Rapid compression of aluminum alloys and its relationship to thixoformability

Shaping of metals by thixoforming relies on the unusual flow behavior of semisolid slurries containing nondendritic solid phase. The microstructure of an alloy stirred during freezing consists of rounded particles of solid, as opposed to the dendrites associated with conventional solidification. In the semisolid state, these slurries are thixotropic, in that their apparent viscosity is dependent on shear rate and time. Here, a technique of rapid compression testing is outlined, carried out under conditions similar to normal industrial thixoforming, to assess slurry flow behavior and to examine the correlation between feedstock production routes, microstructure, and resistance to flow. Samples are heated to the desired temperature in the semisolid state with various soaking times and rammed at constant velocity against a platen backed by a load cell. The load-displacement curves produced from the tests may show an initial peak, believed to originate from a skeletal structure which rapidly breaks down under shear. The load signal during flow decreases with increasing soaking time and with temperature, and the initial peak eventually disappears in all alloys investigated. Quantitative metallography indicates that the lower loads correspond to greater spheroidicity of the solid particles within the slurry. The curves have been analyzed to derive the viscosity as a function of average shear rate and demonstrate that the semisolid slurries exhibit pseudoplastic flow behavior which is dependent on the compression velocity and is far removed from steady-state conditions.     

Grain Boundary Characteristics of As-cast Semi-solid Processed Ultra-high Carbon Steel

The microstructure of 1.4?% carbon steel produced either by ordinary casting or semi-solid casting with different primary fraction of solids using cooling plate technique was investigated. The microstructure of ordinary ultrahigh-carbon steel (O-UHCS), was improved by the semi-solid processing. Grain boundary cementite thickness of UHCS and its morphology is affected by semi-solid process. Grain boundary cementite thickness of about 2???m can be achieved by pouring the semi-solid slurry with 0.31 fraction of solid. The pearlitic interlamellar spacing of semi-solid processed UHCS is shorter compared to O-UHCS.     

Investigation of spherisation in microstructures of aluminium casting alloys for thixoforging process

Thixoforging combined with low superheat casting (LSC) is a promising shaping process for aluminium casting alloys. LSC process is based on rapid solidification of an alloy which cast with low pouring temperature. With this method, a feedstock material is produced with non-dendritic microstructure that ready for spherisation in reheating sequence of further semi-solid process. Al-Si alloys are still castable even at low temperatures due to their excellent fluidities. This study subjects to present spherisation of A356 and A380 alloy billets cast with LSC process that provides appropriate beginning material with relatively high sphericity. Obtained billet parts were reheated for different times at a semi-solid state temperature. Some of these billets were directly quenched for observing the effects of reheating and the others were thixoforged. With sufficient reheating time, deformation of thixoforging process did not significantly affect on the spherical microstructure. Unnecessarily long reheating period caused excessive grain growth. A356 alloy had higher spherisation tendency than A380 alloy under similar process conditions.     

Experimental Investigation on the Rheological Behavior of Hypereutectic Al-Si Alloys by a Precise Rotational Viscometer

Determining the viscosity of low solid volume fraction semisolid alloys is important for predicting the gradient of primary particles within functionally graded materials (FGMs), produced by in-situ casting processes. In this study, a new precise rotational viscometer was developed and used to measure the viscosity of Al-22 pct Si and Al-30 pct Si semisolid alloys up to solid volume fractions of 9 and 20 pct, respectively. Three kinds of typical curves, viscosity (η) vs solid volume fraction (f _s ), shear time (t), and shear rate ([(g)\dot] \dot{\gamma } ), were derived from the results of viscosity measurements for both Al-Si alloys. In the semisolid Al-Si alloys, thixotropic behavior was not detected at low solid volume fractions, but this behavior was obviously observed with increasing solid volume fraction and could be described by an analytical model. Finally, the results showed that the equilibrium viscosity of the semisolid alloys with thixotropic properties decreased by increasing shear rate according to the Ostwald–De Waele power law. A special test, developed in this research, was used to show the effect of agglomeration on the viscosity of the semisolid alloys.     

挤压铸造零件沿流程方向的成分偏析及组织偏聚

采用挤压铸造(液态模锻)工艺制备了A357铝合金螺旋线试件,使用化学成分分析和金相分析方法研究了流程长度对合金成分偏析及组织偏聚的影响.结果表明:在沿流程方向上,流程的开始端和流程末端的Si元素和Mg元素含量大于流程中段的含量.合金的初始α晶粒尺寸随着流程长度的增加先增大后变小.初生固相率随着流程长度的增大呈现波动变化.造成流程成分偏析和组织变化的原因是在挤压铸造凝固阶段中补缩液相的强迫运动.      

Simulation of Cooling of Liquid Metal in an Inclined Slope to Predict the Condition for Semi Solid Forming and Its Experimental Validation

In semi solid processing, the preparation of slurry with globular microstructure is very important. A recent technique named the inclined slope casting, to produce the semi-solid metal slurry, has been evolved. The microstructure of casting depends on several process parameters like initial temperature of liquid metal, length of the slope, cooling rate and the rate of flow of liquid metal. In the present work numerical modelling of cooling of liquid was carried out. Modelling was done for different mass influx velocities, temperature of pouring and different inclinations of the slope set-up. In this work, a CFD model based on finite volume method for simulating flow along the inclined slope was carried out. Heat transfer, fluid flow and solidification were studied by varying the process parameters like slope angle and inlet pouring rate. The numerical results were verified with experiments for same conditions.     

Rheology and microstructure of semi-solid aluminum alloys compressed in the drop-forge viscometer

The rheological behavior and microstructure of semi-solid aluminum alloys were studied using a novel apparatus, the drop-forge viscometer (DFV). The viscometer determines force from the second-derivative-of-displacement data with respect to time and permits calculations of viscosities at shear rates in excess of 1000 s⁻¹. Alternatively, the DFV can be operated like a conventional parallel-plate viscometer, attaining shear rates as low as 10⁻⁵ s⁻¹. Rapid compression experiments (in the DFV) result in first rapidly increasing, then decreasing, shear rates. In a typical experiment, the viscosity decreased from about 100 to 1 Pa·s as the shear rate increased from approximately 200 to 1300 s⁻¹ in less than 4 ms. The viscosity later increased to about 10 Pa·s as the shear rate decreased from 1300 to 30 s⁻¹ over 2 ms. The minimum viscosity obtained depended on the maximum shear rate, not the duration of shear. The dual observed phenomena of (1) a very rapid drop of viscosity with increasing shear rate followed by (2) a relatively slow increase of viscosity with decreasing shear rate thereafter have potential significance for future machine and process design. For example, it should be possible to form higher fraction solid slurries than is now feasible by applying vigorous shear to semi-solid slurries just before the metal is introduced to the die entrance. The DFV was used to calculate viscosity as a function of shear rate for samples produced by the commercial strain-induced, melt-activated (SIMA) and magnetohydrodynamic (MHD) methods, as well as the recently developed Massachusetts Institute of Technology (MIT) method. Isothermal experiments were conducted between fraction solid of 0.44 and 0.67 for the various alloys (corresponding to a temperature range of 579 °C to 611 °C). The viscosity of the commercial semi-solid Al-Si alloys A357 and A356 produced by the various methods was similar. Separation of liquid and solid phases was not observed in rapid compression experiments shorter than 10 ms, either visually or with energy-dispersive spectroscopy (EDS) characterization. At low compression velocities, segregation was observed and increased with increasing amounts of strain. The maximum fraction solid compressed at high and low shear rates were 0.67 and 0.69, respectively.     

Microstructural studies of A357 alloy cast by RSF process

We studied rheocasting of an A357 (Al-7Si-0.6Mg) alloy using the Rapid Slurry Formation (RSF) technology developed by Rheometal. The technology is based on a so-called solid enthalpy exchange material (EEM). Semi-solid slurries of A357 with around 30% solid fraction were produced in this way. The slurry formation was modified by adding several grain refiners (Al-5Ti-1B, Al-3Ti-0.15C and Si-1B master alloys) individually. Stirring speed was varied to study its effect on microstructure. The slurry so produced was air cooled and quenched in water. It was observed that rapid heat extraction, nucleation and forced shearing action developed globular α-Al phase.     

Yield behavior of commercial Al-Si alloys in the semisolid state

Systematic experimental work and modeling efforts have been conducted to characterize the yield behavior of commercial aluminum alloys in the semisolid state. In this study, extensive compression experiments were performed to measure the yield stress of semisolid aluminum slurries at high solid fractions (0.5 to 1.0), and a cone penetration method was employed to measure yield stress at low solid fractions (<0.5). A functional relationship between yield stress and temperature/solid fraction has been established for these alloys. The effect of the processing route on the resultant yield stress of the material in the semisolid state was studied by evaluating commercial A356 billets manufactured via magnetohydrodynamic stirring, grain refining, and UBE’s new rheocasting (NRC) processes, respectively. Detailed microstructure observations and image analyses reveal that the difference in yield-stress values among the alloys evaluated is intricately related to the semisolid structure. At a given solid fraction, the yield stress of semisolid slurries depends on microstructural indices (i.e., entrapped-liquid content, shape factor of the alpha phase, and the alpha particle size). In addition, numerical simulation results indicate that the finite yield stress of semisolid metals plays a significant role in determining the flow pattern during die filling. Depending on processing conditions, five distinct filling patterns (shell, disk, mound, bubble, and transition) have been identified and confirmed through experimental observations. Recent simulations demonstrate that the finite yield stress is also responsible for flow instabilities encountered in commercial forming operations, such as “toothpaste behavior.” Specifically, most flow instabilities can be avoided by properly controlling processing parameters and the initial semisolid microstructure. A stability map that provides a control guide for semisolid processing has been developed and is presented.     

Effects of Solid–Liquid Mixing on Microstructure of Semi-Solid A356 Aluminum Alloy

The desired starting material for semi-solid processing is the semi-solid slurry in which the solid phase is present as fine and globular particles. A modified solid–liquid mixing (SLM) is reported wherein semi-solid slurry can be produced by mixing a solid alloy block into a liquid alloy, and mechanical vibration is utilized to enhance the mixing. Effects such as liquid alloy temperature, mass ratio, and mixing intensity on the microstructure and the cooling curves during SLM were evaluated. 2D and 3D microstructure analysis of treated A356 aluminum alloy shows that microstructure can be refined significantly with a considerable morphology change in primary Al phase. It is critical that the temperature of mixture after mixing is lower than its liquidus temperature to obtain a valid SLM process. Specially, mixing intensity is identified as a primary factor for a favorable microstructure of semi-solid slurry.     

Triaxial Compression on Semi-solid Alloys

Multi-axial compression of the mushy zone occurs in various pressurized casting processes. Here, we present a drained triaxial compression apparatus for semi-solid alloys that allow liquid to be drawn into or expelled from the sample in response to isotropic or triaxial compression. The rig is used to measure the pressure-dependent flow stress and volumetric response during isothermal triaxial compression of globular semi-solid Al-15 wt pct Cu at 70 to 85 vol pct solid. Analysis of the stress paths and the stress–volume data show that the combination of the solid fraction and mean effective pressure determines whether the material undergoes shear-induced dilation or contraction. The results are compared with the critical state soil mechanics (CSSM) framework and the similarities and differences in behavior between equiaxed semi-solid alloys and soils are discussed.

     

Effect of lanthanum addition on microstructure and hardness of brass alloys produced by rheological squeeze casting

The effect of La addition (0–0.30 wt%) on the microstructure and hardness of rheological squeeze casting brass alloys was experimentally investigated. The rheological squeeze casting process is improved by controlling the wall surface crystals and melt flow rate to realise the preparation of semi-solid melt with flow, and a brass alloy workpiece with La is produced. The microstructure and properties of the brass alloy samples were investigated using metallography, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction and hardness testing. The results indicate that the hardness of the rheological squeeze casting brass alloy is increased by 20.4% from 108 to 130 HBW with an increase in the La content from 0 to 0.30 wt%. The microstructural analysis results show that La significantly refines the primary α-phase grains, and the main mechanism is the constitutional undercooling and heterogeneous nucleation caused by the La enrichment in the front of the solid–liquid interface. The squeeze pressure promotes undercooling, which improves the nucleation rate and affects the solute diffusion and nucleus growth. The dual effects of these two aspects aggravate the grain refinement process, consequently increasing the number of grain boundaries and improving the hardness of the brass alloy.     

Influence of Grain Refinement on Slurry Formation and Surface Segregation in Semi-solid Al-7Si-0.3Mg Castings

This study aims to evaluate the effect of grain refinement on slurry formation and surface segregation in semi-solid castings produced by the Rheometal™ process. The effect of two grain refiners, Al-8B and Al-5Ti-1B, on the slurry α-Al grain size, shape factor and solid fraction was evaluated. The results suggest that the addition of a grain refiner can affect the solid fraction obtained in the Rheometal^TM process and, consequently, reduce the solute content near the casting surface. Grain refiner addition resulted in a larger fraction of α-Al grains ≤ 60 µm for the refined alloys compared with the unrefined alloy. Additionally, the growth of α-Al slurry globules was greater for the unrefined alloy compared with the refined alloy during solidification in the die-cavity. A more homogeneous and finer microstructure was observed near the surface in the grain-refined castings compared with the unrefined castings. Evidence of significant liquid penetration was identified in some α-Al globules, indicating that disintegration of α-Al globules may occur during the Rheometal™ casting process.

     

AlMg_5Si_2Mn铝合金等温热处理半固态成形工艺探索

铝合金等温半固态触变成形是一种先进的成形工艺理念,目前尚处于工程化应用前期。本文以AlMg_5Si_2Mn铝合金为研究对象,采用等温热处理法制备实验合金半固态浆料,并研究其半固态等温过程的组织演变规律。然后使用压铸设备开展AlMg_5Si_2Mn铝合金的半固态成形工艺试验,探索可行的半固态成形工艺和合理实现路径。     

The structure of a partially solid alloy

An apparatus is described that permits continuous production of partially solidified slurries of Sn-15 pct Pb alloy. The slurries thus produced can be cast into ingots semi-continuously. Results of earlier fundamental studies carried out on a high temperature viscometer are verified by an extensive number of experiments carried out in this apparatus. The three important process variables affecting structure and viscosity of a slurry are: average shear and cooling rates during primary solidification and volume fraction solid in the slurry. Increasing the average cooling rate makes the primary solid particles smaller and more uniform in size, but increases the amount of entrapped liquid in each particle. Increasing the average shear rate affects particle size and uniformity of size similarly, except this effect is noted only at low cooling rates. Primary solid particle size in a vigorously agitated slurry is approximately the same as primary dendrite arm spacing in a conventional casting solidified at equivalent cooling rate. As the volume fraction solid in a slurry increases, so do its primary solid particle size and viscosity.     

The structure of a partially solid alloy

An apparatus is described that permits continuous production of partially solidified slurries of Sn-15 pct Pb alloy. The slurries thus produced can be cast into ingots semi-continuously. Results of earlier fundamental studies carried out on a high temperature viscometer are verified by an extensive number of experiments carried out in this apparatus. The three important process variables affecting structure and viscosity of a slurry are: average shear and cooling rates during primary solidification and volume fraction solid in the slurry. Increasing the average cooling rate makes the primary solid particles smaller and more uniform in size, but increases the amount of entrapped liquid in each particle. Increasing the average shear rate affects particle size and uniformity of size similarly, except this effect is noted only at low cooling rates. Primary solid particle size in a vigorously agitated slurry is approximately the same as primary dendrite arm spacing in a conventional casting solidified at equivalent cooling rate. As the volume fraction solid in a slurry increases, so do its primary solid particle size and viscosity.     

Numerical Modelling of Semi‐Solid Flow under Processing Conditions

During the industrial process of semi‐solid forming (or thixoforming) of alloy slurries, typically the operation of die filling takes around 0.1s. During this time period the alloy slug is transformed from a solid‐like structure capable of maintaining its shape, into a liquid‐like slurry able to fill a complex die cavity: this involves a decrease in viscosity of some 6 orders of magnitude. Many attempts to measure thixotropic breakdown experimentally in alloy slurries have relied on the use of concentric cylindrical viscometers in which viscosity changes have been followed after shear rate changes over times above 1s to in excess of 1000 s, which have little relevance to actual processing conditions and therefore to modelling of flow in industrial practice. The present paper is an attempt to abstract thixotropic breakdown rates from rapid compression tests between parallel plates moving together at velocities of around 1m/s, similar to industrial conditions. From this analysis, a model of slurry flow has been developed in which rapid thixotropic breakdown of the slurry occurs at high shear rates.     

Preparation and theoretic study of semi-solid Al_2Y/AZ91 magnesium matrix composites slurry by ultrasonic vibration

Semi-solid Al2Y/AZ91 magnesium matrix composites slurry was prepared by ultrasonic vibration, effect of ultrasonic vibration temperature and time on microstructure of semi-solid slurry was investigated. The results showed that with the ultrasonic vibration temperature decreasing the solid volume fraction of semi-solid slurry increased. The best ultrasonic vibration temperature was 600 oC. With the increase of ultrasonic vibration time, the average grain diameter of primary а-Mg particles decreased firstly, then increased, the average shape factor increased gradually and decreased slightly after 90 s, and a few rosette dendrites were observed after 120 s. The best semi-solid slurry with average grain diameter of 75 μm and shape factor of 0.7 were gained after the melt was treated by ultrasonic vibration for about 60 s at near liquidus temperature(600 oC). At last, the microstructure evolution mechanism of semi-solid magnesium matrix composites slurry was analyzed by the theories of thermodynamics and kinetics.     

Mg-Al系镁合金半固态组织二次凝固特征

半固态浆料液相的凝固行为决定了其二次凝固组织,这将直接影响半固态成形件的性能.Mg-Al系镁合金随着成分、凝固行为的不同,其二次凝固组织有较大的不同.该文描述了Mg-Al系合金半固态浆料液相凝固过程以及二次凝固组织特征,并针对其特征对其形成原因展开了分析.     

Non-dendritic Structural Changes in Al–7Si Alloy Cast Through Rapid Slurry Formation (RSF) Process

Rapid slurry formation (RSF) is a semi-solid metal forming technique, which is based on a so-called solid enthalpy exchange material. The beauty of the process is that the slurry is controlled by relative added amounts of the alloy system used. It is not necessary to accurately control the heat flux to surrounding media such as, crucible or mould. In the present investigation 15?% solid fraction in Al?C7Si alloy slurry has been produced by RSF method. The slurry so prepared was cast in gravity die cast mould with water cooling arrangement. Thus ingot of size 30?cm?×?5?cm?×?2.5?cm could be produced. The microstructure showed rosette and globular type ??-Al morphology. Tensile properties evaluated were higher than as cast Al?C7Si alloy. During slurry formation coarsening of particles was experimentally investigated and also theoretically described by Lifshitz-Slyozov-Wagner (LSW) theory and convective mass flow equations.