The Effect of SiC Addition on AlNi Compact Mechanical Properties Produced Via Powder Metallurgy

: This research directed to produce Al-Ni alloys by powder metallurgy technique since of its marketable and industrial significant. Nickel and aluminum powders were determined their particle size then the powders mixed and blended with percent (Al - 20% Ni). Silicon carbide particles (SiC) powder supplemented to master alloy powder by percent (4-6- and 8 wt. %) separately then these powders mixed to obtain homogeneous distribution, then the powders compacted in cold pressure at 700 MPa. The sintering procedure was performed at (530°C) for 8 hrs. At vacuum atmosphere (10 -4 torr). After cooling these samples grinded and polished to estimate microstructure, density, porosity, LOM, SEM, EDS, X-ray diffraction ,hardness and wear tests at dissimilar circumstances. Results showed that the hardness increased by (52%) and wear rate decreased by (55%) at 8 wt % silicon carbide addition, and it was the best results.


Experimental Work:
The samples used in this study were prepared using powder metallurgy method where powders were used (Aluminum, nickel, silicon carbide), Table (1) shows the purity, Particle size and origin of the powders used.

Powders Preparation:
Mixing of elemental powders has been done by using ball mill shown in the Fig. (1). Stainless steel balls with different diameters (10mm and 5mm) have been used to mix and refine metal powders for about 6 hours and 3500rpm according to Kotresh, procedure [4]. The ball-to powder weight ratio was 3:1 by weight [5]. The increment in the number of contact areas between the elemental powder particles enhances the mixing process. Wet mixing is done by using 0.5 cc of ethyl alcohol to every 25 g of powder mixture. The wet mixing is used to minimize the temperature generated by friction between the balls with walls and powder. After mixing the wet mixtures.

Powders Compaction
Electric uniaxial hydraulic press is used to compact the blended powder to green samples with dimensions ( D=12.75mm and t≈ 5-6.5 mm) that used for the tests .The die used was single action die made from stainless steel, Graphite has been used as lubricant in order to minimize the friction between the punch and the die wall as well as the friction between the green compact and the die wall and to avoid the cracks initiated from the ejecting of green compact . The pressure was (800) MPa with loading rate (8 ton/ min) and period of applied pressure time is (4 min) has been used in order to determine the optimum compression stress that gives higher density and low green porosity.

Sintering of Compacts:
After the samples were produced, the sintering treatment was done at 530 ° C for 8 hours and using a vacuum atmosphere. The samples were then left inside the oven to be cooled. with thermal cycle shown in Fig. (2).

2.4.1.Physical Tests
Several physical tests for elemental powders, green compacts, and sintered samples. These tests include the following:

*Particle Size Analysis
Particle size analysis has been done for all elemental powders (Al ,Ni, SiC) by using laser particle size analyzer using distilled water as a dispersion medium . The test has been done in the collage of materials engineering -Ceramic department laboratories.

*Green Density and Green Porosity
The green density of the compact can be expressed by the mass per unit volume of compacted mixed powder.

*Density and Porosity after sintering of Compacts
The density and porosity of samples were calculated according to ASTM B-328 [6]: The test has been done for all samples in order to study the effect of an alloying element on the porosity.

Microstructure Characterization *Light Optical Microscopy Observation
Microstructure observation has been done for sintered samples with different magnifications (100X , 200X , 400X , 800X ) using light optical microscope with high resolution camera fixed in the microscope used to capture photos. The prepared samples were with dimensions ( D=12.75mm , t= 5~7 mm ) has been wet grinding using (1000, 1200 ,1500,2000,2500) grit silicon carbide papers using grinding wheel machine and then polishing by using polishing paper and diamond with 1-3 μm particle size, These samples are then cleaned with water and dried with hot air .the Samples are then etched with were Keller's reagent (1%HF+1.5% HCl + 2.5 HNO 3 +95%H 2 O) [7]to reveal the microstructures clearly for about 40 sec. After that, the samples are ready to use for microscopic observation as shown figure (3).

*Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)
Scanning electron microscope observation is used to reveal the microstructure of etched sintered samples, Several magnification has been captured with (200X, 1000X ,2000X ,5000X and 10000X) KV for nickel aluminum samples without and with 8 wt.% SiC as shown figures 4 and 5,And Energy dispersive analysis has been done for nickel aluminum samples without and with 8 wt.% SiC. Two to three areas have been taken for each sample as shown in Figures(6 -7). SEM examination has been done in The Ministry of Science and Technology / Materials Research Department. And Qualification in Baghdad ,Iraq.

*X-Ray Diffraction (XRD)
X-ray diffraction has been used to identify and analysis the phases exist in the prepared alloys as shown in fig. (8), this test has been done for sintered samples. X-ray diffraction and Energy Dispersive Spectroscopy tests are carried out in The Ministry of Science and Technology / Materials Research Department. Low angle X-ray diffraction is performed and X-ray generator with Cu Kα radiation at( 40.0 Kv ) and (30.0 mA )is used.

*Brinell Micro hardness Measurement
Macro hardness Brinell tester is used to measure the hardness of the samples with (31.25 )kp as applying weight and the Incubation time was (10 sec) in the state applied weight and diameter (2.5mm). Three reading for each sample had been taken and the average value used to analysis the behavior of the alloys.

*Dry Slide Wear Test
The dry sliding wear is studied by using the pin on disk concept using (300 rpm) and constant radius (6.5mm) with different sliding distance and the loads were (10,20 and 30 N). The sample is weighted before and after a period of time (5, 10, 15 and 20 min), using (0.0001) accuracy electric balance the wear instrument that was used in this work are shown in Fig. (9). Figure 10 showed that when increasing pressure, the green density increases too until it reaches a certain limit at which any further increase in the pressure has no or little effect on its value, that was due to the increasing forces causing porosity to close up and it also leads to reduced green porosity as in fig.(11). So the preferred pressure was determined as 700 MPa for all the samples prepared in the present study. Density after sintering also increases and also leads to lower porosity after sintering. This parameter will lead to less porosity as shown figures (12) and (13) respectively. This is because the sintering at 530C° for 8 hr increases diffusion and reduces voids that increase the contact points between particles [8]. The results of the microstructure examination show that the phase (Al3Ni2 ) is formed between aluminum and nickel which reduces the expansion of pores by filling the granules of this phase of the vacancies and gaps formed between the grains of the planet [9].

Results and discussion:
In the current study the hardness of the samples of all alloys after sintering are measured by Brinell hardness test ,and the greatest value was recorded for aluminum nickel reinforced with 8 wt% SiC as shown fig.(14).This increment could be attributed to the high hardness of SiC particles itself which acting as barriers to dislocation motion and strengthens the structure of the samples [10].
Samples with (12.75) mm diameter exposed to wear test beneath different loads (10, 20 and30) N and for different times (5, 10, 15 and 20) min at room temperature. Weight loss was altered to volume loss using the experimental density of each composite sample which was measured in density test. that volume loss is increased with increasing the practical load, where the highest volume loss was documented under (30 N) and vice versa. This is expected performance, where the increase in load pointers to increase the friction between sample surface and the revolving disk [11]. Also, the volume loss was enlarged with increasing time owing to the increase of sample's mass loss with increasing friction time. Figures (15) to (17) exhibit the outcome of SiC particles adding on wear rates at un-similar circumstances. From these figures, it can be seen that volume loss was decreased severely with increasing SiC percentage, even it extents the minimum value at the composite that have maximum SiC percentage (8% This may owing to the role of SiC particles in obstructive dislocation motion so, hardness was increased and there by wear resistance was also increased [12] . Figures (18 -20) showed the wear rate for all sample with and without SiC particles. Figures(18 -20) shows ,That at 10 N the highest wear rate in aluminum nickel sample and the wear rate is decreased by 57% for aluminum nickel reinforced with 8 wt% SiC samples, At 20 N the highest wear rate in aluminum nickel sample and the wear rate is decreased by 78% for aluminum nickel sample reinforced with 8 wt% SiC, At 30 N the highest wear rate in aluminum nickel sample and the wear rate is decreased by 55% at aluminum nickel reinforced with 8 wt% SiC samples.

Conclusions:
1. The sintering process at 530° for 8 hrs of master Al-20% Ni alloys with different compacting pressure is effective enough to complete the transformation of Al and Ni powder to alloy structure and resulted the Al3Ni2 phase.
2. The increasing in compacting pressure from (400 to 800) MPa for master alloy Al-20% Ni resulted increasing in density and decreasing in porosity.
3. Powder metallurgy is an effective method in manufacturing of metal matrix composites, where the optical microscope and SEM images showed uninform distribution of silicon carbide particles in the Al-20% Ni alloys.
5. Wear rate is decreased with the additions of silicon carbide particles by 23%, 41% and 58% for samples of 4, 6, and 8 wt.% SiC respectively under 30 N and 20 min.

CONFLICT OF INTERESTS.
-There are no conflicts of interest.