Effect of cooling rate on the microstructure and solidification characteristics of Al2024 alloy using computer-aided thermal analysis technique

The aim of this work was to investigate the effect of different cooling rates on the microstructure and solidification parameters of 2024 aluminum alloy. Solidification characteristics are recognized from the cooling curve and its first and second derivative curves which have been plotted using thermal analysis technique. In this study, a mold having high cooling rate was designed and used to simulate the direct-chill casting process. The results of thermal analysis show that the characteristic parameters of Al2024 alloy are influenced by cooling rate. The cooling rates used in the present study range from 0.4 to 17.5 °C s^?1. Increasing the cooling rate affects the undercooling parameters both in liquidus and eutectic solidification regions. Investigations showed that solidification parameters such as nucleation temperature, recalescence undercooling temperature, and range of solidification temperature are influenced by variation of cooling rates. Microstructural evaluation was carried out to present the correlation between the cooling rate and dendrite arm spacing.     

Thermal characterization of the interaction of poly(3-hydroxybutyrate) with maleic anhydride

In this research, the effects of Al–5Ti–1B grain refiner and Al–10Sr modifier were studied on solidification characteristics and microstructural features of 319 aluminum alloy. Important solidification events such as recalescence and nucleation undercooling temperature and aluminum–silicon eutectic depression temperature have been evaluated using cooling curve and its first derivative curve obtained from thermal analysis of a sample. The aim of this article is to show the ability of the thermal analysis technique to predict some key parameters controlling solidification and casting process. It has been found that the thermal analysis is the identified method for a rapid on-line monitoring of metallurgical characteristics of aluminum alloy melts without conventional metallographic examination.     

The novel application of Fourier thermal analysis in foundry technologies

The process of casting in sand moulds is used for a large volume of castings manufactured in the world. Internal channels and complex geometries of these products are formed by the placement of cores within the mould cavity. Resin-bound sand mixtures are essential ingredients in engine component manufacturing. In this study, a state-of-the-art application of Fourier thermal analysis in foundry technologies is presented. Investigation of decomposition phenomena of resin-bound moulding materials during casting production is a brand new area to use the potential of thermal sciences. Temperature measurements in test samples of standard types of moulding mixtures were performed. The registered cooling curves were processed by a numerical iteration algorithm to determine the amount of heat absorbed during degradation of the moulding material. Thermogravimetric analysis (TG) and differential thermal analysis (DTA) of sand mixtures were carried out to compare the results of the Fourier thermal analysis with TG and DTA curves.     

Effect of cooling rate and Al content on solidification characteristics of AZ magnesium alloys using cooling curve thermal analysis

The solidification behavior of AZ Magnesium alloys in various cooling conditions was investigated using a computer-aided cooling curve thermal analysis method. In each case, the cooling curve and its first and second derivative curves have been plotted using accurate thermal analysis equipment and solidification characteristics were recognized from these curves. The cooling rates used in the present study range from 0.22 to 8.13 °C s^?1. The results of thermal analysis show that the solidification parameters of AZ alloys such as nucleation temperature (T _N,α), nucleation undercooling (?T _N,α), recalescence undercooling (?T _R,α), range of solidification temperature (?T _S) and total solidification time (t _f) are influenced by variation of cooling rate. Also, the effect of Al content on these parameters was studied. Microstructural evaluation was carried out to determine the correlation between the cooling rate and secondary dendrite arm spacing.     

Research and application of a new rapid heat cycle molding with electric heating and coolant cooling to improve the surface quality of large LCD TV panels

The usage of rapid heat cycle molding (RHCM) has gained increasing attention in overcoming the limits of conventional injection molding (CIM) and improving the surface quality and mechanical properties of molded plastic products. In RHCM, the vario‐thermal mold temperature control system is the key technique because it directly affects the molding cycle time and the final part quality. In this study, a new RHCM technology with electric heating and coolant cooling was studied in detail. Two different RHCM mold structures for a large LCD TV panel were proposed and designed. The numerical simulation method was used to analyze the thermal response of the mold cavity surface at the heating stage and the thermal response of the resin melt at the cooling stage. The heating/cooling efficiency of the proposed electric heating RHCM system was evaluated. The thermal expansion analysis of mold cavity was implemented and the fixation of the cavity in molds was also optimized. The results showed that the electric‐heating mold with a separate cooling plate can efficiently enhance the heating efficiency. The thermal expansion of the cavity surface can be reduced by increasing the alleviating‐gap between the cavity and the cavity‐retainer plate. Then, the service lifetime of the electric‐heating mold can be improved. A RHCM production line with electric heating for the large LCD TV panel was constructed. Both the simulation and test production results indicate that the proposed electric heating RHCM technique can realize high‐temperature injection molding without increasing the molding cycle time. The surface appearance of the LCD TV panels was dramatically improved and the surface marks that usually occur in CIM process were eliminated completely. Copyright © 2009 John Wiley & Sons, Ltd.     

The multiple melting endotherms from poly(butylene terephthalate)

The melting behavior of poly(butylene terephthalate) (PBT) has been investigated, and a simulation has been performed to determine whether the multiple melting endotherms observed during the thermal analysis of PBT can be explained by the simultaneous melting and recrystallization of an initial distribution of crystal melting temperatures that contains only one maximum and two inflection points. Specimens that were cooled at constant rates from the melt showed between one and three melting endotherms upon heating in a differential scanning calorimeter (DSC). The position and breadth of the crystallization exotherms upon cooling from the melt and small-angle x-ray scattering showed that as the cooling rate is increased, the distribution of melting temperatures broadens and shifts to lower temperatures. By combining temperature-dependent recrystallization with an initial distribution of melting temperatures, simulated DSC curves were produced that agreed well with experimental DSC curves. In instances of triple peaked curves, the high temperature peak was due to crystals formed during the scanning process, and the middle and low temperature peaks were due to crystals originally present in the material. Satisfactory agreement between the experimental and simulated curves was found without considering additional crystallization from the amorphous regions during the scanning process.     

Investigation on solidification conditions in functionally Si-gradient Al alloys using simulation and cooling curve analysis methods

During recent years, functionally graded alloys have been widely produced by melt processing routes to achieve advanced material properties. In this paper, the new cast-decant-cast (CDC) method has been used to produce the gradient in concentration of Si particles in cross section of final Al–Si castings. For this purpose, the hypereutectic LM28 and hypoeutectic LM25 alloys were selected for each step casting into the steel mold. Cooling curve thermal analysis and simulation methods were applied to investigate the cooling behavior of first poured LM28 alloy and improve the accuracy of CDC process by determining the curves of solid fraction and temperature profiles. The final products were studied through optical microscopy, image analysis, and Brinell hardness measurement. The results showed that the silicon concentration decreased along transition zone between two alloys by increasing the decanting time in the order of 25, 40, and 50 s. This can be due to the lower temperature of exterior LM28 alloy in semi-solid state and shorter solidification time of interior LM25 alloy. This can lead a to reduction of the diffusion rate of elemental silicon along the transition zone. The microscopic scale of transition zone between two alloys developed the maximum thickness of 438 μm and hardness value of 83 HB comparing with the hardness of 88 and 62 HB for external and internal alloys, respectively. The microscopic observations and hardness evaluations confirmed the creation of functionally Si-gradient products.     

热镀铜层冷凝过程的分子动力学模拟

During the hot-dip process of Cu on the surface of the steel,it involves the solidification from liquid to coating. The cooling rate has great influence on the microstructure and the performance. By means of constanttemperature,constant-pressure molecular dynamics simulation technique,the solidification process of the liquid model system made of 500 Cu particles has been studied with the period boundary condition. With the pairs analysis technology and the bond orientational order method,the difference of the structure and energy of the liquid Cu model system between different cooling velocities has been compared. The significant information of microcosmic structural transformation in the solidification process of liquid Cu system has been obtained. The calculation results show that the Finnis-Sinclair（FS）potential works very well in the solidification process of Cu. Cooling slowly the crystal copper layer can be obtained. Cooling quickly the amorphous copper layer can be obtained.     

Applications of thermal analysis in quality control of solidification processes

Summary Thermal analysis is widely used to determine solidification characteristics of metals and alloys in various metallurgical processes. Computer-Aided Cooling Curve Analysis (CA-CCA) is the most popular thermal analysis technique because of its ease of use and low cost. This paper discusses the principles of CA-CCA and zero curve calculations. The methods for calculating key solidification characteristics of metals from cooling curves are presented, and their importance in the quality control of manufacturing processes are demonstrated. Examples are presented for cast iron, copper and aluminum alloys.     

掺杂元素对铝-硅共晶合金凝固过程的影响

国内外学者对掺杂元素对Al-Si共晶合金的影响进行了大量的研究,这些研究大都侧重于研究变质后的结果,如变质后的晶体结构形态以及掺杂元素对结构形态的影响,再根据变质结果反过来推测可能的变质机理,但对掺杂元素对Al-Si合金凝固过程本身的影响的研究却很少.本文拟采用差热分析法(DTA)和步冷曲线热分析法研究Na、Sr元素对Al-Si共晶合金凝固过程的影响,以探讨Al-Si共晶合金的成核机理。     

二十面体准晶对非晶形成影响的模拟

采用分子动力学模拟技术,以液态金属Ni为例,研究了在不同冷却条件下形成晶体及非晶的过程.模拟采用镶嵌原子法(EAM)作用势,得到了不同温度、不同冷却速度下Ni的径向分布函数以及原子组态变化的重要信息,利用键对分析技术探讨了二十面体准晶对非晶形成的影响.     

Application of thermal analysis to monitor the quality of hypoeutectic cast irons during solidification in sand and metal moulds

By exploring the effects of carbon equivalent (CE?=?3.4?C4.2%) and inoculation (Ca, Ba, Al?CFeSi alloy), the thermal analysis parameters, representative temperatures and undercooling events, during solidification of cast irons are compared with chill tendency (carbides) in cast irons. Cooling curve analyses were conducted directly in the thermal centre of samples solidified in resin sand and metal moulds to explore the utility of thermal analysis in actual casting shapes. As a reference, a Quik-cup? system was employed with the same cooling modulus (CM?=?0.75?cm). The austenite formation temperature is lower at higher CE, with higher temperatures of both the start of the eutectic reaction and end of solidification. This was more pronounced for metal mould solidification and for un-inoculated irons, respectively. The transition from white iron through mottled iron up to grey iron macrostructure is illustrated by variations of eutectic undercooling compared to the metastable equilibrium temperature, as affected by increasing carbon equivalent and applied inoculation. Inoculation affected both austenite and eutectic formation, by preventing excess eutectic undercooling during solidification, even at low carbon equivalent. The differences in solidification patterns of sand and metal mould castings are discussed, for un-inoculated and inoculated irons, at different carbon equivalent values.     

The solidification behavior of a PBT/PET blend over a wide range of cooling rate

In recent years, much attention has been paid to the development of high‐performance polyester blends, among which blends of polybutylene terephthalate/polyethylene terephthalate (PBT/PET) are expected to exhibit remarkable properties as far as their crystallization behavior is concerned. Through trial and error, appropriate commercial compositions have been chosen which could not be otherwise explained by a suitable interpretation of the mechanisms determining their solidification behavior. The solidification behavior of a 60/40 w/w PBT/PET blend was studied in a wide range of cooling conditions, according to a continuous cooling transformation (CCT) procedure developed previously, aiming at emulating the typical conditions encountered in polymer processing. Several samples characterized by a homogeneous structure were solidified from the melt at various cooling rates and the resulting structure and properties were subsequently evaluated by analyzing the density, microhardness (MH), and wide angle x‐ray diffraction (WAXD). The resulting solidification behavior was then compared to that exhibited by the individual constituents of the blend (i.e., PBT and PET). The blend displayed a unique solidification behavior, conversely to those of the pure components which showed characteristics not recognized in the blend except at certain restricted cooling rates ranges. The cooling rate dependence observed in the blend does not bring similarities to the crystallization behavior of individual constituents since the fall down of density with cooling rate should be related to the rate controlling demixing stage of the two moieties just before crystallization occurs. The kinetics observed is thus a measure of the kinetics of demixing. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 799–810, 2009     

Evaluation of adsorbent materials for heat pump and thermal energy storage applications in open systems

The evaluation of solid adsorbents in open sorption systems for heating, cooling and thermal energy storage (TES) applications is crucial for the ecological and economical performance of these systems. An appropriate adsorbent has to reach the temperature limit given by the heating/cooling system of the consumer. It has to provide high energy efficiency and a high energy density for storage applications. A method for an easy evaluation of different adsorbents for a specific application has been developed. The method is based on the adsorption equilibrium of the adsorbent and water vapor. The crucial property for the discussed field of applications is the differential heat of adsorption. Criteria for the evaluation of the adsorbent are the breakthrough curves (responsible for the dynamics of the process), the possible temperature lift (or the dehumidification) of the air, the thermal COP and the storage capacity.     

Superlyophilic Interfaces Assisted Thermal Management

Thermal management has become a critical issue owing to the increasing need for various devices including heat dissipation and adsorption. Recently, the rapid growth of scientific reports is seen to improve thermal management efficiency by developing materials with high transfer coefficient and surface improvement to enhance heat transfer rate. Inspired by nature, constructing superlyophilic interfaces has been proved to be an effective way for thermal management and applied in industry and daily life. Herein, state-of-the-art developments of superlyophilic interfaces assisted thermal management are reported mainly from four perspectives around boiling, evaporation, radiation, and condensation. In particular, we discussed the unique role of superlyophilic interfaces during the heat transfer process, such as increasing bubble detachment rate, superspreading assisted efficient evaporation, directional liquid transfer in textiles during radiative cooling, and so forth. Finally, challenges of thermal management assisted by superlyophilic interfaces toward future applications are presented.     

液态合金NiAl凝固过程中微观结构转变的分子动力学模拟

采用分子动力学模拟方法对液态NiAl凝固过程进行了研究,考察了不同冷却速度下液态NiAl结构变化特点,原子间相互作用势采用F-S多体势,结构分析采用键取向序和对分析技术.计算结果表明,冷却速度对液态NiAl结构转变有重要影响,在不同的冷却速度下, NiAl凝固过程出现了明显不同,冷速为4×1013和4×1012 K/s时, NiAl快速凝固为无序的非晶体结构;而在较慢的8×1011 K/s冷速下, NiAl凝固为晶态结构.给出了不同冷却速度下液态NiAl结构转变的微观信息.     

Crystallization kinetics of virgin and processed poly(lactic acid)

Poly(lactic acid) (PLA) is an emerging material mainly because it can be synthesized from renewable resources and is thus environmentally and ecologically safe. The mechanical properties, above all the thermal resistance of PLA are determined by the crystalline content: the heat deflection temperature of crystalline PLA can reach 100 °C, whereas amorphous PLA loses mechanical properties at temperatures slightly higher than 60 °C. However, PLA has a low crystallization rate, so that after processing it remains mostly amorphous. This characteristic heavily limits the use of PLA for commercial applications. Many studies have been recently published on the crystallization kinetics of PLA. The effect of processing on this feature is however often neglected. In this work, the significance of processing on the crystallization kinetics of a commercial PLA was investigated. Two processing methods were explored: extrusion and injection moulding. The obtained materials, and the starting pellets of virgin polymer, were analyzed by calorimetry in order to obtain the crystallization kinetics. Two protocols were adopted to determine the crystallization rates during cooling from the melt or heating from the solid. The parameters of a kinetic equation were determined for all the materials and protocols adopted and it was thus possible to describe the evolution of crystallinity during heating and during cooling.     

An application of infrared analysis to determine the mineralogical phases formation in fluxes for thin slab casting of steel

The Fourier Transformed Infrared (FTIR) spectra analysis of two fluxes used in the thin slab casting process of steel were carried out in order to identify the mineralogical species present in fluxes as received and after a heat treatment to 1573 K and further solidification at two different cooling velocities. Fluxes as received show the presence of wollastonite (CaO·SiO₂) and a sodium carbonate (Na₂CO₃) as the main components; after the heat treatment, there was almost a whole transformation from the original compounds to cuspidine (3CaO·2SiO₂·CaF₂) and nepheline (Na₂O·Al₂O₃·2SiO₂) phases. These results were confirmed by X-ray powder diffraction (XRD) to the slowly cooling velocity. The FTIR technique is proposed as a useful and complementary technique to X-ray diffraction to study the structure of commercial fluxes for thin slab casting.     

Precise control of agarose media pore structure by regulating cooling rate

A porous structure is the key factor to successful chromatography separation. Agarose gel as one of the most popular porous media has been extensively used in chromatography separation. As the cooling process in the agarose gelation procedure can directly influence the pore structure, ten kinds of 4% agarose media with different cooling rates from 0.132 to 16.7°C/min were synthesized, and the pore structure was determined accurately by using low‐field NMR spectroscopy. The curves of pore structure and cooling rate can be divided into two stages with the boundary of 6°C/min. In stage I, the pore structure met a power equation with the decrease of the cooling rate, and in stage II, the process reached a plateau. Confirmatory experiments proved that, by adjusting the cooling rate, a precise control of the pore structure of agarose media can be realized, furthermore, cooling rate optimization was an effective way to control the pore size of agarose media and can further tailor the pore structure for more effective separation of different proteins.