Taiki Morishige

Microstructural modification of cast Mg alloys by friction stir processing

Abstract Cast Mg alloys were processed using friction stir processing (FSP) to acquire a fine grained structure and high strength. Actually, FSP is a novel grain refinement method for light metal alloys. Using FSP, a cast microstructure with coarse grain size was refined to equiaxial fine grain through dynamic recrystallisation; second phase particles were finely dispersed by FSP. Moreover, FSP is effective to eliminate cast defects such as microshrinkages or porosities. Commercial die cast Mg alloy (AZ91D) and high strength Mg–Y–Zn alloy plates were prepared for FSP. Heat input using a rotational tool during FSP closely affected the microstructure in the stirred zone. Actually, FSP with lower heat input produced a finer grain size and higher hardness. Changes in the friction stir processed microstructures affecting mechanical properties were not only grain refinement, but also second phase particle distributions. Results show that alloys with high hardness by FSP have finely dispersed second phase particles without dissolution during FSP.

Microstructure of friction stir processed Mg-Y-Zn alloy

Friction stir processing (FSP) is the effective method of the grain refinement for light metals. The aim of this study is to acquire the fine grained bulk Mg-Y-Zn alloy by ingot metallurgy route much lower in cost. Such bulk alloy can be formed by the superplastic forging. The microstructure of as-cast Mg-Y-Zn alloy was dendrite. The dendrite arm spacing was 72.5 [(m], and there are the lamellar structures in it. FSP was conducted on allover the plate of Mg-Y-Zn alloy for both surfaces by the rotational tool with FSW machine. The stirring passes were shifted half of the probe diameter every execution. The dendrite structures disappeared after FSP, but the lamellar structure could be observed by TEM. The matrix became recrystallized fine grain, and interdendritic second phase particles were dispersed in the grain boundaries. By using FSP, cast Mg-Y-Zn alloy could have fine-grained. This result compared to this material produced by equal channel angular extrusion (ECAE) or rapid-solidified powder metallurgy (RS P/M). As the result, as-FSPed material has the higher hardness than materials produced by the other processes at the similar grain size.

Friction Stir Processing of Cast Mg-Y-Zn Alloy

A high strength at room temperature and at high temperature was reported for the extremely fine-grained RS P/M Mg-2at%Y-1at%Zn (Mg-6.8mass%Y-2.5mass%Zn) alloy. This property was attributed to the fine grain size. However, such alloys have very poor productivity. Therefore, they are far from practical applications. The aim of this study is to acquire the fine-grained bulk Mg-Y-Zn alloy by ingot metallurgy route much lower in cost. Friction-Stir-Processing (FSP) was performed on this alloy ingot, and the mechanical properties and the microstructures were examined. FSP was conducted on allover the plate of this alloy for both surfaces by the rotational tool with FSW machine. The stirring passes were shifted half of the probe diameter every execution. The microstructure of as-cast material was dendrite. The dendrite arm spacing was 72.5 [μm], and there are the lamellar structures in it. The dendrite structures disappeared after FSP, but the lamellar structure could be observed by TEM. The grain size of FSP materials were measured as ca. 1 μm by TEM micrographs. Introduction Magnesium (Mg) alloys are the lightest of structural materials. For that reason, numerous studies have been carried out to apply this material for low-energy-consuming and easily recyclable structural components. To date, casting and hot forging have been used to produce magnesium products with grain sizes from the millimeter to micrometer level. Finer microstructure engenders further enhanced mechanical properties such as strength and ductility if processing is done in proper working conditions. Consequently, recent magnesium production tends to enlarge the portion of metalworking such as forging, rolling, and extrusion to acquire a refined microstructure. Recently, Abe et al. [1] reported very high yield strength of Mg alloys which have 600 MPa (0.2% proof stress) and 6% elongation at room temperature in the rapid-solidified powder metallurgy (RS P/M) Mg-Y-Zn alloy. This higher strength might derive from fine-grained and long-periodic ordered structure produced by rapid solidification. Although many industrial designers have taken notice of this material as the highest-specific-strength structural material, they intensely require more cost-effective methods to produce high-strength Mg-Y-Zn alloy because the method needs the process of powder consolidation. On the other hand, one of the grain refinement methods for bulk Mg-Y-Zn alloys is Advanced Materials Research Online: 2006-02-15 ISSN: 1662-8985, Vols. 15-17, pp 369-374 doi:10.4028/www.scientific.net/AMR.15-17.369

Effect of Processing Order on Strengthening of Friction Stirred Mg-Y-Zn Alloy

High-strength Mg-Y-Zn alloy plate was obtained by friction stir processing (FSP) after casting. In this study, the effect of processing order on the strength of processed specimens was discussed. It was revealed that the microstructures and strength of doubly stirred zone depended on the direction of overlapping pass. In the area of the doubly stirred zone that was affected by the thermal history of Retreating-side, the hardness was 121Hv. It was twice as hard as Base Metal, and harder than the doubly stirred zone that was affected by thermal history of Advancing-side.

Grain Refinement of Mg-Y-Zn Alloy by Friction Stir Processing

Grain refinement of magnesium alloy by Friction Stir Processing (FSP) was investigated. It is assumed that dynamic recrystallization (DRX) is occurred by frictional heat and plastic flow during FSP. This process is the effective method of the grain refinement for light metals. In this study, FSP was conducted to cast Mg alloys for and the difference of the grain refinement by DRX in these alloys was examined. As a result, in comparison with commercial Mg-Al-Zn alloy and Mg-Y-Zn alloy have finer microstructure. The grain size of FSP-ed Mg-Y-Zn alloy was ~1.7 [/m], however, that of AZ31 alloy was 20~30 [/m].