Papers by Author: C.J. Lee

Abstract: Friction stir processing (FSP) has been applied to fabricate 10~20 vol% nano-sized ZrO2 and 5~10 vol% nano-sized SiO2 particles into an Mg-AZ31 alloy to form bulk composites under the FSP parameters of advancing speed of 800 rpm and pin rotation of 45 min/min. The microstructures and mechanical properties of the resulting composites were investigated. The clustering size of nano-ZrO2 and nano-SiO2 particles, measuring average ~200 nm was relatively uniformly dispersed, and the average grain size of the both Mg alloy of the composites varied within 1.0~2.0 μm after four FSP passes. No evident interfacial product between ZrO2 particles and Mg matrix was found during the FSP mixing in AZ31-Mg/ZrO2. However, significant chemical reactions at the AZ31-Mg/SiO2 interface occurred to form the Mg2Si phase. The mechanical responses of the nano-composites in terms of hardness and tensile properties are examined and compared.

Abstract: There have been numerous methods in fabricating particulate reinforced metallic matrix composites, including stir casting, squeeze casting, spray forming, powder metallurgy, and mechanical alloying. In this paper, one solid state processing technique, friction stir processing, is applied to incorporate 5-15 vol% nano-sized ceramic particles SiO2 into the AZ61 Mg alloy matrix to form bulk composites, using the characteristic rotating downward and circular material flow around the stir pin. The upper working FSP temperature is controlled to less than 400oC in order to avoid chemical reaction. The fixed pin tool is 6 mm in diameter and 6 mm in length, with a shoulder diameter of 18 mm and a 3o tilt angle. The advancing speed of the rotating pin is kept constant to be 45 mm/min, with rotational speed of the pin from 800 rpm (rotation per min), resulting in a strain rate around 101 s-1. After one-pass FSP, the particle dispersion within the central cross-sectional area of the onion ring regions, measuring nearly 6 mm in diameter, was macroscopically uniform. However, the observed particle size is frequently 0.5-5 µm, much larger than the individual SiO2 size (~20 nm), suggesting the clustering of nano particles. The situation after two FSP passes, with an opposite FSP direction for the second pass, appear to be further improved. Electron microscopy characterizations reveal that the aggregating particles were seen to vary from 10 to 1000 nm in size. Some of the large particles, 1-5 µm in diameter, were identified to be the Mn bearing dispersoids (e.g. Al4Mn) by the SEM-EDS. The average grain sizes of the composites with 5-15 vol% SiO2 varied within 00.5-2 µm, and the composites double the hardness as compared with the as-received AZ61 cast billet.

Abstract: The characterizations on the microstructural and mechanical properties of the Mg-Al-Zn multi-element intermetallic alloys fabricated by friction stir processing (FSP) are presented. The composites of the alloys vary within Mg35-70Al5-25Zn25-45. The maximum working temperature can reach 550oC. The current process applies the FSP on vertically stacked foils of various portions of Mg, Al and Zn, 99.9% in purity and 0.2 to 1 mm in thickness. In order to homogenize the alloy composition, three or more FSP passes in opposite directions are applied. Depending on the relative contents of Mg, Al and Zn, numerous intermetallic compound phases are induced. The resulting intermetallic alloys exhibit high hardness up to 350 Hv.