Chip formation and morphology in cryogenic machining of Al-SiC composites

doi:10.21203/rs.3.rs-5309773/v1

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Chip formation and morphology in cryogenic machining of Al-SiC composites

https://doi.org/10.21203/rs.3.rs-5309773/v1

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Machining aluminium matrix composites is a challenging and costly endeavour, primarily owing to silicon carbide reinforce particles, which are abrasive and hard, resulting in excessive tool wear, suboptimal surface quality, and undesirable chip formation. This study investigates the influence of cryogenic cooling on the development and morphology of chips when aluminium-silicon carbide (Al-SiC) composites are turned with an uncoated tungsten carbide cutting tool. Compared with dry machining, cryogenic cooling significantly alters the chip formation process, producing shorter and less curled chips. Chip breakability is mainly caused by cracks at the aluminium reinforcement particle interface, and the particle distribution is more uniform in cryogenically cooled chips. Rake and dual (rake and flank) cooling strategies have proven to be more effective for cryogenic cooling. Chip breakability primarily depends on the chip curl diameter across the used conditions. Chip formation under cryogenic cooling indirectly indicated the significant temperature reduction in the cut.

Chip segmentation

Chip morphology

Chip contact area

Cryogenic cooling

Flank cooling

Turning

Aluminium matrix composite

Composite materials are currently used in various industries (particularly in the construction, military, automotive, and aerospace industries) owing to their excellent specific properties such as strength or stiffness to weight ratio, increased wear resistance, low thermal expansion coefficient, high thermal conductivity, and ability to absorb energy or dampen vibrations [1–3]. Composites are considered multiphase materials because they usually consist of two or more constituents that exhibit significantly different physical and mechanical properties. The synergistic effect of the matrix and filler creates unique properties that the individual constituents do not possess. Matrix surrounds and binds reinforcement, provides geometrical stability, transfers the load on the reinforcement, protects the filler against environmental effects, and determines the surface quality. In contrast, the high-strength filler is the main load-bearing element. Fiber-reinforced (glass, carbon, aramid, boron) and particle-reinforced (metallic, ceramic) composite materials with various matrix types (polymer, metallic, ceramic) are the most commonly used composites for engineering applications [4, 5]. However, the addition of reinforcement to relatively easy-to-machine materials (matrices) turns them into difficult-to-machine materials. Reinforcement significantly increases the abrasiveness of the composite material, but some weaknesses of the matrices are preserved, such as low-temperature resistance (polymers) or brittleness (ceramics). The inhomogeneity of the composite is the cause of unpleasant post-machining quality problems, such as delamination and fiber or particle pull-out. [6, 7].

One of the progressively used particulate metal matrix composites (MMC) is based on an aluminum matrix reinforced with dispersed silicon carbide particles (Al/SiC_p), which benefits from the combination of the hard, stiff, but brittle load-bearing ceramic SiC with a ductile and low-density aluminum matrix. These types of composites are characterized by their hardness, resistance to abrasion, wear, and corrosion, and low cost [8]. Their mechanical properties can be controlled by changing the distribution, size, shape, or volume fraction of the particles [9]. However, owing to the hard and abrasive SiC particles, Al/SiC_p is considered to be a difficult-to-cut material. The contact of Al/SiC_p with the tool cutting edge results in abrasive wear of the tool, which significantly reduces tool life, degrades the surface quality of the machined part, and increases the cutting force and cost [10, 11].

A sharp increase in temperature occurs during machining owing to friction formation between the contact surfaces of the tool and workpiece. As the temperature increases, the SiC particles are prone to absorb more energy; therefore, the machining efficiency decreases. With a higher SiC fraction, higher temperatures are concentrated on the surface of the workpiece because the heat cannot be easily conducted and dissipated. The abrasion of the SiC reinforcement causes material loss from the flank and edge of the tool. As a result of friction, the temperature increases, leading to secondary adhesion [12, 13]. Poor heat dissipation causes significant tool deformation and forms a built-up edge (BUE) on the tool surface, leading to deterioration of the workpiece surface and tool life [14]. This creates the need for a suitable coolant that can reduce the temperature of the tool and improve its tool life and surface integrity. Among the different types of cooling techniques, such as flood, air, minimum quantity lubrication (MQL), and high-pressure cooling systems, cryogenic cooling (LN₂) has proven to be more suitable for machining difficult-to-cut materials owing to its superior ability to reduce heat faster, non-toxicity, environmental friendliness, and inertness (which reduces the possibility of tribochemical reactions) [15].

Duan et al [16] in their study on the effect of cooling and lubrication conditions on tool wear in turning of Al/SiC_p claimed that the application of cryogenic coolant increased the thermal impact and scratch effect on tool face due to SiC presence which increased tool breakage but observed that MQL increased tool life and lower flank wear. Joysula et al [17] studied the sustainable machining of metal matrix composite using liquid nitrogen and observed that cryogenic cooling reduces surface roughness, tool wear, and cutting temperature. Aurich et al [18] investigated the influence of reinforcement and cutting conditions on the surface layer of the workpiece and observed that the feed rate was the major parameter on the surface layer since it decreases the tensile stress on the workpiece but impacts the surface quality negatively. Ghoreishi et al [19] in the evaluation of tool wear in high-speed face milling of Al/SiC_p MMC observed that the presence of SiC particles in the aluminum matrix composite causes severe tool wear and that the tool wear can be reduced by applying a cryogenic coolant.

During the machining process, as the cutting edge meets the material, it tends to shear the material apart by plastic deformation, allowing the cut-off material to slide on the rake face of the cutting tool; this deformed piece of the material is referred to as a chip [20, 21]. The type and morphology of chips produced during machining depend on the cutting parameters (cutting speed, feed, and depth of cut), workpiece material, cutting environment, tool geometry, and material [20, 22]. Chip formation and breakability (the ability of the chip formed during machining to break off from the workpiece) play key roles in determining surface quality, tool life, machining performance, and cost [21, 23]. Different types of chips can be produced during machining operations. For instance, long, continuous ribbon-like chips can pose handling difficulties and necessitate process interruptions. It reduces productivity, increases machining time and cost, negatively affects machine performance, is a hazard to the operator, and can negatively influence surface quality. This creates the purpose of studying and obtaining desirable chip breakability during the machining process. Chip breakability depends on the workpiece material, cutting conditions, tool geometry, cooling conditions, and chip curling [24]. Chip curling is important because it separates chips from the tool surface. It is essential to separate the chips from the tool surface because the longer the chip stays on the tool surface, the higher the heat transferred from the chip to the tool [25]. Chip curling can occur as natural or forced curling. In natural curling, the chip exits the rake face without any obstruction or contact with an obstacle in forced curling, and the chip contacts an obstacle, such as a tool breaker, workpiece, or face of the tool [26]. The presence of these obstacles helps in chip curling, which aids chip breakability and influences the surface quality of the machined workpiece.

Dabade et al. [27] in their work observed that in Al/SiC_p, the number of curls that the chip makes before breaking depends on the number of reinforcement particles present; as the reinforcements increase, chip curling decreases owing to the brittleness of the reinforcements. Wu et al [28] observed that the brittle fracture of the SiC particles in the cutting path can form large cavities and the deformed Al matrix forms the smooth surface of the workpiece by covering the cavities. Al matrix deforms along the shear plane and serrated chip segments are separated along the plane with clustered particles. Xiang et al. [29] observed that during the high-speed cutting of Al/SiC_p, chip formation is induced by high shear strain in the primary deformation zone, the difference in flowability of free and back surfaces of the chip, and the instabilities of plastic deformation and damage promoted by adiabatic shear localization, which leads to the formation of mixed serrated chip morphologies, while Nakayama et al. [30] observed that chip breakage occurs when the strain on the chip increases beyond a certain critical value.

Considering the material properties, machining characteristics, and prior research findings, it is crucial to investigate the connection between the machining characteristics and chip formation in Al/SiCp composites. By examining this relationship, it may be possible to optimize the cutting conditions and enhance the overall performance of the machining process for aluminium composite materials. Therefore, this study aims to establish the impact of cutting speed and different cryogenic cooling strategies on the changes in the microstructure, chip morphology, and key characteristics of Al/SiCp.

2.1 Workpiece and cutting tool

The material used in this study was an aluminum matrix composite brake rotor, which comprised aluminum A359 alloy with a chemical composition of AlSi9Mg0.6, reinforced with 20 wt% silicon carbide particles with an average particle size of 18µm. This specific material was chosen for the investigation owing to its widespread usage, low cost, high strength, and capacity for large-scale production. The blank rotor workpiece was manufactured using the squeeze casting method by Automotive Com Floby AB, Sweden. A photograph of the test piece is shown in Fig. 1. The dimensions of the blank MMC rotor were as follows: thickness of 80 mm, outer diameter of 350 mm, and inner diameter of 80 mm. The bulk hardness of the material was determined as 75 ± 9 HBW by applying a 750 kg load with a 5 mm diameter steel ball for 10 s on a Brinell hardness tester (Table 1).

Table 1

Mechanical properties of workpiece material
Density (g/cm³)	Yield Strength (MPa)	Ultimate Tensile Strength (MPa)	Elongation (%)	Young Modulus (GPa)	Poisson’s ratio (-)	Fracture Toughness (MPa·m^1/2)
2.71	138.7	178.3	1.9	109	0.31	15.9

The cutting tool used was an uncoated carbide tool with a rhombic shape and grade name of K68, with an ISO designation of CNMA120408. Detailed information about the cutting tool is provided in Table 2. This particular grade of cutting tool contains tungsten carbide and cobalt as its primary components, which are alloyed together to form a fine-grained structure. Owing to its high resistance to abrasive wear, this grade of cutting tool is recommended by cutting tool suppliers for machining difficult-to-cut materials. The suitability of uncoated rhombic cemented carbide grades for rough machining operations is attributed to their controlled wear rate and self-sharpening action, which allow for efficient and effective performance during these tasks [31]. Consequently, a rhombic carbide tool with dimensions identical to those used in the industry was selected for the experiments. The tool holder used was a Kennametal DCLNL20X12 JETI, which was equipped with internal channels featuring two coolant nozzles aimed at the rake and flank faces of the insert. The diameters of these nozzles were 0.8 mm and 1.4 mm. The geometrical configuration of the insert secured within the tool holder comprised a rake angle (γ_o) of -6°, an edge inclination (λ_s) of -5°, and a major cutting-edge angle (κ_r) of 95°.

Table 2

Insert nomenclature
Insert cutting edge length [mm]	12.7
Insert thickness [mm]	4.763
Corner radius [mm]	0.8
Insert hole size [mm]	5.16
Clearance angle [°]	0
Type of insert	Uncoated carbide
Insert shape	Rhombic 80°

2.2 Cryogenic and Machining Setup and Process

The experimental setup for the machining and cooling procedures is illustrated in Fig. 2a and b. Four variable conditions, namely dry machining and three cryogenic setups, were employed in this study. Drying was conducted at an ambient temperature of 23°C using liquefied nitrogen (LN₂) as the cryogenic coolant. The medium was stored in a 200 L gas cylinder with a pressure cap valve (Fig. 2d). The valve in the outlet of the liquid nitrogen cylinder was used to control the gas flow into the system. The valve was connected with a vacuum-insulated pipe to the exit nozzle in the lathe machine chamber, and with a flexible pipe to the tool holder (Fig. 2c). The medium was applied through a rake and/or flank nozzle, which led to the distinction between rake, flank, or both (dual) cooling strategies. The coolant density was 1.251 kg/m³ and maintained at a temperature of -196°C and a pressure of 20 bars.

The machining process was a face-turning operation with a setup consisting of an STM 2000 CNC lathe machine (manufactured by STOREBRO Machine Tools AB). The workpiece was secured to the chuck via its internal diameter and dynamically balanced to prevent wobbling or irregular rotation. The cutting rate was maintained constant for the duration of the cutting. The tool holder was fastened to the upper turret, which was linked to the coolant distribution network.

The effect of cutting speed in combination with various coolant strategies on the chip formation process was studied. The depth of cut and feed were kept constant while the cutting speed was varied at three levels. The spiral cutting length (SCL) was calculated and used in machining according to [31, 32]. The cutting parameters used in the experiment are summarized in Table 3.

Table 3

Cutting parameters and cooling conditions for tests
Cutting speed [m/min]	150, 180, 210
Feed rate [mm/rev]	0.3
Depth of cut [mm]	0.5
Spiral cutting length (SCL) [m]	35
Cooling conditions	dry, rake, flank, dual
Nozzle orifice diameter [mm]	0.8 (rake), 1.4 (flank)

Four different experiments under different cooling conditions were performed at each cutting speed:

Dry condition
Cryogenic condition (LN₂) at the rake face
Cryogenic condition (LN₂) at the flank face
Cryogenic conditions (LN₂) at the rake and flank faces.

The used tool and chips formed under each condition were collected and analysed. The chip structure, form, and dimensions were investigated. A microstructural analysis of the chips was performed to observe the changes that occurred in the material after machining. Important parameters such as chip thickness, chip compression ratio, chip curl diameter, tool-chip contact length, and shear angle were evaluated to provide data for analysis and comparison.

3.1 Chip formation analysis

The configuration, shape, and type of chips formed were examined using an Olympus SZX9 microscope at multiple magnifications. The parameters of chip thickness (t_c), chip curl diameter (d_o), and tool-chip contact area length (TCC_length) were evaluated, as shown in Fig. 3. Ten different chips were analysed for each machining condition. Five values for each parameter were measured for each chip.

The chip compression ratio (r_c) and shear angle (φ) were determined based on the measured data from the chip microstructure analysis. The chip compression ratio serves as an indicator of the plastic deformation intensity of the material. This is the ratio between the undeformed chip thickness (t) and chip thickness (t_c), as shown in Eqs. 1 and 2 [32, 33] and Fig. 4a. Next, the shear angle was evaluated as one of the most important parameters defining the chip formation process. The shear angle is the resultant combination of two approaches to reduce evaluation inaccuracies. First, it was calculated from the measured chip thickness using Eq. 3 [34]. Next, the values of the segment orientation angle were determined from the images of the chips (Fig. 4b). The angle was then calculated using Eq. 4. Ten orientation values were obtained from one chip, whereas three chips were evaluated for each cooling and cutting condition. Eventually, the average value of both approaches was determined as the resultant shear angle.

r _c = t/t_c (Eq. 1)

t _c = t_min + (t_max – t_min)/2 (Eq. 2)

φ = arctg (r_c cosγ/(1 - r_c cosγ)) (Eq. 3)

φ = 90° - Φ (Eq. 4)

where γ is the rake angle and Φ is the angle of the segment orientation.

3.2 Microstructural analysis

Metallographic samples were extracted from the friction ring cross-section of the work material (Fig. 1) and prepared for microstructural examination. Before the analysis, the samples were prepared using the standard metallographic procedure: grinding with SiC papers up to #4000 grit, pre-polishing with non-drying fumed silica (SiO2) suspension (0.25 µm), and final polishing with acidic alumina (Al2O3) suspension (0.05 µm). The microstructure of the work material and morphology of the chips were studied using a light optical microscope (LOM) ZEISS Imager M2m Axio, Olympus SZX9, and Olympus DSX1000. Detailed microstructural analysis was performed using a HITACHI TM 3000 and JEOL JSM 7600-F scanning electron microscope (SEM) equipped with secondary (SE) and backscatter detectors (BSE). The phases present in the microstructure were identified by energy-dispersive X-ray spectroscopy (EDXS) using an SDD Oxford X-Max 80 mm² detector.

4.1 Chip morphology, length, and breakability

The cutting speed and cooling strategy have a significant impact on chip morphology, length, and breakability. The work material exhibited a combination of brittle behaviour from hard particles and ductile and adhesive properties from the matrix. Despite the presence of frayed edges, chips with long continuous spiral shapes were formed at all cutting speeds under dry conditions.

Generally, the use of cryogenic coolants results in notable differences in the length, curl dimensions, and overall shape of the chip, as illustrated in Fig. 5. Both sides of the chip exhibited a significant degree of frailty. Consequently, the strength of the chip was lower, and its fragmentation and susceptibility to breaking were higher. Dry machining resulted in tubular short chips of approximately 30 to 40 mm in length, regardless of the cutting speed used (see the area in Fig. 5a). Cryo-cooling supports chip breakability. The influence varies with the cooling strategy and cutting speed. The rake and dual strategies yielded the shortest chips. At lower speeds, the chips were narrower and exhibited less deformation (demonstrating a higher pitch of the chip threads). This indicated that the chips were properly cooled with a low-temperature gradient, as shown in Fig. 5c. On the other hand, the chips were slightly longer and curled more with increasing cutting speed as the cutting temperature increased, as shown in Fig. 5b.

A comparison of the backside (tool-side) of the chips and their fragmentation is shown in Fig. 6. Lighter regions signalize smoother surfaces generated within the sliding region in the last phase of the tool-chip contact, for example, area “A” in Fig. 6. This occurred during the dry and flank cooling strategies, mainly when the cutting temperature was relatively high. In contrast, the increased efficiency of the cooling system and the lower temperature at the tool-chip interface led to greater adhesion between the tool and chip, as evidenced by more pronounced sticking marks or impressions (e.g., area "B"). A similar occurrence was observed in previous research conducted by Alagan et al. [33].

The SEM image of a typical chip generated under cryo-cooling conditions (“Rake,” 180 m/min) is shown in detail in Fig. 7. Based on the image presented in Fig. 7a, it is possible to observe the frayed edges of the chip and its segmentation, which suggests the presence of non-uniform SiC particles, low material cohesion, and lower plasticity, ultimately resulting in a lower capacity for shear deformation. The differences between the regions of sliding and sticking marks on the back side (Fig. 7b) are observable in the picture on the right. A rough surface indicates strong adhesion at the tool-chip interface as the temperature is effectively reduced.

4.2 Chip Characteristics

Effective management of chip characteristics is crucial for achieving efficient, secure, and top-notch machining processes. However, by focusing on chip formation, machinists can diagnose machining issues, optimize cutting conditions, and improve the overall process performance. The data gathered from the chips were examined to assess the chip curl diameter, tool-chip contact length, average chip thickness, chip compression ratio, and shear angle. The findings of these evaluations are summarized in Table 4. The chip curl diameter (d_o) has a significant influence on chip breakability because as it is reduced, the chip tends to curl away either upward or sideways from the rake face. This reduces the tool chip contact area and length, thereby improving chip breakability. The curl diameter decreased with cutting speed under dry conditions as the temperature and its gradient across the chip increased, as shown in Fig. 8a. Next, the parameter is significantly lower than for all cryo-cooling states. This corresponds to the observation of the chip shape above. There was a less clear effect on course for all coolant conditions with cutting speed. The curl diameter showed a decreasing course with rake, flank, and dual cooling, while the particular cryo-cooling strategy did not influence chip curl significantly.

Table 4

The results of chip characteristics
Measured parameter	Coolant condition	Cutting speed [m/min]
		150	180	210
Chip curl diameter d_o [mm]	Dry Rake Flank Dual	5.742 7.315 7.208 7.333	5.722 6.400 7.358 6.973	4.480 6.027 5.984 6.413
Tool-chip contact length TCC_length [mm]	Dry Rake Flank Dual	0.998 0.981 0.997 0.974	0.957 0.959 0.974 0.966	0.946 0.943 0.946 0.972
Average chip thickness t_c [mm]	Dry Rake Flank Dual	0.610 0.608 0.564 0.557	0.548 0.610 0.579 0.571	0.501 0.600 0.509 0.546
Shear angle φ [deg]	Dry Rake Flank Dual	20.3 21.4 24.3 24.0	27.9 22.1 25.8 24.1	29.8 23.4 28.4 26.5
Chip compression ratio r_c [-]	Dry Rake Flank Dual	0.380 0.381 0.411 0.416	0.423 0.380 0.400 0.406	0.463 0.386 0.455 0.425

The average chip thickness is largely determined by the feed and cutting speed. However, the most significant factor was the work material and its behaviour in the shear zone. When turning the aluminium matrix composite, it exhibited similar behaviour in terms of chip thickness as homogeneous metals, with the average chip thickness decreasing with increasing speed. As the metal matrix became softer in the primary shear zone, the width of the chip was reduced and angled towards the rake face, as depicted in Fig. 8.

In contrast, cryocooling resulted in the opposite effect. Specifically, the average chip thickness increased with cutting speed for all cryocooling strategies, and no significant difference was observed in the cooling strategy. This behaviour can be attributed to microstructural changes in the work material and lower chip temperatures. Moreover, higher adhesion occurs at the tool-chip interface. These conditions support the creation of a stable build-up edge and influence the chip-formation process.

The chip compression ratio was found to be influenced by the chip thickness, as demonstrated using a constant feed in all tests. In theory, this ratio should increase with both the cutting speed and temperature in the cutting zone. When a cryocoolant environment was employed, the ratio increased by 40% at a cutting speed of 150 m/min. However, no significant difference was observed in the chip compression ratio when comparing the effects of the cutting strategy and flow direction at different cutting speeds.

The tool-chip contact length showed a decreasing trend with the cutting speed for all cooling strategies, as well as for dry conditions (Fig. 8c). This finding is consistent with previous observations on chip curl diameter (d_o). No significant variation was observed among the strategies at a specific cutting speed. However, the standard deviations for this measurement were relatively substantial. Thus, the differences are not unambiguous. The flank cooling strategy has a similar effect as the rake and dual methods. Hence, the dynamic effect of the rake cryocoolant on the chip and secondary shear zone is not sufficiently strong, as was proven for high-pressure cooling with liquid coolants, for example, by Masek et al. [35].

The shear angle represents a measure of the chip formation efficiency. It shows the degree of material deformation and the amount of heat generated during cutting. In general, the angle increases with the speed and cutting temperature. Subsequently, a lower plastic deformation intensity of the work material occurs. From the results obtained, as shown in Fig. 8d, the shear angle increased progressively with the cutting speed under dry conditions. For an increase in speed of 40%, the angle increased at a similar rate of 50%. A higher shear angle results in a decreased shear zone area, which reduces the shear force and cutting energy. All cryocooling strategies demonstrated an increase in the shear angle with increasing speed. However, the increase was not progressive; hence, the cutting speed did not significantly influence the shear of the material under cryo-cooling conditions. This is a sign of intensive cooling of the chip and cutting region. The direction of the cooling media flow did not significantly influence the shear angle.

4.3 Microstructural analysis of workpiece and chips

The microstructural constituents of a material dictate its behaviour and operational performance. Consequently, the microstructural phases of the work material were analysed and identified, as shown in Fig. 9. The composite consisted of an α-Al matrix and reinforcing SiC particles. Primary silicon Si_P is present in the form of polyhedral particles (2). The eutectic Mg₂Si phase (4) can be found predominantly at the edge of the Si_P and Fe-based phases (1) with either dendritic, skeletal, or partial fish-bone morphology or in the form of globular particles. Fe-rich intermetallic phases were present in the form of polyhedrons, thick needles/platelets (Fig. 9b), and irregular platelets (Fig. 9c). Thick and occasionally fragmented uniform needles were identified as the β-AlFeSi intermetallic phase, while the longer irregular platelets with Mg content were the π-Al₈Mg₃FeSi phase and polyhedrons were α-Al₈Fe₂Si. Scattered Ti particles can occasionally be found either in the α-Al matrix or near the fragmented SiC-reinforced phase.

The microstructural analyses of the workpiece material and chip after dry cutting are compared in Fig. 10a and b, respectively. The distribution of SiC particles in the ductile matrix was not uniform, and particle agglomeration with large clusters was observed across the microstructure of the workpiece (Fig. 10a). This arrangement leads to the presence of large areas without SiC particles, inducing anisotropy in the material properties. This significantly affects the homogeneity of plastic deformation and built-up edge formation during chip formation. The microstructure of the chips after dry cutting showed smaller oriented phases (Si_P/Mg₂Si/Fe-based). Moreover, a more uniform distribution of SiC particles along the shear bands occurred. Redistribution occurred across the thickness of the chip. This is believed to be based on the phenomena that emerge during dry machining. The heat produced was retained more in the material. The aluminium matrix becomes softer and more fluid. Thus, the SiC particles tend to collide with one another and cluster more because the particles have a higher melting point than the aluminium matrix. However, the particles cannot reach the softening state at this temperature. Moreover, these non-metallic particles will not form a perfectly homogenous mixture with the metallic matrix, and this will promote cluster formation. Various degrees of particle fragmentation were observed in the workpiece (Fig. 10c). In comparison, fragmentation can be expected to be significantly higher in chips in general, as observed under dry conditions (Fig. 10d), mainly owing to plastic deformation.

Dry cooling significantly affected the intensity of plastic deformation during chip formation. The shear band localization increases with the cutting speed. On the other hand, it creates regions with more space between the SiC bands and a more compact arrangement of the Si_P/Mg₂Si and Fe-based phases, as shown in Fig. 11a. This is significantly different from the particle distribution for all three cryogenic approaches. Rake cooling resulted in a similar chip morphology (Fig. 11b), but the SiC bands were less oriented according to the shear band. Some of the undeformed α-Al rosette cells can still be observed, and the bands are also accompanied by a smaller number of phase agglomerates. Plastic deformation and shear bands were less apparent (localized) in the case of flank cooling and dual cooling. This indicates a lower temperature in the primary shear zone. Some of the SiC particles and phases did not follow the material flow, which resulted in their less uniform arrangement (Fig. 11c-d). Thus, the microstructure is more similar to that of the workpiece material, but with a more uniform distribution of SiC particles.

The mechanism of chip separation - crack initiation and propagation in the material was observed to be dominant. Chip formation is followed by the initiation of cracks along the matrix/particle interface. The formation of microvoids initially occurs at the locations of existing inhomogeneities or along the boundaries of fragmented SiC particles, and their subsequent coalescence results in the development of microcracks. The Mg₂Si and Si_p phases are obstacles that bridge the microcrack propagation, whereas the hard SiC particles act as barriers and deflect the propagating cracks (Fig. 12d). However, the brittle Fe-based intermetallic exhibited fracturing, resulting in the formation of facets aligned along the boundaries of the remaining phases and SiC particles (Fig. 12c). With a growing trend towards particle fragmentation, these factors serve as possible precursors to the formation of cracks. The fracture resistance of the composite material depends largely on the onset and propagation of cracks. These cracks emerged not only from the unrestricted side of the chip but also from the newly developed rear side (interface between the tool and chip) (Fig. 12a-b).

This study investigated the effect of cryogenic cooling on chip formation and morphology during the turning machining of aluminium-silicon carbide (A359/SiC-20wt%) composites using an uncoated tungsten carbide cutting tool. The results revealed significant differences in chip formation, morphology, and characteristics compared with the machining of the material under dry conditions. The primary outcomes and observations outlined in this paper are as follows:

Under cryo-cooling conditions, shorter and less curled chips were observed in comparison with dry conditions at all tested speeds. Consequently, the most effective cooling strategies are rake and dual cooling, even though the tool-chip contact length remains relatively unchanged. Hence, cryogenic cooling significantly affects the chip morphology.
The results indicate that cryocooling has a diminishing effect on the increase in the shear angle with increasing cutting speed. This phenomenon is primarily influenced by the rake and dual cooling strategies.
The utilization of a cryocoolant (in any tested manner) promotes the adhesion of materials and the formation of built-up edges owing to the reduced temperature of the chip. Greater adhesion was observed on the tool side of the chip. The presence of a built-up edge can shield the tool from the abrasive impact of hard particles. As a result of low cohesion and increased plasticity, the edges of the chips became more frayed.
Microscopic structural changes occurred in the material after deformation (in chips). The original SiC particles underwent fracturing and fragmentation into smaller components within the chip. These particles are redistributed along the shear bands in dry cutting as the matrix becomes softer owing to the high temperature, resulting in non-uniform plastic deformation.
Unlike in dry machining, the hard SiC particles were distributed more uniformly in volume for all cryo-cooling strategies. Despite the composite soft matrix, chip separation was primarily facilitated by the formation and propagation of cracks and microcracks along the interface between the matrix and hard particles, leading to noticeably frayed chip edges and good breakability. Hence, the results indicate that cryogenic cooling significantly improves chip breakability.

Acknowledgment

We would like to acknowledge project NexT- LighT; DNR: 2020-04292 with support from the strategic innovation program LIGHTer, a joint venture by Vinnova, Formas, and the Swedish Energy Agency. CryoMach (Ref.No: 2018-03332) with equipment support, a Vinnova (Swedish Government Agency for Innovation Systems), and Eureka SMART Advanced Manufacturing. Special thanks to industry partners Fortiva Tools Mr Kalle Falk and Air Liquide Mr Niklas Ehrlin and Ms Pernilla Lundberg.

Funding

This publication was created with the contribution of the knowledge obtained within the project, Machine Tools and Precision Engineering (MTPE), Reg. No. CZ.02.1.01/0.0/0.0/16_026/0008404.

Competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Authorship contribution statement

Petr Mašek: Writing, Visualization, Editing, Nageswaran Tamil Alagan: Writing – review & editing, Resources. Vladimír Mára: Writing, Data analysis, Visualization. Samuel A. Awe: Review & editing, Supervision. Emeka Nwabuisi: Design of experiment, Methodology, Data acquisition, Data analysis. Pavel Zeman: Methodology, Writing – review & editing, Resources, Data analysis.

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Chip formation and morphology in cryogenic machining of Al-SiC composites

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Abstract

Figures

1 Introduction

2 Experimental methods

2.1 Workpiece and cutting tool

2.2 Cryogenic and Machining Setup and Process

3 Measurement methods

3.1 Chip formation analysis

3.2 Microstructural analysis

4 Results and Discussion

4.1 Chip morphology, length, and breakability

4.2 Chip Characteristics

4.3 Microstructural analysis of workpiece and chips

5 Conclusions

Declarations

References

Status:

Version 1