Investigation of Microstructural, Mechanical and Corrosion Properties of AA7010-TiB₂ in-situ Metal Matrix Composite

2.1 Material Selection

AA7010 (Al–Zn–Mg–Cu-Zr) is used as matrix alloy and the chemical composition is as shown in Table 1. In- organic salts namely, potassium tetrafluroborate (KBF₄) and potassium hexaflurotitanate (K₂TiF₆) have been used to synthesize the TiB₂ reinforcement by exothermic reaction of two salts with molten alloy. AA7010 and the two halide salts (98% purity)were procured from FENFE Metallurgicals, India and Sisco Research Laboratories Pvt. Ltd. (SRL), India respectively.

2.2 In-situ stir casting synthesis

Aluminium alloy 7010 reinforced with different concentrations of TiB₂ (5, 7.5 and 10 wt.%) were synthesized by insitu stir casting process. The reinforcement was formed through the reaction of two halide salts namely potassium hexaflurotitanate (K₂TiF₆) and potassium tetrafluroborate (KBF₄) with molten aluminium alloy. The formation of reinforcement at the interface is mainly due to the kinetics and thermodynamics of the reaction between two salts and liquid aluminium. The TiB₂ formation in molten aluminium is illustrated in reactions (1) and (2).

The development of TiB₂ particulate reinforcement in the molten aluminium is depicted in reaction (2). And from reaction (1) it can be observed that unbalanced intermetallic components such as AlB₂ and Al₃Ti are formed due to lower reclamation of boron when KBF₄ is added in the stoichiometric proportion. To improve the reclamation of boron, potassium tetrafluroborate was added 20% more than the amount of stoichiometric ratio to attain TiB₂ and evade the development of intermetallic phases [19].

The requisite amounts of two salts needed were weighed to the desired weightfraction of TiB₂. To expel the engrossed moisture present, two fluxes were separately dried in a hot air oven at 150^∘C for 60 mins. The salts of suitable quantity were wrapped in aluminium foils that are to be added to the liquid aluminium alloy. In a vertical muffle furnace, the required amount of aluminium alloy was heated in a pre-heated crucible to a predetermined temperature of 860^∘C. Once the melt reached to 860^∘C the salts were added and stirred at a speed of 180rpm by a zirconia coated graphite stirrer for uniform mixing of salts and molten alloy mixture. To ensure uniform distribution and complete reaction of salts the whole mixture is held at 860^∘C for 60mins. To produce defect free composites the mixture was stirred for first and last 15 mins in a holding time of 60 mins. According to the above Equation (2), TiB₂ particulate formation was confirmed in the molten aluminium alloy by exothermically reaction of two salts (K₂TiF₆ and KBF₄). After the removal of slag, the superheated melt was poured into a plate-shaped cast mild steel mold of 100mm × 100mm × 10mm size. The samples for microstructure, corrosion, hardness and tensile tests were cut by using wire EDM machines.

2.3 Optical Microscopy and FESEM with EDS Analysis

The behavior of most metal alloys and composites can be determined by orientation, size of the grain, twins, shape and size, and distribution of secondary phases in the matrix alloy [20]. Various materials discontinuities, such as inclusions and stringers can also be detected microscopically. Those imperfections could assist initiation of failure sites in the materials, owing to this property of the materials and engineering reliability are to be adjudged through characterization of their shape, size and distribution [21]. Optical microscopy was done at Agro Met Lab, Coimbatore, India by using Dewinter trinocular metallurgical microscope, with specifications of magnification 50× to 1000×, objective lens magnification of 5× to 100× consuming a power of 12V, 50W Halogen lamp with polarizer prism. The polished composite samples were etched with 0.5% HF etchant and Optical micrographs were captured by a digital camera attached to the microscope. Field Emission Scanning Electron Microscope equipped with energy dispersive spectroscopy (EDS) was carried out by using SIGMA HV – Carl Zeiss with Bruker Quantax 200 – Z10 EDS Detector at CIT, Coimbatore, India to observe the microstructure, fracture morphology and elemental analysis of the cast samples.

2.4 XRD Analysis

X-ray diffraction is the scattering of x-ray photons by atoms in a periodic lattice. The scattered monochromatic x-rays that are in phase give constructive interference. This analysis was carried out by using X-Ray Diffraction with small angle scattering and polycrystalline equipment (XRD- Bruker AXS D8 Advance, Coimbatore, India) with a specimen size of 2cm × 1cm × 1cm.

2.5 Hardness Testing

Vickers micro hardness tests were performed by using micro hardness tester (Agro Met Lab, Coimbatore, India, Model: (FIE) Fuel Instrument and Engineering.) with a sample size of 2.5cm × 1.5cm × 1cm were cut as per ASTM standard E384-16 from a casted plate of AA7010-TiB₂in-situ composite. Micro hardness tests were carried out with an indentation load of 300g for a holding time of 30sec. The test was conceded at three different locations on each specimen to obtain average hardness value and avoid possible effect of the indenter resting on the hard reinforcement particles.

2.6 Tensile Testing

The tensile tests with sample size of 9.2cm × 1cm × 1cm were conducted on a universal testing machine (Agro Met Lab, Coimbatore, India Model: PC-2000, Fuel Instrument and Engineering (P) Ltd) according to ASTM standard E370-14 with crosshead speed of 0.3 mm/min.

2.7 Corrosion testing method

Exfoliation corrosion is often observed in 2xxx (Al-Cu-Mg) and 7xxx (Al-Zn-Cu-Mg) series high strength alloys. This type of corrosion is characterized by lamellar surface attack of alloys containing a highly directional grain structure. The primary risk of exfoliation corrosion lies in the possible loss of actual cross section [22]. Exfoliation corrosion is usually intergranular in nature, owing to the galvanic relation between the adjacent matrix and grain boundary precipitates. The common exfoliation corrosion tests in laboratory that have been reported are immersion test method, salt spray method, electrochemical corrosion test and mechanical test. The first two methods usually involve in the interaction between alloys and acidic solution to determine the susceptibility of a certain alloy. The resulting surface is often compared to the environmental exposure data [23, 24, 25]. The specimens for testing the exfoliation corrosion (EXCO) were subjected to duration of 48 hours under artificial mimic environments of industrial or coastal areas. It depicts that the sample exposed to these types of environments resembles an equivalence period of 6 to 9 years. The prepared artificial environment is being formulated with test solution containing adequate quantity of reagent water mixed with chemicals that are highly corrosive. The corrosion rate was determined by conducting EXCO test as per ASTM G34 standard [26, 27]. Chemicals with specimens inside them were placed in an air tight non-reactive container (plastic) for 48 hours test duration. Specimens of size 2cmx1cmx1cm were cut as per the ASTM G34 standard and placed in a container fitted with an air tight cover to impede the evaporation of chemicals. Initial weights of the samples were noted after the specimens were cleaned with kerosene. 3M scotch pressure sensitive tape was used as a masking for the surfaces not to be exposed to the solution. A test solution was prepared which containing 50 g of potassium nitrate (KNO₃), 6.3 mL of nitric acid (HNO₃) and, 234 g of sodium chloride (NaCl), which were mixed together for 1Litre dilute solution for an approximate pH of 0.4. The specimens were immersed in an adequate quantity of solution for providing a ratio of volume to metal surface area with an approximation of 10 to 30 mL/cm² respectively at 28^∘C for time duration of 48 hours. To avoid the loss of material corroded from the metal surface the specimen surface were set to upward direction [28]. Corrosion rate (mm/year) was determined by considering the materials loss based on mass. After performing the procedure, using 50 vol.% nitric acid, specimens were washed, dried, and weighed.

3.1 X-ray diffraction (XRD) analysis of AA7010-TiB₂ Composites

XRD analysis was carried out on metallographically polished composite specimens. Figure 1 shows the XRD patterns attained for the cast in-situ composites and these confirm the presence of TiB₂ reinforcement within the matrix alloy. It is evident that in-situ formed TiB₂ particles and matrix aluminium alloy were clearly visible from the diffraction peaks. The peaks of TiB₂ reinforcement increase with the increase of TiB₂ content while the peaks of AA7010 decrease.

Figure 1

XRD Patterns of AA7010-TiB₂ in-situ composites

3.2 Microstructure Characterization

Figure 2 shows the optical images of as-cast AA7010-TiB₂ composite for different weight percentages of TiB₂ reinforcement. From the figures it is observed that single TiB₂ particles and TiB₂ clusters are homogeneously distributed with increase of reinforcement content with matrix alloy. Formation of dendritic structure is found in matrix alloy whereas, rosette-like irregular dendritic grain structure is observed in the developed composite. The homogenous distribution of TiB₂ is increased with increase of reinforcement concentration. Aluminium grain solidifies around the TiB₂ particulates which are behaving like nucleation center that provides resistance to the grain growth [29].

Figure 2

Optical images of as-cast AA7010/TiB₂ composite (a) 0% TiB₂, (b) 5% TiB₂, (c) 7.5% TiB₂ and (d) 10% TiB₂

Figure 3 shows the FESEM images and EDS patterns of the as-cast sample composites produced with various wt% of TiB₂ in AA7010. FESEM images clearly show that the mechanical interactions between TiB₂ and AA7010 particles have been achieved. It can be understood that due to in-situ stir casting route with optimized process parameters, the development of TiB₂ clusters are avoided in the AA7010-TiB₂ composite. But indeed small salt inclusion can be observed in the FESEM images which are avoided by proper removal of slag. Increase of TiB₂ wt% provide sufficient wettability and bonding with the alloy. It is also observed that the TiB₂ particles are uniformly distributed in the liquid aluminium with very low agglomeration, which is due to the stirring of molten metal before and after 15 min during the holding time of 60min respectively. From EDS spectrums for all the as-cast composites elemental peaks of aluminum, titanium and boron and the chemical composition identified with high intensity peak were zinc, magnesium, silicon, and ferrous which are alloying elements of AA7010. The extra elements like sodium, rhenium and tantalum were identified at very low peaks. It can be understood that from the elemental peaks and wt.% composition of aluminium, titanium and boron confirms the formation of TiB₂ at the interface [30].

Figure 3

FESEM and EDS of AA7010-TiB₂ composites (a) & (b) 5% TiB₂, (c) & (d) 7.5% TiB₂ and (e) & (f) 10% TiB₂

3.3 Mechanical Properties

The average hardness values of AA7010 with different reinforcements of 0%, 5%, 7.5%, and 10% TiB₂ composites are 80 (SD=1.89), 100.5 (SD=6.33), 100.2 (SD=4.46) and 110.9 (SD=9.84) respectively. It is evident that as the weight% of reinforcement increases, the composite hardness also increasing owing to the occurrence of titanium diboride which acts as an advance strengthening phase in aluminum alloy matrix. The effect of strengthening for developed composites was increased due to the presence of TiB₂ which is very hard. Increase in hardness with a net value of 25-35% was reported in Figure 4 for different weight% of TiB₂. Crack growth is resisted due to the interaction between the reinforced TiB₂ particles and the dislocations leading to the improvement of hardness for the composite when compared with base alloy [31]. It is also observed that the hardness of 5% TiB₂ and 7.5% TiB₂ is nearly equal and decrease in the hardness value can be seen in 7.5% TiB₂ which may be due to poor particulate diffusion in aluminum because of limited stir time. Hardness of in-situ composite is also improved due to grain refining action of reinforcement with AA7010 alloy during solidification process.

Figure 4

Effect of hardness for different wt.% of TiB₂

The effect of addition of reinforcement with matrix alloy on Ultimate Tensile Strength (UTS) and Yield Strength (YS) (with error bars of 5%) is depicted in Figure 5 and 6 respectively. Addition of reinforcement with AA7010 develops maximum increase in UTS and YS of 260% and 240% for 10% TiB₂ respectively when compared with base alloy. It is also clear that maximum increase of UTS and YS is found to be with 7.5% and 10% TiB₂. It found that during tensile loading the interaction between strong TiB₂ particulates and dislocations impedes the cracks propagation leading to increase of tensile properties. According to well known Hall-Patch relationship, grain refined in-situ TiB₂ particulates increase the area to resist the tensile loading and improves the strength of the composite. Owing to the existence of hard particulate reinforcement with a significant interaction between the dispersed hard particles and dislocations leads to homogeneously distributed TiB₂ particles which invoke Orowan strengthening mechanism [32]. This effect of particle strengthening in the matrix alloy is found to be improved with increase in the wt.% of reinforcement, where existence of hard TiB₂ particulates impedes the motion of dislocation by acting as a barrier in the composite under plastic deformation. Effect of elongation (with error bars of 5%) behavior of AA7010 reinforced with various wt.% of TiB₂ is as shown in Figure 7. In the current investigation composite elongation percentage improves by about 50% with 7.5% TiB₂, and 10% TiB₂ which is due to the refinement in grain that may hinders the propagation and crack growth in the developed composite. It is been observed that as the wt.% of increases percentage elongation increases for 7.5%, and 10% TiB₂, whereas % elongation decreases for 5% TiB₂ owing to the debonding of TiB₂ particles bands which may causes voids in the ductile matrix and easy propagation of crack leads to early failure [33].

Figure 5

Ultimate Tensile Strength for AA7010 before and after addition of TiB₂ Reinforcement

Figure 6

Effect of Yield Strength for various wt% of TiB₂

Figure 7

Elongation Behavior of AA7010/TiB₂ composites

3.4 Corrosion behavior of AA7010-TiB₂ composites

Corrosion behavior of Aluminium Metal Matrix Composite is mainly rely on various reasons and one of a reason on which the characteristics of cast or wrought depend is the processing methods used, type of matrix alloy, shape, size and type of reinforcement. The effect of corrosion is also happening due to the fabrication methods used for processing of MMC owing to the development of intermetallic phases between the reinforcement and base alloy. The strength or weakness of a composite is mainly depending on the interfacial bonding among the base alloy and the reinforcement. It is understood that interfacial bonding is the most significant parameter in the study of corrosion, the improved corrosion resistance of the AA7010/ TiB₂ composite in the present work owes to a strong interfacial bond between the matrix alloy and reinforcement. Therefore, TiB₂ with increase in weightpercentage leads to improvement in corrosion resistance of the composite [34]. The FESEM surface morphologies of the corroded samples for various wt.% TiB₂ (0%, 5%, 7.5%, and 10%) are shown in Figure 8. It can be perceived that exfoliation begins from pitting corrosion through intergranular corrosion to general corrosion at the end along the grain boundaries. Corrosion cracks are observed around the grain boundaries, which are regularly acting as a propagation path of exfoliation corrosion thereby a significant loss of metal with greater penetration, can be observed on the surface regions. Visual inspection on surface morphologies cannot determine the penetration depth of corrosion. Indeed the corrosion rate of the composite material can be calculated by using the following formula, on the basis of weight loss method for different wt% of TiB₂ [35]. The Table 2 shows corrosion rate behavior of cast aluminium metal matrix composites with various wt%. TiB₂ and it has been observed that while the wt% of TiB₂ increases, corrosion rate decreases. As we know that corrosion resistance of the material is better with the reduction of corrosion rate.

Figure 8

FESEM Analysis of corroded samples AA7010 - TiB₂ for (a) 0% TiB₂, (b) 5% TiB₂, (c) 7.5% TiB₂ and (d) 10% TiB₂

ρ = Specimen density in gm/cm³

A = Area of specimen in cm²

T = Exposure time in 24 hours.

3.5 Fractography

The AMMCs produced by the in-situ stir casting technique were tensile tested and the fracture morphologies for different %wt. TiB₂ are as shown in Figure 9. The morphologies indicate equiaxed dimples in large quantities which are surrounded by tiny dimples that control the fracture surface. The fracture of the composite still follows the ductile nature of the aluminum alloy regardless of the incorporated stiff reinforcement. Indeed the TiB₂ particles are acting as effective barriers in coalescence and growth of voids. From the above results it confirms that the strong interfacial bonding and wetting can be attained using the insitu stirring technique between the matrix and reinforcement [36].

Figure 9

FESEM fractographs of TiB₂/AA7010 composites (a) 5% TiB₂, (b) 7.5% TiB₂ and (c) 10% TiB₂