One of the lightest metallic elements in the periodic table, titanium is also among the most important. Without this durable, high-strength material, our world today would be a far different place. Airlines would charge more for flights, houses would need painting more frequently, hip joints and dental implants wouldn’t last. Unfortunately, titanium sets complex machining demands. It has metallurgical characteristics and material properties that affect the cutting action more severely than other metals – such as cast iron and stainless steel – and make it difficult to machine.
Did you know that titanium alloys can be split into four classes, depending on the structures & alloying elements present?
Untreated, commercially pure titanium
Alpha alloys – with additions of Al, O and/or N
Beta alloys – additions of Mb, Fe, V, Cr and/or Mn
Mixed a+ß alloys, in which a mixture of both classes is present
The mixed α+β alloys, with type Ti-6Al-4V, account for the majority of titanium alloys currently in use, primarily in the aerospace sector, but also in general purpose applications. Titanium has a high strength to weight ratio, with excellent corrosion resistance at 60% of the density of steel. This enables the design of thinner walls.
How to machine titanium
By combining a well-planned process with dedicated application knowledge and tools/set-ups optimised for titanium, gains can be made to take advantage of the great properties that this material has to offer. Follow these eight tips on machining titanium and its alloys:
Mind titanium’s feed and speed comfort zone
Titanium has a narrow band of machinability, with recommended cutting speeds of 60 m/min for roughing and 3-4 times that when finishing. Feed rates are entirely dependent on chip loads and other factors, but should be high enough to prevent work hardening. Significant deviation from titanium’s feed and speed comfort zone can mean melted or broken tools and a pile of expensive scrap. Always follow cutting tool manufacturers recommendations when machining titanium.
Push the heat into the chip for longer tool life
Titanium conducts heat at about the same rate as the oven gloves you use to pull a cake pan out of the oven. During machining operations, this poor thermal conductivity traps heat in the work zone, wreaking havoc on cutting tools. If your machine setup can handle the additional load, try increasing the feed rate to push some of the heat generation into the chip and make tools last longer.
Avoid primary failure modes
If steel were stiff modeling clay, titanium would be frozen putty. Built-up edge, notching at the cut line, galled workpieces and chips welding to the cutter are the primary failure modes when machining this gummy material. A positive rake cutting tool with a tough substrate and hard, lubricious coating keeps tools in the game longer. Also, a small T-land or slight hone on the cutting edge can help improve tool life, but don’t overdo it – titanium needs a sharp tool.
Increase coolant concentration and blast chips out of the work area
With the high temperatures and stringy chips generated when cutting titanium, a copious flow of clean cutting fluid is essential. Filtration to 25-micron or better is a good idea for many machining operations, but is especially important with critical operations such as this. Increase the coolant concentration to 10% or more and install a high-pressure pump of at least 500 psi to blast chips out of the work area. Always use coolant-fed cutting tools and employ inserts with aggressive chip control to avoid catastrophic recutting of chips.
Use the right tool and the right machine equipment
Because of the extreme cutting forces involved, titanium should only be machined on rigid equipment. A machine spindle with abundant surface contact at both the taper and the face used together with CAPTO holders provide the security of multiple contact points with the machine spindle, excellent repeatability and the stiffness needed to absorb heavy radial loads. Dense machine construction will absorb vibration and cutting loads better than one designed for light duty machining. Invest in a high-performance machine tool if you’re serious about machining titanium. High speed can produce a chemical reaction between the chip and the cutting tool material, which can result in sudden insert chippings/breakages. Cutting tool materials should have good hot hardness, low cobalt content and not react with the titanium. Fine-grained, uncoated carbide is usually used. Choose a positive/open geometry with good edge toughness.
Prevent the tool to get pulled out of the tool holder
Titanium tends to grab end mills under heavy loads, pulling them out of the tool holder. This leads to scrapped workpieces and broken tools. Some shops turn to Weldon shank holders as a way to secure tools, only to find cutter vibration loosens even the most tightly torqued set screws. Shrink fit holders are a good choice, but require some small investment in an induction heating station for tool changing. For a no-fail grip, a Safe-Lock or equivalent system secures tool holders tightly and accurately. Hydraulic holders, like the Sandvik Coromant CoroChuck 930, utilise the latest technology and will prevent pull out. On the workholding side of the equation, a hydraulic vise with hardened and ground jaws is the best bet for clamping titanium parts – a serrated or knife-edge jaw gives an extra bite during roughing operations.
Go for the right toolpath
The right toolpath is a big part of success when machining titanium. The same techniques as those used in High Feed Machining (HFM) are effective here. Roll into the cut and don’t slow down in the corners. ‘Drive’ the cutter around the corner by using programmed radius cutter movements. Trochoidal milling paths with constant cutter engagement lessen shock to the machine tool and cutters alike, extending the tool life. And plunge milling can be an effective way to rough out deep cavities.
Finally, be strategic
Before machining any titanium component, it is especially important to plan and optimise the machining process. Analyse all of the part features, taking special consideration of unsupported areas, tall and/or thin walls and hard to reach features. Pick the right cutters, set the appropriate feeds and speeds and then generate code that meets the conditions mentioned earlier.
Granted, these are general guidelines. Titanium presents a complex machining situation, one whose cutting parameters depend on the size and geometry of the workpiece, the specific alloy being cut and the rigidity of the setup and the machine tool. Probably, the best tool available for successful titanium machining is to contact your cutting tool manufacturer or equipment provider knowledgeable in this area – best of all, it’s free.
Courtesy: Sandvik Coromant