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Open Access 2024 | OriginalPaper | Buchkapitel

9. Influence of Reactive Filter Materials on Casting’s Quality in Aluminum Casting

verfasst von : Beate Fankhänel, Ekaterina Schmid, Michael Stelter

Erschienen in: Multifunctional Ceramic Filter Systems for Metal Melt Filtration

Verlag: Springer International Publishing

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Abstract

This chapter presents the most important results of investigations on reactive filter materials for the purification of aluminum melts. Reactive filter materials were developed with the aim to remove impurities dissolved in the melt, such as hydrogen from liquid aluminum, by means of specific chemical interactions between the molten metal and the filter material. Selected ceramic foam filters, consisting of carbon-free and carbon-bonded ceramics, were used in their uncoated state as well as treated with various coatings. Numerous fundamental studies were carried out to evaluate the applicability of the new filter materials: sessile-drop-experiments, immersion and filtration tests, the metallographic evaluation of the used filters and the metal samples retrieved from these experiments. Interfacial reactions and the purity of the treated melts were determined with the help of these experiments, following microstructural analyses to obtain indications for the filtration properties and the potential chemical reactions between the filter material and the melt. As a result, it was possible to determine that spodumene, LiAl(Si₂O₆), positively influences the hydrogen porosity of aluminum castings when applied as a reactive filter material. Filtration alone already helps to prevent areas of increased macroporosity by calming the melt flow, but filter materials containing spodumene further affect microporosity in the castings in positive ways.

9.1 Introduction

The purity of melts plays an essential role in the production of high-quality aluminum castings. Since more than half of the aluminum alloys produced today originate in secondary metallurgy, an increasing number of impurities also find their way into the alloy melts. These impurities can be present in the melts both in dissolved form (e.g. H₂, undesirable alloying elements) and in solid form (e.g. Al₂O₃, MgO, AlN, NaCl, TiB₂). Impurities in solid form can be formed during melting and liquid holding in the melt (endogenous inclusions) or can enter the melt from outside through reactions with the atmosphere or the refractory (exogenous inclusions). Many of these inclusions lead to defects and undesirable macroporosity in the castings. Due to the high solubility of hydrogen in the aluminum melt, its precipitation during solidification in particular leads to increased porosity and a decrease in casting quality. A pronounced hydrogen porosity can thus lead to a reduction in strength, toughness, surface quality and corrosion properties of the components [1].

Even though there are already various ways of reducing the hydrogen content and particulate inclusions of an aluminum melt, solidification in many castings does not always proceed uniformly, so that areas with undesirable macroporosity can still form. In addition, non-metallic inclusions can be generated during melt cleaning by reactions with flushing gases or salt residues. For this reason, it can be advantageous to use filter materials that are specially adapted to the impurities to be removed from the aluminum alloys. The use of ceramic foam filters has been a proven process in aluminum casting for several years [2‐5]. With the help of melt filtration, a laminar melt flow is created and solid impurities can be removed. Both improve casting quality. The possibility of coating filters with active materials to achieve improved melt cleaning is becoming increasingly important [6, 7]. Reactive materials could help to force hydrogen precipitation at the beginning of solidification and/or to bind the precipitated hydrogen in order to effectively reduce the hydrogen content of the melt in time.

Although the processing temperatures of aluminum melts are not in extreme ranges, filtration as a cleaning process on an industrial scale can take 30 min or longer, so that a non-negligible interaction between aluminum melt and filter material must be expected. In order to investigate these interactions more in detail, not using the considerable quantities of metal required for a correspondingly long filtration period, laboratory immersion tests and sessile drop experiments were used. In the case of the immersion experiments, compact pieces of the filter materials were immersed in the molten aluminum for a certain time and moved therein. Sessile drop experiments are a method for investigating the wetting behavior of a fluid (molten) media in relation to solid materials as a function of temperature and time. These investigations were completed by casts in the laboratory to obtain an initial impression of the effectiveness of the new filter materials under operating conditions.

9.2 Interaction Between Aluminum, Its Alloys and Reactive Filter Materials

As already mentioned, wetting, roughness [8] and potential chemical reactions between the filter material and the molten metal play a major role on the filtration effect. For this reason, immersion tests were carried out in addition to sessile drop experiments as preparation for the filtration tests. In order to be able to make more realistic statements, substrates were used for the investigations whose surfaces were adapted to those of the filter materials and for whose production the same routine as for the filter production was applied.

9.2.1 Reactivity of Spodumene

While alumina showed a non-reactive behavior towards pure aluminum and selected alloys in all studied cases, a reactive behavior could be detected for other oxides under certain circumstances [9]. However, spodumene (LiAl(Si₂O₆)) represents a special type of filter material. Spodumene is a naturally occurring material that is chemically a mixed oxide: ½Li₂O · ½Al₂O₃ · 2SiO₂. The reason for researching the lithium oxide-containing material as a potentially reactive filter material was the assumption that Li₂O might be able to form a compound with hydrogen dissolved in the aluminum, thus reducing the hydrogen content in the alloy. Lithium salts have been used for several years for melting point reduction in aluminum production and for oxide removal in aluminum welding [10, 11], so that a basic interaction between spodumene as a filter material and the aluminium melt could be expected. Thermodynamic calculations of possible reactions between the liquid aluminum, the Al₂O₃ as the oxide layer and the LiAl(Si₂O₆) as filter material were carried out with FactSage8.1® for different conditions. These calculations expected the formation of LiAl₅O₈, LiAlO₂ and silicon at the cost of Al₂O₃ in case of a typical casting situation (Eq. 9.1).

$$\begin{aligned} & {4}0{\text{Al }} + \, 0.{\text{1Al}}_{{2}} {\text{O}}_{{3}} + {\text{ LiAl}}\left( {{\text{Si}}_{{2}} {\text{O}}_{{6}} } \right) \to {37}.{\text{33Al}} \\ & \, + {\text{ 2Si }} + \, 0.{\text{72LiAl}}_{{5}} {\text{O}}_{{8}} + \, 0.{\text{28 LiAlO}}_{{2}} \\ & (\Delta {\text{G }} = \, - {315 } - \, - {\text{323 kJ}},{ 75}0 \, - { 65}0 \, ^\circ {\text{C}},{\text{1 bar}}) \\ \end{aligned}$$

(9.1)

In the presence of gaseous hydrogen, as released from the molten aluminum during filtration and cooling, the following reaction scenario was calculated (Eq. 9.2):

$$\begin{aligned} & {4}0{\text{Al }} + \, 0.{\text{5H}}_{{2}} + {\text{ LiAl}}\left( {{\text{Si}}_{{2}} {\text{O}}_{{6}} } \right) \to {\text{37Al }} + {\text{ 2Si }} \\ & + \, 0.{\text{75LiAl}}_{{5}} {\text{O}}_{{8}} + \, 0.{\text{25LiAlH}}_{{4}} + \, 0.{\text{1Al}}_{{2}} {\text{O}}_{{3}} \\ & (\Delta {\text{G }} = \, - {266 } - \, {\text{281 kJ}},{ 75}0 \, - { 65}0 \, ^\circ {\text{C}},{\text{1 bar}}) \\ \end{aligned}$$

(9.2)

Here a hydrogen-containing compound could be formed in addition to the lithium aluminate. However, phases containing lithium and/or hydrogen are undetectable with many analytical methods. Therefore, plasma-based Secondary Neutral Mass Spectroscopy (SNMS) was used as a characterization method for the verification of lithium and hydrogen in the filter materials. This method is based on ion bombardment of the sample and subsequent ionization of the sputtered neutrals [12]. Experiments were performed using the INA-X equipment from SPECS GmbH, Germany [13]. In this configuration an Electron Cyclotron Wave Resonance (ECWR) plasma serves both as source for primary ions and for post ionization. After passing an ion optic, the post-ionized neutrals are separated by a quadrupole mass analyzer and counted by a secondary electron multiplier. The SNMS measurements were done in the so-called High Frequency Mode (HFM) due to the dielectric properties of the investigated samples [14].

SNMS mass spectra (intensity vs. mass number) were plotted for ceramic foam filters containing spodumene (pure LiAl(Si₂O₆) and mixed oxide of 85% Al₂O₃ 15% spodumene) as well as for the Al₂O₃ filters as reference. Each of them were measured three times by using a copper mask (5 mm in diameter) positioned on top of the sample. For the measurements, a krypton plasma with 152 W, 4.5 A and a working pressure of 2 · 10^–3 mbar was used. The applied voltage was set to 500 V at a frequency of 91 kHz and a duty cycle of 60%. The distance between the sample surface and the molybdenum aperture during measurements was 1.5 mm. The sputtering time for each mass spectrum was 210 s.

The presence of lithium content within the samples was examined to make sure that the sintering step of the spodumene did not lead to a complete evaporation of the lithium. The SNMS measurements were carried out at the surface of substrates used for the sessile drop measurements. The measured intensity depends not only on the concentration of the atoms, but also on detection factors which are influenced by geometry factors, post-ionization and on transmission factors [15, 16]. For this reason, a quantification of the lithium content would only be possible after referencing the lithium containing components, which is very demanding and time intensive, and has not yet been carried out. However, the qualitative analysis already showed clear lithium peaks indicating the presence of lithium at the surface of sintered lithium containing coatings in comparison to pure alumina substrates without any lithium peaks (Fig. 9.1).

Furthermore, heat treatments of these filter materials under a hydrogen-containing atmosphere were carried out to simulate the contact between the filter material and gaseous hydrogen, which will be released during solidification of the metal melt. For this purpose, heat treatments of spodumene-containing substrates were carried out for 30 to 90 min at temperatures between 700 and 1000 °C under a forming gas atmosphere consisting of 90 vol.% Ar and 10 vol.% H₂ to investigate the ability of the spodumene to react with gaseous hydrogen. Indeed, hydrogen-containing reaction products were detected on the substrate by SNMS measurements. While in the case of untreated substrates almost no hydrogen was measured, the substrates exposed to the hydrogen atmosphere showed clear intensity maxima relatable to hydrogen. It was irrelevant whether the substrate consisted of sintered Al₂O₃ with 15 wt.% spodumene or of a pure spodumene surface. Similar measurements with pure Al₂O₃ showed hydrogen peaks as well, but to a much lesser extent than with the spodumene-containing samples.

The spectra recorded by SNMS for the element distribution on the sample surface thus indicate that hydrogen is already adsorbed or bound in the filter material from 700 °C onwards (Fig. 9.2).

9.2.2 Sessile Drop Experiments

Procedure

A common method for determining wetting properties is the sessile drop method. The method is based on the evaluation of the shape of liquid droplets on solid surfaces. The shape of the drop is essentially determined from the balance between surface tension and weight. Based on the droplet contour, both the surface tension of the liquid and the contact angle at the triple point of the system can be calculated if the density of the droplet material is known. The basis for the calculations is the Young–Laplace equation (Eq. 9.3), which describes the relationship between the drop shape (r = spherical radius), the surface tension (γ_lg) and the pressure difference (∆p) at the phase boundary.

$$\Delta p= \frac{2 {\gamma }_{lg}}{r}$$

(9.3)

The influence of gravity on the droplet shape can be neglected for small droplets (m < 100 mg) so that a spherical segment can be assumed as the droplet shape [17]. In this case, it is also possible to calculate the contact angle (θ) using the drop height (h) and the diameter at the drop base (d) (Eq. 9.4):

$$\theta \left(rad\right)=2{{\text{tan}}}^{-1}\frac{2h}{d}$$

(9.4)

The existing sessile drop system consists of a 3-zone melting furnace with a maximum working temperature of 1550 °C manufactured by Gero GmbH (Neuhausen, Germany), a vacuum and inert gas system and an optical evaluation unit with CCD camera for computer-aided recording of the melt drop and calculation of the contact angle.

In order to remove the oxide layer on the surface of the aluminum completely and to establish contact between the liquid metal and the substrate surface, a high temperature and a high vacuum pressure are necessary. Only then, the Al₂O₃ at the drop surface is reduced by liquid aluminum to metastable Al₂O. (Eq. 9.5).

$${\text{4Al}}\left( {\text{l}} \right)\, + \,{\text{Al}}_{{2}} {\text{O}}_{{3}} \left( {\text{s}} \right)\, \to \,{\text{3Al}}_{{2}} {\text{O }}\left( {\text{g}} \right)$$

(9.5)

The thus formed Al₂O is gaseous under the above conditions and can be removed via the vacuum pump system. Prior to the sessile drop experiments, the equilibrium partial pressure of Al₂O as a function of temperature was determined for Eq. 9.5 using FactSage® (Fig. 9.3). From the calculation, a temperature of 950 °C and a vacuum pressure of 1.5 · 10^–5 mbar were determined as minimum requirements for the experiments.

Consequently, all sessile drop experiments were carried out according to the following procedure: At first, the metal samples (m < 100 mg) were cut on all of their faces to receive fresh surfaces with oxide layers thinner than 25 Å as described by Bianconi et al. [18] immediately before placing it onto the substrate in the furnace. The furnace was evacuated for 90 min to reach a pressure of p < 1.5 · 10^–5 mbar. Afterwards the heating process started with a heating rate of 350 K/h up to a maximum temperature of 950 °C, which was held constant for 180 min. At this time, a vacuum pressure of 7.4 · 10^–6 ± 0.4 · 10^–6 mbar on average was measured within the furnace. A detailed description of the process and the contact angle calculation is given in [9, 19].

Influence of the Surface Morphology

The wetting behavior of a liquid phase on a solid phase is also influenced by the surface morphology of the solid phase. For this reason, various surface qualities were investigated with regard to their influence on the contact angle between metal and filter material and the resulting wetting statement, which is important for the filtration effect [9]. The classical sessile drop method for determining the contact angle usually uses dense polished substrates to obtain unobstructed contact between the liquid phase and the solid substrate. The surfaces of the filter struts, on the other hand, are open pored and have a natural roughness. For this reason, the investigations to determine the wetting behavior and the interaction between metal and filter material were extended to include the use of substrates with surfaces comparable to those of the filter struts. Both the filter material in itself as well as the surface roughness of the filter struts determine the effect of the filters during filtration. Both can be reproduced very well by coated substrates. The coating process is a combined dipping and centrifuging process. It has been already used to produce the appropriate foam filters and described in detail by Voigt et al. [6].

As a result, compact substrates are obtained whose surface morphology is comparable to that of the filter struts. Examples are shown in Table 9.1 and Fig. 9.4. It can be seen that the surfaces of the coated substrates are quite comparable with those of the filter struts. In the case of Al₂O₃, there is almost no difference, and also in the case of the other materials the average surface height S_a is similar. The area factor S_dr (ISO 25178), which indicates the increase in surface area compared to an ideally smooth surface of the same lateral dimension, differs apart from the alumina from one another. This difference probably is caused by a certain surface roughness of the filter struts in contrast to the pressed tablets. The surface morphology can be enhanced by the coating and cause an increase in the effective surface area.

Table 9.1

Surface roughness (arithmetic average height S_a, area factor S_dr according ISO 25178) of coated substrates compared to filter struts of selected filter materials

Filter material	Coated substrate S_a in µm	S_dr in %	Filter struts S_a in µm	S_dr in %
Al₂O₃	2 ± 0.2	18 ± 1	4 ± 0.5	17 ± 6
85% Al₂O₃ 15% LiAl(Si₂O₆)	10 ± 1	65 ± 9	8 ± 2.5	153 ± 13
TiO₂	1 ± 0.1	35 ± 1	6 ± 0.9	99 ± 14
3Al₂O₃·2SiO₂	13 ± 1	228 ± 6	11 ± 2	379 ± 82

Sessile drop experiments carried out with these substrates showed that a non-wetting behavior occurs in all cases, with the exception of the spodumene-containing materials, which will be discussed separately in the next section [9, 19]. Filters made of mullite (3Al₂O₃ · 2SiO₂), for example, show significantly poorer wetting than other oxidic filter materials only due to their greater surface roughness. Thus, the wetting behavior of all investigated oxidic filter materials (Al₂O₃, 3Al₂O₃ · 2SiO₂, MgAl₂O₄, TiO₂) on an AlSi7Mg alloy was comparable. Furthermore, it was shown that all substrates, but the pure Al₂O₃ form reactive systems in presence of the AlSi7Mg alloy under the conditions of the sessile drop experiments. This involved, for example, the reduction of TiO₂ or SiO₂ by the liquid aluminum, which thereby oxidizes itself to Al₂O₃. The silicon thus formed passes into the melt in metallic form, while the TiO₂ reacts to form Ti₂O₃ and with magnesium-containing aluminum alloys to form MgTiO₃ [9, 20]. If an aluminum melt comes into contact with a magnesium aluminate (MgAl₂O₄), it is depleted of magnesium at the contact zone, which can diffuse into the melt. What remains is a peripheral zone of Al₂O₃. Therefore, it was supposed that the equilibrium value of the contact angle is more dominated by the surface roughness than by one of the investigated materials apart from the alumina.

The chemistry of the filter coatings influences only the course of the wetting process in itself.

Since time and temperature are much lower during filtration than during sessile drop experiments, the occurrence of a chemical reaction between the three filter materials (3Al₂O₃ · 2SiO₂, MgAl₂O₄, and TiO₂) and the melt during casting is, how-ever, rather unlikely. Therefore, the filter materials mentioned here will not exhibit any reactive character during filtration [9].

Nanostructured surfaces represent a special case. Here, the surface roughness is not the determining factor in the contact angle measurement. The wetting behavior is mainly determined by the increased surface energy of the nanostructured surfaces compared to a reference material. Thus, the wetting of Al₂O₃ became worse compared to an AlSi7Mg alloy due to the nanostructuring of the surfaces, while the chemical composition and the surface roughness remained the same. Subsequent filtration tests showed that there is no difference in effect between the filters with nanostructured surfaces and equivalent refractory filters [21].

This means, for non-reactive filter materials only the surface roughness in itself is significant for the filtration process.

Results for Materials Containing Spodumene

Sessile drop experiments confirmed the high reactivity of spodumene as a filter material. In contrast to the other investigated filter materials, the spodumene based substrates showed a completely different wetting behavior. The melt droplet already showed a wetting behavior after a few minutes after deoxidation of the surface and a reaction layer was visible to the naked eye on the spodumene-containing substrates after the experiments (Fig. 9.5). The reaction obviously took place between the LiAl(Si₂O₆) and the metal and/or the oxide on the metal surface. Investigations using Scanning electron microscopy (SEM Ultra55, Zeiss, Germany) showed a reaction zone (Fig. 9.6) consisting of an layered oxide containing aluminum and silicon in varying amounts, where the amount of aluminum is significantly higher (64 ± 2 wt.%) than that of silicon (4 ± 1 wt.%) [19].

Looking at the droplet cross-section of such a sample, comparatively large silicon segregations are noticeable, even in presence of the mixed oxide with only 85 wt.% alumina (Fig. 9.7). The segregations are a result of the reaction of the spodumene with the liquid aluminum, which reduces SiO₂, contained in the spodumene, to metallic silicon (Eqs. 9.1, 9.2).

Since neither lithium nor hydrogen is possible to be detected by EDX-measurements on the SEM, such a reaction layer was analyzed using SNMS. In this way, it was possible to show that the reaction layer contains lithium and hydrogen in addition to aluminum, silicon and oxygen. (Fig. 9.8). Because the sessile drop experiments were carried out under high vacuum, the source of the hydrogen found in the reaction layer only can be the hydrogen dissolved in the melt. Therefore, a hydrogen-containing reaction product can certainly be assumed.

9.2.3 Immersion Tests

In addition to the sessile drop experiments, immersion tests were carried out under environmental conditions. For this purpose, tablets like those used as substrates for the sessile drop experiments were used. The tablets consisted either of 85 wt.% Al₂O₃ and 15 wt.% LiAl(Si₂O₆) or of alumina coated with pure LiAl(Si₂O₆). They were sintered under identical conditions as the filter materials. For the tests, the tablets were immersed in an AlSi7Mg melt and a pure Al melt at 730 °C for 10 or 30 min, respectively.

The reaction marks can already be seen with the naked eye (Fig. 9.9). The surfaces of all tablets showed residues of a reaction product after only a few minutes. Especially the samples coated with spodumene (bottom of Fig. 9.9) showed a strong reactivity, which made it impossible in some cases to separate the metal from the ceramic. But also with the samples consisting of the mixed oxide, 85 wt.% Al₂O₃ and 15 wt.% LiAl(Si₂O₆) (top in Fig. 9.9), reaction traces could be detected in all cases. Already after 10 min exposure time, enrichments of metallic silicon appeared at the interface between an aluminium melt and the filter material. Fragments of the spodumene coatings and oxide particles have been found at the interfaces and at the bottom of the metal caps of the immersion samples. These oxide particles consist of Al, Mg and O according to EDX analysis (Fig. 9.10).

All this indicates a chemical reaction between spodumene and the aluminium melt, even under standard conditions.

9.3 Influencing Hydrogen Porosity in Aluminum Castings

As already mentioned, the production of high-quality aluminum castings requires not only the removal of particulate inclusions but also the reduction of the hydrogen content in the melt. The hydrogen solubility in aluminum is temperature-dependent and is characterized by a solubility leap at the solidification temperature of 660 °C. This means that the solubility for hydrogen in aluminum decreases at the time of solidification by more than one power of ten from 0.7 to 0.04 cm³/100 g [23]. This leap in solubility is the reason of the pores that are formed in the castings. When the melt cools down, not all of the dissolved hydrogen can be kept in solution. The atomically dissolved hydrogen is released in molecular form and gas pores are formed in the metal. The amount and distribution of gas pores depends on the amount of dissolved gas, which is higher than the solubility at the solidification temperature. The solidification rate, the possibilities for bubble nucleation, and the escape of the gas also play a role. At high cooling rates, more gas can be kept in solution because the melt solidifies quickly and less hydrogen is emitted in a short time. As a result, porosity phenomena are lower because of forming micro-porosity. In a highly contaminated melt, compared to a pure melt, more nuclei are present where gas bubbles can form [24]. This can lead to greater porosity. On the other hand, a high number of nuclei often allows faster hydrogen precipitation. A too low number of nuclei can therefore lead to larger pores in the casting. In order to reduce the resulting porosity as effectively as possible, the number of nuclei in the melt must therefore be adapted to the dissolved hydrogen content [25].

Although purging gas treatments and the use of melting salts makes it possible to reduce the proportion of hydrogen and oxidic inclusions, solidification in many castings does not proceed uniformly, so that areas of increased porosity are still formed. As already mentioned, one reason for such an accumulation can be an excess of hydrogen compared to the number of pore nuclei present in the melt, for example. Therefore, it makes sense to find a way not only to remove unwanted inclusions from the melt, which act as pore nuclei, but also to eliminate the gaseous hydrogen released during solidification in time. One solution to this problem can be the use of suitable filter materials that are capable of positively influencing hydrogen precipitation during casting.

In several series of experiments, spodumene (LiAl(Si₂O₆)) was tested as a reactive filter material. Its influence on the hydrogen porosity of various aluminum alloys was investigated in comparison to non-reactive filter materials. Commercial alloys (AlSi7Mg, EN AC-42000, from Trimet Aluminium AG, Germany and AlCu4Ti, EN AC-21100, from Aluminium Rheinfelden GmbH, Germany) were used for the casts.

Since the carbon bonded filter materials Al₂O₃-C, which were also investigated, showed no effect on hydrogen porosity [26], they will not be discussed here in more detail.

9.3.1 Casting Method and Pore Analysis

A special mold was made for the casts, which allows the gaseous hydrogen released in the mold during solidification to be almost completely precipitated as pores and thus made visible. The mold reproduces a problematic situation from practice. The wedge-shaped steel mold (Fig. 9.11) was designed in such a way that the upper area complicates feeding due to early solidification. This prevents the gaseous hydrogen from escaping completely after filtration. In the central area of the mold, cooling is much slower, so that an area of increased porosity is formed, which serves as a measure of the precipitated hydrogen. The sprue and the mold were preheated to 350 °C before each casting to prevent premature solidification of the melt. Before casting, the metal was heated to approx. 200 °C above the temperature of solidification to compensate the temperature loss during handling up to the start of casting. The alloys used differ in their solidification behavior. In contrast to the widely used AlSi7Mg (EN AC-42000) alloy, the AlCu4Ti (EN AC-21100) alloy has a solidification range extended by more than 50 °C and tends to form micro-pitting [27]. The first alloy promotes hydrogen precipitation while the latter favors the formation of smaller separated pores at the expense of shrinkage porosity.

The 10 ppi ceramic foam filters used for filtration were fabricated using Schwartzwalder's replica technique as described by Voigt et al. [6, 28]. The basis for all filters were filter skeletons made of pure alumina, which were coated with the desired materials: Al₂O₃, LiAl(Si₂O₆), 85 wt.% Al₂O₃ 15 wt.% LiAl(Si₂O₆) (mixed oxide ceramic). Casts without filtration were produced for comparison.

The sprue and the filters were placed on the side of the mold edge so that the liquid metal could flow into the mold in the same way for each casting. All castings were made under the same conditions in terms of handling before and during casting and during cooling to ensure comparability. At least three casts were made and evaluated for each investigated variant.

To evaluate the internal porosity of the castings, polished specimen cross-sections were analyzed by light microscopy (AxioImager.M2m light microscope from Carl Zeiss, Germany). The upper critical area, where the melt solidifies most slowly, was analyzed. The pore radii were determined by automatic image analysis. Each image of a sample cross-section was divided into 33 measuring fields. The pores that appeared dark in the microscope were automatically detected and their area determined. From these values, it was now possible to determine an area-equivalent diameter, which characterizes the size of the pores. Only pores completely contained in the measuring field were taken into account in the measurement. The experimental procedure is shown in Fig. 9.11 and a detailed description of this procedure is given in [22] and [26].

Various characteristic values were used for the evaluation: The total number of pores per examined sample cross-section comprises the pores completely contained in the measuring fields and, together with the total pore area, is a measure of the porosity of a casting. In some specimens, the released hydrogen was not completely precipitated as gas porosity, characterized by the approximately circular pore shape.

Areas of shrinkage porosity were formed in which the hydrogen still remained in the residual melt was precipitated exclusively in the intercrystalline or interdendritic spaces, influenced by the solidification morphology. Such areas are typical for the gravity die casting of complex shaped components, as they are reproduced by the mold used. These areas were not taken into account when determining the number of pores. However, their area was determined separately and included in the calculation of the total pore area to give an impression of the total porosity in the casting.

Selected casting sections were scanned additionally by computer tomography (CT-Alpha from ProCon X-Ray GmbH, Garbsen, Germany) to improve the visualization of the area of the macropores. The upper part of the castings (below the sprue) was analyzed with a voxel size of 40 µm in each case, which means that only pores with diameters larger than the voxel size can be visualized. Due to the beam hardening artifacts, the quantitative evaluation of the pore sizes was not possible.

9.3.2 Pore Structure Without Filtration

Figure 9.12 shows typical cross-sections of castings, which have been cast without filters. The macroporosity can already be seen with the naked eye. In the castings without filters, shrinkage porosity had formed in the central area of the castings. Such a concentration of pores is due to shrinkage of the metal, which also leads to poor dimensional accuracy of the castings. The dendritic shape of the pores in this concentration region indicates rapid solidification of the melt. The rapid solidification initially suppresses hydrogen precipitation from the melt. The hydrogen diffuses into the last solidifying areas and finally precipitates in the interdendritic spaces. There, however, it no longer has the opportunity to form round gas pores. No round pores were visible to the naked eye in the entire cross-sections. The phenomenon can be observed particularly well casting the AlSi7Mg alloy. The alloys used differ in their casting properties and microstructure [27]. As standard casting alloys, hypoeutectic AlSi alloys exhibit good flow and mold filling properties and form a solid solution structure with Si precipitates. AlCuTi alloys, on the other hand, have poor casting properties due to their wide solidification interval and a very dense and fine-grained microstructure. They tend to form micro-pitting. For this reason, small pores at the grain boundaries rather than large gas pores and areas of shrinkage porosity are to be expected here. In the CT analysis, which analyzed a larger sample volume, shrinkage porosity could be detected as well. In general, the castings without filter showed the highest shrinkage and the worst dimensional stability. The upper edges of the castings had collapsed and the internal deficit due to shrinkage was more pronounced for samples produced without a filter.

9.3.3 Al₂O₃-Filters

The pore distribution in the castings produced with the aid of an Al₂O₃-filter appears to be more homogenously distributed over the entire cross-section. In the cross-sections shown in Fig. 9.13, which are typical for the use of Al₂O₃ filters, individual large pores are clearly visible. Especially in the upper casting area of the AlSi7Mg castings, the pores have circular shapes. A high number of large pores in the upper region results in a rough casting surface, which can be easily felt by hand [22]. The gaseous hydrogen released during solidification could escape from the upper side of the casting until complete solidification of the metal. This is due to the general influence of a filter on the melt flow during casting. The melt flow is regulated by using a filter, and fluctuations are prevented. A calming of the melt occurs, as the filter acts as a resistance to flow, and turbulences are avoided. Due to the calmer melt flow through the filter, the hydrogen has time to precipitate from the melt and form large gas bubbles. The area with shrinkage porosity was less pronounced here than in the samples cast without a filter.

The Al₂O₃-filters exhibit a non-wetting character towards aluminum and its alloys. Furthermore, the surface roughness is low compared to the spodumene-containing filter materials. The determined pore area is reduced compared to the samples cast without filter while at the same time, the highest number of pores was measured indicating more but smaller pores compared to an unfiltered casting (Fig. 9.14). The main influence of this filter type is the melt calming, which gives the metal more time to fill the mold and thus prolongs the casting process. This leads to enhanced pore formation and stimulates pore growth. In the case of a sufficiently large solidification interval, the gaseous hydrogen thus has the chance to form bubbles and leave the melt before solidification.

9.3.4 LiAl(Si₂O₆)-Coated Filters

The filter materials containing spodumene showed a different influence on the gas porosity. The filters with a pure spodumene surface lead to a higher pore number compared to the unfiltered castings (Fig. 9.14). But none of the bigger pores can be seen in the cross-sections in case of the AlCu4Ti-alloy when using LiAl(Si₂O₆)-coated filters (Fig. 9.15). In this case, the released gaseous hydrogen seems almost exclusively precipitated (influenced by the solidification morphology) in the inter-crystalline or inter-dendritic spaces to form many small pores as expected for the AlCu4Ti-alloy, which tends to micro-pitting. In this case, more pores formed a larger pore area. In the case of the AlSi7Mg alloy, however, the porosity appears to be reduced in total. There are still a few larger pores in the last solidified sample region in the center of the castings, but they no longer represent a typical shrinkage porosity compared to the variants already discussed. In fact, for this alloy the pore area is reduced overall, although there are more pores in number, compared to the unfiltered castings.

LiAl(Si₂O₆) behaves reactively towards aluminum. In addition, high surface roughness increases the effective filter surface area so that more pore nuclei are available, which will influence the gas porosity that is formed after filtration. Therefore, it can be assumed that the LiAl(Si₂O₆)-coated filter enhances the precipitation of gaseous hydrogen during casting. The rough filter surface generates many nuclei for hydrogen precipitation during filtration due to the melt's interaction with the reactive filter surface. This interaction leads to the formation of many small gas bubbles, which would exhibit a lower tendency to escape from the melt during filtration, compared to larger bubbles. This assumption can explain the large porosity when using the filter with the pure spodumene surface in case of the AlCuTi-alloy, which tends to micro-pitting, and the remaining of lager pores in the center of the AlSi7Mg-specimen, that were not able to leave the melt before solidification.

Interfacial investigations on the filter struts after filtration confirmed that a chemical reaction between the molten metal and the spodumene took place during filtration. Scanning electron micrographs show traces of metallic silicon at the filter/metal interface even in case of the silicon free alloy (Fig. 9.16). This indicates that one of the two reactions described in Eqs. 9.1 and 9.2 took place.

9.3.5 Mixed Oxide Filters

The mixed oxide filters seem to enhance the effect on gas porosity described for the LiAl(Si₂O₆)-coated filters. In Fig. 9.17, which shows characteristic cross-sections of the samples cast with a spodumene-containing mixed oxide filter, larger pores can only be seen in the center of the AlSi7Mg alloy casting.

However, the pores generally appear to be distributed more homogeneously over the entire casting cross-section. The filters with 15 wt.% LiAl(Si₂O₆) cause the formation of fewer pores associated with a significantly reduced pore area in general and therefore a shift of the pore size distribution towards bigger pores (Fig. 9.14).

These filters also feature a rough surface with a large specific surface area. However, the surface consists of non-reactive alumina with reactive parts of spodumene. Hence, in addition to melt calming, this filter can generate supplementary nuclei for hydrogen precipitation, but most likely to a lower extent than in filters with a pure spodumene surface. The additional gaseous hydrogen will create new pores and let some of them grow to larger pore sizes as already discussed. However, if a filter with less spodumene is used, fewer nuclei are present for the early precipitation of gaseous hydrogen, which reduces pore formation and results in an increase in pore size through bubble growth, i.e. by means of coalescence, as explained for the Al₂O₃-filters. However, since more hydrogen is available from the beginning, more hydrogen can escape before solidification. In the end, there is less porosity in general in case of the mixed oxide filters. This could be considered as a combined result of balanced hydrogen nucleation on the one hand and pore growth combined with gas escape during filtration on the other hand.

Unfortunately, it was not possible to measure the hydrogen content in the aluminum melt directly. However, due to identical experimental conditions, it can be assumed that the melts contained comparable hydrogen contents before casting.

9.4 Conclusion

The aim of the work was to investigate the possibility to reduce hydrogen porosity in aluminum castings with the aid of reactive filter materials containing spodumene. To determine this, the reactivity of spodumene compared with other oxidic filter materials was investigated in laboratory tests, and casts were made with two different aluminum alloys.

As a result of the laboratory tests, it was found that under the conditions of the contact angle measurement, only the filters consisting of pure Al₂O₃ did not react with a melt of pure aluminum or an alloy. Chemical reactions were detected in all other filter materials investigated (MgAl₂O₄, 3Al₂O₃ · 2SiO₂, SiO₂, TiO₂, LiAl(Si₂O₆). Despite the partially reactive behavior in the laboratory tests, no negative effects were found later in the analysis of the filters used for filtration and in the examination of the castings with regard to a changed composition of the aluminum melts. In addition, all materials except LiAl(Si₂O₆) exhibited non-wetting behavior towards the aluminum melts investigated.

A strong reactivity was found in contact with the spodumene (LiAl(Si₂O₆)), which also occurred in immersion tests under normal pressure and casting temperatures. Here, a reduction of SiO₂ and the formation of metallic silicon occurred. In addition, the formation of new, presumably lithium-containing, aluminosilicates was detected. Under the conditions of the sessile drop experiments, the strong reactivity of the spodumene caused a significant decrease in the contact angle being in direct contact with the molten aluminum. However, the reason for exploring the lithium oxide-containing material spodumene (LiAl(Si₂O₆)) as a potential filter material was the assumption that Li₂O might be able to form a compound with the hydrogen dissolved in the aluminum melt and released during the solidification process, and thus reduce the hydrogen content in the alloy. Initial investigations in this regard actually showed an improvement in terms of the pore structure formed within the castings when a filter material containing spodumene was used compared to a pure Al₂O₃ filter. However, it is assumed that the reduction of hydrogen porosity is mainly caused by the early precipitation of hydrogen from the melt stimulated by the chemical reaction and less by the formation of a hydrogen-containing reaction product at the filter surface. In addition, high surface roughness increases the effective filter surface area so that more pore nuclei may be available. This will influence the gas porosity that is formed during filtration. The smallest volume deficits and best dimensional accuracy were observed for castings produced with the spodumene-containing filters. These castings exhibited exact edges and almost no dents.

Acknowledgements

The authors gratefully acknowledge the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) for financial support of the investigations, which were part of the Collaborative Research Center Multi-Functional Filters for Metal Melt Filtration—A Contribution towards Zero Defect Materials (Project-ID 169148856 – CRC 920, subproject C06). Furthermore, the authors would like to thank Dr. Claudia Voigt (subproject A02) for providing the filter materials and for her fundamental research in the field of aluminum filtration, Birgit Witschel (subproject S01) for preparing the sessile drop and filter cross sections and, Dr. Jana Hubálková (subproject S01) for the performance of the CT measurements.

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Titel: Influence of Reactive Filter Materials on Casting’s Quality in Aluminum Casting
verfasst von: Beate Fankhänel
Ekaterina Schmid
Michael Stelter
Verlag: Springer International Publishing
Buch: Multifunctional Ceramic Filter Systems for Metal Melt Filtration
Print ISBN: 978-3-031-40929-5

Electronic ISBN: 978-3-031-40930-1

Copyright-Jahr: 2024
DOI: https://doi.org/10.1007/978-3-031-40930-1_9

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