Titanium and its alloys exhibit a unique combination of mechanical and physical properties and corrosion resistance which have made them desirable for critical, demanding aerospace, industrial, chemical, energy industry service and sputtering targets in pvd coating industry.
Here are some primary attributes of titanium:
✦ Elevated Strength-to-Density Ratio (high structural efficiency)
✦ Low Density (roughly half the weight of steel, nickel and copper alloys)
✦ Exceptional Corrosion Resistance (superior resistance to chlorides, seawater and sour and oxidizing acidic media)
✦ Excellent Elevated Temperature Properties (up to 600°C (1100°F))
Other attractive properties of titanium:
✦ Exceptional erosion and erosion corrosion resistance
✦ High fatigue strength in air and chloride environments
✦ High fracture toughness in air and chloride environments
✦ Low modulus of elasticity
✦ Low thermal expansion coefficient
✦ High melting point
✦ Essentially nonmagnetic
✦ High intrinsic shock resistance
✦ High ballistic resistance-to-density ratio
✦ Nontoxic, nonallergenic and fully biocompatible
✦ Very short radioactive half-life
✦ Excellent cryogenic properties
All of these characteristics make titanium play an more and more important role in material application field, and it gets an name ”future metal”.
Recently, titanium as a workpiece material is finding its way into many machine shops with milling being the dominant machining method, particularly in implant medical, automotive and structural aerospace parts. While material removal rate is becoming an increasingly competitive factor, this is not easy to achieve in titanium, which exhibits challenging machinability. However, new tool and process developments are providing this increasingly useful design material with new possibilities to improve machining economics.
The physical, chemical and thermal properties of titanium make it a uniquely demanding material to machine. There are a number of titanium alloys with different properties and machinability varies considerably – from the traditional Ti6Al4V to stronger alloys like Ti10-2-3 and now Ti5553. Among the main characteristics is a risk of tool wear due to the cutting edge being exposed to higher temperatures - more heat is absorbed because titanium is a poor thermal conductor.
Tool wear/breakdown is also common due to the smearing tendency of titanium, which is reactive with tool materials. Smearing is where the chip welds to the insert, causing edge-line frittering when re-entering the cut. Other characteristics include material deflection/chatter tendency due to the elasticity of titanium, and the risk of rapid tool wear because of localized high pressure incombination with heat at the contact surface.
There are a few general rules for machining titanium that can help overcome these demands: use relatively low cutting speeds; use sharp cutting edges; optimize feed rates and avoid idling while in cut; use large volumes of coolant, preferably at high pressure through spindle and tool; replace cutting edges at the first sign of any wear; and employ climb (down) milling wherever possible.
When machining any titanium alloy there is the common need for more thorough planning – from selecting the machine tool for the job through to programming of cutting details. Size and shape of component features vary and so do the demands on selection of machine, fixture, coolant supply, tools, method and cutting data.
The first determining factor is the size and shape of configurations and suitable tool size. Indexable insert cutters remove material most efficiently and are today seen as a first choice for roughing as well as unbeatable when it comes to finishing large flat faces. Solid carbide cutters form the solution for semi-finishing and finishing operations and when radii, cavities and slots are too small for indexable insert tools. They have the advantage of a high number of flutes and high axial cutting capability.
The selection of a dedicated milling cutter for titanium needs firstly to be based on the necessary programming options. For titanium, the tool basics are always to include a comparatively positive rake with a sharp but strong cutting edge on a cemented carbide grade – these withstand the particular thermal and chemical demands of titanium. Indexable insert technology has come a long way as regards geometry and tool material, and are taking over as a more cost effective solution from the vast amount of solid carbide and HSS tool options available, even for medium and large size tools. Until recently, progress in machining titanium seems to have been unspectacular, but now a few breakthrough developments have improved the performance of milling.
Due to the nature of typical aerospace components, radial milling is a very suitable machining method for titanium. Parts frequently exhibit several shoulders, edges, profiles and cavities, which often need to be machined from billets. However, large radial depths of cut can result in considerable reductions in tool life, while large axial depths of cut have a relatively limited influence on cutting temperature and hence do not affect tool life in the same way. Therefore, a close-pitch, long-edge milling cutter with a radial engagement of about 30 per cent and as much axial engagement as the application allows is the most effective way of removing titanium.
Indexable insert, long-edge cutters
These are made up of multiple rows of inserts which are based on the continuous, helical edge of solid carbide cutters. Accommodating the indexable inserts to make up a row from the bottom of the cutter rising along the periphery has up to now presented a limitation to achieving acceptable machining capability and security in titanium. Generous flutes for effective chip evacuation are necessary and, in combination with building effective rows of positive, sharp inserts on the cutter, have created pitfalls for the indexable insert, long-edge cutter.
Cutting edges that are accurately and firmly fixed in position to resist the axial forces created by the helix are paramount for milling titanium. Any movement even in roughing operations can lead to uneven wear and put the cutting edge at risk or even cause screw breakage, leading to catastrophic failure. Axial support for inserts is especially difficult to achieve along a row of closely positioned successive inserts, and this can lead to over reliance on the insert screw.
The best way to achieve outstanding performance levels with long-edge milling is to have an uncompromising interface between insert and tool body. Nowadays, some tools here, the insert seat has a definitive support and locking facility as regards axial and rotational forces. Such an insert location provides the capability for high metal removal rate and allows for spacious chip flutes. Furthermore, a range of tooth capability can then be provided for the same tool diameter by having a choice of insert sizes to tackle various operations. A closer insert pitch provides variables for improving productivity through feed rate.
Coolant applied at high pressure, through spindle and tool, during titanium machining affects the distribution of heat, chip formation, welding-on-edges, tool wear and surface integrity, and as such makes a clear difference to performance. The application of high pressure coolant, ranging from a standard 70 to 100 bar pressure, has shown to provide very clear advantages in titanium milling. Coolant at high pressure is a standard on many of today’s machines and as such it is a potential resource to optimize titanium milling.
When the application involves narrow, deep cavities that require long tool reach, a solution is required for small tool capability as well as operational flexibility. Holding a solid carbide end mill in an extended chuck to machine deep into a cavity does not represent optimum stability. This scenario will limit cutting data and can be a risk to component quality. The concept of exchangeable head cutters, however, provides the advantages of both indexability and the finishing capability of solid carbide cutters. The coupling between the head and shank is a key factor for this type of tool concept. Performance relies on the strength, stability, accuracy, repeatability and ease of handling.
From performance and result capability, tool cost perspective and flexibility requirement, an exchangeable head system provides an advantage in the 10 to 25 mm tool diameter area. Flexibility is high with this concept and the reduced tool inventory it offers. The finishing ability is better than indexable insert cutters, and it represents a substantially lower tool cost than a solid carbide cutter and does not need regrinding with loss of size. Being able to select combinations of different heads and different shanks offers a high degree of flexibility and optimization possibilities.
A generous axial support face, a tapered radial support face and a specially developed thread and screw support, such as with the CoroMill 316 exchangeable head cutter provides the coupling needed between head and shank, and the basis for good performance at long tool overhangs.
Some of the general rules of face milling come highly recommended, particularly concerning cutter positioning in relation to the workpiece, cutter diameter to workpiece width and preferred climb (down) milling with a thin-to-thick chip. More care is also needed for the cutter to enter and exit the workpiece. The cutter should be kept on a path providing full contact rather than multiple passes when milling large faces and, if possible, interrupted cuts should be avoided, with holes and cavities made after the face milling.
As regards type of face mill, a round insert cutter, such as the CoroMill 300, is often a first choice because of the strength and geometry of its cutting edge. Cutters with insert size up to 25.4 mm are available, providing very high metal removal rates. The size used is a balance between the depth of cut required, the feature to be machined and the machine tool capability. This is a very effective and reliable roughing and semi-finishing cutter, capable also of machining cavities through helical interpolation. High metal removal rate, long tool life and good security are the potential advantage with this type of cutter. The cutting edge is extra long, distributing the wear which often leads to extended tool life.
The entering angle of the round insert cutter is variable and, like the 45° face mill, gives a chip thinning effect that is beneficial for raising the feed rate. However, power and torque need extra attention when machining titanium because of the higher cutting forces involved. The tensile strength of the alloy, the cutter engagement, feed rate and the number of teeth in cut are especially relevant when it comes to face mill roughing in titanium.
A milling cutter with a really small entering angle (10°) can provide an even higher chip thinning effect, and as such it gives the potential for even greater feeds. Combined with a small depth of cut, high feed milling can be a very effective machining method and does not impose elevated power and torque. High metal removal rates can be achieved on smaller, weaker machines.