Microstructure and Mechanical Behaviour of Al6061-ZrB2 In-situ Metal Matrix Composites

Aluminium matrix composites processed through in-situ molten reaction has emerged as an alternative for eliminating defects existing in ex-situ reinforced metal matrix composites. Development of composites through in-situ method using inorganic salts via liquid metallurgy route is the most widely accepted technique. In the present work, Al6061-ZrB2 in-situ composites have been developed through in-situ reaction of Al-10%Zr and Al-3%B master alloys in Al6061 alloy. Study of microstructure and mechanical properties of in-situ reinforced ZrB2 in Al6061 alloy have been carried out. Composite exhibited grain refinement and improved the mechanical properties of Al6061 alloy. Ductility of composite is reduced with increase in content of ZrB2.


Introduction
Increased demands of current industries for improved material performance have necessitated the use of particulate reinforced metal matrix composites (PMMCs). Aluminium is identified as important material in a manufacturing system for exhibiting light weight with high strength in automobile and aerospace applications [1]. Aluminium metal matrix composites (AMMCs) possess improved properties when compared with monolithic alloys for high strength-weight ratio, high wear resistance and low thermal expansion [2,3]. With this requirement, Al6061 alloy has proved its existence for good workability, good weldability and excellent corrosion resistance [4]. Silicon carbide and aluminium oxides were extensively used as reinforcement during earlier decades of developing AMMCs [5]. Newer trends have been existed in using different reinforcements such as silicon nitride [6,7], , boron carbide [8], aluminium oxide [9], titanium oxide [10], titanium boride [1] for fabrication of PMMCs. Among various reinforcements tried out till date, ZrB 2 stands as a popular material with strong covalent bonding with very high melting temperature, superior hardness and strength along with excellent thermal conductivity and thermal shock resistance, to exist as a suitable candidate in critical environments associated with aerospace industry [11]. Composites can be fabricated by two casting methods [1], firstly by adding the powder externally to the melt through ex-situ synthesis  [12,13]. It has become very essential to eliminate ex-situ synthesis and promote in-situ reactions for meeting the demands of industrial applications [14,15,16]. However, the direct melt reaction process requires agitated time at high temperatures to acquire full incorporation through the response of the additional reactants with liquid metal [17]. Literature has been focused on aluminium composites processed through in-situ technique with different reinforcements in the form of fine particles to investigate the nature and morphology of metal matrix composite and their behaviour. Dinharan et al. [5,18] has developed AA6061 alloy reinforced with ZrB 2 produced by an in-situ reaction between K 2 ZrF 6 and KBF 4 salts. Hardness and strength of composite increases by increasing ZrB 2 content in matrix alloy. Ramesh et al. [1] has developed Al6061-TiB 2 composites through in-situ method using Al-Ti and Al-B master alloys. Tensile strength and Ductility increases in composite with higher content of ZrB 2 elements. Moosavian et al. [19] has prepared ZrB 2 /A356 alloy by in-situ technique using Al-15Zr and Al-8B master alloy. Microscopic study, XRD analysis and Tensile test of extruded composite has been investigated before and after T6 heat treatment and compared with matrix alloy.Tensile test reveals that ultimate tensile strength (UTS) and ductility increases compared to matrix alloy with further increase in UTS and decreasing ductility after T6 heat treatment. Songli Zhang et al. [20] has developed Al-ZrB 2 composites with formation of Al 3 Zr through in-situ reaction and investigated microstructure at room temperature. Morphologies of ZrB 2 and Al 3 Zr elements were in regular tetragonal and hexagonal shapes with a size of about 0.3-0.5µm.Yutao Zhao et al. [21] has fabricated (Al 3 Zr+ZrB 2 )/Al composites from Al-K 2 ZrF 6 -KBF 4 system and investigated the morphology variations of Al 3 Zr crystal in composites due to the effect of molten temperature. Effect of temperature is more sensitive on morphology of Al 3 Zr particles compared to ZrB 2 element.
In this paper, major focus is on developing light weight material with enhanced mechanical characteristics for automobile and marine applications. Until now, very less information is available on synthesis of Aluminium based composites using Al-Zr and Al-B master alloys via in-situ reaction.
The present work focuses on the preparation of Al6061-ZrB 2 in-situ composite processed by Al-10%Zr and Al-3%B master alloy technique. Al6061 was chosen as matrix alloy owing to its excellent castability, formability and weldability for more application on industrial sector. Further, this alloy can be strengthened by suitable heat treatment [1].

Manufacturing of composites
Al6061 alloy, Al-10%Zr and Al-3%B master alloy were used as the raw materials for preparation of composites. Raw materials have been procured from M/s Fenfee metallurgical, Bangalore, India. The weight percentage of Al-10%Zr and Al-3%B Master alloys are reported in Table 1. The chemical composition of raw materials used for fabrication of composite is reported in Table2.  Al-6061 alloy, Al-10%Zr and Al-3%B master alloys were melted in graphite crucible up to 850 0 C in a stoichiometric ratio using 6KW electrical resistance furnace. The composite melt was maintained for duration of 30mins for completion of in-situ reaction. The melt was agitated by stirring intermittently for every 10mins using mechanical stirrer to ensure uniform mixing of in-situ products. After removing the slag, commercially available Hexachloroethane tablets were used for degassing the molten alloy followed by pouring into preheated metal die.

Characterization
The composites were cut into different samples of required size for conducting optical microstructure study, SEM, XRD and Tensile tests. The sample was polished using standard metallographic technique and etched with Kellers reagent. The microstructure of the polished specimen was captured using ZEISS metallurgical microscope. SEM study was carried out on a metallographically polished sample using JSM 840a Jeol scanning electron microscope. Further, samples were characterized by XRD and diffraction patterns were recorded. Microhardness test was performed on a polished specimen. Values were recorded at five different locations and average value was considered. Tensile tests for matrix alloy and composites were carried out on an Instron 30KN load Universal testing machine as per ASTM E8M-01 standard.
An average of three results was considered as strength of each material. Ductility was also calculated to analyze the ability of the samples for subjecting it to the secondary process.         Figure 6 shows EDAX analysis of Al6061-ZrB 2 composite. SEM images clearly show the morphology of ZrB 2 particles and EDAX analysis indicates the presence of Zirconium and Boron element. Due to formation of ZrB 2 particles inside the melt, chances of oxidation of element are avoided. These particles free from oxide surfaces exhibits improved interfacial properties in the composites. Thus, non-oxidized composites reduce the wettability angle between matrix alloy and ZrB 2 casting and resulting in excellent wettability [5]. It is known that uniform distribution of reinforced elements in matrix alloy is mainly due to solidification process in composite. Reduction of wettability angle between ZrB 2 particle and melt resists the movement of ZrB 2 particles during solidification. Subsequently, the ZrB 2 particles are seen inside the grain without entering to the grain boundaries.   Figure 8 shows the grain size of Al6061 alloy and Al6061-ZrB 2 In-situ composite. It is observed that grain size of the composite has been reduced due to the presence of ZrB 2 particles [1]. ZrB 2 itself acts as a nucleating agent for grain refinement in composite [22]. Adding of inoculant particles into the melt results with potential composite developing the heterogeneous nucleation with the presence of solutes [1]. Solute has an ability to undercool the zone constitutionally within the interface [22]. Further, undercooling initiates the nucleation process with activation of nucleants in the region of the interface. This results with constraining of grain growth leading to effective refinement in grains [23]. In the present work, Zirconium act as a solute and ZrB 2 acts as nucleating agent initiating heterogeneous nucleation within the melt. Grain refinement is exhibited due to the presence of solute and nucleating agent. Thus, Zirconium elements have triggered the grain refining action due to constitutional undercooling in the vicinity of the interface. Further, the presence of boron element is also an attribute for grain refinement [24].   Figure 9. Microhardness of Al6061 alloy and Al6061-ZrB 2 in-situ composite. Figure 9 shows the variation of microhardness in Al6061 alloy and Al6061-ZrB 2 in-situ composite. It is observed that the presence of ZrB 2 particles has improved the microhardness of composite compared with unreinforced alloy. During the process of solidification in cast composites, ZrB 2 elements may lead to increase in dislocation density [5]. Since, in-situ technique is adopted; uniform bonding between matrix and reinforcement exists in composite. This is helpful in enhancing the bulk properties of composite. In-situ process -ceramic particles bears the load and offers the resistance [1]. In this work, ZrB 2 acts as a load bearing element and takes maximum load for plastic deformation by increasing the hardness [5]. ZrB 2 being a ceramic material allow material to flow without undergoing deformation, but when it exceeds critical value it causes fracture without further deformation. According to Hall-Petch equation, Hardness can be improved by reduction in grain size [25]. Thus, grain refinement can be an effective cause for enhancing the hardness in ZrB 2 composite. Further, microhardness of composite increases with increase in ZrB 2 particles. Microhardness increases by 10% and 24% respectively in increased content of ZrB 2 in-situ composite.  Figure 10 and Figure 11 shows the variation of yield strength and ultimate tensile strength of Al6061 alloy and Al6061-ZrB 2 in-situ composite. It is observed that increase in ZrB 2 particles has resulted in an increase in tensile strength of the composite. Further, it is also found that increase in ZrB 2 content has increased the ultimate tensile strength with the reduction in % of ductility [5]. The presence of hard ZrB 2 particles, processing of composite through in-situ method and grain refinement may be the attribute for improvement in tensile strength of composites [25]. Fine particle size in composite has always associated with improved mechanical properties.  Figure 11.Variation of Ultimate Tensile Strength. Figure 12 shows the variation of ductility of Al6061 alloy and Al6061-ZrB 2 in-situ composite. It is observed that composite has exhibited with the decrease in ductility with an increase in ZrB 2 content in matrix alloy. Reduction in ductility with an increase in reinforcement may be due to the presence of hard ZrB 2 particles and grain refinement [25]. Grain refinement reduces the ability of structural alloys for ductile-brittle transition. Ductility decreases by 6% and 11% respectively in a composite prepared with an increase in ZrB 2 content.

Conclusion
Al6061-ZrB 2 in-situ composites were successfully developed through liquid metallurgy using Al6061 alloy and Zirconium and Boron master alloy as reactive elements. Microstructure shows the uniform distribution of reinforced ZrB 2 particles with good bonding between matrix and reinforcement.
Appreciable grain refinement has been observed in composite. Microhardness, yield strength and ultimate tensile strength of composites are higher than matrix alloy. Reduction in ductility of the composite is due to effective grain refinement.