2020 Volume 61 Issue 6 Pages 1077-1083
The semi-solid slurry of A356 alloy of various solid fractions was fabricated and water quenched to freeze the morphology of the Al grains in the semi-solid state. Two kinds of specimens with different volume (large and small) were prepared to change the cooling rate during the quenching. The difference in the amount of solid growth around the original spherical Al grains during the quenching was investigated by the color etching method with Weck’s reagent, which is sensitive to the solute segregation. It was found that the amount of solid growth during the quenching increased with a decreasing cooling rate. The amount of solid growth of each spherical Al grain also increased for a decreasing local solid fraction, or the amount of residual liquid and the mutual distance between the solid grains. The solid fraction obtained by the color etching method showed a good agreement with the estimated solid fraction from the phase diagram. It was confirmed that the present color metallography was effective to evaluate the correct solid fraction at the semi-solid state.
Fig. 5 Microstructures of the large specimen hold at and quenched from 588°C (a) etched by NaOH, A: the area of solid, (b) etched by Weck’s reagent, (c) the original solid phase eliminating solid growth during water quenching, A consists of A0 (original solid area) and Aq (solid growth during water quenching).
The semi-solid process is a kind of net-shape manufacturing process, compared with the conventional casting processes using completely melted metal, the semi-solid process offers several advantages to reduce the porosity, such as better mold filling ability (solid-front fill), less air entrapment, and lower solidification shrinkage.1) This process has two basic methods, rheocasting and thixocasting. In the rheocasting, a feedstock is completely melted and cooled down to the semisolid temperature and stirred there to obtain the semi-solid slurry.2,3) As for thixocasting, the feedstock, which is prepared by rheocasting in advance, is heated up to the target temperature to obtain the semi-solid state.4–7) It is known that the integrity and performance of components processed by thixocasting are better than those of the components fabricated through rheocasting.8)
A356 alloys with a relatively wide solidification temperature range have been widely used for the thixocasting process. To achieve a good fluidity for semi-solid casting and to control the quality of the product, the accurate evaluation of the solid fraction is important. The solid fraction can be estimated by the lever rule using a phase diagram. The microstructure observation of the quenched specimen from the semi-solid state is often performed to check the validity of this estimation. However, during water quenching, the growth of the original solid grains, which existed at the semi-solid temperature is inevitable. Therefore, there is no doubt that this leads to an overestimation of the solid fraction. In our previous research, we reported that Weck’s reagent is applicable to solve this problem.9)
Weck’s reagent was developed by Weck and Leistner in the 1980s.10) This reagent is a kind of color etchant and can be used to investigate the solute segregation in aluminum alloys. Weck’s reagent provides a sharp color contrast in the microstructure of cast and welded aluminum alloys. The detail of the etching mechanism was explained by Suárez-Peña and co-authors:11) Weck’s reagent consists of 4% KMnO4, 1% NaOH and 95% H2O. The initial product of the corrosion of aluminum in an aqueous environment is the hydroxide aluminum, Al(OH)3. Then, Al(OH)3 creates a hydrated oxide (Al2O3·H2O). When the sample is immersed in Weck’s reagent, based on an alkaline solution of potassium permanganate (pH value is 13 approximately), the oxide layer is dissolved. The fresh aluminum surface is exposed and it reacts with KMnO4 and a colored film (MnO2) is formed on the surface:
| \begin{equation*} \text{Al} + \text{MnO$^{-}{}_{4}$} + \text{2 H$_{2}$O} \rightarrow \text{Al(OH)$^{-}{}_{4}$} + \text{MnO$_{2}$} \end{equation*} |
In the previous research, the growth layer formed on the original spherical Al grains during water quenching was clearly visualized by etching with Weck’s reagent, and the precise evaluation of the solid fraction was achieved by image analysis of the etched microstructure. For the conventional solid fraction evaluation method such as quantitative metallography, it has the problem of overestimation due to the grain growth during quenching. Using Weck’s reagent to avoid the overestimation can be regard as a modified quantitative metallography.9,12,13)
The microstructure analysis was performed only for the small size specimens, which were cooled at a fixed cooling rate. In order to perform the die-casting for preparing the tensile specimen for future works, much larger size billets were used. The growth behavior of the solid layer during water quenching is considered to change depending on the cooling rate. Therefore, in order to confirm the applicability of Weck’s reagent for microstructures analysis of the semi-solid slurry, the effect of the cooling rate should be investigated. In the present study, A356 alloy slurry specimens with different volumes were quenched from the semi-solid state, and the microstructure change due to the cooling rate difference was investigated.
The material used in this research is A356 Direct-chill cast aluminum alloy supplied in cylindrical shape with a diameter of 105 mm and a length of 450 mm. In order to keep the volume of the specimen is same with the proper volume slurry for die-casting, the smaller cylindrical of A356 cast aluminum alloy billets with a diameter of 30 mm and a length of 100 mm were collected from the A356 Direct-chill cast aluminum alloy. The chemical composition of the alloy obtained by Optical Emission Spectrometer is shown in Table 1. Ti and Sr were added to refine the α-Al phase and eutectic phase, respectively. In order to obtain spherical grains, inducing plastic strain by cold compression before recrystallization and partial melting was adopted.9,14) The billets were compressed axially by 33% at a room temperature as shown in Fig. 1(a). The cold compression was done by a 500 ton compressing machine, and the compressing rate was 0.2 mm/sec. The temperature increase of the billet during compression was about 20°C.
(a) Definition of large specimen and small specimen used in the present study. (b) Schematic of thermocouples insert in the specimens in the present study. (c) Schematic temperature-time curve for heating, holding and cooling process in the present study.
Two kinds of specimens with different volume were prepared as shown in Fig. 1(a). One is the as-compressed billet of 175 g in weight. The other is a rectangular bar specimen with 20 × 10 × 6 mm3 of 3 g in weight, which was collected form the as-compressed billet. They are called “large specimens” and “small specimens”, respectively. In order to measure the temperature of the specimens, a small hole was machined at the side of each specimens and a thermocouple was inserted into the hole as shown in Fig. 1(b). They were put into the electrical furnace and heated to the several kinds of semi-solid temperature (588, 595, 602, and 608°C), which were corresponded to the solid fraction of approximately 60%, 50%, 40% and 30%. These values were determined by the result of solid fraction evaluation in previous research.12) The heating rate was about 24°C/min. When the target temperature was reached, the specimen was isothermally held for 5 min in order to make sure that the specimen was heated homogeneously. After isothermal holding, the specimen was quickly quenched in water, as shown in Fig. 1(c).
Different cooling rates during water quenching were obtained by using different volume specimen. A set of the large (175 g) and small specimens (3 g) were prepared for the measurement of different cooling rates comes from the different cooling conditions. The thermocouples were set to the center and the surface of the large specimen. The thermocouple was also set to the center of the small specimen. They were heated to 588°C and isothermally held for 5 min, then rapidly quenched in water. The cooling rates were obtained by the recorded temperature-time curves.
2.3 Microstructure observation and chemical analysis by EPMAA number of cubic specimens with 8 × 8 × 5 mm3 were collected from the quenched large specimens for the microstructural evaluation. They were ground using 120, 600, 1000, 2000 and 4000 grit paper, polished using 3 and 1 µm diamond paste, and finished using OPS colloidal silica. They were immersed in the Weck’s reagent for 12 s at room temperature. The etched specimens were rinsed well and observed by optical microscopy. The same treatment was made for the cross section of small specimen.
EPMA analyses by X-ray Wavelength Dispersive Spectroscopy (WDS) were carried out to investigate the micro-segregations of Mg, Si and Ti of the quenched specimens. The location of the chemical analysis was the same as the location where the optical micrograph was taken. The acceleration voltage was set to 15 kV. Electron beam current was set to 185 nA. The distribution mapping of the element was made by measuring the intensities of characteristic X-rays, after which the intensity was converted into atomic percentage of each element using the calibration standards.
Figure 2 shows optical micrographs of the as-polished and the etched large specimen held at 588°C and water-quenched. The micrographs were taken from the same location. The spheroidal Al grains were obtained by the process of compressing, partially re-melting and rapidly cooling. The spheroidal Al grains were surrounded by the rapidly solidified eutectic structure, which corresponds to the liquid phase at the semi-solid state. Several round spots are observed in the spheroidal Al grain. They consist of very fine eutectic solidified structure, which results from the rapid cooling of the entrapped liquid droplets inside the solid grains at the semi-solid state. Figure 2(b) shows the microstructure obtained by etching with a conventional NaOH reagent. The morphology of the spheroidal Al grains and the eutectic structure are clearly observable with the help of etching. However, there is no large difference between Fig. 2(a) and (b). In contrast, as shown in Fig. 2(c), characteristic color differences are visible in the microstructure etched by Weck’s reagent. In addition to the complicated color difference inside the spherical Al grain, a ring-shaped pale color contrast is observed around the spheroidal Al grain. It is considered to be the ring-shaped region indicating the grain growth during water quenching. The original liquid region by the water quenching turned to the fine eutectic solidified structure. The eutectic solidified region exhibited a blue color in the picture.
Microstructure of the quenched large specimen from 588°C (a) as-cast (b) etched by NaOH and (c) etched by Weck’s reagent.
The origin of the color difference was examined by the EPMA analysis. Figure 3(a) shows the optical micrograph of the etched specimen. Figure 3(b) to (d) show the solute distribution in the structure for Si, Mg and Ti, respectively. The attached color bars show the level of composition for each element. High concentration of Si and Mg was observed in the eutectic solidification region, which is located around the spherical grains and inside the each spherical grains (Fig. 3(b) and (c)).
Optical micrograph of the specimen etched by Weck’s reagent (a) and results of EPMA analysis, (b) Si, (c) Mg, (d) Ti.
Segregation of Si and Mg could not be observed inside the spheroidal Al grain, except for the entrapped region. The characteristic solute distribution patterns were observed for Ti, as shown in Fig. 3(d). The first is the heavy segregation at the center of the spherical Al grain. The second is the ring-shaped segregation pattern surrounding the spherical Al grain.
The first pattern results from the original segregation of Ti in the dendrite structure of the cast billet. As reported in the previous work,12) the heavy segregation of Ti in the dendrite branches remain even after the holding at the semi-solid temperature. This is due to the low diffusion rate of Ti in Al at this temperature compared to those of Si and Mg.15–19) Therefore, even though Si and Mg are homogenized enough in the spherical Al grain, the core structure of Ti can be preserved like this.
The second pattern is probably due to the normal segregation of Ti corresponding to the phase diagram of Al–Ti system. The partition coefficient of Ti in Al is larger than 1. This means that the concentration of Ti should be high at the start of solidification and then gradually decreases as in the solidification progresses. Comparing Fig. 3(a) and (d), the solute distribution of Ti from the inside to the outside of the ring-shaped region is consistent with this segregation pattern. This fact suggests that the ring-shaped region results from the solid growth during water quenching from the original spherical Al grain surface.
Figure 4(a) and (b) show the microstructures of the large specimen and the small specimen etched by Weck’s reagent, respectively. The ring-shaped layers formed during water quenching can be clearly seen in both specimens. Figure 4(c) and (d) show highly magnified SEM images of the eutectic solidified region formed by quenching of the remaining liquid. The eutectic structure in the large specimen is much coarser than that of the small specimen. This is due to the lower cooling rate in the large specimen during quenching. It is clear that the thickness of the ring-shaped region surrounding the spheroidal Al grain is thicker in the large specimen (Fig. 4(a)) than that of the small specimen (Fig. 4(b)). This fact also supports the hypothesis that the ring-shaped area is formed during quenching and the amount of solid growth increases as decreasing the cooling rate.
(a) and (b): Optical micrographs of the etched microstructure by Weck’s reagent of the quenched large specimen and small specimen from 588°C. (c) and (d): SEM image of the quenched liquid region showing the eutectic solidified structure.
The correct evaluation of solid fraction is important because it controls the fluidity of semi-solid slurry and affects to the mold-filling ability. In the previous research,12) the solid fraction measurement for the quenched small specimens was performed with the help of Weck’s reagent.
The etching by using Weck’s reagent can visualize the spheroidal grain growth during water quenching. This ability helps us to evaluate the solid fraction by using the optical micrography. Figure 5(a) shows an example of the microstructure of the quenched semi-solid slurry. The specimen was etched with NaOH water solution. The globular grains were distinguishable from the matrix, which were the quenched residual liquid at the semi-solid state. In this case, the solid fraction evaluated by conventional metallography etching method fs(NaOH) can be evaluated by the following equation:
| \begin{equation*} f_{s(\textit{NaOH})} = \frac{\displaystyle\sum A}{S} \end{equation*} |
| \begin{equation*} f_{s(\textit{Weck})} = \frac{\displaystyle\sum A_{0}}{S} \end{equation*} |
Microstructures of the large specimen hold at and quenched from 588°C (a) etched by NaOH, A: the area of solid, (b) etched by Weck’s reagent, (c) the original solid phase eliminating solid growth during water quenching, A consists of A0 (original solid area) and Aq (solid growth during water quenching).
In this study, the solid fraction of both quenched small specimens and large specimens were evaluated with the help of Weck’s reagent. For the small specimen, 6 micrographs with 100× magnification (with an area of about 2 mm2) were taken from the cross-section of the quenched specimen. For the large specimen, 5 cubic specimens (8 × 8 × 5 mm3) were collected from the quenched large specimen. 4 micrographs with 100× magnification (with an area of about 2 mm2) were taken from each of them, and provided for evaluation. The solid fraction fs(NaOH) and fs(Weck) were evaluated by using the NaOH-etched and Weck’s reagent-etched specimen, respectively.
Figure 6(a) shows the relationship between fs(NaOH) and holding temperature for the small and large specimen. The results obtained by the previous study12) are also included. A solid curve is shown in the figure. This is the estimated change of solid fraction of Al–7%Si alloy obtained by using the Al–Si binary phase diagram.12) The chemical composition of A356 alloy is comparable to that of Al–7%Si binary alloy. Both fs(NaOH) curves for the small and large specimens show a large deviation from the estimated values. It is also found that fs(NaOH) of the large specimen is higher than that of the small specimen.
Solid fraction evaluated by using two methods. (a) Conventional etching method by using NaOH water solution. (b) Color metallography etching method by using Weck’s reagent.
Figure 6(b) shows the relationship between fs(Weck) and holding temperature for the small and large specimen. In this case, fs(Weck) of the small specimen and that of large specimen are almost the same. In addition, these curves are close to the solid fraction estimated from the phase diagram. These results suggest that fs(Weck) represents the solid fraction at the semi-solid state of each holding temperature. Therefore, it is confirmed that the removal of Aq, resulting from the solid growth during quenching, is reasonable to evaluate the solid fraction correctly, and the color metallography by using Weck’s reagent is effective.
3.3 Effect of cooling rates on the amount of solid growth during quenchingThe cooling rate during quenching of the large specimen was about 520°C/s at the central part, and about 730°C/s at the top surface. It was about 1400°C/s for the small specimen.
The precise microstructure analysis was performed for the specimens collected from the different parts of the large specimen, and the amount of solid growth during water quenching was compared. The amount of solid growth during water quenching was evaluated by using the following parameter:
| \begin{equation*} \Delta q = \frac{A_{q}}{A_{0}} = \frac{A - A_{0}}{A_{0}} \end{equation*} |
This parameter refers to the relative contribution of solid growth during quenching to the original globular grain. A slight difference in solid fraction was observed from location to location. In addition, the amount of solid growth during water quenching (Δq) was also different from location to location as shown in Fig. 7. The local solid fraction fs(local) is the solid fraction only in a small visual field (one 200× magnification micrographs with an area of about 0.5 mm2). Compared with the solid fraction of small specimen (average of 6 micrographs with 100× magnification micrographs) and large specimen (average of 20 micrographs with 100× magnification micrographs) included about 1800 grains and 6000 grains, respectively, the local solid fraction only included about 75 grains. It was evaluated by color metallography etching method by using Weck’s reagent as the etching reagent.
The optical micrograph of the quenched specimens and the painted area representing the original solid phase before quenching. (a) and (c): The location with lower local solid fraction (about 35%). (b) and (d): The location with higher local solid fraction (about 50%).
Figure 8 shows the relationship between Δq and the local solid fraction for three different cooling rate conditions.
Quantitative analysis result of the amount of grain growth during water quenching, Δq, under the diffrent local solid fractions.
Δq decreases for an increasing local solid fraction for all the cooling rate conditions. This result also shows that the amount of solid growth is controlled by the local cooling rate, which corresponds to the amount of residual liquid before quenching. The thermal conductivity of solid is higher than that of liquid, which means the local cooling rate decreases for an increasing amount of residual liquid. With decreasing the amount of residual liquid, the amount of solid growth around the spherical Al grain during water quenching was considered to be reduced. In addition, the larger amount of residual liquid also results in a larger mutual distance between the spherical Al grains and provided more space for solid growth during water quenching. Therefore, the amount of solid growth is probably controlled by the amount of residual liquid before quenching.
The semi-solid slurry of A356 alloy with different volume of various solid fractions were fabricated and water quenched to freeze the morphology of the Al grain in the semi-solid state. Optical microstructure observation was carried out by using Weck’s reagent to visualize the inner microstructure of spherical Al grains, with a focus on the solid growth during water quenching of each spherical Al grain. A ring-shaped pale color contrast region was observed around the spheroidal Al grain, and this region corresponded to the solid growth during water quenching from the original spherical Al grain surface. The amount of solid growth during quenching Δq, increased as decreasing the cooling rate. It was found that Δq of each spherical Al grain also increased when the local solid fraction decreased. These results suggest that the present color metallography is effective for the correct evaluation of solid fraction of the A356 alloy slurry.
