Enhanced Superplasticity in a Magnesium Alloy Processed by Equal‐Channel Angular Pressing with a Back‐Pressure

Abstract

The low density and high strength-to-weight ratio of magnesium alloys have motivated an interest in using these materials for structural applications in the aerospace, automotive and electronic sectors. However, the fabrication of magnesium components is generally limited because the hexagonal close-packed crystal structure leads to relatively low ductility. It is now recognized that enhanced superplastic properties may be achieved by processing metal alloys through the application of severe plastic deformation. [1] An example is equal-channel angular pressing (ECAP) where a billet is pressed repetitively through a die to impose a very high strain. [2] Processing by ECAP has been used effectively to produce a wide range of ultrafine-grained metallic alloys, and a recent review [3] shows that many of these materials, including some magnesium alloys, such as AZ61, [4] AZ91 [5] and ZK60, [6,7] demonstrate excellent superplastic properties when tested in tension at elevated temperatures. Superplasticity has also been achieved in the AZ61 alloy using a different technique of severe plastic deformation, viz. high-pressure torsion. [8] By contrast, only limited success has been achieved with the commercial AZ31 alloy where the maximum elongation reported to date is 460 % after processing using extrusion followed by ECAP. [9] Similarly, a maximum elongation of only 370 % was achieved in an AZ31 alloy after processing for superplasticity using multi-directional forging. [10] The present investigation was initiated to evaluate the feasibility of using ECAP to obtain high superplastic elongations in the AZ31 alloy. ExperimentalThe experiments were conducted using two different AZ31 alloys obtained from commercial sources. These alloys were received as a hot-rolled plate and in an as-cast condition. The chemical compositions of these two alloys, in wt %, were 3.0 Al, 0.8 Zn and 0.2 Mn for the rolled plate and 3.1 Al, 1.0 Zn and 0.3 Mn for the cast alloy. The hot-rolled AZ31 alloy was not heat treated prior to ECAP but the cast alloy was homogenized at 693 K for 4-5 hrs followed by water quenching. To avoid oxidation during the heat treatment, the cast AZ31 billets were placed in a furnace in aluminum oxide powder. Prior to ECAP, the grain sizes in the rolled and cast alloys were ∼ 10 lm and ∼ 640 lm, respectively, and careful microstructural examination revealed the presence of irregularlyshaped and angular incoherent Mg 17 Al 12 particles with average sizes of ∼ 10 lm plus a reasonably uniform distribution of rounded Mn 5 Al 8 precipitates having diameters of ∼ 1-2 lm.Processing by ECAP was performed using a die with an angle of 90°between the two parts of the channel and a ram velocity of 15-20 mm s -1 . Some pressings were conducted with a back-pressure by making use of a backward punch in the exit channel of the die. Molybdenum disulfide and a graphite spray were used as lubricants and the billets were pressed using processing route B c in which each billet is rotated by 90°in the same sense between consecutive passes. [...

Achieving Microstructural Refinement in Magnesium Alloys through Severe Plastic Deformation

“…6 and 7 where extremely good tensile ductilities are evident in tests conducted at elevated temperatures. Furthermore, it is known that improved results are often achieved by ECAP when using an experimental facility which imposes a back-pressure during the processing operation, 28) but the present results, including the record tensile elongation shown in Fig. 7, were attained without the imposition of a back-pressure.…”

Processing magnesium alloys by severe plastic deformation

“…Superplastic elongations in the range of 1000% were observed in the AZ31 alloy processed by ECAP and tested at 623 -673 K at ~10 -4 s -1 [6,[33][34][35]. Elongations even higher are observed in the ZK60 alloy.…”

“…Later it was found that the use of back-pressure [5,6], increasing the die angle [7,8], introducing an intermediate processing step by extrusion [9][10][11][12][13][14][15][16] or gradually decreasing the ECAP temperature [17,18] allow processing these alloys at low and moderate temperatures and produce significant grain refinement. A model for grain refinement of magnesium processed by ECAP was proposed [19,20].…”

The Origin of Fracture in the I-ECAP of AZ31B Magnesium Alloy

“…[8,9] Various solutions were proposed in order to improve formability of magnesium alloys during ECAP, including applying back-pressure, [10] increasing strain rate sensitivity of a material by direct extrusion, [11] increasing die angle, [11] decreasing temperature with subsequent passes, [5,[12][13][14] and decreasing strain rate. [7,15] Gradual temperature decrease was shown to be effective in lowering temperature below 473 K (200°C); therefore, it was also attempted in this work.…”