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Influence of hot rolling on microstructure and mechanical behaviour of Al6061-ZrB2 in-situ metal matrix composites
Author’s Accepted Manuscript Influence of Hot Rolling on Microstructure and Mechanical Behaviour of Al6061-ZrB2 In-situ Metal Matrix Composites R. Vasanth Kumar, R. Keshavamurthy, Chandra S. Perugu, Praveennath G. Koppad, M. Alipour www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(18)31312-1 https://doi.org/10.1016/j.msea.2018.09.104 MSA36985
To appear in: Materials Science & Engineering A Received date: 25 June 2018 Revised date: 19 September 2018 Accepted date: 27 September 2018 Cite this article as: R. Vasanth Kumar, R. Keshavamurthy, Chandra S. Perugu, Praveennath G. Koppad and M. Alipour, Influence of Hot Rolling on Microstructure and Mechanical Behaviour of Al6061-ZrB2 In-situ Metal Matrix C o m p o s i t e s , Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.09.104 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of Hot Rolling on Microstructure and Mechanical Behaviour of Al6061-ZrB2 In-situ Metal Matrix Composites R. Vasanth Kumar1, R. Keshavamurthy2*, Chandra S. Perugu3, Praveennath G. Koppad4, M. Alipour5 1
Department of Mechanical Engineering, Bangalore Institute of Technology, Bengaluru 560004, India 2 Department of Mechanical Engineering, Dayananda Sagar College of Engineering, Bengaluru 560078, India 3 Department of Materials Engineering, Indian Institute of Science, Bengaluru 560012, India 4 Department of Mechanical Engineering, Nagarjuna College of Engineering and Technology, Bengaluru 562164, India 5 Department of Materials Engineering, University of Tabriz, Iran *
Corresponding author. [email protected] (R. Keshavamurthy)
Abstract Synthesis of aluminium based metal matrix composites by in-situ reaction is considered as an alternative method for production of high quality metal matrix composites. In-situ technique eliminates the limitations associated with ex-situ processing technique and it is one of the most widely accepted one. In the present work, Al6061-ZrB2 in-situ composites have been developed by stir casting technique using commercially available Al-10%Zr and
Al-3%B master alloys.
Cast matrix alloy and developed in-situ composites were hot rolled at a temperature of 4000C. Both as-cast and hot rolled matrix alloy and its in-situ composites were subjected to microstructure analysis, microhardness test, grain size studies and tensile test. Tensile behaviour of hot rolled alloy and its in-situ composites were evaluated in the rolling direction (RD), 450 from rolling direction (45D) and transverse direction (TD) and compared with cast ones. Optical and SEM micrographs of hot rolled in-situ composites show that the ZrB2 particles are aligned in the rolling direction. Both as-cast and hot rolled in-situ composites have displayed extensive grain refinement and enhanced mechanical properties when compared with unreinforced alloy.
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However, ductility of the in-situ composites decreases with increase in ZrB2 content. Hot rolled alloy and its in-situ composites exhibited remarkable improvement in ultimate tensile strength and ductility at 450 from rolling direction compared to transverse and rolling directions.
Keywords: Al6061 alloy; Composites; Hot rolling; Microstructure; Mechanical properties.
1.
Introduction
High strength and low weight factor has necessitated the use of Aluminium alloy based composites for automobile, aerospace and transportation applications [1, 2]. Among all aluminium alloys, Al6061 alloy is most versatile heat treatable alloy with good formability, toughness and low coefficient of thermal expansion characteristics which makes it prime candidate matrix material for fabrication of composites [3, 4]. Metal matrix composites are fabricated by either in-situ method where starting materials are transformed into required products or ex-situ method where starting materials experiences no changes. Preparation of composite by ex-situ process leads to various defects like non-uniform distribution, poor wettability and reduced bonding between matrix and reinforcement, which affects the properties of the cast product. To avoid such defects, it is necessary to develop composites by in-situ process which is capable of meeting the demands of industries for various applications. The main advantages of in-situ method over conventional methods are formation of fine size reinforcements which are thermodynamically stable, good interfacial bonding of matrix and reinforcement due to clean interface and uniform dispersion of reinforcements [5-10]. Reinforcements like TiB2, TiC, Mg2Si, AlB2, Al2O3 and ZrB2 are synthesized by facilitating reaction between master alloys, oxide or fluoride salts in the molten metal. In their work, Kumar et al. [9] synthesized in-situ TiC particles by facilitating a reaction between potassium hexafluorotitanate salt and graphite powders in the Al6061 melt maintained at a temperature of
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900ºC. In another work, Afkham et al. [11] reported the development of alumina nanoparticles reinforced aluminium composites by stir casting followed by hot rolling. Addition of ZnO, TiO 2 and sodium borosilicate glass powder to the aluminium melt maintained at 850ºC led to the formation of alumina nanoparticles. However, possibility of formation of casting defects like gas defects such as blowholes or pinholes and shrinkage cavities cannot be ruled out during fabrication of composites. In order to overcome the casting defects, composites are often subjected to secondary forming process like forging, extrusion and rolling. Application of secondary forming processes results in reduction or elimination of porosity, uniform dispersion of reinforcements and further improvement in matrix/reinforcement interfacial bonding over their as-cast counterparts which in turn enhances the mechanical, tribological and corrosion characteristics [12]. Out of all forming process, Rolling is widely used in industrial applications to produce sheets for automobile bodies, refrigerators and other house hold appliances. Hot rolling process parameters such as temperature, speed, thickness reduction and orientation have significant influence on the microstructure and mechanical properties. In particular, studying the effect of hot rolling on mechanical behaviour of aluminium alloy based in-situ composites is of prime importance from industrial application point of view [13]. In the last few years’ significant increase in number of research work were reported on development of aluminum based in-situ composites. Dinharan et al. [14] have reported development of ZrB2 reinforced AA6061 alloy via in-situ reaction between KBF4 and K2ZrF6 salts method. AA6061-ZrB2 in-situ composite processed by salt method exhibited good mechanical properties with increase in ZrB2 content in matrix alloy. Zhao et al. [15] have studied the morphologies of in-situ Al3Zr and ZrB2 particles formed in Al-ZrB2 composites at various molten temperatures. Morphologies of ZrB2 showed no diversity and Al3Zr were sensitive to temperature of aluminium melt. Zhang et al. [16] have reported microstructure of Al(Al3Zr+ZrB2) composites prepared by magnetochemistry in-situ reaction. The ZrB2 particles
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were present in hexagon shape with size in the range of 0.3-0.5μm. The electron microscopy analysis revealed uniform dispersion of ZrB2 and Al3Zr particles in the aluminium matrix. Ramesh et al. [17] have reported processing of TiB2 reinforced Al6061 composites through insitu technique using Al-B and
Al-Ti master alloys. Increase in content of TiB2 was observed
with the increase in addition of master alloys to the aluminium melt. The composite with higher TiB2 content showed highest tensile strength coupled with good ductility.
Increasing the
addition of master alloys results with increase in formation of Al3Ti in composites which are more sensitive to temperature of aluminum melt. In their work, keshavamurthy et al. [18] have reported microstructure and mechanical properties of Al7075-TiB2 in-situ composite using Al-B and Al-Ti master alloys. Grain size was decreased due to presence of fine TiB2 resulting in grain refinement attributed to solute boron. Venkateswarlu et al. [19] investigated the influence of hot rolling and annealing on grain refining efficiency of Al-5Ti-B master alloys. The results showed decrease in mean particle size with the increase in percentage reduction at any given rolling temperature. The overall improvement in grain refinement was attributed to increase in volume fraction of fine TiAl3 particles due to hot rolling. Xu et al. [20] have studied the effect of rolling reduction conducted at temperature of 400ºC on tensile properties of Al6061/ABOwhisker composites. With increase in reduction percentage of rolling, degree of alignment of whiskers took place along the rolling direction. Further, large reduction lead to improvement in mechanical properties like tensile strength. El-Sabbagh et al. [21] have studied the hot rolling behaviour of stir cast Al6061 and Al6082 alloys with SiC particulate reinforced composites. The as-cast composites rolled at intermediate temperature of 450ºC and at strain rate of 1 s-1 showed improved strength. Further, the void percentage was found to decrease linearly with increase in reduction and redistribution of SiC in the matrix leading to less agglomeration. From the review of literature, it is observed that meager information is available on effect of hot
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rolling on microstructure and mechanical behaviour of Al6061-ZrB2 in-situ composites. In the light of the above, present study focuses on synthesis of Al6061-ZrB2 in-situ composites using Al-Zr and Al-B master alloys using low cost stir casting technique followed by hot rolling. The effect of hot rolling on microstructure and mechanical properties have been investigated and compared with that of as-cast composites. 2.
Experimental details
Al6061 alloy, Al-3%B and Al-10%Zr master alloys were utilized as the raw materials for fabrication of in-situ Al6061- ZrB2 composites. The chemical composition of master alloys and Al6061 alloy are given in Table 1. The weight percentage of Al-3%B and Al-10%Zr master alloys are shown in Table 2. Master alloys were added in a stoichiometric proportion to Al6061 alloy for synthesis of Al6061- ZrB2 in-situ composite with a varied percentage. Al6061 and master alloys were placed in a graphite crucible and heated up to 850°C in 6 KW electrical resistance furnace. The composite melt was held at 8500C temperature for duration of 30 minutes to complete the in-situ reaction and followed by stirring alternatingly for every 10 minutes using mechanical stirrer. The slag was removed and hexachloroethane (C2Cl6) tablets were added for the melt as degassing agent to remove hydrogen. The graphite crucible was removed from the furnace and then the molten alloy was subsequently poured into the preheated metal die [21-23]. The weight percentage of ZrB2 was estimated by acid dissolving method and the results are shown in Table 3 [17]. The cast products were cut into rectangular specimens of size 45 X 50 X 11 mm 3 (as shown in Fig. 1a) and heated up to a temperature of 400ºC for 1hour. Once the heating was completed, the specimens were subjected to hot rolling with 10% of reduction in each pass. The hot rolled composite sheet obtained with reduction of 90% as shown in Fig. 1b. Fig. 2 shows the as-cast specimen undergoing hot rolling using 2-High rolling mill.
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Both as-cast and hot rolled in-situ composite samples were polished using standard metallographic technique. The samples were subjected to etching using Keller’s reagent to reveal the microstructure [24]. The microstructures of both as-cast and rolled composites were studied using optical and scanning electron microscope (Make: JSM 840a Jeol). Phase identification of hot rolled composites was performed using X-Ray diffraction (Make: Philips X’Pert Pro X-ray diffractometer). Vickers Microhardness test was performed at a load of 100grams under a dwell time of 10 seconds on polished samples. Tensile tests were carried out on both as-cast and rolled Al6061 alloy and their in-situ composites according to ASTM B557 E8 Standards. The average of three tensile results was considered as ultimate tensile strength of each specimen. Hot rolled sheets were cut through electrical discharge machining process for preparation of tensile specimens in three different directions: Rolling direction (RD), 45° to rolling direction (45D) and Transverse direction (TD). Fig. 3(a, b) illustrates the dimensions and photographs of as-cast and rolled tensile test specimens. More details on composite preparation are available in our earlier work [25].
3.
Results and discussion
3.1 Microstructure and Grain size analysis Fig. 4 shows the optical micrographs of as-cast Fig. 4(a-c) and hot rolled Fig. 4(d-f) Al6061 alloy and Al6061-ZrB2 in-situ composites. It can be observed from Fig. 4(b, c) that most of the ZrB2 particles were found along the grain boundaries of α-Al while some of them were found inside the α-Al grains. The presence of ZrB2 particles at the grain boundaries of α-Al is attributed to movement of solidification front. During solidification the particles are pushed towards the solidification front and stays there until it is completed. Due to this the grain boundaries which are regions of lastly solidifying liquid becomes the final particle location. These observation explains that the dispersion of ZrB2 particles largely depend upon the velocity of solidification
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front which in turn influences whether the particles to be dispersed within the grains or at the grain boundaries. From Fig. 4c we can observe clustering of ZrB2 particles at few locations of grain boundaries due to high ZrB2 (9.9 wt.%) content in this as-cast composite. When compared with Al6061 alloy the composites showed smaller grain size which is attributed to the ZrB2 particles which contribute to refinement of microstructure. In particular the presence of boron in ZrB2 which is well known as grain refiner plays vital role in the grain refinement of in-situ composites [18, 26]. In case of hot rolled Al6061 and Al6061- ZrB2 composites as shown in Fig. 4(d-f), it can be observed that the grain morphology has changed to elongated grain structure over equi-axed grain structure in as-cast materials. Though the grains have elongated structure in the rolling direction, they are excessively fine in size. Further, the extent of homogeneity and dispersion of ZrB2 particles are relatively better in case of hot rolled composites compared to as-cast. The ZrB2 particles act as heterogeneous nucleation sites for the formation of new grains resulting in refined microstructure which is clearly visible in optical micrographs. When evaluated with hot rolled in-situ composites, there is no any noticeable change either in morphology or in size of the in-situ formed ZrB2 particles. Some of the particle clusters observed in the as-cast in-situ composites were disintegrated after subjecting into hot rolling. Fig. 5(a, b) shows the SEM micrographs of hot rolled sample Y and sample Z. SEM images clearly indicates the uniform dispersion of ZrB2 particles after hot rolling. The bonding between Al grains and ZrB2 particles was found to be excellent as there were no reaction products at the interface. Fig. 6 shows the EDAX analysis of hot rolled sample Z in which both the constituents of ZrB2 particle (Zr and B elements) are clearly seen. Table 4 shows an average grain size of Al6061 alloy and Al6061- ZrB2 in-situ composites in both as-cast and hot rolled conditions. It is observed from the table that hot rolled Al6061 alloy shows a reduction of 8% in grain size over the same alloy in as-cast conditions. However, a remarkable reduction of 56% and 26% is observed for hot rolled and as-cast in-situ composites
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respectively when evaluated with unreinforced matrix alloy in cast conditions. When evaluated with unreinforced cast aluminium alloy, in-situ composites exhibited significant decrease in average grain size. Presence of ZrB2 particles in the alloy helps in reducing the grain size and enhancing the mechanical properties which is attributed to obstruction posed by the particles to advancing grains during cooling of composites [27, 28]. Due to this the aluminium grains becomes finer in composites when compared to that of Al6061 alloy without ZrB2 particles. After subjecting both unreinforced aluminium alloy and its in-situ composites to hot rolling process, further decrease in average grain size is observed which may be due to dynamic recrystallization in thermo-mechanical processing. Presence of ZrB2 particles not only contributes to the recrystallization but also plays a vital role in retarding grain growth. Fine size ZrB2 particles facilitate nucleation of new grains which are finer in size than unreinforced alloy. Such newly formed grains will possess high angle grain boundaries around in-situ formed ZrB2 particles [29]. On the other hand, some of the ZrB2 particles pin the grain boundaries of matrix during hot rolling. Simultaneous formation of recrystallized grains and increased dislocation density in the presence of ZrB2 particles reduces the driving force required for further growth of newly formed grains in aluminium matrix [30-34].
3.2 X-ray diffraction analysis Fig. 7 presents XRD patterns for hot rolled Al6061 alloy and Al6061-ZrB2 in-situ composites. Al6061 alloy (sample X) showed most of the peaks corresponding to pure aluminium while in-situ composites (sample Y & Z) showed distinct peaks related to ZrB2 particles. The XRD patterns obtained is in good agreement with the standard JCPDS file for Zirconium diboride (ZrB2) (JCPDS file No: 657806) [35]. The following in-situ reactions taking place between Al6061 alloy and master alloys at 850°C explains the formation of ZrB2 particles. Zr + 3Al
Al3Zr………………………. (1)
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Al3Zr + 2B
ZrB2 + 3Al………………. (2)
AlB2 + Al3Zr
ZrB2 + 4Al……………. (3) 4Al + ZrB2…….. (4)
Al + Al-Zr + 2Al-B
The absence of any other element or compound apart from Al and ZrB2 in the above reaction products indicates that in-situ reaction between master alloys is complete. Further, the sole presence of ZrB2 explains that it is thermodynamically stable since it has not reacted with Al6061 to form any other intermetallic compounds. Further, as shown in Fig. 6 where the SEM image and EDAX of sample Z display absence of any kind of reaction products is well supported by these XRD patterns (Sample Y & Z). Thus, it is presumed that the interface between the Al6061 alloy and in-situ formed ZrB2 is clean and free from any sort of contamination.
3.3 Microhardness Fig. 8 shows the microhardness of Al6061 alloy and Al6061-ZrB2 composites before and after hot rolling. In-situ composites displayed significant improvement in microhardness over the Al6061 alloy in both as-cast and hot rolled conditions. With the increase in ZrB2 content, the microhardness of the Al6061 matrix was found to increase. As-cast and hot rolled sample Z showed an improvement of 18% and 29% when compared to that of as-cast sample X. ZrB2 particles which possess very high hardness tend to impart its attributes to soft ductile aluminium matrix resulting in improved microhardness of sample Y and Z. The strain field around the ZrB2 particles imposes resistance to plastic deformation caused by indentor by restricting the movement of dislocations. The primary reason for improvement in microhardness of in-situ composites is grain refinement caused by ZrB2 particles. It is well known from the Hall-Petch equation that, decrease in grain size increases the strength of the material. In present case, the in- situ formed ZrB2 particles in Al6061 matrix refine the microstructure by acting as nucleation site for new grains during solidification and also pin the grain boundaries for further growth. In
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addition to this in case of hot rolled composites, the interface bonding is very good while in case of as-cast composites it is relatively weak. Due to improved bonding the hot rolled in-situ composites showed improved microhardness. The second reason for improvement in microhardness is restriction to movement of dislocations by ZrB2 particles. In-situ formed ZrB2 particles in the matrix act as barricade to the movement of dislocations and thereby increasing the stress required for the movement of dislocations. With uniformly dispersed ZrB2 particles in the matrix will increase the overall stress required which in turn contribute to increase in microhardness. Further, it is clearly visible from the microstructure that the amount of porosity/defects is minimal in hot rolled in-situ composites which contribute in increment of microhardness. This is mainly because presence of any minor flaws will be healed up during hot rolling.
3.4 Tensile properties Fig. 9 depicts variation of yield strength, ultimate tensile strength and ductility of hot rolled Al6061 alloy and Al6061-ZrB2 in-situ composites. Tensile values of the as-cast and hot rolled in-situ composites are shown in Table 5. The presence of in-situ formed ZrB2 particles improved the overall mechanical properties of in-situ composites in both as-cast and hot rolled conditions. The sample Z showed highest 0.2% proof stress and ultimate tensile strength (UTS) than compared with sample X and Y. An increment in UTS by 61% and 62% was observed in sample Z when compared with that sample X in both as cast and hot rolled conditions. Significant improvement in mechanical properties after hot rolling indicates that it is very effective forming technique in densifying both Al6061 alloy and in-situ composites. Variation of 0.2% proof stress, ultimate tensile strength and ductility of hot rolled composites was studied in three conditions- Rolling direction (RD), 45º direction from RD (45D) and Transverse direction (TD) [36]. From the Fig. 9a, it is observed that 0.2% proof stress and UTS is highest in 45D as
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compared to RD and TD in sample Y and sample Z whereas in case of sample X, RD showed highest 0.2% proof stress and UTS as compared to 45D and TD. About 21% and 28% improvement in UTS in 45D of hot rolled sample Z was observed when compared with that of RD and TD respectively. Enhanced ultimate tensile strength in the 45D and RD may be attributed to strong texture due to presence of elongated grains in the direction of hot rolling [37]. On the other hand, the lower tensile strength in the transverse direction may be attributed to weak texture. Weak texture in the transverse direction might be due to disruption on flow of aluminium matrix as a result of excessive interaction between matrix material and ZrB2 particles [38]. There are various factors which contribute in improvement of mechanical properties of in-situ composites which includes grain size, dispersion of ZrB2 particles and interfacial bonding. The hot rolling not only helps in reducing the grain size by virtue strain hardening but also improves the dispersion more uniformly in the matrix. Fig. 5(a, b) clearly shows that dispersion of ZrB2 particles are homogeneous in nature and hardly any clusters are visible. With the clean interface formed during casting process, the bonding between ZrB2 particles and Al6061 alloy improves further by hot rolling. The improvement in the UTS can be attributed to strengthening mechanisms like load transfer, grain refinement, dislocation and Orowan strengthening. It is well known that there should be continuity between the Al6061 matrix and ZrB2 particles for load transfer during application of load on bulk material. The clean interface with good bonding between matrix and particles will ensure that the load is transferred more efficiently from the matrix to hard ZrB2 particles [39, 40]. The second strengthening mechanism is grain refinement which can take place by recrystallization during thermo-mechanical processing. During hot rolling of in-situ composites the possibility of recrystallization is very high since the in-situ formed ZrB2 particles have size less than 1 micron. The hard ZrB2 particles do not deform during hot rolling and the strain in the Al6061 matrix forms a deformation zone around the
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particles. The particle deformation zones are preferably formed near ZrB2 particles due to the mismatch between them and Al6061 matrix. Thus, recrystallization can easily takes place in these deformation zones due to particle stimulated nucleation. The resultant grain formed due to recrystallization in hot rolled in-situ composites is often less than that found in as-cast Al6061 or in-situ composites. The grain refinement caused by particle stimulated nucleation contributes to increase in strength of hot rolled in-situ composites according to Hall-Petch equation [32]. Dislocation strengthening is other significant mechanism responsible for improvement in strength of in-situ composites. The variation in the co-efficient of thermal expansion between ZrB2 particles (~5.9×10-6 K-1) and Al6061 alloy (~24×10-6 K-1) is quite large. Due to large difference in thermal expansion coefficient prismatic punching of dislocation takes place at matrix/particles interface during cooling. With the increase in ZrB2 content and rolling reduction there will be increase in dislocation density. Presence of high dislocation density in hot rolled in-situ composites results in significant increase in strength [13]. In addition to this, Orowan strengthening mechanism also contributes to ultimate tensile strength of in-situ composite from inhibiting movement of dislocations by finely dispersed ZrB2 particles. The in-situ formed ZrB2 particles which are dispersed uniformly throughout the matrix act as obstacle for the dislocations and restricts their motion. However, with further application of stress the dislocation bypasses the particles by forming a loop around them. This is mainly because these particles are hard in nature and non-deformable due to which it is highly impossible for dislocations to cut them. These dislocation loops formed will raise the stress required for deformation thereby contributing to strength [41]. Fig. 9c shows variation in ductility of Al6061 alloy and Al6061-ZrB2 in-situ composites. It is observed from the graph that with the increase in content of ZrB2 in Al6061 alloy marginally decreases the ductility in both as-cast and hot rolled conditions. It is observed that the hot rolled Al6061 alloy and its composites displayed improvement in ductility when compared to that of
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as-cast counterparts. This is mainly because the presence of casting defects like porosities and solidification shrinkages acts as crack nucleation site which on application of load helps in nucleation and propagation of cracks. On the other hand, in case of hot rolling the casting defects are eliminated due to application of plastic deformation at elevated temperatures. In addition to this the refinement of grains due to the presence of boron also contributes to ductility of the composite [42]. Variation in ductility of hot rolled composites and alloy were also observed in three conditions- RD, 45D and TD. From the observations it is clear that, 45D shows higher ductility as compared to RD and TD. Reduction in the ductility with increased content of ZrB 2 particles may be due to presence of crack nucleating sites particularly at the interface between matrix and reinforcement in large numbers and also the presence of possible microporosities which contributes to failure of matrix material [43]. Particles of ZrB2 present in the matrix are capable of changing the direction of crack propagation resulting in branching, crack bridging and deflection in its favourable direction with respect to the direction of tensile loading. This process requires large amount of energy and offers more resistance to crack propagation resulting in higher fracture toughness and ductility [44]. Higher percentage of agglomeration in ZrB2 particles helps for easy propagation of cracks and earlier debonding of ZrB2 particles from the matrix interface during tensile loading results in decreased ductility [41].
3.5 Fractography Tensile fractographs of as-cast and hot rolled Al6061 alloy and in-situ composites are shown in Fig. 10(a-f). Fig. 10a shows fractograph of as-cast alloy which showed brittle failure macroscopically. The fracture surface depicted debonding of aluminium grains due to weak bonding between the aluminium grains. These particles acts as nucleation sites and causes microcrack leading to debonding of adjacent grains. In case of as-cast in-situ composites sample Y and Z as shown in Fig. 10(b, c), the fracture surface showed presence of dimples indicating
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ductile fracture. However, the size of voids formed after fracture indicated debonding of aluminium grains which were located adjacent to the microporosities and agglomerated ZrB2 particles. The presence of ZrB2 particles on the fracture surface indicates strong adhesion with the aluminium matrix owing to its clean interface. In case of hot rolled alloy as shown in Fig. 10d there exist homogeneous voids and with fine sized dimples all over the fractured surface. When compared with the as-cast alloy the surface didn’t showed casting related microporosities which are probable sites for nucleation of cracks. In same line with that of hot rolled alloy, the in-situ composites as shown in Fig. 10(e, f) exhibited ductile failure. During application of load during testing, the fracture begins with formation of microvoids and coalescence takes place with increase in load. As load reaches its ultimate value, the microvoids are large enough to nucleate crack and propagate at faster rate causing the fracture. Close examination of fractured surface showed the fracturing of ZrB2 particles rather than debonding. Debonding of reinforcements usually occur in case of ex-situ composites where it is hard to obtain good bonding between matrix and reinforcements. In such cases the application of load results in formation of cracks at the interface and removal of complete reinforcement [9]. However in present case due to excellent bonding achieved via in-situ formation of ZrB2 particles and hot rolling resulted in strong interface due to which the particle fracturing is observed.
Conclusions This work presents an experimental study of microstructure, mechanical properties of as-cast and hot rolled Al6061 alloy, Al6061-ZrB2 in-situ composites. The conclusions from the present investigation are as follows: •
Al6061-ZrB2 in-situ composites have been synthesized by stir casting technique and successfully hot rolled at a temperature of 400 oC.
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•
Optical and Scanning electron micrograph shows uniform dispersion of irregular plate shaped ZrB2 particles in the matrix alloy under as-cast and hot rolled conditions.
•
Al6061-ZrB2 in-situ composites exhibited extensive grain refinement both in as-cast and hot rolled conditions.
•
Hardness of as-cast and hot rolled in-situ composite shows a maximum improvement of 18% and 29% respectively when compared with cast alloy.
•
UTS of the in-situ composites increased by 61% and 62% in both as-cast and hot rolled conditions respectively when compared with cast matrix alloy.
•
In-situ composites exhibited an improvement of 21% and 28% in UTS at 450 direction of rolling compared to transverse and rolling direction respectively.
•
Ductility decreases marginally with the increase in content of ZrB2 in Al6061 alloy in both as-cast and hot rolled conditions. Hot rolled Al6061 alloy and its composites displayed improvement in ductility when compared with that of as-cast counterparts.
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Y. Afkham, R.A. Khosroshahi, S. Rahimpour, C. Aavani, D. Brabazon, R.T. Mousavian, Arch. Civ. Mech. Eng. 18 (2018) 215-226. [10] C.S. Ramesh, R. Keshavamurthy, Mater. Sci. Eng. A 551 (2012) 59-66. [11] S.C. Xu, L.D. Wang, H.B. Li, C.Q. Qu, W.D. Fei, Mater. Sci. Eng. A 615 (2014) 208-213. [12] I. Dinaharan, N. Murugan, S. Parameswaran, Mater. Sci. Eng. A 528 (2011) 5733-5740. [13] Y. Zhao, S. Zhang, G. Chen, X. Cheng, Mater. Sci. Eng. A 457 (2007) 156-161. [14] S. Zhang, Y. Zhao, G. Chen, X.N. Cheng, J. Mater. Proc. Technol. 184 (2007) 201-208. [15] C.S. Ramesh, S. Pramod, R. Keshavamurthy, Mater. Sci. Eng. A 528 (2011) 4125-4132. [16] R. Keshavamurthy, S. Megeri, Res. J. Mater. Sci. 1 (10) (2013) 6-10. [17] K. Venkateswarlu, B.S. Murty, M. Chakraborthy, Mater. Sci. Eng. A 301 (2001) 180-186. [18] S.C. Xu, L.D. Wang, P.T. Zhao, Mater. Sci. Eng. A 539 (2012) 128-134. [19] A. El-Sabbagh, M. Soliman, M. Taha, J. Mater. Proc. Technol. 212 (2012) 497-508. [20] H. Moosavian, M. Enamy, M. Mansouri Arani, Key Eng. Mater. 553 (2013) 29-33. [21] S. Suresh, N.S.V. Moorthi, Procedia Eng. 38 (2012) 89-97. [22] G.S. Pradeep Kumar, R. Keshavamurthy, C.S. Ramesh, Mater. Today: Proceedings 2 (2015) 3107-3115. [23] R.V. Kumar, R. Keshavamurthy, C.S. Perugu, IOP Conference Series: Mater. Sci. Eng. 149 (2016) 012062. [24] C.S. Ramesh, A. Ahmed, B.H. Channabasappa, Mater. Des. 31 (2010) 2230-2236. [25] N. Muralidharan, K. Chockalingam, I. Dinaharan, J. Alloys Comp. 735 (2018) 2167-2174. [26] W. Jiangjing, Z. Yutao, Z. Songli, China Foundry 1672-6421 (2012) 01-028-06. [27] K.V. Shivananda Murthy, D.P. Girish, R. Keshavamurthy, P.G. Koppad, Progress in Natural Science: Mater. Inter. 27 (2017) 474-481. [28] A.R. Kennedy, S.M. Wyatt, Compos. Sci. Technol. 60 (2000) 307-314. [29] K.T. Kashyap, T. Chandrashekar, Bull. Mater. Sci. 24 (2001) 345-353. [30] W.S. Miller, F.J. Humphreys, Scripta Metall. Mater. 25 (1991) 33-38. [31] P. Sahoo, M.J. Koczak, Mater. Sci. Eng. A 131 (1991) 69-76. [32] C.S. Ramesh, R. Keshavamurthy, S. Pramod, P.G. Koppad, J. Mater. Proc. Technol. 211 (2011) 1423-1431. [33] J.R. John Xavier Raj, M.C. Rajkumar, Proc. Inter. Mech. Eng. Cong. Expo. (2015) 13-19. [34] Y.G. An, H. Vegter, S. Melzer, P.R. Triguero, J. Mater. Proc. Technol. 213 (2013) 14191425. [35] B. Vivek Kumar, R. Raghavan, A. Tewari, K. Narasimhan, Mater. Sci. Eng. A 679 (2017) 56-65. 16
[36] C.S. Ramesh, R. Keshavamurthy, P.G. Koppad, K.T. Kashyap, Trans. Nonferrous Met. Soc. China 23 (2013) 53-58. [37] S.J.N. Kumar, R. Keshavamurthy, M.R. Haseebuddin, P.G. Koppad, Trans. Indian Inst. Met. 70 (2017) 605-613. [38] P.G. Koppad, H.R.A. Ram, K.T. Kashyap, J. Alloys Comp. 549 (2013) 82-87. [39] M. Wang, D. Chen, Z. Chen, Y. Wu, F. Wang, N. Ma, H. Wang, Mater. Sci. Eng. A 590 (2014) 246-254. [40] R. Zamani, H. Mirzadeh, M. Emamy, Mater. Sci. Eng. A 726 (2018) 10-17. [41] N. Kumar, G. Gautam, R.K. Gautam, A. Mohan, S. Mohan, J. Eng. Mater. Technol., 139 (2017) 011002-1. [42] K. Tian, Y. Zhao, L. Jiao, S. Zhang, Z. Zhang, X. Wu, J. Alloys Comp., 594 (2014) 1-6.
17
(a)
(b)
Fig. 1. Photographs of a) as-cast Al6061-10wt%ZrB2 slab and b) hot rolled sheet of Al606110wt%ZrB2 composite.
Roller
Hot rolled sheet
Fig. 2. Photograph showing hot rolling of in-situ Al6061-10wt%ZrB2 composite.
18
(a)
(b)
Fig. 3. Dimensions of tensile test specimen (a) as-cast (b) Rolled sheet.
19
Fig. 4. Optical micrographs of (a) as-cast sample-X, (b) as-cast sample-Y, (c) as-cast sample-Z, (d) hot rolled sample-X, (e) hot rolled sample-Y and (f) hot rolled sample-Z.
20
Fig. 5. SEM images of hot rolled Al6061- ZrB2 composite (a) sample-Y, (b) sample-Z.
Fig. 6. SEM images of hot rolled sample-Z composite with EDS spectrum.
21
Fig. 7. X-Ray diffraction patterns of hot rolled Al6061-ZrB2 in-situ composite.
Fig. 8. Microhardness of as-cast and hot rolled Al6061-ZrB2 in-situ composites.
22
(a)
(b)
(c)
Fig. 9. Tensile properties of as-cast and hot rolled Al6061, Al6061-ZrB2 composites (a) 0.2% proof stress (b) UTS and (c) ductility.
23
Fig. 10. SEM images of fractured tensile samples (a) as-cast sample-X, (b) as-cast sample-Y, (c) as-cast sample-Z, (d) hot rolled sample-X, (e) hot rolled sample-Y and (f) hot rolled sample-Z.
24
Table 1 Chemical composition of Al6061, Al-10%Zr and Al-3%B master alloys
Zr
B
Mg
Si
Fe
Composition wt % Cu Mn Pb Zn
-
-
0.8
0.8
0.7
0.35 0.27 0.02 0.008 0.01
10.2
-
-
-
0.25
-
-
-
-
-
2.9
-
-
0.25
-
-
-
-
Element Al6061 alloy Al-10%Zr master alloy Al-3%B master alloy
Ti
Others
Al
-
Balance
-
0.5
Balance
-
0.5
Balance
Table 2 Weight percentage of Al-Zr and Al-B master alloys Sample Designation
Weight percentage of master alloys
Sample-X
Al6061 alloy
Sample-Y
Al6061+ (10%(Al-10%Zr) and 30%(Al-3%B))
Sample-Z
Al6061+ (20%(Al-10%Zr) and 60%(Al-3%B))
Table 3 Estimated average weight percentage of ZrB2 in samples Sample Designation
Estimated average weight percentage of ZrB2
Sample-X
0
Sample-Y
5.2
Sample-Z
9.9
25
Table 4 Grain Size of the as-cast and hot rolled in-situ composites
Grain size (µm) Material
As-cast
Hot rolled
Sample-X
52
46
Sample-Y
34
26
Sample-Z
28
23
Table 5 Tensile values of the as-cast and hot rolled in-situ composites Sample
Proof stress (MPa) As-cast
RD
45D
TD
UTS (MPa) As-cast
RD
45D
% Elongation TD
Sample-X
70±3
254±7 234±4 225±7 100±3 257±7 244±4 236±7
Sample-Y
90±2
Sample-Z 100±5
As-cast
45D
TD
8±1
8.2±0.5
6.5±0.2
293±6 301±4 281±5 128±2 304±6 312±4 296±5 5.4±1
6±1.2
6.6±0.7
2.6±0.4
310±4 375±9 295±7 161±5 324±4 395±9 308±7 5.1±2
5.5±.1 6.5±0.7
26
6±1
RD
3±0.6
PII: DOI: Reference:
S0921-5093(18)31312-1 https://doi.org/10.1016/j.msea.2018.09.104 MSA36985
To appear in: Materials Science & Engineering A Received date: 25 June 2018 Revised date: 19 September 2018 Accepted date: 27 September 2018 Cite this article as: R. Vasanth Kumar, R. Keshavamurthy, Chandra S. Perugu, Praveennath G. Koppad and M. Alipour, Influence of Hot Rolling on Microstructure and Mechanical Behaviour of Al6061-ZrB2 In-situ Metal Matrix C o m p o s i t e s , Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.09.104 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of Hot Rolling on Microstructure and Mechanical Behaviour of Al6061-ZrB2 In-situ Metal Matrix Composites R. Vasanth Kumar1, R. Keshavamurthy2*, Chandra S. Perugu3, Praveennath G. Koppad4, M. Alipour5 1
Department of Mechanical Engineering, Bangalore Institute of Technology, Bengaluru 560004, India 2 Department of Mechanical Engineering, Dayananda Sagar College of Engineering, Bengaluru 560078, India 3 Department of Materials Engineering, Indian Institute of Science, Bengaluru 560012, India 4 Department of Mechanical Engineering, Nagarjuna College of Engineering and Technology, Bengaluru 562164, India 5 Department of Materials Engineering, University of Tabriz, Iran *
Corresponding author. [email protected] (R. Keshavamurthy)
Abstract Synthesis of aluminium based metal matrix composites by in-situ reaction is considered as an alternative method for production of high quality metal matrix composites. In-situ technique eliminates the limitations associated with ex-situ processing technique and it is one of the most widely accepted one. In the present work, Al6061-ZrB2 in-situ composites have been developed by stir casting technique using commercially available Al-10%Zr and
Al-3%B master alloys.
Cast matrix alloy and developed in-situ composites were hot rolled at a temperature of 4000C. Both as-cast and hot rolled matrix alloy and its in-situ composites were subjected to microstructure analysis, microhardness test, grain size studies and tensile test. Tensile behaviour of hot rolled alloy and its in-situ composites were evaluated in the rolling direction (RD), 450 from rolling direction (45D) and transverse direction (TD) and compared with cast ones. Optical and SEM micrographs of hot rolled in-situ composites show that the ZrB2 particles are aligned in the rolling direction. Both as-cast and hot rolled in-situ composites have displayed extensive grain refinement and enhanced mechanical properties when compared with unreinforced alloy.
1
However, ductility of the in-situ composites decreases with increase in ZrB2 content. Hot rolled alloy and its in-situ composites exhibited remarkable improvement in ultimate tensile strength and ductility at 450 from rolling direction compared to transverse and rolling directions.
Keywords: Al6061 alloy; Composites; Hot rolling; Microstructure; Mechanical properties.
1.
Introduction
High strength and low weight factor has necessitated the use of Aluminium alloy based composites for automobile, aerospace and transportation applications [1, 2]. Among all aluminium alloys, Al6061 alloy is most versatile heat treatable alloy with good formability, toughness and low coefficient of thermal expansion characteristics which makes it prime candidate matrix material for fabrication of composites [3, 4]. Metal matrix composites are fabricated by either in-situ method where starting materials are transformed into required products or ex-situ method where starting materials experiences no changes. Preparation of composite by ex-situ process leads to various defects like non-uniform distribution, poor wettability and reduced bonding between matrix and reinforcement, which affects the properties of the cast product. To avoid such defects, it is necessary to develop composites by in-situ process which is capable of meeting the demands of industries for various applications. The main advantages of in-situ method over conventional methods are formation of fine size reinforcements which are thermodynamically stable, good interfacial bonding of matrix and reinforcement due to clean interface and uniform dispersion of reinforcements [5-10]. Reinforcements like TiB2, TiC, Mg2Si, AlB2, Al2O3 and ZrB2 are synthesized by facilitating reaction between master alloys, oxide or fluoride salts in the molten metal. In their work, Kumar et al. [9] synthesized in-situ TiC particles by facilitating a reaction between potassium hexafluorotitanate salt and graphite powders in the Al6061 melt maintained at a temperature of
2
900ºC. In another work, Afkham et al. [11] reported the development of alumina nanoparticles reinforced aluminium composites by stir casting followed by hot rolling. Addition of ZnO, TiO 2 and sodium borosilicate glass powder to the aluminium melt maintained at 850ºC led to the formation of alumina nanoparticles. However, possibility of formation of casting defects like gas defects such as blowholes or pinholes and shrinkage cavities cannot be ruled out during fabrication of composites. In order to overcome the casting defects, composites are often subjected to secondary forming process like forging, extrusion and rolling. Application of secondary forming processes results in reduction or elimination of porosity, uniform dispersion of reinforcements and further improvement in matrix/reinforcement interfacial bonding over their as-cast counterparts which in turn enhances the mechanical, tribological and corrosion characteristics [12]. Out of all forming process, Rolling is widely used in industrial applications to produce sheets for automobile bodies, refrigerators and other house hold appliances. Hot rolling process parameters such as temperature, speed, thickness reduction and orientation have significant influence on the microstructure and mechanical properties. In particular, studying the effect of hot rolling on mechanical behaviour of aluminium alloy based in-situ composites is of prime importance from industrial application point of view [13]. In the last few years’ significant increase in number of research work were reported on development of aluminum based in-situ composites. Dinharan et al. [14] have reported development of ZrB2 reinforced AA6061 alloy via in-situ reaction between KBF4 and K2ZrF6 salts method. AA6061-ZrB2 in-situ composite processed by salt method exhibited good mechanical properties with increase in ZrB2 content in matrix alloy. Zhao et al. [15] have studied the morphologies of in-situ Al3Zr and ZrB2 particles formed in Al-ZrB2 composites at various molten temperatures. Morphologies of ZrB2 showed no diversity and Al3Zr were sensitive to temperature of aluminium melt. Zhang et al. [16] have reported microstructure of Al(Al3Zr+ZrB2) composites prepared by magnetochemistry in-situ reaction. The ZrB2 particles
3
were present in hexagon shape with size in the range of 0.3-0.5μm. The electron microscopy analysis revealed uniform dispersion of ZrB2 and Al3Zr particles in the aluminium matrix. Ramesh et al. [17] have reported processing of TiB2 reinforced Al6061 composites through insitu technique using Al-B and
Al-Ti master alloys. Increase in content of TiB2 was observed
with the increase in addition of master alloys to the aluminium melt. The composite with higher TiB2 content showed highest tensile strength coupled with good ductility.
Increasing the
addition of master alloys results with increase in formation of Al3Ti in composites which are more sensitive to temperature of aluminum melt. In their work, keshavamurthy et al. [18] have reported microstructure and mechanical properties of Al7075-TiB2 in-situ composite using Al-B and Al-Ti master alloys. Grain size was decreased due to presence of fine TiB2 resulting in grain refinement attributed to solute boron. Venkateswarlu et al. [19] investigated the influence of hot rolling and annealing on grain refining efficiency of Al-5Ti-B master alloys. The results showed decrease in mean particle size with the increase in percentage reduction at any given rolling temperature. The overall improvement in grain refinement was attributed to increase in volume fraction of fine TiAl3 particles due to hot rolling. Xu et al. [20] have studied the effect of rolling reduction conducted at temperature of 400ºC on tensile properties of Al6061/ABOwhisker composites. With increase in reduction percentage of rolling, degree of alignment of whiskers took place along the rolling direction. Further, large reduction lead to improvement in mechanical properties like tensile strength. El-Sabbagh et al. [21] have studied the hot rolling behaviour of stir cast Al6061 and Al6082 alloys with SiC particulate reinforced composites. The as-cast composites rolled at intermediate temperature of 450ºC and at strain rate of 1 s-1 showed improved strength. Further, the void percentage was found to decrease linearly with increase in reduction and redistribution of SiC in the matrix leading to less agglomeration. From the review of literature, it is observed that meager information is available on effect of hot
4
rolling on microstructure and mechanical behaviour of Al6061-ZrB2 in-situ composites. In the light of the above, present study focuses on synthesis of Al6061-ZrB2 in-situ composites using Al-Zr and Al-B master alloys using low cost stir casting technique followed by hot rolling. The effect of hot rolling on microstructure and mechanical properties have been investigated and compared with that of as-cast composites. 2.
Experimental details
Al6061 alloy, Al-3%B and Al-10%Zr master alloys were utilized as the raw materials for fabrication of in-situ Al6061- ZrB2 composites. The chemical composition of master alloys and Al6061 alloy are given in Table 1. The weight percentage of Al-3%B and Al-10%Zr master alloys are shown in Table 2. Master alloys were added in a stoichiometric proportion to Al6061 alloy for synthesis of Al6061- ZrB2 in-situ composite with a varied percentage. Al6061 and master alloys were placed in a graphite crucible and heated up to 850°C in 6 KW electrical resistance furnace. The composite melt was held at 8500C temperature for duration of 30 minutes to complete the in-situ reaction and followed by stirring alternatingly for every 10 minutes using mechanical stirrer. The slag was removed and hexachloroethane (C2Cl6) tablets were added for the melt as degassing agent to remove hydrogen. The graphite crucible was removed from the furnace and then the molten alloy was subsequently poured into the preheated metal die [21-23]. The weight percentage of ZrB2 was estimated by acid dissolving method and the results are shown in Table 3 [17]. The cast products were cut into rectangular specimens of size 45 X 50 X 11 mm 3 (as shown in Fig. 1a) and heated up to a temperature of 400ºC for 1hour. Once the heating was completed, the specimens were subjected to hot rolling with 10% of reduction in each pass. The hot rolled composite sheet obtained with reduction of 90% as shown in Fig. 1b. Fig. 2 shows the as-cast specimen undergoing hot rolling using 2-High rolling mill.
5
Both as-cast and hot rolled in-situ composite samples were polished using standard metallographic technique. The samples were subjected to etching using Keller’s reagent to reveal the microstructure [24]. The microstructures of both as-cast and rolled composites were studied using optical and scanning electron microscope (Make: JSM 840a Jeol). Phase identification of hot rolled composites was performed using X-Ray diffraction (Make: Philips X’Pert Pro X-ray diffractometer). Vickers Microhardness test was performed at a load of 100grams under a dwell time of 10 seconds on polished samples. Tensile tests were carried out on both as-cast and rolled Al6061 alloy and their in-situ composites according to ASTM B557 E8 Standards. The average of three tensile results was considered as ultimate tensile strength of each specimen. Hot rolled sheets were cut through electrical discharge machining process for preparation of tensile specimens in three different directions: Rolling direction (RD), 45° to rolling direction (45D) and Transverse direction (TD). Fig. 3(a, b) illustrates the dimensions and photographs of as-cast and rolled tensile test specimens. More details on composite preparation are available in our earlier work [25].
3.
Results and discussion
3.1 Microstructure and Grain size analysis Fig. 4 shows the optical micrographs of as-cast Fig. 4(a-c) and hot rolled Fig. 4(d-f) Al6061 alloy and Al6061-ZrB2 in-situ composites. It can be observed from Fig. 4(b, c) that most of the ZrB2 particles were found along the grain boundaries of α-Al while some of them were found inside the α-Al grains. The presence of ZrB2 particles at the grain boundaries of α-Al is attributed to movement of solidification front. During solidification the particles are pushed towards the solidification front and stays there until it is completed. Due to this the grain boundaries which are regions of lastly solidifying liquid becomes the final particle location. These observation explains that the dispersion of ZrB2 particles largely depend upon the velocity of solidification
6
front which in turn influences whether the particles to be dispersed within the grains or at the grain boundaries. From Fig. 4c we can observe clustering of ZrB2 particles at few locations of grain boundaries due to high ZrB2 (9.9 wt.%) content in this as-cast composite. When compared with Al6061 alloy the composites showed smaller grain size which is attributed to the ZrB2 particles which contribute to refinement of microstructure. In particular the presence of boron in ZrB2 which is well known as grain refiner plays vital role in the grain refinement of in-situ composites [18, 26]. In case of hot rolled Al6061 and Al6061- ZrB2 composites as shown in Fig. 4(d-f), it can be observed that the grain morphology has changed to elongated grain structure over equi-axed grain structure in as-cast materials. Though the grains have elongated structure in the rolling direction, they are excessively fine in size. Further, the extent of homogeneity and dispersion of ZrB2 particles are relatively better in case of hot rolled composites compared to as-cast. The ZrB2 particles act as heterogeneous nucleation sites for the formation of new grains resulting in refined microstructure which is clearly visible in optical micrographs. When evaluated with hot rolled in-situ composites, there is no any noticeable change either in morphology or in size of the in-situ formed ZrB2 particles. Some of the particle clusters observed in the as-cast in-situ composites were disintegrated after subjecting into hot rolling. Fig. 5(a, b) shows the SEM micrographs of hot rolled sample Y and sample Z. SEM images clearly indicates the uniform dispersion of ZrB2 particles after hot rolling. The bonding between Al grains and ZrB2 particles was found to be excellent as there were no reaction products at the interface. Fig. 6 shows the EDAX analysis of hot rolled sample Z in which both the constituents of ZrB2 particle (Zr and B elements) are clearly seen. Table 4 shows an average grain size of Al6061 alloy and Al6061- ZrB2 in-situ composites in both as-cast and hot rolled conditions. It is observed from the table that hot rolled Al6061 alloy shows a reduction of 8% in grain size over the same alloy in as-cast conditions. However, a remarkable reduction of 56% and 26% is observed for hot rolled and as-cast in-situ composites
7
respectively when evaluated with unreinforced matrix alloy in cast conditions. When evaluated with unreinforced cast aluminium alloy, in-situ composites exhibited significant decrease in average grain size. Presence of ZrB2 particles in the alloy helps in reducing the grain size and enhancing the mechanical properties which is attributed to obstruction posed by the particles to advancing grains during cooling of composites [27, 28]. Due to this the aluminium grains becomes finer in composites when compared to that of Al6061 alloy without ZrB2 particles. After subjecting both unreinforced aluminium alloy and its in-situ composites to hot rolling process, further decrease in average grain size is observed which may be due to dynamic recrystallization in thermo-mechanical processing. Presence of ZrB2 particles not only contributes to the recrystallization but also plays a vital role in retarding grain growth. Fine size ZrB2 particles facilitate nucleation of new grains which are finer in size than unreinforced alloy. Such newly formed grains will possess high angle grain boundaries around in-situ formed ZrB2 particles [29]. On the other hand, some of the ZrB2 particles pin the grain boundaries of matrix during hot rolling. Simultaneous formation of recrystallized grains and increased dislocation density in the presence of ZrB2 particles reduces the driving force required for further growth of newly formed grains in aluminium matrix [30-34].
3.2 X-ray diffraction analysis Fig. 7 presents XRD patterns for hot rolled Al6061 alloy and Al6061-ZrB2 in-situ composites. Al6061 alloy (sample X) showed most of the peaks corresponding to pure aluminium while in-situ composites (sample Y & Z) showed distinct peaks related to ZrB2 particles. The XRD patterns obtained is in good agreement with the standard JCPDS file for Zirconium diboride (ZrB2) (JCPDS file No: 657806) [35]. The following in-situ reactions taking place between Al6061 alloy and master alloys at 850°C explains the formation of ZrB2 particles. Zr + 3Al
Al3Zr………………………. (1)
8
Al3Zr + 2B
ZrB2 + 3Al………………. (2)
AlB2 + Al3Zr
ZrB2 + 4Al……………. (3) 4Al + ZrB2…….. (4)
Al + Al-Zr + 2Al-B
The absence of any other element or compound apart from Al and ZrB2 in the above reaction products indicates that in-situ reaction between master alloys is complete. Further, the sole presence of ZrB2 explains that it is thermodynamically stable since it has not reacted with Al6061 to form any other intermetallic compounds. Further, as shown in Fig. 6 where the SEM image and EDAX of sample Z display absence of any kind of reaction products is well supported by these XRD patterns (Sample Y & Z). Thus, it is presumed that the interface between the Al6061 alloy and in-situ formed ZrB2 is clean and free from any sort of contamination.
3.3 Microhardness Fig. 8 shows the microhardness of Al6061 alloy and Al6061-ZrB2 composites before and after hot rolling. In-situ composites displayed significant improvement in microhardness over the Al6061 alloy in both as-cast and hot rolled conditions. With the increase in ZrB2 content, the microhardness of the Al6061 matrix was found to increase. As-cast and hot rolled sample Z showed an improvement of 18% and 29% when compared to that of as-cast sample X. ZrB2 particles which possess very high hardness tend to impart its attributes to soft ductile aluminium matrix resulting in improved microhardness of sample Y and Z. The strain field around the ZrB2 particles imposes resistance to plastic deformation caused by indentor by restricting the movement of dislocations. The primary reason for improvement in microhardness of in-situ composites is grain refinement caused by ZrB2 particles. It is well known from the Hall-Petch equation that, decrease in grain size increases the strength of the material. In present case, the in- situ formed ZrB2 particles in Al6061 matrix refine the microstructure by acting as nucleation site for new grains during solidification and also pin the grain boundaries for further growth. In
9
addition to this in case of hot rolled composites, the interface bonding is very good while in case of as-cast composites it is relatively weak. Due to improved bonding the hot rolled in-situ composites showed improved microhardness. The second reason for improvement in microhardness is restriction to movement of dislocations by ZrB2 particles. In-situ formed ZrB2 particles in the matrix act as barricade to the movement of dislocations and thereby increasing the stress required for the movement of dislocations. With uniformly dispersed ZrB2 particles in the matrix will increase the overall stress required which in turn contribute to increase in microhardness. Further, it is clearly visible from the microstructure that the amount of porosity/defects is minimal in hot rolled in-situ composites which contribute in increment of microhardness. This is mainly because presence of any minor flaws will be healed up during hot rolling.
3.4 Tensile properties Fig. 9 depicts variation of yield strength, ultimate tensile strength and ductility of hot rolled Al6061 alloy and Al6061-ZrB2 in-situ composites. Tensile values of the as-cast and hot rolled in-situ composites are shown in Table 5. The presence of in-situ formed ZrB2 particles improved the overall mechanical properties of in-situ composites in both as-cast and hot rolled conditions. The sample Z showed highest 0.2% proof stress and ultimate tensile strength (UTS) than compared with sample X and Y. An increment in UTS by 61% and 62% was observed in sample Z when compared with that sample X in both as cast and hot rolled conditions. Significant improvement in mechanical properties after hot rolling indicates that it is very effective forming technique in densifying both Al6061 alloy and in-situ composites. Variation of 0.2% proof stress, ultimate tensile strength and ductility of hot rolled composites was studied in three conditions- Rolling direction (RD), 45º direction from RD (45D) and Transverse direction (TD) [36]. From the Fig. 9a, it is observed that 0.2% proof stress and UTS is highest in 45D as
10
compared to RD and TD in sample Y and sample Z whereas in case of sample X, RD showed highest 0.2% proof stress and UTS as compared to 45D and TD. About 21% and 28% improvement in UTS in 45D of hot rolled sample Z was observed when compared with that of RD and TD respectively. Enhanced ultimate tensile strength in the 45D and RD may be attributed to strong texture due to presence of elongated grains in the direction of hot rolling [37]. On the other hand, the lower tensile strength in the transverse direction may be attributed to weak texture. Weak texture in the transverse direction might be due to disruption on flow of aluminium matrix as a result of excessive interaction between matrix material and ZrB2 particles [38]. There are various factors which contribute in improvement of mechanical properties of in-situ composites which includes grain size, dispersion of ZrB2 particles and interfacial bonding. The hot rolling not only helps in reducing the grain size by virtue strain hardening but also improves the dispersion more uniformly in the matrix. Fig. 5(a, b) clearly shows that dispersion of ZrB2 particles are homogeneous in nature and hardly any clusters are visible. With the clean interface formed during casting process, the bonding between ZrB2 particles and Al6061 alloy improves further by hot rolling. The improvement in the UTS can be attributed to strengthening mechanisms like load transfer, grain refinement, dislocation and Orowan strengthening. It is well known that there should be continuity between the Al6061 matrix and ZrB2 particles for load transfer during application of load on bulk material. The clean interface with good bonding between matrix and particles will ensure that the load is transferred more efficiently from the matrix to hard ZrB2 particles [39, 40]. The second strengthening mechanism is grain refinement which can take place by recrystallization during thermo-mechanical processing. During hot rolling of in-situ composites the possibility of recrystallization is very high since the in-situ formed ZrB2 particles have size less than 1 micron. The hard ZrB2 particles do not deform during hot rolling and the strain in the Al6061 matrix forms a deformation zone around the
11
particles. The particle deformation zones are preferably formed near ZrB2 particles due to the mismatch between them and Al6061 matrix. Thus, recrystallization can easily takes place in these deformation zones due to particle stimulated nucleation. The resultant grain formed due to recrystallization in hot rolled in-situ composites is often less than that found in as-cast Al6061 or in-situ composites. The grain refinement caused by particle stimulated nucleation contributes to increase in strength of hot rolled in-situ composites according to Hall-Petch equation [32]. Dislocation strengthening is other significant mechanism responsible for improvement in strength of in-situ composites. The variation in the co-efficient of thermal expansion between ZrB2 particles (~5.9×10-6 K-1) and Al6061 alloy (~24×10-6 K-1) is quite large. Due to large difference in thermal expansion coefficient prismatic punching of dislocation takes place at matrix/particles interface during cooling. With the increase in ZrB2 content and rolling reduction there will be increase in dislocation density. Presence of high dislocation density in hot rolled in-situ composites results in significant increase in strength [13]. In addition to this, Orowan strengthening mechanism also contributes to ultimate tensile strength of in-situ composite from inhibiting movement of dislocations by finely dispersed ZrB2 particles. The in-situ formed ZrB2 particles which are dispersed uniformly throughout the matrix act as obstacle for the dislocations and restricts their motion. However, with further application of stress the dislocation bypasses the particles by forming a loop around them. This is mainly because these particles are hard in nature and non-deformable due to which it is highly impossible for dislocations to cut them. These dislocation loops formed will raise the stress required for deformation thereby contributing to strength [41]. Fig. 9c shows variation in ductility of Al6061 alloy and Al6061-ZrB2 in-situ composites. It is observed from the graph that with the increase in content of ZrB2 in Al6061 alloy marginally decreases the ductility in both as-cast and hot rolled conditions. It is observed that the hot rolled Al6061 alloy and its composites displayed improvement in ductility when compared to that of
12
as-cast counterparts. This is mainly because the presence of casting defects like porosities and solidification shrinkages acts as crack nucleation site which on application of load helps in nucleation and propagation of cracks. On the other hand, in case of hot rolling the casting defects are eliminated due to application of plastic deformation at elevated temperatures. In addition to this the refinement of grains due to the presence of boron also contributes to ductility of the composite [42]. Variation in ductility of hot rolled composites and alloy were also observed in three conditions- RD, 45D and TD. From the observations it is clear that, 45D shows higher ductility as compared to RD and TD. Reduction in the ductility with increased content of ZrB 2 particles may be due to presence of crack nucleating sites particularly at the interface between matrix and reinforcement in large numbers and also the presence of possible microporosities which contributes to failure of matrix material [43]. Particles of ZrB2 present in the matrix are capable of changing the direction of crack propagation resulting in branching, crack bridging and deflection in its favourable direction with respect to the direction of tensile loading. This process requires large amount of energy and offers more resistance to crack propagation resulting in higher fracture toughness and ductility [44]. Higher percentage of agglomeration in ZrB2 particles helps for easy propagation of cracks and earlier debonding of ZrB2 particles from the matrix interface during tensile loading results in decreased ductility [41].
3.5 Fractography Tensile fractographs of as-cast and hot rolled Al6061 alloy and in-situ composites are shown in Fig. 10(a-f). Fig. 10a shows fractograph of as-cast alloy which showed brittle failure macroscopically. The fracture surface depicted debonding of aluminium grains due to weak bonding between the aluminium grains. These particles acts as nucleation sites and causes microcrack leading to debonding of adjacent grains. In case of as-cast in-situ composites sample Y and Z as shown in Fig. 10(b, c), the fracture surface showed presence of dimples indicating
13
ductile fracture. However, the size of voids formed after fracture indicated debonding of aluminium grains which were located adjacent to the microporosities and agglomerated ZrB2 particles. The presence of ZrB2 particles on the fracture surface indicates strong adhesion with the aluminium matrix owing to its clean interface. In case of hot rolled alloy as shown in Fig. 10d there exist homogeneous voids and with fine sized dimples all over the fractured surface. When compared with the as-cast alloy the surface didn’t showed casting related microporosities which are probable sites for nucleation of cracks. In same line with that of hot rolled alloy, the in-situ composites as shown in Fig. 10(e, f) exhibited ductile failure. During application of load during testing, the fracture begins with formation of microvoids and coalescence takes place with increase in load. As load reaches its ultimate value, the microvoids are large enough to nucleate crack and propagate at faster rate causing the fracture. Close examination of fractured surface showed the fracturing of ZrB2 particles rather than debonding. Debonding of reinforcements usually occur in case of ex-situ composites where it is hard to obtain good bonding between matrix and reinforcements. In such cases the application of load results in formation of cracks at the interface and removal of complete reinforcement [9]. However in present case due to excellent bonding achieved via in-situ formation of ZrB2 particles and hot rolling resulted in strong interface due to which the particle fracturing is observed.
Conclusions This work presents an experimental study of microstructure, mechanical properties of as-cast and hot rolled Al6061 alloy, Al6061-ZrB2 in-situ composites. The conclusions from the present investigation are as follows: •
Al6061-ZrB2 in-situ composites have been synthesized by stir casting technique and successfully hot rolled at a temperature of 400 oC.
14
•
Optical and Scanning electron micrograph shows uniform dispersion of irregular plate shaped ZrB2 particles in the matrix alloy under as-cast and hot rolled conditions.
•
Al6061-ZrB2 in-situ composites exhibited extensive grain refinement both in as-cast and hot rolled conditions.
•
Hardness of as-cast and hot rolled in-situ composite shows a maximum improvement of 18% and 29% respectively when compared with cast alloy.
•
UTS of the in-situ composites increased by 61% and 62% in both as-cast and hot rolled conditions respectively when compared with cast matrix alloy.
•
In-situ composites exhibited an improvement of 21% and 28% in UTS at 450 direction of rolling compared to transverse and rolling direction respectively.
•
Ductility decreases marginally with the increase in content of ZrB2 in Al6061 alloy in both as-cast and hot rolled conditions. Hot rolled Al6061 alloy and its composites displayed improvement in ductility when compared with that of as-cast counterparts.
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(a)
(b)
Fig. 1. Photographs of a) as-cast Al6061-10wt%ZrB2 slab and b) hot rolled sheet of Al606110wt%ZrB2 composite.
Roller
Hot rolled sheet
Fig. 2. Photograph showing hot rolling of in-situ Al6061-10wt%ZrB2 composite.
18
(a)
(b)
Fig. 3. Dimensions of tensile test specimen (a) as-cast (b) Rolled sheet.
19
Fig. 4. Optical micrographs of (a) as-cast sample-X, (b) as-cast sample-Y, (c) as-cast sample-Z, (d) hot rolled sample-X, (e) hot rolled sample-Y and (f) hot rolled sample-Z.
20
Fig. 5. SEM images of hot rolled Al6061- ZrB2 composite (a) sample-Y, (b) sample-Z.
Fig. 6. SEM images of hot rolled sample-Z composite with EDS spectrum.
21
Fig. 7. X-Ray diffraction patterns of hot rolled Al6061-ZrB2 in-situ composite.
Fig. 8. Microhardness of as-cast and hot rolled Al6061-ZrB2 in-situ composites.
22
(a)
(b)
(c)
Fig. 9. Tensile properties of as-cast and hot rolled Al6061, Al6061-ZrB2 composites (a) 0.2% proof stress (b) UTS and (c) ductility.
23
Fig. 10. SEM images of fractured tensile samples (a) as-cast sample-X, (b) as-cast sample-Y, (c) as-cast sample-Z, (d) hot rolled sample-X, (e) hot rolled sample-Y and (f) hot rolled sample-Z.
24
Table 1 Chemical composition of Al6061, Al-10%Zr and Al-3%B master alloys
Zr
B
Mg
Si
Fe
Composition wt % Cu Mn Pb Zn
-
-
0.8
0.8
0.7
0.35 0.27 0.02 0.008 0.01
10.2
-
-
-
0.25
-
-
-
-
-
2.9
-
-
0.25
-
-
-
-
Element Al6061 alloy Al-10%Zr master alloy Al-3%B master alloy
Ti
Others
Al
-
Balance
-
0.5
Balance
-
0.5
Balance
Table 2 Weight percentage of Al-Zr and Al-B master alloys Sample Designation
Weight percentage of master alloys
Sample-X
Al6061 alloy
Sample-Y
Al6061+ (10%(Al-10%Zr) and 30%(Al-3%B))
Sample-Z
Al6061+ (20%(Al-10%Zr) and 60%(Al-3%B))
Table 3 Estimated average weight percentage of ZrB2 in samples Sample Designation
Estimated average weight percentage of ZrB2
Sample-X
0
Sample-Y
5.2
Sample-Z
9.9
25
Table 4 Grain Size of the as-cast and hot rolled in-situ composites
Grain size (µm) Material
As-cast
Hot rolled
Sample-X
52
46
Sample-Y
34
26
Sample-Z
28
23
Table 5 Tensile values of the as-cast and hot rolled in-situ composites Sample
Proof stress (MPa) As-cast
RD
45D
TD
UTS (MPa) As-cast
RD
45D
% Elongation TD
Sample-X
70±3
254±7 234±4 225±7 100±3 257±7 244±4 236±7
Sample-Y
90±2
Sample-Z 100±5
As-cast
45D
TD
8±1
8.2±0.5
6.5±0.2
293±6 301±4 281±5 128±2 304±6 312±4 296±5 5.4±1
6±1.2
6.6±0.7
2.6±0.4
310±4 375±9 295±7 161±5 324±4 395±9 308±7 5.1±2
5.5±.1 6.5±0.7
26
6±1
RD
3±0.6