Posted on Oct. 19th, 2020, | By Candy, WayKen Rapid Manufacturing
Titanium has long been overlooked due to its properties. However, recent developments in metallurgy and manufacturing technology have given a new view on the material. Right now, titanium is coveted for its outstanding weight-to strength ratio ( 3 times better than steel), its corrosion resistance, human tissue compatibility and outstanding performance at high temperatures. Aerospace titanium parts are especially popular for being lightweight and strong. There are, however, a lot of issues with titanium machining.
The Most Common Titanium Alloys
There is a number of elements that are added to titanium to increase its performance capabilities. Titanium undergoes an allotropic transformation at a temperature of over 800 C. The elements that decrease the temperature of titanium usability are called beta stabilizers and the ones that heighten it are called alpha stabilizers. Depending on the contents of the stabilizers, titanium alloys are divided into 4 groups.
- Unalloyed titanium. Basically just titanium. The unalloyed version has the best corrosion resistance but low strength.
- Alpha alloys. Alpha alloys have better creep resistance so they are used for high-temperature performance.
- Alpha-beta alloys. This group is the most widespread because of its great properties. Alpha stabilizers add to the heat resistance of the material and beta stabilizers add to the strength of the alloy. The most common alpha-beta alloy Ti-6Al-4V accounts for almost 50 % of the whole titanium alloy market.
- Beta alloys. Those alloys are harder than all the previous groups, however, their density is higher as well.
7 Reasons Why Titanium Is Hard to Machine
Let us not delve deep into the mechanics of titanium milling, titanium turning or titanium grinding, tool wear and chip generation mechanics. Instead, here is a summary of 7 points why titanium is such a hassle to machine.
- Titanium retains high strength even at high temperatures and its plastic deformation resistance persists even at cutting speeds. This makes cutting forces much larger compared to any steel.
- When the chip forms after all, it is very thin and the contact area between the chip and the tool is three times smaller than that of steel. Due to that, the tip of the tool has to withstand almost all the cutting force.
- Titanium alloys have a high friction coefficient to the cutting tool materials. This increases the cutting temperature and the forces.
- At a temperature of over 500 degrees centigrade, titanium chemically reacts with the majority of tool materials. Oh, and titanium has a tendency to self-ignite when cut if the heat buildup is too high, so using coolant when cutting titanium alloys is an absolute must.
- Due to a low contact area and the thinness of the chips, all the heat during the cutting process goes to the tool, which significantly lowers its life. Only high-pressure coolants can keep up with the heat buildup.
- Titanium alloys have a very low modulus of elasticity. This can bring about additional vibration, tool chatter, and deflection.
- At low cutting speeds, the material can stick to the cutting edge and it is extremely bad for the surface finish.
So, as you can see there is quite a number of significant problems. However, the manufacturing engineers all over the world have come up with a lot of strategies to increase titanium machining speeds.
Increasing Titanium Machining Speeds With Cutting Tools
Cutting tool choice is the first step to a successful CNC aluminum and titanium machining process. The main thing to choose is the material. Over the years a number of hard alloys have proven themselves to be good at titanium cutting. In order to be good at machining titanium parts, the tools must have a number of properties.
- The tools must be hard at high temperatures.
- They must have a high resistance to chatter.
- They must have high fatigue properties because cip generation with titanium is a cyclic process.
- They must be chemically neutral to hot titanium.
- They must have high strength.
- They must have a good thermal conductivity rate to compensate for the titanium slow rate.
The most suitable tool material range that has all of the above mentioned properties is the WC/Co alloys. Another possible solution is the high-speed steels because they are very resistant to chipping. Diamond tools have also shown good performance against titanium wear. You can look up our Diamond machining tips here.
High-Speed Titanium Machining Techniques
Another viable way to increase the performance of titanium machining is to apply special titanium machining strategies. Our titanium machining services have become quite proficient at choosing the correct cutting parameters to cut the time of titanium machining to a minimum. However, apart from parameter optimization, there are some unconventional machining strategies that can greatly increase the cutting speeds of titanium.
These tools were developed by the General Electric company. They use tools that have a thin protruding ledge. instead of the whole tool, only this thin ledge cuts the titanium. Since the chip is very thin as well, there is no difference, however, the ledge wears slower and it is easier to sharpen. The flank of the ledge wears quickly and then stays constant during the wear of the rest of the ledge, which happens slower with constant flanks.
Rotary inserts are used for titanium turning. They have been impossible before due to the lack of precision in machine tool design. However, the level of manufacturing technology now makes its use possible. The main principle is that the tool insert has a round form and revolves around the fixture as the tool cuts the stock. That way, the heat transfer is much faster and the tool doesn't heat up as much. So, cutting speeds can be increased.
Ultrasonically Assisted Metal Removal
Adding ultrasonic vibration to the cutting tool actually makes chip formation easier. Titanium chip has very bad elasticity that is worsened by the deformation hardening when the cutting edge starts to cut the material. Ultrasonic waves encourage chip fracture instead of deformation and that has a positive influence on the tool life, surface finish, the characteristics of the tool, it prevents material buildup on the cutting edge and allows the machinist to increase cutting speeds.
Want to Learn More?
Choosing the right machining tips and raw materials for your next rapid prototyping is really an important part when you expect the performance. At WayKen, we will be glad to provide more of your professional advice, please feel free to contact us: email@example.com or upload your CAD file there.