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Dieser Artikel geht auf die komplizierte Beziehung zwischen Erstarrungsbedingungen und der Zerspanbarkeit von Aluminium-Silizium (Al-Si) und Aluminium-Silizium-Bismut (Al-Si-Bi) -Legierungen ein und zeigt, wie durch kontrollierte gerichtete Erstarrung Mikrostrukturen maßgeschneidert werden können, um die Schneidleistung zu verbessern. Die Studie untersucht systematisch die Auswirkungen von Wachstumsrate (GR) und Kühlrate (CR) auf den eutektischen Abstand (λSi) und zeigt, dass feinere Mikrostrukturen - die durch höhere GR und CR erreicht werden - zu geringeren Schnitttemperaturen und einer längeren Standzeit führen. Bei Al-Si-Legierungen bleibt die maximale Schnitttemperatur weitgehend unbeeinflusst von Erstarrungsparametern, aber der Zusatz von Wismut (Bi) führt zu einer signifikanten Senkung der Schnitttemperatur entlang der Länge des erstarrten Barrens, was Bis Rolle als mikrostruktureller Modifikator unterstreicht. Die Forschung deckt auch die duale Natur von Bi auf: Während die Mikrostruktur verfeinert und die Bearbeitbarkeit unter bestimmten Bedingungen verbessert wird, kann ein übermäßiger Bi-Gehalt die Kantenbildung und den Werkzeugverschleiß erhöhen, was den Optimierungsprozess kompliziert. Durch die Feststellung quantitativer Korrelationen zwischen Erstarrungskinetik, mikrostrukturellen Merkmalen und Bearbeitungsreaktionen bietet diese Arbeit einen umfassenden Rahmen für die Gestaltung von Aluminiumlegierungen mit überlegener Bearbeitbarkeit und bietet Ingenieuren und Forschern in Werkstoffwissenschaft und Fertigung praktische Anleitung.
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Abstract
This work investigates the effects of growth rate (GR), cooling rate (CR) and eutectic microstructure on the machinability of Al-12.6Si and Al-12.6Si-3.2Bi (wt.%) alloys. Directional solidification experiments were performed using a water-cooled upward solidification apparatus, enabling a range of GR and CR values along the length of the ingots. The as-solidified microstructures consisted of primary α-Al dendrites and interdendritic eutectic Si, while the Bi-containing alloy exhibited additional Bi globules dispersed within the eutectic regions. Machinability was evaluated based on maximum cutting temperature (Tmax), heating rate (HR) and tool flank wear (Vbmax). In the binary alloy, CR and eutectic spacing (λSi) had negligible influence on Tmax, which remained approximately constant at 46 °C. On the other hand, the ternary alloy exhibited Tmax values ranging from 50 to 63 °C. Higher HR values were associated with higher CR and finer λSi for both alloys. Although Bi addition did not significantly affect HR, it promoted the formation of built-up edge (BUE), which contributed to a higher Tmax. The addition of Bi improved machinability by reducing Vbmax by nearly 40% compared with the binary alloy.
This is an invited paper selected from presentations at Symposium J: Alloy Design and Processing for Advanced Applications from the Meeting of the Brazilian Materials Research Society (XXIII B-MRS), held September 28–October 2, 2025, in Salvador, Brazil, and has been expanded from the original publication. The issue was organized by Ana Sofia C.M. D'Oliveira, Federal University of Paraná; Crystopher Brito and Julian Avila, São Paulo State University; Maysa Terada, Independent Researcher; Aline Capella, Federal University of São Paulo; Juliane Ribeiro da Cruz, University of São Paulo; and José Eduardo Spinelli, Federal University of São Carlos.
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1 Introduction
Aluminum-silicon (Al-Si) alloys are widely used in automotive, aerospace and marine applications due to their favorable combination of low density, high specific strength, good castability and adequate mechanical performance, accounting for nearly 80% of all cast aluminum alloys (Ref 1-28). Among these alloys, compositions close to the eutectic (≈ 12.6 wt.% Si) are particularly attractive and dominate aluminum casting applications due to their high fluidity and ability to accurately reproduce complex geometries (Ref 4-6).
Beyond casting performance, the service life and manufacturing cost of Al-Si components are strongly affected by wear and machinability. Several studies have demonstrated that both alloy composition and microstructural characteristics, especially the morphology and spacing of eutectic silicon, play a decisive role in cutting forces, surface quality, tool wear and heat generation during machining (Ref 1-3, 7, 8). In this context, alloying strategies involving free-cutting or low-melting-point elements such as Bi, Sn, Pb and In have been investigated to improve tribological behavior and machinability (Ref 2, 5, 29).
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Machinability is a complex, multivariable phenomenon governed by cutting parameters, tool geometry and intrinsic material characteristics (Ref 20, 28, 30, 31). In Al-Si alloys, the coexistence of a ductile aluminum-rich matrix and hard, brittle silicon particles often leads to severe abrasive tool wear, built-up edge (BUE) formation and elevated cutting temperatures, particularly under unfavorable microstructural conditions (Ref 7, 8, 14, 21, 22, 25, 32). Heat generation at the tool–chip interface is especially critical, as it accelerates wear mechanisms and limits tool life, with a significant fraction of the generated heat being dissipated by the chip and the remainder transferred to the tool and the workpiece (Ref 7, 8, 20, 30, 31).
Recent studies have expanded the understanding of machining-related thermal effects, surface integrity and microstructural damage in Al-Si alloys processed by conventional and non-conventional machining routes (Ref 10, 11, 13, 19). Comprehensive reviews further emphasize that alloying elements and processing conditions strongly influence machinability; however, quantitative correlations between microstructure and machining response remain limited, particularly for alloys processed under controlled solidification conditions (Ref 7, 8, 17, 18, 23).
Despite the extensive literature on the machining of Al-Si alloys, only a limited number of studies have systematically correlated machinability parameters with microstructural features directly controlled by solidification thermal conditions, especially under transient directional solidification regimes (Ref 4, 7, 8). This gap is even more pronounced for Al-Si-Bi alloys, in which Bi additions are known to modify microstructure and hardness, but their influence on cutting temperature, heating rate and tool wear mechanisms is still not fully understood (Ref 2, 5, 8).
Previous investigations have analyzed the influence of transient solidification thermal parameters, growth rate (GR) and cooling rate (CR), on the microstructural evolution of directionally solidified Al-Si eutectic alloys (Ref 24) and Al-12.6 wt.% Si-Bi alloys (Ref 25), establishing quantitative growth laws based on the classical theory of eutectic solidification (Ref 27). These studies have demonstrated a significant refinement of eutectic and dendritic structures with increasing GR and CR; however, they were limited to microstructural characterization and hardness, without addressing machinability.
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This work advances the state of the art by explicitly linking solidification-controlled microstructure to machinability performance in directionally solidified Al-Si and Al-Si-Bi alloys. The novelty of this study lies in establishing quantitative correlations between GR and CR, eutectic spacing (λSi) and key machinability indicators, maximum cutting temperature, heating rate and tool flank wear, measured under standardized turning conditions (Ref 26). By integrating solidification kinetics with machining response, this work provides new insights into the process–structure–property–performance relationships governing the machinability of Al-Si-Bi alloys.
2 Materials and Methods
The investigated alloys were produced based on stoichiometric mass calculations using commercially pure Al, Si and Bi elements (> 99.5%), whose chemical compositions are the same as those reported in Costa et al. (Ref 8). The required masses were determined according to the nominal alloy compositions and the volume of the cylindrical mold, using an analytical electronic balance with a precision of 0.01 g. The weighed elements were then charged into a silicon carbide crucible, previously coated with an alumina-based suspension to prevent contamination and melted in a muffle furnace to ensure complete melting and homogeneous alloy formation.
Thermal analysis, as shown in Fig. 1, was conducted by comparing experimental and simulated cooling curves using a thermodynamic tool in order to identify characteristic transformation temperatures. In particular, the eutectic temperature of each alloy was determined to define the superheating condition (approximately 10% above TE) and to analyze the main solidification events and thermal features. This approach was employed to ensure consistency with the intended alloy compositions and solidification behavior. After confirmation of TE, the prepared alloys were poured into a water-cooled vertical solidification device, which is presented and described in detail in our recently published works (Ref 24, 25).
Fig. 1
(a) Pseudo-binary phase equilibrium diagram of the Al-12.6 wt.%Si alloy as a function of Bi (Fonte: Thermo-Calc/TCAL 8); (b) experimental cooling curve of the Al-12.6 wt.%Si-3.2wt.%Bi alloy
The experimental determination of the growth rate (GR) and cooling rate (CR) was made possible through the thermal records obtained during directional solidification, schematically presented in Fig. 2. The temperature–time curves recorded by thermocouples positioned at different heights along the mold allow the identification of the passage of the eutectic isotherm (TE) as a function of time. The temporal displacement of this isotherm between known positions enables the calculation of GR, while CR is determined from the slope of the thermal curve at the solidification instant.
Fig. 2
Schematic temperature–time profiles recorded by thermocouples at different positions along the mold during directional solidification, indicating the passage of the eutectic isotherm (TE) used to determine the growth velocity (GR) and cooling rate (CR)
This experimental methodology is described in detail in our recently published works (Ref 7, 24, 25) and in (Ref 8). Briefly, the position of the eutectic isotherm was determined from thermocouple readings by identifying the time at which the eutectic temperature (TE) was reached at each thermocouple position. A position–time curve was then constructed and fitted using a power-law function, whose time derivative provided the local growth rate (GR). The cooling rate (CR) was determined directly from the slope of the experimental cooling curves immediately after the passage of the eutectic isotherm at each thermocouple position.
In order to develop a study on the effect of the scale length of the eutectic microstructure (λSi) on the machinability of both alloys, as-cast samples in positions from the heat transfer interface were cut and prepared for metallographic analysis, as schematized in Fig. 3(a), allowing λSi to be measured according to the method presented in Fig. 3(b). Using this method, at least 20 eutectic Si spacings were measured for each as-cast sample. In addition, as-cast samples of the ternary alloy were subjected to microstructural characterization using scanning electron microscope (SEM) Shimadzu (Vega 3-SBU-Tescan) coupled to Shimadzu an energy-dispersive spectroscope—EDS (AZTec Energy X-Act, Oxford).
Fig. 3
(a) Schematic representation of the as-cast samples from the bottom; (b) eutectic spacing (λSi) measurement techniques
The configuration used in the machinability tests is shown in Fig. 4. Cutting temperatures were recorded during the turning operations on a conventional lathe in order to determine the heating rate of each tested sample. A FLIR E40 thermographic camera was employed, and the temperatures were recorded using a data acquisition system. Samples with dimensions of 8.3 × 17 mm and heights varying from 10 to 20 mm were extracted along the length of the solidified ingot. In addition, a rectangular cross section was intentionally adopted for each sample in the machinability tests to systematically evaluate the effects of the solidification thermal parameters (growth rate, GR, and cooling rate, CR) and the resulting eutectic length scale (λSi) along the length of the directionally solidified ingot. Due to material losses, vibrations and other problems during the process, cuts were made at mid-positions along the longitudinal section of the as-cast ingot, as shown in Fig. 4(b). A new cutting tool was used for each machined sample in order to avoid any influence of prior tool wear on the cutting temperature measurements and machinability results
Fig. 4
(a) Schematic of machining device assembly used in machinability tests adapted from (Ref 8); (b) representation of as-solidified samples for necking.
Figure 5 presents the experimental methodology used to determine machining parameters, such as maximum cutting temperature (Tmax), heating rate and tool wear (Ref 8). Machinability tests began as soon as the cutting tool came into contact with the samples, and simultaneously, the thermal camera began to record temperatures in the software. This recording occurred every second until the end of necking, that is, the complete cutting of the part, providing a graph T = f(t) as shown in Fig. 5(a) indicating the maximum and minimum temperatures.
Fig. 5
Techniques used to determine machinability parameters: (a) maximum cutting temperature (Tmax); (b) procedure used to calculate the heating rate (HR), (c) magnified view of the cutting tool tip showing the flank face adjacent to the cutting edge, from which the flank wear parameter VB (ISO 3685) was measured
The workpiece geometry was intentionally designed to allow samples to be extracted at specific positions along the directionally solidified ingot, preserving local solidification conditions (growth rate, cooling rate and eutectic spacing) for machinability evaluation, rather than enabling long, steady-state cutting. As shown in Fig. 5(b), the first ~ 20 s correspond to a non-permanent tool–workpiece contact caused by this geometry, resulting in low and unstable temperatures; therefore, this interval was deliberately excluded from the analysis. Only the subsequent stage with permanent and stable contact was used to determine heating rate, maximum cutting temperature and tool condition, ensuring that the reported machinability parameters are representative and comparable for the different as-cast microstructures. No turning operation was applied to convert the rectangular cross section into a circular geometry, in order to avoid possible recrystallization or any modification of the ingot macro- and microstructure prior to machining.
The mathematical treatment on the \(Temperature\times Time\) profiles was the same used in a previous study presented by Costa et al. (Ref 8) in which the machining heating rate (HR) is calculated by time derivative of the \(T=f\left(t\right)\) function in the region of the graph characterized by a linear behavior. The non-permanent contact between the chisel and the specimen, especially at the beginning of the tests due the geometry of the workpieces, renders an irregular variation of the cutting temperature which should be unconsidered for the determination of the heating rate, as depicted in Fig. 5(b). Table 1 summarizes the main details about the parameters employed during the machining tests, as assumed by Costa et al. (Ref 8).
Table 1
Summary of machining input parameters and specifications
Input parameter
Specification
Machining operation
Necking process in a universal lathe
Cutting tool
ROCAST HSS T6 3/4″ conventional chisel
Cutting temperature acquisition
Flir E40 Thermal Imaging Camera
Advance speed
0.126 mm/rev
Maximum cutting speed
28.4 m/min
Rotation
650 rpm
Number of repeated tests
One test per as-cast sample
Cutting depth
Variable
Tool wear analysis was performed by visual inspection using a stereoscope, with images recorded immediately after machining at 20 × magnification on both sides of the cutting tool. The images were processed using Paint 3D and subsequently analyzed in ImageJ to facilitate accurate measurements. Reference lines were first drawn parallel to the cutting edge, and flank wear measurements were then obtained by drawing perpendicular lines from these references to the worn region, as illustrated in Fig. 5(c). Multiple measurements of the flank wear level (VB) were taken, and the largest value was adopted for data processing, in accordance with ISO Standard 3685 (Ref 26). According to this standard, VB is defined as the maximum width of the worn region on the tool flank face in the vicinity of the cutting edge, regardless of whether the wear is uniform or localized. For comparison with industrial practice, a maximum flank wear criterion (VBmax = 0.6 mm) was used as the end-of-tool-life limit.
3 Results and Discussion
The thermal records shown schematically in Fig. 2 were used to determine the experimental growth and cooling rates (GR and CR), as summarized in Fig. 6. These results correspond to the same sampling positions selected along the as-cast ingot for the subsequent machinability tests (see Fig. 4). As expected, the water-cooled upward solidification system produced a decreasing profile of GR and CR along the ingot length, with the highest values obtained near the cooled mold plate (heat extraction surface) and progressively lower values toward the upper region as solidification progressed. Such behavior is consistent with the variation of the interfacial heat transfer coefficient (hi) along the mold–metal interface, as previously discussed by Rocha et al. (Ref 24).
The detailed thermal analysis and complete set of GR and CR data were reported in (Ref 25), which provides the basis for correlating solidification parameters with the machinability behavior investigated in the present work. The experimental equations presented in the captions of Fig. 6 are results also proposed in (Ref 25), which were used to plot the respective graphs.
It is well known that both the morphology of the as-cast microstructure and the microstructural scale length are strongly governed by the solidification thermal variables (Ref 1, 4-8, 24, 25). Accordingly, the results presented in Fig. 6 were used to correlate the local solidification parameters with the eutectic spacing (λSi). Figure 7 shows the evolution of λSi as a function of position along the as-cast ingot and its corresponding GR and CR values.
Fig. 7
Variation of eutectic spacings (λSi) as a function of growth and cooling rates, respectively
As observed, smaller λSi values and higher GR and CR were obtained near the heat extraction surface, gradually decreasing toward the top of the ingot due to the characteristics of the water-cooled upward solidification system. Power-type functions were used to describe the variation of λSi with GR and CR, with exponents of − 1/2 and − 1/4, respectively, in good agreement with theoretical and experimental predictions (Ref 4, 27).
It is important to note that the mathematical relationships shown in the captions of Fig. 7 were experimentally determined based on the thermal data previously reported in (Ref 25). The mathematical expressions presented in the legends of Fig. 7, which describe the dependence of eutectic spacing on growth rate (GR) and cooling rate (CR), were originally proposed and validated by (Ref 25).
Figure 8 shows the typical solidification microstructures at two positions, 10 and 90 mm, of both investigated alloys from the cooled mold sheet of the ingot mold. The eutectic spacing (λSi) and the morphology of the eutectic silicon phase were both strongly affected by the solidification thermal parameters. As shown in Fig. 7 and 8, higher growth and cooling rates (GR and CR) resulted in finer λSi values and promoted a morphological transition of eutectic Si from flake-like to fibrous or spheroidal structures. Conversely, lower-GR and lower-CR conditions led to coarser eutectic spacing and flake-type Si morphologies.
Fig. 8
Solidification typical macrostructure and microstructures: (a) and (b) Al-12.6Si and Al-12.6Si-3.2Bi, respectively, for positions equal to 10 and 90 mm from the cooled base
These findings are consistent with those previously reported in (Ref 24), where transient solidification conditions were shown to produce morphological instabilities in the Si phase, including the development of halo dendrites and the refinement of eutectic Si with increasing growth rate. The present results, together with the experimental growth laws established by (Ref 25), reinforce that both the scale length and morphology of the eutectic Si phase are directly controlled by solidification kinetics.
Aiming to characterize in more detail the constitution of the typical solidification microstructure, Fig. 9 shows a scanning electron micrograph, element mapping and microanalysis by EDS. The combined analysis with the optical micrographs presented in Fig. 8 indicates the presence of a primary Alα phase surrounded by a mixture of eutectic phases composed of (Alα + eutectic Si + Bi globules) and massive Si crystals.
Fig. 9
Microstructural analysis carried out by SEM/EDS for one of the as-cast samples of Al-12.6Siwt.%-3.2wt.%Bi alloy at the 30 mm position from the heat transfer interface
During solidification, the rejection of Bi may lead to the formation of a Bi-rich liquid due to its negligible solubility in Al and Si, which can be associated with a slight reduction in the characteristic transformation temperature from ~ 577 to ~ 572 °C. This temperature shift may be interpreted as a secondary thermodynamic effect of Bi on the liquid phase, rather than as a dominant factor directly controlling phase selection or interface stability.
From a microstructural standpoint, the influence of Bi addition on the formation of α-Al dendrites appears to be mainly related to kinetic and interfacial effects that are strongly coupled with the solidification parameters. The enrichment of the liquid in Bi ahead of the solid/liquid interface may enhance constitutional undercooling and interfacial instability, effects that tend to become more significant as the growth rate (GR) and cooling rate (CR) increase. Under such conditions, reduced solute diffusion times and higher thermal gradients may favor deviations from coupled eutectic growth and promote α-Al dendritic morphologies. Consequently, the combined influence of Bi segregation and elevated GR and CR is likely to govern the observed microstructural evolution, rather than the modest decrease in the eutectic or liquidus temperature alone.
Figure 10 and 11 provide direct experimental evidence that the machinability response is strongly governed by the interaction between solidification conditions, microstructural scale and alloy chemistry. While the binary Al-12.6Si alloy exhibits an almost constant maximum cutting temperature along the ingot length, indicating a limited sensitivity of Tmax to cooling rate and eutectic spacing, the ternary Al-12.6Si-3.2Bi alloy shows a clear decrease in Tmax with position.
Fig. 10
Variation of maximum cutting temperature as a function of: (a) position in the a-cast ingot; (b) and (c) cooling rate and eutectic spacing, respectively.
This behavior demonstrates that the combined effect of Bi addition and solidification-controlled microstructural refinement modifies the heat generation mechanisms at the tool–chip interface, reducing friction and thermal load during machining. The establishment of a position-dependent relationship for Tmax in the ternary alloy confirms that machinability can be quantitatively linked to CR and λSi, thus reinforcing the process–structure–property relationship proposed in this work.
Similarly, the heating rate results shown in Fig. 10 indicate that, although both alloys follow comparable overall trends, localized deviations in the binary alloy are associated with changes in eutectic Si morphology and microstructural heterogeneities inherent to as-cast structures. The transition from spheroidized to flake-like eutectic silicon alters local ductility and hardness, directly affecting chip formation and transient heat generation.
In contrast, the ternary alloy exhibits a more stable thermal response, suggesting that Bi acts as a microstructural modifier and solid lubricant, mitigating the influence of morphological variations. These observations clarify the mechanisms underlying machinability behavior and demonstrate that alloy composition and solidification parameters jointly control cutting temperature and heating rate.
Figure 8 establishes the microstructural basis required to interpret the machinability trends observed in Fig. 9 and 10. The refinement of eutectic spacing (λSi) and the transition of eutectic Si morphology from flake-like to fibrous or spheroidal structures, promoted by higher growth and cooling rates (GR and CR), directly influence the thermo-mechanical behavior during machining. Finer and more rounded Si morphologies reduce stress concentration sites, improve local ductility of the Al matrix and promote a more stable chip formation process, thereby lowering friction at the tool–chip interface and reducing heat generation.
Conversely, coarser eutectic spacing and flake-like Si morphologies formed under lower-GR and lower-CR conditions intensify abrasive interactions with the cutting tool, leading to higher localized friction, increased cutting temperatures and larger thermal gradients. The consistency between the microstructural evolution shown in Fig. 7, 8 and 9 and the thermal machinability responses in Fig. 10 and 11 demonstrates that the observed differences in Tmax and heating rate are a direct consequence of solidification-controlled microstructural scale and morphology. When combined with previous findings reported in Rocha et al. (Ref 24, 25), these results confirm that solidification kinetics govern eutectic Si morphology, which in turn controls heat generation, tool–chip interaction and machinability performance.
As shown in Fig. 11, the imposed conditions by unsteady-state directional solidification had a strong effect on the heating rate during machining since higher HR values were found for higher CR and lower λSi. In general, HR varied from 13.5 °C/s to 0.87 °C/s along the length of the ingots which made it possible to establish mathematical expressions for both investigated alloys given by HR = 0.90(CR)0.14 and HR = 2.64(λSi)−0.65 but the found value to R2 = 0.32 is much lower than the recommended minimum (0.7). However, the expressions given by HR = 0.93(CR)0.15 and HR = 2.56(λSi)−0.72 proposed for the Al-12.6Si-3.2Bi alloy resulted in R2 = 0.82 greater than 0.7, suggesting that they can be assumed for both alloys since they presented trend profiles very close to the previous ones for R2 = 0.32.
As observed in Fig. 10 and 11, the 3.2wt.%Bi addition to the Al-12.6wt.%Si alloy resulted an increase of approximately 37% in Tmax for higher CR and lower λSi but had no influence on HR. These results are incompatible with those found for Al-9wt.%Si-(1wt.%Bi) alloys (Ref 8) considering that the 1wt.%Bi addition to the Al-9Si (wt.%) decreased both Tmax and HR, with reduction in Tmax being 40%. This suggests that high Bi contents may not favor machinability in as-cast eutectic Al-Si alloys using the maximum temperature criterion.
This contrasting behavior indicates that the effect of Bi on machinability is strongly composition and microstructure dependent. At the higher Bi content investigated in the present work (3.2 wt.%), Bi segregation in interdendritic and eutectic regions, combined with microstructural refinement promoted by higher CR and lower λSi, intensifies tool–chip interactions and frictional heat generation, leading to increased Tmax. In this case, the potential lubricating effect of Bi is outweighed by the increased number of phase boundaries and localized hard constituents acting during cutting. Therefore, unlike lower Bi additions reported in the literature, higher Bi contents do not necessarily improve machinability and may even be detrimental when the maximum cutting temperature is used as the primary machinability criterion.
On the other hand, high cutting temperatures have strong influence on the tool wear resulting in tool life. It is known that increasing cutting speed in machining is one of the ways to increase productivity; however, it can result in an increase in the cutting temperature which in turn accelerates the tool wear mechanisms and, consequently, reduces the your lifetime (Ref 28). According to ISO 3685 (Ref 26), the service life of quick steel, high-hardness and ceramic cutting tools can be defined following some criteria such as the maximum flank wear (VBmax) assumed to be equal to 0.6 mm.
In this work, the effect of the manufacturing process parameters such as the solidification heat flow, microstructural parameters and thermal machinability parameters are investigated on flank wear (VBmax) at the tip of the machining tool in two machined samples of solidified Al-12.Si-xBi alloys (x = 0 and 3.2wt.%) and the results are shown in Table 2 and Fig. 12 and 13. The darkest gray region at the tip of the tool is the built-up edge (BUE) which was formed by the amount of caked material from the as-cast samples after the machining process. The amount of BUE is shown in Fig. 12 and 13 by the total area (A). It was observed that high cutting temperatures (Tmax) obtained on Al-12.6Si-3.2Bi (wt.%) alloy samples promoted greater BUE formation, as indicated in Fig. 13.
Table 2
Results of maximum flank wear volume (Vbmax) for Al-12.6Si and Al-12.6Si-3.2Bi (wt.%) alloys
Position, mm
Alloy, wt.%
Vbmax, mm
HR, °C/s
CR, °C/s
λSi, µm
0-10
Al-12.6Si
Al-12.6Si-3.2Bi
0.096
0.059
1.35
> 6.6
0-2.74
41-60
Al-12.6Si
0.051
0.92-1.07
1.6-1.0
3.64-4
61-80
Al-12.6Si-3.2Bi
0.058
0.87
1.0-0.82
4-4.24
Fig. 12
Cutting tool tip after necking on two as-cast samples of Al-12.6wt.%Si alloy solidified from the heat transfer surface.
To better support the analysis of tool wear mechanisms, enlarged images of the flank face on the opposite side of the cutting tool tip were included for both investigated alloys (Fig. 12 and 13). These images present a clear measurement scale and explicitly show the flank wear regions used as the basis for the quantitative wear measurements. The flank wear values reported were obtained directly from these regions, allowing a clear distinction between adhered material associated with built-up edge formation at the tool tip and the actual flank wear developed during machining. Although the wear observed includes BUE, localized damage and micro-chipping, these features are accounted for in the VBmax measurement, as prescribed by the ISO methodology.
As observed in Fig. 12, for the Al-12.6wt.%Si alloy the heating rate significantly influenced the wear values during necking as well as BUE formation. The Vbmax values were equal to 0.096 mm and 0.051 mm (Table 2), deducing that flank wear is greater for higher HR while for the Al-12.6wt.%Si-3.2wt.%Bi alloy the Vbmax variation was insignificant, that is, 0.059 mm–0.058 mm, indicating that the heating rate did not influence flank wear. Furthermore, although the Bi addition to the Al-12.6wt.%Si alloy did not favor machinability according to the Tmax criterion the Vbmax criterion shows that machinability in the Al-12.6wt.%Si-3.2wt.%Bi alloy was successful because the obtained values of 0.058-0.059 mm were much lower than 0.6 mm. This indicates that the presence of Bi can improve the machining aluminum dry, as reported in Reference (Ref 29).
As reported, BUE often occurs at low cutting speeds; however, the real speed range in which it exists depends on the machined alloy, such as those with low ductility, the tool geometry is negative or there is a need for cutting fluid (Ref 30-32), as in aluminum alloys (Ref 7, 8, 31). This is compatible with the assumed conditions and obtained results in this work since the greater BUE formation in the Al-12.6wt.%Si-3.2wt.%Bi alloy can be justified by the low hardness due to the presence of the Bi element influencing the higher A values, as shown in Fig. 13. This may have influenced the highest Tmax (63 °C) as shown in Fig. 10(a). It was also observed that the greatest wear value (0.096 mm) was found for the binary alloy at positions closer to the heat transfer surface resulting in higher cooling rates, finer microstructures and probably greater hardness, requiring greater tool effort during material removal. It is important to highlight that one of the mechanisms for increasing mechanical strength in metallic alloys, that is, hardening, is the refinement of the as-cast microstructure in which the smaller λSi acts as obstacles to dislocation displacement.
The material adhered to the cutting tool tip corresponds to the formation of a localized built-up edge (BUE). Under the machining conditions investigated, this BUE acted transiently as a protective layer, delaying the development of continuous wear features such as flank or crater wear. However, the BUE was unstable and underwent repeated formation and detachment. This cyclic BUE break-off can promote adhesive wear by attrition, as fragments of tool material may be locally removed together with the detached BUE. Therefore, although BUE is visually observed in Fig. 12 and 13, the wear mechanism is interpreted as adhesive wear mediated by BUE dynamics rather than classical uniform tool wear. Certainly, as the built-up edge is made up of particles of machined material that accumulate on the tool exit surface, more in-depth characterization studies at the tool tip are necessary, such as SEM/EDS microanalysis to evaluate the solute concentration profile.
4 Conclusion
The following major conclusions are derived from the results and discussion of this experimental study:
1.
Typical solidification microstructures were evidenced with complex characteristics for the Al-12.6wt.%Si-xBi (x = 0 and 3.2wt.%) investigated alloys characterized by two primary phases, one consisting of a Al-rich dendritic (diffuse) network (Alα) and the second by massive (faceted) silicon crystals, both surrounded by an irregular eutectic Al-Si mixture. It was also observed Bi dispersed in the interdendritic region with globular morphology.
2.
As expected, it was found that more finer microstructures, that is, lower eutectic spacing (λSi) values, were achieved for higher GR and CR values. It was noted that the Bi phase did not affect λSi.
3.
A single mathematical expression capable of characterizing an eutectic spacing law as a function of GR and CR is proposed.
4.
The machinability results showed for the Al-12.6wt.%Si alloy that both CR and λSi had no influence on the maximum cutting temperature along the length of the as-cast ingot, assuming for Tmax a constant value equal to 46 °C. On the other hand, the Al-12.6 wt.%Si-3,2 wt.%Bi alloy showed a decreasing temperature profile with position in as-cast ingot in which Tmax varied from 63 to 50 °C.
5.
Regarding the heating rate (HR), the obtained results for both alloys showed similar behaviors, that is, higher HR values were obtained for higher CR and lower λSi. It was also found that the Bi element added to the Al-12.6wt.%Si alloy did not affect HR.
6.
Finally, it was noted that the ternary alloy presented a larger area (A) of built-up edge (BUE) formation, suggesting that BUE may have influenced at the higher Tmax achieved in the Al-126.Si-3.2Bi alloy. On the other hand, higher flank wear value (Vbmax) was found for the binary alloy for conditions of higher CR and lower λSi, however, not reaching the maximum useful life value, that is, Vbmax = 0.6 mm. Under these conditions, the Bi addition to the binary alloy reduced Vbmax by approximately 40%.
Acknowledgments
The authors are grateful for the support given by IFPA - Federal Institute of Education, Science and Technology of Pará, UFPA, the Federal University of Pará and the Amazon Foundation for Studies and Research (FAPESPA) for the scholarship granted to Fernando Sousa da Rocha. This study was also financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001.
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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