Growth of tool wear in turning of Ti-6Al-4V alloy under cryogenic cooling
Introduction
Detailed studies on the mechanism of chip formation and other machinability aspects of titanium alloys have been initiated since 1950 [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Siekmann [14] observed that machining of titanium alloys would remain a problem due to typical combinations of its mechanical and thermal properties and its mode of chip formation. Komanduri and von Turkovich [15] and Kitagawa et al. [16] noted that machining of titanium alloys can be undertaken in the cutting velocity range of 30–60 m/min without the problem of rapid tool wear but with low productivity. Rapid tool wear, when machining of titanium and its alloys, in the range of 60–120 m/min and beyond for enhanced productivity, has been a persistent problem denoting titanium alloys as difficult-to-machine materials. Tool wear in machining of these alloys [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19] is a consequence of several detrimental effects arising out of:
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continuous segmented chips with short chip-tool contact length that give rise to high dynamic forces and stress acting on the cutting edge;
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high cutting temperatures concentrated at a narrow region adjacent to the cutting edge;
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high chemical reactivity of titanium with tool materials at temperatures above 500 °C.
It is thus obvious that reduction in tool wear in machining of titanium alloys requires effective control of cutting zone temperature through an efficient cooling system. Cryogenic cooling with liquid nitrogen as coolant can be a viable option for the purpose both for its excellent cooling ability and environmental friendliness. Cryogenic cooling has been attempted in machining of steels [20], [21], [22], [23], [24] and also titanium alloys [25], [26], [27], [28], [29] with substantial technological benefits.
The present investigation is an attempt to investigate the role of cryogenic cooling with liquid nitrogen as the cooling medium on the mechanism of tool wear and tool life in machining Ti-6Al-4V alloy using uncoated microcrystalline K20 tungsten carbide inserts compared to dry and wet (soluble oil) machining.
Section snippets
Experimental procedure
Plain turning experiments were carried out on Ti-6Al-4V alloy bar of 150 mm diameter on an 11 kW centre lathe by ISO K20 SNMA 120408 type uncoated microcrystalline grade carbide inserts of geometry −6° −6° 6° 6° 15° 75° 0.8 (mm) with an edge radius in the range of 33 μm under dry, wet and cryogenic cooling (in the form of liquid nitrogen jets) environments. Liquid nitrogen jets were impinged on the tool rake and flank surface using a specially designed nozzle. Fig. 1 shows the liquid nitrogen
Results and discussion
Tool wear in machining titanium alloys is reportedly due to adhesion–dissolution–diffusion of tool material into the flowing chip at the tool–chip interface. The temperature at the cutting zone even at moderate cutting velocities is generally around 900 °C [16], [30]. At such high temperature, titanium chip maintains a very intimate contact with the tool on the rake face and flank surface [31], [32] through an interfacial layer.
Once, the chip material establishes an intimate interface layer with
Conclusion
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Irrespective of the machining environments the following were observed, which are characteristics of machining of Ti-6Al-4V alloy.
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The chip-tool contact length is very small compared to machining of steels. Such small contact length increases machining temperature and enhances the chances of adhesion–dissolution–diffusion wear on the tool.
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Adherent chip material was visible throughout the crater surface indicating severe adhesion between the chip and the rake surface of the tool.
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The appearance of
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Acknowledgement
The authors are thankful to the SERC division of Department of Science and Technology (DST), Government of India for providing the financial support [Sanction III.5 (104)/2000-ET dated 20.04.2001] to carry out this investigation.
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