Low Frequency Vibration Assisted Drilling of Aluminium Alloys

Abstract

In the present study the vibration drilling process of light weight materials and compound stacks has been investigated. Fiber-metal compound materials provide excellent mechanical properties which make them a major choice in lightweight applications. Especially in aircraft industry the use of multi-layer materials is significantly increased during the last few years. To join parts of dissimilar materials usually rivets or bolts are applied as fasteners. Therefore it is necessary to machine boreholes with partially very high quality requirements. Because of the different material properties the machining process of serial stacks imposes high demands to the cutting tools and requires certain process strategies. Previous investigations revealed that the bore surface can be damaged during the extraction of the hot and sharp metallic chips. Besides the risk of thermal damage the main issue lies in an erosive expansion of the borehole diameter due to the reaming of metallic chips at the borehole surface. The chip extraction can be significantly improved by low frequency assisted vibration drilling. In that case the axial tool movement is superimposed by a sinusoidal oscillation (in this case 1.5 per revolution) which is provided by the tool holder. Under certain cutting conditions this leads to a controlled chip breakage. Compared to conventional drilling the process parameters, cutting speed vc and feed f are supplemented with the amplitude A of the oscillation and the frequency f which represents the amount of vibrations per revolution of the tool. This causes radical changes to the kinematics of the process and therefore of the cutting conditions and chip formation. For a better understanding of the process a kinematic model for a two-flute cutter was developed which allows calculating the undeformed chip shape in dependency of the four cutting parameters vc, fz, A and f. The model also helps to predict whether a discontinuous cut will be achieved or not. To characterize the process and chip shape the following parameters are optionally calculated within the model: maximum chip thickness, chip radian, effective feed, feed speed at the moment of tool entrance and exit (for one chip). Experimental drilling trials in Al2024 T351 were used to evaluate the calculated parameters. The chip thickness and radian as well as the cutting time show a very good correlation to the calculations. It is interesting that the measured cutting forces are much lower compared to the theoretical values according to the Kienzle cutting force equation. Additionally it was found that the measured cutting force is strongly decreasing with an increasing cutting speed. Infrared images of the drilling process in Ti6Al4V were used to analyze the temperature close to the cutting zone and to observe the chip evacuation during the process. It was found that the cutting temperature is up to 50% lower when using vibration drilling. Furthermore it was shown that this effect is strongly dependent on the chip extraction. It is important that the chips do not stack in the drilling flute during the process. A chip breakage is facilitated by a decreasing ratio between feed and amplitude. At the same time an increasing material removal rate degrades the chip extraction even if the chips are separated. Besides the advantages of vibration drilling a major issue was found to be chipping at the cutting edges or even tool breakage. This could be avoided by a reduction of the oscillation amplitude and /or feed. Under consideration of these correlations the productivity of the drilling process and the bore hole quality in CFRP/Ti6Al4V-stacks could be significantly increased. The investigations have shown that vibration assisted drilling represents a huge opportunity, especially in the field of drilling composite materials. However further investigations are necessary to better understand this very complex process.