An investigation on dry sliding wear behaviour of pressure infiltrated AA1050-XMg/B₄C composites

2.1 Material preparation

In this study, for preparing a metal-matrix composite, AA1050 aluminium alloys were used as matrices containing various Mg alloying additions (0, 1, 2 and 4 wt.% Mg); boron carbide in powder form was used as the reinforcement. Table 1 presents the designations used to represent each grade of the composites produced. B₄C particulates with a mean particle diameter of 48 μm, supplied from BOROPTİK, Turkey. The volume fraction of the B₄C particles was 60±3%. The composites were fabricated into a cylindrical preform of 7 mm diameter. The infiltration vessel was pressurised with argon to 8 bars (0.8 MPa), and the molten alloys were forced into the B₄C compacts at 800°C.

2.2 Structural analysis

For microstructural analysis, composites were cut and hot mounted. The specimens were automatically polished using standard metallographic practices prior to their microstructural examination by optical microscopy (Nikon Epiphot 200 model, Japan) and scanning electron microscopy (SEM, Carl Zeiss Ultra Plus Gemini Fesem model, Germany). Volume fraction was performed using the optical microscopy with Clemex software (Japan). X-ray diffraction (XRD, Rigaku Ultima IV model, Japan) was used to follow the structural changes for various Mg alloying additions.

2.3 Mechanical tests

The mechanical properties of samples were investigated by hardness and compression testing. The Rockwell A hardness test was carried out in this work to find out the deformation of the composite under constant load (60 kg). Cylindrical specimens with the length-to-diameter ratio of 2:1 (ASTM E9) [13] were prepared from consolidated AA1050-B₄C composites and used for compression tests. The compression tests were conducted on the Zwick/Roell Z600 universal testing machine with a cross-head speed of 0.5 mm/min.

2.4 Sliding wear tests

Sliding wear tests were conducted in pin-on-disc wear testing apparatus (ASTM G99) at different loads (10–40 N) for a fixed sliding speed of 0.5 m/s against a 1040 steel disc of hardness 59 HRC. The pin samples were 30 mm in length and 7 mm in diameter. Wear tests were repeated three times for each sample. The samples were cleaned and weighed before and after each test. The wear rate was calculated from the weight loss technique.

3.1 Microstructural features

Microstructures of samples 1 (S1) and 4 (S4) are shown in Figure 1. Microstructural examination of all samples showed that B₄C particles were uniformly distributed in the aluminium matrix. Microstructural examinations showed that the composites contained some porosity mainly concentrated near the tips of the B₄C particles. The resulting porosity, reinforcements and matrix obtained by image analysis are given in Table 1. Porosity is also a very important parameter that influences the mechanical properties of the composite. Porosity amount can be reduced with the use of matrix elements with higher wettability [14], and a stronger bonding is achieved between matrix and reinforcing elements. In the production of ceramic-reinforced Al matrix composites, there is an Al₂O₃ oxide layer between the Al matrix and reinforcement material and the wettability of ceramic particles of the matrix is difficult due to the high affinity of aluminium to oxygen [15]. However, especially with the addition of surface active elements such as Mg into the alloy MgO or spinel (MgAl₂O₄) which include Mg can occur and the contact angle decreases. Many researchers have highlighted the role of MgAl₂O₄ in modifying the oxide layer that covers the surface of the liquid aluminium, and hence the wettability is achieved [16], [17], [18], [19], [20], [21], [22], [23]. The porosity ratio showed a decrease at significant levels with the addition of Mg and composite materials with the porosity of 0.77% were produced with the addition of 4% Mg. XRD patterns obtained from the S1, S3 and S4 composites are shown in Figure 2. XRD analysis (Figure 2) showed that Al₃BC, AlB₂ and AlB₁₀ [24], [25], [26], [27], [28] phases were present in the AA1050 matrix composite, whereas the Al₃₇Mg₃ phase appeared when Mg was added to the AA1050 matrix in addition to these phases. The fact that the Al₃₇Mg₃ phase which was determined with XRD analysis was a grey sharp-edged structure illustrated with 1 in Figure 1B was supported by the Energy Dispersive X-Ray Analysis (EDX) results (Table 2) obtained through this phase.

Figure 1:

Microstructure of (A) S1 and (B) S4 composites.

The arrows show some of the porosities present in the compacts.

Figure 2:

XRD patterns of (A) S1, (B) S3 and (C) S4 alloy matrix composites.

3.2 Mechanical behaviour

In Figure 3A, the hardness of the investigated composites is plotted with respect to Mg content of the matrix. While the addition of 1% Mg to the AA1050-XMg alloy matrix composites increased the hardness at the rate of 25%, the increase in hardness of the composites containing 2% and 4% Mg decelerated. This increase observed in hardness as a result of the addition of Mg to the AA1050 matrix was caused by the precipitation of secondary phases (Al₃BC, AlB₂, AlB₁₀ and Al₃₇Mg₃) specified in microstructure and XRD analyses.

Figure 3:

Hardness (A) and compression (B) results of the investigated samples.

The results obtained from the compression tests as a function of Mg content of the matrix are shown in Figure 3B. When compared with AA1050 matrix composite materials, the maximum compression strength increased at the rate of 65% with the addition of 4% Mg. This situation was also compatible with hardness results. This enormous increase in the compression strength was achieved through the increase of matrix/B₄C interface bond strength caused by the wettability that increased with the addition of Mg as well as the precipitating secondary phases [26], [27].

3.3 Wear behaviour

The weight loss of the investigated samples obtained from adhesive wear tests is shown in Figure 4 as a function of the sliding distance. The weight loss of the samples increased linearly with increasing sliding distance. Wear rate is obtained from the slope of data plots in Figure 4. The effect of Mg addition on the wear rate of the investigated samples is shown in Figure 5.

Figure 4:

The effect of the sliding distance and the applied load on the weight loss of (A) 10, (B) 20, (C) 30 and (D) 40 N.

Figure 5:

The effect of the applied load on the adhesive wear rate of the investigated samples.

The increase of wear resistance with the addition of Mg could be attributed to two reasons. The first reason was that the wettability of the matrix element increased with the addition of Mg, and as a result, the porosity decreased to a large extent [26]. Especially after the porosities formed on particle edges were reduced to minimum with the addition of Mg, an increase in wear resistance was observed. The second reason of the increasing wear resistance was the formation of Al₃₇Mg₃ phases which the measured hardness was 1204 HV_0.05. The formation of phases in the structure as a result of the addition of Mg increased the matrix hardness of the composites.

In addition, the formation of phases such as Al₃B₄₈C₂ that contain aluminium and boron carbide is frequently mentioned in the literature. It is thought that these phases also affected positively the wear resistance especially in low loads [25], [28].

As shown in Figure 4, the slope of “weight loss-sliding distance” graphics in low loads (10–20 N) consists of two different areas. While weight loss increased rapidly up to 3000 m, the weight loss alteration decreased between the sliding distances 3000 and 12,000 m, and the graphic gained a stable state. It was determined that while the weight loss increased in high loads (30–40 N), the beginning wear period observed in low loads continued throughout 12,000 m and the stable wear period did not occur. On the other hand, the increase in the wear resistance obtained in low loads as a result of the addition of Mg could not be achieved in high loads and the weight loss and wear rate in composites containing different rates of Mg were observed to be almost close to the AA1050 matrix composites. As a general rule of wear, the increasing hardness of the material increased the wear resistance. However, if the material becomes too brittle, the hardness increases, and the wear resistance can be affected adversely in high loads [29]. It is thought that secondary phases and particles precipitating within the structure during the wear in high loads ruptured and weight loss of the material dramatically increased.

General views of the worn surfaces of the investigated samples are shown in Figures 6 and 7. When the SEM images of worn surfaces were examined, a groove formation and piece plastering were observed, which indicated that the wearing had adhesive wear characteristics. The weight loss in composites produced with the addition of Mg in low loads decreased at least at the rate of 50%, whereas the weight loss was close to the original sample level in high loads. Figures 6 and 7 illustrate the SEM images of worn surfaces obtained from the surfaces after the adhesive wear experiment. On examination of the SEM images taken from the samples worn, it was observed that while the wear had plastering characteristics in low loads, wear characteristics underwent some changes in high loads. Wear mechanism under severe loads (40 N) involved cracking and pulling out of B₄C particles in addition to plastering (Figure 7C). The EDX analysis of the worn surfaces (Tables 3 and 4) showed that the formation of the plastering layer contained high amount of O, B and C without Fe originated from the counter surface. Although Sun et al. [29] reported that the plastering layer with O and Fe increased wear resistance due to the lubricating quality, in this study, the plastering layer formed without Fe was less stable. Thus, the role of this layer reducing weight loss is not considered to be significant. The precipitation of secondary hard phases and reducing porosity as a result of the addition of Mg to the AA1050 matrix has the major effect on the increase of the wear resistance.

Figure 6:

Worn surfaces of S1: (A) 10, (B) 20 and (C) 40 N.

Figure 7:

Worn surfaces of S4: (A) 10, (B) 20 and (C) 40 N.

On the other hand, when both the wear rate and the SEM images taken from the surface were examined, it was obvious that the applied load of 40 N is the threshold value in composites produced by using the AA1050-4Mg matrix element. This threshold value even decreased to 20 N for AA1050-1Mg matrix composites, while it was 30 N for AA1050-2Mg matrix composite. The fact that the threshold load value to be applied on sample 4 here was higher compared with the other composite materials can be explained by the decrease in porosity to a great extent with the addition of Mg.