An Investigation of Wear Behaviors of AA7075 Al Hybrid Composites

Introduction

Aluminum composite materials (ACMs) constitute a widely preferred material group in many advanced material applications due to their low density and high performance. The poor wear resistance of aluminum alloys can be increased with addition of reinforcement phases [1, 2, 3, 4]. The ratio, shape, size and type of the reinforcement phase are quite important for increasing the resistance of such materials. The properties of the reinforcement phase are directly related with the intended use of produced ACMs [5]. These materials are widely used in the production of machine parts, especially in automotive industry, and most of them are exposed to friction. In ACM production, hard ceramic reinforcements such as Al₂O₃ [6], TiB₂ [7], SiC [8], B₄C [9] and TiO₂ [10] are generally used as reinforcement phase. These ex situ composite materials produced by adding hard-ceramic reinforcement phases stand out with their structural stability (especially in high temperature applications).

The most significant problem experienced with ex situ ACMs is the interface formed between the matrix and the reinforcement phase. The reinforcement phase can easily be removed with friction due to the interface formed between the matrix and the reinforcement phase during the production of ex situ ACMs. In such cases, the matrix cannot carry the load since the reinforcement phase does not perform its duty. In situ composite is the preferred production method to eliminate this problem in ACMs [11, 12, 13, 14, 15, 16]. The interest in these composites has been increasing since no interface is formed between the matrix and nano-sized reinforcement phases in the matrix formed in situ by adding different elements to the alloy. The most significant disadvantage of these materials is their low performance in high temperature applications (due to high increase in temperature). However, matrix–reinforcement phase interface incompatibilities observed in ex situ are not seen in in situ composites. For this reason, this study investigates wear performances of hybrid ACMs produced by adding 4 % Ti and different amounts of B₄C (ex situ) ceramic reinforcement phase in order to create in situ TiAl₃ reinforcement phase in the matrix.

Materials and method

In this study, gas atomized 7075 aluminum alloy powders (<100 µm) were used as matrix material. The chemical composition of gas atomized AA7075 aluminum alloy can be seen in Table 1.

Powders prepared by adding 4 % Ti (≤20 µm) and different amounts of (3 %, 6 %, 9 %) B₄C (≤10 µm) to AA7075 Al alloy were mixed in Turbula mixer at 67 rpm for 45 minutes. Mixed powders were cold-pressed under 600 MPa pressure and Ø12×7 mm green compacts were produced. Green compacts were sintered in argon atmosphere at 580 °C for 4 hours. After the sintering process, the samples prepared with standard metallographic procedures were etched with 95 ml pure water, 2.5 ml HNO3, 1.5 ml HCl, 1 ml HF (Keller’s) solution for 15–20 seconds. After the etching process, the samples were characterized. Scanning electron microscopy (SEM+EDS) (Zeiss-Ultra/Plus [FEG] model) and X-ray diffraction n (XRD) (Rigaku D-MAX RIN-2200 Series X-RAY Diffractometer) were used for structural characterization. SHIMADZU hardness measurement device (HMV 0.5) was used to measure hardness. Density measurements were performed according to the Archimedes’ principle. The mean value of five hardness measurements results is taken for each sample.

Wear tests were performed with standard pin-on disc-type wear test device in accordance with ASTM-G99-05 standards. Prior to wear testing, the samples were grinded with 1200 grit sandpaper and polished using 6 µm diamond paste to achieve the same surface quality for each sample. 1 ms^-1 sliding speed, 30 N load and five different sliding distances (500 – 3000 m) were used in wear tests. The wear disc and sample surfaces were cleaned with acetone before wear tests. Worn samples were weighed on 1/10,000 g precision scale to determine weight losses. Once wear tests were completed, sample surfaces were examined under SEM. The equation used to calculate the wear rate can be seen in eq. (1).

where, Wa is wear rate, Δm is weight loss after the wear test (g), M is the load weight (N), s is sliding distance (m) and ρ is density (gr/cm³).

Results and discussion

Figure 1 shows SEM images of hybrid ACMs produced by adding 4 % Ti and 9 % B₄C to the AA7075 Al alloy.

Figure 1:

Microstructure SEM images of hybrid ACMs produced by adding 4 % Ti (a) and 9 % B₄C (b).

As seen in the microstructure SEM images in Figure 1, in situ (a) TiAl₃ and ex situ (b) B₄C reinforcement phases showed a homogeneous distribution in the Al-Zn-Mg matrix and the structure. The images in Figure 1 also indicate that powder metallurgy is a reliable method to produce both ex situ and in situ composites. In addition, it is understood from the microstructure SEM images that B₄C reinforcement phase added ex situ partially localized in the structure. 10 µm or smaller powder sizes of B₄C, which was added to the alloy as the ex situ reinforcement element, allowed for localization of the reinforcement phase due to polarization (agglomeration) of powders. This case was pointed out in a previous study as well. Figure 2 shows XRD results of hybrid ACMs.

Figure 2:

XRD results of hybrid ACMs.

The XRD results given in Figure 2 show how certain reactions took place during the sintering process of hybrid composites between Ti and B₄C, which led to formation of new phases in situ. This new phase formed in the structure in situ is given in eq. (2).

B₄C added to the AA7075 is a structurally stable material with a high melting temperature (2723 °C). The B₄C phase is seen in the structure from different angles in the XRD results. Also, it is understood that TiAl₃ phases was formed in the composite material in situ. It was reported in some previous studies that TiAl₃ intermetallic phase was formed at temperatures between 500–600 °C [11, 17, 18]. Reinforcement phases formed in the structure in situ significantly contributes to the increase in strength of composites. The formation of in situ reinforcement phases during the sintering of ex situ composites produced by adding B₄C improves properties of hybrid composites and the high strength factor expected from these composites. Also, the most significant contribution of phases formed in situ is that no matrix-reinforcement phase interface is formed unlike ex situ composites [19].

Final properties of hybrid composites depend on individual properties of selected matrix and reinforcement phases. In addition, the method used for production of hybrid ACMs depends on the nature of the matrix alloy and the reinforcement material, which are effective on final properties of ACMs. Figure 3 shows hardness and density changes in the hybrid composites produced using the powder metallurgy method.

Figure 3:

Hardness (a) and density (b) changes in the hybrid composites produced using the powder metallurgy method.

It can be understood from hardness changes shown in Figure 3 that the hardness varied from 112 to 123 HV with the addition of 4 % Ti and different amounts of B₄C to the AA7075 alloy. The increase in the amount of ex situ B₄C reinforcement phase and the TiAl₃ phase formed in the structure in situ increased the hardness of hybrid composites. Density measurements showed that the increase in the amount of B₄C decreases density. This is due to the lower density of B₄C compared to the matrix phase of AA7075. It was reported in a previous study that the increased sintering time of in situ composites led to an increase in hardness and density [17]. In ex situ composites, on the other hand, the contribution of the reinforcement element in the structure depends on the amount added. For this reason, the increase in the hardness of B₄C added composites changes in parallel with the increase in the amount of reinforcement phase added [20]. In addition, the increase in the ratio (wt%) of reinforcement phase in composites produced ex situ results in an increase in hardness [21, 22,23, 24]. Figure 4 shows weight losses and wear rates of the hybrid composites after wear tests.

Figure 4:

Weight losses (a), wear rates (b) and friction coefficients (c) of hybrid composites produced using powder metallurgy under 30 N load.

The weight loss results given in Figure 4 (a) show that the weight loss increased with increasing sliding distance in all composites. The weight loss decreased in all composites as the amount (wt%) of B₄C increased. It was also pointed out in previous studies that the increase in reinforcement phase resulted in a decrease in weight loss in ACMs produced ex situ [6, 7]. The results obtained are confirmed by the hardness results in Figure 3. The lowest weight loss among ACMs was obtained for the hybrid composites produced by adding 4 % Ti and 9 % B₄C. However, the weight loss increased rapidly after 2500 m especially for the hybrid composites produced by adding 4 % Ti and 6 % B₄C. The reason for this sudden increase in weight loss is the particles detached from the sample surface during the sample-disk contact. There are two reasons for these fractures. The first reason is that the composites were produced using the powder metallurgy method. Such fractures may be observed in samples when bonds formed between powders during sintering are weak. The second reason is the interface formed between matrix-reinforcement phases in composite materials with metal matrix produced by adding B₄C ex situ. Weak bonds formed between the matrix and the reinforcement element due to the interface result in fractures when forced. For this reason, reinforcement phases detach from the matrix due to breaking of weak bonds during wear tests with the increase in sliding distance, which results in an increase in weight loss. Wear rate results of ACMs given in Figure 4 (b) show that the wear rate increases with the sliding distance. The results obtained are supported by the weight loss results as well. This linear relationship between the weight loss and the wear rate is not observed in the friction coefficient results given in Figure 4 (c). The lowest friction coefficient was obtained for the 4 % (wt%) Ti 3 B₄C alloy. Hardness results given in Figure 3 show that graph followed a different route while we expected a decrease in friction coefficient with the increase in hardness. This is believed to be due to the interface effect between the matrix and the reinforcement phase which occurred during friction. B₄C particles detached due to the weak interface bond between B₄C and the matrix and were stuck between the disc and the sample leading to a different friction coefficient. Figure 5 shows the wear surface SEM images of ACMs produced by adding 4 % Ti and different amounts of B₄C.

Figure 5:

Wear surface SEM images of hybrid composites tested under 30 N load: 4 % Ti (a), 4 % Ti+3 % B₄C (b), 4 % Ti+6 % B₄C (c) and 4 % Ti+9 % B₄C (d).

Wear surface SEM images in Figure 5 clearly demonstrate the sliding direction. It is understood that the adhesive wear mechanism was predominant on wear surfaces in general. Micro chip particles detached from the wear surface reattached to the surface due to heat and damages such as scratches, smearing, flakes and tears occurred on wear surfaces of the samples. Deformation usually occurred on small microridges. As the amount (wt%) of Ti and B₄C in the structure increased, particles detached from the surface led to an increase in both the weight loss and the wear rate.

Conclusion

The results of the present study which investigates wear behaviors of in situ and ex situ hybrid composites are given below.

1. It was observed in microstructure examinations of composites produced by adding Ti and B₄C to gas atomized AA7075 powders that reinforcement phases showed a homogeneous distribution in the structure.

2. In hybrid aluminum composites, the increase in the amount (wt%) of B₄C resulted in a decrease in density and an increase in hardness. Also, the weight loss decreased as the amount (wt%) of B₄C increased.

3. Among in situ aluminum composites, the lowest friction coefficient was obtained for the 4 % (wt%) Ti 3 B₄C alloy, whereas the highest weight loss was obtained for 4 % Ti composites.