Enhancing strength without compromising ductility in copper by combining extrusion machining and heat treatment

https://doi.org/10.1016/j.jmatprotec.2018.12.001Get rights and content

Abstract

It is a challenge to produce metallic materials with high strength and good ductility. Improving the strength of metallic materials usually sacrifices the ductility or work-hardening capacity. Here combining extrusion machining and heat treatment, we improve the strength of copper without losing strain hardening capacity and therefore the ductility remains. Copper was first deformed by extrusion machining at shear strain 3.1 and then annealed at 523 K for 5 min. Compared with the initial workpiece, the processed copper possesses five times higher yield strength and alike work hardening behavior. Microstructural characterizations illustrate that high strength and high strain hardening are attributed to the hierarchical microstructure that the recrystallized grains are surrounded by elongated subgrains. Finally, an analytical modeling was employed to rationalize the mechanical properties of copper processed by the proposed strategy. The theoretical results are in agreement with the experimental measurements.

Introduction

High strength and high ductility are demanded for the application of engineering materials. Strength can be improved by the well-known methods such as grain refinement (Güzel et al., 2012), solid solution alloying (Su et al., 2008), work hardening (Lobos et al., 2010), and introducing strengthening phases (Dolata-Grosz et al., 2006). As demonstrated by Valiev et al. (2000), strength and ductility are contradictory in metallic materials. According to Meyers et al. (2006), improving the strength of materials usually leads to poor ductility or work-hardening capacity. Jia et al. (2001) found that the poor ductility always stemmed from the low strain hardening in materials. Zhu and Liao (2004) further found that high strain hardening rate helped to restrain localized deformation and postpone necking. Wu et al. (2014) reported that high-strength metals would need a higher strain hardening rate to maintain the ductility.

Specific microstructures could be designed to improve the strain hardening rate in high-strength metals. Wang et al. (2002) proposed a bimodal grain size distribution to induce strain hardening mechanisms in high-strength copper. Lu et al. (2004) noted that nano-twinned copper possessed extra-high strength with keeping the ductility within a certain regime. Petryk et al. (2008) investigated the effect of dislocation cell and cell-block boundaries on grain refinement and strain hardening in interstitial free steel. Fang et al. (2011) reported that gradient nano-grained structures above coarse grained copper contributed to both strength and ductility. Kauffmann et al. (2013) presented that the formation of non-coherent twin boundaries could result in an increase of strength but a loss of ductility in cryogenic drawn copper. Liu et al. (2016a) used topologically controlled surface mechanical attrition treatment (SMAT) to produce planar heterogeneous structure in copper with high strength but no sacrifice of ductility. Huo et al. (2017) employed a simplified thermo-mechanical treatment to prepare fine-grained Al-Zn-Mg-Cu alloy with notably improved ductility and maintained high strength. Tian et al. (2018) stated that the recrystallized ultrafine-grained copper with a minimum mean grain size of 0.51 μm showed high strength and good ductility.

As proved by Childs (2013), machining is an effective strategy to impose severe plastic deformation (SPD) in metallic materials. Compared with the conventional SPD methods, e.g. equal channel angular pressing (ECAP) (Segal et al., 1981), high-pressure torsion (HPT) (Smirnova et al., 1986), accumulative roll binding (ARB) (Saito et al., 1999) and cyclic extrusion compression (CEC) (Richert et al., 1999), machining needs only a single pass of room-temperature deformation process to achieve SPD. Ceretti et al. (1999) and Umbrello et al. (2008) conducted numerical simulation of machining to investigate the influence of machining parameters on large deformation filed. Moscoso et al. (2007) employed the SPD method of extrusion machining to produce bulk nanostructured copper. Brown et al. (2009) further investigated the effects of extrusion machining parameters (strain, strain rate and temperature) on the microstructural evolution and Vickers hardness. By means of high speed extrusion machining, Liu et al. (2016b) fabricated a bimodal grain size distributed magnesium alloy with high Vickers hardness.

Machining, as a method of SPD, has been used to process materials with specific microstructure and high Vickers hardness. However, it is unknown whether extrusion machining could fabricate materials with high strength and good ductility. In this paper, extrusion machining and heat treatment were combined to research the relationship between machining parameters and mechanical properties. Uniaxial tensile tests and microstructural characterizations were further performed to investigate the effect of controlled microstructure on strength and strain hardening. Finally, a theoretical model was employed to reveal the mechanism underlying the proposed strategy.

Section snippets

Experimental procedure

Oxygen-free-high-conductivity (OFHC) copper with the purity of 99.95% was used here. A copper cuboid with dimensions of 25 × 25 × 80 mm3 was annealed in vacuum at 773 K for 1 h to obtain homogeneous coarse grain (CG) microstructure. The copper samples were processed by means of quasi-static extrusion machining (QSEM) at the room temperature (Fig. 1a). The tool with a precut thickness h0 is cutting the workpiece at the cutting speed V0 (Fig. 1b). The chip thickness hc is controlled by the

Mechanical property

Fig. 2 shows the tensile behaviors of copper sheets as well as the initial workpiece for comparison. The engineering stress-strain curves were plotted in Fig. 2a. For the initial workpiece, the initial workpiece has the 0.2% yield strength of about 47 MPa. After QSEM and HT, the strength of copper was improved significantly (Fig. 2a). As shown in Table 2, the 0.2% yield strength increases with the increasing shear strain. The yield strength of copper sheet is ∼245 MPa for the shear strain of

Theoretical model

During QSEM, dislocation density ρ increases with plastic deformation ε. Referring to the model of Estrin and Mecking (1984), dislocation density ρ can be expressed as:dρdε=1bd+k1ρk2ρIn Eq. (1),ε is effective plastic strain, b is Burger’s vector, d is the average grain size of initial workpiece, k1 and k2 are dislocation storage rate and dynamic recovery rate respectively.

The effective plastic strain ε increases from zero to γ/3 during QSEM. The dislocation density ρ0 for ε=0 is the

Discussion

Physical metallurgy has been widely used to process metallic materials for enhancing the mechanical properties. Heat treatment, as a type of physical metallurgy, is an effect method to improve the ductility of metallic materials. Annealing temperature and annealing time are two important parameters during heat treatment. As shown in the experiments of Yuan et al. (2015), with the increasing temperature, the strength decreases but the elongation increases. Nouroozi et al. (2018) found that both

Concluding remarks

The mechanical properties of copper sheet, processed by combining severe plastic deformation (SPD) and heat treatment (HT), were investigated in this study. The following conclusions can be drawn.

  • (1)

    Quasi-static extrusion machining (QSEM) at the shear strain of 3.1 and subsequent heat treatment (HT) at 523 K for 5 min were used to enhance yield strength five times without compromising ductility in copper.

  • (2)

    Microstructural characterization shows that high strength and good ductility are attributed to

Acknowledgments

This work has been supported by the National Key Research and Development Program of China (No. 2017YFB0702003), the National Natural Science Foundation of China (Grant Nos. 11602236, 11132011 and 11802013), Fundamental Research Funds for the Central Universities (Grant No. FRF-BR-17-015A), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant Nos. XDB22040302, XDB22040303) and the Key Research Program of Frontier Sciences (Grant No. QYZDJSSW-JSC011).

References (49)

  • R. Jiang et al.

    Grain size effects in a Ni-based turbine disc alloy in the time and cycle dependent crack growth regimes

    Int. J. Fatigue

    (2014)
  • A. Kauffmann et al.

    Properties of cryo-drawn copper with severely twinned microstructure

    Mater. Sci. Eng. A

    (2013)
  • X. Liu et al.

    High strength and high ductility copper obtained by topologically controlled planar heterogeneous structures

    Scr. Mater.

    (2016)
  • Y. Liu et al.

    A new method for grain refinement in magnesium alloy: high speed extrusion machining

    Mater. Sci. Eng. A

    (2016)
  • M.A. Meyers et al.

    Mechanical properties of nanocrystalline materials

    Prog. Mater. Sci.

    (2006)
  • S. Nag et al.

    Correlation between ferrite grain size, microstructure and tensile properties of 0.17 wt% carbon steel with traces of microalloying elements

    Mater. Sci. Eng. A

    (2014)
  • M. Nouroozi et al.

    Effect of microstructural refinement and intercritical annealing time on mechanical properties of high-formability dual phase steel

    Mater. Sci. Eng. A

    (2018)
  • J.-W. Park et al.

    Effect of electric current on recrystallization kinetics in interstitial free steel and AZ31 magnesium alloy

    Mater. Charact.

    (2017)
  • H. Petryk et al.

    Grain refinement and strain hardening in IF steel during multi-axis compression: experiment and modelling

    J. Mater. Process. Technol.

    (2008)
  • M. Richert et al.

    Microstructural evolution over a large strain range in aluminium deformed by cyclic-extrusion–compression

    Mater. Sci. Eng. A

    (1999)
  • Y. Saito et al.

    Novel ultra-high straining process for bulk materials—development of the accumulative roll-bonding (ARB) process

    Acta Mater.

    (1999)
  • J.-h. Su et al.

    Aging study of rapidly solidified and solid-solution Cu–Cr–Sn–Zn alloy

    J. Mater. Process. Technol.

    (2008)
  • Y.Z. Tian et al.

    Remarkable transitions of yield behavior and Lüders deformation in pure Cu by changing grain sizes

    Scr. Mater.

    (2018)
  • D. Umbrello et al.

    Hardness-based flow stress for numerical simulation of hard machining AISI H13 tool steel

    J. Mater. Process. Technol.

    (2008)
  • Cited by (20)

    • A novel method to study recrystallization behavior: Continuous heating stress relaxation

      2020, Materials Characterization
      Citation Excerpt :

      Cheng et al. [1] improved the stretch formability of AZ31 magnesium alloy by multidirectional pre-compression process induced static recrystallizations. Liu et al. [2] improved the strength of copper without losing strain hardening capacity by the hierarchical microstructure that the recrystallized grains are surrounded by elongated subgrains. Recrystallization defects can reduce the service life of single crystal turbine blades, Li et al. [3] proposed a mathematical thermo-mechanical model to study the behavior of recrystallization to reduce the likelihood of recrystallization.

    • Wetting of h-BN by molten Cu-8.8Zr-xTi ternary alloys at 1373 K

      2020, Vacuum
      Citation Excerpt :

      Cu matrix composites have a widely application, due to its good ductility, high thermal and electrical conductivity, and high thermal shock resistance [1–3].

    • Selective laser melting of Ti–22Al–25Nb intermetallic: Significant effects of hatch distance on microstructural features and mechanical properties

      2020, Journal of Materials Processing Technology
      Citation Excerpt :

      As discussed in Section 3.2, the average grain size of the as-fabricated samples decreases significantly with the increasing hatch distance. Liu et al. (2019) has proven that grain refinement is an efficient way to enhance the strength without sacrificing ductility of polycrystalline materials. The finer β grains can introduce more grain boundaries, which effectively hinders the movement of dislocations.

    View all citing articles on Scopus
    View full text