Elsevier

Advanced Powder Technology

Volume 32, Issue 9, September 2021, Pages 3314-3323
Advanced Powder Technology

Original Research Paper
Effect of powder processing and alloying additions (Al/ZrB2) on the microstructure, mechanical and electrical properties of Cu

https://doi.org/10.1016/j.apt.2021.07.012Get rights and content

Highlights

  • All the Cu samples could be densified to more than 92.16%

  • The microstructure of hot-pressed Cu-Al (mixed) alloys exhibited core-rim structure.

  • The conductivity of Cu varied between 97.0% and 12.3% IACS.

Abstract

The present work elucidates the effect of powder processing conditions (milling/mixing) and conductive alloying element (Al: aluminium) and ceramic (ZrB2: zirconium diboride) reinforcement addition on the densification, microstructure and electrical conductivity of copper (Cu) processed via hot pressing route. Disregard of alloying element/reinforcement/content or powders preparation method, the density of Cu materials varied between 92.16 and 99.76% ρth (theoretical density) after hot pressing at a low temperature of 500 °C. In case of Cu-Al alloys, the powder processing method significantly influenced its microstructure and conductivity. Particularly the Cu-Al alloys processed using mixed powders consisted of various phases Cu, α-Cu, γ1 (Cu9Al4), δ (Cu3Al2), ζ1 (Cu4Al3), η2 (CuAl) and θ (CuAl2) and the Cu alloys prepared using milled powders composed of either only α-Cu or α-Cu and γ1 (Cu9Al4) phases (depending on the Al content). Whereas, only Cu and ZrB2 phases were observed with the Cu-ZrB2 composites processed using either milled or mixed powers. In case of Cu-Al alloys, the hardness (0.88–3.41 GPa) and strength (540.30–1120.18 MPa) of Cu increased with the addition of Al. Interestingly, the hardness (0.88–2.55 GPa) and strength (508.50–970.60 MPa) of Cu increased upto 5 wt% ZrB2 and then they lowered with further addition of ZrB2. In particular, the hardness and strength of Cu-ZrB2 composites are lower than Cu-Al alloys reflecting the effectiveness of solid solution strengthening in the Cu alloys as compared to dispersion strengthening mechanism in Cu composite. The pure Cu prepared using milled powders exhibited low conductivity (75.70% IACS) than Cu processed using as-received/un-milled powders (97.00% IACS). Also, the Cu-ZrB2 composites measured with better electrical conductivity than Cu-Al alloys. Depending on the milling conditions and alloying/reinforcement amount, the conductivity of Cu-ZrB2 composites varied between 44.10 and 88.70% IACS.

Introduction

Copper (Cu) based materials are known for their excellent electrical and thermal conductivities. The primary advantage of these materials including their conductivity, moderate strength along with good ductility and corrosion resistance enables them suitable for many engineering applications [1], [2]. Especially, the rail overhead current collector shoes, commutator in alternators and electrical discharge machining (EDM) tool electrodes require high electrical conductivity along with good strength, hardness and wear resistance. Primarily solid solution strengthening, precipitation hardening, dispersion strengthening and grain refinement mechanisms are adopted to improve the strength of the pure Cu without affecting much on the ductility and workability. However, they can affect the electrical conductivity of Cu and the level of effectiveness depends on the amount of alloying elements, its distribution and the microstructure.

Generally, Cu is alloyed with the alloying elements such as Al, Cr, Fe, Ni, Pb, Sn and Zn etc. [3], [4], [5], [6], [7], [8], [9]. Most of the times, a tie exists between strength and electrical conductivity, while strengthening Cu via alloying route. As strength increased, the electrical conductivity of Cu alloys decreased and vice-versa. For example, pure Cu possessed low tensile strength of 66.6 MPa and high electrical conductivity of 99.5% IACS; whereas Cu-2 wt% Be alloy exhibited high tensile strength of 196.4 MPa and reduced electrical conductivity of 16.4% IACS [10]. Formation of intermetallic compounds in Cu-2Be alloy drastically decreased the electrical conductivity. Recently, the Cu-Al alloys were attempted for electrical and structural applications [11]. The Cu samples were fabricated using cold compaction followed by pressureless sintering process at a temperature of 900 °C. Hardness of 1.09 GPa was noted with the Cu-11Al alloys, but the electrical conductivity significantly reduced to 12.9% IACS due to the formation of intermetallic phases [11].

Alternatively, the strength of the Cu could also be improved with the addition of Al2O3, B4C, SiC, Si3N4, TiB2, TiC, ZrB2 reinforcements through dispersion strengthening [12], [13], [14], [15], [16], [17], [1], [18], [19], [20]. Fathy et al. [18] investigated the effect of ZrO2 on wear of in-situ prepared Cu-ZrO2 nanocomposites. Cu-ZrO2 composites exhibited improvement in the wear properties at the expense of electrical conductivity (~22% IACS) with the addition of 9 wt% ZrO2. The wear resistance of Cu-TiB2 composites was significantly increased with the addition of only 5 wt% of TiB2 content [19]. A careful review of the literature revealed that the effective composition of reinforcement for Cu should be limited to below 10 wt% in order to improve its mechanical, wear and electrical properties.

In continuation of our research efforts in developing Cu based materials, it was realised that the addition of Al is beneficial in improving hardness, strength and wear properties of Cu [3], [21]. In the present work, a systematic investigation was made to study the effect of Al/ZrB2 (up to 10 wt%) on the densification, microstructure and electrical conductivity of copper (Cu) processed via hot pressing route. As far as the present work is concerned, Al is selected as an alloying element since it is a light metal and exhibits high electrical conductivity (3.5 × 107 S/m) and widely used as an alloying element for Cu to enhance the strength and wear resistance. Also, the zirconium diboride (ZrB2) found to be an interesting ultra-high temperature conductive ceramic material having a high melting point (>3000 °C), high hardness (23 GPa), high electrical conductivity (10.3 × 104 S/cm) and thermal conductivity (58.2 W/m K) [20], hence it is of interest to know the characteristics of Cu-ZrB2 materials. In addition to understanding the effect of conductive metal (Al) and ceramic reinforcement (ZrB2) on structure and electrical properties of Cu, an emphasis was also made on studying the influence of powder preparation methods on microstructure and electrical properties of Cu. To the best of authors knowledge such studies were not existing in particular, concerning the electrical properties of Cu-Al/Cu-ZrB2 materials.

Section snippets

Materials and methods

Commercial copper (Padmasree enterprises, Hyderabad, India) with mean particle size of ~9.98 µm (purity greater than 99%), aluminium powders (SRL™, India) with mean particle size ~6.47 µm (purity greater than 99%), ZrB2 powders (H.C. Starck, Grade B) with mean particle size of 3–5 µm and purity greater than 97% were used as starting materials. The size of ZrB2 powder particles was considerably reduced to about 0.29 µm after subjecting to high energy ball milling (Model: Pulverisette 7, Fritsch,

Microstructure of Cu-Al alloys

A representative microstructure of Cu-10Al powders after subjecting to milling and mixing operations is shown in Fig. 2a and b. It is evident that the milled Cu-10Al powders appeared in the flaky and irregular form and varying in different sizes. It also represents the uniform dispersion of the Al alloying element in Cu. On the other hand, the morphology of Cu-10Al (mixed) resembles very similar to starting powders as there was no much milling effect (Fig. 2b). The X-ray diffraction (XRD)

Conclusions

The microstructure of hot-pressed Cu-Al (mixed) alloys exhibited core-rim structure and consisted of Cu, α-Cu, γ1 (Cu9Al4), δ (Cu3Al2), ζ1 (Cu4Al3), η2 (CuAl) and θ (CuAl2) phases. In case of the Cu-Al (milled) samples, no core-rim structure was observed. However, the Cu-Al (milled) samples consisted of α-Cu in Cu containing Al (up to 5 wt%) and α-Cu and (Cu9Al4) phases in Cu containing up to 10 wt% Al. Both the Cu-ZrB2 (ML) and Cu-ZrB2 (MX) composites consist of Cu and ZrB2 phases. In case of

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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