A comparative study on deformation mechanisms, microstructures and mechanical properties of wide thin-ribbed sections formed by sideways and forward extrusion

https://doi.org/10.1016/j.ijmachtools.2021.103771Get rights and content

Highlights

  • Wide thin-ribbed aluminium profiles were manufactured by sideways and forward extrusion at the same condition.

  • Deformation mechanisms, resulting microstructures and mechanical properties for the two processes were compared.

  • Sideways extrusion requires a lower extrusion force but results in higher effective strains in the profile rib.

  • Sideways extrusion leads to greater grain refinement, yield strength and ultimate tensile strength.

  • Strength and ductility are improved simultaneously for the profile formed by sideways extrusion at elevated temperature.

Abstract

Extruded profiles/sections are increasingly used in the transport industry for lightweight structures. In this paper, a wide thin-ribbed aluminium profile with asymmetric Z-shape, was manufactured by a novel sideways extrusion process proposed by the authors. A comparative study was conducted by utilising the direct/forward extrusion process at the same extrusion temperature and speed, in which the different process mechanics, resulting microstructures and mechanical properties of profiles have been investigated by experiments and finite element modelling. It was revealed that, compared with sideways extrusion, although the design of a die pocket in forward extrusion induces preform and avoids the use of the large-diameter billet and extrusion container/press needed for extruding wide profiles, it requires a greater extrusion force due to work-piece upsetting necessary to fill the die pocket and leads to a lower effective strain in the profile rib. EBSD characterisation of the regions with an equal effective strain indicated that an increased shear strain is more efficient for obtaining fine grains with a higher average misorientation angle. In the same region of the profile rib made from the two different processes, sideways extrusion results in greater grain refinement due to greater effective strains, and a slightly greater texture intensity was found due to the intensive shear deformation. Tensile tests on formed profiles revealed that sideways extrusion leads to a higher yield strength (YS) and ultimate tensile strength (UTS) but a relatively lower elongation to failure, due to the combined effects of grain refinement, GND and texture intensity enhancement. Compared with the billet, the profile formed by forward and sideways extrusion has a YS increased by about 60% and 79% respectively, and an UTS increased by about 74% and 80% respectively in the extrusion direction, demonstrating an advantage of the sideways extrusion process in improving material strength under the same extrusion condition.

Introduction

Due to their characteristic lightweight and high stiffness/strength to weight ratio, extruded aluminium alloy profiles/sections are used to increasing extent in transport structures. They contribute to the effort to reduce structural weight and thus reduce environmentally degrading emissions from automobiles, aircraft and railway trains. Generally, a 10% weight reduction for conventional internal combustion engine (ICE) vehicles has been found to result in a 6%–8% fuel efficiency improvement [1]. With the continuously increased deployment of electrification in transportation (e-mobility), in the effort to achieve net-zero emissions by 2050, increased energy efficiency is an urgent issue in ensuring compatibility of increasing demand for electricity with available supply [2]. The potential contribution for lightweight extruded profiles in the drive to reduce vehicle weight is great and the need for new forming technologies is high [3].

Both straight and curved extruded profiles are widely used in vehicle structures. Curved profiles are generally obtained by bending forward extruded straight ones in a subsequent process. Vollertsen et al. [4] have noted that the most commonly used bending processes include rotary draw bending, roll bending, stretch bending and press bending. Generally, the force applied to bend straight extrusions tends to cause defects such as cross-sectional distortion [5], wrinkling [6,7] and springback [8,9]. Avoiding/eliminating these requires ad hoc methods which entail time and cost. These defects are more prevalent and of greater magnitude in thin-walled profiles, as has been described in a review by Yang et al. [10]. To deal with this problem two major research directions have emerged. One is based on refining the traditional bending process, including superposition of extra stresses [11] and torque [12], free bending with a spherical connection between the bending die and guider [13], flexible stretch bending having multiple rotational axes [14], radial hydro-forging bending where circumferential compression is applied during tube bending [15], and the recently developed differential heating-based rotary draw bending (DH-RDB) method which contributes to breaking the forming limit of difficult-to-form tubular materials utilising neutral layer shifting reconstruction (NLSR) [16]. Another is developing integrated extrusion-bending methods, in which the profiles are bent during forward extrusion using an external deflection device such as a guiding tool [17], or bending discs [18]. Alternatively, extrusion tooling has been designed to cause unbalanced material flow across the die orifice in forward extrusion, including die orifices inclined to the punch axis [19], die land of nonuniform length [20] and punches having stepped front ends [21]. Generally, compared with the traditional bending processes, the extrusion-bending integrated methods result in improved efficiency and can reduce the bending defects caused by external bending force. For curved profile extrusion using an external deflection device, expensive and complex tooling for guiding is necessary to enable profile curvature to be changeable during extrusion. For curved profile extrusion utilising the internal differential material flow caused by design of die tooling, process flexibility is limited since profile curvature is basically pre-set or hard to be varied during the extrusion process, thus different tool designs are needed for forming profiles with variable curvatures. In addition, the mechanical properties of curved profiles formed by these processes are, at best, the same as those obtained by conventional forward extrusion.

CONFORM is a continuous extrusion process where the work-piece is pushed by frictional force from the groove wall into the stationary die channel, and extruded through an outlet either in the forward direction or perpendicular to the forward direction [22]. In the latter case the stationary die channel intersects the groove at a 90° angle, and the work-piece is subjected to a shear stress as it is forced to make a 90° turn [23]. The grain refinement induced by material flow turning an angle by shearing has long been realised, and various techniques have been developed based on this shearing. The most well-developed one is equal channel angular pressing/extrusion (ECAP/E) with the most commonly used 90° channel intersection angle [24]. To realise the continuous ECAP process, ECAP-Conform has been proposed where the work-piece experiences shearing when it turns 90° in the intersection channel of the die and groove as in the conventional ECAP [25]. Similar shearing also occurs in radial extrusion where the material flows radially, i.e. perpendicular to the forward extrusion direction [26]. The CONFORM and ECAP are normally one-sided, while the radial extrusion can be one- or two-sided. Previous work on radial extrusion was conducted mostly at room temperature for simple shapes such as rods, tubes, cups, inner races, and flanges using equal ram speeds, largely demonstrating the effects of the die geometry and configuration on the shape, dimensions and flow-dependent defects of the produced parts [[27], [28], [29], [30], [31]]. Although CONFORM and ECAP/E processes utilise shear deformation to improve microstructures of materials, they cannot be applied to directly manufacture curved profiles. To enable curved profiles to be extruded directly, a novel sideways extrusion process has been proposed by the authors. In this process two opposing punches with variable velocities have been used enabling profiles with different curvatures to be produced and designed [32]. Round bars/tubes with varying curvature were manufactured, and in addition to the absence of defects and increased process efficiency resulting from integrated extrusion and bending, it was found that extrudate grain refinement occurred during sideways extrusion at room temperature due to severe plastic deformation, resulting in increased mechanical properties of AA1050 bar [33]. Sideways extrusion in general can be reasonably regarded as two equal or non-equal channel angular extrusion (N-ECAE) at the same time, depending on the location of the central dividing line determined by the velocity ratio of the two opposing punches [32]. The grain refinement during sideways extrusion utilises shearing in the intersection region of the inlet channel and the outlet channel as material flow changes 90° after extrusion, which is similar to that of channel angular extrusion [33]. However, it is realised that for the commonly used procedures for manufacturing curved aluminium alloy profiles by forward extrusion, and subsequent bending, grain refinement also occurs during the extrusion process due to dynamic recovery and dynamic recrystallization (DRX) [34]. Zhang et al. [35] studied the effects of extrusion parameters on the microstructure of AA6N01 profiles in hot forward extrusion and showed fine equiaxed grains were generated from initially coarse grains, due to dynamic recovery and DRX. Güzel et al. [36] studied the grain structure evolution along a predetermined flow line in hot forward extrusion of AA6082 at a temperature of 537 °C, speed of 5 mm/s, and extrusion ratio of 25, it was found that the initial grain size of 110 μm was gradually refined to a size of 46 μm along the flow line, due to DRX.

For a given alloy, factors influencing the microstructure and mechanical properties of extrusion profiles include; extrusion temperature, speed (strain rate), and extrusion ratio (effective strain). Bai et al. [37] studied the effect of extrusion condition on the microstructure of a magnesium alloy (Mg–8Al-0.5Zn-0.5RE) profile and showed grain refinement occurred due to complete DRX but average size of dynamic recrystallized (DRXed) grains increased significantly with the increase of extrusion temperature. Kaneko et al. [38] investigated the forward extrusion of AZ31 at a temperature of 480 °C and speed of 0.67 mm/s and found the grain size was refined from 24.6 μm to 13.4 μm when increasing the extrusion ratio from 4.6 to 25, and the grain was less refined at 17.0 μm with a higher extrusion speed of 2.67 mm/s. Chen et al. [39] evaluated the microstructure and mechanical properties of 2196 Al–Li alloy during hot extrusion, it was found that grain refinement increased with a proper increase of extrusion temperature and speed due to promoted degree of DRX and dynamic recovery, which also resulted in an increase of the ultimate tensile strength and yield strength of the extrudate. In view of the understanding cited above, as previous work on sideways extrusion focused on simple cross-sections formed at room temperature, to increase the applicability of the proposed sideways extrusion for manufacturing profiles, especially those with thin-walled sections, it is useful to obtain a comparison of the microstructure and mechanical properties of profiles formed by the proposed sideways extrusion with those of forward extruded profiles, at elevated temperature commonly used in industry.

In this paper, a comparison is presented of deformation mechanisms, microstructures and mechanical properties of wide thin-ribbed Z-shaped profiles formed utilising sideways and forward extrusion. To minimise the effect of bending, sideways extrusion was carried out with equal velocities of the two opposing punches so that a straight profile was produced as was that for forward extrusion. This also facilitates the cutting of samples for tensile tests and minimises the bending effect on sampling. Experiments were conducted with billets/work-pieces of commercially pure aluminium AA1100 at a temperature of 480 °C and a constant extrusion velocity of 0.1 mm/s. Finite element modelling was employed to facilitate understanding of the different mechanisms behind the deformation behaviour and the resulting microstructure and mechanical properties.

Section snippets

Extrusion tool design

As shown in Fig. 1, tool sets were made to enable both forward and sideways extrusion to be undertaken. The geometry of both die orifices and extrudate cross-sections is shown in Fig. 1(a). To facilitate clarity of discussion, as shown in the figure, three parts of the cross-section across its width are referred to in the text below as, upper flange (UF), thin rib (TR), and lower flange (LF). Fig. 1(b) and (c) show the extrusion tools for forward and sideways extrusion, respectively. The tools

Comparison of the extrusion load, dead zone and flow pattern

The finite element model was firstly validated by comparing the profile geometry and load-time curve for sideways extrusion obtained from experiments and modelling, as shown in Fig. 4. It can be seen that both of the profiles are slightly bent towards the upper flange. Probably this is due to the fact that although this flange is thinner than the lower one, 1.5 mm compared with 2 mm (Fig. 1(a)), it has a bigger cross-sectional area, 5.25 mm2 compared with 5 mm2, and its lesser effective

Conclusions

Wide thin-ribbed Z-shaped aluminium profiles were formed by novel sideways extrusion and conventional forward extrusion at the same temperature (480 °C) and speed (0.1 mm/s). Finite element process modelling was conducted to investigate the different deformation mechanisms and aspects of extrusion load, flow pattern, strain field, strain rate field, and temperature field. Microstructure characterisation and tensile tests were carried out to compare the resulting microstructure and mechanical

Credit author statement

Wenbin Zhou: Conceptualization, Methodology, Investigation, Formal analysis, Visualization, Software, Data curation, Writing – original draft. Junquan Yu: Conceptualization, Methodology, Investigation, Validation, Supervision, Formal analysis, Writing- Reviewing and Editing. Xiaochen Lu: Investigation, Resources. Jianguo Lin: Conceptualization, Project administration, Funding acquisition, Supervision, Writing- Reviewing and Editing. Trevor A. Dean: Conceptualization, Writing- Reviewing and

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The financial support from the UK Engineering and Physical Sciences Research Council (EPSRC) (grant No: EP/S019111/1; UK FIRES: Locating Resource Efficiency at the heart of Future Industrial Strategy in the UK) is greatly appreciated.

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