Elsevier

Materials & Design

Volume 63, November 2014, Pages 83-88
Materials & Design

Microstructures and mechanical properties of pure Mg processed by rotary swaging

https://doi.org/10.1016/j.matdes.2014.05.057Get rights and content

Highlights

  • Grain size of pure Mg can be effectively reduced by rotary swaging processing.

  • The dominated texture of the swaged pure Mg was a basal fibre.

  • Twinning and non-basal plane sliding accommodated the swaging process.

  • Gradient texture distribution was observed through the rod diameter.

  • There existed a slight shear deformation on the surface of the swaged rod.

Abstract

Microstructures and tensile properties of commercial pure magnesium processed by rotary swaging (RS) technique were investigated. Bulk and gradient textures in the RS processed Mg were characterised by neutron and synchrotron diffractions, respectively. Grains of the pure Mg were gradually refined with increase in the RS passes, which largely contributed to an increase in the tensile yield strength. A dominated basal fibre texture was observed in the RS processed pure Mg. Accommodated twinning deformation was also observed. Both the optical observations and texture analyses through the diameter of the swaged rod showed a gradient evolution in microstructure.

Introduction

Swaging is a forging process in which the dimensions of an item are altered using a die or dies by applying force. It is usually a cold working process; however, it also can be a hot working process. Rotary swaging is primarily suitable for cylindrical parts made from rods, tubes, and wires [1], [2]. The low tooling cost and fast setup make swaging an economical process for the production of even hundreds of pieces. The high output rate and material savings often make it also suitable for the highest production applications. Swaging process is widely used to produce Al alloys, steel and Cu alloys based products but not extensively used for Mg and its alloys till nowadays [2], [3], [4].

Mg and its alloys are one of the lightest structural materials. However, they have relatively low strength and ductility which limits their applications. Efforts have been taken to improve the mechanical properties of Mg by alloying [5], [6] and microstructural optimization by thermo-mechanical deformations [7], [8].

Conventional rolling, extrusion and forging on Mg can help to improve the tensile properties, but the deformed Mg always exhibits a high strong anisotropy [9], [10]. Recent investigations on severe plastic deformations (SPD), such as equal channel angular pressing/extrusion (ECAP/ECAE) and high pressure torsion (HPT) of Mg and its alloys have been widely performed [7], [8], [9], [10], [11]. Results showed that the SPD processed Mg alloys demonstrated a high ductility but relatively low strength [7], [9]. Due to the limitation of deformation conditions the large scaled application of previous SPD techniques on Mg and its alloys is still in progress [12]. Swaging deformation on Mg and its alloys has not been so far widely carried out even though it is a traditional forming technology.

Mg shows anisotropy because of its HCP (hexagonal close-packed) crystal structure which has limited slip systems at low temperatures [13]. Texture development of Mg alloys by conventional thermo-mechanical processes such as hot extrusion, forging, rolling, etc., are well understood [9], [14]; and as well their anisotropic tensile or compression stress–strain behaviours [15]. Basal plane sliding accommodated by twinning deformation was mainly considered to be the formation of preferred orientation/texture in Mg and its alloys. Unique texture developed via ECAP/ECAE has widely been investigated in the last years by experimental and as well by the simulation [7], [9], [16], [17], [18] During ECAP/ECAE processing the basal planes gradually orientating normal to the shear direction via different processing routes were reported [15], [17]. This was also proved by the texture gradient analysis in ECAP processed pure Mg [19]. A stationary oblique B fibre which is a typical simple shear deformation has been reported in HPT processed pure Mg [11], [20]. Few investigations were carried out to explore the texture development during swaging of Mg [21], [22]. Moreover, most of them did not systematically clarify the bulk texture evolution and as well the texture homogeneity.

Current investigations were undertaken to study first the microstructure and texture (bulk and local) of the RS processed pure Mg. Their evolutions will be related to the RS deformation mechanism. Second, the tensile properties at room temperature of the RS processed pure Mg will be investigated and discussed.

Section snippets

Experimental details

Rods of diameter of 10 mm and height of 100 mm machined from the commercial pure Mg (99.99%, Xinxiang Jiuli Magnesium Co., Ltd., China) ingots were used as raw materials (Fig. 1(a)). A stationary spindle swaging machine was from Heinrich Müller. The principle of its operation was illustrated in Fig. 1(b); and as well as the sample coordinates definition (SD – swaging direction, R – radial direction, H – hoop direction). The rod sample was heated to a desired temperature and held for 10 min before

Microstructures

Fig. 3 shows the optical microstructures of the samples S1 to S7. Coarse grains with an average size of about 4.5 mm were observed in the as-cast pure Mg. After swaging processing with 2 cycles (S2) the grain was refined with a bimodal distribution, i.e., some large grains about 120 μm were surrounded by small grains of about 40 μm in average diameter. Many twins inside the large grains were clearly observed. In the sample S3, large grains of about 50 μm surrounded by small grains of 25 μm were

Discussion

From the above description the bulk texture development shows a strong relationship with the RS process pass. In principle the swaging deformation looks like a multiple directional forging plus extrusion of a rod. The grain size of RS processed pure Mg can be greatly refined even with few cycles. Dominant basal planes sliding accommodated by twinning deformation lead to the formation of strong basal fibre texture [13]. Partially recrystallization also occurred due to a relatively high swaging

Conclusions

With the rotary swaging processing, the grain size of pure magnesium can be effectively refined. The microstructure was inhomogeneous from the surface to the centre of rods due to the existence of shear deformation at the near surface region. A strong {0 0 1} basal fibre texture was observed. Its maximum intensity slightly increases with increasing the RS process pass. The tensile yield stress monotonously increased and tended to be stable during swaging. The tensile elongation first increased,

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