On thermoplastic shear instability in the machining of a titanium alloy (Ti-6Al-4V)

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INTRODUCTION

IN the machining of some of the difficult-to-machine materials, such as titanium alloys, nickel-based superalloys, hardened alloy steels, and stainless steels, two types of chip forms are commonly encountered, depending on the type of work material used and its metallurgical condition, as well as the cutting conditions used. They are the continuous chip (Figure 1(a)) and the shear-localized chip (Figure 1(b)). For some materials, such as the titanium alloys, shear localization is found to occur at such low cutting speeds (⬍0.5 m/ min) that only the shear-localized type of chip predominates over the conventional cutting-speed range. For other materials, the cutting speed for the onset of shear localization is much higher. Consequently, continuous chips are obtained below the cutting speed for the onset of shear localization, and shear-localized chips are obtained above this cutting speed. The term shear localization is used here to denote intense plastic deformation in a narrow band between the segments and negligible deformation within the chip segments. The thermomechanical instability of the work material under the conditions of cutting plays an important role in shear localization, especially with many of the difficultto-machine materials, such as titanium alloys, nickel-based superalloys, and hardened steels. Shear-localized chips are generally formed in materials with limited slip or in hcp metals, such as titanium alloys RANGA KOMANDURI, Regents Professor and Endowed Chair in Engineering, and ZHEN-BING HOU, Visiting Professor, are with the Mechanical and Aerospace Engineering Department, Oklahoma State University, Stillwater, OK 74078. Contact e-mail: [email protected] Manuscript submitted September 24, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A

with unfavorable thermal properties. Table I gives a comparison of the thermal properties of some steels, some aluminum alloys, a titanium alloy, and a nickel-based superalloy. Considering the thermal properties of AISI 1018 steel as unity, it can be seen that the thermal conductivity (␭ ) of aluminum alloys is ⬃3 times that of steel, while that of the titanium alloy is only about one-tenth of it. Similarly, the thermal diffusivity (a) is about 4 to 4.5 times that of steel for the aluminum alloys, while that for titanium alloys is only about 15 pct that of steel. Also, the product of the thermal properties, namely, the thermal-contact coefficient (␭␳c), is about 2 times that of steel for aluminum alloys, while for the titanium alloy it is only about 8 pct that of steel. This, in turn, will affect the temperature generated and the heat transfer from the shear-localized regions to the rest of the segments. An examination of the photomicrograph of the longitudinal midsection of a polished and etched titanium chip shows significant deformation between the chip segments and practically no strain in the bulk of the chip segment. Figure 2 is an optical photomicrograph of the longitudinal midsection of a Ti-6Al-4V chip, showing practically little or n