Yuichi Miyahara

Developing superplasticity in a magnesium alloy through a combination of extrusion and ECAP

Abstract A new processing procedure was applied to a cast Mg-9% Al alloy. This procedure involves the sequential application of extrusion and equal-channel angular pressing and is designated EX-ECAP. Experiments show that the Mg-9% Al alloy has an initial grain size of ~50 μm after casting but this is reduced to ~12 μm after extrusion and it is further reduced to ~0.7 μm when the extruded alloy is subjected to ECAP for 2 passes at 473 K. Although the cast alloy exhibits extremely limited ductility and the extruded alloy is only moderately ductile, it is demonstrated that processing by EX-ECAP produces excellent superplastic ductilities including the occurrence of both low temperature superplasticity and high strain rate superplasticity. The EX-ECAP process is less effective when the ECAP step is conducted at 573 K because, although the pressing is then very easy, there is significant grain growth at this higher temperature.

Grain refinement of commercial Al-Mg alloy using severe torsion straining process

Severe plastic deformation (SPD) makes it possible to refine grain size in many metallic materials. Recently, we have developed a new SPD process designated the severe torsion straining process (STSP). This process requires no die but one side of a rod is rotated with respect to the other while producing a local heated zone in the rod and cooling both sides of the heated zone. Torsion strain is then introduced in the local heated zone. The STSP can be a continuous process because the rod is moved in the longitudinal direction while introducing torsion strain through the rotation. For grain refinement using the STSP, various factors may affect, which are the rotation speed, moving speed, straining temperature, cooling rate and diameter of the rod. In this study, the STSP is applied to grain refinement of an A5056 Al-Mg commercial alloy and the factors affecting the grain refinement are optimized. STSP was conducted at a temperature in the range from 573K to 723K. Microstructure was observed by optical microscopy, scanning electron microscopy with an orientation imaging system, and transmission electron microscopy. Microscopy observations revealed that the grain size was reduced to ~0.9 μm, when STSP was conducted at 573K with a rotation speed of 10 rpm and moving speed of 50 mm/min. There is a critical ratio of rotation speed to moving speed above which the rod breaks. The grain size tends to be finer as the straining temperature is lower, the cooling rate is faster and the ratio of rotation speed to moving speed is closer to the critical value.

Microstructures and Mechanical Properties of Mg Alloy after Severe Torsion Straining Process

Grain refinement is attempted using severe plastic deformation (SPD) through the severe torsion straining process (STSP) which we have developed recently. The STSP is a continuous process for grain refinement without requirement of any die. In this study, an AZ61 Mg alloy was subjected to STSP at a temperature of 573 K with a rotation speed of 10 rpm and a moving speed of 200 mm/min. With this process, an initial grain size of ~20 μm was reduced to ~2~3 μm. Room temperature compression tests revealed that there were no cracks after 15% of compression for the STSP sample whereas fracture occurred for a conventionally extruded sample. For compression tests at 473 K, no cracks occurred in the STSP samples after 80% compression but compression was feasible without cracking only up to 20% for an extruded sample. It is shown that the STSP can be useful for grain refinement and ductility improvement of the AZ61 Mg alloy.

Mechanical properties and microstructure of Mg-Zn-Y alloys processed by ECAE

Development of high strength I/M Mg alloys has been tried by ECAE processing. The mechanical properties and microstructure of ECAE-processed Mg97Zn1Y2 alloy with LPSO (long-periodic stacking ordered) structure were investigated. The tensile yield strength and elongation of as-cast Mg97Zn1Y2 were improved substantially by ECAE process. ECAE-processed with yield strength of 290 MPa and elongation of 22 % was obtained. The microstructure of ECAE-processed Mg97Zn1Y2 alloy consisted of refined α-Mg with the grain size around 6.5 μm and finely dispersed LPSO phase. Furthermore, the some texture was formed by ECAE process. The improved mechanical properties seem to be originated by the microstructure refinement and texture.

Effect of microstructures on tensile properties of AZ31 Mg alloy processed by ECAP

A commercial AZ31 Mg alloy was subjected to ECAP (equal-channel angular pressing) and the tensile properties were examined at room temperature. It is shown that the microstructure before ECAP significantly affects the performance of ECAP without breaking samples. When the initial structure before ECAP is homogeneous with equiaxed grains, subsequent ECAP is feasible at lower temperatures. The grain refinement is achieved more effectively as the temperature for ECAP is lowered and the number of ECAP pass is increased. The tensile ductility was reduced after 1 pass of ECAP at 473 K or 498 K. However, the ductility was improved with an increasing number of ECAP pass or by annealing at 473 K or 523 K after ECAP. The results demonstrate that a homogeneous distribution of fine equiaxed grains is important for ductility improvement in the AZ31 Mg alloy.