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19719. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2018

Wear Behavior of SiC Reinforced AZ91 Magnesium Matrix Composites Fabricated by High Pressure Die Casting

Ali Serdar Vanlı, Bedri Onur Küçükyıldırım, Anıl Akdoğan

Yildiz Technical University, Faculty of Mechanical Engineering, Department of Mechanical Engineering, Materials Science and Manufacturing Engineering Division, Istanbul, Turkey

Abstract

This study investigates the wear behavior of AZ91 Magnesium (Mg) matrix composites reinforced with silicon carbide (SiC) fabricated by the infiltration of matrix material into a SiC containing ceramic preform by cold chamber high-pressure die casting method. Wear tests were conducted on a ball-on-disc wear testing device at room temperature under dry sliding conditions in accordance with ASTM G99 standard using Ø6 mm 100Cr6 ball under 5 N load within a sliding velocity of 60 mm/s with 500 m distance and 3 mm implementation radius. Specific wear rates were calculated using the disk wear loss determined from the wear track measurements. The microstructure and the morphology of the wear surfaces are studied by scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray spectroscope (EDS). Under the applied dry sliding conditions we improved the wear resistance of composites and reduced the specific wear rate down to 5.07x10-4 mm3/Nm by using high-pressure die casting process. Moreover, we achieved an incredible increase in the macro hardness (more than 140%) by the reinforcing properties of SiC particles in the preforms.

1. Introduction

Mg and its alloys have great interest in the industrial area as well as the scientific area regarding to their superior properties such as low density, good castability and high specific strength [1]. These properties have quite importance for components used in automotive and aerospace industry such as engine parts, bearings and brake rotors. However, their wear resistances are inadequate for specific applications listed above [1-3]; therefore, Mg and Mg alloy composite studies are carried out to improve the limited properties of these materials.The first Mg composite studies started at the 90’s, using SiC, Al2O3,and boron fibers and particles to enhance the mechanical and tribological properties including wear resistance [4-6]. It is noted that SiC particle reinforced Mg composites obtain higher wear resistance compared to Al2O3 reinforced ones [1].

Saravanan et. al. [6] fabricated SiC reinforced pure Mg composites by melt stir technique and observed that wear rates are decreased for two orders of magnitude compared to the pure Mg. In another study carried out

by Lim and co-workers [4], SiC reinforced AZ91 Mg alloys are produced by powder metallurgy using cold compaction of elemental powders, sintering in the vacuum environment and extruding them to the final sample shape. Wear tests in this study are carried out under different loading (10 N and 30 N) and sliding speed (0.2, 0.5, 1, 2 and 5 m/s) conditions. Results of the study showed us that under 10 N loading forces, SiC reinforced composites have high wear resistance in all sliding speed conditions; however, under 30 N loading forces SiC showed an efficacy just at moderate speeds (between 1 to 2 m/s).

Labib et. al. [7] have also studied dry tribological behavior of SiC reinforced pure Mg composites at room and elevated temperatures owing to the economical obtainability and good stability of SiC in Mg. They used powder metallurgy methods to fabricate composites in order of cold compacting of powders, hot pressing and hot extrusion and investigated the wear rates under room temperature (25°C) and elevated temperature (100, 150 and 200°C) conditions. Results of the study show that addition of 15 vol.% SiC increased the wear resistance at all testing conditions.

Increasing the particle reinforcement ratio without losing the strength of interface bonding between matrix and reinforcement is important to improve the properties of composites. Thus, some researchers use infiltration of molten matrix metal into a preform which contains reinforcements. Turan et. al [8], used this technique by pressure infiltration of molten AZ91 into B4C and SiC preforms to produce 50 vol.% B4Cand 50 vol.% SiC reinforced AZ91 Mg matrix composites, respectively; and observed that wear characteristics of SiC reinforced composites are better than B4C reinforced ones under two different loading (20 and 30 N) conditions. In this study, we preferred cold-chamber high-pressure die casting method for the fabrication of SiC reinforced AZ91 composites to improve wear resistance and carry out a new manufacturing method for the future of new industrial scale applications.

2. Experimental Procedure

Size of the required die casting machine is basically determined by the shot volume and the projection area of the casting product including the designed gating system considering the maximum pressure during the

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intensification phase. METAL PRES (Turkey), MP100 Type, cold chamber die casting machine is preferred for production line. And many types of co-equipment are used in the melting process of Mg alloys. The used melting resistance furnace unit is a stable crucible type electrical resistance furnace, MELTEC (Austria), MDF-200C. It is integrated to the production line for the experiments. The protection of molten Mg alloy with the protective gases (fluxless method) is preferred for the casting procedure. The used protection gas mixing unit PGM-3000 and dosing channel 1400-V3 type are produced by MELTEC GmbH. One of the most effective protective atmospheres in casting process of the Mg alloys is the mixture of fluorinated gases. Furthermore, the geometrical and mathematical design of the gating system calculated for the cavity. Die halves are made of EN X40CrMoV5-1 hot work tool steel and hardened 48 HRC. The die heating and cooling device, ISITAN, CH210-S Type, is used to have a homogeneous temperature distribution of the fixed and the moveable die halves [9].

SiC reinforced AZ91 Mg matrix composites are fabricated by using ceramic preform reinforcements with 10 ppi void ratio that includes sharp edged SiC particulates with the mean diameter of nearly 15 μm. The macrograph and micrograph of the preform sample is given in Figure 1(a-b), respectively. The manufactured casting die suitable for the snug fit of the preform and realized the fabrication of SiC/AZ91 Mg composites by high-pressure die casting process. Die temperature is arranged as 200°C and SiC preforms are heated up to 400°C in another electrical resistance furnace in order to provide the flow of liquid Mg alloy smoothly. The specific injection pressure of the molten Mg alloy is selected as 1200 bar according to the porosity measurements of the previous studies of the authors [10]. Melting bath temperature was set to 680°C and dosing channel temperature was preferred as 700°C. We used a mixture of %99.75 N2 + %0.25 SF6 (volume fraction) gasses as the protective atmosphere for the melting furnace and dosing channel and applied the gas into the furnace with 600 L/h flow rate. The samples are machined into a bar form (15x15 mm2 in cross section and 100 mm in length) by milling and then cut into pieces by sample cutting device for final porosity measurements. After porosity measurements we carried out using electronic analytical balance with 0.001 g precision, we mounted the SiC/AZ91 Mg samples into bakelite and prepared the surface by grinding and polishing and reached an average roughness (Ra) below 0.8 μm for the wear test according to ASTM G99 [11].

Roughness measurements were carried out with “Taylor Hobson Form Talysurf Intra 50” type profilometer and achieved data was processed using “TalySurf Intra” software [12]. 60 mm stylus arm length, 2 μm radius size of conisphere diamond stylus tip, 1 mm/s of speed with 1 mN force and 0.08 mm cut-

off value were used in the measurements of the contact stylus instrument. Owing to the high accuracy traverse datum, skidless measurements are processed [13]. We measured hardness of the samples by Brinell hardness testing method under 62.5 g load and 5 s dwelling time and results are given in Table 1. Wear tests are performed by Tribotechnic (France) ball-on-disk testing device at room temperature in accordance with ASTM G99 [11]. We realized the wear tests by using Ø6 mm 100Cr6 ball with parameters of 5 N load, 60 mm/s velocity and 500 m distance that ball covers it on the disk at 3 mm implementation radius. The volume of wear loss was calculated by determining the disk wear loss by measuring the wear track width by a stereomicroscope. Finally, the microstructure and the morphology of the wear surfaces on the samples were studied by Phenom XL scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscope (EDS).

Figure 1. SiC contained ceramic preform (a) macrograph and (b) micrograph (B&W print)

3. Results & Discussion

At the beginning of fabrication processes, AZ91 Mg matrix material is subjected to a phase analysis by using X-Ray Diffraction (Philips PANalytical - PW 3060/40 XRD) in order to confirm the phases of the material and the XRD spectrum is given in Figure 2. We determine -Mg ve -Mg17Al12 phases in the AZ91 structure, unsurprisingly. Then, the particle sizes of the SiC’s are measured by length measurements over SEM micrographs and an example of these measurements is shown in Figure 1. The average of the measured lengths demonstrates that the average particle size is around 13.6 μm. Firstly on the fabrication part, virgin AZ91 Mg alloy samples are manufactured by the high-pressure die casting method and the selected parameters are successfully verified by the gravimetric tests carried out using electronic analytical balance. Later on, SiC/AZ91 Mg composites are fabricated by the infiltration of molten matrix material into the ceramic preforms that placed into the die cavity. Composite fabrication process steps are visualized in the Figure 3. Gravimetric tests show us the success of the parameters for the composite manufacturing as well. The final porosity measurement results indicate that the fabrication is accomplished with the porosity ratio less than 3% (±1) and we can rate as industrially applicable due to the porosity values below 5% [14].

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Afterwards, we mounted the samples, grinded and polished the surfaces and achieved the Ra of 0.26 ±0.03 μm from all samples which is suitable for the wear test according to ASTM G99 [11].

Figure 2. XRD spectrum of AZ91 used as a matrix material

Figure 3. Composite fabrication process steps: (a) taking preform from the curing furnace, (b) transfer of preform to

the die, (c) placing the preform into the die cavity, (d) closed view of the die ready for casting, (e) transfer of the molten

matrix material (arrow indicates the molten AZ91 Mg alloy), (f) removal of the cast composite from the die, (g) rear and (h) front view of the composite as cast (arrows indicate the

reinforcements)

Hardness test results given in Table 1 shows us the virgin AZ91 alloy has approximately 69 HB of hardness and compatible with the high-pressure die casted AZ91 hardness results takes place in the literature (average 70 HB) [15, 16]. Composite hardness results given in Table 1 indicate that the average hardness is 208 HB with a high standard deviation value of 37 HB. Nevertheless, the minimum value of hardness (166.4 HB) has an incredible increase more than 140% and is quite higher than the literature results (125 HB with 20 vol. % SiC reinforcement) found out [17].

Specific wear rate results (Table 1) for virgin AZ91 and composite materials are determined by calculating the disk volume loss from the wear track by using the simplified equation for the disk volume loss given as:

𝑉 𝜋 𝑅 𝑤 𝑟 (1)

where R is the wear track radius, w the track width and r the ball radius is an approximate geometric relation and the result is correct to 1 % for w/R <0.3, and is correct to 5 % for w/R <0.8. Thus, we added on the correction of 5 % to the virgin matrix and 3 % to the composite for the w/R ratios of 0.39 and 0.28, respectively.

Table 1. Average Hardness and specific wear rate Sample Hardness

[HB]Specific Wear Rate

[mm3/Nm] Virgin Matrix 68.6 ±2.2 1.4233 x 10-3

Composite 208.2 ±37.3 0.5072 x 10-3

Specific wear rates specified by the disk volume loss method indicate that we achieved significantly less specific wear ratio of 0.507x10-3 mm3/Nm by the SiC/AZ91 Mg composites. We found from the literature that the maximum enhancement in the wear properties is achieved in the study of Labib et. al [7]. They used Mg matrix material with a specific wear rate of nearly 1.5x10-3 mm3/Nm and which is nearly the same as our matrix material with 1.42x10-3 mm3/Nm. After the reinforcement, Labib et. al. achieved specific wear ratio between 1.2 x10-3 – 1.6x10-3 mm3/Nm. It is clearly seen that by our reinforcement method, it is possible to achieve a better improvement and lower specific wear rate result compared to the previous study of Labib et. al. [7].

SEM micrographs given in Figure 4(a) show us the condition of the wear track on the virgin matrix material surface. Narrow and shallow wear scars indicate the abrasion formed by the hard and rigid abrasive pieces split by the friction of the ball and sample surface, penetrate and scratch/drag on the surface with the surface of the slightly flattened ball surface. The wear track mostly comprises abrasion wear scars and abrasion is known as the most frequent type of the wear type seen between metallic surfaces. The pieces break away from the sample is ordinarily removed from the sample surface just as chips. By this means, the mass loss method is quite applicable for the abrasion wear type which is mostly seen in our virgin matrix samples. Besides the abrasion, we discover dark and rugged surfaces seem to be covered by thin layer of fine particles in Figure 4(b, d). The EDS analysis shown in Figure 4(e) identified that strong oxygen (39.52 wt.%) and Mg (45.03 wt.%) peaks and the stoichiometric oxide concentration remark us the presence of high amount of MgO (70.97%). This feature points out the oxidative wear caused by the heat generation due to the friction between ball and sample surfaces during dry sliding process in the atmospheric condition. The oxide fragments showed up on the wear tracks indicate us the existence of the oxidative wear; however these regions are in low quantity so as to

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effect the volume change of the wear track. We can also observe another mechanism of plastic deformation on the wear track of virgin matrix material samples as a third kind of adhesive wear type (Figure 4(c)). Tips of the rough surfaces stick together during the sliding friction between metal-metal surfaces. By this means, the stress on this region achieve to the yield point occasionally. Thus, a fair amount of material slip in shear from the surface; however the stress cannot reach to the rupture point and the material is smear to the surface as a partial plastic deformation. This small quantity of adhesive wear seen in the wear track is not capable of material removal and may not change the volume of neither the wear track nor the sample.

Figure 4. SEM micrographs of virgin matrix material: (a) general view of the wear track mostly showing the abrasion,

(b) oxidative wear, (c) adhesive wear, (d) EDS target point on the oxide fragment, and (e) EDS spectrum and analysis result

including the stoichiometric oxide concentrations

Wear track SEM micrographs of SiC/Mg alloy composites are given in Figure 5 for both the low and high reinforced zones. It is firstly understood that both low and high reinforced zones have nearly the same wear characteristics. The matrix region of the composite come across the wearing line entirely has the abrasion scars as we determined on the surfaces of virgin matrix materials. Abrasion scars are observed easily where the reinforcement ratio is low (Figure 5(a)). We expected to see less abrasion scars over the highly reinforced regions owing to the high hardness and rigidity of SiC reinforcements (Figure 5(b)). In fact, the presence of the hard particles has a wear effect on the abrasive balls nearly in the amount of 1.5 x 10-5

mm3/Nm specific wear rate. It is obviously seen from the SEM image (Figure 5(c)) taken by greater magnification (2400x) than other images, abrasion on the high reinforced regions are rare and shallow. The slight increase on the sliding surface of the ball owing to the abrasion give rise to the reinforcements reduce

the penetration of the ball and save the matrix material from the wearing effect.

Figure 5. SEM micrographs of SiC/AZ91 Mg composite: wear track from (a) low reinforced region, (b) high reinforced region, and (c) scars of matrix in the high reinforced region

with a greater magnification (2400x)

4. Conclusions

a. We fabricated SiC/AZ91 Mg composites by the infiltration of AZ91 Mg matrix material into a SiC containing preform successfully using cold-chamber high-pressure die casting method for the first time and achieved porosity level less than 3% for both the virgin matrix materials and the composites.

b. We achieved an improvement more than 140 % on the macro hardness of fabricated composites with the help of reinforcing characteristics of SiC particles preforms.

c. The presences of SiC particles not only improve the wear resistance by their hardness and rigidity but also they enclose the matrix in high reinforced regions and prevent the ball penetration to the matrix geometrically. By this means, our composites exhibit better wear resistance (nearly 64% enhancement) and we achieve a remarkable amount of reduction in the specific wear rates.

d. The dominant wear mechanism is abrasion under the applied dry sliding wear conditions. However, we observe oxidative and adhesive wear types fragmentary. The amount of oxidation and the plastic deformation seen in the wear tracks are too few to effect the volume change and the wear rates.

Acknowledgements

This work was supported by the Yildiz Technical University, Coordination of the Scientific Research Projects 2014-06-01-GEP03 and 2015-06-01-GEP02.

References

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[3] P. Poddar, V.C. Srivastava, P.K. De, K.L. Sahoo, Processing and mechanical properties of SiC reinforced cast magnesium matrix composites by stir casting process, Mat Sci Eng a-Struct, 460 (2007) 357-364. [4] C.Y.H. Lim, S.C. Lim, A. Gupta, Wear behaviour of SiCp-reinforced magnesium matrix composites, Wear, 255 (2003) 629-637. [5] V.K. Lindroos, M.J. Talvitie, Recent Advances in Metal-Matrix Composites, J Mater Process Tech, 53 (1995) 273-284.[6] R.A. Saravanan, M.K. Surappa, Fabrication and characterisation of pure magnesium-30 vol.% SiCP particle composite, Mat Sci Eng a-Struct, 276 (2000) 108-116. [7] F. Labib, H.M. Ghasemi, R. Mahmudi, Dry tribological behavior of Mg/SiCp composites at room and elevated temperatures, Wear, 348–349 (2016) 69-79. [8] M.E.Z. Turan, H. ;Cevik, E. ; Sun, Y.; Turen, Y. ; Ahlatci, H., Wear Behaviors of B4C and SiC Particle Reinforced AZ91 Magnesium Matrix Metal Composites, International Journal of Chemical, Molecular, Nuclear, Materials and Metallurgical Engineering, 10 (2016) 1160-1163.[9] A.S. Vanli, Optimization of Process Parameters Effective on the Product Quality in Die Casting of Magnesium Alloys, Mechanical Engineering, PhD Thesis, Yildiz Technical University, 2013, Istanbul, Turkey.

[10] A.S. Vanli, A. Akdogan, H. Sonmez, Mechanical and Morphological Properties of Cold Chamber High Pressure Die Casting AZ91 Magnesium Alloy Products, Academic Journal of Science, 3 (2014) 113-119. [11] ASTM G99-05: Standart Test Method for Wear Testing with a Pin-on-Disk Apparatus, 2005. [12] T.G. Mathia, P. Pawlus, M. Wieczorowski, Recent trends in surface metrology, Wear, 271 (2011) 494-508. [13] ISO 3274: Geometrical Product Specifications (GPS)-Surface texture: Profile method-Nominal characteristics of contact (stylus) instruments, 1996. [14] A.S. Vanli, A. Akdogan, M.N. Durakbasa, Integrated Die Casting Manufacturing System for Sustainable High Quality of Magnesium Alloy Products, QIEM'16 Proceedings Book, Romania, 2016, pp. 43-46. [15] K.N. Braszczy, M. ska, I. Zawadzki, W. Walczak, J. Braszczy, ski, Mechanical properties of high-pressure die-casting AZ91 magnesium alloy, Archives of Foundry Engineering, 8 (2008) 15-18. [16] A.F. Society, Magnesium Alloys, Casting Source Directory, Engineered Casting Solutions, 2006, pp. 41-43. [17] A. Kandil, Microstructure and Mechanical Properties of SiCp/AZ91 Magnesium Matrix Composites Processed by Stir Casting, Journal of Engineering Sciences, 40 (2012) 255-270.