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.     

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.     

Intermetallic compound layer growth at the interface of solid refractory metals molybdenum and niobium with molten aluminum

The growth mechanisms and growth kinetics of intermetallic phases formed between the solid refractory metals Mo and Nb and molten aluminum have been studied for contact times ranging from 1 to 180 minutes at various temperatures in the range from 700 to 1100°C. The growth of the layers of the resulting intermetallic phases has been investigated under static conditions in a saturated melt and under dynamic conditions using forced convection in unsatured aluminum melts. The Nb/Al interfacial microstructure consisted of a single intermetallic phase layer, Al₃Nb, whereas two to four different phase layers were observed in the Mo/Al interface region, depending upon the operating temperature. It was found that, in a satured melt, the intermetallic phase growth process was diffusion-controlled. The parabolic growth constants of the first and second kind and integral values of the chemical diffusion coefficients over the widths of the phases were calculated for both Mo/Al and Nb/Al systems. It also was found that the AlNb₂ phase grew between the Nb and Al₃Nb phases after consumption of the saturated Al phase. Similarly, the AlMo₃ phase grew between the Mo and Al₈Mo₃ phases with diminishing of all the other existing compound phases. In an unsaturated melt, the intermetallic phase layer grows at the solid surface while, simultaneously, dissolution occurs at the solid/liquid interface. This behavior is compared to the growth mechanisms proposed in existing theories, taking into consideration that interaction occurs between neighboring phases. It was found that the intermetallic phase, Al₈Mo₃, adjoining the base metal, was not bonded strongly to the base metal Mo and was brittle; its hardness also was larger than that of the layer near the adhering aluminum and the adjacent phases.     

Experimental Investigation of the Ce-Mg-Mn Isothermal Section at 723 K (450 °C) via Diffusion Couples Technique

The isothermal section of the Ce-Mg-Mn phase diagram at 723 K (450 °C) was established experimentally by means of diffusion couples and key alloys. The phase relationships in the complete composition range were determined based on six solid–solid diffusion couples and twelve annealed key alloys. No ternary compounds were found in the Ce-Mg-Mn system at 723 K (450 °C). X-ray diffraction and energy-dispersive X-ray spectroscopy spot analyses were used for phase identification. EDS line-scans, across the diffusion layers, were performed to determine the binary and ternary homogeneity ranges. Mn was observed in the diffusion couples and key alloys microstructures as either a solute element in the Ce-Mg compounds or as a pure element, because it has no tendency to form intermetallic compounds with either Ce or Mg. The fast at. interdiffusion of Ce and Mg produces several binary compounds (Ce _x Mg _y ) during the diffusion process. Thus, the diffusion layers formed in the ternary diffusion couples were similar to those in the Ce-Mg binary diffusion couples, except that the ternary diffusion couples contain layers of Ce-Mg compounds that dissolve certain amount of Mn. Also, the ternary diffusion couples showed layers containing islands of pure Mn distributed in most diffusion zones. As a result, the phase boundary lines were pointing toward Mn-rich corner, which supports the tendency of Mn to be in equilibrium with all the phases in the system.     

Reaction mechanisms for the coatings formed during the hot dipping of iron in 0 to 10 Pct Al-Zn baths at 450° to 700°C

The reaction mechanisms and the structures of the phases formed during the hot dipping of iron in 0 to 10 pct Al-Zn alloy baths at temperatures of 450° to 700°C were studied by X-ray diffraction and electron microprobe analysis techniques. A new mechanism for the inhibition reaction between iron and zinc is proposed. At bath temperatures below 600°C, a thin layer of an Fe-Al-Zn ternary compound forms on the iron surface and inhibits the growth of Fe-Zn phases. Breakdown of inhibition occurs during the dipping process when the ternary compound becomes rich in aluminum and transforms to a more stable structure which is isomorphous with Fe₂Al₅. While this breakdown is occurring, the zinc atoms react with iron and form the conventional Fe-Zn phases. In 1 to 10 pct Al-Zn baths at temperatures≥600°C a very violent, highly exothermic reaction occurs during hot dipping. This type of process is due to the electronic bond rearrangements which occur during the formation of the intermetallic Fe₂(AlZn)₅. This intermetallic forms from the reaction of aluminum-bearing FeZn₇ with the Zn-Al alloy bath.     

Corrosion of Mo,Nb, Cr,and Y in molten aluminum

The corrosion (dissolution) kinetics of solid molybdenum, niobium, chromium, and yttrium in molten aluminum were investigated at temperatures between 700 °C and 915 °C under hydrodynamic conditions using the rotating disc method. Dissolution was governed by diffusion under laminar flow conditions (in the angular velocity range of 10 to 32 rad/s) regardless of the number of intermetallic compound layers formed at the solid-liquid interface. The solubility limit (C _S ), dissolutíon rate constant(K), and diffusion coefficient(D) were determined. It was found that temperature dependencies of the solubility, dissolution rate, and diffusion coefficient for each system obeyed Arrhenius-type relationships; from these, the activation energies were calculated. The single or multiphase intermetallic layer growth occurring at the solid-liquid interface during dissolution in an unsaturated melt between 700 °C and 915 °C was characterized.     

On the wear of a cobalt-based superalloy in zinc baths

Journal bearings made of a cobalt-based superalloy, trade marked STELLITE 6, were evaluated in zinc baths with and without the presence of aluminum and iron. The sleeve and the bushing wore evenly when tested in a pure zinc bath. The surfaces were generally smooth and covered by some rather fine grooves after the test. The wear of the bearings was much more severe when tested in aluminum-containing zinc baths. The worn surfaces of the bearings were fully covered by wide and deep grooves. The coefficient of friction of the superalloy generally increased with increases in the aluminum content of the molten zinc. The iron addition to the molten zinc appeared to affect the friction and wear characteristics of the superalloy. Detailed metallographic and microchemical analyses were performed to elucidate the wear mechanisms. The superalloy was found to react readily with the molten zinc alloys to form intermetallic compounds. In a pure zinc bath, zinc-based intermetallic compounds formed on the bearing surfaces; in baths containing aluminum, a compact intermetallic layer, based on the cobalt-aluminide phase, was detected on the bearing surfaces. Evidence collected in this study indicated that wear debris reacted with aluminum in the molten zinc and transformed itself into hard and abrasive cobalt-aluminide particles. During the test, these particles reattached to the bearing surfaces and built up. Acting as blunt microcutters, these particles plowed the bearing surfaces and created deep grooves. The dominant wear mechanism in the aluminum-containing zinc baths was identified as abrasion; fatigue and corrosive wear, however, served as precursors of the dominant wear mechanism.     

Iron-rich intermetallic phases and their role in casting defect formation in hypoeutectic Al−Si alloys

Iron is the most common and detrimental impurity in aluminum casting alloys and has long been associated with an increase in casting defects. While the negative effects of iron are clear, the mechanism involved is not fully understood. It is generally believed to be associated with the formation of Fe-rich intermetallic phases. Many factors, including alloy composition, melt superheating, Sr modification, cooling rate, and oxide bifilms, could play a role. In the present investigation, the interactions between iron and each individual element commonly present in aluminum casting alloys, were investigated using a combination of thermal analysis and interrupted quenching tests. The Fe-rich intermetallic phases were characterized using optical microscope, scanning electron microscope, and electron probe microanalysis (EPMA), and the results were compared with the predictions by Thermocalc. It was found that increasing the iron content changes the precipitation sequence of the β phase, leading to the precipitation of coarse binary β platelets at a higher temperature. In contrast, manganese, silicon, and strontium appear to suppress the coarse binary β platelets, and Mn further promotes the formation of a more compact and less harmful α phase. They are therefore expected to reduce the negative effects of the β phase. While reported in the literature, no effect of P on the amount of β platelets was observed. Finally, attempts are made to correlate the Fe-rich intermetallic phases to the formation of casting defects. The role of the β phase as a nucleation site for eutectic Si and the role of the oxide bifilms and AlP as a heterogeneous substrate of Fe intermetallics are also discussed.     

Thermodynamic and kinetic study of diffusion paths in the system Cu-Fe-Ni

An understanding and modeling of diffusion paths in ternary systems requires a combined thermodynamic and diffusion kinetic approach. The driving force for the intrinsic diffusion of each component is the gradient of the chemical potential (or activity) which can be calculated from the concentration profile if the thermodynamic properties of the system are known. For studying the diffusion behavior in ternary metallic systems, the Cu-Fe-Ni-system was chosen because of its experimental and thermodynamic simplicity. Concentration profiles and diffusion paths in single-phase areas and across an α/β interface were studied experimentally at 1000 °C using the diffusion couple technique. Coefficients for interdiffusion and tracer diffusion have been calculated at the intersection points of two independent diffusion paths with a common composition. A concentration dependence for the tracer diffusion coefficients for each component was calculated and found to be consistent with the literature data in the binary Cu-Ni and Fe-Ni systems. The calculated vacancy flux in the couples was consistent with the experimentally observed marker shift.     

渗铝液Fe含量对镶圈铝合金活塞铝铁结合区的影响

研究不同Fe含量的渗铝液成分对镶嵌活塞铁铝粘结层的组织及性能的影响,采用金相显微镜及扫描电镜观察结合区的组织形貌,采用显微硬度计及压力机测定结合区的硬度和粘接强度。结果表明:活塞铝铁结合区的形成包括铝原子向铸铁环的扩散及铁原子向铝基体的溶解过程;结合区主要由Fe2Al5相和FeAl3相组成;渗铝液中铁含量增加时,铝铁结合层厚度明显下降;结合区硬度显著高于高镍铸铁环及铝合金活塞基体,且随渗铝液铁含量增加,结合区硬度较高;结合层粘接强度随渗铝液铁含量的增加,呈先增后减的趋势。     

Effect of powder particle size on the growth of titanium compacts during liquid-phase sintering with aluminum

Conclusions In the liquid-phase sintering of a Ti+30 at. % Al powder mixture compact growth increases with increasing mean particle size. The volume of a compact from such a mixture exhibits an anomalous increase (surpassing its growth due to Kirkendall flow during unipolar diffusion of aluminum atoms from the molten phase to the titanium). The increase is attributable to growth of layers of an intermetallic compound, which sets up a stress in the particles disturbing the continuity of their material. A fall in the density of particles linked with the disturbance of the continuity of their material is characteristic of particles exceeding in size a certain critical value — in the case under consideration, about 45m. Smaller particles grow only as a result of Kirkendall flow, whose extent is determined by the concentration of aluminum in the mixture. The reason why no disturbance in the continuity of material occurs in fine particles is apparently that the intermetallic compound layers forming on them are very thin, so that the stress set up in the particles does not reach a level sufficient for the initiation and growth of discontinuities.Translated from Poroshkovaya Metallurgiya, No. 9 (225), pp. 33–37, September, 1981.     

钢包耐火材料与钢液反应的研究现状

随着国家工业的快速发展,对高品质钢的需求增大,侵蚀的耐火材料作为钢中外来夹杂物的主要来源受到广泛关注.基于此,阐述了耐火材料与钢液的反应机制、不同耐火材料与钢液的相互作用以及对钢液质量造成的影响.耐火材料会向钢中溶解并与钢中成分发生反应,之后形成界面层,当界面层是高熔点物质时,会阻碍耐火材料的溶解扩散,当界面层是低熔点...     

Partial Redetermination of the Fe-W Phase Diagram

The objective of the current study is to perform a careful investigation of the (Fe) and (W) solvus and of the intermetallic compounds in the Fe-W system as it could have a significant impact on higher-order systems based on this binary system. Two key alloys, Fe-14 at. pct W and Fe-50 at. pct W, and a diffusion couple were synthesized based on the literature. They were studied by long time annealing experiments with a selected heat treatment route, X-Ray diffraction analysis, and electron microprobe analysis with a careful control of impurity levels of C, Si, and O. The two intermetallic phases μ-Fe₇W₆ and λ-Fe₂W are characterized and the composition range of the μ-Fe₇W₆ phase is specified. The phase previously reported as FeW is most probably a ternary carbide with low carbon content. An accurate determination of the solubility of W in αFe and of Fe in αW is presented.     

Effects of Applied Strain on Interface Microstructure and Interdiffusion in the Diffusion Couples

The effects of applied strain on the interface microstructure and atomic interdiffusion in the binary alloy diffusion couples were studied using the phase-field model. In the two-phase diffusion couples, the single-phase regions are formed beside the interface without applied strain, and the width of single-phase regions enlarges as temperature increases. When the strain is applied, the phases are elongated and they are across the initial interface, which makes the diffusion couples to syncretize as the temperature increases or concentration difference decreases. In the diffusion couples formed by single and two phases, the larger composition difference results in the larger movement distance of interface, the atomic diffusion direction is determined by the initial composition difference. Under the applied strain, the elongated two phases are also across the initial interface with the small concentration difference. However, when the concentration difference is large, the two-phase region is recessional as the single-phase region moves forward. When the applied strain makes the morphology parallel to the initial interface of the diffusion couple, the single-phase regions are formed beside the interface.     

Iron-rich intermetallic phases and their role in casting defect formation in hypoeutectic Al-Si alloys

Iron is the most common and detrimental impurity in aluminum casting alloys and has long been associated with an increase in casting defects. While the negative effects of iron are clear, the mechanism involved is not fully understood. It is generally believed to be associated with the formation of Fe-rich intermetallic phases. Many factors, including alloy composition, melt superheating, Sr modification, cooling rate, and oxide bifilms, could play a role. In the present investigation, the interactions between iron and each individual element commonly present in aluminum casting alloys, were investigated using a combination of thermal analysis and interrupted quenching tests. The Fe-rich intermetallic phases were characterized using optical microscope, scanning electron microscope, and electron probe microanalysis (EPMA), and the results were compared with the predictions by Thermocalc. It was found that increasing the iron content changes the precipitation sequence of the β phase, leading to the precipitation of coarse binary β platelets at a higher temperature. In contrast, manganese, silicon, and strontium appear to suppress the coarse binary β platelets, and Mn further promotes the formation of a more compact and less harmful α phase. They are therefore expected to reduce the negative effects of the β phase. While reported in the literature, no effect of P on the amount of β platelets was observed. Finally, attempts are made to correlate the Fe-rich intermetallic phases to the formation of casting defects. The role of the β phase as a nucleation site for eutectic Si and the role of the oxide bifilms and AlP as a heterogeneous substrate of Fe intermetallics are also discussed.     

Discussion of “Interfacial layer in coatings produced in molten Zn-Al eutectoid alloy containing Si”

Studies of the interfacial layer in coatings produced in a molten Zn-22.3 pct Al-0.4 pct Si alloy revealed a double layer with an Fe₂Al₈Si sublayer on the top and an Fe₂Al₅ sublayer on the bottom. This suggests that a diffusion path is established during the hot-dipping stage, starting from the liquid phase and passing through the phase fields of the ternary inhibition compound of Fe₂Al₈Si and all binary Fe-Al intermetallic compounds available in the Al-Fe system. Thermodynamic calculations of the free energies of the relevant phases have confirmed the feasibility of such a diffusion path. M. RANJAN, R. TEWARI, W.J. VAN OOIJ, and V.K. VASUDEVAN: Metall. Mater. Trans. A, 2004, vol. 35A, pp. 3707–20.     

Mg对Zn--11%Al合金镀层凝固组织及合金层生长的影响

将工业纯铁分别在510℃的Zn-11% Al、Zn-11% Al-1.5% Mg、Zn-11% Al-3% Mg和Zn-11% Al-4.5% Mg合金熔池中进行不同时间的热浸镀,使用X射线衍射仪、扫描电子显微镜、能谱仪等仪器设备,研究Mg含量对Zn-11% Al合金镀层凝固组织和镀层中Fe-Al合金层生长的影响.结果表明:Zn-11% Al合金镀层凝固组织由富Al相和Zn/Al二元共晶组成;随着Zn-11% Al-x% Mg合金中Mg含量的增加,合金镀层的凝固组织中逐渐出现Zn/Al/MgZn₂三元共晶、块状MgZn₂相和Al/MgZn₂二元共晶.四种合金镀层中合金层主要由Fe₂Al₅Zn_x和FeAl₃Zn_x相组成,合金层的厚度随浸镀时间的增加而增加,Mg含量的增加使Fe-Al合金层生长速率指数和生长速率降低.在Zn-11% Al合金镀层中Fe-Al合金层形成的初期,可形成致密稳定的Fe-Al化合物层;热浸镀120 s后,扩散通道的移动使Fe-Al化合物层失稳破裂.Zn-11% Al-x% Mg合金中Mg元素可明显推迟液Zn进入镀层中Fe-Al合金层的时间,使Fe-Al合金层更加稳定和致密.      

Impurity controlled diffusion reaction in thorium-vanadium system

The effect of ppm level impurities present in the thorium metal on the diffusion reaction in thorium-vanadium system has been investigated by making “sandwich” type diffusion couples and annealing them in the temperature range of 900 to 1200°C. The microstructure of the diffusion zone has been examined by optical and electron microscopy. X-ray powder techniques have been employed to detect the presence of secondary intermetallic phases in thorium metal and in the diffusion zone. The chemical composition of these phases and the elemental distribution in the diffusion zone have been established by using an electron microprobe analyzer. The time and the temperature dependence of the diffusion zone width is used to evaluate the kinetics of layer growth. Present studies have shown that impurities like iron, copper and aluminum form low melting point intermetallic compounds in thorium metal and segregate at the grain boundaries. Further in a diffusion reaction at the elevated temperatures, these thorium rich secondary phases melt and form a liquid film between the thorium and the bonding metal, vanadium in the present case. The rate controlling process appears to be an interfacial chemical reaction involving dissolution of vanadium in the thorium rich liquid.     

The nitriding behavior of Ti-Al alloys at 1000 °C

The nitriding behavior of a series of alloys in the binary Ti-Al system has been determined at 1000 °C, under a controlled atmosphere of pure nitrogen gas, for times ranging between 7 and 100 hours. The scales and subscales were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive and wavelength dispersive X-ray analysis, electron energy loss spectroscopy, and optical microscopy. Upon formation of a surface nitride scale, the subscale became enriched in Al and resulted in the formation of a series of Al-rich intermetallic phases. This enrichment has been linked to the transport processes in the scale and subscale and a shifting of the diffusion path toward the Al-rich corner of the ternary isotherm. The formation of Al-rich intermetallic phases in the subscale was shown to result in rapid “breakaway” nitriding of the TiAl and TiAl₂ alloys. The stoichiometry of the binary nitrides AlN and TiN was measured, as well as the composition of the ternary nitride “Ti₂AlN.”