Powder Metallurgy Progress, Vol.11 (2011), No 3-4 238

DEFORMATION AND FRACTURE OF COPPER-TITANIUM POWDER MATERIALS

L. Ryabicheva, D. Usatyuk, O. Gaponova

Abstract Copper with mass content of titanium up to 1.5% allows obtaining of materials with high strength, wear resistance and low electrical resistance. The influence of deformation parameters and porosity on structure formation, physical and mechanical properties of copper-titanium electrotechnical powder materials has been investigated. A fine-grained structure was found after predeformation in the temperature interval of 500-600°C. Results of investigation of fracture surfaces are presented. It has been established that deformation softening and presence of porosity influence the fracture character of powder billets. Copper-titanium powder materials of 2.8-3.0% porosity after deforming in deformation softening conditions are characterized by relatively high strength and plasticity, low probability of brittle fracture and low resistivity. It makes it possible to propose them as electrotechnical materials. Keywords: electrotechnical powder material, deformation, microstructure, mechanical properties, fractography, density, electrical resistance

INTRODUCTION Copper-based powder materials are used for manufacturing of electrical

components and contacts of railway transport, collectors, electrodes, etc., due to their high electrical and heat conductivity in combination with heat resistance. Today, the tungsten-copper and molybdenum-copper compositions are widely used. The copper-based powder materials with addition of 0.25-1.5% Ti are employed for production of electrical contacts and electrodes for contact welding [1]. Investigations of compact material have shown a 4 times increase of proof stress at 4 at.% titanium content in copper solid solution [2], while titanium is promoting intensive growth of resistivity that limits application of copper-titanium materials with titanium contents higher then 1.5% as electrical materials. Powder metallurgy techniques are used for development of materials with titanium contents up to 1.5% that allows obtaining a combination of high mechanical properties with high electrical resistance.

The purpose of the work is investigation the influence of deformation parameters on structure formation, physical and mechanical properties of copper-titanium powder materials for electrical engineering.

EXPERIMENTAL PROCEDURE Experimental investigations have been performed on samples made from

mechanical mixture of copper powder PMS-1 and titanium powder VT1-0 with the mass

Lyudmila Ryabicheva, Dmytro Usatyuk, Volodymyr Dahl East Ukrainian National University, Lugansk, Ukraine Oksana Gaponova, Sumy State University, Sumy, Ukraine

Powder Metallurgy Progress, Vol.11 (2011), No 3-4 239 fraction of titanium 0.5%. The prismatic samples with dimensions of 10×13×55 mm were pressed by bilateral pressing on an hydraulic press, sintered at temperature of 900±10ºC for 3 hours in a synthesis gas medium (the gas composition is 72% Н2, 21% СО, 5.5% СО2, 1.5% Н2О) and stamped on a screw press at temperatures of 20, 200, 400, 500, 700 °C. The average porosity of samples after stamping was 2.8, 6.2 and 9.1%. The strength and ductility parameters were determined by a tension test. The chemical composition of diffusion zone on the boundaries of Cu and Ti particles after sintering and deformation, microstructure and fracture surfaces after tension have been investigated by scanning electron microscope REMMA-102. Electrical resistivity was measured by a bridge scheme.

RESULTS AND DISCUSSION The microstructures of samples after sintering and subsequent forging at different

temperatures are presented in Fig.1. They consist of copper matrix grains and titanium particles and are characterized by copper grain size of 21-22 μm and titanium particles of 55 μm (Fig.1a). Forging at elevated temperatures leads to changing of microstructure due to mechanisms defined by the deformation temperature and initial porosity. Dynamic softening processes that are developing with increasing temperature of deformation in the hard phase lead to changes of structure and properties [3]. The dynamic recovery at 200°C is partially relieving stresses, while refining copper grains to 16-17 μm and titanium particles to 40-42 μm during deformation (Fig.1b). Dynamic recrystallization at 600°C promotes a fine-grained structure with the copper grain size of 5.4 μm and size of titanium particles 28-30 μm (Fig.1c). Structure formation at the deformation temperature 400°C takes place under the influence of dynamic recrystallization and strain aging. The microstructure is characterized by complex-shaped copper grain boundaries (Fig.1d), due to inhibiting movements of high-angle boundaries by segregations, while copper grain size becomes 9-10 μm and size of titanium particles is 34 μm.

(a) (b)

(c) (d)

Fig.1. Microstructures of samples at 9.2% porosity: (a) - after sintering and after forging at temperatures: (b) - 200ºC; (c) - 600ºC; (d) - 400ºC.

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The growth of porosity from 2.8% to 9.2% influences structure formation associated with increasing of free energy due to developed free surfaces in the form of pores’ boundaries, which are the nucleation centres of dynamic recrystallization [4]. At the same time pores’ boundaries are preventing migration of high-angle boundaries and inhibiting growth of recrystallized grains. The grain size of copper at 9.2% porosity is lower than in the material of 2.8% and 6.1% porosity due to the above mentioned processes.

Formation of the diffusion zone after sintering has been observed as the result of surface and bulk diffusion with different concentrations of components determined by the content of alloying element and porosity value. The diffusion is accelerating with increasing of deformation temperature and appearing of a stress field. The diffusion zone of copper inside titanium particles is 10 μm, with its concentration of 45.9-49.3% and titanium zone in the copper particles is 15 μm with concentration of 1.0-2.2% (Fig.2). In the case of higher porosity, the amount of diffused copper is lower, due to obstacles for transferring of matter created by pores.

(a) (b)

Fig.2. Quantitative distribution of copper in the particles of titanium (a) and titanium particles of copper (b) at porosity of 6.1%: 1 - after sintering, and 2 - 200ºC; 3 - 400ºC; 4 -

600ºC.

The difference in diffusion of copper and titanium is explained by their partial diffusion coefficients. The partial diffusion coefficient of copper is higher than titanium and copper is mostly diffused into titanium that leads to higher activity of copper particles during sintering and subsequent deformation processes. Concentrations of components in the diffusion zone is growing due to intensification of diffusion under the influence of deformation temperature (Fig.2) that is leading to formation of solid solutions of copper and titanium according to the phase diagram [3]. Structure formation at various temperatures and porosity influence mechanical properties. The general trend of decreasing hardness and tensile strength while increasing of porosity has been observed (Fig.3). The strength is reduced by 8-10% at the temperature range of dynamic recovery and by 15-25% at recrystallization. A slight increase of strength properties, as a result of dynamic strain aging, is observed after forging at 400ºC.

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(a) (b)

Fig.3. The dependence of hardness HRB (a) and tensile strength (b) on the temperature of forging: - θ0 = 2.8%, - θ0 = 6.1%, - θ0 = 9.2%.

Structure formation under dynamic softening promotes increasing plasticity (Fig.4), especially after forging at 500-700ºC due to dynamic recrystallization of the hard phase.

Fig.4. Changing of relative elongation δ depends on forging temperature at porosity θ0: -

2.8%, - 6.1%, - 9.2%.

Strain softening is the main factor controlling the material failure process, as its major structural features are determining, to a considerable extent, details of failure. Tensile stresses during the tension test are leading to increase of total porosity of sintered material, while increasing deformation. A three dimensional stress state occurs under the influence of external axial tension and the initial crack appearing in the central part of section, where the normal and radial tensile stresses reach maximum values. This type of fracture is typical for ductile materials [2, 3]. Samples of the powder materials under tension were fractured almost without formation of macronecks. Investigation of fracture surface at high magnifications have shown that materials of 2.8, 6.1 and 9.2% porosity have shown a ductile fracture (Fig.5) due to a high plasticity of copper matrix at all temperatures of forging. The character of fracture is under the influence of stress concentration from one side, and loosening action of the growth and coalescence of pores from the other side.

Fracture of copper-titanium powder material is different from the similar compact material (Fig.5). The failure has gone through the copper matrix, indicating strong interparticle connections of powder mixture components and tough phases formed during diffusion. The spatial structure of pores changed as a result of plastic deformation prior to failure. Large pits that transform into cracks are observed, together with small pits on the fracture surface after forging at 20°C (Fig.5,a). After tension of sample stamped at 200°C, a fairly uniform alternation of large and small holes with ridges between them was observed (Fig.5b). A number of small pits increases while decreasing number of large pits (Fig.5 c,d) due to increasing plasticity of the hard phase at temperatures of 500 and 700°C. Dynamic strain aging in the range of 300-400°C enhances the strength of samples and, at the same time, leads to formation and growth of large cracks on the ductile fracture (Fig.5 e,f).

Powder Metallurgy Progress, Vol.11 (2011), No 3-4 242

(a) (b) (c)

(d) (e) (f)

Fig.5. The fractographs of fractures at temperatures of forming at 6.1% porosity: (a) - 20°C; (b) - 200°C; (c) - 500°C; (d) - 700°C; (e), (f) - 400°C.

Structural defects in the form of pores and weak interparticle bonds are leading to increase of electrical resistance during production of electrical parts. However, softening processes during deformation at elevated temperatures are promoting transition of material into a near-equilibrium state, relieving internal stresses, while increasing contact area due to growing plasticity of hard phase and changing of pores’ morphology. Reduction of resistivity is starting only at 500ºC at intensive softening. However, the resistivity remains high and limiting implementation of copper-titanium powder materials of 6% porosity as electrical materials due to the presence of porosity (Fig.6).

Fig.6. The dependence of resistivity ρ on temperature of forging: - θ0 = 2.8%; - θ0 =

6.1%; - θ0 = 9.2%.

Copper-titanium powder material of 2.8% porosity was used for the production of electrode wheels for contact welding [5]. Physical and mechanical properties of finished part are the following: density 8.86 g/cm3, tensile strength 270 MPa, relative elongation

Powder Metallurgy Progress, Vol.11 (2011), No 3-4 243 4%, hardness HB 90-100, electrical resistivity 0.021 Ω.mm2/m, arc resistance 0.145 cm3/s. Hardness is 2 times higher, tensile strength increased by 1.4 times, resistivity grown by 15%, arc resistance by 35%, comparing with parts made of copper M1 by conventional technology.

CONCLUSIONS The copper-titanium powder material with mass content of titanium up to 1.5%

with high strength, wear resistance and low electrical resistance has been developed. Investigation of microstructure, fracture surfaces, physical and mechanical properties of materials with different porosity and mass content of titanium 0.5% has shown that forging of porous powder billets at 500-600°C ensures a fine-grained structure with grain size of 5.4 μm. The porous powder materials of 2.8-6.1% porosity after deformation under dynamic softening conditions are characterized by relatively high strength and ductility, reduced probability of brittle fracture, low electrical resistance, which allows us to recommend them as electrotechnical powder materials.

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