(665d) Experimental and Computational Study of Progress in Material Refinement by Mechanical Milling

Mechanical milling is used to prepare a wide range of materials, including mechanically alloyed powders, chemicals synthesized by reactive milling, and reactive nanocomposite powders prepared by arrested reactive milling. Typically, shaker and planetary mills are used for laboratory scale production; however, attritor mills are more useful for practical scale manufacturing. Transferring milling parameters between devices required for material synthesis on different scales is the major obstacle to implementing the mechanical milling methods into materials manufacturing in industry. Therefore, accurate models describing the progress in material refinement as a function of the milling parameters and applicable to different milling devices are of substantial interest. A discrete element method (DEM) model was used to characterize the material evolution in the mill by tracking a milling progress function, defined by the energy transferred from the milling media to the material under processing. It was observed that the energy transfer from the milling tools to the powder occurs differently in shaker and planetary mills. Specifically, head-on collisions between milling balls controlled the energy transfer and the rate of powder refinement in the shaker mill, while rolling motion of the balls caused the powder refinement in the planetary mill. The earlier DEM models did not account properly for the rolling friction, contributing to the energy transfer substantially in both planetary and attritor mills. Furthermore, the friction coefficients for both static and rolling friction were poorly quantified, especially for the milling tools covered with the material that is being processed. Milling efficiencies of different types of mills were compared to one another based on both laboratory experiments and simplified models. However, the results of such comparisons were inconsistent with experimental observations related to mechanical alloying. In this effort, commercial EDEM software by DEM Solutions Inc. was used to describe all three milling devices, including a shaker mill, planetary mill, and attritor mill. Separate experiments established appropriate values for rolling and sliding friction coefficients to be used in the code. The influences of milling parameters including number and dimensions of the milling balls and rate of impeller rotation (in the attritor) on the energy transfer from the milling media to the powder were investigated. The energies predicted to be transferred from milling tools to the milled powder in different mills were directly compared to one another. The milling progress achieved in different devices was also tracked experimentally by measuring the yield strength of non-reactive metal-oxide composite powders as a function of the milling time. Milling results in the formation of work-hardened, oxide reinforced composites so that their yield strength increases as a function of the milling progress. Aluminum and magnesium oxide powders were milled in the attritor, planetary and shaker mills for various milling times. The samples obtained were pressed using an Instron 5567 device and their yield strength was measured. Results obtained from both modeling and experimental efforts and their implications for design of larger scale production facilities for preparation of reactive nanocomposite materials will be discussed.