Experimental analysis and numerical modeling of the friction drilling process.

Abstract

Friction drilling is a nontraditional hole-making process. A rotating conical tool is applied to penetrate a hole and create a bushing in a single step without generating chip. The friction drilling process relies on the heat generated from the frictional force between the tool and sheet metal workpiece to soften, penetrate, and deform the work-material into a bushing. Under the constant tool feed rate, the experimentally measured thrust force and torque were analyzed. The infrared camera system was applied to measure the temperature of the tool and workpiece. High temperature and strain affect material properties and grain microstructure of the material surrounding a hole from friction drilling. Samples of cross sectioned holes were polished and etched for material analysis in carbon steel, alloy steel, aluminum and titanium. Knoop micro-hardness values were recorded and near hole microstructure was observed with optical micrographs. The technical challenge for the brittle cast aluminum and magnesium alloys is to generate a cylindrical shaped bushing without significant radial fracture or petal formation. Two ideas of pre-heating the workpiece and high speed friction drilling were investigated. The thrust force and torque decreased and the bushing shape was improved with increased workpiece temperature. The wear of a hard tungsten carbide tool used for friction drilling a low carbon steel workpiece was investigated. Measurements were made to characterize tool wear and monitor the effects of tool wear. Results indicate that the carbide tool is durable, showing minimal tool wear after drilling 11000 holes, but observations also indicate the progressively severe abrasive grooving on the tool tip. The analytical model was created to predict the thrust force and torque in friction drilling based on the measured temperature, material properties, and estimated area of contact. The explicit finite element method (FEM) was applied to model the large deformation, large plastic strain, and high temperature work-material deformation in the friction drilling process. Thrust force, torque, and temperature in FEM were compared to experimentally measured values. Workpiece temperature was found to approach the work-material solidus temperature. Distributions of plastic strain, temperature, and stress demonstrated the thermo-mechanical response of the workpiece.