Dry sliding wear behaviour of thixoformed Al-5.7Si–2Cu-0.3 Mg alloys at high temperatures using Taguchi method

Dry sliding wear behaviour of thixoformed Al-5.7Si–2Cu-0.3 Mg alloys at high temperatures using Taguchi method

Journal Pre-proof Dry sliding wear behaviour of thixoformed Al-5.7Si–2Cu-0.3�Mg alloys at high temperatures using Taguchi method M.A. Abdelgnei, M.Z. ...

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Journal Pre-proof Dry sliding wear behaviour of thixoformed Al-5.7Si–2Cu-0.3�Mg alloys at high temperatures using Taguchi method M.A. Abdelgnei, M.Z. Omar, M.J. Ghazali, M.N. Mohammed, B. Rashid PII:

S0043-1648(19)30589-7

DOI:

https://doi.org/10.1016/j.wear.2019.203134

Reference:

WEA 203134

To appear in:

Wear

Received Date: 4 April 2019 Revised Date:

1 November 2019

Accepted Date: 12 November 2019

Please cite this article as: M.A. Abdelgnei, M.Z. Omar, M.J. Ghazali, M.N. Mohammed, B. Rashid, Dry sliding wear behaviour of thixoformed Al-5.7Si–2Cu-0.3�Mg alloys at high temperatures using Taguchi method, Wear (2019), doi: https://doi.org/10.1016/j.wear.2019.203134. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.

Dry Sliding Wear Behaviour of Thixoformed Al-5.7Si-2Cu-0.3Mg Alloys at High Temperatures Using Taguchi Method M. A. Abdelgnei a,b, *, M. Z. Omar a, M.J. Ghazali a,M. N. Mohammedc and B. Rashidd a

Centre for Materials Engineering and Smart Manufacturing (MERCU), Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM, 43600 Bangi, Selangor, Malaysia

b

Department of Mechanical Engineering, Faculty of Mechanical Engineering, Omar Almukhtar University, +218 Albaida, Libya

c

Department of Engineering & Technology, Faculty of Information Sciences and Engineering, Management & Science University, 40100 Shah Alam, Selangor, Malaysia d

Institute of Technology, Middle Technical University, 29008 Alzafaranya, Baghdad, Iraq, * Corresponding author: [email protected]

Abstract Present work focused on the impact of high temperature on the tribological properties for thixoformed Al-5.7Si-2Cu-0.3Mg alloy. A pin-on-disc tribometer was used for dry sliding wear experiments that were executed according to Taguchi method. Wear behaviour was investigated under four process parameters at three levels: process (as-cast, thixoforming, and thixoformingT6), load (10, 50, and 100N), sliding temperature (25, 150, and 300°C), and sliding distance (1000, 3000, and 5000m). Microstructure was characterized and evaluated by optical microscopy, scanning electron microscopy with energy dispersive X-ray detector, and X-ray 1

diffraction. Results revealed that the thixoformed-T6 alloy consisted of fine globular structure and equiaxed grains, which showed excellent wear resistance at elevated temperature. The experimental values, which were within the permissible limits for the volume loss and coefficient of friction, were congruent to the predicted values because both performed at the rates of 96% and 89%, respectively. Despite increasing wear resistance of the thixoformed-T6 alloy at a sliding temperature of 300°C, applied load of 100N, and sliding distance of 3000m, the best coefficient of friction was attained by the thixoformed-T6 alloy at a sliding temperature of 150°C, applied load of 100N, and sliding distance of 5000m. The dominant wear mechanisms in tribological tests included abrasion, adhesion, and minor delamination.

Keywords: Al-Si-Cu alloy; thixoforming; high temperature; microstructure; tribological 1.

Introduction

The global environmental efforts at recycling have recently motivated researchers to investigate new alternative materials [1]. Given that the automotive engineering industry seeks to improve engine efficiency and fuel economy, new elements are being applied to reduce engine size and enhance mechanical and physical characterised of their structures. Over the past decades, Al-Si alloys have been applied widely in foundries for aerospace and general engineering industries [2–4]. At present, Al–Si–Cu alloys have different uses in the automotive industry, such as for clutches, pistons, and piston rings, because of their excellent fluidity and mechanical strength [5,6]. Despite such advances in this research, the tribological performance of thixoformed Al-Si alloys has not been comprehensively explored. The wear properties of Al–Si alloys formed by conventional casting apparently deteriorate compared with those of ferrous materials. The conventional cast Al-Si alloys usually have 2

casting defects such as porosity and inclusion that can degrade the mechanical properties of material

[7]. These alloys required certain tribological improvements to overcome these

drawbacks of conventional casting. Modification methods include heat treatment and coating application. Recent works have demonstrated that refinement in the size of the primary Si particles improved the mechanical properties and wear resistance [8,9]. However, semi-solid metal (SSM) process is markedly superior to conventional casting because of its lower solidification shrinkage, reduced porosity, extended die life, proper filling of the die, and improved mechanical properties [10]. Furthermore, two technical casting methods are improved for producing semisolid components [11]. The first method is thixoforming process, which is typically divided into three stages: (1) preparation of a semi-solid billet into appropriately-sized billets, (2) partial re-melting, and (3) forming of the semi-solid billets into their final shape [12]. The second method is rheocasting process, which involves shearing the molten alloy in the time of solidification to get a non-dendritic slurry, which can pour straight inside a die [11]. The main focus of the SSM method is the preparation of a slurry with an optimal thixotropic microstructure, fine non-dendritic grains as well as a convenient volume fraction of spherical solid grains that are evenly dispersed within the liquid matrix [13]. Numerous methods, such as magnetohydrodynamic (MHD) stirring [14], stir casting [15], ultrasonic vibration [16], recrystallization and partial melting (RAP) [17], strain-induced melt activation (SIMA) [18], also cooling slope (CS) method [19], were used to obtain a suitable non-dendritic microstructure. Among the preparation methods of feedstock material for thixoforming process, cooling slope casting is the most cost-efficient approach owing to its simplicity and low equipment requirement [5,19]. This process produces a non-dendritic microstructure by shearing the molten metal during solidification before pouring it directly into the die [20].

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Although recent works have shown that microstructural refinement through SSM process is beneficial to the mechanical properties of alloys, however, just a few researches have investigated the tribological characteristics of thixoformed and rheocast Al–Si [11,21]. Hence, the present study aimed at evaluating the influence of an improved microstructure by thixoforming on the dry sliding behaviour of Al–5.7Si–2Cu–0.3Mg alloy at high temperature. Taguchi method (the design of the experiment) also analysis of variance (ANOVA) were conducted to estimate the wear and friction results. 2.

Materials and methods

2.1. Materials Selection The base material selected for this process was Al–5.7Si–2Cu–0.3Mg alloy. Table 1 lists the chemical composition of the base alloy, which was obtained by X-ray fluorescence (XRF) technique. Differential scanning calorimetry (DSC) was utilised to determine each of solidus and liquidus temperatures besides the liquid fraction in the semi-solid range. Conventional cast sample was cut to small cylindrical shape (Ø4mm× 3mm) to test by using a Netzsch-STA (TGDSC) 449 F3 simultaneous thermogravimeter. A heating rate of 10 °C/min was utilized in a nitrogen atmosphere to prevent oxidation. As-cast, thixoformed, and thixoformed-T6 (thixoformed sample subjected to T6 heat treatment) samples were used in this study. Table 1 Chemical composition of studied aluminum alloy (wt.%) Alloy

Si 5.67

Fe 0.32

Cu 1.92

Mg 0.32

Mn 0.12

Zn 0.03

2.2. Cooling slope (CS) casting process

4

Ni 0.02

S 0.16

Cl 0.12

Cr 0.05

Ca 0.04

Al Bal.

CS method was utilised to obtain a globular microstructure. Al–5.7Si–2Cu–0.3Mg alloy was melted into a SiC crucible with dimensions of Ø100 mm × 250 mm by placing this material in a laboratory electric resistance furnace. The molten alloy was superheated at 700 °C with degassing by passing argon gas through graphite lance for 15 min for decreasing the porosity in the CS sample. Then the molten alloy was cooled to the chosen pouring temperature of 650 °C prior to being poured on the sloped plate surface, which was covered by a thin layer of boron nitride (BN) to inhibit adhesion of the molten metal. The CS plate was 400 mm in length with a tilt angle of 60° and was water-cooled underneath to produce a globular microstructure. The cold plate increased the nucleation rate of α-Al solid particles. Next, the poured molten metal was collected inside cylindrical stainless-steel moulds that were preheated to 100 °C before being cooled to room temperature. 2.3. Thixoforming Process The cylindrical billet (Ø25 mm × 110 mm) produced by the CS process was placed on the ram into induction coil with dimensions Ø 100 × 200 mm and an average frequency (30–80 kHz, 35 kW), which was placed below the die to heat these bars to the semi-solid state in situ. The slug was heated quickly at the rate of ~130 °C/min to prevent unfavourable grain growth. Moreover, when the slugs achieved the targeted temperature of 575 °C (with the liquid fraction at approximately 50%), this temperature was maintained for the spherodisation of grains. The billet sample temperature was determined by a K-type thermocouple that was placed into a drilled hole in the centre of the sample at a distance of 2 and 6 mm from the outer surface. The thermocouple was quickly removed from the billet immediately before the semi-solid ingot was formed in the die by using a laboratory hydraulic press, which provided 20 kN load at speed of 85 mm/s. Furthermore, the applied pressure was maintained until ~15 s after filling. Cartridge heaters 5

preheated the die to 300 °C. A silicon spray lubricant was used to decrease the adhesion between the ingot material and the die. The thixoformed sample was then cut into a piece measuring Ø25 mm × 15 mm to examine the microstructure and measure the hardness. 2.4. Heat Treatment Several thixoformed alloy samples were subjected to the optimum T6 heat treatment. The solution heat treatment carried out for 12 h at 485 °C, quenched by warm water (60 °C for 30 s) to avoid early precipitation, afterward aged for 10 h at 190 °C. 2.5. Characterization of the microstructure All samples were horizontally cut from the middle of all billets for microstructure studies. The samples were then ground and polished as stated in the ASTM-E407-2002 standard (ground with 400–1200 grinding SiC paper, polished with diamond paste at 6, 3, and 1 µm until a mirror-like surface was achieved). These samples were subsequently etched with Keller’s reagent (1 mL of hydrofluoric acid, 1.5 mL of hydrochloric acid, and 2.5 mL of nitric acid in 95 mL of distilled water) for ~10 s. Optical microscopy using an Olympus optical microscope was used to observe the microstructures of the conventional cast, thixoformed, and thixoformed-T6 alloys. Different phases of samples were determined by scanning electron microscopy (SEM). Additionally, X-ray diffraction (XRD) analysis was applied to illustrate the different phases of the samples. X-ray diffraction (XRD) analysis of samples were recorded using a Bruker D8 Advance diffractometer using CuKα radiation, at wavelength of 0.15406 nm and equipped with a Lynx-Eye Linear Position Sensitive Detector. The surface roughness of samples used for XRD were approximately 0.3µm. A search/match procedure was performed using Bruker DIFFRAC.EVA

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software version 4 interfaced with a diffraction pattern database to calculate the Vol.% of phases from XRD. The closest match that covered most of the peaks was chosen for further analysis. 2.6. Macrohardness Test The as-cast, thixoformed, and thixoformed-T6 samples were measured on an HRB scale with a ball diameter of 1/16 inch on a diamond indenter and a load of 100 kg for 30 s and using a digital Rockwell hardness testing machine. The average of at least 10 measurements was considered the macrohardness value. 2.7. Wear Test Sample In the experiment shown in Fig. 1, dry sliding wear behaviour was evaluated using a pin-on-disc apparatus (DUCOM, model: TR-20LE) in accordance to ASTM G99 [22]. Wear samples were machined by wire cutting into cylindrical shapes with dimensions of Ø8 mm × 20 mm. One sides of the cylindrical specimen were ground to a roughness of surface of approximately ~0.1µm. Roughness was determined by a Perthometer tester with a contour and roughness measuring system. M2 tool steel with a hardness of 62 HRC was used as a countersurface with dimensions of Ø160 mm × 10 mm. The pin and disc were cleaned ultrasonically by using acetone and were dried before the test. The diameter of the rounded track traversed by the pin was fixed at 80 mm for all tests. The environment inside the laboratory was safeguarded at a constant temperature of 25 °C and humidity of ~58% as the dry sliding conditions. The loss weight of the pin material was measured prior (W1) and after (W2) the wear test using a digital weight balance (B154-S Mettler Toledo, Switzerland) to accuracy of up to 0.0001g. The weight loss was then confirmed and convert to volume loss. Each experiment was repeated for at

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least three times. Volume loss (VL) and specific wear rate (SWR) were computed via Eq. (1) and Eq. (2), respectively [23].

VL=∆W/ρ

(1)

SWR=VL/(N×S)

(2)

Where VL is the volume loss (mm3); SWR, the specific wear rate (mm3/N m); ∆w, the weight loss = (W1-W2); ρ, the density (g/cm3); N, the applied load (N); and S, the sliding distance (m). During wear experiment, friction and wear exhibited constantly tangential friction force. The friction force value was listed via the weight sensor of the pin-on-disc tribometer for determining the (COF, µ). The subsurface of the specimens was prepared by longitudinal cuts parallel to the axial direction of the wear sample. However, the surface, subsurface, and debris were characterized by using a SEM-EDX microscope and XRD analysis.

Fig. 1. Schematic diagram of a pin-on-disc apparatus.

2.8. Design of Experiment (DOE) 8

The experimental design was established to determine the impact of incorporating control factors on the results of the method [1]. Taguchi method is an important statistical approach that is applied to exhibit the influence of different factors on multiple levels. This method includes a series of experiments to obtain the ideal parameters aimed at low wear and friction features. In Taguchi method, the output is estimated for all collections of the examined levels of factor. This method is appropriate to examine input variables via determining the significant factors, which affects the reaction. Furthermore, the examination might similarly yield the best set of these factors [24]. Taguchi method used to reduce the number of experiments by using orthogonal array design [25]. Three samples were applied for each condition to calculate the main value and obtain reliable wear results. In this study, the experiments were designed on the basis of the L27 orthogonal array (OA) system, that is, 34 = 27 runs, to survey whole factors at each level. Dry sliding wear tests were conducted with four factors at three varying levels (shown in Table 2). The Minitab16 software used to analyse volume loss (VL) outcomes for various groups among the factors: process (as-cast, thixoforming, and thixoforming-T6), load (10, 50, and 100 N), under applied pressures of 0.19, 0.99 and 1.98 MPa respectively (taking into account the contact area of 50.26 mm2), sliding distance (1000, 3000, and 5000 m), and elevated temperature (25 °C, 150 °C, and 300 °C) at a sliding speed of 1 m/s. These factors were significant and separate characteristics of the wear behaviour that affected the performance of the alloy. Further, the Taguchi method enabled the response (VL) to be investigated utilising the conceptual procedure, which included plotting each of the main interaction effects and identifying the main significant factors. Table 2 9

Control factors with their specific values at three levels Factors Process Load (N) Sliding distance (m) Temperature (°C)

Factors Designation A B C D

Level 1 As-cast 10 1000 25

Level 2 Thixoforming 50 3000 150

Level 3 Thixoforming-T6 100 5000 300

Taguchi method has three categories of the S/N ratio, (the lower-the-better, the higher-the-better and the nominal-the-better). In the present study, a ‘lower-the-better’ performance feature was taken, since the goal is to reduce the volume loss the coefficient of friction Eq.(4) [26].

= −10

1

.

(4)

Where ‘i’ is the serial number of a trial; ‘Yij’ is the measured value of quality characteristic for the ith trial and jth experiment; ‘n’ is the number of repetitions for the experimental combination.

Statistical ANOVA used to investigate the significance variables of process over the yield characteristics. The optimum state of the experiment was determined as guided by S/N ratio and ANOVA analysis. Eventually, an acceptance experiment was completed to confirm the optimal treatment of process condition obtained from the design parameters. 3.

Results and discussions

3.1. Microstructure analysis Fig. 2 observes the difference of liquid fraction versus temperature got from the DSC curve of Al-5.7Si-2Cu-0.3Mg alloy. It showed that both the solidus temperatures and the liquidus temperature of the examined alloy are 510 °C and 635 °C, respectively. Fig. 3 displays the microstructure of as-cast and rheocast alloys. The bright phase primarily consisted of the primary 10

α-Al phase, and the black phases surrounding the α-Al phase were the eutectic phase. The primary α-Al phase morphology was a dendritic shape in the conventionally cast alloy (Fig. 3a). However, the dendritic primary α-Al phase morphology transformed into almost globular and rosette like (Fig. 3b). The change in morphology was owing to the continuous shear of the molten metal along the sloped plate causing the dendritic arms in the melt slurry to break and form rosette-like structures [5,27]. 2.5

100 90 80

2.0

) % ( o i t c 50a r f d 40i u q i L 70

60n

1.5

n o i t c a s n a r t c i t c e t u E

) W m ( w o l F t a e H

1.0

30

n o i t c a r F d i u Cq Si DL

0.5

20 10 0

0.0 520

540

560

580

) C o ( e r u t a r e p m e T

500

600

620

Fig. 2. DSC and liquid fraction vs. temperature curves for Al-5.7Si-2Cu-0.3Mg alloy.

Fig. 3. Optical micrographs of the: (a) as-cast, and (b) rheocast Al-5.7Si-2Cu-0.3Mg alloys. Fig. 4 present the optical micrographs and SEM images of the thixoformed and thixoformed-T6 Al–5.7Si–2Cu–0.3Mg alloys. The structure of the thixoformed sample is mainly composed of fine globular primary α-Al phase with homogeneous distribution of eutectic phase as well as a

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few porosities Fig. 4 (a,c). It was noted that some numbers of eutectics seemed as black dots, these were entrapped liquid into the globular morphology of the thixoformed alloy (Fig. 4a). In addition, it was clear that, there is small different in the microstructure of thixoformed after being subjected to T6 heat treatment as shown in Fig. 4.

Fig. 4. Optical and SEM micrographs of (a) thixoformed and (b) thixoformed-T6 Al-5.7Si2Cu-0.3Mg alloys; SEM images showing (c) thixoformed and (d) thixoformed-T6 Al-5.7Si2Cu-0.3Mg alloys.

Fig. 5 and Fig. 6 present the SEM-EDX micrographs of thixoformed alloy and thixoformed-T6 alloy, respectively. The morphology of eutectic Si particles in the thixoformed alloys were less spherodisation (Fig. 5) compared with the thixoformed-T6 alloys (Fig. 6). The thixoformed-T6 alloys observed a homogeneous distributions of fine primary Si particles due to their nucleation and growth behaviour during solidification. These uniform distributions of primary Si particles are appropriate for improving the mechanical properties and wear resistance of alloy [28].

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The EDX analyses showing the presence of Si, Cu, Mg and Fe elements as small amount in the Al matrix as clear in SEM and EDX patterns. These small number of elements concentrated clearly at the grain boundaries of the eutectic phase. It was also proven that a needle-like shape of particles in Fig. 5 and Fig. 6 were Cu phase and Fe intermetallic compound, respectively. The Cu phase was more equally diffused at the grain boundaries between the globules that lastly can develop the mechanical properties of the alloys. On the other hand, the needle-like i,e. Fe intermetallics that occurred in the eutectic Si decreased the mechanical properties of the alloys [29].

Fig. 5. SEM−EDX of (a) thixoformed Al-5.7Si-2Cu-0.3Mg alloys.

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Fig. 6. SEM−EDX of thixoformed-T6 Al-5.7Si-2Cu-0.3Mg alloys.

Fig. 7 displays the peaks of the β-Al5FeSi, Al5Cu2Mg8Si5, and Al2Cu phases as well as Al and Si in the thixoformed and thixoformed-T6 alloys. This was due to positive response of the thixoformed sample toward the T6 heat treatment [2]. However, the Al15(Mn-Fe)3Si2 intermetallic phase did not appear in the XRD analysis because of low content of Mn (0.12%) used in examined alloy. The XRD graph confirmed that, Si and Al2Cu peaks in thixoformed-T6 sample were higher than thixoformed sample, reflecting the precipitation of this phase during the T6 heat treatment. A small increase in the intensity of the Si and Al2Cu peaks in the thixoformed samples compared to the as-cast Al-5.7Si-2Cu-0.5Mg samples was owing to the thixoforming process. This increase of intensity of Al2Cu phase in thixoformed-T6 samples led to the raise in the result of the precipitate-hardening phase, strength of alloy.

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Al Si

Intensity (AU)

c

Al2Cu Al5Cu2Mg8Si5 βAl5FeS

c c c c

Thixoformed-T6 Thixoformed As-cast

20

30

40 50 2Theta (degree)

60

70

80

Fig. 7 XRD pattern of as-cast, thixoformed, and thixoformed-T6 of Al-5.7Si-2Cu-0.3Mg alloy.

The precipitation order for the Al2Cu in Al–5.7Si–2Cu–0.3Mg alloy is explained as follows; solid solution (αSS)→ GP zones → θ′′→ θ′ → θ (Al2Cu). The order starts with disintegration of solid solution forming disk shape Guinier-Preston (GP) zones as clusters, which contained a high fraction of solute Cu atoms. When the aging temperature increases, the GP zones dissolve and substituted as metastable precipitates θ′′ that may be coherent or semi-coherent with the α-Al matrix in binary Al–Cu alloys [19]. High coherency degree causes comprehensive coherency–strain, conferring the highest strength to the alloy [30]. When the aging continue, θ′′ begins to dissolve, and θ′ starts to form via nucleating on disruption and/or cell walls [31]. θ′ shows a plate-like shape and was formed from the Al and Cu particles, which loses coherency with the matrix during growth [32]. Continuous aging temperature for 10 hours at 190°C yields increasing precipitate small fine homogeneous θ (Al2Cu) in the alloy. Hence, the hardness value of thixoformed-T6 were higher as compared to those in the thixoformed alloy. 15

3.2. Macrohardness Measurement The macrohardness of as-cast, thixoformed and thixoformed-T6 alloys are presented in Table 3. In this research, the hardness of Al-5.7Si-2Cu-0.3Mg alloy was measured by using a macrohardness test instead a microhardness test because there are many intermetallic phases that would make the average of the microhardness values inaccurate. It can be noted from Table 3 that the values of hardness in the thixoformed-T6 is greater than the as-cast and thixoformed alloys. As the microstructure improved, exhibiting increasingly fine grain size of α-Al, fine distribution of Si and intermetallic phases and a reduced porosity (Fig. 3 and Fig. 4). It is known that the wear volume of materials decreases with the increase in hardness [33,34].This improvement was due to high the pressures that were used in thixoforming. Furthermore, the increased measurements are attributed to precipitation of the Al2Cu intermetallic phase between the α-Al globules through the alloy Fig. 6. Table 3 Measured macrohardness values of tested alloys Process As-cast Thixoformed Thixoformed-T6

Hardness (HRB) 39.79±2.3 74.02±3.1 80.35±3.5

3.3. Volume Loss (VL) The wear resistance distinctly increases when thixoforming is applied to the Al-Si alloy [11]. Thus, this research purposes to determine the substantial factors and their combination, which affect the process of achieving the lowest wear rate and COF. Increasing wear loss is usual with normal applied load, and this is in agreement with a similar way in other researches [11,35,36]. 16

Fig. 8 shows that the minimum VL was achieved for the following factors combination: thixoforming-T6 process, applied load of 100N, sliding distance of 3000m, and sliding temperature of 300°C. This contact areas of the thixoformed-T6 sample and disc with high temperature increase the interface coherence of mixed material of both pin and disc materials, which attributed to decline the volume loss of sample due to the high hardness of mixed metal on the surface. Additionally, there was less porosity relative to the as-cast alloy under the same conditions. Consequently, the development in the microstructure of the thixoformed-T6 Al5.7Si-2Cu-0.3Mg alloy was attributed to the improvement in the wear resistance. These developments provided reasonable evidence of the minimum VL value of the thixoformed-T6 alloy compared to the thixoformed and conventional cast alloys. The results showed that the VL values ranged from 2.9 mm3 to 1.8 mm3 for the thixoformed-T6 alloy, from 7.1 mm3 to 5.7 mm3 for the thixoformed alloy, and from 8.9 mm3 to 7.2 mm3 for the as-cast alloy. It was clear that the wear resistance of the thixoformed and thixoformed-T6 alloys had improved by 20% and 67%, respectively. These results showed that changes in the microstructure formed by different processing methods had affected the wear resistance. In addition, T6 heat treatment had improved the wear resistance of as-cast Al-5.7Si-2Cu-0.3Mg alloy due to the precipitation of Al2Cu, while also has transformed the morphology of acicular Si particles to homogenous sphere particles. This improvement in microstructure had attributed to the increase in the hardness of the as-cast-T6 to approximately 8% higher than as-cast alloy as reported by the authors’ previous work [37]. Hekmat-Ardakan et al. 2010 [38] reported that heat treatment positively affected Al-Si alloys. It is known that wear rate of metal at high temperatures declines due to the oxides layer (oxidative wear) plastic deformation and delamination wear (Fig. 9), which formed on surface. In 17

addition, oxide layer does not always have a negative effect. Its performance depends on its composition, cohesion and adhesion. In this work, the reduction in wear rate was usually associated with the development of oxide layers produced by frictional heating and therefore, prohibiting metal-to-metal contact. This complex oxide layers covered potentially high adhesion forces between contacting solids and significantly influences friction and wear of alloys.

This phenomenon is known an oxidative wear. So, the shortest sliding distance (1000m) at room temperature observed higher volume loss compared with 3000m and 5000m, which gave effect together 150 °C and 300 °C respectively, as shown in Fig. 8b. The analysis of variance (ANOVA) was showed the thixoformed-T6 process was the greatest contribution to the VL results. Further explanation is shown in Appendix A.

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Fig. 8. (a) Interaction plots for volume loss (VL) and (b) Main effects plot for volume loss (VL).

Fig. 9. SEM image of stable transferred material formed at the surface of thixoformed-T6 sample under dry condition at 50N, 1000m and 300°C. 19

3.4. Specific Wear Rate (SWR) Specific wear rate (SWR) is typically recognized as wear resistance. Fig. 10 shows the SWR for the as-cast, thixoformed and thixoformed-T6 samples as a function of the applied load, pin temperature, and sliding distance. For all the samples, the SWR for all sample increased with increasing temperature, applied load as well as sliding distance respectively. This finding was due to the reinforcement of the transferred layers from the samples and the counter surface material. Thus, largely compensating for the weight loss at the surface of the worn sample (Fig. 9). This stable re-adhered friction layer improved the wear resistance at a high load and temperature. In addition, the thixoformed-T6 alloy showed the highest SWR value among all samples under different conditions. The reason for the increase in the SWR was due to the collection of large debris that later could further broken-up during sliding. These small particles can be easily oxidised, hated-up, and later sintered, covering the worn surface [39]. Meanwhile, the thixoformed alloy showed an intermediate SWR value. 120 As-cast

Specific Wear rate ꓫ10-3

100 80

Thixoformed Thixoformed-T6

60 40 20 0 1000m

1000m

1000m

3000m

3000m

3000m

5000m

5000m

5000m

10N

10N

10N

50N

50N

50N

100N

100N

100N

25°C 25°C 150°C 150°C 150°C 300°C 300°C A Sliding Distance (m), Applied load (N) and Temperature (ᵒC)

300°C

25°C

Fig. 10. SWR for different process as a function of sliding distance, loads and temperatures. 20

3.5. Coefficient of Friction (COF) Fig. 11 shows the coefficient of friction values for the as-cast, thixoformed, and thixoformed-T6 of Al–5.7Si–2Cu–0.3Mg alloys at the applied load of 50 N, sliding distance of up to 5000m and the temperature of 25 °C. All the COF graphs include two stages as shown in Fig. 11 called running-in period and steady-state stages. In the first stage, the initial COF of the alloys observed a rapid increase. The reason for higher COF in the running-in period is that the static friction is higher than the dynamic friction [40]. After the sliding distance of 100m, it decreased rapidly until the sliding distance of about 1000m, and then became steady. The reason for the decreasing COF were due to surface of the alloy becoming smoother and harder as a result of continue sliding. Later, the more steady fluctuation of the COF that occurred may be related to adhesion, oxidational wear, accumulation of debris in interface of pin and disc, which reduce the direct contact between the surfaces of the sample and countersurface, hence may further lower the COF. Typically, the higher the volume of debris in the contact the higher the friction. However, at longer sliding distance, all materials show almost similar values of friction coefficient. Furthermore, the friction coefficient at longer sliding distance appeared to have fluctuated around 0.2-0.4 range, which possibly associated with the entrapment of a large amount of wear debris between the contacts. This may be attributed to the presence of debris that was rolled into the interface of the sample. Fig. 12 shows an example of rolled debris on the worn surface. A similar finding was reported by Alhawari 2015 [11] when they examined the dry sliding wear behaviour of as-cast and thixoformed A319 aluminum alloys. The initial COF of the as-cast alloy at 25 °C was close to ~0.47, and a COF of ~0.34 was achieved at the end of testing.

21

The main effects and interaction plot for the COF of the control factors at all levels are shown in Fig. 13a and 13b. The figures observe that the minimum coefficient for Al-5.7Si-2Cu-0.3Mg alloy was associated with the following combination of factors: thixoforming-T6 process, load of 100 N, sliding distance of 5000 m, and temperature of 150 °C. Additionally, it can be concluded that the COF values seems to be stabilize after the run-in period with an increase in the applied load. The results showed that sliding distance exerted a minimal influence on COF of the alloys. The average COF values of the as-cast, thixoformed, and thixoformed-T6 alloys were approximately 0.67–0.69, 0.35–0.41, and 0.33–0.28, respectively.

Friction coefficient (µ)

0.5 0.4 0.3 0.2 As-cast

0.1

Thixoformed Thixoformed-T6

0 0

1000

2000

3000

4000

5000

Sliding distance (m)

Fig. 11. An example of the COF record showing the average COF for the as-cast, thixoformed, and thixoformed-T6 Al-5.7Si-2Cu-0.3Mg alloys at a normal load of 50N and temperature of 25 °C.

22

Fig. 12. An example of the rolled debris material on the worn surface of the thixoformed-T6 alloy at 50N, 1000m and 300°C.

Fig. 13. (a) Plot of main effects for the coefficient of friction (COF), and (b) Interaction plots for the coefficient of friction (COF). 23

In summary, the T6 heat treatment improved the hardness of the alloy, so enhancing the strength and reducing the average COF of the thixoformed and thixoformed-T6 alloys by 48% and 51% respectively, compared with the COF of as-cast alloy. The thixoformed-T6 alloy at 300 °C exhibited improved wear resistance despite the low COF. From the ANOVA of COF (Appendix A), it was shown that the process has the main contribution followed by applied load at COF results. 3.6. Worn Surface Morphology To evaluate the wear mechanism, the worn surfaces of samples were tested under SEM. Fig. 14 shows the SEM micrographs for the worn surfaces of the as-cast (applied load of 10 N, sliding distance of 1000 m, and sliding temperature of 25 °C), thixoformed (applied load of 50 N, sliding distance of 5000 m, and sliding temperature of 150 °C) and thixoformed-T6 (applied load of 100N, sliding distance of 3000 m, and sliding temperature of 300 °C) Al–5.7Si–2Cu–0.3Mg alloys. The figure shows that, the worn surface of as-cast alloy is more destroyed than that in thixoformed and thixoformed-T6. It was clearly that the volume loss of alloy show decreased in volume loss of thixoformed and thixoformed-T6 because of the effect of elevated temperature. Small craters formation (Fig. 14 (b)) indicating the removing materials from the pin surface of the thixoformed alloy at a 25 °C temperature and 50N of applied load. This proves that the occurrence of craters formation on the worn surface is due to a high stress concentration on the coarse acicular or flake morphology such as β-(Al5FeSi) phase (needle-like morphology) caused a harmful effect on wear resistance [41]. This makes the particles loosely bonded with the matrix and the interfacial areas between this phase and the matrix become prone to microcracking.

24

The thixoformed-T6 alloy with a high level of hardness exhibited lower abrasion wear than the as-cast alloy and the thixoforming process control the abrasive wear of a material. However, the amount of material removed for the thixoformed-T6 alloy decreased at elevated temperature, thereby resulting in a high COF. This phenomenon is because the mechanically mixed layer (MML), which formed on the surface by concentrating large particles of wear products fracture into small particles, oxidise, sinter and covering the wear surface of thixoformed-T6 alloy. The COF values in all processes increased when the temperature of the sample increased. The hardness of thixoformed alloy increased after the heat treatment. This increased in hardness is owing to lower porosity and fine grain size of α-Al phase and more precipitated of intermetallic phase amongst the α-Al globules in the thixoformed-T6 alloy. Thixoformed-T6 alloy also observed lower wear rate than the as-cast alloy. The friction layer in the thixoformed alloy at 150 °C was removed due to the motion between the sample and the counter surface material upon being in contact during wear test. This finding is due to the compactness, few cracks, and abrasive characteristics of the thixoformed alloy (Fig. 14b). The friction layer detached with the elimination of a massive amount of the material, leaving a surface with a mix of abrasive and slightly delaminated wear. In the thixoformed-T6 alloy at 300 °C, no transfer of a flaky layer occurred because of the decrease in abrasions and strikes on the worn surface (Fig. 14c). Therefore, the surface of the thixoformed-T6 alloy at 300 °C was almost covered with an oxide layer. Thus, the wear rate of this alloy decreased. EDX used to define the different composition of the worn surface of the pin of the thixoformed-T6 alloy at 100 N, 3000 m, and 300 °C. The oxide layers will protect the pin samples surface from direct contact with the counter surface, leading to an increase of wear resistance. The formation of oxide at the worn surface of thixoformed-T6 alloy was obvious from EDX analysis (see Fig. 14d).

25

Fig. 14. SEM micrographs of worn surface at different processing conditions: (a) as-cast alloy at 10N, 5000m and 150°C, (b) thixoformed alloy at 100N, 3000m and 25°C, and (c) and (d) SEM micrographs and EDX spectra, respectively of the worn surface of the thixoformed-T6 alloy at 50N, 1000m and 300°C. 3.7 Morphology of The Tribolayer

26

The SEM images for the longitudinal cross sections of worn surfaces sample of the as-cast (applied load of 10 N, sliding distance of 1000 m, and sliding temperature of 25 °C), thixoformed (applied load of 50 N, sliding distance of 5000 m, and sliding temperature of 150 °C) and thixoformed-T6 (applied load of 100N, sliding distance of 3000 m, and sliding temperature of 300 °C) Al–5.7Si–2Cu–0.3Mg alloys are shown in Fig. 15. It can be seen that all alloys exhibit subsurface plastic deformation at different conditions. The concentration of particles of wear debris of the tribolayer on the worn surfaces can be transferred from one surface to another. These particles resulted in the comminution and merging of plastic deformation and oxidised particles to hard, protecting layer that decreased the wear rate [42]. The protected layer may cause drastic changes in the roughness and properties of surfaces during the wear test, and subsequently improves the wear resistance of the tribo-system. As shown in Fig. 15d the MML contains more amount of iron and Al oxide as compared to that in alloy. Therefore, the MML works as protective layer that may cause drastic changes in the roughness and properties of surfaces during the wear test, and subsequently improves the wear resistance of the tribo-system. Deuis et al. 1997 [43] reported that this transition in the wear was significant at high temperatures, where the wear scar was developed a flat tribolayer. Densely plastic deformation appeared under the MML at high applied load at the alloy under different conditions. The density of MML gained at the thixoformed alloy at load of 50 N is higher than for as-cast alloy at 10 N load, but it is not regular manner in alloys especially in the as-cast alloy. By comparing the as-cast alloy, the greater hardness of the thixoformed-T6 alloy, that is connected with the better the precipitated morphology of the intermetallic phase amongst the αAl globules, globular α-Al phase, a smaller amount porosity and uniform distribution of Si particles, attributed to a more stable layer. This stable layer can be supported the MML formed

27

that decrease the concentration of stresses transferred to the bulk material and raise the tribological properties of the thixoformed-T6 alloy. The amount of Fe volume fraction occurred in thixoformed-T6 alloy (Fig. 15d), which confirm the increase in wear resistance of the thixoformed-T6 alloy. The increase of Fe volume fraction can be due to the wearing-off of the countersurface material.

Fig. 15. SEM micrographs of the corresponding tribolayer at different processing conditions: (a) as-cast alloy at 10N, 5000m and 150 °C, (b) thixoformed alloy at 100N, 3000m and 25 °C, (c) thixoformed-T6 alloy at 50N, 1000m and 300°C, and (d) EDX spectra of the thixoformed-T6 Al-5.7Si-2Cu-0.3Mg alloy at 50N, 1000m and 300 °C (arrows indicate the direction of the sliding test). 4.

Conclusions

Dry sliding wear behaviour of thixoformed Al-5.7Si-2Cu-0.3Mg alloys at elevated temperatures was tested in the present study, the following conclusions were drawn from the above results and analysis:

28



The microstructure of the thixoformed alloy contained of fine globular and equiaxed grains with homogeneous distribution of eutectic phase.



In thixoformed-T6 alloy, the microstructure of the eutectic Si particles shows fine distribution due to their nucleation and growth behaviour during solidification. The thixoformed-T6 alloy showed higher precipitation of Al2Cu than the thixoformed alloy.



The hardness of thixoformed-T6 Al–5.7Si–2Cu–0.3Mg alloy is ~80 HRB, being higher than those of as-cast and thixoformed alloys.



The VL values of the thixoformed and thixoformed-T6 alloys decreased by 20% and 67%, respectively, while the COF values decreased by 48% and 51% for the thixoformed and thixoformed-T6 alloys, respectively, compared to those for the as-cast alloy.



The combination parameters for minimum VL for the thixoformed-T6 alloy included 100 N applied load, 3000 m sliding distance, and 300°C sliding temperature, whereas the combination of parameters for a low COF for the thixoformed-T6 alloy included an applied load of 100 N, sliding distance of 5000 m, and sliding temperature of 150 °C.



SEM investigation of the alloys revealed the predominant wear mechanisms in the tribological tests included abrasion, adhesion, and minor delamination. The surface of the thixoformed-T6 alloy at 300 °C was almost covered with a stable layer of a mixture of Al, O2 and Fe, which enhanced wear resistance. The worn surface and the corresponding tribolayer of the alloys were strongly dependent on the temperature and applied load.



From investigation of the worn surfaces by scanning electron microscope, it is revealed that oxidative, plastic deformation and delamination wear are the dominant wear mechanisms at elevated temperatures.

29

Appendix A: Analysis of variances (ANOVA) The analysis of variance (ANOVA) has been utilised to evaluate the effect of different operating factor on the wear loss. This investigation carried out for level of confidence of 95% (i.e., level of significance of 5%). Table A1 shows the results of ANOVA analysis of VL. The results show that the process (92%) has the highest influence on wear of the alloy.

Table A1 Analysis of variance for VL using Adj SS for the test Source

DF

Seq SS

Adj SS

Adj MS

Process

2

145.63

145.631

72.816

Fratio

Pvalue

Contribution %

0.000

92.24

0.008

2.962

0.844

0.078

202.2

Load

2

4.678

4.678

2.339 6.50

Sliding Distance (m

2

0.124

0.124

0.062

Temperature(°C)

2

0.967

0.967

0.483

0.17

Error

18

6.482

6.482

0.360

1.34 -

Total

26

157.82

-

-

-

0.286

0.612

-

4.11

-

100

S = 0.600090 R2 = 95.89% R2 (adj) = 94.07% DF: degree of freedom, Seq SS: sequential sum of squares, Adj SS: adjusted sum of squares, and Adj MS: adjusted mean squares. ANOVA results showed that the thixoformed-T6 alloy statistically and significantly contributed to the reduction in the COF. Table A2 summarises the ANOVA of the COF of alloy. as shown in table, one can noted that the process and the load main effects are statistically important, though the temperature has an unimportant effect on the COF results; (P=0.616). The atest column in Table A2 observes all the contribution of the factors. It was found that the process contributes

30

most of the COF results of alloy by about 84.54%, followed by normal load 2.46%, sliding distance 1.89% and temperature 0.5%.

Table A2 Analysis of variance for COF using Adj SS for the test Source

DF

Seq SS

Adj SS

Adj MS

Process

2

0.733901

0.733901

0.366951

Load

2

0.021419

0.021419

0.010709

Fratio

0.000

Contribution % 84.54

0.150

2.46

0.225

1.89

0.616

0.5

Pvalue

72.39 2.11 Sliding Distance (m

2

0.016450

0.016450

0.008225 1.62

Temperature(°C)

2

0.005049

0.005049

0.002525

Error

18

0.091240

0.091240

0.005069

0.50 -

-

10.5

Total

26

0.868059

-

-

-

-

100

S = 0.0711963 R2 = 89.49% R2(adj) = 84.82% Declaration: The authors declare no conflict of interest. Acknowledgments: The authors gratefully acknowledged Universiti Kebangsaan Malaysia (UKM) and the Ministry of Education (MOE), Malaysia, for the financial support under research grant DIP-2016-007.

31

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35

Highlights •

The microstructure of Al-5.7Si-2Cu-0.3Mg alloy was improved via a thixoforming and T6 heat treatment processes.



The dry sliding wear behaviour of as-cast, thixoformed and thixoformed-T6 alloys at different elevated temperatures, applied loads, and sliding distance was evaluated by using pin-on-disc tribometer.



At high temperature the thixoformed-T6 alloy dominate the best wear results by Taguchi method.



Oxidative, plastic deformation and delamination wear at elevated temperatures highly affects wear results.

Conflict of Interest and Authorship Conformation Form Please check the following as appropriate: o

In this work, M. A. Abdelgnei, M. Z. Omar, M.J. Ghazali conceived and designed the experiment; M. Z. Omar provided the materials; Bushra Rashid and M. N. Mohammed conducted part of analysis particularly the microstructure evaluation; M. A. Abdelgnei and M. Z. Omar wrote the paper.

o

This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.

o The following authors have affiliations with Universiti Kebangsaan Malaysia (UKM) and the Ministry of Education (MOE), Malaysia, for the financial support under research grant DIP-2016-007. With direct financial interest in the subject matter discussed in the manuscript.

Author’s name Mnel A H Abdelgnei

Mohd Zaidi

Mariyam Ghazali

Muhammed Abdulrazaq

Bushra Rashid

Affiliation Centre for Materials Engineering and Smart Manufacturing (MERCU), Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM, 43600 Bangi, Selangor, Malaysia. Department of Mechanical Engineering, Faculty of Mechanical Engineering, Omar Almukhtar University, +218 Albaida, Libya Centre for Materials Engineering and Smart Manufacturing (MERCU), Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM, 43600 Bangi, Selangor, Malaysia Centre for Materials Engineering and Smart Manufacturing (MERCU), Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM, 43600 Bangi, Selangor, Malaysia Department of Engineering & Technology, Faculty of Information Sciences and Engineering, Management & Science University, 40100 Shah Alam, Selangor, Malaysia Institute of Technology, Middle Technical University, 29008 Alzafaranya, Baghdad, Iraq,

Thank you for your consideration of this manuscript.

Mnel A H Abdelgnel Faculty of Engineering University Kabangsaan Malaysia -01112380014-