Influence of melting conditions of aluminum alloys on the properties and quality of castings obtained by lost foam casting

The development of modern foundry production is characterized by a constant increase in requirements for the quality of fabricated casting and rational use of material resources, which determines the search for new technical and process solutions, making it possible to acquire the required properties of cast wares along with resource saving. Herewith, the question of revelation and investigation into the regularities of the influence of thermal-temporal parameters of smelting and pouring of aluminum alloys into the casting mold during the lost foam casting on tightness and mechanical and qualitative characteristics of thin-wall castings remain poorly known and complex for implementation, especially allowing for the performance of resourcesaving measures. In this publication, the influence of process parameters of smelting on the strength, tightness, and content of nonmetallic inclusions in castings of the gas-analyzer case made of AK7 alloy during the lost foam casting is considered. The data set acquired based on the experimental investigations has been subjected to statistical processing. The use of statistic models makes it possible to acquire the results of the influence of the holding time and content of secondary materials in the charge on strength and tightness of mentioned castings. The results of an investigation into the influence of holding the AK7 melt at the overheating temperature of 880–890°С on the content of nonmetallic inclusions in castings show that it can be regulated varying the holding time. This procedure decreases the melt microinhomogeneity and provides the acquisition of numerous castings with a minimal content of nonmetallic inclusions.     

Effects of Thermalrate Treatment Technique on Microstructure and Mechanical Properties of Hypoeutectic Al–Si Alloys

In this study, effects of thermalrate treatment (TRT) technique on microstructure and mechanical properties of hypoeutectic Al–Si alloys with addition of Ti were studied. The superheating temperatures of the melt were ascertained based on the DSC result. The results show that when the alloy castings in sand mold were treated with TRT technique at the superheating temperature of 930 °C, α-Al changes into smaller equiaxial crystals from coarse dendrites, and hardness of the alloy increases by 12.7 %, compared to that of the alloy treated with conventional casting technique. In addition, the supercooling increases to 8.5 °C and the characteristic temperatures of eutectic solidification are all the lowest with TRT technique at the superheating temperature of 930 °C. As holding time increases at the pouring temperature of 730 °C in TRT at the superheating temperature of 930 °C, the effects on microstructure and mechanical properties of the alloy casting in sand mold decrease. TRT technique plays a limited role in the alloy casting in permanent mold.     

Determination of interface heat transfer coefficient between aluminum casting and graphite mold

To produce castings of titanium, nickel, copper, aluminum, and zinc alloys, graphite molds can be used, which makes it possible to provide a high cooling rate. No die coating and lubricant are required in this case. Computer simulation of casting into graphite molds requires knowledge of the thermal properties of the poured alloy and graphite. In addition, in order to attain adequate simulation results, a series of boundary conditions such as heat transfer coefficients should be determined. The most important ones are the interface heat transfer coefficient between the casting and the mold, the heat transfer coefficient between the mold parts, and the interface heat transfer coefficient into the environment. In this study, the interface heat transfer coefficient h between the cylindrical aluminum (99.99%) casting and the mold made of block graphite of the GMZ (low ash graphite) grade was determined. The mold was produced by milling using a CNC milling machine. The interface heat transfer coefficient was found by minimizing the error function reflecting the difference between the experimental and simulated temperatures in a mold and in a casting during pouring, solidification, and cooling of the casting. The dependences of the interface heat transfer coefficient between aluminum and graphite on the casting surface temperature and time passed from the beginning of pouring are obtained. It is established that, at temperatures of the metal surface contacting with a mold of 1000, 660, 619, and 190°C, the h is 1100, 4700, 700, and 100 W/(m² K), respectively; i.e., when cooling the melt from 1000°C (pouring temperature) to 660°C (aluminum melting point), the h rises from 1100 to 4700 W/(m² K), and after forming the metal solid skin on the mold surface and decreasing its temperature, the h decreases. In our opinion, a decrease in the interface heat transfer coefficient at casting surface temperatures lower than 660°C is associated with the air gap formation between the surfaces of the mold and the casting because of the linear shrinkage of the latter. The heat transfer coefficient between mold parts (graphite–graphite) is constant, being 1000 W/(m² K). The heat transfer coefficient of graphite into air is 12 W/(m² K) at a mold surface temperature up to 600°C.     

Influence of crystallization conditions on the structure and modifying ability of Al–Sc alloys

The influence of the modes of thermal-and-temporal treatment and cooling rate of metallic alloys on crystallization regularities of Al–Sc alloys and their structure, properties, and modifying ability are established. Castings of Al–Sc alloy, which were prepared by the electrolysis of salt melts KF–NaF–AlF₃–Sc₂O₃ at 820–850°C, are used as the initial charge for casting. It is established that, by varying the magnitude of melt overheating and casting temperature, it is possible to vary the crystal shape, amount, and size in wide limits. The modifying action of cast and rapidly quenched master alloys, as well as the master alloy produced by electrolysis, is tested for Al–4.5% Cu alloy. The largest effect of milling the structure of the Al–4.5% Cu–0.4% Sc alloy is attained when using the rapidly quenched master alloy.     

Mathematical modeling of particle segregation during centrifugal casting of metal matrix composites

A one-dimensional transient heat-transfer model coupled with an equation for force balance on particles is developed to predict the particle segregation pattern in a centrifugally cast product, temperature distribution in the casting and the mold, and time for complete solidification. The force balance equation contains a repulsive force term for the particles that are in the vicinity of the solid/liquid interface. The solution of the model equations has been obtained by the pure implicit finite volume technique with modified variable time-step approach. It is seen that for a given set of operating conditions, the thickness of the particle-rich region in the composite decreases with an increase in rotational speed, particle size, relative density difference between particles and melt, initial pouring temperature, and initial mold temperature. With reduced heat-transfer coefficient at the casting/mold interface, the solidification time increases, which, in turn, results in more intense segregation of solid particulates. Again, with increased initial volume fraction of the solid particulates in the melt, both the solidification time and the final thickness of the particulate-rich region increase. It is noted that for Al-Al₂O₃ and Al-SiC systems, in castings produced using finer particles, lower rotational speeds, and an enhanced heat-transfer coefficient at the casting/mold interface, the volume fraction of particles in the outer layer of the casting remains more or less the same as in the initial melt. However, for castings produced with coarser particles at higher rotational speeds and reduced heat-transfer coefficients at the casting/mold interface, intense segregation is predicted even at the outer periphery of the casting. In the case of the Al-Gr system, however, intense segregation is predicted at the innermost layers.     

AZ91合金消失模铸造充型过程研究

利用电触点法研究TT艺参数对消失模铸造AZ91合金的流动形态和流动速度的影响．结果表明,AZ91合金熔液以凸弧形平稳地向前逐层置换泡沫,这是由于裂解产物在流动前沿尤其模样边缘处的大量堆积造成的．充型时间随浇注温度、真空度的提高而变短．充型过程中充型速度呈现交替变化．以熔融金属作为振子,熔体和泡沫模之间的气层作为弹簧建立弹簧一振子模型,可以诠释充型速度的交替变化．     

The air-gap formation process at the casting-mold interface and the heat transfer mechanism through the gap

The formation process of the air gap at the casting-mold interface and the heat transfer mechanism through the gap were investigated by measuring the displacement of, and the temperature in casting and mold for cylindrical and flat castings of aluminum alloys. The thickness of the air gap was measured as the difference between the location of the casting surface and that of the mold inner surface. For cylindrical castings, the mold began to move outward immediately after pouring, while the casting stayed until solidification progressed to a great extent. For flat castings, the mold began to move greatly toward the casting pushing the casting immediately after pouring and moved reversely after a maximum appeared. It was possible to calculate the displacement of the mold by thermal expansion. It was found that when the thickness of the air gap was not large, the heat through the gap was transferred mainly by heat conduction.     

Solidification microstructure and temperature field during direct chill casting of Al-16Si alloy

By properly controlling casting parameters such as pouring temperature, casting velocity and water flux, direct chill (DC) casting can be employed to produce refined microstructure in the hypereutectic Al-Si alloys without chemical modification. This refined microstructure is characterized of fine primary Si particles, fully developed dendritic Al halos and fine coupled eutectics. In this work, in situ measurements of temperature field in the mould during DC casting of Al-16Si alloy at casting velocity of 2.17 mm/s, 3.5 mm/s and 4.34 mm/s at a pouring temperature of 800°C were performed. The results show that the primary Si phase nucleated at considerable undercooling (about 27°C to 38°C) and the growth temperature of dendritic Al halos was 7°C to 8°C below the equilibrium eutectic temperature. In the center regions of the DC cast billet, halos are fully developed because the G_l/R value is low.     

Determination of the heat-transfer coefficient between the AK7ch (A356) alloy casting and no-bake mold

The heat-transfer coefficient h between a cylindrical cast made of AK7ch (A356) aluminum alloy and a no-bake mold based on a furan binder is determined via minimizing the error function, which reflects the difference between the experimental and calculated temperatures in the mold during pouring, solidification, and cooling. The heat-transfer coefficient is h _L = 900 W/(m² K) above the liquidus temperature (617°C) and h _S = 600 W/(m² K) below the alloy solidus temperature (556°C). The variation in the heat-transfer coefficient in ranges h _L = 900–1200 W/(m² K) (above the alloy liquidus temperature) and h _S = 500–900 W/(m² K) (below the solidus temperature) barely affects the error function, which remains at ~22°C. It is shown that it is admissible to use a simplified approach when constant h = 500 W/(m² K) is specified, which leads to an error of 23.8°C. By the example of cylindrical casting, it is experimentally confirmed that the heat-transfer coefficient varies over the casting height according to the difference in the metallostatic pressure, which affects the casting solid skin during its solidification; this leads to a closer contact of metal and mold at the casting bottom.     

Concept and present state of a coal-based smelting reduction process for iron ore fines

The concept of a two-stage smelting reduction process is presented. In the first stage highly metallized iron ore fines are produced in a circulating fluidized bed. In the second stage a hot metal is produced in a melter-gasifier where – together with metallized ore – coal and oxygen are injected to generate the required heat and the CO-rich reducing gas. The process was tested stepwise in pilot scale installations. Although only a reduction temperature of 830 °C instead of the required 880–900 °C could be realized in the pilot unit, test results make it very probable that a metallization of 90% can be reached with any fine ore without sticking problems, if the ore is covered with a carbon layer by CO decomposition in a pretreatment stage with the reduction offgas at 500–600 °C. The CO decomposition on the fresh ore leads to a high gas utilization which renders a CO₂ washing stage and gas recycling unnecessary. To prove the technical and economic feasibility of the combined process, the next development step should be the design and operation of a larger pilot plant with a capacity of at least 5 t hot metal/h for continuous and joint operation of both the melter/gasifier and the reduction stage.     

Reducing the melt heating in the mold to enhance rail-steel quality

The macrostructure of continuous-cast high-carbon steel (in particular, rail steel) billet may be improved by introducing macrocooling additives to prevent overheating of the melt in the casting-machine mold. The liquidus temperature of the macrocooling additives is 5–20°C lower than for the cast steel. Macrocooling strip is alloyed with boron, which ensures additional modification of the rail steel.     

Prediction of the As-Cast Structure of Al-4.0 Wt Pct Cu Ingots

A two-stage simulation strategy is proposed to predict the as-cast structure. During the first stage, a 3-phase model is used to simulate the mold-filling process by considering the nucleation, the initial growth of globular equiaxed crystals and the transport of the crystals. The three considered phases are the melt, air and globular equiaxed crystals. In the second stage, a 5-phase mixed columnar-equiaxed solidification model is used to simulate the formation of the as-cast structure including the distinct columnar and equiaxed zones, columnar-to-equiaxed transition, grain size distribution, macrosegregation, etc. The five considered phases are the extradendritic melt, the solid dendrite, the interdendritic melt inside the equiaxed grains, the solid dendrite, and the interdendritic melt inside the columnar grains. The extra- and interdendritic melts are treated as separate phases. In order to validate the above strategy, laboratory ingots (Al-4.0 wt pct Cu) are poured and analyzed, and a good agreement with the numerical predictions is achieved. The origin of the equiaxed crystals by the “big-bang” theory is verified to play a key role in the formation of the as-cast structure, especially for the castings poured at a low pouring temperature. A single-stage approach that only uses the 5-phase mixed columnar-equiaxed solidification model and ignores the mold filling can predict satisfactory results for a casting poured at high temperature, but it delivers false results for the casting poured at low temperature.     

Optimization of Mold Flux for the Continuous Casting of Cr-Contained Steels

To compensate the negative effect caused by the absorption of chromium oxide inclusions during the casting process of Cr-contained steels, a new mold flux system has been designed and investigated. The melting temperature range of the newly designed mold flux system is from [1124 K to 1395 K (851 °C to 1122 °C)]. The viscosity at 1573 K (1300 °C) and the break temperature increase with the addition of MnO and Cr₂O₃ but decrease with the addition of B₂O₃. The crystalline fraction of mold flux decreases from 81 to 42.1 pct with the addition of MnO and Cr₂O₃, and then further decreases to 25.3 pct with the addition of B₂O₃; however, it improves from 54.4 to 81.5 pct when the basicity increases. Besides, the heat-transfer ability of mold flux is inverse to the crystallization ratio of the slag. The comprehensive study of the properties for the four designed mold fluxes suggests that the mold flux with 1.15 basicity-3.01 pct B₂O₃-1.10 pct MnO-2.10 pct Cr₂O₃ shows the best properties for the continuous casting of Cr-contained steels.     

Effect of Mould Coatings and Pouring Temperature on the Fluidity of Different Thin Cross-Sections of A206 Alloy by Sand Casting

Thin-wall castings of the A206 alloy can pose a manufacturing problem associated with mould filling which results in fluidity defect. The mould coating generates a smooth surface, and reduces the friction between the melt and mould contact, thus reducing the heat-transfer coefficient, which in turn leads to enhancement of fluidity and mechanical properties. The fluidity of A206 alloy was observed in various cross-sections of the thin channels at three altered pouring temperatures i.e. 700, 750 and 780 °C for uncoated and coated green sand moulds. The Graphite and Soapstone powder coatings were used as sand mould coatings in the present investigation. It was found that the fluidity of aforesaid alloy was significantly increased with the Soapstone powder mould coating at pouring temperature of 750 °C. The characterization of the coating materials was performed by the X-ray diffraction analysis and scanning electron microscope test with EDAX.     

泡沫铝合金的渗流铸造工艺方法研究

用正交实验法则研究了泡沫铝的渗流铸造工艺，分析了粒子预热温度、浇注温度和渗铸压力对金属液充型过程的影响。认为合理选择粒子预热温度是生产泡沫金属铸件的前提，适当提高浇注温度是保证泡沫组织均匀良好的关键，保持适度的渗铸压力有利于提高材料的孔隙度和工艺稳定性。     

The Effect of Purification on the Glass-Forming Ability of a Pd-Cu-Si Alloy

The copper mold casting method is now commonly used for preparing bulk metallic glasses (BMGs). In the present work, it was found that, by employing the copper mold casting method, Pd_77.5Cu₆Si_16.5 (at. pct) glassy rods with 1-mm diameter could be prepared, while the ?2-mm Pd_77.5Cu₆Si_16.5 casting rod possesses some crystalline phases embedded within the glass matrix, confirming that the critical size of the glassy alloy is about 1?mm. By melt purification with fluxing treatment, the critical size of the glassy rod prepared by copper mold casting is increased to 4?mm. Based on thermal property analysis, it was found that melt purification by the fluxing method can greatly enhance the thermal stability and increase the glass forming ability (GFA) of the Pd-Cu-Si alloys. The as-prepared ?4-mm Pd-Cu-Si glassy rod exhibits a reduced glass transition temperature (T _rg ) of 0.599, a supercooled liquid region (??T) of 74?K (74?°C), and a ?? parameter of 0.419.     

Hot Ductility Evolution Mechanism of Titanium-Bearing Microalloyed Steels

Titanium-bearing (Ti-bearing) microalloyed steels have high strength and toughness by grain refinement effect of carbonitride precipitates. However, they can induce surface cracks of continuous casting slab when the Ti alloyed content is high. A microalloyed steel with Ti content (0.10–0.15 wt%) is carried out by thermalmechanical simulator over 600–1350 °C to analyze hot ductility evolution mechanism. Fracture surface morphology, phase transition, and behavior of precipitates of the tensile samples are investigated by experimental detection and thermodynamic calculation. The ductility–temperature curves show that the third brittle temperature range is 600–890 °C, which is mainly attributed to the thin proeutectoid ferrite film and precipitated titanium carbonitride particles, widening the embrittlement temperature ranges through of steel. In addition, the tensile samples at 890–1350 °C have good hot ductility, indicating the dynamic recrystallization of deformed austenite can trigger grain boundaries migration away from cracks and avoid the side effect of the Ti (C,N) particles on hot ductility. The first brittle temperature range of 1350 °C-melting point is mainly ascribed to the partial melting of the grain boundaries with element segregation of sulfur and phosphorus, and microporosity loose among dendrites.     

Macrosegregation Improvement by Swirling Flow Nozzle for Bloom Continuous Castings

Based on mathematical model coupling electromagnetism, fluid flow, heat transfer, and solute transport, the metallurgical performances of conventional straight nozzle, swirling flow nozzle (SFN), and M-EMS have been evaluated and compared. The soundness improvement of bloom castings has been investigated by casting tests of adopting the newly designed SFN. As compared to the normal nozzle, center porosity has been eliminated along with the popular center radial crack, and a better chemical homogeneity was obtained by employing the SFN accordingly, where the maximum segregation degree of C and S at the strand cross section is decreased from 1.28 to 1.02 and from 1.32 to 1.06, respectively. Combined with the results of numerical simulation, the positive effect obtained can be attributed to the remarkable superheat dissipation under the implementation of SFN, where, compared with the normal nozzle, the melt superheat degree at the mold exit is reduced by 15.5 K, 9.8 K, and 17.3 K (15.5 °C, 9.8 °C, and 17.3 °C) under the other three casting measures of SFN, normal nozzle with M-EMS, and SFN with M-EMS, respectively.     

Nonisothermal Crystallization Kinetics of Glassy Mold Fluxes

Crystallization of the solid glassy mold flux film occurring in the gap between the initial shell and mold wall is important, as it determines the in-mold heat transfer and mold lubrication during the process of continuous casting. In order to study the nonisothermal crystallization behavior of the glassy mold flux film in the continuous casting mold, the continuous heating transformation diagram, crystallization mechanism, and precipitate phases were investigated using the single hot thermocouple technique, kinetic models, a scanning electron microscope, and an energy-dispersive spectrometer (EDS). The results show that the initial crystallization temperature for CaO-SiO₂ based flux A ranges from [1086 K to 1147 K (813 °C to 874 °C)], which is lower than the case of CaO-Al₂O₃ based flux B ranging from [1205 K to 1245 K (932 °C to 972 °C)]. The crystallization kinetics for flux A are constant nucleation rate, two-dimensional growth, and control by diffusion. For flux B, they are constant nucleation rate, three-dimensional growth, and control by interface reaction. Besides, the EDS results indicate that the precipitate crystals in fluxes A and B are CaSiO₃ and Ca₂AlSiO₄, respectively.     

Effects of pattern coating and vacuum assistance on porosity of aluminium lost foam castings

Lost foam casting (LFC) process has several advantages when compared to conventional sand casting techniques however formation of large amount of gaseous products during foam pattern removal increases porosity fraction of castings, especially for low melting point A1 and Mg alloys. In this study pattern coating and vacuum assistance at the time of filling were investigated and their characterizations in constant casting conditions have been determined. Green sand moulding technique was carried out for all moulds because it is necessary to obtain sound castings by using expandable polystyrene (EPS) foam patterns without refractory coating. Simple prismatic shaped patterns were prepared from cutting pieces from an EPS isolation board. A well-known A380 Al-Si-Cu casting alloy was cast at 730°C. As expected, pattern coating reduce the gas permeability and increase porosity however metal penetration into sand grains and surface roughening occurs without coating. Slight vacuum were applied to moulds with vacuum casting machine until solidification. Vacuum assistance enhanced gas removal and it has clear effect on decreasing porosity.