Abstract
The dependence of the interfacial formation of ZnAlCu alloys on S355 steel substrates on the aluminum content of the alloy and the cooling conditions is investigated for the gravity die casting process. Steel substrates pre-galvanized with ZnAl0.2 as well as with ZnAl5 are used as initial condition. The area fractions of the interfacial phases formed in the process are quantitatively determined by a binarization method and correlated with the compositions and the casting conditions in simple mathematical models. Under the test conditions, especially for hypoeutectic alloy variants with a maximum of 5.5 wt% Al, clear dependencies of the phase fractions in the interfacial formation on the aluminum content, but also on the cooling conditions could be demonstrated. The correlations of the forming phases with the type of pre-galvanization could also be shown over a wide range of compositions and solidification rates.
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Introduction
In the last decade, research into alternative material systems for plain bearing applications has been steadily intensified.1 Previous studies have shown the excellent suitability of alloys of the ternary system ZnAlCu for a wide range of applications in the field of plain bearings. Due to the diverse and precisely adjustable microstructural characteristics, a large part of high-speed, but also more heavily loaded load collectives can be covered.2,3,4
Alloy compositions in the range up to 22 wt% Al are characterized not only by high compressive and fatigue strength, but also by their high adaptability and outstanding dry sliding properties. The latter properties result from the high proportion of soft, zinc-rich η matrix phase, while the increased strength is caused by the finely distributed, aluminum-rich α phase. In addition, contents of copper below 2 wt% lead to significant solid solution strengthening. Above 2 wt% copper, on the other hand, there is an increased formation of ε-phases, which also lead to an increase in strength, but can act as an abrasive third phase in tribological contact.5,6,7
The production of plain bearings consisting exclusively of alloys of the composition ZnAlCu is not possible due to their relatively low modulus of elasticity of only 100 GPa.
In order to achieve a higher resistance to deformation, steel supports are used which are coated with a sliding layer of the ZnAlCu alloy in the running surface.
The bonding of the sliding layer to the steel support body is of high importance for the application in the sliding bearing.
The production of plain bearings in the casting process is still state of the art. An investigation of the bonding behavior of ZnAlCu alloy to a steel substrate in gravity die casting is intended to provide information on the boundary layer formation processes taking place there. In a first step, the influence of the aluminum content on the formation of intermetallic phases at the interface is to be evaluated. In addition, the heat content of the system must also be considered.
The effect of aluminum in zinc alloys on the bonding to steel is sufficiently documented from galvanizing technology. As shown in numerous publications such as Adachi et al., an addition of up to 0.2 wt% aluminum leads to the formation of a thin Fe2Al5 interface due to the high affinity of aluminum for iron. This boundary layer serves as a short time diffusion barrier for zinc atoms. This allows the formation of the boundary layer to be precisely controlled.8,9,10
The effect of the aluminum content in the casting process, on the other hand, has barely been researched, which makes it difficult to precisely control the boundary layer.
The analytical approach pursued here to quantify the phase fractions at the interface is intended to develop the prerequisites for controlled adjustment of cast structures and interfaces of ZnAlCu alloys on a steel support body. With the data on the properties of the intermetallic phases of the FeZn system and the FeAl system which are available in the literature, it is therefore possible to selectively adjust the properties of the interface formed during casting.11,12
Materials and Methods
To prepare the samples, melts of different compositions of pure zinc (99.995 %), pure aluminum (99.8 %) and pure copper (99.99 %) were prepared in an induction furnace. The melts were cast at 90 °C, 140 °C and 190 °C superheat, respectively. The castings were made in a steel mold (Figure 1) in which a steel substrate of composition S 355J0 (Table 1) (100 mm × 28 mm × 2 mm) was placed.
Test setup; melt weight per cast 1.5 kg.
Half of the substrates used were pre-galvanized in a bath of ZnAl0.2 and the other half in ZnAl5 by an industrial galvanizing plant. These variants represent the most common types of pre-galvanization and will be used for the following investigations. The initial conditions of the pre-galvanized sheets are shown in Figure 2.
Initial conditions of the steel substrates with a pre-galvanization using (a) ZnAl0.2 and (b) ZnAl5. The chemical composition of the plotted points was measured by EDS. Due to the small spatial extent of the phase in point 4, it had to be identified via the EBSD method as Fe2Al5.
Cross sections of the as-cast composites were prepared in order to study the interfaces formed. The preparation route used is described in more detail in.13
The composition and morphology of the formed phases, especially in the composite zone, were examined by scanning electron microscopy (SEM) (Zeiss, Oberkochen, Germany) and energy-dispersive X-ray spectroscopy (EDS) (Oxford Instruments, Wiesbaden, Germany) analysis. The backscattered electron contrast (BSE) is used for all SEM pictures shown in this work.
A quantitative evaluation of the intermetallic phases was carried out via gray-scale contrasts using the image analysis software ImageJ.
One series of tests was carried out with mold temperatures and substrate temperatures of 200 °C and a second series of tests at 350 °C in order to additionally evaluate the influence of the cooling rate on the interface formation.
An overview of the tests carried out in this context is given in Table 2.
Results and Discussion
As can be seen from the initial states of the steel substrates shown in Figure 2, the substrates pre-galvanized with ZnAl0.2 have a ZnFe13 boundary layer of about 20 µm thickness. In the case of the sheets pre-galvanized with ZnAl5, on the other hand, a stable, approximately 500 nm thick interface of Fe2Al5 is formed. These conditions show good agreement with numerous publications in the fields of galvanizing and galvannealing.8,9,14,15
Effect of the Al-content on ZnAl0.2 Pre-galvanized Substrates
In order to investigate the influence of the aluminum concentration of the alloys on the interface formation, the area fractions of the formed Fe2Al5 and FeZn13 phases were measured on an interface length of 70 µm (5000 × magnification). For this purpose, the individual images were binarized according to the procedure shown in Figure 3 for the respective phases and the area fraction was determined.
Method for determining the phase fractions of the interfacial phases by binarization.
As can be seen from the performed investigations of the sample sections, the increase of the aluminum content of the alloys also leads to an increased formation of the intermetallic phase Fe2Al5 along the interface. This relationship is exemplified for the alloys ZnAl0.5Cu0.7 and ZnAl2.5Cu0.7 in Figure 4.
Comparison of micrographs of (a) ZnAl0.5Cu0.7 and (b) ZnAl2.5Cu0.7.
The increase of Fe2Al5 phase with higher Al contents could be demonstrated up to the eutectic composition, which is 5.5 wt% Al and 0.7 wt% Cu. The interface formed from this composition onwards showed no further growth with an increase in Al concentration in the experimental setup investigated. This could be attributed to the fact that there is sufficient Al in the melt and the growth of the Fe2Al5 interface becomes increasingly dependent on the cooling rate. The cooling rate of the cast specimens is high enough so that the diffusion rate drops quickly and thus no further reaction between aluminum and iron can take place.
In contrast to the behavior of the Fe2Al5 phase, the formation of the FeZn13 layer shows an opposite behavior with increasing Al content. Based on the studies of the temporal change of the intermetallic phases during galvannealing, a diffusion process of zinc atoms takes place along the grain boundaries of the Fe2Al5 boundary layer during cooling. This process leads to the nucleation of an intermetallic FeZn phase between steel and the Fe2Al5 layer. With increasing growth of this FeZn layer, mostly an FeZn13 (ζ-phase), the Fe2Al5 layer finally flakes off.8,14 The phase fraction of the ζ phase shows a decreasing tendency at higher Al contents, which, however, could not be directly correlated with the Al-concentration. Rather, the local energy during the detachment of the Fe2Al5 layer is decisive here. In those cases where the layer breaks off earlier, there is still more thermal energy in the system, so that a more pronounced FeZn13 layer can form. As a result, a significantly higher scattering of the phase fractions can be observed for FeZn13. The measured phase fractions for Fe2Al5 and FeZn13 are compared in Figure 5. For a better overview, only the values in 1 wt% steps are shown there.
Area fractions of the intermetallic phases along the interface as a function of the Al content.
A first mathematical approach, which describes the dependence of the Fe2Al5 formation on the Al content for hypoeutectic compositions up to 5.5 wt% Al, is shown in Figure 6.
Approach to the mathematical description of the dependence of the interface formation on the Al content for substrates pre-galvanized with ZnAl0.2.
The chosen mathematical approach is described by the fourth-degree polynomial
When using polynomials of lower degrees, in particular the relationship for low Al concentrations is not adequately represented.
Effect of the Cooling Rate on the Interface Formation of ZnAl0.2 Pre-galvanized Substrates
In a first step, cooling curves were recorded using type K mineral-insulated thermocouples in order to be able to estimate the temperature curves over time. The sampling rate of the thermocouples during the measurements was 10 Hz. As can be seen from Figure 7, however, the inertia of the thermocouples used has a particular effect on the start of the measurements, so that especially at 90 °C superheating the start of solidification could not be recorded completely.
Influence of superheat and mold temperature on the cooling rate for the casts with ZnAl11Cu0.7.
In order to determine the solidification times and the time from casting to completed solidification, the first and second derivatives of the cooling curves were calculated and the corresponding times were determined on the basis of these. Since, according to the relevant literature in the field of thermodynamics, diffusion in the solid state is several orders of magnitude lower than in the liquid state, the influence of solid-state diffusion was neglected in the following analysis.
From the cooling curves determined, it is clear that the influence of substrate preheating in the experimental setup considered and the parameters investigated has a more significant effect on the resulting cooling times than the overheating of the melt. This is mainly due to the rather low wall thickness of 4 mm of the cast alloys and the thus relatively low heat energy gain with further superheating.
The effects of increasing superheat or substrate preheat did not show a clear trend in the hypoeutectic alloy range. Thus, as illustrated by the example of the alloy ZnAl1Cu0.7 in Figure 8a and b, a slight increase in the Fe2Al5 phase fractions occurred, which can be attributed to a longer convection time and higher diffusion rate in the molten state. In contrast, the formation of the FeZn13 phase at this point decreases at higher temperature, but this varies strongly locally along the interface.
Comparison of the influence of the substrate temperature on the proportions of the intermetallic phases formed when the substrate temperature is varied for (a and b) ZnAl1Cu0.7, and (c and d) ZnAl11Cu0.7.
In contrast to the observations for hypoeutectic alloys, a clear dependence of the formed Fe2Al5 interface thickness on superheat and substrate preheating was observed for alloys with more than 5.5 wt% aluminum. As shown in Figure 8c and d, the phase fraction of Fe2Al5 in the interface for ZnAl11Cu0.7 increases from about 1250 µm2 to over 1800 µm2 when the substrate temperature is increased from 200 to 350 °C while the superheat remains the same.
With the times from the start of casting to complete solidification determined by the cooling curves shown in Figure 7, which is about 15 s for a superheat of 190 °C and a substrate temperature of 200 °C and about 37 s for a substrate temperature of 350 °C, a clear relationship between solidification time and formation of the Fe2Al5 phase can be deduced for hypereutectic alloys in the ZnAlCu system. Based on the observation that the proportions of the intermetallic phase formed for a melt superheat of 90 °C at 350 °C substrate temperature (3.5 K/s) are similar to the proportions for a superheat of 190 °C at 200 °C (12.7 K/s), a direct dependence of the interface formation on the cooling rate cannot be seen. As already described by Fick’s laws in 1855, the diffusion length can be assumed as
where LD is the maximum diffusion length, D is the diffusion coefficient and t is the time, which an atom has to diffuse. Since D is a temperature-dependent parameter, the cooling rate must also exert an influence on the diffusion width, which has to be investigated in a broader range of experiments.
Effect of Al Content on ZnAl5 Pre-galvanized Substrates
Following the studies already shown for pre-galvanization with ZnAl0.2, the behavior of the interface formed for S355 steel substrates pre-galvanized with ZnAl5 was considered with a variation of the aluminum content of the cast alloy. In contrast to the previous investigations, however, substrates pre-galvanized with ZnAl5 all show a homogeneous and stable Fe2Al5 interface in the initial state. This Fe2Al5 boundary layer serves as a diffusion barrier and is intended to prevent the diffusion of iron atoms into the zinc melt during galvanizing, so that uncontrolled growth of intermetallic FeZn phases is prevented.
As can be seen from the investigations carried out, the existing boundary layer also has the same effect on the interface formation in the considered test area, irrespective of the aluminum content of the cast alloy. As an example, Figure 9 shows the interfaces formed for a superheat of 190 °C and a substrate temperature of 350 °C for both the hypoeutectic alloy ZnAl1Cu0.7 and the hypereutectic alloy ZnAl11Cu0.7.
Interface formation at a superheat of 190 °C and a substrate temperature of 350 °C for (a) the hypoeutectic alloy ZnAl1Cu0.7 as well as (b) the hypereutectic alloy ZnAl11Cu0.7 on a S355 steel substrate pre-galvanized with ZnAl5.
As can be seen from Figure 9, the Fe2Al5 boundary layer of the pre-galvanization remains stable in both cases. Thus, no diffusion through this boundary layer can be detected under the present thermal boundary conditions. As can be seen from some studies in the field of galvanizing, the diffusion-inhibiting effect depends on the temperature of the melt and the duration for which the interface is exposed to this melt. The influence of this temperature dependence is accordingly considered below.8
Effect of the Cooling Rate on the Interface Formation of ZnAl5 Pre-galvanized Substrates
Similar to the variation of aluminum contents, the change of cooling rates and solidification times primarily shows no effect on the interfacial formation of substrates pre-galvanized with ZnAl5. In a second step, tests were carried out with ZnAl11Cu0.7 at a melt superheat of 240 °C and a substrate temperature of 350 °C. As can be clearly seen in Figure 10, the further increase in melt temperature during casting locally breached the original boundary layer, initiating significant growth of a new intermetallic boundary layer.
Interface between ZnAl11Cu0.7 and a steel substrate pre-galvanized with ZnAl5 with a substrate temperature of 350 °C at a superheat of the melt of (a) 190 °C and (b) 240 °C. In (c) the cooling curves of the corresponding casts are given.
From the cooling curves shown in Figure. 10c it can be seen that, although the duration from pouring to final solidification does not increase due to the increase in superheat, the temperature is significantly higher for several seconds after pouring. The resulting increase in atomic mobility and diffusion rate is sufficient to initiate the growth of a new boundary layer. By means of EBSD measurements, the composition of the boundary layer near the substrate could be identified as Fe2Al5, whereas with increasing distance from the substrate the proportion of FeAl3 predominates. These observations are in agreement with the studies of Shawki et al, which describe a similar growth during galvanization of steel in a Zn-Al bath with high Al concentrations.16
Conclusion
From the investigations carried out, some approaches could be derived which describe the development of the interface between ZnAlCu alloys and pre-galvanized steel substrates during gravity die casting:
-
For substrates pre-galvanized with ZnAl0.2, the proportion of intermetallic phases formed can be specifically adjusted by varying the Al concentration of the melt. Under the test conditions, the proportion of FeAl phases formed was primarily limited by the amount of aluminum present in the diffusion range, due to the generally higher affinity of iron to aluminum. When the eutectic composition is reached, the Al content in the melt is sufficient for the formation of stable Fe2Al5 interfaces, whereby the proportions of this intermetallic phase are increasingly determined by the solidification time and the associated maximum diffusivity range of the iron atoms in the melt.
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For the investigated temperature range, on the other hand, no clear dependencies of the formed phase fractions on the prevailing cooling times during casting of hypoeutectic ZnAlCu alloys on ZnAl0.2 pre-galvanized S355 steel could be demonstrated. Thus, no mathematical approach to describe these correlations could be derived from the investigations. From the investigations, it can therefore only be concluded that at higher temperatures and thus longer solidification times, a slight increase in Fe2Al5 interphases can be measured, which can also be attributed to the resulting increased diffusion rate and time. The area fraction of FeZn13 is locally fluctuating along the interface, which can be attributed to the thickness of the primary Fe2Al5 interface and the time of its flaking off.
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For the hypereutectic alloy ZnAl11Cu0.7, due to the small number of samples, no quantitative correlation between solidification time and the resulting interface could be derived either. However, it could be clearly shown that an increase in Fe2Al5 can be reproducibly set with longer solidification time, but also with an initially higher casting temperature. The diffusion length is a function of the square root of time, which mainly limits the growth of the boundary layer under the experimental conditions.
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In the case of pre-galvanization with ZnAl5, the resulting interface was so stable that a variation of the aluminum concentration under the casting conditions had no influence on the formation of a new intermetallic interface. Only a further increase of the casting temperature finally leaded to a destabilization of the primary interface, allowing the growth of a new interfacial layer.
At the present time, however, no quantitative correlation can be drawn between the superheating of the melt, the solidification time and the intermetallic phases formed. In order to be able to describe this behavior more precisely, further, more detailed investigations must first be carried out regarding the influence of the variations in superheating and solidification time on the formation of a new interfacial layer.
Furthermore, the influence of the copper content on the interface formation during gravity die casting in the investigated ternary alloy system must also be examined. The copper content has a decisive influence on the mechanical and tribological properties of the alloys and must therefore also be considered for a comprehensive description of the bonding behavior.
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Funding
Open Access funding enabled and organized by Projekt DEAL. The presented results are derived from IGF project 20165 N of the “FVA Research Association of the Drive Technology” which was funded within the scope of joint collaborative program “IGF” by the Federal Ministry for Economic Affairs and Energy, following a decision of the German Bundestag.
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Gimmler, S., Vroomen, U. & Bührig-Polaczek, A. Interface Formation of Zinc-Rich ZnAlCu Bearing Alloys on Pre-galvanized S355 Steel Substrates in Gravity Die Casting. Inter Metalcast 17, 1256–1265 (2023). https://doi.org/10.1007/s40962-022-00853-2
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DOI: https://doi.org/10.1007/s40962-022-00853-2