The Effect of a Laser Beam on Chip Formation during Machining of Ti6Al4V Alloy
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INTRODUCTION
THE segmented chip, characterized by localized, heavily deformed shear bands, is the typical chip morphology observed when machining titanium alloys.[1–11] The onset of shear localization for a given undeformed chip thickness is determined by the critical cutting speed, which is very low for conventional cutting of titanium alloys.[8] Periodic oscillation in the cutting and thrust components of the forces associated with segmentation causes tool vibration and chatter.[8] This limits the material removal rate and plays an important role in tool wear and surface finish; therefore, continuous chip formation at high cutting speed is preferred to achieve longer tool life and smother machined surface. The increase of critical cutting speed for the onset of shear localization is required to achieve longer tool life and high material removal rate. The mechanisms associated with the segmented chip formation are associated with the reduction in shear strength of the material by either the thermal softening[4,5] or growth of periodic microcracks.[6,11] Laser-assisted machining (LAM) is a process in which an external laser source is employed to heat and soften the workpiece locally in front of the cutting tool. The process has attracted research interest for decades, S. SUN, Research Engineer, is with CAST CRC, the Industrial Laser Applications Laboratory, IRIS, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia. Contact e-mail: [email protected] M. BRANDT, Professor, is with CAST CRC, School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Bundoora, Victoria 3083, Australia. M.S. DARGUSCH, Technology Manager, is with DMTC, CAST CRC, School of Mechanical and Mining Engineering, University of Queensland, Brisbane, Queensland 4072, Australia. Manuscript submitted March 10, 2009. Article published online March 12, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
because it offers a number of benefits when machining hard-to-cut materials, such as reduced cutting forces, smoother machined surfaces, and production of a continuous chip in steel and titanium alloys at high cutting speeds.[12–14] An increase in the critical cutting speed for the onset of shear localization has also been achieved with laser assistance.[15] However, the difference in chip formation mechanisms in LAM and conventional machining has not been discussed in detail and the mechanism leading to continuous chip formation has not been investigated. This article reports on a detailed examination of the change in chip formation by employing a laser beam in front of the cutting tool when machining the Ti6Al4V alloy. The effects of laser power on chip formation at different cutting speeds and the continuous chip transition speed have been investigated for a constant depth of cut and feed rate. Continuous chip transition by application of laser beam is discussed in the context of a new physical model of the segmented chip formation proposed by the authors.
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