Tribological and microstructural characteristics of Al₂O₃–LM6 semi-solid cast composite synthesized by dual mixed slurry process

Abstract

The purpose of this work is to analyze the impact of reinforced Al₂O₃–LM6 aluminum composite made with dual mixed slurry for semi-solid casting with stirring, which is documented according to microstructure and wear properties. Under dry sliding pin-on-disk conditions, the composition and surface morphology of worn surfaces were carefully examined for the 10 N applied force, 1.2 m/s sliding speed, and sliding distances up to 750 m. Experimental findings indicate that Al₂O₃ reinforcement with semi-solid dual slurry effects give outstanding resistance against the adhesive wear. Microstructure, EDS, and XRD results from worn surface evaluation all show evidence of protective layers, which helps explain the reason for the rate of wear decreased due to processing. In this experiment, LM6 serves as the matrix material and Al2O3 as reinforcement. Al₂O₃–LM6 semi-solid cast composite was created via a dual mixed slurry technique with temperature control close to the solidus line for varying compositions. This work reports a substantial relationship between metallurgical and mechanical parameters. Mechanical properties and wear characteristics have been investigated.

Introduction

Engineering applications such as engine parts of the automobile, structures of the bridges, machine aliments, and commercials and house hold utilities always seek continuous improvement according to the light-weight and high-strength characteristics [1,2,3,4]. Along with this, mechanical and metallurgical properties of the fabricated component are also considerable for continuous improvement with the help of variation in material and process control [3, 5]. Selection of material for the ceramic waste composite according to the TiB₂, Al₂O₃, B₄C, and SiC and other glass-based ceramics is always attractive due to reinforcement capabilities for aluminum- and copper-based metal [1,2,3,4,5,6,7]. Other than material properties, process control also provides excellent results of the synthesis for these ceramic-based composite [7]. This involved processing parameter provides process control and quality product synthesis [8,9,10]. The synthesis of MMC (metal matrix composite) to measure or evaluate the impact of processing routes and parameters, such as liquid metallurgy and dry metallurgy process, are in attraction for the metal industries. Liquid metallurgy or casting processes are best suited for fabrication technique in industries due to flexibility based on the choice of material and various processing parameters [11]. In the advance casting process, temperature control is a measured parameter in the method and formation of slurry to the pouring of melt. With this, the temperature of the slurry is governed by the associated secondary parameters such as steering action, rotational speed, and slow behavior during the process [12]. On the other hand, mixing of slurry for two different components is also opted for synthesizing ceramic component. Moreover, mixing of slurry for additional material provides semi-solid slurry under a control temperature near the solidus line (650 °C–700 °C) [13]. Formation of dual mixed slurry has the potential to offer refined grains, pressurized solidification, and uniform distribution of particles [3, 7]. The formation of dual mixed slurry for similar and dissimilar metals gives excellent results to the net-shaped component synthesis [10]. LM6 is a silicon-rich alloy which is used for the synthesis of components where excellent frictional and wear properties are desirable. However, in this experimental work, Al₂O₃ is considered as a reinforcement and LM6 as a matrix material. The Al₂O₃–LM6 semi-solid cast composite has been synthesized for deferent composition by dual mixed slurry approach with temperature control near the solidus line. Mechanical properties and wear characteristics have been evaluated, and a significant relation between metallurgical and mechanical characteristics is reported in this work.

Materials and methods

The processing composition of the material with its combination used for the synthesis of Al₂O₃–LM6 dual mixed slurry cast composite is reported in Table 1. Initially, in the process, LM6 was used as a primary material in which 13% Si was present, and micro size Al₂O₃ was considered as a reinforcement material. In the 1^st stage synthesis process, chill bricks of LM6 were considered for the fabrication of primary slurry (matrix slurry). Small size chill bricks of LM6 were heated separately up to a temperature of 850 °C to 900 °C, and here in the experiments, LM6 considered 90% of the total weight of each particular composition. Simultaneously, in other compositions, Al₂O₃ powder particle (size ≤ 20 μm, 120 mesh, average particle size = 0.13 mm, density = 3.95 g/cm³) and LM6 powder particle (size ≤ 22 μm, 160 mesh, density = 2.66 g/cm³) have been taken for reinforcement slurry. Both the powder particles were mixed by the help of ball milling process which provides mixing as well as deduction in the size of the powder particle. Here, for the ball milling process, material-to-ball weight ratio was taken as 0.1. A constant rotational speed of 200 rpm in planetary ball milling machine mix powder particles were closed, and after 40 min of continues rotation, powder particles were cooled down to avoid any type of creation in the particles. The process of rotation was repeated 3 times for the total milling time of 120 min. During the milling process, due to the impact of the ball, powder particles were flattened and broken into small particles. In Fig. 1, microstructure image for the mixed powder particle is presented. It is clearly visible that the maximum size of the flattened particle was about less than 180 μm and minimum or less than 70 μm. This confirmed that during the milling process, the size of the powder particles reduces. After the milling process, powder particles for each combination were placed inside a hot air oven for 1 h to avoid any type of moisture generated after milling. In the next step, dual mixed slurry was prepared by the mixing of reinforcement and matrix slurry, having a mixed temperature of 700 °C before pouring. During the mixing of slurry, a stirring action was provided for a short interval of 10 min. It is possible to generate some bubbles during a steering process; however, the speed was very slow and equal to 20 rpm. The prepared mixed slurry was poured inside a mold for solidification process. During the flow, mixed slurry was poured by the help of a channel due to which the free vertex flow for the slurry was observed. In effect of this, initial solidification was delayed which generally appear for direct pouring process. For each combination of the Al₂O₃ (x = 2 and 4%), the initial pouring temperature was a little bit different, which is mentioned in Table 1. Similar steps involved in the process are discussed in detail in our previous publication [10, 11].

Table 1 Elemental composition of LM6 (wt. %) and process parameters

Fig. 1

Processing scheme of the Al₂O₃–LM6 semi-solid cast composite

Solidified cooled ingot was taken out after some time (about 6 h) and considered for the analysis in this work. Microstructure and tribological characteristics have been considered for the analysis. Microstructure at the different interfaces and locations has been conducted. For the friction and wear characteristic, pin-on-disk (POD) apparatus has been used for dry sliding wear behavior. The size of the pin was 6 mm in diameter, where the length of each pin was restricted between 30 and 35 mm. Dry sliding wear behavior analysis is considered for a sliding distance of 750 m on the EN31 counter surface. The load provided in the machine was limited to 10 N, and mass loss from the pin was recorded after an interval of 150 m.

The microstructure characteristic (energy-dispersive X-ray spectroscopy (EDX) with a ZEISS SUPRA 40 VP instrument) of the plane and worn surfaces were considered for each combination and conducted at different locations of pin cross-section. Along with the microstructure, EDX measurement has been taken into consideration for elemental phase analysis of worn surfaces.

Result and discussion

Figure 2 presents the microstructure of the developed composite at different locations. In Fig. 2a, it is clearly visible that various cavities are generated during the casting appeasers. But it is also observed that these cavities are filled by solid particles of Al₂O_3. Moreover, closer microstructure for Al₂O₃–LM6 cast composite (Fig. 2a) distribution of Al₂O₃ micro-size particulate is clearly identified. In Fig. 2b is the interface of Si-rich Al₂O₃ distributed (gray in color) with independent interfaces of Al and Al₂O₃ (white distribution). These faces are irregular in distribution but segregated with other interfaces. According to Fig. 2c, d, effect of 4% Al₂O₃ addition is clearly visible which confirms the incomplete recrystallization generated during the solidification [7, 13]. Micro-size un-melted solid particles are present in a similar way, but generated cavities during the solidification are quite less [7, 13]. In Fig. 2c, this phenomenon is observed in which less than # micro meter size cavities are observed and filled by the gray distribution which might be oxidized particles/interface of Al and Si [13]. For this, a closer observation is presented in Fig. 2d, in which a particle inside a cavity is marked by green boundary [6, 9]. The size of these particles is almost equal to the 1 μm in the radius but simply segregated from nearby the grains [10]. This depicted the effect of thermal mismatch in between the matrix and reinforcement material [9].

Fig. 2

Microstructure of a, b 2% Al₂O₃–LM6 semi-solid and c, d 4% Al₂O₃–LM6 semi-solid

For more investigation of the particles/interface distribution inside the cavities, microstructure of the synthesize composite is presented in Fig. 3. Independent clustering particle (marked by red boundary) is observed in Fig. 3a, although some amount of gray phases (oxidized particles due to temperature) are surrounded along the grain boundary. But some amount of white phases of Al-Si interface is also present in this microstructure which confirms the effect of the ball milling due to size (nano) of the Al₂O₃ particle [10]. Similarly, in Fig. 3b, Al₂O₃-rich interface is presented along the cavity for 2% Al₂O₃ addition. Moreover, this independent distribution contains some pours which is a direct effect of solidification. During the cooling and solidification, particles and slurry interfaces are shrunk, and these pours are generated [10, 14,15,16,17]. A similar effect for 4% Al₂O₃ is presented as Fig. 3c, d. Here, Fig. 3c, d of Si phases is presented as a comparison to Fig. 3a. A difference is observed here, for particle distribution and interface generation. A bigger gap boundary is obtained in between the brittle Si particle and LM6 solidified matrix. This shows and confirms the effect of drop in temperature as well as the pressurized solidification for 4% Al₂O₃. Furthermore, like Fig. 3b, here in Fig. 3d, a similar effect for distribution for Al₂O₃ is presented but quit done [3, 11].

Fig. 3

Microstructure for particle distribution and interfaces a, b 2% Al₂O₃–LM6 semi-solid and c, d 4% Al₂O₃–LM6 semi-solid

Coefficient of friction (COF) is presented in Fig. 4a. According to Fig. 4a, it can be observed as with a function of sliding distance, the value of COF increases continuously, but in a random manner [13]. While with an amount of Al₂O₃, a difference is observed in COF, according to the amount of Al₂O₃ addition. With an increase in the amount of Al₂O₃ (4%), the coefficient of friction decreases that reflects a formation of third body generation [7, 13]. This drop in COF is possible due to following reasons:

• A mechanically mixed layer (tribolayer) in between the sliding surfaces

• Brittle asperities of nano/micro size particles in between sliding surfaces

• Adhesion between the sliding surfaces [7, 13, 17]

Fig. 4

a Coefficient of friction as a function of sliding distance. b Specific wear rate as a function of sliding distance

Among the above reasons, the formation of tribolayer is best possible in this condition because the variation in specific wear rate is almost similar in each case as shown in Fig. 4b. On the later stage of the sliding distance, a specific wear rate is almost equal (0.1329 × 10⁻⁶ mm³/Nm) and little bit lower for 750 mts sliding distance and 4% Al₂O₃. In the earlier stage, the specific wear rate is lower due to the present hard abrasive particles (Al₂O₃ and Si) on the pin cross-section. A maximum specific wear rate of 0.1338 × 10⁻⁶ mm³/Nm is obtained for 2% Al₂O₃–LM6 semi-solid cast composite. With the increase in the sliding distance, the specific wear rate also increases which conforms to the COF behavior as discussed earlier [13].

The microstructure of the worn surfaces is presented in Fig. 5. Several wear furrows are observed along with numerous wear debris (marked by yellow box). Here, these debris are stuck along the line of furrows which indicated that formation of these furrows is an effect of brittle particles present in the surface [7, 13, 17,18,19,20,21]. In Fig. 5a, b, worn surfaces of 2% and 4% is presented in which a significant difference for the size and gap between the furrows is clearly seen. Furthermore, the density of the particles along the tribolayer (MML) is much higher for 4% Al₂O₃ as a comparison to the Al₂O₃ addition. A closer view for the MML is presented in Fig. 5c, d, for an interface distribution generated during the sliding [7, 13]. Here, this layer works as a third body in between the sliding surfaces and reduces the specific wear rate. More analysis for the elemental identification on worn surfaces is presented by EDX analysis in Fig. 6. Electron image for 2% Al₂O₃ along with its spectrum is shown in Fig. 6a, b. The obtained data from EDX result is shown in Table 2; it is clearly identified that the participation of Al, O, and Si is more dominating than other elements. Although some amount of Fe is present (2.94%), which conforms the oxide formation for some intermetallic overworn surface [7, 22,23,24], while for 4% Al₂O₃ addition distribution, these particles and element are much higher with respect to 2% Al₂O₃–LM₆ cast composite (Fig. 6c). The corresponding spectrum (Fig. 6d) showed an almost equal distribution of Al and O with Si. Some amount of oxide particles might contain C and Fe. However, the analysis of FESEM, EDX, and specific wear rate confirms the deformation of tribolayer/MML as function of Al₂O₃ addition. This also confirms that the oxide layer is dominated by brittle Si particles available in the matrix material [12, 25].

Fig. 5

Microstructure for worn surfaces of a, b 2% Al₂O₃–LM6 semi-solid and c, d 4% Al₂O₃–LM6 semi-solid

Fig. 6

EDX mapped area and spectrum for a, b 2% Al₂O₃–LM6 semi-solid and c, d 4% Al₂O₃–LM6 semi-solid

Table 2 The elemental identification of the chip structure by wt%

The XRD spectrum for the dry sliding wear surfaces of Al₂O₃–LM6 semi-solid is presented in Fig. 7. X-ray diffraction (XRD) analysis was conducted on different combinations of Al₂O₃–LM6 cast composite to identify the individual phases of aluminum (Al) and silicon (Si) and their oxide agglomeration [7, 17]. The obtained results show that various locations within each combination exhibit individual Al- and Si-rich phases due to dry sliding wear and the presence of metal particles [13, 19, 26,27,28,29]. The predominant phases observed are influenced by the oxides of Al and Si particles, which are indicated by the peaks in the XRD analysis (Fig. 7). Based on the results and analysis, it was found that the Al₂O combination has a majority of phases in a dominant condition, while individual Si-rich phases are present at different locations within the defined scanning range [13, 27]. Some peaks also indicate the presence of elemental phases such as Al₂O and Al₃SiO₂ with Al- and Si-independent brittle particles. Consequently, during sliding, a brittle tribolayer forms and adheres to the softer aluminum surfaces [29]. These phases are primarily influenced by Si particles, which are abundant in LM6 alloys.

Fig. 7

XRD evolution of the worn surfaces of Al₂O₃–LM6 semi-solid

Conclusion

Following findings are drawn for the experimental works:

• Microstructure of Al₂O₃ reinforcement-based LM6 cast composite showed an efficient distribution of Al₂O₃ and homogenous particle dispersion inside the cavities generated during the solidification.

• Friction and wear results for Al₂O₃–LM6 cast composites suggested that wear behavior of the cast composite strongly depends on the amount of Al₂O₃ addition as well as other processing parameters. With an increase in the amount of reinforcement (Al₂O₃), the amount of specific wear rate reduces also with the sliding distance.

• A generation of a third body layer on the worn surfaces is identified on the worn surfaces with brittle abrasive debris and concluded a strong observation for the mechanically mixed layer (MML) due to dry sliding wear.

Synthesized Al₂O₃–LM6 composites will be useful in automotive applications as well as sliding contact devices for high wear resistance in components like pistons and piston rings. The impact of Si and Al₂O₃ abrasive particles increases the wear-resistant quality of the developed composite, which results in longer component life and less maintenance.