Titanium alloys and CNC machining

Titanium alloys are extensively used in the aerospace industry besides other industries like medical, marine, jewelry, etc. Most aircraft engines have a significant amount of titanium alloys in their parts with the most commonly used being Ti-6Al-4V alloy. However, the unpredictable machining behavior of the titanium alloy makes it very difficult to machine.

One factor responsible for limiting the cutting parameters during CNC machining of titanium alloys is the cutting temperature. Success in CNC machining of titanium alloys depends largely on overcoming several of the inherent properties of the metal. Titanium alloys have low chip-tool contact area and the thermal conductivity of these metals is considerably low (about 7,3 W/m K). This combination of small contact area and low thermal conductivity results in very high cutting temperatures and impose a constraint on the usage of higher cutting speeds.

In addition, during the cutting of metal, high temperatures are generated in the region of the tool cutting edge, and these temperatures have a controlling influence on the rate of wear of the cutting tool and on the friction between chip and tool.

The generation of high temperature near the tool cutting edge not only accelerates the chemical wear in the cutting tool but also causes higher penetration of tensile residual stresses beneath the machined surface, which is undesirable. About 80% of this generated heat is retained in the tool and 20% in the chips.

Supplementing to this factor is the danger associated with eruption of titanium fire, which is caused when the temperatures exceed the ignition temperature of the material, especially with the very thin titanium chips. Due to the low elasticity module of the titanium, the tool is subject to a pulsation load (spring back) during the machining causing attrition and vibration.

The combination of pulsation and high temperature accelerates the wear mechanisms of the tool. It has been found that for turning of titanium alloy, the cemented carbide (WC/Co) represents the best cutting tool for machining of Ti-6Al-4V alloy.

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