Fabrication of Cu-Al2O3 Composites by Simplified Internal Oxidation

Process

Baohong Tian1, a, Chengdong Xia 1, b and Shuguo Jia 1, c 1 School of Materials Science and Engineering, Henan University of Science and Technology,

Luoyang, 471003, China

atianbh@mail.haust.edu.cn,

bxcd309@163.com,

c sgjia@mail.haust.edu.cn

Keywords: Cu-Al2O3 composite; internal oxidation; hot extrusion; cold roll; simplified process.

Abstract. Cu-Al2O3 composites were prepared by a new simplified internal oxidation process

integrating with powder metallurgical process, and then the hot extrusion and the cold rolling

processes were carried out. The microstructure, electrical conductivity, hardness, tensile strength and

thermal stability of the composites were investigated. The results show that Cu-Al2O3 composites

were fabricated successfully by the simplified process in which internal oxidation completed during

the sintering. There are a mass of fine Al2O3 particles in size varying from 5 nm to 20nm dispersed in

copper matrix after sintering 950 for 4h. After sintered at 950 for 4h and extruded at 950

followed with the cold deforming of 80%, the electrical conductivity, hardness, tensile strength and

softening temperature of composite reach 81％IACS, 137HV, 561MPa and 850 respectively. It is

considered that the dispersion strengthening and strain hardening have greatly contribution to the

Cu-Al2O3 composites fabricated with the simplified process.

Introduction

Cu-Al2O3 composite is a kind of excellent functional and structural materials with high strength as

well as high electrical and thermal conductivity at elevated temperatures. So it has been widely used

for electric materials for lead frames, relay blades, contact supports, electrode for spot welding [1-3]

etc. The properties of Cu-Al2O3 composites fabricated by other processes do not exceed that of the

internal oxidation process [4, 5]. The traditional internal oxidation process for fabrication of the

Cu-Al2O3 composites mainly includes the following procedures: powders internal oxidation→

deoxidization→encapsulation, vacuumizing, pressing→ hot extrusion. The mechanical properties

and electrical performance of the Cu-Al2O3 composites prepared in this way are outstanding.

However, it is difficult to control the quality of products and the cost is too high because of the

complicated processes and long production cycle. So the Cu-Al2O3 composites are limited greatly for

further mass production and application [6]. In this paper, a simplified process internal oxidation

combined with powder metallurgical technique was adopted to prepare the Cu-Al2O3 composites. In

the novel simplified process, after the mixing powder of Cu-Al alloy and proper oxidant, the forming

and sintering were carried out in the same working procedure combined with the internal oxidation.

The followed hot extrusion and cold roll processes were carried out to improve the composite

properties.

Experimental Details

Cu-(0.74wt.%) Al2O3 composites were used as the experimental material fabricated by simplified

internal oxidation with hot extrusion and cold roll processes. The fabrication was carried out

according to the following procedures: (a) induction melting of known amounts of copper and

aluminum (0.37wt.% of Al), (b) atomizing the melt into powder by high pressure nitrogen gas flow,

(c) drying and sieving, (d) mixing with proper Cu2O (oxidant) powder, (e) forming, (f) sintering

(internal oxidation) at 950°C for 2hrs, 4hrs, 8hrs and 16hrs, respectively (g) hot extrusion (950

°C,

extrusion ratio of 11 to 1, extruding speed of 5mm/s), (h) cold roll (60% and 80% deformation).

Advanced Materials Research Vols. 148-149 (2011) pp 416-419Online available since 2010/Oct/27 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.148-149.416

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Samples after hot extrusion were annealed at 400°C, 500

°C, 600

°C, 700

°C, 800

°C, 850

°C, 900

°C and

950°C respectively for 1hour in a muffle electric resistance furnace under a flowing atmosphere of

argon.

The tensile properties of the composites were determined with a Shimadzu AG-I 250kN precision

universal tester according to the standard test methods of ASTM E8-03, the crosshead speed was

1mm/min at ambient temperature.

The electrical conductivity was determined by measuring the resistance of sample in 100mm length

using a ZY9987-type standard direct-current four-probe tester with the accuracy of less than

±0.0002 ohm. The microhardness of the specimens was measured with an HVS-1000 type digital

microhardness indenter under the load of 200g and the loading time of 10s. Every specimen was

tested for five times and the average value was calculated with the accuracy within 5%. The

microstructure and tensile fracture surface morphology were observed with a JSM-5610LV scanning

electron microscope (SEM). The microstructure and the precipitated phase of the composites were

analyzed with an H-800 transmission electron microscope (TEM) operating at 200kV.

Results and Discussion

Microstructure. Fig.1 show the TEM images of the dispersion particles produced in the Cu-Al2O3

composites after sintering for 4hrs and 16hrs respectively. A mass of precipitated disc-like particles

are dispersed in the copper matrix, which were indexed as γ-Al2O3 Fig.1b. Fig.1a and Fig.1c show the

alumina particle size increases from 5nm to 20nm in diameter with increasing of the oxidation time

from 4hrs to 16hrs during internal oxidation process at the temperature of 950°C. In general, the

alumina particle size is determined by a competition between the rate of nucleation as the internal

oxidation front passes and the subsequent growth and coarsening rates of the particles [7].The driving

force for the alumina coarsening is the Ostwald ripening [8, 9].

Fig 1 TEM images of Cu-Al2O3 composites as sintered and as hot extruded (a) sintering at 950°C for

4 hrs, (b) electron diffraction pattern of sintering for 4 h, (c) sintering at 950°C for 16 h

Electrical Conductivity. Fig.2 shows that the electrical conductivity of the Cu-Al2O3 composites

increases at the beginning and then decreases after sintering for 4h with increasing the sintering time.

It also shows that the electrical conductivity improves with increasing the deformation. The peak

value of the electrical conductivity reaches 80%IACS after sintering for 4hrs and 80% cold rolling. It

is due to the interaction between macro defects and micro defects. On one hand, the crystalline grains

are refined and high density dislocations are formed. A mass of interfaces of different phases are

introduced into the Cu-Al2O3 composites like usual metallic materials with increasing the cold

deformation. All of those interfaces scatter the electron movements, which lead to the electrical

conductivity decreases. On the other hand, the pores and the uncompacted local areas are eliminated

with increasing the cold deformation. Thus the grain boundaries are arranged regularly and result in

Advanced Materials Research Vols. 148-149 417

the increasing of the electrical conductivity. The effect of the latter factor is more notable than that of

the former for powder metallurgy materials.

0 2 4 6 8 10 12 14 16 1840

45

50

55

60

65

70

75

80

85

Electrical conductivity /%IACS

Sintering time/ h

hot extrosion

hot extrosion and 60% deformation

hot extrosion and 80% deformation

Fig.2 Variation of electrical conductivity of Cu-Al2O3 composites with different deformation and

sintering time

Hardness and Tensile Strength. Table 1 shows the mechanical properties of Cu-Al2O3

composites under different conditions. The differences are obvious that the hardness and the tensile

strength of the composites as sintered are the lowest respectively, which are improved greatly after the

hot extrusion and the cold deformation of 80% with hardness of 137HV and tensile strength of 586

MPa respectively. The hot extrusion and the cold deformation are effective processes to improve the

mechanical properties of the sintered composites.

Table 1 Mechanical properties of Cu-Al2O3 composites

Sintering

time

/h

As sintered As hot Extruded Extrusion and 60％

cold deformation

Extrusion and 80％

cold deformation

Hardness

HV

Tensile

strength

/MPa

Hardness

HV

Tensile

strength

/MPa

Hardness

HV

Tensile

strength

/MPa

Hardness

HV

Tensile

strength

/MPa

2 50 － 104 250 128 418 137 519

4 58 155 108 242 124 460 131 561

8 52 － 103 256 126 456 135 586

16 61 － 100 266 124 487 135 558

Fig. 3 SEM fractograph of Cu-Al2O3 composites (a) as sintered, (b) as hot extruded

418 Manufacturing Processes and Systems

According to the SEM fractograph of the Cu-Al2O3 composites as sintered and as hot extruded shown

in Fig.3, it can be concluded that the fractures both belong to the ductile fracture, while the toughness

of the hot extruded is better than that of the sintered. Comparing with the electrical conductivity,

hardness, tensile strength and the softening temperature of the composites of different conditions, it

can be deduced that the process of sintering at 950°C for 4hrs and then hot extrusion and cold rolling

of 80% is optimal for preparing the Cu-Al2O3 composites with high performances.

Conclusions

Cu-Al2O3 composites were successfully prepared by a simplified process in which internal oxidation

were completed during sintering. The particles which rang in size from 5nm to 20nm in copper matrix

after sintering 950°C for 4h. The processes are easy to realize the mass production and automatic

production.

After sintering at 950°C for 4hrs and then hot extrusion and cold rolling of 80% were carried out,

the electrical conductivity, hardness, tensile strength and the softening temperature of composite

reach 81%IACS, 137HV, 561MPa and 850°C, respectively. The process is optimal for preparing

Cu-Al2O3 composites with high performances.

It is considered that the dispersion strengthening and the strain hardening have greatly contribution

to the strengthening of the Cu-Al2O3 composites fabricated with the simplified process.

References

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Advanced Materials Research Vols. 148-149 419

Manufacturing Processes and Systems 10.4028/www.scientific.net/AMR.148-149 Fabrication of Cu-Al2O3 Composites by Simplified Internal Oxidation Process 10.4028/www.scientific.net/AMR.148-149.416

DOI References

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