Microstructure and Corrosion Characterization of Squeeze Cast AM50 Magnesium Alloys

Squeeze casting of magnesium alloys potentially can be used in lightweight chassis components such as control arms and knuckles. This study documents the microstructural analysis and corrosion behavior of AM50 alloys squeeze cast at different pressures between 40 and 120 MPa and compares them with high-pressure die cast (HPDC) AM50 alloy castings and an AM50 squeeze cast prototype control arm. Although the corrosion rates of the squeeze cast samples are slightly higher than those observed for the HPDC AM50 alloy, the former does produce virtually porosity-free castings that are required for structural applications like control arms and wheels. This outcome is extremely encouraging as it provides an opportunity for additional alloy and process development by squeeze casting that has remained relatively unexplored for magnesium alloys compared with aluminum. Among the microstructural parameters analyzed, it seems that the β-phase interfacial area, indicating a greater degree of β network, leads to a lower corrosion rate. Weight loss was the better method for determining corrosion behavior in these alloys that contain a large fraction of second phase, which can cause perturbations to an overall uniform surface corrosion behavior.     

Segregation band formation in Al-Si die castings

Banded defects are often found in high-pressure die castings. These bands can contain segregation, porosity, and/or tears, and changing casting conditions and alloy are known to change the position and make-up of the bands. Due to the complex, dynamic nature of the high-pressure die-casting (HPDC) process, it is very difficult to study the effect of individual parameters on band formation. In the work presented here, bands of segregation similar to those found in cold-chamber HPDC aluminum alloys were found in laboratory gravity die castings. Samples were cast with a range of fraction solids from 0 to 0.3 and the effect of die temperature and external solid fraction on segregation bands was investigated. The results are considered with reference to the rheological properties of the filling semisolid metal and a formation mechanism for bands is proposed by considering flow past a solidifying immobile wall layer.     

Experimental Study and Numerical Verification of Heat Transfer in Squeeze Casting of Aluminum Alloy A443

As an effective tool, simulation helps do the analysis and optimization in advance and undertake preventive action. A critical portion of casting simulation is the heat transfer at the metal/mold interface. However, it is difficult to determine the values of interfacial heat-transfer coefficients (IHTCs) in squeeze casting of aluminum alloys due to many influence factors. In this work, IHTCs were determined by using the inverse algorithm based on measured temperature histories and finite-difference analysis in a five-step squeeze casting of aluminum alloy A443. The results showed the IHTCs initially reached a maximum peak value followed by a gradually decline to a lower level. Similar characteristics of IHTC peak values were also observed at 30, 60, and 90?MPa applied pressures. With the applied pressure of 60?MPa, the peak IHTC values of aluminum alloy A443 from steps 1 to 5 varied from 5629?W/m²K to 9419?W/m²K. The comparison of the predicted cooling curves with the experimental measurement manifested the cooling temperatures calculated by the IHTC values determined in the current study were in the best agreement with experimental ones. The verification of the determined IHTC values demonstrates that the inverse algorithm is an effective tool for determination of the IHTC at the squeeze casting?Cdie interface.     

Relationship Between the 3D Porosity and β-Phase Distributions and the Mechanical Properties of a High Pressure Die Cast AZ91 Mg Alloy

Currently, most magnesium lightweight components are fabricated by casting as this process is cost effective and allows forming parts with complex geometries and weak textures. However, cast microstructures are known to be heterogeneous and contain unpredictable porosity distributions, which give rise to a large variability in the mechanical properties. This work constitutes an attempt to correlate the microstructure and the mechanical behavior of a high pressure die cast (HPDC) Mg AZ91 alloy, aimed at facilitating process optimization. We have built a stairway-shaped die to fabricate alloy sections with different thicknesses and, thus, with a range of microstructures. The grain size distributions and the content of β-phase (Mg₁₇Al₁₂) were characterized by optical and electron microscopy techniques as well as by electron backscatter diffraction (EBSD). The bulk porosity distribution was measured by 3D computed X-ray microtomography. It was found that the through-thickness microhardness distribution is mostly related to the local area fraction of the β-phase and to the local area fraction of the pores. We correlate the tensile yield strength to the average pore size and the fracture strength and elongation to the bulk porosity volume fraction. We propose that this empirical approach might be extended to the estimation of mechanical properties in other HPDC Mg alloys.     

Estimating the Effective Metal-Mould Interfacial Heat Transfer Coefficient via Experimental-Simulated Cooling Curve Convergence

The design of improved casting systems requires accurate modeling of metal cooling processes. This can only be accomplished after determining the interfacial heat transfer coefficient (IHTC) between a solidifying casting and its mould. In the current work, a simple and robust inverse heat conduction technique was applied for the estimation of the effective IHTC between an aluminum alloy casting and a steel permanent mould during solidification. The solidification of the alloy at varying mould preheating temperatures was monitored using a thermocouple, and the experimental cooling curves were compared with curves simulated by casting solidification modeling software. The IHTC value applied to the software was varied until its output converged with the experimental data, leading to an estimation of 6000 W/m²K for this system. This technique is useful as a preliminary tool in materials modeling, and it will promote the development of improved casting processes without the need for excessive experimentation.     

Mechanisms of Soldering Formation on Coated Core Pins

Die soldering is one of the major casting defects during the high-pressure die casting (HPDC) process, causing dimensional inaccuracy of the castings and increased downtimes of the HPDC machine. In this study, we analyzed actually failed core pins to determine the mechanism of soldering and its procedures. The results show that the soldering process starts from a local coating failure, involves a series of intermetallic phase formation from reactions between molten aluminum alloys and the H13 steel pin, and accelerates when an aluminum-rich, face-centered cubic (fcc) phase is formed between the intermetallic phases. It is the formation of the aluminum-rich fcc phase in the reaction region that joins the core pin with the casting, resulting in the sticking of the casting to the core pin. When undercuts are formed on the core pin, the ejection of castings from the die will lead to either a core pin failure or damages to the casting being ejected.     

Formation of defect bands in high pressure die cast magnesium alloys

Die cast magnesium components are being increasingly used worldwide because of the excellent castability and properties that magnesium alloys offer. High pressure die casting of thin-walled components is particularly suitable because of the excellent flow characteristics of molten magnesium alloys. Typical automotive applications for thin-walled castings include components such as instrument panels, steering wheels, door frames and seat frames. These applications require optimisation of the quality and performance of the castings. It has been found that bands of porosity or segregation which follow contours parallel to the surface of the casting are formed under certain casting conditions in thin-walled magnesium high pressure die castings. The presence of this type of defect can have a significant effect on the mechanical properties. This paper investigates the effect of varied casting conditions on casting integrity and the appearance of the bands. A rationale for understanding the origin of these defects is related to the solidification behaviour, the mushy zone rheological properties and the filling pattern of the casting with associated shearing of the mushy zone. Methods to optimise the process parameters to control the occurrence of the banded defects, and thereby optimise the quality of high pressure die cast magnesium components, are outlined.     

Neutron diffraction measurement of strain required for the onset of hot tearing in AZ91D magnesium alloy

Magnesium alloy castings are increasingly used in automotive, aerospace and electronics industries. These castings are mainly produced via high-pressure die-casting (HPDC). During this casting process, molten alloy solidifies within a rigid mold, which resists the alloy’s volumetric contraction. As a result, thermal and mechanical stresses develop in the casting and potentially lead to the nucleation of hot tears.     

Melt-Conditioned, High-Pressure Die Casting of Mg–Zn–Y Alloy

The liquid Mg–Zn–Y alloy was conditioned by an application of high-intensive shearing with a pair of intermesh twin screws prior to high-pressure die casting (HPDC). Melt conditioning produces a uniform microstructure with fine grain size and high integrity. The microstructure was analyzed thoroughly, and the solidification characteristics of the melt-conditioned HPDC (MC-HPDC) structure were discussed. The enhancement in I-phase precipitation and the improvement in mechanical properties of MC-HPDC Mg–Zn–Y alloy can be achieved through cyclic annealing.     

Formation of Nondendritic Primary Aluminum Phase in Hypoeutectic Alloys in Controlled Diffusion Solidification (CDS): A Hypothesis

Controlled diffusion solidification (CDS) is a novel process wherein specific Al alloys can be cast by mixing two precursor alloys of specific compositions and temperature and subsequently casting the resultant mixture. This process enables a nondendritic morphology of the primary Al phase in the cast samples, which is beneficial in mitigating hot tearing tendencies and enabling castability of dilute Al (wrought) alloys to obtain castings with superior mechanical and performance properties. In this study, a hypothesis is proposed to describe the mechanism of the CDS process, specifically the processes of mixing two precursor alloys and a subsequent solidification process. Al – 4.5 wt pct Cu was used as an example alloy system to propose a hypothesis and to verify the various features in the mechanism of mixing two alloys. Experimental results show that the mixing process naturally causes copious nucleation of one of the alloys mixed and that the turbulence energy during mixing distributes these nuclei uniformly to enable a favorable solidification condition for a nondendritic cast microstructure. It is critical that the alloy with the higher thermal mass (mass and temperature) is mixed into the alloy with lower thermal mass to obtain a valid CDS process and that the reverse will not yield a favorable homogeneous cast sample. Certain critical parameters during the CDS process have also been identified and quantified for a favorable microstructure of the casting.     

高强韧铝合金轮毂的轻量化铸旋新工艺

综述了铸造铝合金的强韧化研究现状和最新进展,介绍了铝合金轮毂的铸旋成形新技术,分析了铸旋工艺在铝合金轮毂轻量化中的作用。分析表明热塑性形变韧化可成为A356合金强韧化的新途径,以此为基础发展的铸旋成形工艺可满足汽车轮毂进一步轻量化的要求。采用铸旋工艺成形的铝合金车轮轮辋部位的各项性能指标比低压铸造车轮有大幅提高,轮辋壁厚也可大幅度减薄,相同规格的车轮,采用铸旋工艺生产可减重5%～15%,实现了产品的高强度、轻量化要求,具有更轻的竞争力。之后重点介绍了铝合金轮毂铸坯热旋压工艺原理、A356合金的可旋性、工艺参数的选择及有限元分析在热旋压工艺设计中的应用,并指出存在的问题和所需做的进一步研究。结果表明铸造A356合金的热旋压可加工窗口较窄,成形温度控制是关键,为了促进铝合金轮毂铸旋工艺的广泛应用与发展,在铸造铝合金的热旋压变形性能、热旋压时金属的变形机理和流动行为,以及热旋工艺数值模拟和参数优化等方面还需要做大量的、深入系统的研究工作。     

Heat transfer and microstructure during the early stages of metal solidification

Transient heat transfer in the early stages of solidification of an alloy on a water-cooled chill and the consequent evolution of microstructure, quantified in terms of secondary dendrite arm spacing (SDAS), have been studied. Based on dip tests of the chill, instrumented with thermocouples, into Al-Si alloys, the influence of process variables such as mold surface roughness, mold material, metal superheat, alloy composition, and lubricant on heat transfer and cast structure has been determined. The heat flux between the solidifying metal and substrate, computed from measurements of transient temperature in the chill by the inverse heat-transfer technique, ranged from low values of 0.3 to 0.4 MW/m² to peak values of 0.95 to 2.0 MW/m². A onedimensional, implicit, finite-difference model was applied to compute heat-transfer coefficients, which ranged from 0.45 to 4.0 kW/m² °C, and local cooling rates of 10 °C/s to 100 °C/s near the chill surface, as well as growth of the solidifying shell. Near the chill surface, the SDAS varied from 12 to 22 (μm while at 20 mm from the chill, values of up to 80/smm were measured. Although the SDAS depended on the cooling rate and local solidification time, it was also found to be a direct function of the heat-transfer coefficient at distances very near to the casting/chill interface. A three-stage empirical heat-flux model based on the thermophysical properties of the mold and casting has been proposed for the simulation of the mold/casting boundary condition during solidification. The applicability of the various models proposed in the literature relating the SDAS to heat-transfer parameters has been evaluated and the extension of these models to continuous casting processes pursued.     

Study on Improving Solidification Structure of Rare Earth Al-Si Alloys with Electropulse Acting on Liquid Alloy

Electropulse modification (EPM) process, a new physical field method for improving the solidification structure of metals was introduced.Different from other research, EPM is only acting pulse current on melt under liquid state.The solidification structure of Al-Si alloys, A1-Cu alloys,cast iron and steel can be modified obviously with this method: the solidification structure of ZL101 alloy presented the Na and Sr modification and the mechanical properties were enhanced; a large number of primary silicon appeared in the microstructure of ZL109 alloy; the equiaxed grain zone was expanded and the grains were fined in Al-5.0wt% Cu alloy; the graphitization took place in solidification process of molten cast iron; the grain sizes of solidification structure of T8 steel were reduced significantly and the shape of steel pearlites also changed; the equiaxed grain zone increased to 88% from original untreated 19%, the equiaxed grains were fined and the intercrystalline crack was avoided in concasting billet by continuously treating liquid electrical sheet steel in tundish.Effects of rare earths on casting Al-Si alloys were also summarized.The method of modifying the solidification structure of rare earth Al-Si alloys with EPM in producing the alloys was proposed.     

Migration of crystals during the filling of semi-solid castings

In cold-chamber high-pressure die castings (HPDC), the microstructure consists of coarse externally solidified crystals (ESCs) that are commonly observed in the central region of cross sections. In the present work, controlled laboratory scale casting experiments have been conducted with particular emphasis on the flow and solidification conditions. An A356 aluminum alloy was used to produce castings by pouring semi-solid metal through a steel die. Microstructures similar to those encountered in HPDC have been produced and the resulting microstructure is found to depend on the melt and die temperature: (1) the fraction of ESCs determines the extent of migration to the central region; (2) a maximum packing determines the area fraction of ESCs in the center; and (3) the die temperature affects the position of the ESCs—a higher die temperature can induce a displaced ESC distribution. It is found that the migration of crystals to the central region requires a flow, which is constrained at all melt/die interfaces. Furthermore, potential lift mechanisms are discussed. An assessment of the Saffman lift force on individual particles shows it has no significant effect on the migration of ESCs.     

Developments in Solidification Processing of Functionally Graded Aluminium Alloys and Composites by Centrifugal Casting Technique

Centrifugal casting is one of the potential solidification processing techniques used for producing near-net shaped symmetrical cast components with improved properties. The emergence of new class of functionally graded materials has propelled this technique for the fabrication of engineering components and structures with graded property. The specific properties obtained by the use of functionally graded metal matrix composites (FGMMC) are high temperature surface wear resistance, surface friction and thermal properties, adjustable thermal mismatching, reduced interfacial stresses, increased adhesion at metal?Cceramic interface, minimized thermal stresses and increased fracture toughness and crack retardation. Among various processing techniques available for the fabrication of FGMMC, centrifugal casting has emerged as the simplest and cost effective technique for producing large size engineering components of FGMMC. The present paper gives an overview on the developments in use of centrifugal processing technique for processing various functionally graded aluminium alloys and composites. The influence of various process and solidification parameters on microstructure and properties of graded alloys and composites are described.     

Understanding the Initial Solidification Behavior for Al–Si Alloy in Cold Chamber High-Pressure Die Casting (CC-HPDC) Process Combining Experimental and Modeling Approach

Metallurgical and Materials Transactions A - In the cold chamber high-pressure die casting (CC-HPDC) process, alloy solidification in the shot sleeve due to heat loss leads to the formation of...