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Titanium and its machinabilityThis is a featured page

Titanium and its alloys are used extensively in the aerospace industry because of the excellent combination of their properties, such as, high specific strength, which is maintained up to elevated temperature, fracture resistance and exceptional resistance to corrosion below 500 0C. However, they pose considerable problems in manufacturing due to their poor machinability.
Machinability
Machinability is defined as the ease (or difficulty) with which a material can be machined under a given set of operating conditions including cutting speed, feed rate and depth of cut. It can be also described as a measure of the response of material to be machined with a given tool material resulting in an acceptable tool life and at the same time providing good surface finish and acceptable functional characteristics of the components. Machinability rating depends on tool life, surface finish, cutting force and power consumption during the machining operation, and chip control. However, machinability is mainly assessed by measuring the tool life, surface finish generated and component forces during a cutting operation. Machinability ratings are based on a tool life of T = 60 minutes. The standard material is AISI 1112 steel (resulfurized) which is given rating 100. For a tool life of 60 minutes, this steel should be maintained at a cutting speed of 100 ft/min (30 min/min). Higher speeds will reduce tool life, and lower speeds will increase it. For example, 3140 steel has a machinability rating of 55. This means that when it is machined at cutting speed of 55 ft/min (17 m/min), tool life will be 60 minutes.
Machinability of Titanium alloys
Titanium and its alloys are extremely difficult-to-machine materials. Poor machinability of titanium alloys subjects cutting tool materials to extreme thermal and mechanical stresses close to the cutting edge, often leading to plastic deformation and accelerated tool wear. Typical failure modes observed when machining titanium alloys are notching at the nose and/or depth of cut line, flank wear, crater wear, chippjng and catastrophic tool failure. Cutting tools used for machining titanium alloys should possess adequate hot hardness to withstand elevated temperatures generated at high speed conditions. Under these conditions most tool materials generally lose their hardness resulting in the weakening of the inter-particle bond strength and consequent acceleration of tool wear. Poor machinability of titanium alloys is associated with their several inherent properties. They have low thermal conductivity and high chemical reactivity with many cutting tool materials. Further more, the low modulus of elasticity of titanium alloys and their high strength at elevated temperature further impair their machinability.

Low chip-tool contact length and high cutting pressure
It is well known that chip-tool contact length during machining titanium and its alloys is extremely low, as a result high cutting temperatures are generated close to the cutting edge of the tool and due to the serrated behaviour of the chip which. The cutting forces recorded when machining titanium alloys though are slightly lower than those when machining steels but due to the low chip tool contact length the stresses acting on the tool are much greater than that when machining steel. These stresses occur in the immediate vicinity of the cutting edge during machining of titanium alloys. The hotspot temperature on the other hand is much higher in the case of titanium alloys compared to that of in the case of steel.

Chatter
Chatter is another major problem to be overcome when machining titanium alloys, especially for finish machining. The low modulus of elasticity of titanium alloys and the serrated type of chip produced are the principal causes of chatter during machining. When subjected to cutting pressure, titanium deflects nearly twice as much as carbon steel, the greater spring-back behind the cutting edge resulting in premature flank wear, vibration, and higher cutting temperature. In effect, there is a bouncing action as the cutting edge enters the cut. The appearance of chatter is also be partly ascribed to the high dynamic cutting forces in the machining of titanium as a result of serrated toothed chip formation. Besides high cutting temperatures, high mechanical pressure and high dynamic loads in machining of titanium alloys, result in high plastic deformations leading to rapid failure of the tool.
Mastering chatter is the key to maximizing the responsiveness of the machine. Chatter is not caused due to fault of the machine tool, it is rather generated due to a phenomenon known as resonance. The machine tool system comprising the spindle, toolholder and tool fixture and work has a set of frequencies at which it naturally wants to vibrate. At certain cutting speeds, the frequency of chip serration come close to or are equal to multiple one of these natural frequencies, and lead to chatter. Control of chatter is very essential since in many machining processes, chatter is the barrier that defines the maximum cutting parameters, when the limits should instead be defined by the strength of the tool and the power of the machine. The narrow zones of stable, optimal spindle speed are the gaps in the wall of chatter that allow this barrier to be surpassed.
Attempts have been made to minimize the problem associated with chatter by employing very rigid machines, using proper cutting tools and set-ups, minimizing cutting pressures, providing copious coolant flow and designing special tools. Preheating of work material is found to be a new approach to control chatter.

Chemical Reactivity
Performance of cutting tools is also effected by strong chemical reactivity of titanium. Titanium and its alloys react chemically with almost all available tool materials at cutting temperature in excess of 500 0C due to their strong chemical reactivity.



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