Wear behaviour of a Mg alloy subjected to friction stir processing

Wear 303 (2013) 65–77

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Wear behaviour of a Mg alloy subjected to friction stir processing H.S. Arora a, H. Singh a,n, B.K. Dhindaw b a b

School of Mechanical, Materials and Energy Engineering, Indian Institute of Technology Ropar, Rupnagar-140001, Punjab, India School of Mineral, Metallurgical and Materials Engineering, Indian Institute of Technology Bhubaneswar, Bhubaneswar-751013, Orissa, India

a r t i c l e i n f o

abstract

Article history: Received 25 September 2012 Received in revised form 5 February 2013 Accepted 26 February 2013 Available online 13 March 2013

In the present investigation, wear behaviour of a Mg alloy AE42 was examined under as-cast as well as friction stir processed conditions. Friction stir processing (FSP) was carried out at an optimized set of FSP parameters. Wear tests were performed in a pin-on-disc configuration using Universal Tribometer. The load range was varied from 5 to 20 N, whereas sliding velocity from 0.33 to 3 m/s. Worn surfaces and wear debris were analyzed using SEM and EDS for the determination of different wear mechanisms. The friction stir processed (FSPed) AE42 alloy demonstrated significant decrease in the wear rate which may be attributed to the microstructural refinement resulting in enhanced hardness and ductility of the FSPed alloy along with higher work hardening capability. At low loads, wear mechanism transformed from oxidation and abrasive wear at low sliding velocity to delamination at high velocity. At intermediate loads, oxidation and abrasion characterized the worn surface at low velocity, whereas delamination and plastic deformation were found to be major wear mechanisms at high velocities. At high loads, the corresponding mechanisms were abrasion, delamination and plastic deformation at low velocity, whereas severe plastic deformation and delamination at high velocities. & 2013 Elsevier B.V. All rights reserved.

Keywords: Mg alloy Friction stir processing Wear testing Sliding wear Electron microscopy

1. Introduction Materials with improved tribological properties have become the pre-requisite of the advanced engineering design [1]. Magnesium alloys have earned reputation owing to their high specific strength, which leads to the weight reduction resulting in a considerable economic advantage. They have become alternative candidate for applications in automotive, aerospace, audio and electronic industries [2]. The current trend in the automotive industry shows two main application ranges for Mg pressure die castings, the powertrain and the body structure [3]. Typical magnesium applications in the power train are gear box and certain engine components such as crank case and cylinder bore [3]. In such applications, the relative sliding motion of the components results in material loss through friction and wear. The tribological properties of such sliding systems depend on the properties of the specimen materials [4,5], counterface materials [6], their interaction with the environment [7,8] as well as the experimental conditions, including the applied load and sliding velocity [9]. The inferior surface properties of the Mg alloys, such as wear and corrosion resistance, have restricted the utmost use of these alloys. The appearance of casting defects, such as porosities, in the as-cast alloys further deteriorates their properties and renders

n

Corresponding author. Tel.: þ91 98557 09052; fax: þ91 01881 223395. E-mail address: [email protected] (H. Singh).

0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2013.02.023

them unsuitable for such applications. Wear behaviour of different Mg alloys has been widely studied. Selvan et al. [2] investigated dry sliding wear behaviour of as-cast ZE41A magnesium alloy. The dominant wear mechanisms were found to be abrasion, oxidation, delamination, plastic deformation and melting. Lo´pez et al. [10] also investigated the wear behaviour of ZE41 magnesium alloy. Oxidative wear was found to be the major wear mechanism at low sliding velocities (  0.1 m/s) with a small participation of abrasion and delamination mechanisms. Abrasion became more dominant wear mechanism at intermediate speeds along with participation of oxidation. At high speeds the main mechanism changed to delamination at intermediate loads and to plastic deformation at high loads. An et al. [11] investigated the dry sliding wear behaviour of as-cast magnesium alloys Mg97Zn1Y2 and AZ91. Tests were performed at a sliding velocity of 0.785 m/s. It was observed that Mg97Zn1Y2 exhibited good wear resistance compared to AZ91 for applied loads in excess of 80 N. Zhang et al. [12] investigated the dry sliding wear of as-cast Mg–Zn–Y magnesium alloy using block-on-wheel system. Wear tests were conducted within a load range of 10 to 70 N, sliding time range of 10 to 40 min and at a sliding velocity of 0.42 m/s. They revealed that Mg–25Zn–2Y quasicrystal material exhibited better wear resistance at all applied loads. Along with as-cast magnesium alloys, the tribological performance of magnesium based metal matrix composites (MMC’s) has also been widely investigated. Mondal and Kumar [13] analyzed the dry sliding wear behaviour of magnesium alloy

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AE42 based hybrid composites. Combination of Saffil short fibres and SiC particles, mixed in different ratios, were used as reinforcements. Wear tests were conducted on a pin-on-disc set-up under dry sliding condition having a constant sliding velocity of 0.837 m/s for a constant sliding distance of 2.5 km in the load range of 10–40 N. Wear rate of the composites was found to be lower than the alloy and the hybrid composites exhibited a lower wear rate than the Saffil short fibres reinforced composite at all loads. Habibnejad-Korayem et al. [14] evaluated the wear performance of pure Mg and AZ31 magnesium alloy strengthened by Al2O3 nano-particles. Wear tests were conducted on as-cast materials using a pin on-disc configuration under normal stresses of 0.5, 1.0 and 1.5 MPa at sliding speeds of 0.5 and 1.5 m/s for sliding distances up to 2000 m. Composite materials showed much lower wear rates mainly due to the strength improvements caused by nanoparticles. Increased work-hardening capacity due to the interaction of dislocations and nano-particles was found to be the main mechanism improving wear behaviour of the nanocomposites [14]. Besides the conventional routes, the fabrication and wear assessment of Mg and Al based MMC’s has also been studied using one of the severe plastic deformation processes known as friction stir processing (FSP). Number of investigations, such as [15–19,1], have demonstrated that FSP can achieve significant microstructural refinement together with improved mechanical and tribological properties of materials. ShafieiZarghani et al. [20] fabricated Al/Al2O3 nano-composite using FSP and evaluated its wear performance. Significant improvement in wear resistance was observed by surface nano-composite layer in comparison to the as-received substrate. Dolatkhah et al. [21] investigated the wear behaviour of Al5052 and SiC nanocomposite fabricated using FSP. It was found that microhardness value improved up to 55% and wear rate was reduced about 9.7 times compared to as received 5052 aluminum alloy. Soleymani et al. [22] investigated the microstructural and tribological properties of Al5083 based surface hybrid composite produced by FSP. It was observed that surface hybrid composite demonstrated higher wear resistance. In all such investigations, sub micron or nano sized reinforcement has been added to the matrix material using the FSP tool resulting in the formation of ex-situ composites. The stirring action during FSP together with the intense plastic deformation and application of high strain rates resulted in the uniform distribution of externally added reinforcements. However, ex-situ composites suffer from certain disadvantages such as interfacial reactions between the reinforcements and the matrix, and poor wettability between the reinforcements and the matrix due to surface contamination of the reinforcements [23–25]. To overcome the inherent problems associated with ex-situ composites, in-situ techniques were developed in which the reinforcement phases are developed within the matrix material [23]. Compared to the conventional MMCs produced by ex-situ methods, the in-situ MMCs exhibit the following advantages: (a) the in-situ formed reinforcements are thermodynamically stable at the matrix, leading to less degradation in elevated temperature service; (b) the reinforcement-matrix interfaces are clean, resulting in a strong interfacial bonding; and (c) the in-situ formed reinforcing particles are finer in size and their distribution in the matrix is more uniform, yielding better mechanical properties [24,25]. In one of the recent investigations by the authors [26], FSP of AE42 alloy was performed under different conditions. It was observed that FSP resulted in the formation of fine in-situ precipitates of aluminum-rare earth (Al-RE) and magnesium-rare earth (Mg-RE) intermetallics along with grain size refinement. The mechanical properties of friction stir processed AE42 alloy such as micro-hardness and bulk hardness were found to enhance by nearly 60%. The major strengthening phenomenon was found to

be grain size strengthening. The fine in-situ precipitates produced during FSP were found to contribute towards evolution of ultra fine grain structure in the FSPed AE42 alloy through Zener pinning of the recrystallized grain boundaries. As an incremental step to the previous study, the aim of the current investigation is to analyze the wear behaviour of the as-cast as well as FSPed AE42 alloy under dry sliding conditions and to understand the dominant wear mechanisms.

2. Experimental details 2.1. Materials and processing The chemical composition of the Mg alloy AE42 is shown in Table 1. Rectangular specimens having dimensions of 80 mm  40 mm  3 mm were prepared from the alloy ingot. Friction stir processing was carried out on a computer numerical control (CNC) vertical milling machine (5 H.P) using a specially designed FSP fixture and tool. The schematic of the experimental set-up, actual components and details of the FSP fixture are given elsewhere [26]. The fixture has the configuration of a hollow rectangular box for the flow of cooling liquid so as to facilitate the undersurface cooling of processed specimen. For this cooling, a cryostat cooling bath of 250 W with 8 L capacity was used to produce the rapid cooling of the FSPed specimens. The circulating liquid used in the cryostat bath was Lab reagent (LR) grade methanol. Polyurethane (PU) pipes with thermal insulation were used for connecting the fixture to the cryostat bath. The tool material used for FSP was stainless steel. The FSP tool was a commonly used cylindrical tool without threads with 12 mm shoulder diameter, 4 mm pin diameter and 2.6 mm pin length. An optimized set of FSP parameters comprising; 700 tool rpm, 60 mm/min linear speed, 0.35 mm plunge depth,  10 1C cooling temperature and three FSP passes, as determined in a separate investigation [27], was used in the current study. For microstructural studies, as-cast as well as FSPed AE42 specimens were sectioned perpendicular to the processing direction. Specimen surfaces were prepared by standard metallographic techniques and etched with a solution of 5 ml acetic acid, 6 gm picric acid, 10 ml water, 100 ml ethanol, for 30 s. Microstructural study of the FSPed specimen was done using electron back scatter diffraction (EBSD) analysis. Metallurgical examination of the as-cast as well as FSPed AE42 specimens subjected to wear tests was carried out by scanning electron microscopy (SEM, JEOL make, Model: JSM-6610LV) equipped with EDS. For EBSD analysis, all the samples were ground down to 4000 grit followed by disc polishing. The ground and polished samples were further polished using colloidal silica and ethanol on a fine velvet cloth for nearly 15 min. Further processing includes polishing on an ion-beam polishing machine (Precision etching coating system, Gattan make, model: 682). Ion beam polishing was done at 651 rock angle for nearly 6 min. EBSD scan was performed within the nugget zone at a position located 1.5 mm below the top surface using FEI Quanta 3D FEG (Field emission gun), on a mid plane across the specimen cross-section. The scan area was 50 mm  50 mm with the step size of nearly 0.1 mm and nearly 2, 50,000 data points. For subsurface analysis, Table 1 Chemical composition of Mg alloy AE42. Element

Al

Ce

La

Nd

Mn

Th

Pr

Si

Zn

Mg

wt%

3.9

1.2

0.6

0.4

0.3

0.2

0.1

0.01

o 0.01

Balance

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some selected specimens from the wear test were cut into two halves, perpendicular to the worn surface. 2.2. Mechanical testing Mechanical properties of the as-cast as well as FSPed AE42 alloy were determined using micro-hardness and tensile tests. Microhardness measurements were performed on Microhardness Tester [Wilson (Instron) Model 401/402 MVD] at 300 gf load for a dwell period of 10 s. The variation in microhardness along the cross-section was also determined for the FSPed alloy. Tensile testing of the as-cast as well as FSPed alloy was done on a Universal testing machine (UTM, Tenius-Olsen make) equipped with computerized data acquisition system at a strain rate of 1  10  3/s rate until specimen failure. Fig. 2. XRD analysis of the as-cast AE42 alloy.

2.3. Wear tests Rectangular pin specimens with cross-section as 4 mm  3 mm and 10 mm in length were machined from the as-cast as well as FSPed AE42 alloy for the wear tests. Dry sliding wear tests were carried out on a universal tribometer (CETR UMT3) using a pin-on-disc test configuration. The counter face material used was a stainless steel disc with hardness value of nearly 220 HV. Before the wear tests, the cross-section of all the specimens were ground using different grades of emery papers down to 1000 grit. All wear tests were conducted at three sliding speeds of 0.33, 1 and 3 m/s under normal loads of 5, 10 and 20 N, for sliding distance up to 2500 m. Weight loss was determined as a function of sliding distance to a precision of 0.01 mg. Before and after each test, both the disc and specimens were cleaned with acetone and were dried in the air in order to avoid contamination. All wear tests were repeated two times to ensure reproducibility of the results.

3. Results 3.1. Microstructure The SEM image of the as-cast alloy is shown in Fig. 1. The image shown in Fig. 1 reveals the presence of elongated precipitates embedded in the Mg matrix. These precipitates are formed in-situ during the transformation cooling of the alloy. Some of the precipitates shown in Fig. 1 can be observed to be as long as 80–90 mm. The XRD analysis of the as-cast AE42 alloy is shown in Fig. 2. From the figure, it is evident that the major phases present in the as-cast alloy are Al11Ce3 and Mg17Al12 along

with the Mg matrix. As shown in Table 1, the major constituents of the AE42 alloy are Al and different rare earth (RE) elements. RE elements were added to the alloy to enhance its creep resistance. However, owing to the higher reactivity of RE elements to Al than to Mg, formation of Al11RE3 (Al11Ce3, for instance) is favored without formation of Mg-RE phase along with the generation of an intermetallic compound with the stoichiometric composition of Mg17Al12, commonly known as a b-phase. Thus, the microstructure of as-cast AE42 alloy comprises elongated precipitates of Mg17Al12 (b-phase) and Al11RE3 embedded in the a-Mg matrix, which represents the solid solution of Al in Mg. The SEM image of the FSPed AE42 alloy is shown in Fig. 3. It is perceptible from the image that the elongated precipitates in the as-cast alloy are fragmented during FSP resulting in generation of fine in-situ precipitates. Some of the precipitates as seen in Fig. 3 are of few nanometers only. EDS analysis of a precipitate present in the nugget zone of FSPed AE42, shown in Fig. 3, reveals the presence of different rare earth elements along with Al and Mg. The XRD analysis of the nugget zone of FSPed alloy shown in Fig. 4 depicts the presence of some new precipitates of aluminium-rare earth and magnesium-rare earth intermetallics, not originally present in the as-cast alloy, such as Mg12Pr, Al4Nd and Al4Ce. The EBSD map as well as the plot representing the grain size distribution for the FSPed alloy is shown in Fig. 5(a) and (b), respectively. It can be observed that in comparison to the as-cast alloy having grain size of nearly 20 mm, the grain structure got significantly refined during FSP and most of the grains appear to be equiaxed. The plot showing the grain size distribution reveals the presence of nearly 40% sub-micron grains with the average grain size of nearly 1.5 mm. 3.2. Hardness The variation of microhardness for the FSPed alloy as well as the average microhardness value of the as-cast alloy is shown in Fig. 6. The average microhardness of the as-cast alloy can be observed to be 62.5 HV which got appreciably increased after FSP. Moreover, FSP has resulted in the nearly uniform enhancement throughout the depth of the specimen as revealed by marginal variation in the microhardness value with increase in distance from the top surface. Grain boundary strengthening in accordance with well known Hall–Petch equation is believed to be the major phenomenon that contributed towards enhanced hardness of the FSPed alloy. 3.3. Tensile test

Fig. 1. SEM image of the as-cast AE42 alloy.

The representative true stress strain curves for the as-cast and FSPed AE42 alloy is shown in Fig. 7. It can be observed that

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Mg 74% Al 12% Ce 9% Pr 2% Nd 2%

Fig. 3. SEM image of the nugget zone of friction stir processed (FSPed) AE42 alloy along with the spot EDS analysis of a precipitate marked by a symbol.

Fig. 4. XRD analysis of the friction stir processed (FSPed) AE42 alloy.

initially, both the specimen exhibit nearly linear elastic deformation. Once the yield point is reached, the plastic deformation sets in. The mechanical properties obtained from the tensile testing for both the investigated cases are listed in Table 2. It can be noticed that the yield strength, ultimate strength as well as ductility got enhanced after FSP. The possible reason for the improved mechanical properties of FSPed AE42 may be attributed to the microstructural refinement. Furthermore, the as-cast alloy delayed the necking phenomenon nearly up to 7.5% but once initiated the as-cast alloy was not able to sustain elongation for a longer duration and failed at nearly 9% elongation indicating somewhat a brittle failure of the as-cast alloy whereas; the FSPed alloy exhibited higher elongation of nearly 12%. 3.4. Wear testing The wear rate plots for as-cast as well as FSPed alloy are plotted against sliding distance under different normal loads and sliding velocities in Fig. 8. The wear rates obtained for the as-cast and FSPed alloy at 0.33, 1 and 3 m/s sliding velocities at different normal loads are shown in Fig. 8(a)–(c), respectively. It is evident from the figure that FSPed alloy has consistently shown better wear resistance throughout the range of parameters investigated. Moreover, the as-cast as well as FSPed alloy has shown uniform wear rates over a large part of sliding distance, at all loads and

Fig. 5. (a) EBSD map of the nugget zone of FSPed AE42 alloy on the mid plan of specimen cross-section at 1.5 mm below the top surface (b) grain size distribution for the FSPed AE42 alloy at the same location.

velocities tested. Fig. 9(a) shows the wear rate plot of the as-cast as well as FSPed alloy as a function of sliding velocities at different normal loads. It can be observed from this figure that maximum mass loss/sliding distance or wear rate occurred at lowest velocity and it decreases with the increase in the sliding velocity for both the as-cast as well as FSPed alloy. The drop in the

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Fig. 6. Plot showing the micro-hardness variation along the cross-section of the FSPed AE42 alloy.

69

showing variation of wear rate with respect to both these parameters is represented in Fig. 10. The figure elegantly demarcates different regions with varying wear rates. Highest wear rates corresponds to maximum load and lowest velocities. The contour plot shown in Fig. 11(a) represents the variation of coefficient of friction (COF) for the AE42 alloy. It can be observed that highest COF corresponds to lowest load and lowest sliding velocity. At constant load of 5 N, COF decreases with increase in sliding velocity and thereafter it increases with further increase in the velocity. However, at other load values, that is, at 10 and 20 N, the COF decreases with the increase in the sliding velocity. Further, it can be observed that the COF value of FSPed alloy is lower than the as-cast alloy at 1 m/s as well as at 3 m/s velocities for all normal load values. At 0.33 m/s velocity, it can be observed that COF of FSPed alloy is lower than that of as-cast alloy at 5 and 10 N loads, whereas at 20 N load, the COF of the FSPed alloy became more than that of the as-cast alloy. The possible reason for the same was revealed by the EDS analysis for this specimen. As discussed in the forthcoming section, the EDS analysis of the wear debris collected for the FSPed AE42 alloy at 20 N load and 0.33 m/s velocity reveals the presence of small amount of iron particles. The presence of iron particles was not found in any other investigated case in the current study. The presence of iron particles in the wear debris at these parameters indicates the possibility of formation of an iron rich transfer layer on the surface of the Mg alloy specimen. Therefore, the occurrence of higher COF value for the FSPed AE42 alloy at these parameters may be attributed to the presence of hard wear debris particles of iron in the transfer layer, formed on the surface of the test specimen. 3.4.1. Wear mechanisms In order to identify the different mechanisms operating during wear testing of the AE42 alloy, the worn surfaces was analyzed using SEM and EDS. It was observed that although the wear rates of the as-cast and FSPed alloy were different, both the alloys showed nearly similar wear mechanisms. Therefore, the prominent wear mechanism described here are irrespective of the type of alloy. Where ever some small differences are observed, the same have been mentioned.

Fig. 7. Stress–strain curve for the as-cast as well as FSPed AE42 alloy.

Table 2 Mechanical properties for the as-cast as well as FSPed AE42 alloy obtained from tensile test performed at 1  10  3 s  1 strain rate. Sr. Material no.

YS (MPa)

1 2

1007 10 160 712 97 0.5 125 7 10 230 715 127 0.5

As cast AE42 FSP AE42

UTS (MPa)

Elongation (%)

Young’s modulus (MPa)

Strain hardening exponent

50 62.5

0.18 0.244

wear rate with change in sliding velocity from 1 to 3 m/s can be observed to be higher as compared to when it changes from 0.33 to 1 m/s. This indicates that different wear mechanisms may be operating at different velocities and loads. Fig. 9(b) shows the wear rate plot of the as-cast as well as FSPed alloy as a function of normal load at different sliding velocities. It can be observed from this figure that maximum mass loss/sliding distance or wear rate occurred at highest load and it decreases almost linearly with decrease in the normal load values. To elucidate the combined influence of sliding velocity and normal loads, a wear rate map

3.4.1.1. At 5 N load. The SEM images of the AE42 specimens subjected to wear test at 5 N normal load and 0.33 m/s sliding velocity are shown in Fig. 12(a–c). It can be observed that the wear surface is characterized by the presence of long continuous groves, fractured surface and features of microcutting and ploughing, all indicative of abrasion. Another simultaneously occurring wear mechanism under these conditions appears to be oxidation wear as evident from presence of oxide layer on the worn surface. Surface oxidation under these conditions was also verified by the EDS analysis (not shown here). Moreover, EDS results have revealed a highest wt% of oxygen under these conditions amongst all the other investigated cases (discussed in Section 3.4.2). Some traces of material removal in the form of small platelets, indicative of delamination, are also visible on the worn surface. Thus, abrasion and oxidation were observed to be the major wear mechanisms at 5 N load and 0.33 m/s velocity with some involvement of surface delamination. The dominant wear mechanisms at 5 N load and 1 m/s velocity were found to be abrasion and surface oxidation along with surface delamination. At 3 m/s velocity, delamination was observed to be the major wear mechanism, as revealed by the SEM image of this case shown in Fig. 12(d). It is evident from the image that the traces of abrasion grooves have decreased with increased severity of delamination. Increase in the intensity of

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Fig. 8. Wear rate plots for as-cast as well as FSPed AE42 alloy at (a) 0.33 m/s sliding velocity, (b) 1 m/s sliding velocity and (c) 3 m/s sliding velocity and different normal loads.

subsurface cracks at higher velocity may have intensified the fatigue induced delamination wear.

SEM image also shows the presence of surface delamination under these conditions.

3.4.1.2. At 10 N load. The SEM images of the AE42 specimens subjected to wear test at 10 N normal load and 0.33 m/s velocity are shown in Fig. 13(a) and (b). The abrasive wear features viz., microcutting, ploughing and grooves are noticeable on the surfaces. These features can be observed to be more intense at 10 N load in comparison with that at 5 N. The fractured surface, evident in the form of broken particles, can also be seen on the worn surface. EDS analysis of the worn surface reveals significant wt% of the oxygen, which indicates surface oxidation and presence of oxidation wear of the surface. Delamination of the surface can also be observed in the SEM image. Along with abrasion, as revealed by the presence of abrasion grooves and surface cracks, presence of fractured particles, delamination and oxidation, the worn surface of the specimen tested at 10 N load and 1 m/s velocity is also characterized by presence of plastic deformation indicated by the lip formation as shown in the image in Fig. 13(c). Large heat generated at velocity of 1 m/s may have resulted in plastic deformation of the worn material. At 3 m/s, the extent of plastic deformation became much more severe as compared to the lower sliding velocity of 1 m/s. Owing to the large plastic deformation of the material, the surface microstructure comprises extruded layers of the material and presence of shear lips, as shown in Fig. 13(d). The

3.4.1.3. At 20 N load. Fig. 14 shows the SEM images of the AE42 specimens subjected to wear test at 20 N normal load and 0.33 m/s sliding velocity. Fig. 14(a) shows the plastically deformed rolled edges near detachment from the worn surface. It is worth to mention here that such plastic deformation was observed for the FSP specimen only and not for the as-cast alloy. This may be attributed to the higher ductility of the FSPed AE42 specimen in comparison to the as-cast alloy, as revealed by tensile tests. Extensive delamination of the surface can also be observed under these conditions. Besides signs of abrasion in the form of ploughing and scratch marks, the distinctive feature observed for this specimen was the presence of partially broken oxide layer as shown in Fig. 14(c). The EDS analysis of the worn surface also supported surface oxidation under these conditions. SEM images of the worn surfaces at 20 N and 1 m/s, shown in Fig. 14(d) revealed the dominance of plastic deformation and delamination as major wear mechanisms. EDS analysis revealed the occurrence of surface oxidation also. SEM image of the specimen tested at 20 N and 3 m/s is shown in Fig. 14(e). It is evident from the image that the surface appearance is relatively clean and shiny without much traces of abrasion. However, it can be observed that the specimen surface was

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Fig. 9. Variation of wear rate (a) vs. sliding velocity at different normal loads (b) vs normal loads at different sliding velocities for as-cast a well as FSPed AE42 alloy.

Fig. 11. (a) Contour plot showing coefficient of friction for AE42 alloy at all investigated parameters (b) plot showing comparison of coefficient of friction for the as-cast as well as FSPed AE42 alloy.

Fig. 10. Contour plot showing the wear rate for AE42 alloy at all the investigated parameters.

subjected to severe delamination and plastic deformation during wear test. The presence of subsurface cracks, extending up to the surface, can also be seen on the worn surface. Surface oxidation, evident in the form of oxide layer, is also noticeable. High shear force which is attributed to high normal load and sliding velocity may have resulted in broken oxide layer on the worn surface.

3.4.2. Wear debris analysis The wear debris analysis can reveal significant mechanistic information about a process and the material. An important aspect related to wear mechanism is revealed by the EDS analysis of the wear debris performed for all the investigated conditions which showed that oxidation of the worn surface took place under all the investigated test conditions, the extent of which however varies with the operating conditions. EDS analysis of wear debris for some of the selected cases have been shown in Fig. 15(a)–(c). The EDS analysis reveals the presence of significant amount of oxygen in the wear debris of all the specimens, largest being for the specimen tested at 5 N load and 0.33 m/s velocity. However, the oxygen content can be observed to decrease with the increase in normal load. SEM images of the wear debris from some of the selected cases have been shown in Fig. 15(d)–(f). The plate like wear debris, as shown in Fig. 15(d), supports the occurrence of delamination of the worn surfaces. The presence of short lathy strips in the wear debris, as shown in Fig. 15(d), indicates abrasion in the form of cutting and ploughing. The wear debris shown in Fig. 15(e) reveals the presence of a long extruded and rolled up chip. The image also shows the presence of an extensively deformed and extruded surface layer of the material

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5 N,0.33 m/s

Microcutting

5 N,0.33 m/s Fractured surface

Ploughing Sliding direction

Sliding direction

Delamination

5 N, 1 m/s

5N, 3m/s

Delamination Microcutting Sliding direction Sliding direction

Fig. 12. SEM images of the worn surfaces of AE42 alloy tested at 5 N load and different sliding velocities.

10N, 0.33m/s

Sliding direction

10N, 0.33m/s Fractured surface

Delamination Ploughing

Oxide layer

Sliding direction

Microcutting

Plastically deformed and extruded layers

10N, 1m/s Surface crack

Delamination

Sliding direction

Shear lip indicating plastic deformation

Sliding direction 10N, 3m/s

Fig. 13. SEM images of the worn surfaces of AE42 alloy tested at 10 N load and different sliding velocities.

in the wear debris. The presence of such deformed material in the wear debris is an indicative of plastic deformation of the surface. A distinctive feature was revealed by the EDS analysis of the wear debris collected at 20 N load and 0.33 m/s sliding velocity, as shown in Fig. 15(f). The EDS analysis shows the presence of iron along with other elements. It is interesting to know that the

presence of iron is revealed in the wear debris of FSPed AE42 specimen only and not in the as-cast specimen, tested under same parameters. The presence of iron in the wear debris indicates that wear of the counter surface, that is, stainless steel disc, might have taken place. Higher hardness of the FSPed AE42 alloy in comparison with the as-cast AE42 specimen may have resulted in

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20 N, 0.33 m/s

Plastically deformed rolled up edge

Sliding direction

Delamination

Broken oxide layer

Sliding direction Sliding direction Ploughing 20 N, 0.33 m/s

Sliding direction

Shear lips

20 N, 0.33 m/s

Sliding direction

20 N, 3m/s Delamination

20 N, 1m/s

Delamination with visible subsurface cracks

Fig. 14. SEM images of the worn surfaces of AE42 alloy tested at 20 N load and different sliding velocities.

wear of the counter surface, whereas the as-cast alloy itself worn out owing to low hardness. Although not exact analysis, a fairly acceptable estimate of the extent of surface oxidation can be obtained from the amount of oxygen revealed during the EDS analysis [10]. The contour plot shown in Fig. 16 gives an indication of the extent of surface oxidation (based upon the amount of oxygen) that had occurred under all investigated conditions. It can be observed that specimens subjected to wear test at low loads and smaller sliding velocities, have higher amount of oxygen, which is an indicator of higher surface oxidation. The extent of surface oxidation is found to decrease with the increase in the sliding velocity and normal load. These observations are in agreement with the observations of Lo´pez et al. [10].

3.4.3. Wear mechanism map Wear mechanism map basically aids in differentiating different mechanisms operating under different conditions. The map

defines regions where a particular mechanism is predominant, separated by the transition lines. Based on the observations, wear mechanism map was drawn for the AE42 alloy as shown in Fig. 17. As evident from the figure, the major wear mechanism at low velocity of 0.33 m/s is the combination of abrasion and oxidation wear at low and intermediate loads. Along with those mentioned above, delamination also adds as a major wear mechanism at high loads. At an intermediate velocity of 1 m/s, abrasion, delamination and oxidation were observed to be major mechanisms at low and intermediate loads whereas; delamination and plastic deformation became more dominant at high loads. At high velocity of 3 m/s, delamination and combination of delamination and plastic deformation were found to be dominant wear mechanisms at low and intermediate loads, respectively. At high loads accompanied with high velocity, severe plastic deformation and delamination became the governing wear mechanism. Further, it has been observed that at any particular parameter, a combination of different wear mechanisms operate, contribution of

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Fig. 15. (a)–(c) EDS analysis of the wear debris for some of the selected cases (d)and (e) SEM images showing wear debris (f) EDS analysis of the wear debris collected at 20 N load, 0.33 m/s sliding velocity.

Fig. 17. Wear map showing the major wear mechanisms observed for the AE42 alloy.

Fig. 16. Contour plot showing wt% of oxygen obtained in the wear debris under all investigated conditions.

wear mechanisms. Major wear mechanism in the different investigated conditions has been indicated in bold letters in Fig. 18.

one may be higher than the others. Different wear mechanisms, major as well as minor, that were observed under different operating conditions have been represented in the form of a matrix plot in Fig. 18. The matrix plot along with the wear mechanism map, shown in Fig. 17, may be helpful in better understanding of different

3.4.4. Subsurface analysis The SEM images of some of the selected worn surfaces revealing the substructure characteristics are shown in Fig. 19. The SEM image shown in Fig. 19(a) corresponds to the as-cast alloy tested under 20 N normal load at 0.33 m/s velocity.

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The image shows the presence of some needle type elongated trenches, possibly produced by the splintering of elongated b-phase precipitates during wear testing. The presence of some broken precipitates at these sites can also be observed. An SEM image shown in Fig. 19(b) represents the subsurface details of the FSPed alloy. The figure illustrates the subsurface material deformation which is evident in the form of flow field near to the upper surface of the specimen. The presence of such flow field supports that during the wear period subsurface is subjected to extensive plastic deformation and the softened material get extruded in the sliding direction. The image also shows the presence of a subsurface crack, generally initiated at some material non homogeneity or imperfection such as void [28]. The presence of such subsurface cracks can result in sudden material loss by repetitive motion of some abrasive particles over the surface leading to fatigue instigated material failure. Subsurface plastic deformation can contribute towards work hardening of the subsurface layers which can significantly influence the behaviour of a material against wear phenomena. In order to get quantitative thought about subsurface work hardening, microhardness testing of subsurfaces of as-cast as well as FSPed alloy was done, as shown in Fig. 20. It can be observed from the figure that subsurface of both the as-cast as well as FSPed alloy got appreciably work hardened. The as-cast AE42 subsurface shows maximum hardness of nearly 93.7 HV and 86 HV at 20 N load, 0.33 m/s velocity and 3 m/s velocity, respectively compared to 110 HV and 98.5 HV, respectively for the FSPed alloy. Further, it is noticeable that for the same load of 20 N, the extent of work hardening of the material is more at a lower sliding velocity of 0.33 m/s than at 3 m/s although, the material has been subjected to higher strain rate at higher velocity. This might have occurred because of higher amount of heat generated on the specimen surface at high sliding velocity resulting in high surface temperatures. The specimen material got thermally

Fig. 18. Matrix plot showing major as well as minor wear mechanisms for AE42 alloy under all investigated conditions. Major wear mechanism is shown in bold letters. Abbreviations used: oxidation (O), abrasion (A), delamination (D), plastic deformation (PD), severe plastic deformation (SPD), severe delamination (SD).

20N, 0.33m/s

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softened owing to high surface temperature. Thus, work hardening produced by high straining of the material is more than compensated by the thermal softening effect resulting in a less work hardened material at 3 m/s than at 0.3 m/s velocity. Furthermore, the maximum work hardening can be seen to occur at locations lying 50–200 mm below the top surface.

4. Discussion The worn surfaces of AE42 alloy contain long continuous grooves, features of microcutting and ploughing, all of which are indicative of abrasive wear. Abrasion is usually caused by the presence of hard particles that plough into the specimen surface. The movement of these particles over the surface causes the removal of material along its path on the surface of the specimen along with grooves and scratch marks predominantly in the sliding direction. The hard debris particles may also cause generation of surface cracks resulting in appearance of the fractured surfaces and particles on the specimen surface. In the current investigation, abrasion was mainly observed at low sliding velocities. At low sliding velocities, the entrapped wear debris may not be able to gain sufficient outward thrust to get out of the interacting surfaces, resulting in more abrasive wear. Further, the COF value in the current investigation was found to be higher at

Fig. 20. Plot showing microhardness profiles for some of the wear tested specimens. Microhardness variation of FSPed AE42 is also shown for comparison.

20N, 0.33m/s Sliding direction

Material flow Sliding direction

Elongated trenches

Subsurface crack

Fig. 19. SEM micrographs showing subsurface microstructure of some worn specimens.

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low sliding velocities. It appears that at low sliding velocities, the higher abrasion of the wear specimen by wear debris particles might have resulted in higher COF. The amount of surface oxidation was also found to be higher at lower velocities. It is believed that at low sliding velocities, repeated abrasion by wear debris particles would readily expose the underlying material to the oxidizing atmosphere and thereby resulting in higher surface oxidation at lower velocities. At high loads and low sliding velocities, the abrasion phenomena became more aggressive because of the increased asperity contact for longer durations between the interacting surfaces resulting in more wear debris. Surface delamination was observed to be another important wear mechanism for the investigated alloy. During wear testing, repeated sliding motion of the debris may result in generation of subsurface cracks, possibly at some material non homogeneity or imperfection [28], leading to subsurface initiated fatigue wear. Eventually, the subsurface cracks reach surface and cause removal of material as debris, the phenomenon commonly known as delamination. It was observed that in the current study delamination was a dominant wear mechanism at high velocities. At higher velocities, the number of rotation or cycles/time also increases for a constant radial distance. With the increase in number of cycles/time, fatigue loading of the surface is also intensified resulting in generation of subsurface cracks and surface delamination. Further, it was observed that COF get reduced at higher sliding velocities and higher normal loads. The decrease in the value of COF at higher sliding velocities and higher normal loads might be attributed to the thermal softening of the material owing to the generation of high surface temperatures. The amount of surface oxidation was also found to decrease at higher sliding velocities. It is anticipated that the possibility of surface oxidation at high sliding velocity and normal load is higher. However, there is an opposing argument also; according to which the oxidation amount may decrease due to the delamination of the oxide scale either at higher sliding velocity or at higher load. The presence of broken oxide layer at 20 N load, shown in Fig. 14(c), supports this argument as well as the observation of reduced oxidation rates at higher sliding velocities and normal loads. At higher loads, plastic deformation of the surface was observed. The temperature produced at the specimen surface is expected to be higher at high loads. At high temperature, the yield strength of the material decreases and it get softened [13]. The softened matrix spreads out and deform plastically under the action of high loads. Further, it was observed that the FSPed alloy was more plastically deformed than the as-cast alloy. Higher plastic deformation of the FSPed AE42 alloy may be attributed to its higher ductility than the as-cast alloy, as demonstrated by the tensile test results. Due to large plastic deformation, the subsurface layers of the FSPed alloy may also have got more work hardened. Further, it was observed that the FSPed alloy has shown higher work hardening capability. The possible reason for the same may be attributed to the microstructural refinement of the FSPed alloy. The work hardening capability can be thought of as being closely related to the stacking fault energy of the material. Materials having high stacking fault energy, such as aluminium and magnesium, readily undergo dislocation rearrangement to form dislocation sub-cell structures through well known climb and glide phenomena. Owing to the lesser dislocation density, the material with high stacking fault energy does not show much tendency to work harden [29]. Zhang et al. [30] found that the addition of rare earth elements to Mg resulted in decrease in the stacking fault energy. Therefore, it is believed that the evolution of fine in-situ precipitates in the FSPed alloy may also influence the stacking fault energy of Mg alloy. During FSP, high energy regions in the form of dislocation walls, dislocation tangles and dislocation pile ups are

produced. It is believed that the fine in-situ particles produced during FSP get clustered along the dislocation network and thus hinder their movement during the dynamic recovery phase (DRV) of the hot deformed material. The presence of high dislocation density may be responsible for higher work hardening capability of the FSPed alloy. The same phenomenon is also believed to be responsible for the evolution of refined grain structure, superior mechanical properties and hence improved wear resistance of the FSPed AE42 alloy. Due to the restricted dislocation movement, the stored energy, which may get consumed during dislocation movement, may have got utilized mainly during the dynamic recrystallization phase. The fine in-situ particles present in the FSPed material may pin the fine recrystallized nuclei. The growth of the recrystallized nuclei is prohibited under the Zener drag of fine insitu particles, thus resulting in the evolution of refined grain structure. The presence of fine grain structure may have contributed towards grain boundary strengthening (GBS) in accordance with the Hall–Petch relation. GBS appears as the major phenomenon responsible for enhanced mechanical properties, such as hardness of the FSPed AE42 alloy [26]. As observed from the wear rate plots, the drop in the wear rate with change in linear velocity from 1 to 3 m/s is higher as compared to when it changes from 0.33 to 1 m/s. At low velocity of 0.33 m/s, the entrapped abrasive particles caused excessive material loss by surface abrasion. In comparison to the sliding velocity of 0.33 m/s where the major wear mechanism was abrasion, the major wear mechanism at 1 m/s sliding velocity was essentially abrasion along with delamination. From the wear rate plots, it is evident that material loss was higher at 0.33 m/s than at 1 m/s. This indicates that the severity of abrasive wear was somewhat decreased at higher sliding velocities which may be due to fact that entrapped wear particles were able to escape out more easily at higher velocity of 1 m/s than at 0.33 m/s. Simultaneously, the delamination may not be high enough to significantly enhance the wear rate above than that occurred at 0.33 m/s. At 3 m/s, the material loss was dominated by plastic deformation and delamination with much reduced abrasion. The appearance of clear shiny surface at 3 m/s velocity indicate that the abrasive wear particles easily move out of the entrapped region contributing only marginally to the abrasive wear. Delamination, which was the major wear mechanism at 3 m/s, may not result in instant material loss as that of abrasion. Since delamination is fatigue initiated process in which subsurface cracks grow and propagate towards surface, so it can be thought of as a sluggish process as compared to abrasion which is much rapid and result in instant material loss. Thus, absence of abrasion may be the major reason for the reduced wear rate at high sliding velocities.

5. Conclusions

1. For the range of normal loads and sliding velocities investigated, friction stir processed (FSPed) AE42 alloy demonstrated reduced wear rate under all the conditions. 2. The major factors that contributed to the reduced wear rates for FSPed AE42 alloy were found to be microstructural refinement resulting in higher hardness, greater work hardening capability and improved ductility. 3. It was found that maximum mass loss/sliding distance (wear rate) occurred at highest load and lowest velocity. Moreover, wear rate was found to decrease with decrease in load and increase in sliding velocity for most of the cases. 4. Abrasive and oxidation were found to be dominant wear mechanisms in the low velocity regime. At intermediate sliding

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velocity, delamination also became significant along with abrasion and oxidation. The major wear mechanisms at high velocities were found to be plastic deformation along with delamination. 5. The as-cast alloy has shown higher value of coefficient of friction (COF) than the FSPed alloy at 5 N as well as at 10 N loads and all sliding velocities. However, at 20 N load and 0.33 m/s velocity, the FSPed alloy has shown higher COF than the as-cast alloy. This may be attributed to the higher work hardening of the FSPed alloy at 20 N load and 0.33 velocity. 6. The subsurface analysis revealed higher work hardening tendency at low sliding velocities. This may be attributed to the dominance of thermal softening effect over work hardening at higher velocity because of high surface temperatures.

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