Papers by Keyword: Rapid Solidification

Abstract: Recycled Al–Si (9.2%) alloy contaminated with Fe (0.3%), Pb (3.1%) and Sn (11.4 %) was cast and poured at 650 oC, approximately 50 oC above the liquidus line. A cooling slope was used to obtain a semisolid material that feeds a ceramic nozzle designed to function as a good contact area for solidification and improve the quality of strip casting. The internally cooled material rolls in soluble oil (1 oil / 9 water) at a rate of 0.2 l/s and works as a heat exchanger which drags the metallic slurry puddle generated at the roll surface at a speed of 0.12 m/s. This forms a metallic strip with a thickness of 2 mm and a width varying from approximately 45 mm to 60 mm. The cooling system of the rolls, combined with four springs placed at the housing screw, prevented adhering of the metallic strip during production at a pressure of approximately 450 N. Cracks were observed on the strip surfaces; however, these defects did not interrupt the continuous flow of the solidified strip during manufacturing. The strip’s poor surface quality could be related to the Pb and Sn contamination as well as cold cracks due to the low pouring temperature. Al-Si eutectics positioned at a grain boundary of α-Al globular structures, as well as the presence of a Sn phase, resulted in a metallic strip with a yield stress, maximum stress and elongation of 94.5 MPa, 100.2 MPa and 1.6%, respectively.

Abstract: Mould casting and drop-tube techniques were used to solidify a AlCoCrFeNi_2.1 eutectic high-entropy alloy under conditions of high cooling rate. The samples obtained from two different methods present the same phase constituent, FCC and B2 phases. During mould casting experiments the alloy almost solidified into the eutectic structure consisting of lamellar and anomalous morphology, with a tiny fraction of cellular and dendrite morphology being observed at certain sites of the sample surface due to the corresponding high cooling rate. Instead, during drop-tube experiments a typical, coarse dendrite structure of FCC single phase was formed across the entire 106-150 μm particle. The cellular structure can also be formed directly from the melt. The rest region solidified into the general eutectic morphology as was observed in the casting rods. The results clearly indicate the transition from coupled eutectic growth to single-phase dendrite growth with increasing departures from equilibrium for the multi-component AlCoCrFeNi_2.1 eutectic high-entropy alloy.

Abstract: Rapidly solidified thin micro-alloyed surface layers are generated by laser melting of plasma thermal sprayed steel surfaces. Samples of carbon steel are plasma sprayed with fine nickel and aluminum powders. Laser surface melting generated a thin localized molten pool of metal with steep horizontal thermal gradient. The latter triggered intense vortex formation in the molten pool which thoroughly mixed the nickel and aluminum powders within the molten pool in a fraction of a second. As the sample is moved away with a predefined velocity, the cold substrate quenched the melt pool, generating rapidly solidified micro-alloyed surfaces. A 2.5 kilowatts continuous carbon dioxide laser was used for surface melting; laser power was maintained at 800 watts while the samples were moved with respect to the laser beam at linear velocities in the range of 100-200 mm/min. The technique generated metallurgical bonded novel surfaces. Depth of the laser modified layer was achieved in the range of 0.2-0.4 mm. Refined microstructures of pre-austenite grain size in the range of 4±2 µm were generated. Micro-hardness measurements of the modified layer indicate an almost three times enhancement in the hardness values; the latter are, in general indicative of mechanical strength of the material. The shape of the solid/liquid interface of the advancing molten pool determines the orientation of the growing dendrites; at higher velocities of sample translation with respect to the laser beam, these are almost parallel to the sample surface. The orientation of the dendrites, the uniformity in surface alloying (within fraction of a second) and the resulting hardness values are explained with the help of the modeled shape of the liquid metal pool. The laser processed material proved to be a flexible technique to synthesize novel surfaces for surface sensitive applications.

Abstract: Phase Change Materials (PCMs) can be applied in Thermal Energy Storage and Thermal Management systems, exploiting the storage and release of latent heat associated to a phase transition. Among them, metallic PCMs can be used at medium and high temperatures (i.e. above 150°C), storing higher heat per unit volume at higher temperatures with respect to the most widely investigated polymeric and salt-based PCMs. Miscibility Gap Alloys (MGAs) can be used to obtain multiple-phase mixtures in which the active phase (the actual PCM) is mixed to a second, high-melting temperature phase, with negligible interaction between them. These can actually be considered as fully metallic composite materials specifically developed for thermal management. Suitable microstructures can prevent leakage of active phase when the solid-liquid transition occurs, resulting in a form-stable PCM (FS-PCM). However, obtaining these microstructures it is not trivial. The present study focuses on a solid-liquid FS-PCM consisting of a ‘classical’ fully metallic FS-PCM, an Al-Sn based MGAs produced by powder metallurgy. The goal was to evaluate the effect of different production processes on thermal and mechanical behaviour of the PCM. Particularly, powder metallurgy routes including both simple mixing and ball milling were compared and further combined. Moreover, several compression and sintering conditions were considered, also substituting Al powders with Al-alloy powders, in order to optimize the material microstructures in view of suitable thermal and mechanical properties. Finally, the casting route with a rapid solidification approach was investigated for the same alloy.

Abstract: Ti-6Al-4V alloy ribbons were successfully prepared by rapid solidification at different roller speeds. The morphology and microstructure of Ti-6Al-4V alloy ribbons were studied by X-ray diffractometer, scanning electron microscope and energy spectrum analysis. The effect of SiO₂ on the microstructure and properties of Ti-6Al-4V alloy ribbons were discussed. It is found that the Ti-6Al-4V alloy ribbons consist of β phase with alternated distribution of α' phase and α'' phase. And quicker roller speed promotes the formation of α' phase for the higher rapid solidificationg cooling rate. According to the analysis of energy spectrum data, it is found that there is chemical reactions between the Ti-6Al-4V material and the quartz tube wall during electromagnetic induction melting at high temperature, and the Si elements are segregated at the embossment induced by quick solidification and the phase interfaces between α′ phase and β phase.

Abstract: Refining grain plays an important role in improving the mechanical properties of aluminum alloys. However, the conventional casting method with a slow cooling rate can be easy to cause coarseness of the microstructure and serious segregation. In this paper, the rapid solidification of Al-Zn-Mg-Cu alloy was prepared by the single-roller belt method. The alloy strip was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and hardness test to study the microstructure and properties of the rapidly solidified aluminum alloy. The results show that the roller speed was an important parameters affecting the formability of the alloy. When the roller speed was 15 m/s, the aluminum alloy produced a thin bandwidth of 5 mm and a thickness of 150 um. As the rotation speed of the roller increased, the cooling rate of the melt increased, and the microstructure of the rapidly solidified Al-Zn-Mg-Cu aluminum alloy strip improved in grains refinement. Compared with the conventionally cast Al-Zn-Mg-Cu aluminum alloys, the Al-Zn-Mg-Cu aluminum alloys prepared by rapid solidification showed much finer crystal grains, and enhanced solid solubility of alloying elements with less precipitation of second phase and high hardness.

Abstract: The binary Cu-Fe system is characterized by a metastable liquid miscibility gap. WhenZr is added into the Cu-Fe alloy, the miscibility gap can be extended into Cu-Fe-Zr ternary system. In the present study Cu-Fe-Zr alloys were prepared by single-roller melting-spinning method, and the samples were characterized by the SEM, EDS, HRTEM and nanoidentation. The results show that liquid-liquid phase separation into CuZr-rich and FeZr-rich liquids takes place during rapid cooling the Cu-Fe-Zr alloy, and the mechanism depends on the atomic ratio of Cu to Fe. With increasing Zr content, the size of secondary phase formed by the liquid-liquid phase separation reduces to nanoscale. The structure with amorphous Cu-rich nanoparticles embedded in the amorphous Fe-rich matrix was obtained in the as-quenched Cu₂₀Fe₂₀Zr₆₀ alloy. For its structure particularity of the Cu₂₀Fe₂₀Zr₆₀ sample, mechanical evaluation was carried out by using nanoindentation.

Abstract: In this paper, twin roll casting of magnesium alloys with high aluminum content such as, Mg-11 mass%Al-0.2 mass%Mn, Mg-12 mass%Al-0.2 mass%Mn, Mg-13 mass%Al-0.2 mass% have been performed for the purpose of use as an original material for hot forging. Also the mechanical properties of the cast materials were examined. A 10 miri-meters thick strip was cast by the use of a horizontal twin roll caster. The microscopic observation was conducted to investigate into the precipitation of the metal compounds such as Mg₁₇Al₁₂, and the Vickers hardness of the cast strips test were performed. From the result of the roll casting experiments, a 10 mm thick strip was continuously cast at a roll speed of 3.1 m/min. The average grain size of the casting strips was about 46 micron meters. When the aluminum content was 13%, the hardness of the twin roll cast (TRC) strips became 1.7 times higher than that of extruded AZ 31. Also, a uniaxial compression test at elevated temperature was conducted to obtain a true strain-true stress curve for examining possibility of direct hot forging (DHF) of TRC magnesium alloys with high aluminum content.

Abstract: Spherical copper particles with diameter ranging from 120.6 to 437.0 μm were prepared by the pulsated orifice ejection method (termed “POEM”). These spherical copper particles exhibit a good spherical shape and a narrow size distribution, suggesting that the liquid copper can completely break the balance between the surface tension and the liquid static pressure in the crucible micropores and accurately control the volume of the droplets. Furthermore, the relationship between cooling rate and microstructures of spherical copper particles was carried out with a specific focus on different cooling atmosphere and particle diameter during the rapid solidification. The cooling rate of spherical copper particles is evaluated by a Newton’s cooling model. It is revealed that the cooling rate was depended on cooling medium and particle diameter. The cooling rate decreases and the grain size increases with the increase of particle diameter during the rapid solidification, while the grain boundary of same particle diameter with larger cooling rate in argon gas is smaller, while the grain boundary of particles with smaller cooling rate in helium gas is larger. When the particle diameter is larger than 100 μm, the cooling rate of the cooper droplet in argon gas achieves 1.0×104 K/s. Meanwhile, the cooling rate decreases rapidly when the particle diameter increased between 70.6 and 149.6 μm. It is an effective route for fabrication of high-quality spherical copper particles.

Abstract: Spherical mono-sized Sn-1.0Ag-0.5Cu (wt.%) particles with diameter ranging from 124.0 to 337.4μm were prepared by the pulsated orifice ejection method (termed “POEM”).These spherical Sn-1.0Ag-0.5Cu particles exhibit a good spherical shape and a narrow size distribution, suggesting that liquid Sn-1.0Ag-0.5Cu can completely break the balance between the surface tension and the liquid static pressure in the crucible micropores and accurately control the volume of the droplets. Furthermore, the relationship between cooling rate and microstructures of spherical Sn-1.0Ag-0.5Cu particles was studied with a specific focus on different particle diameter during the rapid solidification. The cooling rate of spherical Sn-1.0Ag-0.5Cu particles with different diameter was evaluated by the Newton’s cooling model. It is revealed that the cooling rate decreases gradually with the increase of particle size during the rapidly solidified process. When the particle diameter is equal to 75 μm, the cooling rate of the Sn-1.0Ag-0.5Cu particle achieves 4.30×10³ K/s which indicates that smaller particles can rapidly solidified due to their higher cooling rate. Meanwhile, the cooling rate decreases rapidly when the particle diameter increases between 75 and 100 μm. Furthermore, the different particle diameter with different cooling rate has a great influence on the solidification microstructure of Sn-1.0Ag-0.5Cu particles. The cooling rate and grain boundary size decreases with the increase of particle diameter during the rapid solidification. In addition, the phase size of βSn increases with the decrease of particle size. Smaller particles have relatively high cooling rate and it gives less solidification time as compared to larger particles. It is an effective route for fabrication of high-quality spherical Sn-1.0Ag-0.5Cu particles. Keywords: Spherical Sn-1.0Ag-0.5Cu particles; Rapid solidification; Structural; Cooling rate