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Study some mechanical properties of copper-alumina functionally graded materials.

INTRODUCTION

Copper is the best commercial metal for electrical and thermal applications [1,2]. The fact that copper is regarded as a criterion for the electrical conductivity measurements in the IACS standard shows its capability for electrical conductivity [3]. For applications such as electrical contacts, high performance switches, and spot welding electrodes, mechanical properties and thermal stability at elevated temperature, as well as electrical and thermal conductivity are required [4,5]. The alloying of copper with soluble materials enhances the mechanical properties, but these alloys contribute to an intense decrease in electrical conductivity [6]. Traditional highstrength copper alloys rely on precipitation hardening to increase their strength for low and intermediate temperature application but not at high temperatures [1]. Recently, the strengthening of copper with oxide and carbide reinforcement such as TiC [4], SiC [7,8], [Al.sub.2][O.sub.3] [9,10], Cu2O [6], and NbC [11] has attracted attention of many researchers. Many engineering components require contradictory material properties, such as lightweight, high wear resistance, electrical conductivity, hardness and toughness. Functionally gradient materials (FGMs) fill the gap in materials science that the components require different properties in different positions and the optimal property is not achievable with the homogeneous cross-section materials. FGMs are materials with variation of composition and microstructure along their thickness [12,13]. For electrical contacts, the hardness, wear resistance and thermal stability of the surface, as well as the electrical conductivity of bulk are required [5]. FGMs are composite materials with properties and microstructure continuously changing along a specific material direction. The main advantages of functionally graded structures are the possibility of taking advantages of each base material's properties simultaneously [14]. From Literature Review, it is observed, two types of functionally graded materials FGMs were studied. The first type was metal/metal FGM, such as Steel/Al, Al/Si [15,16]. The second type was metal/ceramic FGM, such as Al/SiC and [Al.sub.2][O.sub.3]/Ti, Cu/[Al.sub.2][O.sub.3], Cu/Nbc, Al/[Al.sub.2][O.sub.3] [17, 18, 19, 20, 21]. the aim of this type was to increase the compression strength and shear strength, in addition, don't cracking or failure under thermal stresses when functionally graded materials FGMs were used instead of welding.

In this paper, an attempt is made to prepare five-layers functionally graded Cu/[Al.sub.2][O.sub.3] with gradient of [Al.sub.2][O.sub.3] from 5, 10, 20, 30 and 40 wt.% By using powder metallurgy technique. Furthermore, different parameters have been studied such as: a- [Al.sub.2][O.sub.3] weight percentage and Different compacting pressure.

Experimental Part:

In this work, Elemental powders of Copper and Alumina used to prepare the functionally graded Cu/[Al.sub.2][O.sub.3] with gradient of [Al.sub.2][O.sub.3] from 5, 10, 20, 30 and 40 wt.% By using powder metallurgy technique. Mixing of copper (34.32 [micro]m) and alumina (1.439 [micro]m) powders for two hours and then several disk sample with dimensions (14mm diameter and 10 thickness) have been compacting at different compacting stresses (550, 650 and 750 MPa). However, sintering of specimens for three hours at 850 C[degrees]under vacuum about [10.sup.-3] torr has been achieved. The samples surface subsequently ground, polished and then characterize with scanning electron microscopy. Compositions for each layer of elemental powders used in this study have been shown in the Table 1.

The total weight of each (FGMs) sample is assumed to be constant approximately (10gm), every layer weight (2 gm).

The density and porosity was calculated before sintering according to (1, 2) equations [22]

[[rho].sub.g] = [m.sub.g]/[V.sub.g] (1)

Where:

[[rho].sub.g]: is the green density (g/[cm.sup.3]), and [m.sub.g]: mass of green compacted (g) [V.sub.g]: volume of green compacted ([cm.sup.3]). [P.sub.g] = (1-pg/[rho]th) x 100 (2)

Where:

[P.sub.g] = green porosity (%), [[rho].sub.g] = green density (g/[cm.sup.3]) [[rho].sub.tb] = theoretical density of mixed powders (g/[cm.sup.3])

and after sintering according to ASTM B328, in order to determine the porosity and density of sintered samples the following procedure was followed:

1-After drying at 100[degrees]C for 5 hours in a vacuum furnace the sample was weighed, and the weight represent mass A.

2-the sample is completely immersed in Paraffin oil with density 0.8 g/[cm.sup.3] and an evacuating system (to decrease the pressure) for 30 minutes at room temperature.

3-the impregnated (soaked) sample has been weighed in air, and the weight was mass B.

4- Mass C was determined by weighing the impregnated sample in water.

5- The porosity and density have been calculated by the following equations (3, 4) [ASTM B - 328(23)]:

P = [B-A / (B-C)[D.sup.[omicron]] x 100] [D.sub.W] (3)

D = [A/B-C]DW (4)

Where:

D[omicron] = the density of the used oil

[D.sub.W] = the density of water

Appropriate grinding and polishing were carried out before subjecting the samples to the hardness test. The test was conducted on Micro Vickers hardness device using the weight of (300g) for 10 sec with a square-base diamond pyramid. Then reading were done along each layer. The Vickers microhardness (H.V.) is defined as follows:

HV = 1.854 P/[d.sup.2] (5)

Where P = applied load Kg.

d = average length of the diagonal [mm.sup.2].

Samples of (18 mm in diameter and 27-30 mm height) were prepared for compression test according to ASTM (D695-85) [24]. Have been compressed using computer control electronic universal Testing machine [model:WDW-200KN]. Universal testing machine was used to piston speed of (0.2mm/min). Been designed a special mold for double shear test in order to appropriate to the thickness of the samples are prepared for this test, as shown in Fig1 Samples of (18 mm in diameter and 27-30 mm height) were prepared for this test. Have been compressed using computer control electronic universal Testing machine [model:WDW-200KN]. Universal testing machine was used to piston speed of (0.2mm/min).

RESULTS AND DISCUSSION

Fig 2 a: is shown that if the compacting pressure increases, 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. This can be attributed to the fact that powder under compaction pressure is subjected to the force that contributes in plastic flow, which increases compact density So the preferred pressure is determined as 650 MPa for all layers of FGMs. And Fig 2 b: indicates that an increase in compacting pressure causes a decrease in the green porosity due to increase in the contact points between particles that reduce voids.

Fig 3 a: shows the effect of additions [alpha]-[Al.sub.2][O.sub.3] on the porosity after sintering. the porosity increase with [alpha]-[Al.sub.2][O.sub.3] increases As know The main disadvantage of copper matrix composites reinforced with aluminum oxide particles is residual porosity, which influences the material properties, and pores that are present within the area of the ceramic phase.

One can observe that a-form of aluminum oxide powder ([alpha]-[Al.sub.2][O.sub.3]) shows a strong tendency to form agglomerates at the preparation stage of Cu and [Al.sub.2][O.sub.3] powder mixtures, which results in residual porosity in composites. Fig 3 b: show the effect of compacting pressure on true porosity. When the pressure increases, the porosity decreases because of the increasing the Agglutination atoms.

Fig.4 a and b: show the X-ray diffraction results for layer (70%Cu+30%[Al.sub.2][0.sub.3]) before and after sintering process. From results of tests before and after sintering is similar, there is no new phase formed after sintering, this indicates that there is no solubility between copper and alumina This means that the strengthening here by dispersion and there is no depositions.

Fig. 5, and Fig 6: show the SEM images for Layers each one individually and FGMs sample using a compacting pressure of 650 Mpa with different magnifications. From these images it is seen that the distribution and existence of additive particle and pores through the sample surface. Note that additive particle distributed uniformly in the copper matrix, Through the SEM test for functionally graded sample as shown in Fig.6 seen that the existence of a gradation clear for layers within the sample functionally graded, and also the presence of the overlap between the layers, this is proof of the gradation properties.

Hardness results:

The results show that the hardness increases with the increasing the adding percentage because of the added material particles ([alpha]-[Al.sub.2][O.sub.3]) work to obstruct and prevent the movement of dislocations on the sliding planes and due to the fact that the rise in additive content make the ability of the indenture of the hardness tester to hit additive particles increase and the additive particles has high hardness compared with copper hardness. This result has been corresponding with [25, 26].

Fig. 7 a: shows the measured hardness for layers (Cu-[Al.sub.2][O.sub.3] composite) at compacting pressure 650 Mpa, and Fig.7 b: for FGMs sample using various values of compacting pressure. Fig. 7a shows Vickers microhardness in each layer was measured at 1mm interval from the first to fifth layer. The interface between each layer was monitored carefully in this work. All samples generally show an interesting graded microhardness across the thickness. Dependent on the microstructure of those samples, material properties vary with position within the gradient and can be used to tailor functionality. Removing the sharp interface by an interlayer compositional gradient. However, residual stresses still develop at the new heterogeneous interfaces introduced by the composite compositional gradient. It is realized that when the compacting pressure increases the hardness increases. Increasing pressure works on minimizing the pores and increases the density of the sample and leads to increase of the hardness.

Compression results:

Fig. 8: shows Compression test Results for layers to show the effect additions [Al.sub.2][O.sub.3] Wt% on the compression strength prepared at compacting pressure 650 MPa. And fig.8: Represents an illustrative picture of the samples that have been tested. The fig.9: illustrates the decreasing of compression strength from (215.14) MPa of layer 1, (169.29) MPa of layer 2, (134.02) MPa of layer 3, (112) MPa of layer 4 and (91.70) MPa of layer 5 with increase in alumina percentage, due to increase the percentage of porosity, it leads to decrease surface area, which subjected to load. It is lead to increase applied stresses on the materials, also the porosity is a stress riser and work as stress concentration in point and happen failure [27,28],

Figure 9: show In the first layer in which the proportion of 5% alumina and also the second layer in which the proportion of alumina 10% observe the failure in the form of barrel. Due to the presence of a high percentage of copper with a small percentage of alumina. But when alumina ratio increases more as in the last three layers are failing faster without a clear change in size. Fig. 10: shows Compression test Results for samples of FGMs prepared at different compacting pressure. And Fig.11: It represents an illustrative picture of the FGMs samples that have been tested.

Figure 10: shows the effect of compacting pressure on compression strength. It can be noticed that when the pressure increases, the compression strength increasing. This is due to the decrease of the voids and pores between particles.

Through the figure 11 note that cracks start from the fifth layer which are the highest percentage of alumina which are the most layer hardness and then move toward the fourth layer, third and then second down to the first, which is the lowest rate alumina and the most resistant to compression.

Double shear results:

The purpose of this test with the aim of knowing the extent of cohesion between the layers, the test was performed and noted that failure to get through layers and not at the boundary between the layers this is proof that the sample a coherent and works good as a sample singly, as shown in Fig 12.

Fig.13 shows double shear test Results for layers to show the effect additions [Al.sub.2][O.sub.3]Wt% on shear strength prepared at compacting pressure 650 Mpa.

Fig.13 show with the increasing proportion of alumina decreasing shear strength, due to pores in which increases with ceramic phase. We note the much lower value of the state of compression because these pores work differently. In this test opens up the pores very quickly and cause fail faster. And fig.14 show with the increasing compacting pressure increases the shear strength because it increased the pressure less pores as well as increased the contact surfaces between particales through the grains and so get the best results when you increase the pressure. And Fig.14 for samples of FGMs prepared at different compacting pressure.

Conclusions:

From the experimental, several conclusions may be drawn as follows:

1. FGM can eliminate the microscopic interface, such as a result as that traditional Cu-[Al.sub.2][O.sub.3] joint.

2. Removing the sharp interface by an interlayer compositional gradient. However, residual stresses still develop at the new heterogeneous interfaces introduced by the composite compositional gradient.

3. The best compacting pressure for [alpha]-[Al.sub.2][O.sub.3] addition was 650 Mpa.

4. Hardness number is changed in each layer of produced FGM according to the percentage of the harder alumina content. The hardness increases with the increasing the adding percentage.

5. Porosity increase with increasing of addition percentage of [alpha]-[Al.sub.2][O.sub.3].

6. Compression and shear strength decrease with increase alumina percentage.

7. The double shear test was performed and noted that failure to get through layers and not at the boundary between the layers this is proof that the sample a coherent and works good as a sample singly.

8. With the increasing proportion of alumina decreasing shear strength, due to pores in which increases with ceramic phase.

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(1) Roaa Hatem Kadhim Al-Nafiy and (2) Abdul Raheem. K. Abid Ali

(1) Research Follow, Department of Metallurgical Eng., College of Material's Eng. /University of Babylon, Hilla, Babil, Iraq.

(2) Assistant Professor, Advance materials, College of Material's Eng. /University of Babylon, Hilla, Babil, Iraq,

Received 18 September 2016; Accepted 15January 2017; Available online 29 January 2017

Address For Correspondence: Roaa Hatem Kadhim Al-Nafiy, College of Material's Eng. /University of Babylon Department of Metallurgical Eng., Babil, Iraq.

E-mail: Roaa.eng91@yahoo.com

Caption: Fig. 1: Show the special mold for double shear test.

Caption: Fig. 2a: Green density of FGMs compacts as a function of compacting pressure. b: Green Porosity of FGMs compacts as a function of compacting pressure.

Caption: Fig. 3a: Effect of additions [alpha]-[Al.sub.2][O.sub.3] on the porosity. b: Effect of compacting pressure on true porosity for samples of FGM.

Caption: Fig. 4 a: X-ray diffraction pattern of layer (70%Cu+30% [Al.sub.2][O.sub.3]) before sintering process. b: X-ray diffraction pattern of layer (70%Cu+30%[Al.sub.2][O.sub.3]) after sintering.

Caption: Fig. 5: SEM images of layers (a) 5% [Al.sub.2][O.sub.3] (b) 10% [Al.sub.2][O.sub.3] (c) 20%[Al.sub.2][O.sub.3] (d) 30%[Al.sub.2][O.sub.3] (e) 40%[Al.sub.2][O.sub.3].

Caption: Fig.6: SEM images for FGMs samples

Caption: Fig. 7 a: Hardness for Layers each one individually using a compacting pressure of 650 MPa. b: Hardness as a function of distance of FGMs from one layer (5% [Al.sub.2][O.sub.3]) at various compacting pressures.

Caption: Fig. 8: Represents an illustrative picture for layers each one individually

Caption: Fig. 9: Effect of additions [alpha]-[Al.sub.2][O.sub.3] on the compression strength.

Caption: Fig. 10: Show compression strength for FGMs samples at different compacting pressure.

Caption: Fig. 11: An illustrative picture of the FGMs samples.

Caption: Fig. 12: An illustrative picture of the FGMs samples that have been tested.

Caption: Fig. 13: Effect additions [Al.sub.2][O.sub.3] Wt% on shear strength

Caption: Fig. 14: Shows shear strength for samples of FGMs prepared at different compacting pressure.
Table 1: Weight composition for each layer in FGM.

Layer    Layer weight   Layer weight
Number    Percentage     Percentage
             Cu%           A1203%

1             95             5
2             90             10
3             80             20
4             70             30
5             60             40
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Author:Nafiy, Roaa Hatem Kadhim Al-; Ali, Abdul Raheem. K. Abid
Publication:Advances in Natural and Applied Sciences
Article Type:Report
Date:Jan 1, 2017
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