XU Lixiao, YAN Hong*, LIU Wei, XIONG Junjie

(1. School of Mechanical and Electrical Engineering, Nanchang University, Nanchang 330031, China; 2. Key Laboratory of Light Alloy Preparation and Processing, Nanchang 330031, China)

Abstract: The microstructure and microhardness of ADC12 alloy that was mixed with 0, 0.3, 0.6, and 0.9 wt.% rare earth praseodymium/cerium (Pr/Ce) were studied. The addition of Pr/Ce improved the microhardness of the alloys. The ADC12+0.6 wt% Pr/Ce alloy displayed the smallest grain size and maximal microhardness.The tribological behavior of the alloys was tested by the pin-on-disc dry sliding friction pair with a sliding velocity of 0.21 m/s under various loads (20,40,60,80 N). The wear morphology was observed by a scanning electron microscope (SEM) and the wear mechanism was discussed. The result indicated that the wear resistance of ADC12+0.6 wt% Pr/Ce alloy was the most optimal. The wear rate relative to the matrix is reduced by 67.5%under a load of 20 N. The wear mechanism is adhesive wear.

Key words: Pr/Ce; ADC12; microstructure; microhardness; wear mechanism

1 Introduction

The Al-Si-Cu alloy ADC12 is a cast aluminum alloys. It has a low density, high electrical conductivity,easy processing, and corrosion resistance, and therefore have an important role in industrial structural materials, aerospace, home appliances and automobiles[1].However, the application of these cast aluminum alloys in the manufacture of critical safety parts has been restricted in automobiles. This is because the friction and wear problem of aluminum alloys cannot be ignored.The application of aluminum alloys may directly or indirectly cause damage to mechanical parts, and lead to decreased tolerance, then shortening the service life of the mechanism. The goal is to increase the wear resistance of aluminum alloys and the service life of the workpiece[2]. Early reports revealed that the addition of single rare earth elements could enhance the mechanical and tribological properties. Huang[3]studied

the modification of rare earth elements lanthanum (La)on the ADC12 alloy. Anasyidaet al[4]reported that 2 wt% cerium (Ce) increased the wear resistance and hardness of the Al-12Si-4Mg alloy. Research on the addition of mixed rare earth to aluminum alloys was also performed. Liet al[5]improved the microstructure and mechanical properties, such as wear resistance of AlSi10Cu3 alloy by adding mixed rare earth elements lanthanum/ytterbium (La/Yb). ADC12 alloy has a good casting property and is used in car shock absorber electric tool meter body, but it lacks a good wear resistance. Songet al[6]improved the microstructure and mechanical properties of Al-7Si0-0.7Mg alloy by adding rare earth elements praseodymium/cerium (Pr/Ce). Fanget al[7]demonstrated that the grain of AlSi5CuMg alloy was refined by rare earth elements Pr/Ce,and its ultimate tensile strength reached a maximum.Several authors have investigated the influence of Pr/Ce on the microstructure and mechanical properties of Al-Si alloy, but not the effect of Pr/Ce on tribological behavior. The present study reports the effects of rare earth elements Pr/Ce (0, 0.3, 0.6, and 0.9 wt%) on the wear resistance of ADC12 alloy. The ADC12 alloy with Pr/Ce was prepared for tribological behavior. The tribological behavior of the composites was tested by the pin-on-disc dry sliding friction pair with a sliding velocity of 0.21 m/s under different loads (20, 40, 60 and 80 N). The wear mechanism under different conditions was discussed in detail.

2 Experimental

2.1 Preparation of experimental materials

The composition of matrix ADC12 alloy was determined by using ICP-AES (inductively coupled plasma atomic emission spectrometry, Beijing Huake ETS Analysis Instrument Co., Ltd., Beijing, China) testing,as shown in Table 1. Mixed rare earth elements Pr/Ce was added to the matrix by means of an intermediate alloy. First, the binary intermediate alloy (Al+10 wt%Pr and Al+10 wt% Ce) were prepared by using pure aluminum (99.9%), Pr (99.9%), and Ce (99.9%), respectively. The Al+10 wt% Pr and Al+10 wt% Ce intermediate alloys were mixed in a 1:1 mass ratio and cast into a ternary intermediate alloy Al+5 wt%Pr+5 wt%Ce. Second, the matrix ADC12 alloy was melted in a corundum crucible at 740 ℃ and then ultrasonically oscillated for 5 mins after adding the ternary intermediate alloy wrapped with aluminum foil paper. Finally, mixed melt heat preservation was kept for 40 mins, then the melt was poured into a preheated metal mold, in order to produce the composites ADC12+xwt% Pr/Ce (x= 0,0.3, 0.6, and 0.9).

Table 1 Chemical composition of ADC12 aluminum alloy/wt%

2.2 Experimental procedure

Table 2 Detail of friction and wear test

The samples were processed into a pin shape ofФ4.5 mm × 11 mm. The tested surface of the pin was polished with sandpaper to a roughness not more than 0.1 μm, and ultrasonically washed with ethanol and dried. The microstructure of the composites was observed by using an optical microscope (OM Eclipse MA200, Nikon Metrology, Inc. Brighton, UK). The Vickers microhardness of the composites was obtained by using an HV1000 hardness tester device (Lanzhou Huayin Testing Instrument Co., Ltd., Laizhou, China).MMD-1(Jinan Yihua Tribology Testing Technology Co., Ltd., Jinan, China) multi-functional and wear tester was used for the tribological test (Table 2). The experiment was carried out in the form of pin-on-disc dry sliding friction. At the same time, the friction pair polishing was required and the friction coefficient of the experiment was automatically generated on the device directly. After the test, it was ultrasonically cleaned for 5 mins, then dried and weighed. The weight loss of the samples was obtained by using the FA2204B electronic balance weighted pin before and after the test. In order to get the volume loss, the weight loss of the samples was transformed by using the density of each sample.The wear surface was observed by scanning electron microscopy (SEM) with an energy dispersive spectrum(EDS) to analyze the wear mechanism.

3 Results and discussion

3.1 Microstructure

Optical micrographs of as-cast ADC12 alloy with various contents of mixed rare earth Pr/Ce additions are shown in Fig.1. With the increase in Pr/Ce content from 0 to 0.6 wt%, the microstructure of ADC12 alloy is continuously modified. Pr/Ce additions affected all phases: a-Al, iron, and silicon. Theα-Al has a disorderly arrangement with a thick dendrite (Fig.1(a)). The eutectic Si phase in a matrix is a rough long needle-like structure. The iron-rich phase in the structure is massive and strip-shaped and of a relatively large size.Addition of 0.3 wt% Pr/Ce (Fig.1(b)) causes a decrease of the dendritic a-Al phase and a shorten structure of the silicon phase, although some primary silicon does not disappear. The iron-rich phase in the structure has a coral-like appearance and became smaller than the matrix.

Fig.1 Microstructure of ADC12 alloy sample with different contents of Pr/Ce: (a) 0 wt%, (b) 0.3 wt%, (c) 0.6 wt%, (d) 0.9 wt%

Fig.2 Bake scanning electron microscope (BSEM) micrograph of ADC12+0.6 wt% Pr/Ce alloy and energy dispersive spectrum (EDS) analysis of region A

When 0.6 wt% Pr/Ce was added (Fig.1(c)),α-Al is distributed uniformly and become finer. The primary silicon almost disappears, and the rough needle-like eutectic Si becomes shorter, smoother, and rounder.The iron-rich phase becomes finer and granular. Addition of 0.9 wt% Pr/Ce (Fig.1(d)) leads to being elliptically shaped a-Al, with increasing size. The eutectic Si becomes short rod-shaped, coarser than shown in Fig.1(c). It could be attributed to a poisonous effect by surplus Pr/Ce rather than the prevention of silicon nucleation, although the primary silicon is considerably reduced. The iron-rich phase exhibits a dendritic morphology and its size is larger than the phase in Fig.1(c).

Fig.2 illustrates the Bake scanning electron microscopy (BSEM) image and EDS analysis of the alloy with 0.6 wt% (Pr/Ce). From the binary phase diagram of Al-Pr and Al-Ce, there is a reaction between rare earth (Pr/Ce) and Al[8]: L→a-Al+Al11RE3. When adding to the rare earth, Pr/Ce has a reaction with Al, and it generates five to six types of compounds such as Al11RE3, Al3RE, and AlRE3[9]. It may be attributed to intermetallic compounds generation. This can improve the alloy properties, and it can also promote wear resistance.

3.2 Microhardness

Hardness is positively correlated with wear behavior[10]. The hardness of the material must be known before the wear test[11]. The amount of added rare earth elements could affect the hardness of the as-cast alloy.The effect of different rare earth additions on the microhardness of the alloy is shown in Fig.3. It indicates that the microhardness of the ADC12+0.6 wt% Pr/Ce alloy is 43% higher than that of the matrix alloy, and the microhardness of the alloy reached a maximum. However,the microhardness declines when the addition of Pr/Ce increases to 0.9 wt%. The change in hardness is due to the nascent alloy. The size of the Si particles changed as seen in Fig.1. But when the contents of Pr/Ce were 0.9 wt%, the size of the Si phase becomes larger than that of the ADC12+0.6 wt% Pr/Ce alloy. The change in Si morphology improves the stress concentration on the interface of the particle matrix. The diversification of microhardness can be explained by the Hall-Petch equation[12,13]:

where syis the yield load and can also be expressed in Vickers microhardness[14]. s0is the starting load of the dislocation motion which is a constant.d'is the grain size,kyis a strengthen coefficient which is a specific constant of the material. When the grain size becomes fine, the hardness increases.

Fig.3 Microhardness of ADC12 with different contents of mixed rare earth elements Pr/Ce

3.3 Wear rate

Wear rate is the amount of wear per unit friction distance. In addition, the wear rate and the coefficient of friction are highly correlated. This amount is used to compare the wear rates of different types of materials.According to the wear rate equation[15]as follows:

wherekis the wear intensity and it can be expressed by the wear coefficient,Wis the vertical load;His the average microhardness. Under the dry sliding friction of 126 m (equivalent to 600 s×0.21 m/s), the relationships between the wear rate and different contents of Pr/Ce are shown in Fig.4. Fig.2 and Fig.3 show that the hardness of the alloy is inversely proportional to the wear rate[16]. The wear rate reaches a minimum at 0.6 wt%Pr/Ce and then increases with the increasing amount of Pr/Ce. The wear rate increases as the load increases for all alloys. It is indicated that the wear resistance of the added Pr/Ce alloy is better than that of the matrix.

Fig.4 Wear rate of ADC12 alloy with different contents of mixed rare earth elements Pr/Ce under different normal loads

Fig.5 Wear rate of ADC12+0.6 wt% Pr/Ce alloy

In addition, the wear behavior of the Al-Si alloy is also related to the microstructure characteristics of the alloy[17]. Wear rate relative to the matrix alloy under different loads is reduced by 67.5%, 53.6%, 33.8%, and 31.5%. When 0.6 wt% Pr/Ce is added, the wear rate is the minimal. This is because the primary Si phase of the alloy is finer than other contents, and it is tightly bonded by the aluminum matrix in their position to resist the damaging effect of friction. However, when the addition increased to 0.9 wt%, the wear rate increases.This is due to the increase in primary silicon.

As shown in Fig.5, the wear rate of the ADC12+0.6 wt% Pr/Ce alloy under different normal loads is positively correlated with the load. It is revealed that the increase in normal load could make the wear resistance of the alloy deterioration. Two sliding surfaces generate frictional heat, which increased the interface temperature[18]. When sufficient internal energy reaches a critical value, the material cracks. In this study, the material is detached under external load, then the wear rate increased with the increase of normal load.

3.4 Coefficient of friction (COF)

Sliding velocity and normal load have a serious impact on the friction properties of the material[19]. The relationships between the average coefficient of friction(COF) and different loads are shown in Fig.6. The coefficient of friction is always changing because friction force depends on three main factors[20]: asperity deformation, surface adhesion, and particle hardness. While these three factors are changing with the sliding distance, the coefficient of friction is changing with sliding time. As can be seen, the COF of the ADC12 alloy increases by 16% when the applied normal load increases from 20 to 80 N. These results can be attributed to the increase of the real contract area. When the normal loads increase, the uneven surface contact area between the pin and the friction increase. While the slide occurs,the uneven surface can hinder the friction, which can increase the shear force. Plastic deformation occurs and thereby increases the coefficient of friction. As shown in Fig.6, the friction coefficients of different contents of mixed rare earth of ADC12 alloy are smaller than that of the matrix alloys, thus the frictional coefficients of the alloys adding to Pr/Ce in the content of 0.6 wt%are satisfactory. This is attributed to the addition of the mixed rare earth elements improving the microstructure and refining the size of the grain. This corresponded to the Bhushan B[21]statement that different grain morphology in Al-Si alloy affects the wear and mechanical properties of the alloy.

Fig.6 Different contents of Pr/Ce coefficient of wear

When the addition of the Pr/Ce is 0.6 wt%, the fluctuation of the friction coefficient increases as the normal load increases (Fig.7). As can be seen, at the beginning of the experiment, the coefficient of friction of the alloy fluctuates greatly. This is because the pin and friction pair have a running-in- phase. During this time, friction surface detached particles contribute to increased friction coefficients.

Fig.7 Dynamic coefficient of friction of ADC12 +0.6 wt% Pr/Ce alloy under different load: (a)20 N; (b)40 N; (c)60 N; (d)80 N

3.5 Wear surfaces

The study of the wear morphology reveals information about many information about the mechanism of friction and wear[22]. The scanning electron micrographs (SEM) of wear surface for ADC12+xwt% (x=0, 0.3, 0.6, and 0.9) Pr/Ce alloys is in 0.21 m/s velocity and 60 N normal load (Fig.8). Typical surface damage such as cracks, grooves, and abrasive scoring marks are observed. The wear surface of the ADC12 alloy has parallel grooves and pits (Fig.8(a)). The surface of the groove has cracks, and the plastic deformation area is scratched indicating that the wear mechanism is mainly abrasive wear and surface fatigue wear. The grooves are formed by abrasive contact between the hardness steel plate and the aluminum pin. The cracks are caused by the plastic deformation of the hard material surface. The grinding debris is produced by the effect of a circling normal load, and a small amount of debris is pressed into the crack from the surface to the inside slowly in-depth, thus contributing to the further expansion of the crack. When the crack reaches a certain depth, the crack is broken and pits are formed. The appearance of wrinkles in the area of plastic deformation is an indicator of the determination of adhesive wear.When the addition of Pr/Ce increased to 0.3 wt%, the disappearance of the crack indicated that the dominant wear mechanism is adhesive wear (Fig.8(b)). In a reciprocating movement, debris is pulled off one surface and attached to the other. Then the wear debris may peel off the surface from which they are formed and return to the original surface, or form loose wear particles. Generally speaking, fine, spherical and uniformly distributed microstructure can improve the wear and mechanical properties[23].

Fig.8 Scanning electron microscopy of the worn surface: (a)ADC12; (b) ADC12+0.3 wt% Pr/Ce; (c) ADC12+0.6 wt%Pr/Ce; (d) ADC12+0.3 wt% Pr/Ce alloys subjected to 60 N

When the addition of Pr/Ce is increased to 0.6 wt%, as shown in Fig.8(c), wear surface has many ploughs and grooves parallel to the direction of friction.The main wear mechanism of ADC12+0.6 wt% Pr/Ce alloy with 60 N is abrasive wear. This is mainly due to two reasons: refinement of microstructure by primary silicon that is finer in size and tightly bonded to the matrix to resist the damage of sliding; alloys have maximum hardness. When the addition of Pr/Ce increased to 0.9 wt% ( Fig.8(d)), groove and plastic deformation are seen, the alloy exhibits adhesive wear, and the abrasion grooves are wider and the plastic deformation is lighter than that of the matrix alloys, which indicates that the wear mechanism is a deteriorating effect of the cation[24]. When the aluminum pin plate is in contact with the steel friction pair, the atomic and molecular bonding forces at the interface react, so there is no tendency to adhere. However, under the force of sliding, two surfaces embedded in the process of behavior. The surface of the aluminum pin produces a certain amount of wear and tear and may adhere to the greater hardness (steel disk). This phenomenon is called metal transfer[25]. The surface of the alloy and the friction pair eventually fall off as abrasive debris under continuous sliding, so the dominant wear mechanism is abrasive wear and adhesive wear.

Fig.9 shows the wear surface of ACD12+0.6%Pr/Ce alloy at 20, 40, 60 and 80 N. The wear surface is relatively smooth with the effect of 20 N (Fig.9(a)), and a few small dimples and a finely scratched surface are smooth. As the normal load increases, the characteristics of adhesive wear are more obvious. With increased load, the work done against friction generates more heat, then the temperature of the contact surface also increases. The interface temperature has a significant impact on the wear behavior of casting Al-Si alloy[26].As shown in Fig.9(b), under normal load 40 N, the wear surface is hardly dimpled, and the edges of the scratches are wider than that of the normal load 20 N.The wear mechanism is mainly abrasive wear. When the normal loads increase to 60 N (Fig.9(c)), the wear mechanism is abrasive wear. When the normal load is 80 N (Fig.9(d)), a large area of pits appears on the wear surface, and wavy layered wear surface appears in the pits, indicating adhesive wear.

Fig.9 Scanning electron microscopy of the worn surface of different loads of ADC12+0.6 wt% Pr/Ce: (a) 20 N; (b) 40 N; (c)60 N; (d) 80 N

Fig.10 Schematic diagram showing the wear crack formation

When two sliding surfaces contact, the normal loads transfer adhesion and plowing action through the contact point. Soft surfaces are easily deformed and some are fractured by the reciprocating loads. Then the relatively smooth surface is generated, and uneven point contact becomes uneven surface contact. However, the hard surface is always uneven. The contact is not only the uneven point contact but becomes the uneven surface contact. Every point on the soft surface experiences cyclic loading under the condition of the uneven hard surface. The plastic shear deformation is caused by the traction force produced by the unevenness. Plastic shear deformation accumulates with repeated loading. As the subsurface deformation continues, cracks are nucleated below the surface. Once a crack occurs,further loading and deformation contributes to the expansion of crack and connects with adjacent cracks(Fig. 10). Cracks tend to diffuse on parallel surfaces at depths controlled by the properties and coefficients of the material, when they eventually shear to the surface(at a weak point), and the long and thin wear plates are layered[27]. Then the wear surface forms delamination.

4 Conclusions

a) Addition to the mixed rare earth elements can refine the grain size. When the contents of Pr/Ce are 0.6 wt%, it had an optimal microstructure. The primary silicon halmost disappeares, and it is replaced by finer,round, shorter and smooth granules. The iron-rich has became fine granular. The a-Al particles become uniformly finer.

b) The microhardness of ADC12+0.6 wt% Pr/Ce alloy reaches a maximum which is 1.22 times higher than that of matrix alloy. In dry sliding, the wear rate increases with increasing of the load, and the content of 0.6 wt.% has a good reduction.

c) The coefficient of friction is always changing with time. The coefficient of friction of matrix ADC12 increases by 16% with load from 20 to 80 N. The ADC12+0.6 wt% Pr/Ce alloy has the optimal coefficient of friction.

d) The results of the wear tests demonstrated that by increasing the amount of normal load, the wear rate loss is increased. With the normal load, the wear resistance of the modified alloy is better than the unmodified alloy. In addition, the ADC12+0.6 wt% Pr/Ce alloy has optimal wear resistance

e) The abrasive wear is the main wear mechanism for matrix ADC12 alloy and ADC12+ 0.6 wt% Pr/Ce alloy. The wear mechanism of ADC12+0.3 wt. Pr/Ce alloy and ADC12+0.9 wt% Pr/Ce alloy is adhesive wear.