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.     

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.     

Application of cooling curve analysis in solidification pattern and structure control of grey cast irons

In this article, the composites based on long glass fibre reinforced polypropylene/intumescent flame retardant (LGFPP/IFR) were prepared by melt blending. The influence of thermal oxidative ageing on the LGFPP/IFR composites with different thermal oxidative ageing time at 140 °C was studied by means of oven heating. The thermal stability and flammability of the composites were respectively investigated by thermal gravimetric analysis (TG), limiting oxygen index (LOI), UL-94 test, cone calorimeter test (CCT), scanning electronic microscopy (SEM), mechanical properties test and energy-dispersive X-ray analysis (EDAX). A trend of increase first and then decrease in LOI values was shown in 0–50 days ageing, with the same trend as thermal stability obtained from TG in nitrogen condition. The CCT results indicated that the LGFPP/IFR composites after ageing achieved a higher heat release rate, which means a higher fire risk. The mechanical properties showed a global decrease in just 10 days ageing. Morphologies obtained from SEM showed that both the rupture of PP matrix and fibre interface debonding led to the decrease in mechanical properties. The EDAX proved that IFR particles could emerge and gather on the surface of sample in ageing procedure, which had great effects on the thermal stability and flame retardancy of the composites.     

Influence of secondary cooling strategies on thermal gradients in the direct chill casting of magnesium alloys

Journal of Thermal Analysis and Calorimetry - Direct chill (DC) casting is the widely used technology for the production of ingots or billets of nonferrous alloys. Non-uniform thermal gradients...     

Characterization of light and heavy hydrated magnesium carbonates using thermal analysis

Upon heating, hydrated magnesium carbonates (HMCs) undergo a continuous sequence of decomposition reactions. This study aims to investigate the thermal decomposition of various commercially produced HMCs classified as light and heavy, highlight their differences, and provide an insight into their compositions in accordance with the results obtained from thermal analysis and microstructure studies. An understanding of the chemical compositions and microstructures, and a better knowledge of the reactions that take place during the decomposition of HMCs were achieved through the use of SEM, XRD, and TG/differential thermal analysis (DTA). The quantification of their CO₂ contents was provided by TG and dissolving the samples in HCl acid. Results show that variations exist within the microstructure and decomposition patterns of the two groups of HMCs, which do not exactly fit into the fixed stoichiometry of the known HMCs in the MgO–CO₂–H₂O system. The occurrence of an exothermic DTA peak was only observed for the heavy HMCs, which was attributed to their high CO₂ contents and the relatively delayed decomposition pattern.     

Effect of Ag concentration on the thermal behavior of the Cu-10 mass% Al and Cu-11 mass% Al alloys

In this work the effect of Ag concentration on the thermal behavior of the Cu-10 mass% Al and Cu-11 mass% Al alloys with additions of 4, 6, 8 and 10 mass% Ag was studied using differential scanning calorimetry (DSC), in situ X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The results showed that for the Cu-10 mass% Al alloy Ag addition induce the β′₁ phase formation and for the Cu-11 mass% Al alloy these additions increase the amount of martensite formed on quenching and decrease the stability range of this phase on heating.     

Effect of rapid solidification on heat capacities of Al-Sr alloys

Heat capacities of both the ingot-like and melt-spun Al-Sr alloys have been measured through the temperature range 373 to 1060 K using differential scanning calorimetry. The experimental results show that rapid solidification has a slight effect on the temperature dependence of the heat capacities of the Al-Sr alloys. The heat capacities of the melt-spun Al-Sr alloys increase more slowly than those of the ingot-like alloys with increasing temperature from 373 to 900 K. Furthermore, the effect of rapid solidification on the heat capacities becomes more obvious with increasing Sr concentration in the Al-Sr alloys. The data of the heat capacities between 373 and 900 K have been fitted with the least square method and a linear dependence on temperature was assumed for that temperature range. This revised version was published online in July 2006 with corrections to the Cover Date.     

Studies on the thermal characteristics of polymer alloys

PVC: polystyrene alloys of varying compositions were prepared by the film casting technique. Thermogravimetric analysis of these materials showed that the decomposition patterns of the alloys are quite different from those of the constituent polymers. These results are explained in terms of polymer interaction phenomena.

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Zusammenfassung Mittels eines Feinfoliengiebverfahrens wurden PVC-Polystyrol-Mischungen verschiedener Zusammensetzung gefertigt. Eine thermogravimetrische Analyse dieser Stoffe zeigte, daß sich der Typ der Zersetzung der Mischungen vollständig von dem der reinen Komponenten unterscheidet. Diese Ergebnisse werden als Folge von Polymerwechselwirkungen interpretiert.

     

The effect of cooling rate on the solidification and microstructure evolution in duplex stainless steel

The effects of the cooling rate on the solidification and microstructure evolution in the duplex stainless steel SAF 2205 was studied using DSC and light microscopy. A ferritoscope was used to measure the ferrite content. It was revealed that the cooling rate has an influence on the ??-ferrite nucleation temperature and the width of the solidification interval. Moreover, with an increase in cooling rate, the content of ??-ferrite increases, while the quantity of austenite in the ferrite matrix decreases and its morphology changes to acicular. A two-cycle DSC experiment made possible a more accurate interpretation of the collected data.     

Influence of cooling rate on crystallisation kinetics on microstructure of cast zinc alloys

In this study, the change of the cooling rate in the range of about 0.1–1 °C s^?1 and the addition of Sr on the crystallization kinetics of the cast zinc alloys of the ZnAlCu type, as well as its relation to the microstructure were also investigated. Therefore, the aim of the rapid crystallisation is the achievement of materials with better properties, which can be obtained by refinement of the dendritic or eutectic microstructure, elimination of segregation, or creation of metastable phases and their morphology changes. In the investigated alloys, the change of cooling rate of 1 °C s^?1 has caused microstructure’s refinement as well as increase in hardness. Increase in the cooling rate causes also morphology changes of the η + α eutectic, and makes generally a global overcooling of the alloy as well as change in the temperatures at the beginning of crystallization T _DN and of the alloy crystallization T _S. The presented investigations concerning the electron microscopy methods, including transmission electron microscopy, allow revealing the crystallographic structure, based on the d-spacing changes, as well as the diffraction method used for phase determination, which is a helpful tool for the explanation of the important points in the thermo-derivative analysis curve, where the relation between the amount of phase and the occurrence of new phases can be determined.     

过硫酸铵-三乙醇胺氧化还原引发体系对丙烯酸镁固化的影响

为研究过硫酸铵-三乙醇胺氧化还原引发体系中,氧化剂、还原剂用量以及氧化还原剂配比对丙烯酸镁溶液固化时间以及固化物吸水膨胀性能的影响,实验设计氧化剂和还原剂用量为0.3％,0.5％,1％,1.5％,2％,2.5％,3％;氧化还原剂配比为3:1,2:1,1:1,1:2,1:3,以此分别研究不同的氧化剂和还原剂用量对丙烯酸镁...     

Isothermal kinetic analysis of the thermal decomposition of magnesium hydroxide using thermogravimetric data

In the present study, the kinetics of the thermal decomposition of magnesium hydroxide is investigated, using isothermal methods of kinetic analysis. For this purpose, experiments in thermogravimetric analyser were carried out in standard values of temperature (350°, 400°, 450° and 500°C) which resulted in weight loss percent as a function of time. The data were further modified to give fraction reacted ‘' versus time to be tested in various forms of ‘' functions. In order to determine the mechanism of the magnesium hydroxide decomposition and the form of the conversion function which governs the dehydroxylation of Mg(OH)₂, four different methods of isothermal kinetic analysis were used. Applying each of these methods to the data, it was concluded that the nucleation mechanism predominates the Mg(OH)₂, decomposition for all values of temperature tested; at 350°C the kinetic model which represents the experimental data is that of reaction at phase boundaries (random nucleation), F₁: ln(1−)=kt) while for the higher temperatures 400°, 450° and 500°C the kinetic equation of nucleation and development in two dimensions, A₂: [−ln (1−)]^1/2=kt was found to fit better the experimental results. The activation energy was evaluated applying two alternative methods; the Arrhenius plot, using maximum rates of reaction, from which the activation energy was evaluated to be 20.54 kcal/mol. An alternative method based on plots of ln t versus 1/T corresponding to the same value of ‘' gave values of 10.72, 13.82 and 16.31 kcal/mol for ‘' values of 0.25, 0.50 and 0.75, respectively.     

火焰原子吸收光谱法测定镁及镁合金中镉含量不确定度评定

采用火焰原子吸收光谱法测定镁及镁合金中镉含量,对测定结果不确定度进行评定.建立数学模型,分析不确定度来源,计算合成标准不确定度和扩展不确定度.当镁及镁合金中镉含量为0.00418％时,其扩展不确定度为0.00034％(k=2).火焰原子吸收光谱法测定镁及镁合金中镉含量不确定度主要来源于样品称量、样品消解溶液定容、标准工...     

Determination of the latent heat of fusion and solid fraction evolution of metals and alloys by an improved cooling curve analysis method

Journal of Thermal Analysis and Calorimetry - In this work, a new computer-aided cooling curve analysis method (CA-CCA) called metal/mold energy balance method (MEB) is presented. Its originality...     

Effect of NQ content on the thermal decomposition of nitroguanidine propellant using isoconversional methods

Journal of Thermal Analysis and Calorimetry - The effect of nitroguanidine (NQ) content on thermal decomposition of nitroguanidine propellants was studied using three nitroguanidine propellants...     

Modeling and thermal analysis of solidification in a low alloy steel

In this study, solidification of a low alloyed steel used for armor material was simulated at various transformation temperatures by the help of thermodynamical calculations using Thermo-Calc software. Differential thermal analysis (DTA) was carried out to determine the critical transformation temperatures for the phases and results were compared with those calculated by Thermo-Calc. It is concluded that the temperatures calculated by Thermo-Calc software are in good agreement with the results obtained from DTA. These results will be very useful to determine the proper austenization temperature having an important effect on the properties of steel serving as an armor material.     

The effects of concentration and heating or cooling rate on the DTA curves of Al-Ce alloys

The crystallization of Al-Ce alloys was studied by DTA. The melting and freezing DTA curves of the investigated alloys exhibit two peaks, corresponding to the transformations of a high-purity phase and the eutectic phase, respectively. The peaks could be separated during both freezing and melting by changing the heating or cooling rate. The final temperature of the phase transformation is marked by the starting temperature of the second peak. A slight shoulder on the DTA peak, even on the opposite side to the maximum point, may correspond to the final temperature.The connection between the reaction time and the peak temperature for the sample was studied at different cooling and heating rates.It was concluded that a knowledge of the structure and composition of the studied system is necessary in the interpretation of the DTA curves.

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Zusammenfassung Die Kristallisation von Al-Ce-Legierungen wurde mittels DTA untersucht. Die Schmelz- und Erstarrungs-DTA-Kurven der geprüften Legierungen weisen zwei Peaks auf, welche der Umwandlung einer Phase hohen Reinheitsgrades, bzw. der eutektischen Phase entsprechen. Die Peaks konnten sowohl während des Erstarrens als auch während des Schmelzens durch Änderung der Aufheiz- oder Abkühlgeschwindigkeit getrennt werden. Die Endtemperatur der Phasenumwandlung wird durch die Anfangstemperatur des zweiten Peaks angedeutet. Eine kleine Schulter des DTA-Peaks, selbst an der entgegengesetzten Seite des Maximums, kann der Endtemperatur entsprechen.Der Zusammenhang zwischen der Reaktionszeit und der Peak-Temperatur der Probe wurde bei verschiedenen Aufheiz- und Abkühlgeschwindigkeiten untersucht.Es wurde gefolgert, daß bei der Deutung von DTA-Kurven die Kenntnis der Struktur und Zusammensetzung des untersuchten Systems nötig ist.

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Résumé La cristallisation d'alliages Al-Ce a été étudiée par ATD. Deux pics apparaissent sur les courbes ATD lors de la fusion et de la cristallisation des alliages étudiés. L'un est dû à la transformation de la phase de haute pureté et l'autre à la phase eutectiques. Les pics peuvent être séparés en changeant les vitesses de chauffage et de refroidissement, tant à la fusion qu'à la cristallisation. La température finale de la transformation de phase est marquée par la température initiale du second pic. Un faible épaulement du pic ATD, même du côté opposé au maximum, peut correspondre à la température finale.Le rapport entre la durée de la réaction et la température du pic a été étudié pour diverses vitesses de refroidissement et de chauffage.On en conclut que l'interprétation des courbes ATD exige de connaître la structure et la composition du système étudié.

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1-. , , , . , , . . , , . . , , .

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Paper presented at the Scientific Session on Thermal Analysis held at Balatonfüred, Hungary, on 14–16 October, 1976.

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The authors are grateful to Mrs. M. Laár for the excellent preparation of the alloys. Thanks are due to Miss K. Papp and Mr. J. Szeleczki for their help in the thermal analysis. We are also indebted to Mrs. M. Horváth for the metallographic work.     

Electrodeposition kinetics of Li—Al alloys on Al: Effect of the third component

During cathodic intercalation of lithium from a 1 M LiC1O₄ solution in a mixture of propylene carbonate and dimethoxyethane (1 : 1 volume ratio), the process is limited by the lithium diffusion in the solid phase, no matter the nature of the third component. The activation energies for the process during formation of α-phase and subsequent formation of intermetallic compound β-LiAl are close and increase from 20.4-22.9 to 27.3-28.3 kJ mol^-1 after introducing Zn, Pb, and Cd, respectively. The initial concentration of lithium defects decreases from 3.6 × 10^-2 to 1.14 × 10^-2 mol cm^-3 in the series Al(Pb) > Al(Zn) > Al(Cd) > Al(Cr) > Al(Co) > Al > Al(Mn). The diffusion coefficient for lithium in β-LiAl varies from 8.25 × 10^-9 to 3.43 × 10^-9 cm² s^-1 in initial aluminum modified by lead or chromium, respectively. The incorporation of the third component in the aluminum matrix presumably facilitates the formation of additional vacancies and defects that make the lithium diffusion easier