Machining Titanium: It's Complicated
Machining Titanium is a Popular Choice these Days but it comes with a Number of Issues.
The popularity of titanium is spreading across every industry due to the metal's exceptional strength to weight ratio and corrosion resistance. However, it also means any shop machining components on a schedule has to be creative enough to achieve desired productivity while keeping costs down due to the challenges this titanium poses.
The intensive research and development by the cutting tool industry has led to big improvements that have facilitated the expansion of titanium into everyday industrial applications.
The field now boasts many tooling solutions that effectively machine titanium and titanium alloy. Poly Crystalline Diamond (PCD) inserts, diamond coated inserts and specially designed milling tools that enable quality finish on thin titanium alloy components within specified tolerances have made machining titanium and titanium alloys less complicated.
Sharper insert geometries have been introduced that lower cutting forces while PCD and diamond coatings have proven to be effective in increasing productivity and, when possible, speed.
Concepts such as cryogenic gas show promise but further testing is required.
Metallurgically speaking, titanium is divided into four types: pure and un-alloyed, alpha alloy, alpha and beta alloys and beta alloy.
Amongst the above described, alpha and beta alloys are the most commonly applied in the manufacturing sector.
Yet, despite its advantages, machining this metal poses certain difficulties. While titanium is 30-40 percent easier to machine in speed and metal removal rate terms (MRR) over HRSA, titanium alloys such as Ti5553 is far trickier. One of the down sides of machining titanium and titanium alloys is its thermal conductivity characteristic.
If the incorrect tooling solution is applied to titanium alloys such as Ti-6Al-4V, the heat generated during machining would cause total failure of the cutting tool. Success depends on the titanium grade, application, workpiece thickness and other aspects of the cutting conditions; this will determine the speed and feed of the machine.
In the aerospace field, titanium alloy is used for blades and casings which have complicated shapes. Even if thermal conductivity were not an issue, in some cases, high feed cutting tools cannot be effective because in 5-axis machines, which dominate this field, applications with complicated tool paths cannot change direction at the intended speed.
In milling applications involving thin titanium workpieces, improper tooling solutions “push” up the component from the fixture resulting in it being out of tolerance, hence ruining the workpiece.
Due to thermal conductivity, up-milling is not an advisable course of action.
From a concept perspective the issues for cryogenic gas machining has the potential to increase productivity but there are drawbacks such as safety concerns and cutting tool design limits.
Getting Tool Around it
The above examples demonstrate the challenge when machining titanium and titanium alloys but there are solutions.
MM Chip Breaker
One being to use round type inserts at a reduced speed when using a 5-axis machine – this will extend tool life and enable very favorable MRR.
In milling thick titanium parts, the use of high positive radial/axial rake angle cutter bodies with strong cutting edge inserts in roughing, will achieve longer tool life and high MRR.
Since the 1960s, titanium has proved invaluable across the industrial spectrum. In the aerospace field, it is used for components such as air frames, wing access panels, landing gears and most importantly, the “cold” section of jet engine components such as engine casings, compressor discs and blades.
By using Titanium types like Ti-6Al-4V, these components resist both metal fatigue and the resulting stress cracks when operating in temperature deviation situations.
As a result of these very favorable features, the aerospace field dominates the demand for titanium by accounting for over 50 percent of the market.
Outside the aerospace field, its biocompatible nature means it is very popular in the medical field where it is used in manufacturing prosthetic parts such as hip and dental implants.
The corrosion resistance characteristic means it is suitable for salt water and certain types of chemical environments – it is used for submarine hull and ship propeller manufacturing as well desalination plants and pipes, and drills for off-shore rigs. It is also widely used in petrochemical and paper making plants.
In its finished form, it has luminosity so it has a very smooth, fine and attractive look. This makes it very much in demand for everyday household items.
Higher Productivity Tips
The goal when machining titanium is productivity where high speed and feed is feasible, or through metal removal rates. The point is to adopt the right cutting tools that diffuse the generated heat and encourages the longest tool life.
Additionally, the machine set-up in multi-axis complicated tool path milling, needs to be very rigid to withstand vibration. This also means the fixture inside the machine must be as secure as possible.
In turning applications on Grade 5 titanium (Ti-6Al-4V), using PCD inserts with high pressure coolant cuts chips effectively and reduces the generated heat when machining at speeds of up to 150m/min versus carbide's speed of 60-80 m/min and a feed rate between 0.2-0.3 mm/rev.
In Grade 2, titanium diamond coated inserts with high radial/axial rake angle and high pressure coolant achieve higher MRR and longer tool life.
In drilling, to achieve longer tool life in both roughing and finishing applications and higher productivity with speeds averaging between 60-80 m/min and a feed rate averaging 0.15-0.2 mm/rev, it is advisable to use sharp positive cutting edge indexable/non-indexable drills with less honing.
Applying an unequal space end mill on a component such as an impellor or blades with high pressure coolant minimizes chattering.
When taking all the above factors into account, the primary objective when machining titanium does not change – to achieve the highest MRR under the best velocity and speed while minimizing production cycle times.
Depending on the application, a good MRR runs from 30cm³. If such rates are achieved, a shop is on the path to optimal tool life and production cycle time.
There are a number of solutions that can be used when machining titanium. While not being the easiest material to machine, the recent introduction of grades, geometries, cutter body designs as well as tool machine set-ups has made titanium and titanium alloy machining that much simpler.
This article appeared in the November-December 2013 edition of
“Asia Pacific Metalworking Equipment News”
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