Effect of graphite on fracture toughness of 6061Al-graphite

Abstract

This paper exhibits the investigative work conducted on the fracture toughness and microstructure of 6061Al-graphite particulate composites. The requisite specimens were prepared using stir casting technique with graphite proportions ranging from 3 to 12% by weight. The microstructure and interfacial bonding of 6061Al-graphite particulate composites were studied. Fracture toughness investigations were carried out on 6061Al-graphite for 3%, 6%, 9% and 12% of graphite by using single-edge notch bend (SENB) specimens. The maximum fracture toughness was found for 6061Al-9%Gr for a∕W = 0.45 and the value is 17.94 MPa $\surd$ m. Examination of specimen surface of 6061Al-graphite was done using scanning electron microscope (SEM). Due to good interface bonding and uniformity in distribution, improved fracture toughness properties are achieved.

Keywords

Aluminium-graphite particulates MMC SEM fracture toughness SENB

1. Introduction

Fracture toughness is typically not a general phrase, but a well-defined quantitative concept for assessing the resistance of a material against crack propagation (starting from an existing crack). It is limited to outcomes of fracture mechanics investigations in this work, which are specifically pertinent to crack control and fracture test in depicting the material’s behavior for a crack to oppose fracture. The investigational estimation and standardization of fracture toughness assume a basic part in the use of fracture mechanics techniques to assess the structural integrity, design for damage tolerance, assessment to fitness-for-service, and examination of residual strength for various engineering structures and components. Values of fracture toughness may likewise fill in as a premise in performance evaluation, quality affirmation and material description for representative engineering structures, together with oil and gas pipelines, aircraft, ship and automotive structures, piping and pressure vessels, petrochemical tanks etc. In this manner, fracture toughness investigation and assessment have been a critical issue being developed for fracture mechanics technique and its engineering applications.

The most important parameters [1] used in fracture mechanics are the elastic energy release rate G (or its equivalent accomplice – stress intensity factor K), the J-integral and the crack-tip opening displacement (CTOD). To measure these parameters many experimental techniques have been adopted to explain the material’s fracture toughness (K_Ic). Customary terminology relating to K_Ic testing and assessment has been defined in E399-17 [2,16] by the American Society for Testing and Materials (ASTM). All concepts and requisites relating to fracture tests utilized as a part of this work are characterized by ASTM E399.

ASTM fracture test standards prescribed many types of conventional fracture test specimens. These include single-edge notch bend (SENB) specimen, compact tension (CT) specimen, disk-shaped compact tension (DCT) specimen, arc-shaped bend (AB) specimen and arc-shaped tension (AT) specimen. Different specimen size requirements are prescribed for different fracture test standards so as to get valid fracture toughness, also to restrict the effects of crack-tip limitation on that fracture toughness parameter.

Metal matrix composites (MMCs) have their applications where it requires weight savings, wear resistance and thermal management. Considerably the majority of commonly used metal matrix composites [3] has their base material as aluminum, magnesium, and titanium alloys reinforced with silicon carbide (SiC), alumina (Al₂O₃), carbon, or graphite.

The literature study exhibits a survey of the published material available i.e. aluminum based MMCs which are a mixture of two phases, matrix, and the reinforcement. Properties of MMCs were the effect of various reinforcement types, their weight/volume fraction, their particle size and aging behavior. Aluminium-graphite particulate composites for various mass fractions are prepared by using stir casting method [4–7], advanced shear technology [8] etc., and their properties such as hardness [4], tensile strength and elongation [5] and fracture properties [9,10] etc. were examined for different mass fractions of reinforcement.

Fracture toughness [10], tensile fracture behavior on circumferential notched tensile (CNT) specimens [11,12], effect of thickness on fracture toughness [14], and fracture toughness using compact tension specimens [17] of aluminium based MMCs were reported by many researchers in order to compare the results of the experiments with the unreinforced aluminium alloy.

The effects of time and stress state on fracture behaviors and mechanisms of polyvinyl chloride foam [18] have been evaluated by varying displacement rates (0.1 to 1000 mm/min) and specimen thicknesses (5 to 37.5 mm). The fracture toughness was lower than those of thick specimens at low displacement rate.

The literature review also reveals that the more work has been done on the tensile and fracture characterization of Al- SiCp [11], Al-TiC [12], Al-Fly Ash [13] MMCs. Still, a considerable scope is present for a study on the Al-6061/graphite particulate composites, predominantly in the areas of fatigue and fracture so as to enhance the fracture aspects of the material to avoid the cracking. In this background, this research work is intended to find the fracture toughness (K_Ic) of 6061Al-graphite particulate MMC at varied mass fractions by using single edge-notch bend (SENB) specimens.

2. Materials

Precipitation-hardened aluminum alloy called 6061Al and its main alloying elements are silicon (0.70%) and magnesium (0.81%). Physical properties [14] of 6061Al are hardness 95BHN, Elastic modulus 68.9 GPa, ultimate tensile strength 315 MPa, yield strength 275 MPa, extension 17%. Graphite is available in the shape of fibers and particles which has been identified as high strength material. Physical properties of graphite [15] are elastic modulus 15 GPa, yield strength 55 MPa.

6061Al and graphite particulate metal matrix composites produced by solidification techniques present greater tribological properties such as better machinability, low wear rate, high damping capacity, low coefficient of friction, and their outstanding antifriction properties used for a range of automobile applications [10].

Aluminum 6061 as a matrix and graphite particles as reinforcement are utilized for this work. The reason to involve these materials is their density. The density of 6061Al is 2.65g/cc and graphite is 2.26 g/cc. The 6061Al-graphite particulate composites exhibit isotropic properties.

3. Processing

The stir casting method was utilized to prepare the 6061Al-graphite particulate metal matrix composites at different mass fractions of graphite (3, 6, 9 and 12%). The 6061Al blocks were allowed to melt in the stir casting furnace. After melting, molten 6061Al was super-heated to the temperature about 720 °C. Hexachloroethane (C₂Cl₆) as a degasifier has been added to the molten aluminum to take away the gases. Impurity in the molten aluminum that is slag also removed. The requisite quantity of graphite particles was added to the molten 6061Al while stirring with a stirrer at speed of 500rpm. Before pouring into the mold cover flux (4%NaCl+45%KCl+10%NaF) has been added to avoid the gas to enter the molten metal. In the split type graphite mold, molten 6061Al-graphite was poured and it was allowed to solidify. From the bars are taken out from molds were utilized for determining required properties of 6061Al-graphite alloy bars.

Shearing temperature (620 °C) and shearing speed (500 rpm) were the two process parameters which affect the composites. To examine the effect of processing parameters, tests were conducted. The different process parameters were chosen to exert a hydrodynamic force on the molten material and to retain the best possible fluidity for the casting.

Fig. 1.

SENB specimen.

Fig. 2.

Three types of load-displacement behavior in a K_Ic test [17].

4. Experimentation

The 6061Al-graphite metal matrix composites (MMC) reinforced with 3%, 6%, 9% and 12% of graphite particles were fabricated by using stir casting method with the stirrer speed of 500 rpm. The molten metal was then poured into the graphite mold of size 70 × 70 × 150 mm. Single-edge notch bend (SENB), as shown in Fig. 1, [16] specimens of size 12 mm × 12 mm × 64 mm, span = 48 mm and notch size is 5 mm were utilized for fracture toughness testing. All the specimens were prepared as per the ASTM standard. Further, a fatigue crack is introduced at the end of the notch by maintaining crack length to width (a∕W) ratios i.e., 0.45, 0.47 and 0.50. i.e fatigue crack of 1 mm, 1.8 mm, 2 mm has been introduced using a servo-hydraulic testing machine. The fracture toughness tests were conducted by maintaining the displacement rate 1 mm/min. SENB specimens are successfully pre-cracked in this manner with the cyclic loading of 0.3 times the yield load of the material by maintaining the frequency of 5 Hz. When crack self-arrest occurred the loads were changed to complete the pre-cracking process. Once the crack starts and propagates by pre-requisite dimension the fatigue loading would stop by the machine control.

In a room temperature, fracture toughness tests were conducted on a computerized universal testing machine (UTM). Load-deflection curves of different graphite proportions for different a∕W ratios (i.e., 0.45, 0.47 and 0.50) were recorded. The fractured surfaces of all specimens were studied in detail using a scanning electron microscope to characterize the fracture mechanisms.

The load-deflection records were evaluated to find the fracture toughness (K_Ic) of the 6061Al-graphite MMCs. The value of K_Ic is calculated from crack length and critical load (P_Q). The value of P_Q is obtained by plotting a secant slope on the load and deflection record.

Fig. 3.

(a): Load v/s displacement graphs for different compositions for a∕W = 0.45. (b): Load v/s displacement graphs for different compositions for a∕W = 0.47. (c): Load v/s displacement graphs for different compositions for a∕W = 0.50.

The estimation for P_Q engages representing the loading slope of the load versus displacement record. A slope of 5% less than the available slope, referred as secant slope, is then drawn. In the case of the slope of the load-versus-displacement record of Al6061-graphite, the curve obtained was Type III [17] curve as shown in Fig. 2. The maximum value of the load is itself will be the critical load (P_Q). This corresponds to about 2% ductile crack extension; this may be an effective crack extension linked to plastic zone development.

From the measured crack length and the P_Q value for each experiment, the value of fracture toughness K_Q is determined using the Eq. 1: $\begin{eqnarray}\displaystyle \text{K}_{\text{q}}=\left(\frac{\text{P}_{q}}{\text{B}\sqrt{w}}\right)f\left(\frac{a}{w}\right) & & \displaystyle\end{eqnarray}$ (1) where f (a∕W) is a geometry function [16] that depends on the crack size to specimen width ratio a∕W only. For SENB specimens, $\begin{eqnarray}\displaystyle f\left(\frac{a}{W}\right)=\displaystyle \frac{3\displaystyle \frac{S}{W}\sqrt{\frac{a}{W}}}{2(1+2\frac{a}{W})(1-\frac{a}{W})^{3/2}}\left[1.99-\displaystyle \frac{a}{W}\left(1-\displaystyle \frac{a}{W}\right)\left\{2.15-3.93\left(\displaystyle \frac{a}{W}\right)+2.7\left(\displaystyle \frac{a}{W}\right)^{2}\right\}\right] & & \displaystyle \nonumber\end{eqnarray}$ K_Ic values were determined from the P_q values as prescribed in ASTM E399 Standard. Experiments were carried out and the results are listed in Table 1. Figure 4 shows the load-displacement curves of Al6061-graphite obtained using SENB specimens for different crack length to width ratios.

From Fig. 3 it is clear that as the load increases displacement increases for a∕W = 0.45, 0.47 and 0.50 of 6061Al-graphite. For all the a∕W ratios increase of load carrying capacity of 6061Al-graphite has been observed. At 12% graphite, there is a decrement in the load-bearing capacity of the material has been observed. This decrement may be due to the clustering of graphite particles in the aluminum matrix. As compared to aluminum alloy, Al-graphite composite has more fracture toughness values. This is due to the aluminum alloy don’t have the reinforced particles to barricade the crack propagation. Also crack will propagate faster in aluminum alloy than the Al-graphite composite. Addition of reinforcement will lead to loss of ductility, and hence brittle fracture was observed (Fig. 3).

Table 1

Fracture toughness of Al6061-graphite MMC for different a∕W ratios

Specimen	For a∕W = 0.45		For a∕W = 0.47		For a∕W = 0.50
	Fracture load (P_Q) kN	K_IC MPa $\surd \text{m}$	Fracture load (P_Q) kN	K_IC MPa $\surd \text{m}$	Fracture load (P_Q) kN	K_Ic MPa $\surd \text{m}$
6061Al	2.51	17.45	2.33	17.20	2.10	17.01
Al-3%Gr	2.28	15.85	2.14	15.79	1.95	15.79
Al-6%Gr	2.44	16.96	2.23	16.46	2.02	16.36
Al-9%Gr	2.58	17.94	2.39	17.64	2.17	17.58
Al-12%Gr	2.49	17.31	2.37	17.49	2.14	17.33

5. Optimization

5.1. Taguchi’s experiment

The Taguchi method of optimization is one of the most effective techniques because of its simplicity to conduct the design of experiments. The main aim of the Taguchi technique is to evaluate the statistical data which is the input function for optimization. The technique developed for the design of experiments to examine the different parameters and their effect on process mean and variance. Analysis of variance (ANOVA) on the data from the Taguchi design of experiments can be used to select new parameter values to optimize the performance behavior.

In the present work, optimizing the parameters of the single-edge notch bend (SENB) specimens is carried out using the Taguchi method. Four parameters and two factors are considered to optimize the parameters. Factors considered are material composition and a∕W ratio. Levels considered are a∕W = 0.45, 0.47 and 0.50, and material compositions considered are 3%, 6%, 9% and 12% of graphite reinforcement in the 6061Al matrix. The experimental data i.e load carrying capacity and fracture toughness are input functions for the Taguchi design. For the given input functions, Taguchi design has been analyzed. The results of the analysis are shown in Fig. 4.

Fig. 4.

(a) Load carrying capacity vs composition and a∕W ratio. (b) Fracture toughness K_Ic vs composition and a∕W ratio.

From the outcomes of the Taguchi design, it is observed that the load carrying capacity of the composite decreases as a∕W ratio increases. As composition increases load carrying capacity increases up to 9% of graphite and decrease for 12% graphite. From Fig. 4(b) it is observed that the fracture toughness of the composite decreases as a∕W ratio increases. It is obvious that as the load carrying capacity decreases fracture toughness decreases. As composition increases fracture toughness increases up to 9% of graphite and decreases for 12% graphite.

The optimize composition is at 9% graphite based on load and fracture toughness of the Al6061-graphite composite. Also, load carrying capacity is maximum for a∕W = 0.45 and as there is not much difference in fracture toughness values for a∕W ratio = 0.45 and 0.47. From the results of the experiment of single-edge notch bend (SENB) specimens fracture toughness values increased as the increment in the graphite content by up to 9% of graphite. From the Taguchi analysis, on SENB specimens, 6061Al-9% graphite is the optimized composition and fracture toughness is maximum for a∕W ratio = 0.45. Hence Al6061-9% graphite will be considered as optimized composition.

5.2. Analysis of variance (ANOVA)

Analysis of variance, in short called ANOVA, is a statistical tool used to evaluate the level of the individual involvement of the process parameter on the responses such as fracture toughness and load carrying capacity, and furthermore, give precisely the arrangement of the process parameters. Individual optimal values for the process parameters and their predefined performance attributes can be found. Table 2 shows the results of ANOVA for load carrying capacity, and fracture toughness.

Table 2
(a) ANOVA for load carrying capacity. (b) ANOVA for fracture toughness

Source DF Seq SS Adj MS F-value P-value % confidence level

(a)

Composition 3 0.117533 0.039178 58.52 0.161 28.80

a∕W ratio 2 0.286517 0.143258 214.00 0.134 70.21

Error 6 0.004017 0.000669 0.98

Total 11 0.408067 100.00

(b)

Composition 3 6.5378 2.17926 75.79 0.136 95.57

a∕W ratio 2 0.1304 0.06520 2.27 0.257 1.91

Error 6 0.1725 0.02876 2.52

Total 11 6.8407 100.00

Source	DF	Seq SS	Adj MS	F-value	P-value	% confidence level
(a)
Composition	3	0.117533	0.039178	58.52	0.161	28.80
a∕W ratio	2	0.286517	0.143258	214.00	0.134	70.21
Error	6	0.004017	0.000669			0.98
Total	11	0.408067				100.00
(b)
Composition	3	6.5378	2.17926	75.79	0.136	95.57
a∕W ratio	2	0.1304	0.06520	2.27	0.257	1.91
Error	6	0.1725	0.02876			2.52
Total	11	6.8407				100.00

DF = Degrees of freedom, SS = sum of squares, MS = mean Square, F = variance, P = test statics.

From the ANOVA outcomes, it is observed that the factors affecting the load carrying capacity are the composition (28.8%) and a∕W ratio (70.21%). The ANOVA analysis demonstrates that the load carrying capacity of the material mainly influenced by a∕W ratio than the composition of the material. It is obvious that as crack length (a) increases load carrying capacity decreases. Also, the factors affecting the fracture toughness are the composition (95.57%) and a∕W ratio (1.91%). The ANOVA analysis demonstrates that fracture toughness of the material mainly influenced by the composition of the material and also by the a∕W ratio. It is obvious that as crack length (a) increases load carrying capacity decreases, in turn, reduces the fracture toughness.

Fig. 5.

Scanning electron micrograph showing a uniform distribution of graphite particles (a) 3% graphite (b) 6% graphite (c) 9% graphite (d) 12% graphite.

6. Results and discussions

In the microstructure, shown in Fig. 5, of the 6061Al-graphite particulate composite, confirms uniform distribution of the reinforcement. During the time spent the mixing, a spinning of liquid material is formed by the revolution of the stirrer through which the graphite particles are drained into the dissolve.

The force gave by mixing the molten material with a mechanical stirrer beats the surface vitality hindrance because of poor wettability of graphite by Al composite. Once the graphite particles are moved into the molten aluminum, the distribution is firmly influenced by certain flow transitions. From the momentum transfer and the outspread flow of melt, lifting of graphite particles will take place and also causes prevention of particle settling in the matrix. Meanwhile, local hydrodynamic forces are induced on the particle grouping of graphite particulates. These forces induced are capable of separating the clustering of graphite particles which in turn leads to homogeneous microstructure all through the cast segment.

The corresponding EDX profile analysis shown in Fig. 6 provides the atomic percentages of the elements found on the 6061Al surface. In all the compositions of graphite, oxygen (O) content has been obtained. The content of O is due to the formation of Al₂O₃ on the top of the pits as the main compound on the surface. The presence of carbon might have come from the 6061Al-graphite composite itself. The low content of Silicon and Magnesium indicates that the presence of Si and Mg in the 6061Al [5,14].

Fig. 6.

EDX profile analysis for the surfaces (a) 3% graphite (b) 6% graphite (c) 9% graphite (d) 12% graphite.

A strong homogeneous microstructure between the matrix and reinforcement helps in the load transfer from the reinforcement to the matrix. Thus, the break happens in the composite via the reinforcement and not along the interface. Despite the fact that the graphite is a non-load bearing ingredient, a solid particle/matrix interface helps the graphite particles install themselves into the matrix legitimately, enhancing the crack resistance. It has been reported that during solidification, an improvement in the interfacial relationship between the aluminum matrix and graphite.

From Fig. 7 it is clear that there exists a good bonding between aluminum matrix and graphite particles. Also, it is found that there is no indication of extensive segregation or void formation at the matrix-particle interface. The good interfacial bonding and homogeneous distribution of graphite particles in the matrix have a direct impact on the mechanical behavior of a composite material.

Fig. 7.

SEM of an Al/Gr composite demonstrating interface between the aluminum matrix and a graphite particle. (a) 3% graphite (b) 6% graphite (c) 9% graphite (d) 12% graphite.

The outcome of fracture toughness testing it is clear that the K_Ic of 6061Al-graphite was increased with an increase in weight percent of graphite. The increase of fracture toughness is due to the result of increased graphite particulates which prevent the initiation of dimples/voids into internal cracks in the microstructure. The decrease of fracture toughness at 12% graphite may be the result of increased graphite particles which causes particle grouping in the aluminum matrix. The maximum fracture toughness was found for 6061Al-9%Gr for a∕W = 0.45 and the value is 17.94 MPa $\surd$ m.

Figures 8(a) to (d) show SEM of fractured surfaces of 6061Al-graphite indicating voids/dimples of different sizes combined together form dents of different size and shape initiates microcracks. These microcracks propagating not only through the matrix but also through the matrix and reinforcement interface. Examination of the fractured surface exposes the restricted obliteration to be congregation at the reinforcing graphite particulates and decohesion at aluminum and graphite interfaces.

Fig. 8.

Scanning electron micrograph of fractured surfaces of 6061Al/Gr composite showing dimples, micro cracks. (a) 3% graphite (b) 6% graphite (c) 9% graphite (d) 12% graphite.

Due to the good bonding and uniform distribution of graphite particles in the aluminum matrix, 6061Al-graphite particulate composites have greater tensile properties, better machinability, improved hardness, and have good fracture toughness properties.

From the above mentioned properties one can consider the 6061Al-graphite particulate MMC for automobile applications such as chassis of the Audi A8; Bearing surfaces, bushes, cylinder liners, pistons, camshafts; a diving cylinder, scuba tanks; bicycle frames and components; and aerospace applications such as wings and fuselages of aircraft structures; Internal aerospace engine components, Exhaust systems; military applications such as in AR-15 rifle variants, docks and gangways, helicopter rotor components; also some general applications include packaging of food and beverages, fly fishing reels.

7. Comparison of fracture toughness of 6061Al-graphite and other Al MMCs

The literature review also reveals that the more work has been done on the tensile and fracture characterization of Al-SiC_p [11], Al-TiC [12], Al-Fly Ash [13] MMCs. For as-cast conditions K_Ic = 6. 61, 6.59, 5.63, 6.71 MPa $\surd$ m and for age hardened conditions K_Ic = 7. 6, 7.65, 7.8, 8.2 MPa $\surd$ m respectively for 3, 6, 9, 12% of weight fractions of silicon carbide [11]. In both cases, 6061Al-graphite have more fracture toughness values as compared to aluminum silicon carbide. Also, M. S. Raviraj et al. [12] reported that Al-TiC has fracture toughness 19.2, 16.4, 17.6 MPa $\surd$ m for 3, 5, 7% of mass fractions of titanium carbide particulates. Ajit Bhandakkar et al. [13] reported that Al6061/fly ash composites have K_Ic = 13. 77, 14.27 MPa $\surd$ m for 5, 10% of fly ash reinforcement.

From the literature, it is found that the fracture toughness values for Aluminium-Graphite are more compared to Al-SiC_p [11], Al-TiC [12], Al-Fly Ash [13] MMCs.

8. Conclusions

An effective blending innovation to accomplish a uniform circulation of uncoated graphite particles inside an aluminum matrix has been developed. Quantitative investigation using EDX uncovered enhanced particle distribution in the 6061Al-graphite composite. Quantitative examination of the reinforcement distribution and mechanical properties affirmed the benefits of the stir casting processes. The maximum fracture toughness was found for 6061Al-9% Gr for a∕W = 0.45 and the value is 17.94 MPa $\surd$ m. From the Taguchi analysis, 6061Al-9%graphite is the optimized composition and fracture toughness is maximum for a∕W ratio = 0.45. The ANOVA analysis demonstrates that load carrying capacity of the material mainly influenced by a∕W ratio than the composition of the material and fracture toughness of the material mainly influenced by the composition of the material and also by the a∕W ratio. Enhanced fracture toughness is accomplished because of structural consistency, good bonding and uniform distribution and strong interfacial bonding of graphite particles in the matrix. From the enhanced properties we can consider the 6061Al-graphite as a potential material for e.g. aerospace and automobile applications.

Conflict of interest

None to report.

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