The cutting forces when machining titanium alloys are only slightly higher than steels of equivalent hardness, but the physical phenomena occurring during titanium alloy machining are much more complex than in steel machining, thus posing huge difficulties for titanium machining.
Titanium alloys are alloys that contain a mixture of titanium and other chemical elements. Titanium alloys are lightweight and have very high strength and toughness (even at extreme temperatures), as well as corrosion resistance and high-temperature resistance. However, due to the high costs, titanium alloys are only used in military applications, aircraft, spacecraft, bicycles, medical devices, jewelry, high-stress components (such as connecting rods in racing cars), and some high-end sports equipment and consumer electronics.
Physical Phenomena in Titanium Machining
Most titanium alloys have very low thermal conductivities, only 1/7 that of steel and 1/16 that of aluminum. Therefore, the heat generated during titanium cutting cannot be quickly conducted away into the workpiece or removed by the chips, instead being concentrated in the cutting zone, with temperatures reaching over 1000°C. This leads to rapid wear, fracture, and build-up edge formation on the cutting edge, creating a worn cutting edge that generates even more heat, further shortening tool life.
The high temperatures also damage the surface integrity of the titanium part, causing decreased geometric accuracy and severe work hardening that significantly reduces fatigue strength.
The elasticity of titanium alloys may be beneficial for part properties, but during cutting, the elastic deformation of the workpiece is an important source of vibration. The cutting forces make the “elastic” workpiece spring away from the tool and rebound so that rubbing between the tool and workpiece is greater than the cutting action. The rubbing also generates heat, compounding the poor thermal conductivity issue of titanium alloys.
This problem becomes even more severe when machining thin-walled or annular parts prone to deformation. Achieving desired dimensional accuracy on titanium-thin-walled parts is not an easy task. As the material is pushed away by the tool, local deformation of the thin walls exceeds the elastic range and becomes plastic, significantly increasing material strength and hardness at the cutting point. At this stage, continuing cutting at the original parameters becomes excessive, leading to rapid tool wear.
Machining Techniques for Titanium Alloys
Based on understanding the machining mechanisms of titanium alloys, and past experience, the main techniques for machining titanium alloys are:
Use positive rake-cutting tool geometries to decrease cutting forces, heat, and workpiece deformation.
Maintain constant feed rates to avoid work hardening. The tool should always be in a feeding state during cutting. In milling, the radial depth of cut ae should be 30% of the radius.
Use high-pressure and high-volume cutting fluids to ensure thermal stability during machining, preventing surface alteration of the workpiece and tool damage from excessive temperatures.
Keep the cutting edge sharp. A blunt tool causes heat concentration and wear, easily leading to tool failure.
Machine the titanium alloy in its softest state if possible, since after hardening the material becomes more difficult to cut, with heat treatment increasing material strength and abrasive wear on the tool.
Use large tool nose radii or chamfers to engage more of the cutting edge into the cut. This decreases the cutting forces and heat for each point, preventing localized damage. In titanium milling, cutting speed has the greatest influence on the tool life of all the parameters, followed by radial depth of cut.
Solving Titanium Machining Challenges through Tooling
Notching wear occurring on cutting tools during titanium alloy machining is localized wear on the flank and face in the depth of cut direction. It is often caused by work-hardened layers generated in previous machining. Chemical reaction and diffusion between the tool and work material at temperatures exceeding 800°C also contribute to notching wear. During machining, titanium atoms accumulate on the tool face and are “welded” to the cutting edge under high pressure and temperature, forming a built-up edge. When the built-up edge detaches from the cutting edge, it takes away the hard coating on the tool, so titanium machining requires specialized tool materials and geometries.
Suitable Tool Designs for Titanium Machining
The focus in titanium machining is heat. High-pressure cutting fluid must be applied precisely and timely at the cutting edge to rapidly conduct heat away. There are specially designed end mills on the market targeted for titanium machining.
Why is it difficult to machine titanium alloys?
Titanium alloys have low thermal conductivity and high chemical activity. It is easy to generate high temperatures and strong reactions with the cutting tools during machining, causing severe wear of the cutting tools.
What should be noted when machining titanium alloys?
Choose suitable positive-rake tools, maintain a constant low feed rate, use plenty of cooling liquid, and machine in a softer state if possible.
What are the common forms of tool wear when machining titanium alloys?
Mainly tool tip cracking, plastic deformation of cutting edge, and severe grooving wear.
How to improve the machining efficiency of titanium alloys?
Optimize machining parameters, use special structured tools designed for titanium alloys, employ high-pressure cutting liquid cooling, etc.
What is the most suitable tool material for machining titanium alloys?
Advanced cemented carbides, PCD diamond tools, high-speed steel and other high-performance materials.
What factors should be prioritized when machining titanium alloys?
Mainly to reduce cutting heat, increase the cooling effect, improve tool strength and reduce friction.