MECHANICAL BEHAVIOR ENHANCEMENT OF MAGNESIUM ALLOY AZ61 WITH MICRO-SiC PARTICLES
Song-Jeng Huang,1 Murugan Subramani1, Dawit Bogale Alemayehu 1, Tien-Hsi Lee2, *, Kou- Chen Liu3,
1Dept. of Mechanical Engineering, National Taiwan University of Science and Technology 2Dept. of Mechanical Engineering, National Central University 3Dept. of Electronics, Chang Gung University
* corresponding author: [email protected] Abstract In the present work, the mechanical behavior of magnesium alloy AZ61 with the effect of different weight % (0, 1 and 2) of micro-silicon carbide particles (SiCp) reinforcement were fabricated by gravity casting method using the stirring process was investigated at room temperature. After homogenization (T4) heat treatment of the prepared all samples are characterized by optical microscope (OM), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX), mechanical behavior. Microstructure observation results found that increasing micro-SiCp into the magnesium matrices, significantly decreasing the grain size of the metal matrix composites. In the presence of micro-SiCp have been assisted to enhance the hardness value of metal matrix composites (MMCs). In addition, micro SiCp ultimate tensile strength (UTS), yield tensile strength (YTS) and elongation of the MMCs increased. As is evident, that added 1% of micro SiCp notable increment in the UTS and YTS were 170.06 and 108.17 respectively and elongation were decreased in 4.47%. Furthermore, adding 1% of micro SiCp led to decrease the UTS, YTS and elongation were 166.64, 100.64 and 3.44 respectively. Because increasing micro SiCp content latter it may be attributed to the agglomeration of the MMCs. However, the maximum value of strength has been achieved to adding 1% of micro SiCp MMCs. Obtaining tensile test results from experimental process were compared with finite element method results. Finite element results were good agreement with the experimental results.
Keywords: Mg alloy AZ61, silicon carbide particles, homogenization, mechanical behavior, finite element method
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1. Introduction In the recent year’s magnesium alloys has great interest in lightweight materials, because of their high strength to weight ratio, high stiffness and low density. Density of magnesium is less than that of aluminum and steel. For this reason magnesium alloys are very attractive materials for load bearing components in automobile industry [1] Reinforcement for metal matrix composite have different stipulation in profile. which is found by processing, production and matrix system of the composite materials. The following stipulations are commonly applicable for reinforcement materials: low density, chemical compatibility, high compression and tensile strength, good process ability and economic efficiency [2]. Magnesium, Aluminum, Copper and Titanium are the mainly used matrix metals and Al2O3, SiO2 SiC, TiB2, WS2 and INT are the commonly used secondary phase ceramic reinforcement particles, whiskers and fibers. They have low density, high levels of strength, hardness, young's modulus and thermal stability. While during the particle reinforcement with the matrix material the grain boundaries and smooth the way for formation of a grain structure. This increase the mechanical properties such as young's modulus, ultimate strength, yield strength and also influence to improve the fatigue and creep properties of the composite materials at low temperatures. However, SiC particles are the most commonly chosen reinforcement particle for magnesium alloy, because of its superior properties, low cost and easily availability [3] . Huang et al. [4] investigated the magnesium alloy AZ61 with the reinforcement of different weight percentage (0, 0.5 and 1) of SiC powder with a particle size 4.5μm fabricated by stir casting method for MMCs tubes hot extrusion. Obvious grain size refinement discovered on both additions of reinforcement and extrusion process. However, the grain refinement effect is significant improvement on 0.2% yield strength and ultimate tensile strength of magnesium MMCs. Wang, X. J., et al. [5] processing the grain size of magnesium alloy metal matrix composites decreased with increasing SiC particle content and as well as enhance the ultimate tensile strength, yield strength and elastic modulus of the MMCs. Afshin and co- workers are fabricated pure magnesium and magnesium alloy AZ80 with reinforcement of nano SiC particles with different weight percent (1.5, 2.5 and 3.5) by stir casting method. They reported their fabricated magnesium matrix composite materials are significant improvement in hardness, ultimate tensile strength and yield tensile strength. Additionally, increase the nano SiC particle into the monolithic materials more than 2.5 % approximately
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there is no effect on grain size. As compare pure Mg and AZ80 alloy can be seen adding nano SiC particle led to remarkable increase in ultimate tensile strength and yield strength on the AZ80 alloy [6]. Huang, S. J., & Ali, A. N. [7] investigated the solution and ageing heat treatment process used to improve the micro plastic deformation behavior of as cast AZ61/SiCp metal matrix composites by stir casting method. Gupta et al. [8], Manoj Kumar et al. [9] studied the use of magnesium / silicon carbide composite. Magnesium 9.8 and 26.3wt% of silicon carbide particles (25μm) synthesized by using of decomposition melting Deposition (DMD) technique. Tensile tests are taken from the room temperature showed when SiCp increased the hardness, elastic modulus (E) and 0.2% yield tensile strength (YTS) increased by 43.3%, 30.2% and 33.9% but grain size, Ultimate Tensile Strength (UTS) and elongation decreased by 40%, 2.8% and 600%
respectively. Hardness improvement and ductility decline due to the Mg2Si phase is formed at the interface between Mg and SiCp. Han Lin, [10] investigated as cast and heat treated microstructure and mechanical properties of magnesium alloy with reinforcement of with and without minor Sc addition. Their study reported that increasing the Sc to the as cast could refine the grains and refine the
Mg17Al12 phase also suppress the formation of the Mg17Al12 phase. As a result, mechanical properties such as UTS, 0.2%YTS and elongation% at room temperature for the Sc containing as cast magnesium alloy significantly improved. Bita Pourbahari and co-workers [11] are investigated the synergistic effect of Gd/Al ratio on the mechanical properties of as cast Mg-Al-Gd-Zn alloys by changing the chemical composition of AZ61 to GZ61. For precedential, the UTS and elongation to failure of Mg- 3Al-3Gd-1Zn alloys were significantly enhanced by 4% and 180% respectively. But the yield strength become decrease. Further increment in Gd in Gd/Al ratio, which led to decrease the UTS and elongation. Aatthisugan I et al. [12], Studied mechanical and wear behavior of magnesium alloy
AZ91D with reinforcement of boron carbide (B4C) and graphite particle reinforced hybrid composites were fabricated by stir casting method. their results show that the graphite reinforced hybrid composite revealed a lower wear loss compared to pure AZ91D and
AZ91D/B4C composite. Addition of both particle reinforcements into the AZ91D alloy remarkably enhance the wear resistance, hardness and ultimate tensile strength. Wang, Xiaojun, et al. [13], investigated the effect of SiC nano-particles on micro structural and mechanical strength of AZ31 magnesium alloy matrix composites during hot rolling. they revealed the test results SiC nano-particles impede the formation of slip bands, which are
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much more prevalent in matrix alloy sheets. For the mechanical properties of the hot rolling leads to notable improvement on both of alloy sheets and nano-composite sheets. In this work Studying the reinforcement of SiCp effect on mechanical properties as a function of SiCp wt.% on the magnesium alloy AZ61. Analyzing the tensile deformation behavior of AZ61/SiCp metal matrix composites, with the characterization of their microstructures. Obtain results from experiments process were compare with finite element method analysis. The numerical and experimental can also be applied for other types of materials.
2. Experimental procedures and materials 2.1 Material and fabrication of composites The magnesium alloy AZ61 was matrix material with chemical composition of Al- 5.95, Zn-0.64, Mn-0.26, Fe-0.005, Si-0.009, Cu-0.0008, Ni-0.0007 and Mg balance and silicon carbide particles were used as reinforcement. Particle size and weight percentage of silicon carbide particles shown in Table 1. In this study AZ61/SiCp magnesium matrix composites fabricate with different wt.% (0wt.%, 1wt.% & 2wt%) of silicon carbide particles by stir casting method. Table 1 Particle size and wt.% of SiCp in AZ61/SiCp Mg MMCs casts. Types of casting ingots Particle size of SiCp (μm) Wt.% of SiCp AZ61 ---- 0 AZ61/SiCp 10 1 AZ61/SiCp 10 2
During casting use fiber cotton to prevent heat loss and start with a temperature 100˚C and by waiting for about 10-15 minutes then increase the temperature 100˚C step up to
400˚C. At 400˚C SF6/CO2 gas allow inside the crucible to prevent burning of magnesium alloy AZ61. And then increase the temperature at 700˚C argon gas was applied to prevent oxidation. after the temperature was reach at 700˚C the AZ61/SiCp melt was stirred with two stir blade at 300 RPM and 3 min respectively.
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Fig. 1. Final casting ingots (a) pure AZ61, (b) AZ61/1% of SiCp, (c) AZ61/2% of SiCp Finally the melt was poured into the mold, which placed inside the lower chamber of the furnace and then we get the final cast ingots. Fig. 1. shows that three types of ingots prepared by stir casting method. The preparation of ingots which, were made into specimen dimension of 15mm * 10 mm * 5 mm for microstructure and hardness test. Middle part of the ingot is used to make the specimen for tensile test specimens. All of the specimens are heat treated (homogenized) at 410˚C for 24 hrs.
2.2. Microstructure analysis Microstructure of cross section was observed by optical microscope (OM), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). The EDS and SEM are used to analyze the composition and fracture surface effect of reinforcement on the tensile properties of magnesium alloy composites. Samples for OM observation were mounted, polished(240, 400, 600, 800, 1000, 2000 and 4000 CW silicon carbide abrasive papers) and etched (using solutions of 100 ml of ethanol, 10 ml of distilled water, 5 ml of acetic acid and 6 g of picric acid for 25 seconds of etching time). Samples for SEM observation mounded, polished and etched in the same solution and it will reveal the particle distribution.
Fig.2. Specimens for tensile test
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2.3. Hardness test Hardness test were conducted by using Vickers micro-hardness testing machine (Akashi MVK-H1) throughout the cross section of the composite samples. The test was executed automatically with 300gf and 10s as testing parameters. In order to estimate the distribution homogeneity of the reinforcement particles.
2.4. Tensile test The uniaxial tensile test was conducted to evaluate the mechanical properties of magnesium alloy metal matrix composites. Tensile test specimens were prepared according to ASTM E 8 standard and it shown in Fig. 2. During the tensile testing applied load and elongation were recorded. When the plastic deformation ends that point is called yield point and stress corresponding to the point is called yield strength. But Mg alloy MMCs materials obviously no yield point, alternatively determine the yield strength is to draw straight line parallel to the curve which starts 0.2% strain. The point intersects with stress strain curve is called 0.2% yield strength. At the end of the failure elongation data are obtained then using formulation engineering stress strain can be calculated. From the stress strain value, the Young´ s modulus (E), 0.2%yield strength (YS), ultimate tensile strength (UTS) and elongation were calculated. Test was performed to using MTS 810 material testing machine axial capacity of 100 KN load and constant strain rate of 1 mm/min at laboratory air and room temperature. At least 3 samples were tested to taken the average of results.
3. Results and Discussions 3.1 Microstructural analysis Fig. 3 shows the optical microscope image of as cast and homogenized AZ61/SiCp magnesium metal matrix composites. As cast microstructure consists of α-Mg as the matrix and large amount of β-Mg17Al12 as the secondary phase. The as cast material was subjected to homogenization (T4) heat treatment at 410˚C in 24hrs. During homogenization, secondary β-
(Mg17Al12) phase particles dissolved into the matrix and it traces on the grain boundary and it can enhance ductility of the material [16]. Clearly observed from Fig. 3 that added SiCp have a significant effect on the grain refinement magnesium alloy matrix composite. Linear intercept method was used for measuring average grain size. Overall, at least 20 locations examined for each sample and the average value will be reported as average grain size. The average grain size of pure AZ61 was 56.42μm. When, added SiCp 1wt.% into pure AZ61 the
1-135 grain size was 41.93μm corresponding to average grain size decreased by 28.68%. Further increment in SiCp 2wt.% into the AZ61 the grain size was 50.93μm. Which, grain size decreased by 9.7% compared to pure AZ61, but increased by 21.53% compared with AZ61/1% SiCp composite. The average grain size measurement of Mg alloy MMCs as shown in Fig. 4. The main reason for the grain refinement of the AZ61 alloy containing SiCp. The microstructures composites SiCp has to be mostly dispersed into the matrix, because of the proper selection of the stirring parameters such as stirring speed, stirring time and etc. The uniformly dispersed SiCp increased the dislocation density during the tensile test that is led to increase the strength of the material.
As cast Pure AZ61 Homogenized Pure AZ61
As cast AZ61 & 1%SiC Homogenized AZ61 & 1%SiC
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As cast AZ61 & 2%SiC Homogenized AZ61 & 2%SiC
Fig. 3 Microstructure images of as cast and homogenized Mg alloy MMCs
Fig. 4 Average grain size of magnesium alloy metal matrix composites
3.2 Mechanical properties 3.2.1 Micro hardness Vickers micro-hardness test was carried out across the cross section of as cast and homogenized samples and Fig. 5 shows the micro-hardness value of as cast and homogenized samples value. Generally, the micro-hardness value of the magnesium MMCs significantly
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higher compare to the pure magnesium alloyAZ61. With an increasing SiCp wt.% on AZ61/SiCp MMCs the micro-hardness value significantly increases. However, the weight percentage of the SiCp shows the significant difference between the micro-hardness value of adding 1%SiCp and 2%SiCp for the homogenized and as cast conditions. Hardness value for as cast pure AZ61 was 57.46, In addition, added 1wt.% of SiCp the hardness value 59.54 which increased by 3.49% and 2wt.% of SiCp were added hardness value 62.8 which is increased by 7.77%. Hardness value for homogenized pure AZ61, AZ61/1%SiCp and AZ61/2%SiCp were 56.94, 57.44 and 59.92 respectively. In addition, added 1 and 2 wt.% of SiCp in homogenized condition hardness value increased by 0.87% and 4.13% respectively.
Fig. 5 Microhardness value of as cast and homogenized AZ61/SiCp samples 3.2.2 Tensile test 3.2.2.1. Experimental Tensile tests were conducted to evaluated the influence of SiCp into the magnesium alloy AZ61 MMCs homogenized condition at room temperature. Fig. 6 shows the engineering stress vs. engineering strain curve for the pure AZ61 and AZ61 MMCs. Ultimate tensile strength (UTS), yield tensile strength (YTS) and elongation of the AZ61/SiCp composite are shown in Fig. 7-9 and Table 2. As expected, adding SiC particles has significantly increased the strength of MMCs. From the results of MMCs the UTS, YTS and elongation of the pure AZ61 are 156.61, 91.45 and 4.62 % respectively. As is evident, that the added 1% of SiC particles into the monolithic material have notable increment in UTS and YTS are 170.06 and 108.17 respectively and elongation decreased at 4.47%. However, the maximum value of UTS and YTS has been achieved at the adding 1wt.% of SiC particles metal matrix
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composites. The SiC particles uniformly distributed and interaction with the matrix dislocations is the major source of the increasing strength in MMCs. As it noticed in Fig. 4, the SiCp are distributed significantly in the matrix material. This helps to the SiC particles more effectively impede the movement of matrix dislocation and leads to higher strength. Furthermore, increment in SiC particle led to decrease the UTS, YTS and elongation are 166.64, 100.64 and 3.44 respectively. Because increasing reinforcement particles latter may be attributed to the agglomeration of SiC particles. On the other hand increasing SiCp into the Mg alloy AZ61 ductility were decreased. The major reason for lower ductility due to the brittle nature of the SiCp. Initiation of cracks are formed in the brittle particles and then propagate into the matrix and then leading to the final fracture it has been reported [17].
Fig. 6 Engineering stress-strain curves of the homogenized micro-SiCp composites
Fig. 7 Ultimate tensile strength of AZ61/SiCp MMCs with different weight percentage of micro-SiCp
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Fig. 8 Yield tensile strength of AZ61/SiCp MMCs with different weight percentage of micro- SiCp
Fig. 9 Elongation of AZ61/SiCp MMCs with different weight percentage of micro-SiCp
Table. 2. Mechanical behavior of AZ61 MMCs
Materials Ultimate stress 0.2% yield Stress Elongation (%) (MPa) (MPa) Pure AZ61 156.61 91.45 4.62
AZ61&1% SiC 170.06 108.17 4.47
AZ61 & 2% SiC 166.64 100.64 3.44
3.2.2.2. Finite element method analysis Finite element methods are the advanced technique for solving engineering designs. ANSYS AIM 18.0 with finite element software package were used for the numerical
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simulation of tensile test method. Process of finite element method the material properties and geometry creation of the specimen using the ANSYS AIM 18.0 workbench. While the material was linear behavior defined to use ANSYS Bilinear kinematic hardening model. In this study materials are the nonlinear behavior was simulated using ANSYS Multi-linear kinematic hardening material model. Geometry of the modal is dog bone shape as shown in Fig. 10 (a), were created and meshed by quadratic 3D elemental preprocessing hyper mesh as shown as Fig. 10 (b). Boundary conditions and material properties were applied for similar of the experimental boundary conditions [18].
Fig. 10 (a) Tensile meshed specimen and (b) Geometry of the specimen
3.2.2.3. Simulation tensile test results of Mg alloy AZ61 matrix composite Simulations of tensile test were conducted for homogenized Mg alloy MMCs using of finite element method under the conditions of experimental process. Fig. 11-13 presents the AZ61 magnesium alloy MMCs simulation results and comparison of experimental and numerical results of stress-strain curve and deformation. From the fallowing figures red color is the maximum strength to failure of the specimen, at that stage material should break. Finite element analysis is one of the mostly used tool for complex materials. In addition, the meshed element size refinement with 0.2 mm was maintained and nodes and elements are 144354 and 33734 respectively, for all type of the AZ61 MMCs specimens. Stress vs. strain curve plotted by using of ANSYS result data and comparison with experimental results. Obtained finite element tensile results are good agreement with the experimental results.
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Fig. 11 Simulation and experimental tensile test results of pure AZ61 (a) Equivalent Von- mises stress and Deformation, (b) Stress Strain curve
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Fig. 12 Simulation and experimental tensile test results of AZ61/1% SiCp (a) Equivalent Von-mises stress and Deformation, (b) Stress Strain curve
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Fig. 13 Simulation and experimental tensile test results of AZ61/2%SiCp (a) Equivalent Von- mises stress and Deformation, (b) Stress Strain curve
6. Conclusion In the present work, the mechanical behavior of magnesium alloy AZ61 with the reinforcement of different weight % (0, 1 and 2) SiC particles investigated by stir casting method. Experimental mechanical behavior results are conducted in homogenized (T4) heat treatment condition. Obtained tensile test results are compared with finite element method analysis. The major results of this investigation can be summarized as follows:
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1. As cast AZ61 MMCs microstructure are consists of large amount of β-Mg17Al12 as secondary phase. Secondary β-(Mg17Al12) phase particles dissolved into the matrix and it traces on the grain boundary and it can enhance ductility of the material during homogenization heat treatment.
2. Clearly observed from the micrograph that added SiCp have a significant effect on the grain refinement of magnesium alloy matrix composite. The average grain size of pure AZ61 was 56.42μm. When, added SiCp 1wt.% into pure AZ61 the grain size was 41.93μm corresponding to average grain size decreased by 28.68%. Further increment in SiCp 2wt.% into the AZ61 the grain size was 50.93μm. Which, grain size decreased by 9.7% compared to pure AZ61, but increased by 21.53% compared with AZ61/1% SiCp composite. The main reason for the grain refinement of the AZ61 alloy containing SiCp.
3. Increasing SiCp wt.% on AZ61/SiCp MMCs the micro-hardness value significantly increases. Micro hardness value of AZ61 MMCS in as cast condition is much high compare to the homogenized AZ61 MMCs. Because, during homogenization the brittle β-Mg17Al12 phase dissolved into the matrix.
4. Adding SiC particles has significant increment on UTS and YTS of AZ61 MMCs. UTS, YTS and elongation of the pure AZ61 are 156.61, 91.45 and 4.62% respectively. As is evident, that the added 1% of SiC particles into the monolithic material have notable increment in UTS and YTS are 170.06 and 108.17 respectively and elongation decreased at 4.47%. Because the SiC particles uniformly distributed and interaction with the matrix dislocations is the major source of the increasing strength in MMCs. Uniform distribution of SiCp interaction with the matrix dislocations is the major source of the increasing strength in MMCs. Furthermore, increment in SiC particle led to decrease the UTS, YTS and elongation are 166.64, 100.64 and 3.44 respectively. Because increasing reinforcement particles latter may be attributed to the agglomeration of SiC particles. 5. Finite element analysis tensile test results were good agreement with the experimental results. 6. Fracture surface of the Mg alloyAZ61 shows much more dimples with ductile fracture, when compared with the AZ61/SiCp MMCs. Mg alloy MMCs containing 1%SiCp and 2%SiCp, the dispersion of SiCp favors a granular flat surface with the cleavage fracture surface, which are indicates the brittle fracture of the materials.
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