Magnesium alloy ZK60 tubing made by Shear Assisted Processing and Extrusion (ShAPE)

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Abstract

ZK60 Magnesium tubing has been friction extruded from as-cast billets and T5 conditioned bars using Shear Assisted Processing and Extrusion (ShAPE). Tubes having an outer diameter of 50.8 mm and wall thickness of 1.9 mm were extruded with >20 times less ram force compared to conventional extrusion due to the unique shearing conditions and tooling inherent to ShAPE. Microstructures of the as-cast billet and T5 bar feedstock materials were significantly different from each other in terms of grain size, texture, and second phase distribution; yet the resulting microstructures after ShAPE were remarkably similar. An average grain size of 4–5 μm, 20° tilt of basal texture away from the extrusion axis, and refined second phases having a uniform distribution were achieved independent of the feedstock material. Hardness for as-extruded and artificially aged tubes are presented with isotropic behavior explained by detailed microstructural analysis. This work suggests that bulk ZK60 magnesium alloys extrusions may be fabricated in a single step, with microstructures that are unobtainable with conventional extrusion.

Introduction

One pathway for improving the energy efficiency and increasing the driving range of automobiles is by decreasing overall vehicle weight. To this end, magnesium (Mg) alloys are being investigated as a lightweight alternative for structural applications [1] within the automotive industry [2]. Despite the potential for large weight savings, 30% compared to aluminum and 80% for steel, widespread adoption of Mg alloys has not yet occurred primarily due to a combination of high cost and inadequate mechanical properties [3]. This lack of market penetration into the automotive industry is evident by Mg alloys accounting for less than 0.5% of a typical passenger vehicle's weight; a level that has remained relatively constant over the past twenty years [4]. A significant barrier for Mg resides with the balance between cost and properties. For example, Mg alloys containing rare-earth (RE) elements have attained mechanical properties sufficient for some structural automotive applications [5], but their cost is too high for typical passenger vehicles [6]. Conversely, non-RE Mg alloys are more cost effective, but their mechanical properties are insufficient for structural automotive components. If a non-RE Mg alloy could be extruded with improved mechanical properties, using a process amenable to industrial production, then progress could be made toward higher usage of Mg components in automobiles. The research reported herein suggests that improved material properties in non-RE Mg extrusions may be possible using Shear Assisted Processing and Extrusion (ShAPE); a relatively new process that could be scaled to industrial production. The non-RE Mg alloy, ZK60, is investigated in this work due to its comparatively low cost, moderate strength, and favorable compressive to tensile strength ratio relative to other non-RE Mg alloys [7].

It is well known that grain size, basal plane alignment, and refinement of second phases can improve strength and ductility in extruded Mg alloys [8]. As a result, various severe plastic deformation (SPD) techniques have been investigated as a means to achieve microstructures with highly oriented texture, refined grain size, and refined second phases in the non-RE Mg alloy ZK60 [9]. Equal channel angular pressing (ECAP) of ZK60 has shown an average grain size of 0.6–5.6 μm [10], ultimate tensile strength (UTS) of 320–450 MPa [11], and elongation of 35% [12]. Accumulative roll bonding (ARB) of alternating Mg/ZK60 sheet has shown an average grain size of 2.4 μm, UTS of 290 MPa, and elongation of 15% [13]. Cyclic extrusion and compression (CEC) has shown an average grain size of 3–6 μm, UTS of 275 MPa, and elongation of 38% [14]. These examples illustrate the ability of SPD to achieve high strength and ductility in non-RE ZK60 compared to conventional extrusion [7]. However; ECAP, ARB, CEC and other SPD techniques are limited to small batch sizes and require multiple deformation passes to achieve these properties which reduces throughput and increases cost. Although these SPD processes are able to generate remarkable properties in ZK60, they are not readily scalable to industrial levels and have therefore remained largely an academic interest. ECAP has shown the ability to tilt basal texture away from the extrusion direction, but only with repeated passes [15]. A degree of grain refinement can be achieved with conventional extrusion, but the extrusion rate must be kept low [17] or second phase particles or costly rare earth elements can be added [[16], [17], [18]]. Additionally, conventional extrusion has limited ability to refine and distribute second phase particles within the extrudate [18].

Hollow-section Mg extrusions are of particular interest to the automotive industry for structural components such as bumper beams and crush cans, but ECAP, ARB, CEC and other SPD methods are not capable of producing hollow-sections. Ideally, what is needed is a single step process that achieves microstructures with fine grain size, refined and distributed second phases, and controlled texture. Such a process should also be scalable to an industrial level and capable of hollow-section geometries. ShAPE is one possible method with the potential to meet these criteria and is the extrusion process utilized in this work.

ShAPE is a cousin to friction stir back extrusion (FSBE) [[19], [20], [21], [22], [23], [24], [25], [26]], friction stir extrusion [[27], [28], [29], [30], [31], [32], [33], [34]] and friction extrusion [[35], [36], [37], [38]] (all referred to as FSBE in this work for simplicity). However, ShAPE is distinctly different and overcomes many of the challenges facing tubes made by FSBE such as uniformity of grain size and texture through the wall thickness, short extrusion length, and limited scalability of the cross section. The microstructural benefits and potential for scalability of ShAPE have been described in detail elsewhere [[39], [40], [41]]. In short, the high shear deformation and intense mixing inherent to ShAPE enables a degree of microstructural refinement, crystallographic orientation, breakdown and dispersion of second phases, and reduced extrusion pressure which are not possible with conventional extrusion techniques. For example, ShAPE has been used to extrude, in a single step, hollow tubes with 7.5 mm diameter and 0.75 mm wall thickness directly from as-cast Mg-2Si [39] and AZ91 melt spun flake [40]. These tubes had an average grain size below 5 μm, up to a 30° rotation of the (0001) basal planes to the extrusion axis, and significant breakdown and dispersion of brittle Mg2Si and Mg17Al12 s phases. The potential for scalability of the ShAPE process was demonstrated for ZK60 where 50.8 mm diameter tubes with 1.52 mm wall thickness were extruded having an average grain size of 3.8 μm and the (0001) basal planes aligned 20° away from the extrusion axis [41]. In these examples, the ShAPE process is shown to be scalable compared to SPD and enables unique control over grain refinement and crystallographic orientation compared conventional extrusion. A greater propensity to breakdown and disperse second phases has also been shown. In the current work, the ability of ShAPE to extrude two very different starting microstructures of the same alloy (non-RE ZK60 casting and wrought bar stock) and achieve similar extrudate microstructures is investigated.

Table 1 shows the body of literature, to the extent the authors are aware, for aluminum, magnesium, and copper tubes made by FSBE and ShAPE. The last row in the table is the data from this work as indicated by an asterisk. By plotting outer diameter vs. extrusion ratio from Table 1, the dimensional scalability of ShAPE extruded tubes relative to FSBE tubes becomes apparent as seen in Fig. 1. The published literature for FSBE tubes is tightly clustered over a narrow range of diameters and extrusion ratios. Both the outer diameter (this work) and extrusion ratio [39,40] are much larger for ShAPE tubes (blue circles) compared to FSBE tubes (black circles). This is because the ShAPE process for tubing is significantly different than FSBE. In FSBE, a spinning mandrel is rammed into a contained billet, much like a drilling operation. Scrolled grooves force material outward, and material back extrudes onto the OD of the mandrel to form a tube having undergone very little area reduction; not having been forced through a die. As a result, only very small extrusion ratios are possible. The tube is not fully processed through the wall thickness, and the tube length is limited to the length of the mandrel. In contrast, ShAPE utilizes spiral grooves on a die face to feed material inward through a die and around a mandrel that is traveling in the same direction as the extrudate. As such, a much larger outer diameter and extrusion ratio are possible, and the material is uniformly processed through the wall thickness. The extrudate is free to push off the mandrel as in conventional extrusion, and the extrudate length is only limited only by the starting volume of the billet. These advantages suggest that the ShAPE process may one day be scalable to an industrial level.

Section snippets

Experimental approach and materials

In ShAPE, a rotating die is rammed against a stationary billet causing frictional heating at the die/billet interface (Fig. 2). The amount of heat generation is controlled by regulating the applied torque, rotational speed, extrusion rate, and flow of chilled water through the mandrel. As temperature increases, the billet face softens and material plastically flows inward toward the extrusion orifice through spiral grooves machined into the die face. Upon exiting the grooves, material then

Results

Billets made from ZK60 as-cast and ZK60-T5 bar were extruded using the ShAPE process. During extrusion, the rotating die was rammed into the billet container at a constant speed of 3.81 mm/min while rotating at 300 rpm. Pressure and temperature build rapidly as shown in Fig. 4; with pressure rising to a peak (e.g. breakthrough) of 9.5 MPa for both billet materials. After breakthrough, pressure falls off sharply indicating that the material has softened enough to flow through the spiral grooves

Conclusion

A key outcome of this research is that similar grain size and second phase distributions were achieved in ShAPE extruded ZK60 tubing made from feedstock materials with vastly different microstructures. Tilted basal textures were also observed to develop in both materials. This capability was demonstrated for ZK60 feedstock materials in the form of as-cast billet and T5 conditioned bar. Hardness measurements suggest that ShAPE processing was effective at reducing anisotropy compared to

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Acknowledgements

The authors thank the U.S. Department of Energy Vehicle Technologies Office (DOE/VTO) for supporting this work. The design, procurement and installation of the ShAPE machine was funded through the MS3 (Materials Synthesis and Simulation Across Scales) Initiative at Pacific Northwest National Laboratory as was a portion of the characterization work. The authors are grateful for the dedication of Jens Darsell and Md. Reza-E-Rabby in assisting with extrusions on the ShAPE machine, and Anthony

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