Why do we consider titanium alloy a difficult material to process? Because there is a lack of profound understanding of its processing mechanism and phenomena.
1. Physical Phenomena in Titanium Processing
The cutting force during titanium alloy machining is only slightly higher than that of steel of equal hardness, but the physical phenomena involved in machining titanium alloys are much more complex than machining steel, which makes titanium alloy processing quite challenging.
Most titanium alloys have low thermal conductivity, only 1/7 of steel and 1/16 of aluminum.
Therefore, during the cutting process of titanium alloys, the heat generated does not quickly transfer to the workpiece or get carried away by chips, but instead concentrates in the cutting area.
The resulting temperature can reach over 1,000℃, leading to rapid wear, chipping, and buildup on the cutting edge of the tool. This, in turn, further generates more heat in the cutting area and shortens the life of the tool.
The high temperature generated during the cutting process also compromises the surface integrity of titanium alloy parts, resulting in reduced geometric accuracy and severe work hardening, which significantly decreases their fatigue strength.
While the elasticity of titanium alloys may be beneficial for part performance, it is a primary cause of vibration during the cutting process. Cutting forces cause the “elastic” workpiece to move away from the tool and rebound, thereby increasing friction between the tool and the workpiece over the actual cutting. This friction also generates heat, further exacerbating the poor heat conduction issue of titanium alloys.
This problem becomes more serious when processing easily deformable parts such as thin walls or rings. Achieving the desired dimensional precision when machining thin-walled titanium alloy parts is not an easy task.
As the material of the workpiece is pushed away by the tool, the local deformation of the thin wall has exceeded its elastic limit, causing plastic deformation, and the strength and hardness of the material at the cutting point increase noticeably. At this point, proceeding with the original determined cutting speed becomes excessive, leading to rapid tool wear.
Heat is the primary culprit making titanium alloys difficult to process!
2. Tips for Titanium Alloy Machining
Based on understanding the mechanism of titanium alloy machining and past experiences, the key tips for processing titanium alloys are as follows:
(1) Use positive-rake geometrically-shaped cutting inserts to reduce cutting force, cutting heat, and deformation of the workpiece.
(2) Maintain a constant feed to avoid workpiece hardening. During cutting, the tool should always be in a feeding state. During milling, the radial depth of cut (ae) should be 30% of the radius.
(3) Use high-pressure, high-flow cutting fluid to ensure thermal stability during the machining process and prevent surface changes and tool damage due to excessive temperatures.
(4) Keep the cutting edge of the insert sharp. Dull tools cause heat concentration and wear, leading to tool failure.
(5) Process in the softest possible state of titanium alloys, as hardened materials become more difficult to process. Heat treatment increases the strength of the material and accelerates tool wear.
(6) Use a large cutting edge radius or chamfered entry, allowing as much of the cutting edge as possible to engage in the cutting process. This can reduce the cutting force and heat at each point, preventing local damage. In milling titanium alloys, among various cutting parameters, cutting speed has the most significant impact on tool life (vc), followed by radial depth of cut (milling depth) (ae).
3. Addressing Titanium Machining Challenges by Focusing on Cutting Inserts
The grooving wear observed in cutting inserts during titanium alloy machining is local wear in the direction of the cutting depth on both the back and front of the insert. It is often caused by the hardened layer left from previous machining.
The chemical reaction and diffusion between the tool and workpiece materials at machining temperatures above 800℃ is also one of the reasons for grooving wear.
During the machining process, the titanium molecules of the workpiece accumulate in front of the cutting edge and “weld” to the cutting edge under high pressure and temperature, forming a chip buildup.
When this buildup peels off the cutting edge, it carries away the hard alloy coating of the insert. Hence, titanium alloy machining requires special cutting insert materials and geometries.
4. Tool Structure Suitable for Titanium Machining
The focus of titanium alloy machining is heat. A large amount of high-pressure cutting fluid needs to be sprayed accurately and promptly onto the cutting edge to quickly remove heat.
There are milling cutters with unique structures specifically designed for titanium alloy machining on the market.
