Mechanical and Microstructural Properties of Ultra-fine Grained AZ91 Magnesium Alloy Tubes Processed via Multi Pass Tubular Channel Angular Pressing (TCAP)

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Ultra-fine grained (UFG) cylindrical tubes were produced via recently developed tubular channel angular pressing (TCAP) process through different passes from as-cast AZ91 magnesium alloy. The microstructure and mechanical properties of processed tube through one to four passes of TCAP process at 200 °C were investigated. Microhardness of the processed tube was increased to 98.5 HV after one pass from an initial value of 67 HV. An increase in the number of passes from one to higher number of passes has no more effect on the microhardness. Yield and ultimate strengths were increased by 4.3 and 1.4 times compared to those in as-cast condition. Notable increase in the strength was achieved after one pass of TCAP while higher number of passes has no more effect. Microstructural investigation shows notable decrease in the grain size to around 500 nm from the primary value of ∼150 μm. Dissolution and distribution of hard Mg17Al12 phase in the grain boundaries of dynamically recrystallized UFG AZ91 with a mean grain size of ∼500 nm was an interesting issue of TCAP processing at 200 °C compared to other severe plastic deformation processes.

Introduction

In recent years, improvement of material properties by grain refinements using severe plastic deformation (SPD) has been much attractive whereas ultra-fine grained (UFG) and nanostructured materials exhibit outstanding properties[1]. There are different SPD methods such as equal channel angular pressing (ECAP)[2], high-pressure torsion (HPT)[3], and accumulative roll bonding (ARB)[4]. Most of SPD methods are suitable for 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 UFG tubular parts using SPD methods. An especially attractive method named as tubular channel angular pressing (TCAP) method was recently developed by Faraji et al.[11] as an effective and novel SPD process suitable for processing tubes to very large strains. They processed AZ91 magnesium alloy using TCAP method at 300 °C and reported more grain refinement and significant improvement in the mechanical properties after only single pass[11]. They also investigated the effects of process parameters such as channel geometry[12] and channel angles[13] on plastic deformation behaviour and process load using finite element modelling. In this study, AZ91 magnesium alloy is processed via multi pass TCAP process at 200 °C and the effects of the number of passes on grain refinement, microhardness and compression behaviour were investigated.

Fig. 1(a) shows the principle of TCAP process. The tube material is pressed by a hollow cylindrical punch into a gap between die and mandrel, which forms a tubular angular channel with three shear zones I, II and III (Fig. 1(b)). The cross-section of the TCAP processed tube remains unchanged at the end of process. In multi pass TCAP, the process is repeated as many times as necessary to achieve distinct strains without any reduction in the cross-section of workpiece.

As mentioned in literature[12], the strain–stress state during TCAP is different from that during the ECAP. In the ECAP process, the strain state could be considered as simple shear, while in the TCAP process, there are some additional radial and circumferential tensile and compressive strains. The exact value of total accumulated strain (ε¯T) after N passes of TCAP processing can be calculated by the following relationship, which results from common engineering plasticity formulae and the geometry of Fig. 1(b)[11]:ε¯TN=N{i=13[2cot(φi/2+ψi/2)+ψicosec(φi/2+ψi/2)3]+43lnRR0}

From Eq. (1), the total equivalent plastic strain in TCAP with the parameters used in this work after one, two, three and four passes are ∼2.2, 4.4, 6.6 and 8.8, respectively.

Section snippets

Experimental

An as-cast AZ91 magnesium alloy with a composition Al 9.1 wt%; Zn 0.68 wt%, Mn 0.21 wt%, S 0.085 wt%, Cu 0.0097 wt%, Ni 0.001 wt%, Fe 0.0029 wt%, and Mg balance was used as starting material. Cylindrical tubes of 20 mm in outer diameter and 50 mm in length were machined from as-cast ingots. A TCAP die was manufactured from hot worked tool steel and hardened to 55 HRC. Die parameters and their values are listed in Table 1. The TCAP experiments were carried out with an INSTRON press with a

Results and Discussion

Fig. 2(a) and (b) shows the AZ91 workpiece before and after TCAP. During the process, the tube diameter increases and returns to the initial size at the end of TCAP. As shown in this figure, the cross-section area of the tube before and after TCAP remains constant though the tail part has some inconsistency. Fig. 3 shows hollow cylindrical punch load vs its displacement during the process. Maximum required load is about 100 kN. It is clearly seen that the force diagram could be divided to six

Conclusion

Ultra-fine grained (UFG) cylindrical tubes were produced via multi pass tubular channel angular pressing (TCAP) process from as-cast AZ91 magnesium alloy. The microstructure and mechanical properties of processed tube through one to four passes of TCAP process at 200 °C were investigated. Microhardness of the processed tube was increased to 98.5 HV after one pass from an initial value of 67 HV. An increase in the number of passes from 1 to higher number of passes has no more effect on the

Acknowledgement

This work was financially supported by Iran National Science Foundation (INSF).

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