EFFECT OF WC REINFORCED ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF CUALMN ALLOYS PRODUCED BY HOT PRESSING METHOD

In this study, the effect of WC reinforcing particles on the microstructure and mechanical properties of CuAlMn and CuAlMn-WC alloy produced by powder metallurgy method was investigated by adding 5 %, 10 % and 15 % by volume WC to CuAlMn alloy. Cu, Al, Mn and WC powders of approximately 99.9 % purity with a grain size of 325 mesh were used in the production of the alloys. The samples were produced by hot pressing method at 900 °C temperatures under 35 MPa pressure for 6 minutes. Microstructure, phase formation, hardness and corrosion properties of the samples were investigated in detail. Scanning electron microscopy (SEM) was used for microstructure analysis and X-ray diffractogram (XRD) was used for phase formation detection. The hardness measurements of the samples were measured by microhardness measuring device. The corrosion tests were performed potentiodynamic polarization curves of the composite materials in 3.5% NaCl solution. As a result, it has been determined that the mechanical properties of WC reinforcing particles added to CuAlMn matrix increase with increasing volume ratio.

Metal matrix composites have many positive properties such as high elastic modulus, high strength and reproducibility [13]. In addition, these materials have very good abrasion resistance due to particle reinforcements [14,15]. There is also a lot of research on composites with copper matrix [16]. In a study, the abrasion resistance was improved by adding Ni3Al particles to the copper matrix [9]. CuCr SiC composite material has been produced and it has been stated that the hardness increases in the examination [17]. In addition, FeMnp and FeCrp were added to the Cu matrix to investigate their microstructure and mechanical properties [18]. By adding different additions to the Cu matrix, its hardness, strength, abrasion resistance and conductivity can be improved [19]. In addition, tensile strength increased up to 4% within the varying Cu matrix ratios; It was observed that after 4% it decreased and the hardness increased with increasing Cu matrix ratio [20].
Although powder metallurgy (PM) is not a newly known process, it was only used as an industrial process in the early 20th century. PM method has been used extensively in different fields since the beginning. As an example to these; tool steels, stainless steels, superalloys, aluminum and titanium alloys, copper and copper alloys, nuclear materials and cermets can be given [1][2][3][4][5]21]. Parts produced by this method have a smoother surface than parts produced by other methods and often do not require secondary treatment. It was determined that approximately 97% of the first material used in mass production with PM was used. Accordingly, the production of the part is cheaper and in the desired composition, and some parts that are difficult to produce and process by other methods are easily produced. By making fluctuations on the value of the punch pressure, it is possible to produce products in various forms in a better quality and in a proper manner [4].
Demand increases for copper (Cu) based alloys, especially Cu-Al-Ni, CuZn-Al and Cu-Al-Mn alloys due to their cheap cost and good shape recall effects and superelasticity in engineering applications [22][23][24]. Cu-based shape recall alloys have been in great demand after the 1960s and have been used as alternatives to NiTi alloys, because they have good electrical and thermal conductivity [25,26]. Cu based shape remembering alloy systems are in great demand in the practical applications of shape remembering alloys. Because it can be produced both cheaply and easily. However, as the high Al additive to this alloy causes the grain boundaries to weaken, it creates a very fragile alloy. It is important to add new elements to strengthen the Cu-Al alloy with a good ductility [27,28]. The addition of new elements to Cu-Al alloys made this alloy group more useful and attractive. However, in addition to the elementary additive, CuAl based shape recall alloys can improve the performance of the applied heat treatment. Generally, high temperature shape recall alloys are important in the robotics, automotive and aircraft industries. The application temperatures of these alloys should be above 390 K [29,30]. In this study, the effect of heat treatment temperature and heat treatment time on the conversion temperature and crystal structure of CuAlMn alloy, which is a new high temperature shape remember alloy, will be examined. There are some studies in the literature about the production of Cu-Al-Mn alloys with the PM method and the relationship between the microstructure and mechanical properties [31,32]. For example, Yu-Yang Gao et al. [33,34] in their study, they made a characterization study by adding different ratios of SiC to Al-Cu-Mg-Si-Mn composites by using PM method. They determined that the particles were dispersed homogeneously and the tensile strength increased. This has been linked to the formation of precipitates of SiC particles during sintering and post-sintering cooling. Yan et al. In their study, they investigated the change of martensitic phase by adding Ni to Cu-Al-Mn shape memory alloy casting method. They found that when 2% Ni was added, the triple Cu-Al-Mn alloys increased the shape memory from 85% to 92%. Cu-Al-Mn alloys have a high tensile strength. It is used in the production of machine parts, vehicles, lifting gear, railway wagons in constructions where high strength is required. Strength in Hardened State It has been determined that it reaches the strength of 52 steel [32].

Experimental Procedure
In the starting matrix powders, Cu, Al, Mn and WC an average particle size 44 μm were used. The matrix in produced segments that was sintered by hot pressing process. Powder mixtures were weighed by using precision scales and mixing process was applied for 30 minutes by using a turbula mixer (Celmak Group 7T, Turkey). PEG 400 (Polyethylene Glycol) at a rate of 1,5 wt% was added in the powder mixture in order to reduce friction forces during hot pressing and to provide a homogenized mixture. Previously, powders were weighed 20 grams, and then CuAlMn-WC were produced with a pressure of 300 MPa by using double-effect hydraulic press (Dim-Net WP-45SA, Korea). After that, CuAlMn-WC were placed within graphite dies and hot pressed under vacuum atmosphere by a PLC controlled direct hot pressing machine (Zhengzhou Golden Highway, SMVB 80, China) (Fig. 1). Following that production process was completed by applying a sintering to the samples in a furnace (Prothem, PLF 120/27, Turkey) under argon atmosphere at 900 ℃ and under pressure 35 MPa and for 4 minutes. Composite parameters of production were given in the Tab. 1. The produced composite samples were characterized for hardness, optical microscope. Surface of the samples were polished with 200, 400, 600, 800, 1000 and 1200 mesh wet sandpaper by using grinding device (Metkon Forcimat, Turkey). Microhardness of the samples were measured in Vickers

Results and Discussion
The SEM images of the CuAlMn-WC composites produced by the powder metallurgy method (Fig. 4) were taken and evaluations were made according to the obtained images.
When the SEM images are examined, it is seen that WC particles are distributed homogeneously in the interior of the sample containing CuAlMn. Samples produced are non-cracked and partially porous. As the addition of WC increases, the pore amount decreases and it is seen from SEM   Samples for CuAlMn -15% WC. They are almost identical in the CuAlMn and CuAlMn -5% WC hardness of composites by addition of WC. But CuAlMn -15% WC composite is sharply increased in the hardness increased the amount of WC. Fig. 6 shows SEM-EDS analysis taken from the samples. When the analysis results given in Fig. 6 were examined, it was figured out that the material was CuAlMm and WC. When the analysis results given in Fig. 6 were examined, the peaks of Cu, Al, Mn, W and C that were present also in the samples were clearly seen. The matrix phase acquires a brighter backscattered electrons contrast level in splats where WC dissolution was more pronounced. Fig. 7 shows XRD graphs of the CuAlMn-WC composites produced by hot pressing method.
When the XRD graphs given in Fig. 7 were examined, the peaks of Al77Mn23, Al5Cu2Mn3, AlCu2Mn, AlCu4, Mn, Al8Mn5, Cu0.4W0.6 and WC were determined. In this study, the phases formed are shown by XRD analysis; The formation of other phases has been revealed according to the results of EDS analysis.In their study on the effect of CuAlMn/WC ratio on the plastic deformation in strengthening mechanism, Ji et al., determined similar peaks [38]. The dominant phases were seen to be Al77Mn23, Al5Cu2Mn3, AlCu2Mn and AlCu4 in the graphs. When all peaks were examined, it was found that as WC amount increased in the samples. The corrosion tests of the samples were carried out in the prepared 3.5% NaCl solution. Potentiodynamic polarization curves resulting from the experiments are given in Fig. 8.    As a result, there is a reaction between Cu, Al, Mn and WC. When the Tab. 2 is examined, the Rp (corrosion resistance) of the sample CuAlMn was 2.08 kΩ.cm 2 , while the value for the sample CuAlMn + 5 % WC was calculated as 3.22 kΩ.cm 2 . It is clear from the data obtained that as the amount of WC in the reinforced increases, the corrosion resistance of the samples increases. Similar results have been found in the literature for composites of reinforced WC [28].

Conclusion
The effects of WC on the microstructure and mechanical properties of composites fabricated by hot pressing were investigated.
 Microstructure observation demonstrates a relative homogenous distribution in CuAlMn of WC particulates.  The hardness measurement values were 206 HV0,1 for CuAlMn, 248 HV0,1 for CuAlMn -5% WC, 322 HV0,1 for CuAlMn -10% WC and 490 HV0,1 for CuAlMn -15% WC.  The analysis of EDS the peaks of Cu, Al, Mn, W and C that were present also in the samples were clearly seen.  The dominant phases were seen to be Al77Mn23, Al5Cu2Mn3, AlCu2Mn and AlCu4 in the XRD graphs.  It is clear from the data obtained that as the amount of WC in the reinforced increases, the corrosion resistance of the samples increases.