Experimental determination of strain rate and flow stress in the primary shear zone while machining AISI 4340 and Ti-6AL-4V
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Metal cutting subjects the material being cut to high strain rates (103 -106/s) and strains>100% within the primary shear zone (PSZ). This thesis is aimed at quantitative measurement of the strain, strain rate and flow stress in the PSZ in order to provide data that can be used to verify and improve high strain rate material models. An experimental study of the cutting forces, chip thickness, and velocity fields in the PSZ, has been carried out during the cutting of AISI 4340 steel in three different tempers; hot rolled steel, hardness 18HRc, referred to as AISI 4340HR; hot rolled steel with additional 50% cold rolling, hardness 27HRc referred to as AISI 4340CR; Heat treated (quenched and tempered) steel, hardness 45HRc, referred to as AISI 4340HT; and Titanium alloy Ti-6Al-4V. Kennametal make KC510M inserts with edge radius 6μm is used for all the cutting experiments reported and is mounted on a tool holder which provides a rake angle of 30o and a clearance angle of 4o. The KC510M inserts are used in an effort to minimize the side spread of the material. A new type of ultra high-speed camera is used to capture sequential and high resolution microphotographs of the PSZ during machining at frame rates ranging from a few kHz to 1MHz. The velocity fields are obtained by 2D cross-correlation of sequential images and the gradient of the velocity field yields the strain rate field. Each experiment yields four to seven strain rate fields from which the average strain rate field is obtained. Average strain rate fields found in multiple experiments are averaged to obtain the overall average strain rate for the cutting condition. As the shear velocity (Vs) needs to be same throughout the chip width for the chip to be moving as a rigid body, the shear angle has to be constant from the sides to the mid-section of the chip. Two independent estimates of the shear angle, from the velocity field and from the chip thickness, show reasonable correlation. The shear angle from chip thickness is an average over each experiment, while the shear angle from velocities is an instantaneous value and is taken to be the true value. The 'indentation' component of the cutting forces is calculated from the measured cutting forces using the ratio of flow stress in the PSZ to the contact pressure observed in finite element analysis (FEA) simulations. The flow stress along the nominal shear plane is computed from the measured shear angle and cutting forces, after removal of the 'indentation' component. The thickness of the PSZ is smaller and the peak strain rate is higher for AISI 4340CR compared to AISI 4340HR, due to reduced work hardening. While the forces are found to be nearly the same, the vii shear angle is larger and the flow stress is smaller for the AISI 4340CR material. For the AISI 4340HT, the flow stress is found to be about 10-20% larger compared to AISI 4340HR. The PSZ is typically parallel sided and shows a single peak in the CD direction along the PSZ while cutting AISI 4340HR, AISI 4340CR, AISI 4340HT and Ti-6Al-4V. The strain rate near the cutting edge is high and gradually decreases as we move towards the free end while cutting AISI 4340HR and AISI 4340CR. The strain rate near the cutting edge and the free end of the PSZ is high but lower in the middle of the PSZ, while cutting AISI 4340HT. The mode of deformation while cutting AISI 4340HR and AISI 4340CR is inhomogeneous, meaning the same regions of the work material accumulate deformation and the shear ‘plane’ moves with the material over time, till eventually switching over to a new shear ‘plane’. The high-speed images of the PSZ while cutting Ti-6Al-4V clearly show localized bands of shear separated by larger bands of less deformed material, implying the deformation continues to concentrate within the same region of the material. The chip is thinner and quantitatively shows that the shear band is at the same region with respect to the workpiece. We have compared and contrasted the deformation process of Ti-6Al-4V with that of AISI 4340HR and AISI 4340CR. For the same cutting speed and feed, the strain rate while machining Ti-6Al-4V is higher and the PSZ narrower when compared to AISI 4340CR.
Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Industrial and Manufacturing Engineering.