The Determination of Dendrite Coherency Point Characteristics Using Three New Methods for Aluminum Alloys

Abstract

The aim of this work is to give an overview of existing methods and to introduce three new methods for the determination of the Dendrite Coherency Point (DCP) for AlSi10Mg alloys, as well as to compare the acquired values of DCP based on a thermal analysis and on the analysis of cooling curves working with only one thermocouple. Additionally, the impact of alloying and contaminant elements on the DCP will be also studied. The first two proposed methods employ the higher order derivatives of the cooling curves. The DCP was determined as the crossing point of the second and third derivative curves plotted versus time (method 1) or that of the temperature (method 2) with the zero line just after the maximum liquidus temperature. The third proposed method is based on the determination of the crossing point of the third solid fraction derivative curve with the zero line, corresponding to a minimum of the second derivative. A Taguchi design for the experiments was developed to study the DCP values in the AlSi10Mg alloy. The DCP temperature values of the test alloys were compared with the DCP temperatures predicted by the previous methods and the influence of the major and minor alloying elements and contaminants over the DCP. The new processes obtained a correlation factor r2 from 0.954 and 0.979 and a standard deviation from 1.84 to 2.6 °C. The obtained correlation values are higher or similar than those obtained using previous methods with an easier way to define the DCP, allowing for a better automation of the accuracy of DCP determination. The use of derivative curves plotted versus temperature employed in the last two proposed methods, where the test samples did not have an influence over the registration curves, is proposed to have a better accuracy than those of the previously described methods.

Zr as an Alternative Grain Refiner in the Novel AlSi5Cu2Mg Alloy

“…The addition of Zr also resulted in an increase in T liq (Table 2), which could have a direct effect on the increase in the DCT temperature [39]. The results obtained are consistent with the research of Gómez [41], who found that a smaller grain size results in more dendrites during the solidification process. This allows the tips of the dendrites to touch each other more quickly, increasing the T DCP value.…”

Determination of Solidification of Rigidity Point Temperature Using a New Method

“…Values were recorded for temperatures between 630 and 400 • C. A high-speed National Instruments Data Acquisition System (Field logger Novus) linked to a personal computer was used to collect temperature data every 1 s. Each TA test was done at least three times with a cooling rate of about 3 • C/s. First, the methods described in the literature were applied and then the results obtained compared with the recently proposed methods for AlSi 10 Mg alloys. Figure 1 shows the first method based on the determination of the second lowest point in the second derivative [11].…”

“…In TA, plotting temperature versus time, a curve is obtained that with its derivatives is used to characterize the solidification path of the alloy [7][8][9][10]. To obtain the cooling curve, liquid aluminum is preheated approximately to 100 • C above its liquidus temperature, and then it is poured into a defined thermal analysis cup made from ceramic, steel, graphite, or sand.…”

Special Issue on “Mechanical Behaviour of Aluminium Alloys”

“…Below are several characteristic examples. Staab et al [4] studied the stability of Cu-precipitates in Al-Cu alloys via the densityfunctional approach implemented through projected-augmented waves and plane wave expansions; the paper written by Li et al [5] presented outcomes of a study on the effect of T6 heat treatments on tribological properties of Al-Si alloys reinforced with SiCp and processed by the electromagnetic stirring method; the paper written by Dumitraschkewitz et al [6] provided an interesting insight into the impact of ternary-alloying elements on stacking fault energies intermetallic TiAl compounds using ab initio techniques; and finally, the paper written by Gómez et al [7] suggested alternative methods to assess the dendrite coherency-point characteristics in AlSi 10 Mg alloys on the basis of higher-order derivatives of cooling rates and on the basis of third solid-fraction derivative curves.…”