Microstructural properties and tribological behaviours of Ultra-High frequency induction rapid sintered Al-WC composites

Uğur Çavdar^a,*, Levent Ulvi Gezici^b, Burak Gül^b, Mehmet Ayvaz^c

^a İzmir Demokrasi University, Engineering Faculty, Mechanical Engineering Department, İDU Campus, 35140, İzmir/Turkey

^b Celal Bayar University, Engineering Faculty. Department of Mechanical Engineering, 45140 Manisa, Turkey

^c Vocational School of Manisa Technical Sciences, Manisa Celal Bayar University, Manisa, Turkey

(*Corresponding author: ugur.cavdar@idu.edu.tr)

1. INTRODUCTIONTOP

Metal matrix composites (MMCs) are developed for optimum combinations of low weight and cost, high strength, abrasion and corrosion resistance. In the production of aluminum metal matrix composites (AMMCs), it is aimed to improve the mechanical properties of the monolytic aluminum alloys, which is the basic material of air, land and sea transportation vehicles due to their low density (Sun and Lei, 2008; Idusuyi and Olayinka, 2019; Imran and Khan, 2019; Panwar and Chauhan, 2018; Krishan et al., 2019). The mechanical properties which are intended to be developed are in particular weak creep, fatigue, tensile strength, abrasion and corrosion resistance (Sun and Lei, 2008; He et al., 2008; Yandouzi et al., 2009; Barbera et al., 2016; Roseline and Paramasivam, 2019; Ravindran et al., 2019; Giugliano et al., 2019). For this purpose, aluminum alloys are reinforced with oxide such as Al₂O₃, SiO₂ and TiO₂, boride such as TiB, nitride such as BN and carbide particles such as B4C, SiC and TiC (Busquets et al., 2005; Rodrigo et al., 2005; Fernandez et al., 2005; Egizabal et al., 2010; Gezici et al., 2018; Sivakumar et al., 2018; Jalilvand et al., 2019; Philip et al., 2019; Ao et al., 2019; Kvashnin et al., 2019; Zhou et al., 2019; Durmuş et al., 2019; Khodabakhshi et al., 2019; Shinde et al., 2019). One of the popular subjects of recent years is AMMCs, which is used as WC reinforcement as carbide particle (Li et al., 2019; Hegde et al., 2019; Banerjee et al., 2019; Ziejewska et al., 2019; Cardoso et al., 2019; Gopal Krishna et al., 2019).

In their study, Bernoosi et al. (2014) produced 1, 3, 5 and 7 wt.% WC reinforced composite samples having Al4.5Cu matrix composition by hot pressing method. Composite powder mixtures were obtained by planetary ball milling method. As a result of the tests carried out, 5 wt.% WC reinforced composite sample was determined as having the highest hardness and yield strength. When the WC reinforcement ratio of 7 wt.% was reached, the hardness and strength of the composite sample decreased.

Pakdel et al. (2017) produced nanoscale and micro WC particulate reinforced aluminum matrix composites using the spark plasma method. In the study, 1,5 and 10 wt.% WC was used in both nano and micro composites. In addition, 400, 450 and 500 ^oC spark plasma sintering temperatures were used in the study. As a result of the tests performed, it was observed that the sintering temperature had no significant effect on the hardness values of 1wt.% WC reinforced composite samples. However, hardness increased with increasing sintering temperature in 5 and 10% WC reinforced composite samples. In addition, in this study, a decrease in hardness was observed when the reinforcing ratio increased over 5 wt.% in nano WC reinforced composites.

Selvakumar et al. (2016) produced 2, 4, 6, 8 and 10 wt.% nano WC reinforced aluminum matrix composites by liquid metallurgy method. As a result of the tests, the Brinell hardness and density values of composite samples were increased and coefficient of friction (COF) and wear losses decreased with increasing WC reinforcement content. Pal et al. (2018) produced 1, 1.5 and 2 wt.% WC reinforced aluminum matrix composites with ultrasonic cavitation assisted stir casting method. Similarly, in this study, it has been determined that the wear resistance of composite samples increased with increasing amount of WC reinforcement. Simon et al. (2015) produced 5, 10 and 15 wt.% WC reinforced Al matrix composite samples by powder metallurgy method. In this study, the hardness values of composite samples decreased after 5 wt.% WC reinforcement ratio. Wear loss values determined after the wear tests of the samples also increased after 5% WC reinforcement rate, similar to previous studies. In addition, Al₂Cu and Al₁₂W intermetallic phases were determined in the study.

In the production of carbide reinforced aluminum matrix composites, it is desirable to prevent carbon reductions in the reinforcement, oxidation of the matrix and reinforcement, and the formation of intermetallic phases such as Al₄C₃ (Trujillo-Vazquez et al., 2016; Guo et al., 2018; Zhang et al., 2019). To achieve this, the sintering temperature or sintering time must be reduced. Sintering methods in which the sintering time and the temperature of an amount are reduced are called fast sintering methods. Induction, microwave and spark plasma sintering are widely used fast sintering methods (Torralba and Campos, 2014; Sarı Çavdar and Çavdar, 2015; Taştan et al., 2015; Taştan et al., 2019). In the literature, there are studies on Al-WC and Al-WC-Co composite samples sintered by microwave and spark plasma method (Yuying et al., 2010; Ghasali et al., 2015; Pakdel et al., 2017). However, there is no study of Al-WC composites produced using induction sintering which is another fast sintering method. Therefore, in this study, ultra-high frequency induction sinterability of 1, 3, 5, 9 and 15 wt.% WC reinforced composite samples and comparison of produced Al-WC composites with conventional sintering were performed.

2. MATERIALS AND METHODSTOP

In this study, aluminum matrix composite samples with 1, 3, 5, 9 and 15 wt.% WC reinforcing amount sintered using ultra-high frequency induction sintering and conventional sintering methods were produced and microstructural properties and tribological behavior of these composite samples were examined and compared. Both aluminum and WC powders used in the study have high chemical purity (99.8%) and average powder size of 75 microns. The powder contents of the composite samples were prepared using Acculab precision Weighing Balances with a sensitivity of 0.0001 g. The weighed powders were mixed and ball milled in the AISI 316 stainless steel box at 56 rpm for 30 min. AISI 316 stainless steel balls in a ratio of 1:10 were used to obtain a homogeneous powder mixture. The prepared powder mixtures were pressed with a pressure of 200 MPa in AISI D2 steel mold having a diameter of 16 mm by using unidirectional cold pressing method and green compacts were obtained. Some green samples were sintered at 600 °C for 1800 sec. by conventional method. For conventional sintering process, Protherm laboratory type furnaces with 10 °C min⁻¹. heating rate were used. The other portion of the green samples was sintered using the ultra-high frequency sintering (UHFIS) method at 600 °C for 300 sec. with a heating rate of 100 °C sec⁻¹. In the induction rapid sintering process of green samples, 2.8 kW power and 900 kHz ultra-high frequency induction generator and 20 mm diameter cylindrical coils were used.

The apparent density measurements of the sintered samples were carried out using the Archimedes principle in accordance with ASTM B962-17 (2017) standards. Hardness measurements were carried out by Brinell method in accordance with ASTM E10-18 (2018) standards. For these Brinell hardness measurements, 62.5 kgf and 2.5 mm diameter steel balls were used. Measurements were repeated a minimum of 5 times for each sample and average values were presented. For microstructural examination of samples sintered by sieved by conventional method and UHFIS method, samples were sanded with 100-1200 grid sanders. The surfaces of the sanded samples were polished with 3 and 1 μm diamond polisher, respectively. Keller’s reagent - 2ml HF (48%) + 3ml HCl + 5ml HNO₃ + 190ml H₂O were used for etching the sample surfaces. Carl Zeiss 300VP scanning electron microscope (SEM) was used in microstructural examinations in İzmir Katip Çelebi University Central Laboratory.

CSM Instruments tribometer tester was used to examine the tribological properties of aluminum matrix composite samples containing 3, 9 and 15 wt.% WC sintered by UHFIS and traditional methods. The tests were carried out in accordance with ASTM G99-05 standards. Pin-on-disk method was used as abrasion test method. 100Cr6 stainless steel balls with a diameter of 6 mm were used as spherical end pins. In tribological tests carried out at a sliding distance of 100 m, a load of 5N was applied and a shear rate of 10 ms⁻¹ was used. As a result of tribological tests, friction coefficients of the samples were determined depending on the sliding distance. Wear properties and regimes of WC reinforced composite samples were determined by SEM.

3. RESULTS AND DISCUSSIONSTOP

The volumes and weight values of WC reinforced composite samples sintered by conventional and UHFIS methods after compaction and after pressing are given in Tables 1 and 2. From these tables, it can be clearly seen that there were shrinkages in both UHFIS and composite samples sintered by conventional method. Due to the nature of the powder metallurgy production process, there are many open pores in the hard particle reinforced composite structure after pressing. These open pores, also found in WC reinforced composites, were somewhat closed after UHFIS and conventional sintering processes, and shrinkage occurred in composite samples. When the weight changes after sintering are examined, it is seen that the weight of composite samples sintered by UHFIS process was generally decreased. On the other hand, there was an increase in weight of all sintered samples by conventional method. In conventional sintered samples, the aluminum matrix was oxidized by sintering in an open atmosphere, thus increasing the weight of the composite samples. The weight was reduced due to less oxidation of the fast sintered samples by the UHFIS method and the evaporation of the mold lubricant that had penetrated into the composite sample during pressing. Similarly, in previous studies, it was found that aluminum oxide thickness increased gradually with increasing process time (Baran et al., 2014).

The apparent density values of green compacts and composites sintered by conventional and UHFIS methods depending on the amount of WC reinforcement are given in Fig. 1. In green samples, there was some increase in apparent density when WC amount increased from 1 to 3 wt.%, but after this point, apparent density decreased with increasing WC amount. This was due to the reduced compressibility of the composite material due to the increased amount of WC, thereby increasing the amount of porosity. After sintering, similar to literature, the apparent densities of the composite samples increased with the decrease of open pores (Simon et al., 2015). This increase was parallel to the increase in the amount of WC. Figure 2 shows the Brinell hardness of the composite samples sintered by UHFIS and conventional sintering methods. The Brinell hardness of composite samples decreased with increasing amount of WC. This is due to the fact that WC particles reduce the sinterability of the composite sample. Furthermore, it is seen that this decrease occurs more sharply in the induction fast sintered composite sample.

The SEM micrographs of microstructures of conventional and UHFIS sintered, 1, 3, 5, 9 and 15 wt.% WC reinforced aluminum matrix composite samples are given in Figs. 3 and 4, respectively. In the SEM micrographs, the grey area is Aluminum metal matrix, the white areas are WC particles and the black areas are open pores. When the SEM micrographs are examined, it can be seen that a homogeneous WC distribution is obtained in all samples. In addition, there is no apparent difference in the amount of porosity between the composite samples sintered by conventional and UHFIS methods. However, as shown in Fig. 5, intermetallic compounds having a needle-like structure were observed in the samples sintered by the UHFIS process. As an interesting and important data, in the EDS analysis obtained from this intermetallic phase formed by electromagnetic induction current as short as 300 seconds, this compound was found to be Al-Fe compound. The Al-Fe intermetallic phase formation has been observed in the literature in powder metallurgy method (Wang et al., 1998; Fathy et al., 2015).

The coefficients of friction of 3, 9 and 15 wt.% WC reinforced Al matrix composites sintered by conventional and UHFIS methods can be seen in Figs. 6 and 7, respectively. The point where the friction coefficient line makes the first peak is the coefficient of static friction of that sample. After this point, it gives the kinetic friction coefficient of the composite material. In general, friction coefficient values are parallel to the data in the literature (Selvakumar et al., 2016; Pal et al., 2018; Li et al., 2019). When the graphs are analyzed, it is seen that 15 wt. % WC reinforced composite sample has the highest coefficient of both static and kinetic friction. There was no significant change in the mean friction coefficients of 9 and 15 wt.% WC reinforced composite samples sintered by conventional and induction sintering method. However, with induction sintering, it was found that the average coefficient of friction of 3 wt.% WC reinforced composite sample decreased and the coefficients of friction of composite samples sintered by UHFIS method increased in parallel with increase in the amount of WC.

The SEM micrographs of the corroded surfaces of 3, 9 and 15 wt.% WC reinforced aluminum matrix composites sintered by conventional and UHFIS methods are shown in Fig. 8, respectively. When the micrographs of the worn surfaces are examined, it is seen that the dominant wear type was delamination in the sintered composites by traditional method. On the other hand, abrasive type wear as a dominant wear regime was observed in UHFIS method. This is understood from the flaking off and plastic deformation zones of the samples sintered by the conventional method and the deep abrasive grooves on the surfaces of the sample sintered by the UHFIS method. Of all composite samples, the largest wear scar was seen in the 3 wt.% WC reinforced composite sample sintered by the conventional method. In composite samples sintered by UHFIS method, wear scar width decreased with increasing amount of WC reinforcement and the narrowest wearing scar was determined in 15 wt.% WC reinforced sample sintered by UHFIS method in all composite samples. SEM investigations of worn surfaces generally show that composite samples sintered by the UHFIS method have a higher wear resistance than conventional sintered samples.

4. CONCLUSIONSTOP

In this study, sinterability, microstructure, wear and friction behavior of composite samples containing 1, 3, 5, 9 and 15 wt.% WC sintered at 600 ^oC, 1800 sec conventional method and 300 sec UHFIS method were investigated. The results can be listed as follows:

REFERENCESTOP