–7Si–Mg castings in this study due to the dominant effect of the macrosegregation regions on fatigue crack initiation. [ABSTRACT FROM AUTHOR]

This study explores the development and characterization of Aluminum alloy (AA) 5083 based nanocomposites, reinforced with equal proportion of graphene nanosheets (GNs) and multi-walled carbon nanotubes (MWCNTs) using stir-rheo-squeeze casting. To control the problem of segregation of the nano dispersoids, the study successfully employs a novel approach of making nano-preform billets of the reinforcements by green compacting and then introducing their sections to the alloy melt having compositions 0.5, 1, 1.5, 2, and 2.5% by weight, while stirring. MWCNTs and GNs in 1:1 ratio mixed and milled with ten times the high purity Aluminum (Al) powder volume. HRTEM images and XRD patterns conformed to the size and purity of MWCNT and GNs. SEM images confirmed their sound mixture. The structure was refined by about 60% in 3 wt.% composite, but agglomeration and cracks appeared at this composition. BHN increased consistently as the reinforcement was increased. Tensile and compressive strength increased initially but declined in the composite with 3 wt.% nanofillers due to agglomerations and weak interface bonding. Elongation first decreased and then increased due to counter slip systems introduced by the presence of MWCNTs and GNs. Prominent crack growth was observed in the fractography in 3 wt.% composite. An optimum reinforcement concentration of 1.5–2 wt.% has emerged superior on a combined evaluation of dispersion, refinement of structure, and mechanical properties.

Rheocasting with the aim of developing and offering solutions to problems related to the casting process. Rheocasting is a casting process that utilizes solidification agitation to change the dendritic structure to non-dendrite (globular) by giving a variation of the stirring speed. Al-Cu is a metal material that has high strength-to-weight ratio characteristics from steel, good fatigue resistance, and heat-treatable. But fluidity is low, this triggers casting defects such as shrinkage and miss-run. The defect occurs because the cooling process of liquid metal is not uniform or there is gas trapped in molten metal. Defects in casting can significantly reduce material mechanical properties. The results show morphological changes in the dendrites to rosette to globular with increased stirring speed and grain diameter decreasing with increasing stirring speed. Increasing the stirring speed also produces a positive trend of the fluidity value of Al-Cu alloys in each mold thickness.

Investment casting has long been known as a process that can produce complex parts with fine details. However, it has not been used widely for parts that need to be anodized and whose nonuniform color is affected by the type of cast aluminum. Although 6063 aluminum is widely used in color anodizing for decoration purposes, it is almost entirely wrought. Some studies have used aluminum 6063 in cast aluminum, but no investment casting studies have been reported. The objective of this work is to develop a new semi-solid investment casting process using 6063 aluminum alloys to achieve a cosmetic anodized workpiece. To study the feasibility of the gas-induced superheated slurry (GISS) investment casting process, the effects of rheocasting time, mold temperature, and vacuum assistance were investigated. The results showed that the samples produced using the GISS process had a uniform microstructure and less porosity. The uniform color after anodizing and coloring also confirmed these processes. From the results, it can be summarized that the GISS process is feasible for application in aluminum 6063 investment casting for anodizing and coloring purposes.

In the modern era, the need for new products with novel processing and multipurpose materials is increased. The current market requirements for engineering applications are lightweight, high strength and low-cost materials. This paper explores the novel development process of high-performance nano cermet material for aerospace applications. Herein, lightweight aluminium 6061 + 2% of SiC (40 nm) nano cermet was fabricated through the casting method. The effects of ultrasonication, double stir casting or rheocasting, and squeezing pressure on nano cermet fabrication were successfully investigated by comparing their physical, thermal and mechanical properties. Scanning electron microscopy was employed to analyse the morphology of the cermets, and the presence of reinforcements was verified through EDS. The reinforcement of SiC into Al 6061 improved density, hardness, and reduction in porosity and grain refinement. This study reveals a novel fabrication process of ultrasonic-aided rheo-squeeze casting technique which enhanced the mechanical properties of the cermets compared to Al 6061 alloy due to nanoparticles homogeneous distribution, nominal agglomeration and porosity.

Until recently, the strengthening efficiency of carbonaceous reinforcements in aluminum-based matrix nanocomposites (AlMNCs) was lower than expected. The intrinsic characteristics of matrix and carbonaceous reinforcements and fabrication technique, which affected the microstructural characterization, played main roles in determining the performance of Cs/AlMNCs. In this study a novel strategy was developed via flake powder metallurgy followed by slurry blending combined with rheocasting and hot-extrusion processing to prepare hybrid AA7075-matrix composite reinforced with CNTs + GNPs. No Al4C3 phase was detected in Raman spectrums, while uniform dispersion of carbonaceous hybrid reinforcements with minimal structural damage was observed. There were ~ 9% and 39% decrease in the average grain size of the as-casted and extruded CNTs + GNPs/AA7075 composites, respectively, compared to AA7075 alloy. In the extruded CNTs + GNPs/AA7075 composite, plate precipitates were predominant and some lath-like dynamic precipitates were observed at the GNPs/AA7075 interfaces. The micro-hardness, tensile yield strength (TYS), ultimate tensile strength (UTS) and uniform elongation (UE) of the CNTs + GNPs/AA7075 composite were improved by ~ 17%, 51%, 7% and 20% compared to AA7075 alloy. An ultra-high yield strengthening efficiency of carbonaceous hybrid reinforcements in AA7075 alloy up to 3950 was obtained, where load bearing was the main strengthening mechanism and carbonaceous reinforcements illustrated bridging and pulling-out in the fracture surfaces of composite. [ABSTRACT FROM AUTHOR]

Four-point bending fatigue tests were performed on semi-solid Al–7Si–Mg castings with varying magnesium contents and heat treatment conditions. Additionally, the effect of anodising on the fatigue resistance of semi-solid Al–7Si–Mg castings was evaluated. Fracture surface and microstructure analysis showed that fatigue crack initiation occurred mainly at the periphery of regions of positive macrosegregation at the casting surface, resulting most likely from exudation. The microstructure of these macrosegregation regions was mostly eutectic and was frequently found surrounded by a layer of oxides. This layer of oxides promoted weak bonding between the macrosegregation region and the surrounding material and acted as a crack initiation site. In this study, primary α-Al globule agglomerates at the casting surface and surrounded by a layer of oxides also promoted fatigue crack initiation. Fatigue resistance of semi-solid Al–7Si–Mg castings in the T5 and T6 conditions increased with the increase in the magnesium content of the alloy from 0.3 to 0.45 wt.% due to the higher precipitation hardening response. However, the increase in the magnesium content from 0.45 to 0.6 wt.% resulted in a slight decrease in the fatigue resistance. The oxide layer formed during anodising had no significant effect on the fatigue resistance of the semi-solid Al–7Si–Mg castings in this study due to the dominant effect of the macrosegregation regions on fatigue crack initiation.

The purpose of the present study is to determine the cause of failures of semisolid rheocast cylinder head covers of hypereutectic modified LM30 alloys. The covers, manufactured by rheocasting, a process that consists of the incorporation and homogeneous distribution of the reinforcements in semisolid metal (SSM) before die casting, were shot-peened after heat treatment to increase the fatigue resistance. However, these engine components failed during operation after a short service time. The major flaws were located by radiography. The microstructure was controlled by metallography. Significant interactions between shot peening and surface/subsurface contaminants were identified by surface morphology and fractography. The fatigue properties of hypereutectic alloys were investigated by a high-frequency vibrophore. It was identified that the failure was related to the cleanliness of SSM as well as the rheocasting process. Recommendations were given to prevent the recurrence of similar failures. [ABSTRACT FROM AUTHOR]

In the present paper, the microstructure evolution of semi-solid metal slurry was investigated. The material was tested by rheocasting route. In order to better estimate the materials behavior, the investigation was performed on three series of samples. The rheocasting was conducted different stirring speed of 500, 1000 and 1500rpm and 0.3 solid fraction, before casting. The microstructure was analysed using an optical and scanning electron microscope. The results show that the relationship between the equivalent diameter of Si and α - Al particles with stirring speed can be well fitted to quadratic or linear equation. The results from this study can also be used to optimize the process and study mechanism of the rheocasting process. [ABSTRACT FROM AUTHOR]

Semisolid cast AA2024 matrix composites reinforced with 2.0 wt% titanium boride (TiB2) and varying amounts of yttria (Y2O3) (0.5 wt%, 1.0 wt%, 1.5 wt%) were developed using stir rheocasting. The metallurgical characterisation was done by X-ray diffraction, optical microscopy, and scanning electron microscope, and mechanical characterisation by Vickers's hardness testing machine and universal testing machine. The microstructure evolution and mechanical properties such as hardness, tensile strength, and fracture toughness (KQ) of the developed composites were correlated. Pure AA2024 and AA2024 reinforced with TiB2 samples were also fabricated as a benchmark to compare the effect of yttria. The highest hardness, ultimate tensile strength, and yield strength achieved were 122 HV, 310 MPa, and 230 MPa, respectively, for the AA2024/2 wt% TiB2 + 1.5 wt% Y2O3 composite. They are 25%, 45%, and 57% higher compared to the unreinforced sample. The maximum conditional fracture toughness (KQ) achieved was 25.56 MPa m1/2 for the unreinforced specimen and 19.77 MPa m1/2 for AA2024/2 wt% TiB2 + 1.5 wt% Y2O3 composite. With the addition of Y2O3 particles in the matrix, the porosity in the samples increased, resulting in lower elongation. The processing route enhanced the mechanical properties of the composite in comparison with the liquid cast matrix due to a uniform distribution of particles and grain refinement. [ABSTRACT FROM AUTHOR]

This paper aims to evaluate the morphological evolution of a magnesium-aluminum alloy (Mg-6Al-3La-1Ca) processed by a mechanical stirring process (MSP) with different stirring times. An apparatus for semisolid metal alloy processing was used to melt and obtain rheocast material. Transformation temperatures were determined at a slow cooling rate using computer-aided cooling curve thermal analysis (CA-CCTA) and differential scanning calorimetry (DSC). Isothermal mechanical stirring (605 °C), at 950 rpm, was carried out with a solid fraction of 0.29 and stirring times of 0, 1, 2, 4, 8 and 10 min, followed by fast cooling in water. The results showed that the microstructure after conventional casting (0 min—no stirring) is composed of an α-Mg dendritic matrix, Al11La3, (Al,Mg)2Ca and Mg2Ca compounds. With the 1-min stirring time, it was not possible to obtain globular structures. For stirring times longer than 2 min, a primary α-Mg globular morphology was obtained. Microstructures with 2 min of stirring time showed better globularization than those obtained under the other conditions. Morphological changes were observed for Al11La3 and (Al,Mg)2Ca after the MSP followed by fast cooling. [ABSTRACT FROM AUTHOR]

The paper analyses specific defects of castings produced by semi-solid casting process, especially rheocasting method SEED, which uses mechanical swirling for reaching proper structure in semisolid state with high content of solid fraction. Heat treated alloy AlSi7Mg0.3 was applied for producing an Engine Bracket casting part. For observing structure, metallographic observation by light and SEM microscopy was used. To analyse the process, software ProCAST was used to simulate the movements in shot chamber and filling of the mold.

In this study, TiB2/7075 aluminum matrix composites were prepared via in situ synthesis. It was found that TiB2 particles are mainly quadrate. Large TiB2 particles (1–2 μm) agglomerate at grain boundaries, but most of the particles are on the submicron scale. Adding 4.5 wt.% TiB2 particles effectively optimizes α-Al grains in the 7075 aluminum alloy. By combining in situ reinforcing particles with the self-stirring effect of a serpentuator, rheocasting of the 7075 aluminum alloy was achieved in a simple and economical way. The average grain size of the specimen after rheocasting and heat treatment was smaller than 33 μm, and the shape factors were greater than 0.85. The wear resistance of the 4.5 wt.% TiB2/7075 aluminum matrix composite that was prepared via rheocasting and gravity casting was tested with loads of 30, 60, 90, and 120 N at a friction speed of 0.15 m/s for a duration of 30 min. Because of the optimized microstructure and increased hardness, the wear resistance of the 4.5 wt.% TiB2/7075 aluminum matrix composite was significantly better than that of the 7075 aluminum alloy, and the wear resistance of the rheocast TiB2/7075 aluminum matrix composite was better than that of the gravity cast one.

Magnesium and silicon concentrations in the interior of primary α-Al of Al-7Si-Mg alloys were studied at temperatures in the liquid-solid range and just after solidification was completed. Analysis of the results showed that the magnesium concentration in the interior of primary α-Al is very low in the temperatures range from the liquidus to the start of the Al-Si eutectic reaction. Formation of silicon-rich phases during eutectic reactions, such as eutectic silicon and β-Al5FeSi, phases trigger a significant increase in the magnesium concentration in the interior of primary α-Al, when sufficient time is allowed for solid-state diffusion to occur. When fast cooling rates are applied during the Al-Si eutectic reaction, most of the magnesium is retained in π-Al8FeMg3Si6 and Mg2Si phases formed during solidification. Semi-solid Al-7Si-Mg castings were produced with varying magnesium contents, and the mechanical properties were evaluated in the as-cast, T5 and T6 conditions. It was found that the 0.2% offset yield strength of the semi-solid Al-7Si-Mg castings in the T5 and T6 conditions increases linearly with the square root of the magnesium concentration in the interior of the α-Al globules formed during the slurry preparation process.

Al matrix composites reinforced with Cu-coated pitch-based carbon fibres (Al/Cu-CFs) were fabricated, using a novel combination of rheocasting and equal channel angular extrusion (ECAE) techniques, in order to exploit the thermal conductivity ( K ) of the material. Rheocasting allowed the introduction and dispersion of Cu-CFs within the Al 3 Mg matrix. The subsequent ECAE processing reduced the porosity of the composites from 3 to 0.03% and induced a high degree of fibre alignment within the matrix, although considerable damage to the fibres occurred during this processing step. After 6 ECAE passes, in which the billet orientation remained constant, the composite with the highest degree of fibre alignment show a thermal conductivity ( K ) improvement of ~20% with respect to the rheocast composite. The improvement is due to porosity reduction, improved fibre alignment and forced intimate contact of clean CF surfaces with the matrix. [ABSTRACT FROM AUTHOR]

This work analyzes the thermodynamic behavior of two candidate aluminum alloys (A355 and B319) for SSM processing. Both alloys undergo a semisolid tertiary phase transformation below the main eutectic solidification temperature. The use of the differentiation method (DM) to identify critical points associated with the thixoformability of these alloys is evaluated. The method is compared with the conventional tangent method (TM) for determining the solidus and liquidus temperatures and with the visual recognition method for identifying the unstable “knee” of the liquid fraction- versus -temperature curve. For both alloys, numerical simulation is performed with Thermo-Calc® (Scheil condition), and experimental DSC data is generated for cooling (rheocasting) and heating (thixoforming) cycles under different kinetic conditions (5, 10, 15, 20 and 25 °C/min) to compare the DM and the conventional approaches. The findings indicate that the DM is an efficient tool for identifying critical points such as the solidus, liquidus , knee and beginning and end of the SSM-processing working window for A355 and B319 alloys and that it not only yields results that agree well with those obtained using traditional techniques, but also, unlike the latter, provides accurate results even in the presence of tertiary phase transformations when the kinetic effects are strong. A correction to the Flynn method of integration of partial areas using two complementary baselines is introduced for rheocasting and thixoforming operations. [ABSTRACT FROM AUTHOR]

Dry sliding wear behavior of rheocast AZ91 magnesium alloy and AZ91/SiCp composites reinforced with 5 and 10 vol% SiC particles were investigated under normal loads of 10–250 N and sliding speeds of 0.1, 0.3, 0.5 and 1 m/s using pin-on-disc configuration against a 1045 steel disc counterbody. In this work, rheocast alloys and composites have been tested to determine the role played by the globular microstructure, and to evaluate if the increase observed in other mechanical properties is also translated to wear behavior. Wear rates and friction coefficients were registered during wear tests. Worn tracks and wear debris were studied by Scanning Electron Microscope (SEM) and Energy Dispersive X-Ray Spectrometry (EDS) in order to obtain the predominant wear mechanisms maps of the studied materials. The following wear mechanisms were found in the worn surface of the three materials: abrasion, oxidation delamination and melt wear. The composites with globular microstructure exhibit slightly superior wear resistance at low testing sliding speeds (0.1 and 0.3 m/s) and medium loads (40–80 N) than the AZ91 Mg alloy. But, for other conditions, the presence of SiC particles seems to be detrimental to the wear behavior of AZ91 magnesium alloy. A wear mechanisms map that allows identifying the main wear mechanisms for each wear condition and material composition has been developed. Rheocast microstructure improves the wear resistance of alloys in most conditions but the addition of SiCp reinforcement is only favorable in few of them. [ABSTRACT FROM AUTHOR]

The effects of ultrasonic vibration (UV) treatment on microstructure of semi-solid aluminum alloys and the application of UV in rheocasting process are reviewed. Good semi-solid slurry can be produced by high-intensity UV process for aluminum alloys. The microstructures of Al-Si, Al-Mg and Al-Cu alloys produced by rheocasting assisted with UV are compact and with fine grains. The mechanical properties of the UV treated alloys are increased by about 20%-30%. Grain refinement of the alloys is generally considered because of cavitation and acoustic streaming caused by UV. Apart from these mechanisms, a hypothesis of the fuse of dendrite root caused by capillary infiltration in the ultrasonic field, as well as a mechanism of crystallites falling off from the mould-wall and crystal multiplication by mechanical vibration effect in indirect ultrasonic vibration are proposed to explain the microstructure evolution of the alloys.

Application of a coating on a mold surface is widely used in the foundry industry. Changes in coating change the heat transfer at the mold−melt interface, which influences the microstructure of the casting. In this study, the effect of boron nitride coating thickness on the interfacial heat transfer and slug microstructure in the Swirled Enthalpy Equilibration Device (SEED) process was investigated. The temperatures of the semi-solid slug and mold were measured, and the interfacial heat transfer coefficient and heat flux of the mold−slug interface was estimated based on these data. Microstructures of the quenched slugs were also examined. The results indicated that the interfacial heat transfer coefficient decreased with an increase in coating thickness and was sensitive to a coating thickness of less than 0.1 mm. The interfacial heat flux decreased sharply at the early stage, and then slowed down as the swirling time increased and the coating thickened. The coating thickness affected the temperature evolution of the slug at the early stage of the SEED process. As the coating thickness increased from near zero to 1.0 mm, the grain size of the slug increased by ~20 µm and the globular structure of the slug transformed into a dendritic structure.

In the present work, microscale deformation behavior, plastic strain localization, and plastic instability of rheocast Al–Si–Mg (A356) alloy have been investigated using micromechanical approach. For this purpose, two-dimensional microscale models (representative volume elements) have been developed using actual microstructure of the cast samples made under three different process conditions. Microstructure of the above-mentioned alloy consists of two different phases, such as aluminum-rich primary phase and silicon-rich eutectic phase. In line with that, composite micromechanical models have been developed to analyze them within the finite element framework. Rheocasting has been performed using cooling slope with two different slope angles of 45° and 60°, and comparison has been made with the conventional cast samples of the alloy that has been cast directly from the superheated molten state. Different boundary conditions have been assumed to perform finite element based simulation, using a popular finite element solver ABAQUS, depending upon the position of representative volume elements on the cylindrical tensile specimen. Under uniaxial tensile loading, ductile failure mode is predicted in the form of plastic strain localization due to incompatible deformation between the phases. This indicates inhomogenity of microstructure that determines the damage initiation process within this material, as there is no damage or failure criterion specified during the finite element analysis. Grain size, shape, and orientation of the primary aluminum phase are found to play a vital role on deformation behavior and failure mode of the materials investigated in this study. [ABSTRACT FROM AUTHOR]

To obtain the semi-solid slurry with uniform and fine structure morphology that satisfies the requirement of rheocasting process, a new process for preparing semi-solid Al alloy slurry was developed, in which local chilling was combined with low superheat pouring and slight electromagnetic stirring (LSPSEMS). The morphology and the size of primary α-Al in the A356 alloy slurry prepared with the new process, i.e., LSPSEMS with local chilling, were investigated using MIAPS image analyzing software, and the grain refinement mechanism was discussed. The results indicate that the semi-solid primary phase consists of particle-like or globular-like α-Al, and the morphology and grain size of primary α-Al in the slurry can be markedly improved by the new process. The fine primary α-Al distributes uniformly in the slurry, which satisfies the requirement of rheocasting. Compared with the alloy prepared by LSPSEMS, the average equal-area-circle grain diameter of primary α-Al in semi-solid A356 alloy ingot prepared by the new process is decreased from 85.6 μm to 68.8 μm at the central area, 112.6 μm to 77.6 μm at the transition area and is 84.7 μm in the edge area, respectively. The corresponding shape factor of primary α-Al is increased from 0.78 to 0.83, 0.54 to 0.77 and 0.28 to 0.59, respectively. In addition, the pouring temperature could be suitably raised from 620-630 ℃ of the traditional process to 650 ℃ using this technique, which is convenient for practical operation. The mechanism of grain refinement, in the new process, is that the local chilling quickens up the temperature decrease in the center of the melt. The nuclei could not grow up in a short time so the finer grains are formed in the melt.

Semi-solid metal processing involves processing metallic alloys between thesolidity and the liquidity. The microstructure must be non-dendritic and consist ofspheroids of solid in a liquid matrix. In this work two potential routes are used:First, The superheated Al-Si-Mg alloy was continuously cooled down to varioustemperatures above its liquidity (620,630,640) o C . When the desired temperature ofthe melt has been reached, then mechanical stirring with rotational speed of(800)rpm for (30)sec. was started. The resultant slurry was bottom poured into astainless steel mold placed below the crucible. The solidified samples were reheatedto the semi-solid metal processing temperature, held there for (10) min, and rapidlyquenched in into water tank to retain the semi solid microstructures. In the second setof experiments , the superheated melt was continuously cooled down to varioustemperature in semi solid range (610,600,595,590,580)o C, then mechanically stirredat rotating speed(800)rpm for (30)sec. the resultant slurry was bottom poured into astainless steel mold placed below the crucible. The metallic mold is then rapidlyquenched in water to room temperature. Metallographic examinations indicate thatthe mixing method leads to highly globular semi-solid slurries of fine particle size.The particle sizes of slurry quenched at various temperatures within the semi solidmetal range are much smaller when compared to the reheated structures. The particlesize gradually increases as the slurry temperature decrease.

An investigation was made on the evolution of microstructures of hypoeutectic white cast iron slurry containing 2.5wt.%C and 1.8wt.%Si produced by rheocasting in which the solidifying alloy was vigorously agitated by electromagnetic stirrer during isothermal cooling processes. The results indicated that under the proper agitating temperatures and speeds applied, the dendrite structures in white cast iron slurry were gradually evolved into spherical structures during a certain agitating time. It also revealed that the bent dendrites were formed by either convection force or by the growth of the dendrites themselves in the bending direction; then, as they were in solidifying, they were gradually being alternated into separated particles and into more spherical structures at the end of the isothermal cooling process. Especially, the dendrites were granulated as the bending process proceeding, which suggested that they were caused by unwanted elements such as sulfur and phosphor usually contained in engineering cast iron. Convective flow of the melt caused corrosion on the dendritic segments where they were weaker in strength and lower in melting temperature because of higher concentration of sulfur or phosphor. And the granulation process for such dendrites formed in the melt became possible under the condition. Certainly, dendrite fragments are another factors considerable to function for spherical particles formation. A new mechanism, regarding to the rheocast structure formation of white cast iron, was suggested based on the structural evolution observed in the study.

Corrosion properties of two Al–Si alloys processed by Rheo-high pressure die cast (HPDC) method were examined using polarization and electrochemical impedance spectroscopy (EIS) techniques on as-cast and ground surfaces. The effects of the silicon content, transverse and longitudinal macrosegregation on the corrosion resistance of the alloys were determined. Microstructural studies revealed that samples from different positions contain different fractions of solid and liquid parts of the initial slurry. Electrochemical behavior of as-cast, ground surface, and bulk material was shown to be different due to the presence of a segregated skin layer and surface quality.

Thermal conductivity of a rheocast component made from Stenal Rheo1 (Al-6Si-2Cu-Zn) alloy was investigated in as-cast, T5 and T6 conditions. The thermal conductivity measurement in different locations showed variation of this property throughout the rheocast component. The results of microstructural investigation revealed that the ratio of solute-lean α1-Al particles formed during slurry preparation to fine solute-rich α2-Al particles formed during secondary solidification had significant influence on thermal conductivity. The reduced amount of solutes in the α1-Al particles was determined as the root cause of higher thermal conductivity. A linear relation between the fraction of precipitates and the increase in thermal conductivity was obtained and silicon in solid solution is shown to have a dominant influence. As silicon was precipitated during the heat treatment, thermal conductivity increased. For an optimal combination of thermal and mechanical properties, it is therefore important to use an ageing temperature above the temperature of Si precipitation. [ABSTRACT FROM PUBLISHER]

In this paper semisolid slurry of Al-6.3Zn-1.9Mg-1.8Cu-0.15Zr alloy with different levels of Sc was prepared, and the effect of Sc content on the morphology of microstructure and mechanical properties of the rheo-diecasting samples was investigated. The results indicate that the excellent semisolid slurry of the wrought Al alloy could be obtained with the addition of 0.22%Sc. Sc additions not only influenced the morphology of the primary α-Al particles in the slurry, but also promoted the formation of fine α-Al globules in the solidification process of the remaining liquid, which contributed to the considerable improvement in tensile strength and elongation. The tensile strength and elongation of the rheo-diecasting samples with 0.45%Sc after T6 heat treatment increased by 82% and 147% respectively compared to the die-casting samples of the base alloy. [ABSTRACT FROM AUTHOR]

The relation between microstructural inhomogeneity and thermal conductivity of a rheocast component manufactured from two different aluminum alloys was investigated. The formation of two different primary α-Al particles was observed and related to multistage solidification process during slurry preparation and die cavity filling process. The microstructural inhomogeneity of the component was quantified as the fraction of α-Al particles in the primary Al phase. A high fraction of coarse solute-lean α-Al particles in the primary Al phase caused a higher thermal conductivity of the component in the near-to-gate region. A variation in thermal conductivity through the rheocast component of 10% was discovered. The effect of an inhomogeneous temperature-dependent thermal conductivity on the thermal performance of a large rheocast heatsink for electronics cooling in an operation environment was studied by means of simulation. Design guidelines were developed to account for the thermal performance of heatsinks with inhomogeneous thermal conductivity, as caused by the rheocasting process. Under the modeling assumptions, the simulation results showed over 2.5% improvement in heatsink thermal resistance when the higher conductivity near-to-gate region was located at the top of the heatsink. Assuming homogeneous thermo-physical properties in a rheocast heatsink may lead to greater than 3.5% error in the estimation of maximum thermal resistance of the heatsink. The variation in thermal conductivity within a large rheocast heatsink was found to be important for obtaining of a robust component design. [ABSTRACT FROM AUTHOR]

This article examines factors affecting the thermal conductivity of aluminium alloys. More specifically, thermal conductivity is measured with the Hot Disk method. The work shows that improvements as high as 50 % in thermal conductivity can be achieved through heat treatment of the studied alloys. Optical microscopy and scanning electron microscopy indicate that some elements in solid solution (particularly silicon) precipitate in aluminium grains during annealing. Modifying the microstructure with strontium and sodium increases thermal conductivity slightly, whereas rheocasting and grain refinement treatments do not affect thermal conductivity. AlB treatment improves thermal conductivity especially in low-silicon primary aluminium alloys. As-cast and heat-treated thermal conductivities of the studied alloys are calculated by applying Matthiessen's rule and the Wiedemann-Franz law and compared to the measured thermal conductivity. [ABSTRACT FROM AUTHOR]

The goal of this work is to introduce a simple method for evaluating the thixoformability of Al–Si alloys based on semisolid transformation curves obtained by differential thermal analysis (DTA), differential scanning calorimetry (DSC) and Thermo-Calc ® simulation. The Al–Si alloys evaluated had a silicon content ranging from 2 to 7 wt% Si and were produced by grain refining with the addition of Al5Ti1B master alloy. The semisolid transition for each alloy was analyzed according to classical thermodynamic thixoformability criteria, i.e., by looking at the highest “knee” on the liquid fraction vs. temperature curve, the sensitivity of liquid fraction (d f L /d T 0.4–0.6 ) at 40/60% liquid fraction and the solidification interval of each alloy. The results showed that thermodynamic characterization using DTA, DSC and Thermo-Calc ® was effective in predicting semisolid behavior but that use of the commonly adopted criteria reduces the number of alloys considered suitable for semisolid metal (SSM) processing. The criterion proposed here, “ The sensitivity of the liquid fraction, dfL/dT, at the desired liquid fraction, fL, exclusively for the primary phase must be as low as possible (<0.03 °C −1 ) ”, increases the number of thixoformable alloys by focusing on the control of SSM processing. Thixoforming or Rheocasting must be avoided at temperatures at which the common secondary phase, i.e., usually the eutectic phase present in SSM raw materials, undergoes transformation as this transformation is more complex and less stable than the corresponding one for the primary phase. The paper also demonstrates that kinetics parameters, such as heating (or cooling) rate, have a major effect on semisolid transformation and should be incorporated in the control of SSM processing so that the best processing window can be determined more accurately. [ABSTRACT FROM AUTHOR]

In this paper, the effects of pouring temperature of magnesium melt, preheating temperature of the barrel of the screw mixer, and shear rate on the solidified microstructures of semi-solid slurry were investigated by a mechanical stirring semi-solid process. The appropriate processing parameters of slurry preparation were obtained, and the mold filling ability of semi-solid slurry for thin-walled casting was examined. Results indicate that the solid volume fraction of non-dendritic mi-crostructure increases with a decrease in pouring temperature of magnesium melt and the barrel preheating temperature of the screw mixer. Also the grain size of primary a-phase is reduced. Furthermore, the solid volume fraction of semi-solid non-dendritic structure decreases with an increase of shear rate. The fine and round granular microstructure with 30一50 Nm insize of semi-solid AZ91D magnesium alloy was presented. Finally, a 1.0 mm thin-walled casting with a clear contour and good soundness was successfully made by semi-solid rheo-diecasting.

En el presente trabajo se ha estudiado la microestructura y el comportamiento de material producido mediante rheocasting, una nueva vía de producción industrial, en estado semisólido, que presenta diversas ventajas respecto a los procesos más habituales. Se ha caracterizado microestructuralmente la aleación hipoeutéctica de Al-Si A357, antes y después del proceso de inyección en estado semisólido, observándose en ambos estados una estructura fina y homogénea. Se han realizado ensayos de tracción de probetas mecanizadas de componentes reales producidos mediante rheocasting, tanto sin tratamiento térmico como tras un tratamiento T5. Los resultados, en ambos casos, fueron superiores a los obtenidos, para esta misma aleación, mediante otras vías.

In this work, a simulations study on the impact on gravity cast salt cores was carried out for the different casting parameters in high pressure die casting as well as in rheocasting. To compare the simulation results, salt cores were created in test casts and characteristic values were determined by means of a three-point bending test. The calculated loads on the cores were determined and aligned with the bending strength. Salt cores in rheocasting showed reduced loads and tend to increase core viability compared to conventional hpdc. [ABSTRACT FROM AUTHOR]

Rheocast and liquid cast AZ91 magnesium alloy were compared to explore the feasibility of Rheocasting commercial Mg alloys using the RheoMetal process. A close-to industrial scale squeeze casting machine was used, with a mold having rectangular shaped cavities of section thicknesses 7, 13 and 20 mm. The pore content and its distribution as well as the microstructure of the castings were analyzed. The average size of the globular primary α-phase particles in the rheocast samples was around 50 μm. The shape factor of the primary particles in the rheocast samples had a bell-shaped distribution with a peak at 0,4-0,6 while the conventionally cast material had more stochastic distributions. The porosity in the rheocast samples was coarser than in the liquid cast samples, even though both types had similar levels of overall porosity. There was a layer of segregated porosity at around 30% of the section thickness from the sample surface; severe in the liquid cast samples and slight or non-existent in the rheocast samples. [ABSTRACT FROM AUTHOR]

This article reports the effects of stirring speed and T6 heat treatment on the microstructure and mechanical properties of Al-2024 alloy synthesized by a rheocasting process. There was a decrease in grain size of α-Al particles corresponding to an increase in stirring speed. By increasing the stirring speed, however, the globularity of matrix particles first increased and then declined. It was also found that the hardness, compressive strength, and compressive strain increased with the increase of stirring speed. Microstructural studies revealed the presence of nonsoluble Al(CuFeMn)Si phase in the vicinity of CuAl in the rheocast samples. The required time for the solution treatment stage was also influenced by stirring speed; the solution treatment time decreased with the increase in stirring speed. Furthermore, the rheocast samples required a longer homogenization period compared to conventionally wrought alloys. Improvements in hardness and compressive properties were observed after T6 heat treatment. [ABSTRACT FROM AUTHOR]