Some Observations on Microstructural Changes in a Mg-Based AE42 Alloy Subjected to Friction Stir Processing

Abstract

The main aim of the current study is the analysis of friction stir processing (FSP) of Mg-based alloys as a possible tool for nanocomposites production. The study reports microstructural changes taking place in a Mg-based alloy (AE42) subjected to FSP under different cooling conditions. The FSP process was carried out with single as well as multipass options. The friction stir processed samples were characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM), focused ion beam (FIB)-scanning ion microscopy (SIM), and X-ray diffraction (XRD). It was observed that FSP tends to fragment the elongated precipitates and produces near homogeneous distribution of fine particles. The smallest particle size was observed to be produced by double-pass FSP supplemented by rapid cooling, thereby generating in situ nanocomposites. Vickers microhardness testing was done along the thickness (transverse direction) of the specimen to study and understand the variation of hardness with thickness. Nearly a two-times increase in the microhardness of AE42 was observed in the case of double-pass, FSP AE42 with cooling at temperature of approximately 253 K (-20°C). To confirm these observations, another magnesium alloy AM50 was also friction stir processed under similar conditions. The fine submicron grain structure produced in AE42 alloy contributed immensely toward grain boundary strengthening and Orowan strengthening had only marginal influence. Subgrain boundary pinning by in situ nanoparticles contributed significantly in the strengthening process.

Microstructure‐Property Relationship for Friction Stir Processed Magnesium Alloy

“…However, the current study shows higher strain hardening for finer grained specimens. In an earlier investigation by the authors, [16] it was observed that FSP of AE42 alloy resulted in a refined microstructure comprising a uniform distribution of fine in situ precipitates of Al-rare earth and Mg-rare earth elements. The precipitate size for SPUSC specimen was found to be in the range of 200 nm to 1mm which got further refined to nearly 50 nm for the DPUSC specimen.…”

“…However, only limited improvement in mechanical properties such as elastic modulus could be achieved. In one of the earlier investigations by the authors, [16] it was observed that FSP of AE42 alloy supplemented by external undersurface cooling resulted in a refined microstructure comprising of uniformly distributed fine in situ second phase precipitates. However, the influence of FSP on texture and grain size distribution has not been evaluated.…”

“…Details about the cooling mechanism are given in an earlier publication. [16] Rectangular specimens having dimensions of 80 mm  40 mm  3 mm were prepared from the alloy ingot. The FSP tool was cylindrical in shape with a 12 mm shoulder diameter, 4 mm pin diameter, and 2.7 mm pin length.…”

“…SEM images of (a) SPUSC (b) DPUSC specimen, showing the refinement of fine in situ precipitates [16]. …”

Enhancing the Mechanical Properties of AE42 Magnesium Alloy through Friction Stir Processing

“…In an earlier investigation by the authors, [18] it was found that FSP of AE42 alloy supplemented by external undersurface cooling resulted in a refined microstructure comprising of uniformly distributed fine in situ second phase precipitates. It is known that FSP parameters including tool rpm, linear speed, axial load, number of passes, and cooling conditions significantly affect the evolved microstructure.…”