Analysis of the mechanism of die soldering in aluminum die casting

A mechanism of soldering of an aluminum alloy die casting to a steel die is proposed. A soldering critical temperature is postulated, at which iron begins to react with aluminum to form an aluminum-rich liquid phase and solid intermetallic compounds. The liquid joins the die with the casting upon solidification. The critical temperature is determined by the elements in both the casting alloy and the die material and is equal to the solidus temperature of the resulting alloy. The critical temperature is used to predict the onset of die soldering, and the local liquid fraction is related to the soldering tendency. Experiments have been carried out to validate the concept and to determine the critical temperature for die soldering in an iron-aluminum system. Thermodynamic calculations are used to determine the critical temperature and soldering tendency for the cases of pure aluminum and a 380 alloy in a steel mold. Factors affecting the soldering tendency are discussed, and methods for reducing die soldering are suggested.     

Die soldering: Mechanism of the interface reaction between molten aluminum alloy and tool steel

Die soldering is the result when molten aluminum sticks to the surface of the die material and remains there after the ejection of the part; it results in considerable economic and production losses in the casting industry, and is a major quality detractor. In order to alleviate or mitigate die soldering, one must have a thorough understanding of the mechanism by which the aluminum sticks to the die material. A key question is whether the die soldering reaction is diffusion controlled or interface controlled. A set of diffusion couple experiments between molten aluminum alloy and the ferrous die was carried out. The results of the diffusion couple experiments showed that soldering is a diffusional process. When aluminum comes in contact with the ferrous die material, the iron and the aluminum atoms diffuse into each other resulting in the formation of a series of intermetallic phases over the die material. Initially iron and aluminum react with each other to form binary iron-aluminum intermetallic phases. Subsequently, these phases react with the molten aluminum to further form ternary iron-aluminum-silicon intermetallic phases. Iron and aluminum have a great affinity for each other and the root cause of die soldering is the high reaction kinetics, which exists between iron and aluminum. Once the initial binary and ternary intermetallic phase layers are formed over the die material, the aluminum sticks to the die due to the abnormally low thermal conductivity of the intermetallic phases, and due to favorable interface energies between the intermetallic layers and aluminum. The experimental details, the results of the interface reactions, and the analysis leading to the establishment of the mechanism giving rise to die soldering are reviewed discussed.     

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.     

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.     

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.     

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.     

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 displace 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. H.I. LAUKLI, Research Scientist, formerly with the Department of Materials Technology, NTNU, N-7491 Trondheim, Norway     

Mechanical alloying of nb-al powders

The effect of mechanical alloying (MA) on solid solubility extension, nanostructure formation, amorphization, intermetallic compound formation, and the occurrence of a face-centered cubic (fcc) phase in the Nb-Al system has been studied. Solid solubility extension was observed in both the terminal compositions and intermetallic compounds: 15 pct Nb in Al and 60 pct Al in Nb, well beyond the equilibrium and even rapid solidification levels (2.4 pct Nb and 25 pct Al, respectively) and increased homogeneity range for the NbAl₃ phase. Nanostructured grains formed in all compositions. In the central part of the phase diagram, amorphization occurred predominantly. Only NbAl₃, the most stable intermetallic, formed during MA; in most cases, a subsequent anneal was required. On long milling time, an fcc phase, probably a nitride, formed as a result of contamination from the ambient atmosphere.     

Microstructure formation in high pressure die casting

In order to optimise the high pressure die casting (HPDC) process, more understanding of microstructure and defect formation is essential. This article gives an overview of the key microstructural features and the mechanisms of microstructure formation in this process. The incavity solidified grain size in HPDC can be as low as 10 μm, but externally solidified crystals (ESCs) as large as 200 μm are often also present. The eutectic microstructure is very fine with an interlamellar spacing around 500 nm. Bands of positive macrosegregation and sometimes with cracks/porosity, so-called defect bands, are also often observed. Intensification pressure (IP) is one of the major factors governing the porosity level in the casting. At high IP, defect bands form in the gate region and appear to be assisting the feeding during the intensification stage.     

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.     

Microstructural Evolution and Solidification Behavior of Al-Mg-Si Alloy in High-Pressure Die Casting

Microstructural evolution and solidification behavior of Al-5 wt pct Mg-1.5 wt pct Si-0.6 wt pct Mn-0.2 wt pct Ti alloy have been investigated using high-pressure die casting. Solidification commences with the formation of primary α-Al phase in the shot sleeve and is completed in the die cavity. The average size of dendrites and fragmented dendrites of the primary α-Al phase formed in the shot sleeve is 43 μm, and the globular primary α-Al grains formed inside the die cavity is at a size of 7.5 μm. Solidification inside the die cavity also forms the lamellar Al-Mg₂Si eutectic phase and the Fe-rich intermetallics. The size of the eutectic cells is about 10 μm, in which the lamellar α-Al phase is 0.41 μm thick. The Fe-rich intermetallic compound exhibits a compact morphology and is less than 2 μm with a composition of 1.62 at. pct Si, 3.94 at. pct Fe, and 2.31 at. pct Mn. A solute-enriched circular band is always observed parallel to the surface of the casting. The band zone separates the outer skin region from the central region of the casting. The solute concentration is consistent in the skin region and shows a general drop toward the center inside the band for Mg and Si. The peak of the solute enrichment in the band zone is much higher than the nominal composition of the alloy. The die casting exhibits a combination of brittle and ductile fracture. There is no significant difference on the fracture morphology in the three regions. The band zone is not significantly detrimental in terms of the fracture mechanism in the die casting. Calculations using the Mullins and Sekerka stability criterion reveal that the solidification of the primary α-Al phase inside the die cavity has been completed before the spherical α-Al globules begin to lose their stability, but the α-Al grains formed in the shot sleeve exceed the limit of spherical growth and therefore exhibit a dendritic morphology.     

锌合金压铸用二元复合水溶性盐芯的制备与性能

针对复杂内腔锌合金压铸件难清理及型芯强度要求高的问题,以高熔点盐氯化钾和低熔点盐硝酸钾组成混合盐为制芯材料,采用熔融重力浇注工艺制备了高强度二元复合水溶性盐芯.对比分析了氯化钾盐芯、硝酸钾盐芯和20% KCl-80% KNO₃二元复合水溶性盐芯的性能特征.采用扫描电镜和X射线衍射等测试方法分析了水溶性盐芯的微观形貌与物相组成.研究结果表明:与单质盐芯相比,二元复合盐芯综合性能更佳,其表面基本无裂纹与褶皱,抗弯强度可达21.2 MPa,24 h吸湿率为0.568%,80℃的水溶速率可达208.63 kg·min-1·m-3.复合盐芯在裂纹扩展时走向发生了偏转,这是复合盐芯抗弯强度提高的主要原因.      

Deformation and fracture in Al−CuAl2 eutectic composites

Ambient temperature compressive stress-strain behavior to failure, and associated structural detail, have been characterized in Al?CuAl₂ composites of small interlamellar spacing (≤2μ). Differences in the compressive and tensile yield stress levels of the composite are attributed to thermally induced residual stress. Analysis gives a residual tensile stress ～3500 psi and anin-situ yield stress ～13,500 psi in the aluminum-rich phase. Evidence for a dislocation-interface interaction is provided by the form of deformation substructure in the aluminum-rich phase. Failure in these multi-grained eutectic composites is shown to be controlled primarily by shear-mode buckling of the lamellar structure. Buckling leads to cleavage of the CuAl₂ phase, shear in the aluminum-rich phase, accompanied by void formation, coalescence and crack formation.     

数值模拟在旋转芯轴压铸工艺优化中的运用

用数值模拟软件ProCAST对汽车安全带旋转芯轴压铸件原工艺方案的充型过程进行模拟并预测可能产生缩孔、缩松的位置,发现在压铸件的两端出现了缩松。通过在铸件两端增设溢流槽的工艺方案后,模拟结果表明铸件中消除缩松,保证了铸件的质量,为该压铸件模具设计提供了优化的工艺设计方案。     

Effect of manganese on sintering processes in the Ti-Fe system. III. High-temperature x-ray diffraction investigation of the sintering process

Conclusions The temperatures of accelerated formation of intermetallic compounds in the Ti-Fe-Mn system correspond to the temperatures of sharp changes in the direction and rate of the sintering process and also of the appearance of heat evolution peaks. The rate of heating of compacts to sintering temperatures affects the mechanism of formation of intermetallic compounds: At lower rates of heating intermetallic compounds are formed by solid-phase reaction, while at higher rates the role of liquid phase grows. A relation has been established between the composition of the intermetallic compounds forming during sintering and the behavior of specimens during this process at various heating rates. At higher rates of heating of Ti-Fe-Mn compacts Ti-Fe intermetallic compounds are formed containing no Mn; the behavior of these specimens during sintering is similar to that of Ti-Fe compacts (the specimens grow). At lower heating rates intermetallic compounds alloyed with Mn are formed, and growth of specimens is followed by their shrinkage.Translated from Poroshkovaya Metallurgiya, No. 4(304), pp. 23–28, April, 1988.     

7A52合金铸锭漏斗底结物形成原因

7A52合金铸锭时,分液漏斗底部常产生底结物。通过化学及SEM-EDS分析,发现底结物中Mn、Cr、Ti元素浓度分别超正常值约5倍、10倍、15倍。XRD分析表明其主要化合物相组成为Al7Cr、MnAl6、TiAl3。结果表明,底结物产生和形成原因主要与分配漏斗与液穴底的间距、分配漏斗的材质、铝熔体中的杂质等因素有关,同时也与铸造温度、分配漏斗预热等工艺条件有关。根据分析结果,提出了提高铸造速度、改变分配漏斗尺寸、对分配漏斗进行喷涂和预热等消除底结物的具体措施,并成功应用于生产实践。     

The Effects of Microstructure Heterogeneities and Casting Defects on the Mechanical Properties of High-Pressure Die-Cast AlSi9Cu3(Fe) Alloys

Detailed investigations of the salient microstructural features and casting defects of the high-pressure die-cast (HPDC) AlSi9Cu3(Fe) alloy are reported. These characteristics are addressed to the mechanical properties and reliability of separate HPDC tensile bars. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes throughout the tensile specimen. The results indicate that the die-cast microstructure consists of several microstructural heterogeneities such as positive eutectic segregation bands, externally solidified crystals (ESCs), cold flakes, primary Fe-rich intermetallics (sludge), and porosities. In addition, it results that sludge particles, gas porosity, as well as ESCs, and cold flakes are concentrated toward the casting center while low porosity and fine-grained structure is observed on the surface layer of the castings bars. The local variation of the hardness along the cross section as well as the change of tensile test results as a function of gage diameter of the tensile bars seem to be ascribed to the change of porosity content, eutectic fraction, and amount of sludge. Further, this behavior reflects upon the reliability of the die-cast alloy, as evidenced by the Weibull statistics.