Investigation of a discus-milling process using a powder mixture of Al and TiO 2
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I. INTRODUCTION
SINCE the 1970s, various mechanical milling processes have been invented and the alloying process during milling has been investigated intensively, due to the successful application of ball milling in producing oxide dispersion strengthening (ODS) superalloys.[1] The influence of the milling parameters has been determined, either theoretically or experimentally. The milling media, the milling atmosphere, the milling time, and some other factors have been found to contribute to the alloying process and to the microstructure evolution of the resultant powder particles. More recently, a novel mill, called a discus mill, has been applied to the production of nanostructured composite materials.[2,3] Different from the ball-milling processes conventionally used in laboratories, a discus mill employs as milling media two matching alloy discuses in a cylindrical bowl. During milling, the discuses rotate and vibrate irregularly in the bowl. Material powder particles trapped between the discuses and between the discus and the bowl wall experience compression and shear, which cause plastic deformation, fracturing, and cold welding. With the accumulation of a large number of plastic deformations, and with fracturing and cold-welding events, the formation of composite powder particles is realized. Further milling refines the composite structure and eventually leads to alloying. Compared to the conventional, laboratoryscale ball mills, this discus mill possesses several advantages: (1) a large amount of powder charge (typically more than 0.5 kg per batch); (2) a low risk of contamination, since the impacts between the discuses always involve trapped powders; and (3) a high efficiency, stemming from the first advantage. However, so far, there has been no published report on a detailed investigation of the milling process involving a discus mill. Z.G. LIU, Research Fellow, S. RAYNOVA, Research Assistant, and D.L. ZHANG, Associate Professor, are with the Materials & Process Engineering Department, The University of Waikato, Hamilton, New Zealand. Contact e-mail: [email protected] Manuscript submitted July 14, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
In the current study, we investigated the discus milling process by monitoring the evolution of the powder particle size and the microstructure during the milling of a mixture of Al and TiO2 powders. This starting powder mixture was designed with a composition that followed the nominal displacement reaction during subsequent heating: 3TiO2 4Al S 3Ti 2Al2O3
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Heat treatment was performed on the as-milled powders, as well, to evaluate the quality of the milled powders and the influence of milling conditions. II. EXPERIMENTAL DETAILS The discus mill employed in the present study is made by Rocklabs Ltd. (Aukland, New Zealand). As illustrated in Figure 1, there are two matching discuses (195-mm ID) in a cylindrical bowl (with an i.d. size of 238 76 mm), which all are made of high-Cr, high-carbon steel. The bowl is set in a fixing racket and is locked
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