MACHINABILITY OF TITANIUM ALLOYS-A REVIEW

PRESENTEDBY

A.HARSHITHAB.MANOJ BABU

INTRODUCTIONINTRODUCTION

Titanium alloysTitanium alloys are metallic materials which contain a  are metallic materials which contain a mixture of titanium and other chemical elements. mixture of titanium and other chemical elements.

eg:Ti-6Al-4V, Ti-5Al-2.5Sn,..etc.eg:Ti-6Al-4V, Ti-5Al-2.5Sn,..etc.

PROPERTIES OF TITANIUM ALLOYS:PROPERTIES OF TITANIUM ALLOYS:

Such alloys have very high tensile strength and toughness (even Such alloys have very high tensile strength and toughness (even at extreme temperatures), light weight, corrosion resistance, and at extreme temperatures), light weight, corrosion resistance, and ability to withstand extreme temperatures. ability to withstand extreme temperatures.

Titanium alloy, exhibits highly favorable properties such as a high Titanium alloy, exhibits highly favorable properties such as a high strength-to-weight ratio, good heat treatment capability, low density, strength-to-weight ratio, good heat treatment capability, low density, good bio- compatibility etc. good bio- compatibility etc.

Titanium and its alloys are used extensively in aerospace Titanium and its alloys are used extensively in aerospace and biomedical industries because of their excellent and biomedical industries because of their excellent combination of hi specific strength (strength-to-weight ratio). combination of hi specific strength (strength-to-weight ratio).

They are also being used increasingly in other industrial They are also being used increasingly in other industrial and commercial applications, such as petroleum refining, and commercial applications, such as petroleum refining, chemical processing, surgical implantation, pulp and paper, chemical processing, surgical implantation, pulp and paper, pollution control, nuclear waste storage, food processing, pollution control, nuclear waste storage, food processing, and and marine applications.marine applications.

CLASSIFICATION OF TITANIUM ALLOYSCLASSIFICATION OF TITANIUM ALLOYS

Titanium Alloys are generally classified into four main Titanium Alloys are generally classified into four main categories: categories:

ALPHA ALLOYSALPHA ALLOYS NEAR-ALPHA ALLOYSNEAR-ALPHA ALLOYS

ALPHA & BETA ALLOYSALPHA & BETA ALLOYS BETA ALLOYSBETA ALLOYS

2. MACHINING OF TITANIUM ALLOYSDifficulties The machinability of titanium and its alloys is generally considered to be poor

owing due to several inherent properties of the materials, such as High cutting temperature, Chatter, High cutting pressures. etc. Source: W. Konig, Proc. 47th Meeting of AGARD Structural and

Materials Panel, Florence, Sept. 1978, AGARD, CP256, London, 197 1.1-1.10.

Fig.1. Distribution of thermal load when machining titanium and steel;

Fig.2. Normal and tangential stresses in machining; Source [18] : W. Konig, Proc. 47th Meeting of AGARD Structural and Materials Panel,

Florence, Sept. 1978, AGARD, CP256, London, 1979, pp. 1.1-1.10.

 

Titanium is very chemically reactive and, therefore, has a tendency to weld the cutting tool during machining, thus leads to chipping and premature tool failure.

Its low thermal conductivity increases the temperature at the tool/work piece interface, which affects the tool life adversely.

Besides high cutting temperatures, high mechanical pressure and high dynamic loads in the machining of titanium alloys, which result in plastic deformation and rapid tool wear, cutting tools also suffer from strong the chemical reactivity of titanium. .

Tool Failure Modes And Wear Mechanisms Cutting tool materials encounter severe thermal and

mechanical shocks when machining titanium alloys, the high cutting stresses and high temperatures generated, close to the cutting edge greatly influencing the wear rate and hence the tool life.

Notching, flank wear, crater wear, chipping and catastrophic failure are the prominent failure modes occur when machining titanium alloys, these being caused by a combination of

i. High temperature, ii. High cutting stresses, iii. The strong chemical reactivity of titanium, iv. The formation process of catastrophic shear (lamellar) chips,

etc.

Tool Materials For Machining Titanium Alloys Over the last few decades, there have been great advancements in Over the last few decades, there have been great advancements in

the development of cutting tools, including coated carbides, the development of cutting tools, including coated carbides, ceramics, cubic boron nitride and polycrystalline diamond. These ceramics, cubic boron nitride and polycrystalline diamond. These have found useful applications in the machining of cast irons, steels have found useful applications in the machining of cast irons, steels and high temperature alloys such as nickel-based alloys.and high temperature alloys such as nickel-based alloys.

The paramount qualities required for tool materials in improving The paramount qualities required for tool materials in improving the machinability of titanium alloys are:the machinability of titanium alloys are:

a)a) High hot hardness to resist the high stresses involved; High hot hardness to resist the high stresses involved; b)b) Good thermal conductivity to minimise thermal gradients and Good thermal conductivity to minimise thermal gradients and

thermal shock;thermal shock;c)c) Good chemical inertness to depress the tendency to react with Good chemical inertness to depress the tendency to react with

titanium; titanium; d)d) Toughness and fatigue resistance to withstand the chip Toughness and fatigue resistance to withstand the chip

segmentation process; andsegmentation process; ande)e) High compressive, tensile and shear strength.High compressive, tensile and shear strength.

CUTTING FLUID.

The high temperature and the high stresses developed at the cutting edge of the tool are the principal problems when machining titanium alloys, to minimise the problem, a cutting fluid must be applied, as a basic rule.

The cutting fluid not only acts as a coolant but also functions as a lubricant, which reduces the tool temperatures and lessees the cutting forces and chip welding that are commonly experienced with titanium alloys, thus improving the tool life.

The uniterrupted flow of coolant has a significant effect on tool life.

The correct choice of cutting fluid has a significant effect on tool life to remove chips, minimise thermal shock of milling tools and prevent chips from igniting, especially when grinding titanium.

Additionally, a high pressure coolant supply can result in small, discontinuous and easily disposable chips, unlike the long continuous chips produced when machining with a conventional coolant supply.

Several tests were carried out and the work concluded that the prohibition of machining titanium with lubricants containing chlorine additives can no longer be maintained.

CONCLUDING REMARKS It is found that titanium and its alloys are considered as difficult to-

cut materials due to the high cutting temperature and the high stresses at and/or close to the cutting edge during machining. The high cutting temperature is due to the heat generated during machining (catastrophic thermoplastic shear process), a short chip-tool contact length and the poor heat-conductivity of the metal, whilst the high stresses are due to the small contact area and the strength of titanium even at elevated temperature.

Cutting tool materials undergo severe thermal and mechanical loads when machining titanium alloys due to the high cutting stresses and temperatures near the cutting edge, which greatly influence the wear rate and hence the tool life. Flank wear, crater wear, notch wear, chipping and catastrophic failure are the prominent failure modes when machining titanium alloys. Flank and crater wear may be attributed to dissolution-diffusion, attrition and plastic deformation, depending on the cutting conditions and the tool material, while notch wear is caused mainly by a fracture process and/or chemical reaction.

As a basic rule, a cutting fluid must be applied when machining titanium alloys. The correct use of coolants during machining operations greatly extends the life of the cutting tool. Chemically active cutting fluids transfer heat efficiently and reduce the cutting forces between the tool and the workpiece.

As a basic rule, a cutting fluid must be applied when machining titanium alloys. The correct use of coolants during machining operations greatly extends the life of the cutting tool. Chemically active cutting fluids transfer heat efficiently and reduce the cutting forces between the tool and the workpiece.

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