Mile B. Djurdjevic

Determination of dendrite coherency point characteristics using first derivative curve versus temperature

The dendrite coherency point (DCP) temperature refers to the state of a solidifying alloy at which a coherent dendrite network is established during the formation of grains. Several relatively complex methods for detection of the DCP temperature have been developed. There are four main DCP temperature testing approaches: (i) the rheological technique, (ii) thermal analysis of the minimum temperature difference between two cooling curves, (iii) thermal analysis of the second derivative of one cooling curve, and (iv) the thermal diffusivity measurement technique. This paper follows up the proposed thermal analysis of one center cooling curve for the determination of the DCP characteristics such as: temperature, time, instantaneous solidification rate, and fraction solid. The first derivative of the cooling curve is plotted versus the temperature and time and the thermal characteristics of all metallurgical reactions, including the DCP are determined with the same accuracy achieved using the two thermocouple technique developed by Bäckerud et al. [4, 5]. Statistical analysis of the DCP temperature using the one versus two thermocouple techniques shows R2 equal to 0.99. This research revealed that utilization of dT/dt versus the temperature curve methodology also allows for analysis of the α-Al dendrite nucleation and growth characteristics and consequent determination of the grain size. On-going work on this new methodology for characterization of other solidification events will be presented in subsequent papers.

Comparison of Newtonian and Fourier thermal analysis techniques for calculation of latent heat and solid fraction of aluminum alloys

The cooling curve analysis (CCA) has been used extensively in the metal casting industry, usually to predict alloy compositio n and microstructure constituents. The use of CCA can be expanded to other areas of solidification if the zero curves can be properly calculated. In this paper the Newtonian and Fourier techniques of zero curve determination are described. These techniques were developed to calculate latent heat and to determine the correlations between solid fraction and temperature/time for Al-7 wt%Si alloy. The importance of the changes in heat capacity and density of solid and liquid phases during solidification on the latent heat calculations was examined. The latent heat calculated by Computer-Aided Cooling Curve Analysis (CA-CCA) method is compared with those reported in the literature. The effect of experimental procedure and type of sampling cup on the latent heat calculations were studied for both techniques.

Synergy between thermal analysis and simulation

Available databases presently used by commercial simulation software packages for the aluminum casting industry are usually armed with material properties for only a few selected standard alloys. In the case of other alloys with different chemical compositions and refinement or modification treatment, thermal analysis could be an invaluable tool to gain necessary properties. The aim of this article is to demonstrate the potential application of the cooling curve analysis in the existing simulation software by improving its accuracy.

The Effect of Chemistry and Cooling Rate on the Latent Heat Released during the Solidification of the 3XX Series of Aluminum Alloys

The latent heat of solidification of any alloy depends on its chemistry that consequently affects the macro and microstructures for the given solidification conditions. In order to analyze the effects of chemistry on the release of latent heat during solidification of the industrial 3XX series of aluminum alloys, four different levels of silicon (5, 7, 9 and 11wt% Si) and three different levels of copper (1, 2 and 4 wt% of Cu) were taken into consideration. The solidification process was studied at cooling rates of 6 and 10°C/minute. The solidification path of these alloys was determined and the corresponding latent heat released during the solidification process was measured using a Differential Scanning Calorimeter (DSC). The tested hypoeutectic alloy chemical composition was expressed by the novel concept of silicon equivalency. The findings indicate that increases in the cooling rates shift the characteristic temperatures toward lower values without having a significant effect on the amount of released latent heat.

Al-Fin Bond in Aluminum Piston Alloy & Austenitic Cast Iron Insert

This paper presents the results of investigating an Al-Fin bond between an aluminum piston alloy and austenitic cast iron. The part investigated utilized an austenitic cast iron insert for the first ring groove for application in a highly loaded diesel engine for increased wear resistance. A metallographic investigation using an optical microscope in combination with SEM/EDS (Scanning Electron Microscopy/Energy Dispersive Spectroscopy) analysis and 3D visualization of the quality of the intermetallic bonding layer was performed. The test results show that a metallic bond can be formed between the aluminum piston alloy and the austenitic cast iron.

The Impact of Major Alloying Elements and Refiner on the SDAS of Al-Si-Cu Alloy

Abstract This paper investigates the effect of some major alloying elements (silicon and copper) and the effect of grain refiner (titanium boride) on the size of the secondary dendrite arm spacing (SDAS) in series of Al-Si-Cu alloys. It has been shown that both silicon and copper have significant influence on this solidification parameter. The addition of grain refining master alloys to aluminium alloys is common practice in many commercial foundries aiming to reduce the grain size of Al-Si alloys. However, it was shown in the present paper that master alloy based on TiB had an unexpected impact on the SDAS, decreasing the size of SDAS. In addition, there is a minimum of SDAS corresponding to the presence of 0.12 wt% of titanium in Al-Si alloy. Such findings could have important implications for Al-Si alloys in particular, due to their wide spread applications in the automotive industry.

Influence of different contents of Si and Cu on the solidification pathways of cast hypoeutectic Al-(5–9)Si-(1–4)Cu (wt.%) alloys

Abstract A comprehensive understanding of the solidification process is of paramount importance for the control and prediction of actual casting characteristics. The present work presents the potential of cooling curve analysis to characterize the solidification path of a cast hypoeutectic series of Al–Si–Cu alloys. The aim of this work was to examine how variation in chemical composition of Si (from 5–9 wt.%) and Cu (from 1–4 wt.%) may affect characteristic solidification temperatures, their corresponding solid fraction, and thermal freezing ranges of the investigated alloys. All solidification parameters that have been calculated using cooling curve analysis show good correlation with the corresponding parameters calculated using commercial Pandat software. These parameters, either collected from the cooling curve analysis or calculated using Pandat software, can be easily incorporated into existing simulation software packages in order to improve their accuracy. In addition, cooling curve analysis can be used to estimate the effect of cooling rate on the above mentioned solidification parameters and used as additional input data for simulation.

Quantification of the Impact of Strontium on the Solidification Path of the Aluminum-Silicon-Copper Alloys Using Thermal Analysis Technique

Abstract The impact of strontium on the solidification path of the AlSi6Cu4 aluminium alloy was examined. Strontium levels up to 210 ppm have been considered. The purpose of this study is: (i) to optimize the amount of strontium additions in AlSi6Cu4 melt in order to achieve the necessary degree of silicon modification and (ii) to observe the effects of strontium level on the other characteristic solidification temperatures of this alloy, with special emphasis on the nucleation temperature of copper rich phases. This investigation showed that a strontium level of approximately 140 ppm was sufficient to modify eutectic morphology of silicon in thermal analysis test samples. Increasing the strontium levels up to 210 ppm raises the nucleation temperature of copper rich phases, leading also to the higher area fraction of those phases.

Solidification Path of NemAlloy

The use of aluminium castings in automotive industries has dramatically increased in the last few decades due to the beneficial effects that arise by combining light weight and mechanical properties. Many of these castings have been used as parts of diesel engine. Diesel engines are subjected to high thermo-mechanical stresses, especially in the combustion chamber area of their cylinder heads. Due to the increasing demands needed for high power engines, weight reduction and elevating thermal and mechanical loads current casting produced using standard aluminium alloys (family of Al-Si and Al-Si-Cu alloys) are already reaching their limits, especially at elevated temperatures (>250 °C). New aluminium alloys able to fulfill these customers’ demands are therefore needed. Recently, the NemAlloy (AlCu7MnZr) has been developed by the Nemak Linz R&D-Team for cylinder head production for high performance engines. Beside good mechanical properties at elevated temperatures, the new alloy offers: hot tearing resistance, high thermal conductivity, low thermal expansion, high thermal shock resistance, corrosion resistance and good suitability to massive production. In the present paper the precipitation sequences that take place during solidification of this alloy have been identified applying Thermal analysis technique. Thermal analysis has been successfully applied in aluminium foundries for characterization of aluminium cast alloys. Characteristic solidification temperatures (TLIQ, TDCP, \( {\text{T}}_{\text{Eu}}^{\text{AlSi}} \),\( {\text{T}}_{\text{Rig}} \), \( {\text{T}}_{\text{Eu}}^{\text{AlCu}} \), TSOL…) and fraction solid distribution have been collected to support simulation as well as to provide necessary data for process development. The solidification path of the NemAlloy (AlCu7MnZr) has been compared with the precipitation sequence observed during solidification of the standard AlSi7MgCu alloy currently applied for cylinder head production.

Impact of Silicon, Magnesium and Strontium on Feeding Ability of AlSiMg Cast Alloys

This paper shows how characteristic solidification temperatures, including rigidity temperature can be used to quantify the various feeding mechanisms that occur during solidification of AlSiMg alloys. In addition, the impact of Silicon, Magnesium and Strontium on the temperature intervals of various feeding regions have been analyzed.