Enhancing strength without compromising ductility in copper by combining extrusion machining and heat treatment
Graphical abstract
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 is cutting the workpiece at the cutting speed (Fig. 1b). The chip thickness 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 increases with plastic deformation . Referring to the model of Estrin and Mecking (1984), dislocation density can be expressed as:In Eq. (1), is effective plastic strain, is Burger’s vector, is the average grain size of initial workpiece, and are dislocation storage rate and dynamic recovery rate respectively.
The effective plastic strain increases from zero to during QSEM. The dislocation density for 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).
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