Tubular channel angular pressing (TCAP) as a novel severe plastic deformation method for cylindrical tubes
Highlights
► TCAP was proposed as a novel severe plastic deformation technique for cylindrical tubes. ► TCAP can refine the grain size of AZ91 alloy to 1.5 micron from initial value of 150 micron through one pass. ► TCAP could increase the hardness to 78 Hv from 51 Hv through one pass.
Introduction
There has been much interest in recent years in improving material properties by grain refinements using severe plastic deformation (SPD) [1] such as equal channel angular pressing (ECAP) [2], high-pressure torsion (HPT) [3], and accumulative roll bonding (ARB) [4]. Of these various processes, ECAP is an especially attractive processing technique for several reasons [2]. ECAP was previously applied to bar and rod form parts [5], [6], along with application to plate and sheet parts [7], [8], [9], [10]. Despite the need for high strength tubes in a wide range of industrial applications, few efforts have been undertaken to produce ultrafine grained tubular parts using SPD methods. Toth et al. [11] proposed an SPD method based on HPT for producing ultrafine grained (UFG) tubes. Mohebbi and Akbarzadeh [12] developed an accumulative spin-bonding (ASB) method based on ARB to produce UFG tubes.
Appreciating the outstanding capabilities of ECAP, we introduce in the present article a tubular channel angular pressing (TCAP) method as a novel SPD process suitable for processing tubes to very high strains. The TCAP process for the first time was registered as a patent by the first author [13]. To primarily demonstrate the applicability of this novel method, an AZ91 magnesium alloy was processed.
Section snippets
Principles of TCAP
A schematic of TCAP is shown in Fig. 1(a). The tube, constrained by the inner and outer dies, is pressed by a hollow cylindrical punch into a tubular angular channel. The tube material is pressed into the tubular angular channel, where three shear events take place during one processing path. The initial tube at the start of the process is put into the gap between the inner and outer dies. Then, the hollow cylindrical punch is put over the tube's upper surface in the gap between the dies. The
Experimental procedures
The material used in this study was an AZ91 magnesium alloy. Cylindrical tubes of 20 mm in outer diameter, 2.5 mm in thickness, and 35 mm in length were machined from as-cast ingots. A TCAP die was manufactured from hot worked tool steel and hardened to 55 HRC. The channel angles φ1, φ2, and φ3 in the TCAP die shown in Fig. 1(b) were 135°, 90°, and 135°, respectively. The angle of curvature, the outer corner angle ψ2, was 90° [15], and both ψ1 and ψ2 were equal to 0°. The TCAP experiments were
Results and discussion
Fig. 2(a) shows an AZ91 workpiece from the initial tube feeding stock to the form in tubular channel, and final TCAP processed tube. During the process, the tube diameter increases and returns to the initial size at the end of the TCAP process.
The microstructure of the initial as-cast material is shown in Fig. 2(b), which shows a typical dendritic structure with primary Mg embedded in an Mg17Al12 network with a mean grain size of 150 μm. Fig. 2(c) and (d) give the optical micrographs of the
Conclusions
Tubular channel angular pressing (TCAP) was introduced as a novel high strain method suitable for deforming tubular components. In this new method the tube material is subjected to SPD by utilizing alternate pressing into the tubular angular channel with three shear zones. The flow pattern and the strain–stress state of TCAP are different from those of conventional ECAP. In the TCAP process there are additional radial tensile and compressive strains because the tube diameter changes during the
Acknowledgments
The authors would like to acknowledge the financial support of the University of Teheran for this research. HSK acknowledges the support by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea.
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