Semi-solid Casting Research Articles

Improving the Corrosion Resistance of Magnesium-Aluminum Alloys Using Microstructural Design

Magnesium-aluminum (Mg-Al) alloys are crucial lightweight materials with exceptional properties. However, unfortunately, they exhibit rather poor corrosion resistance, and, thus, they have attracted enormous research efforts and triggered a great deal of technological developments during the last decade. Despite the impressive progress in understanding the corrosion mechanism of Mg and Mg alloys, several shortcomings and bottlenecks still exist, preventing more widespread use of Mg alloys in various applications. Recent studies have shown that the corrosion properties of commercial Mg-Al alloys can be greatly improved by applying a semi-solid casting procedure. This promising method involves the rheocasting (RC) technique, in which the required slurry is prepared by the RheoMetal process, enabling the manufacturers to design the alloys’ microstructure with a rather high degree of freedom. A series of comprehensive accelerated corrosion tests in constant/cyclic relative humidity and temperature show that the alloys produced by the RC technique exhibit up to four times better corrosion resistance than their high pressure die-cast (HPDC) counterparts. In this study, the influence of alloy microstructure, in terms of the effect of all the microstructural constituents (α, β, η and casting pores), on the mechanisms of degradation of the RC and HPDC Mg-Al alloys in ambient atmosphere is discussed in detail. The results imply that it is (in principle) possible to use the component fabrication step (i.e., casting) to modify alloy microstructure so as to mitigate corrosion properties. The idea is generic and may also be applicable to other alloy systems.

A Novel Method for Semi-Solid Casting of Hypereutectic Gray Cast Iron in Expendable Mold

In the present research, the possibility of single stage semi-solid casting of a hypereutectic gray cast iron in a sand mold was investigated. For this purpose, a specially designed chilled sprue sand mold rigged to a vertical centrifugal casting machine was used. The molten cast iron was poured in the chilled sprue sand mold at 1275 oC at a rotation speed of 400 rpm. Two control samples were also poured at 1275 oC under gravity and centrifugal casting conditions in the same mold without sprue chilling. Microstructures and mechanical properties of the specimens were then characterized. The results showed that the average length and thickness of the primary solid phase, i.e. the Kish graphite, and those of the eutectic flake graphite were smaller in semi-solid centrifuged castings and their hardness were higher than those of the control samples. The results are discussed in terms of the shearing and chilling effects of the designed sprue during the initial stages of pouring.

Prediction of the equivalent elastic modulus of mush zone during solidification process coupled with phase field simulations

A phase field model with the consideration of applied and thermal stress is developed in this paper. The elastic stress and strain fields inside the solid skeleton can be calculated out by phase field model together with the phase morphologies. By adding an external constant stress boundary condition, the total strain can be obtained by solving the mechanical equilibrium equations. We can get the equivalent elastic modulus under different phase morphologies from these calculations, which is useful for FEM or FDM simulations on a larger scale. This model is applied to as-cast and semi-solid casting process, respectively. The equivalent elastic modulus under dendritic and spherical solid skeleton are both obtained.

Influence of friction stir processing on the microstructure and mechanical properties of a compocast AA2024-Al2O3 nanocomposite

The effect of friction stir processing (FSP) on the microstructure and mechanical properties of a semi-solid cast AA2024-1wt%Al2O3 nanocomposite was investigated. For comparison, plates of unreinforced AA2024 alloy were also cast and processed at the same FSP conditions (400rpm, 20mm/min). The microstructure of all the produced materials was investigated using optical microscopy (OM), scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Microhardness and tensile tests were carried out on the unreinforced AA2024 alloy and AA2024-Al2O3 nanocomposite before and after FSP. The addition of 1wt% of Al2O3 nanoparticles significantly reduced the grain size of both the cast and FSPed microstructures, leading to a grain size reduction from 28μm to 18μm in the cast condition, and from 3.7μm to 2.3μm after FSP. The application of FSP to AA2024-Al2O3 nanocomposite enhanced the tensile strength and yield strength by 71% and 30%, respectively, in comparison to the as cast matrix, as a result of the uniform distribution of Al2O3 reinforcement and grain refinement of Al matrix. The combined application of compocasting and FSP resulted to be a promising method to treat casting defects and to produce nanocomposites characterised by good reinforcement dispersion and high strength and ductility.

Optimization of the EMS process parameters in compocasting of high-wear-resistant Al-nano-TiC composites

Understanding of the electromagnetic stirrer (EMS) process parameters–wear relation in nanocomposite is required for further creation of tailored modifications of process in accordance with the demands for various applications. This study depicts the performance of hybrid algorithm for optimization of the parameters in EMS compocasting of nano-TiC-reinforced Al–Si alloys. Adaptive neuro-fuzzy inference system (ANFIS) coupled with particle swarm optimization (PSO) was applied to find the optimum combination of the inputs including mold temperature, mix time, impeller speed, powder temperature, cast temperature and average particle size. The optimized condition was obtained in minimization of objective function. The objective function is calculated by ANFIS and then minimized by PSO. The optimized parameters were used to produce semisolid cast aluminum matrix composites reinforced with nano-TiC particles. The optimized nanocomposites were then studied for their tribological properties.

A new semi-solid casting technique for fabricating SiC-reinforced Mg alloys matrix composites

The capability of the newly developed rheocasting (RC) technique in combination with the RheoMetal process for producing SiC particulate-reinforced AM50 and AZ91D matrix composites (Mg-based MMCs) was investigated. The quality of the MMCs was studied by analyzing the fraction of casting pores, number density of SiC clusters and the uniformity of SiC particles. Solid fraction, particle size and oxidation of SiC particles had strong impacts on the overall quality of the MMCs. The MMCs produced by 40% solid fraction and oxidized micron-sized SiC particles exhibited an excellent casting quality. A low-quality MMC was obtained when non-oxidized sub-micron sized SiC particles were employed. The results showed the formation of various types intermetallic particles and carbides such as MgO, Mg2Si, Al2MgC2, Mg2C3, Al4C3 as the interfacial reaction products of SiC/Mg alloy's melts. Mg hydride (alpha-MgH2) was also identified in inter-dendritic regions of the MMCs for the first time.

Effects of Manufacturing Processes on Microstructure and Properties of Al/A356–B4C Composites

Liquid and semi-solid stir casting processes were applied to fabricate B4C particles-reinforced aluminum–matrix composites. The effects of manufacturing processes on particle distribution, particle/matrix interface, and mechanical properties of the prepared composites were studied. The results show that particle distribution can be significantly improved by using K2TiF6–flux and Ti powders in the liquid stir casting process, whereas in the semi-solid stir casting process it could be improved by decreasing the temperature of the slurry. With additions of Ti, the decomposition of B4C was prevented, and the interfacial bonding strength was significantly improved due to the fact that a TiB2 layer formed at the particle/matrix interface. Compared to the matrix, the hardness and tensile strength of the Al–B4C composite fabricated by the liquid stir casting process were increased by 89.6% and 128.8%, respectively; those of the A356–B4C composite fabricated by the semi-solid stir casting process had no significant improvement due to the weak particle/matrix interface and the presence of particle porosity clusters.

Phase Segregation during Processing of Semi-Solid Slurry by Rheocasting Method SEED

The paper describes specific phenomena of semi-solid casting process, especially rheo-casting method SEED, which uses mechanical swirling for reaching proper structure in semi-solid state. The eutectic segregation during processing of semi-solid slurry can cause specific casting defects. Heat treated alloy AlSi7Mg0.3 was applied for producing castings. For observing structure, metallographic observation by light and SEM microscopy was used as well as spectral analysis.

Fatigue Characteristics and Quality Index of A357 Type Semi-Solid Aluminum Castings Used for Automotive Application

The present work aims to investigate the fatigue characteristics of automotive lower suspension arm made of semi-solid A357 aluminum castings using metallurgical and analytical approaches. The fatigue life calculations of analytical model are used to identify and introduce the model parameters based on the suspension arm material followed by analyzing the load-number of cycles fatigue curve. The critical stress areas capable of initiating cracks during fatigue tests are detected using ABAQUS software followed by the installation of strain gages on the suspension arm to calculate maximum stress. The fatigue experiments are carried out to compare the results of the analytical method with the experimental endurance curves traced by lower suspension arm samples. Microstructure characteristics of the semi-solid A357 under T6 heat treatment conditions are examined using scanning electron microscope. The results show that the fatigue life and the quality index of alloys investigated are affected by casting technique, castings design, microstructural characterization, and heat treatment condition.

Effect of Heat Treatment on the Resistivity and Mechanical Property of Low-Pressure Semi-Solid Die Cast Aluminum Alloy Wheel

Study the property and resistivity of Low-pressure semi-solid die casting ( LPSSDC) aluminum wheel hub after heat treatment. The Brinell hardness has reached more than 100HB in all position. Tensile properties at inner rim and outer rim are both reached 280MPa. The result of resistance analysis on LPSSDC aluminum wheel hub is below. The resistivity of the material is only influenced by the supersaturated solid solution (SSSS), GP zone, metastable phase β 'and stable phase β (Mg2Si). The resistivity has a decline during aging 0-0.5h and 0.75-8h, and an increasing during 0.5-0.75h. The resistivity remains stably when the decomposition of the solid solution achieved a dynamic balance after 8h aging time.

Mechanical properties of rolled A356 based composites reinforced by Cu-coated bimodal ceramic particles

Abstract Three kinds of A356 based composites reinforced with 3 wt.% Al2O3 (average particle size: 170 μm), 3 wt.% SiC (average particle size: 15 μm), and 3 wt.% of mixed Al2O3–SiC powders (a novel composite with equal weights of reinforcement) were fabricated in this study via a two-step approach. This first process step was semi-solid stir casting, which was followed by rolling as the second process step. Electroless deposition of a copper coating onto the reinforcement was used to improve the wettability of the ceramic particles by the molten A356 alloy. From microstructural characterization, it was found that coarse alumina particles were most effective as obstacles for grain growth during solidification. The rolling process broke the otherwise present fine silicon platelets, which were mostly present around the Al2O3 particles. The rolling process was also found to cause fracture of silicon particles, improve the distribution of fine SiC particles, and eliminate porosity remaining after the first casting process step. Examination of the mechanical properties of the obtained composites revealed that samples which contained a bimodal ceramic reinforecment of fine SiC and coarse Al2O3 particles had the highest strength and hardness.

Microstructure Evolution of Conventional and Semi-Solid Cast of A356 Aluminium Alloy with Addition of Inoculant

This paper investigated the effect of inoculant, Al-5Ti-1B in conventional and semi-solid casting A356 aluminium alloy. A356 aluminium alloy was melted at 850 oC and poured at 680 °C directly into the steel mould and on the inclined slope into steel mould. Inoculant was added in various percentages of 1 wt.%, 2 wt.%, 3 wt.% and 3.5 wt.% in A356 aluminium alloy melt. Microstructure and microhardness were characterized using optical microscope and Vicker’s microhardness tester. The addition of master alloy up 3.5 wt.% Al-5Ti-1B in conventional casting refined dendritic structure with average grain size of 33.41 μm. The microstructures of semi-solid A356 aluminium alloy with addition of Al-5Ti-1B consist of equiaxed structure of α-Al. Further addition of Al-5Ti-1B refined the globular structure of α-Al. The higher hardness was achieved for A356 alloy prepared using semi-solid with addition of 3.5 wt.% of Al-5Ti-1B.

Influence of Semi-Solid Casting and Equal Channel Pressing on Microstructure of a Hypoeutectic Al-Si Alloy

The aim of this work was to develop understanding of microstructural evolution of the alloy casted in semi-solid condition using a cooling slope and conventional casting followed by ECAP in a 120o die. Feed materials were prepared by cooling slope casting and conventional casting for ECAP process. The microstructures and Vickers hardness of the worked materials extruded by two routes (A and BC) were evaluated. The primary α-Al phase tends to be elongated after processing by route A. However, its morphology was similar in nature to the microstructure of the as-cast sample after processing by route BC. The Si particles become fragmented during ECAP processing and are more nearly globular in shape and uniform in size than in the as-cast sample. The microstructure of the semi-solid cast ECAPed samples was more homogenous than that of the conventional cast ECAPed sample followed by ECAP for both routes. The hardness of semi-solid cast ECAP samples was also higher than that of conventional cast ECAPed samples for both routes.

A comparative study on the friction and wear properties of semi-solid cast A356 alloy

Abstract In this study, the friction and wear properties of A356 alloy processed via semi-solid casting and conventional casting were experimentally investigated. Experimental results indicate that under the same experimental conditions the wear of the semi-solid cast A356 in mass does not exceed 85 % of that of the conventionally cast A356 alloy. The friction coefficient curve of semi-solid cast A356 is smoother than that of the conventionally cast A356. The friction coefficient of the semi-solid cast A356 is around 0.53 for stationary friction, less than the 0.59 for the conventionally cast A356. The improvement in the friction and wear properties is mainly a result of the microstructural change induced by semi-solid processing.

Corrosion Behavior of Alloy AM50 in Semisolid Cast and High-Pressure Die Cast States in Cyclic Conditions

The atmospheric corrosion behavior of the Mg-Al alloy AM50 produced by high-pressure die casting (HPDC) and a semisolid metal (SSM) technique was studied under alternating wet-dry conditions for up to 672 hours. Alloy AM50 in the SSM state was fabricated using rheocasting (RC) technique in which the slurry was prepared by the newly developed RheoMetal process. To simulate the real-world atmospheric environment, the exposure program included a 6 hour wet phase (including rain with 0.5% NaCl), a 2.5 hour transition phase, followed by a 15.5 hour dry phase at 50°C and 70% relative humidity. The results showed that the RC alloy was substantially less prone to corrosion than the HPDC material in the cyclic wet-dry atmosphere. Based on the gravimetric results and microstructural characterization studies, this was attributed to a lower fraction of porosity and to the barrier role of β phase particles in the RC.

Determination of Optimal Parameters for Diffusion Bonding of Semi-Solid Casting Aluminium Alloy by Response Surface Methodology

Liquid state welding techniques available are prone to gas porosity problems. To avoid this solid state bonding is usually an alternative of preference. Among solid state bonding techniques, diffusion bonding is often employed in aluminium alloy automotive parts welding in order to enhance their mechanical properties. However, there has been no standard procedure nor has there been any definitive criterion for judicious welding parameters setting. It is thus a matter of importance to find the set of optimal parameters for effective diffusion bonding. This work proposes the use of response surface methodology in determining such a set of optimal parameters. Response surface methodology is more efficient in dealing with complex process compared with other techniques available. There are two variations of response surface methodology. The one adopted in this work is the central composite design approach. This is because when the initial upper and lower bounds of the desired parameters are exceeded the central composite design approach is still capable of yielding the optimal values of the parameters that appear to be out of the initially preset range. Results from the experiments show that the pressing pressure and the holding time affect the tensile strength of jointing. The data obtained from the experiment fits well to a quadratic equation with high coefficient of determination (R 2 = 94.21%). It is found that the optimal parameters in the process of jointing semi-solid casting aluminium alloy by using diffusion bonding are the pressing pressure of 2.06 MPa and 214 minutes of the holding time in order to achieve the highest tensile strength of 142.65 MPa

High strength and good ductility at elevated temperature of nano-SiCp/Al2014 composites fabricated by semi-solid stir casting combined with hot extrusion

The tensile properties at elevated temperature of nano-SiCp/Al2014 composites have been investigated. The nano-SiCp can remarkably improve the tensile strength at 493K of Al2014 alloy without sacrificing the ductility. The strengthening effect for the matrix alloy of 0.5vol% nano-particles is superior to that of 4vol% micron-particles.

The Effect of Process Parameters on the Microstructure and Mechanical Properties of Semisolid Cast Al6061

An examination of the microstructure and mechanical properties of aluminum 6061 alloy samples produced by a miniature cooling slope is presented. The effects of several process parameters including pouring rate, cooling slope angle, superheat and cooling slope length on the microstructure and mechanical characteristics of the samples were investigated. An attempt was made to use the two-level factorial design method to determine the relationships between the process parameters and the properties of the semisolid aluminum 6061 alloy manufactured by the miniature cooling slope. Finally, an optimum processing region was identified. The interaction between the cooling slope length and the pouring rate (BC); the interaction among the superheat, the pouring rate, and the cooling slope length (ABC); and the cooling slope length (C), in the order mentioned, were identified as the most important effects.

Solution and ageing heat treatments of ZK60 magnesium alloys with rare earth additions produced by semi-solid casting

Rare earth elements reportedly improve high temperature strength and creep resistance of Mg alloys. In the present work, ZK60 Mg alloys containing different amounts (0.5, 1.5 and 2.5 wt.%) of rare earth additions were prepared by thixocasting and submitted to solution and ageing heat treatments (T4, T5 and T6). The as-cast and heat treated microstructures were investigated by scanning electron microscopy and hardness was evaluated as a function of heat treatment parameters. In the as-cast state, the alloys are formed by globular α-Mg grains reinforced by a network of composite Mg-Zn-RE precipitates with either smooth or lamellar/acicular morphologies. Solution of the smooth precipitates took place in alloys containing 0.5 and 1.5wt.%RE for T4-500 ºC but no precipitates were dissolved with T4-380 ºC. The optimum temperature for T5 and T6 was identified as 175 ºC, while T6-500 ºC led to the highest hardness, followed by T5 and T6-380 ºC, respectively.

Effects of reinforcement surface modification on the microstructures and tensile properties of SiCp/Al2014 composites

The effects of reinforcement surface modification on the microstructures and tensile properties of 3 vol% SiCp/Al2014 composites fabricated by semi-solid stir casting were investigated. The distribution of oxidized as well as Ni coated SiC particles in the casting composites was the most homogeneous, followed by the Cu coated SiC particles and then the untreated SiC particles. The reinforcing effect for the tensile properties of the composites reinforced by SiC particles with different surface modifications is as follows: oxidized SiC (1273 K and 3 h)>Ni coated SiC>Cu coated SiC>untreated SiC. Furthermore, the optimal particles oxidation parameters (1373 K and 3 h) were achieved based on the microstructures and tensile properties of the extruded composites. The yield strength and ultimate tensile strength (318 and 529 MPa) of the extruded composites reinforced with optimal oxidized SiC particles are 35.9% and 15% higher than those of the extruded Al2014 matrix alloy (234 and 460 MPa), respectively.