It is well known that various mechanical, thermal and chemical properties of the workpiece materials have an influence on cutting forces, temperatures generated during machining and consequently on tool wear mechanisms observed during machining of that particular group of workpiece materials. Strength, hardness and work hardening characteristics influence the cutting forces. Thermal conductivity of the material determines the chips’ ability to carry away the heat during machining, thereby greatly influencing the temperature at the cutting edge.
The polar diagram below is a graphical depiction of these thermo-mechanical properties. Based on the study of polar diagrams, the workpiece material’s behaviour during machining can be understood. A typical polar diagram for one such common titanium alloy is reproduced below, along with its comparison with the polar diagram for a commonly used steel.
In addition to the basic properties mentioned earlier, additional considerations of the workpiece material, such as characteristics of stickiness and the chemical interaction potential in relation to the cutting tool materials (especially the coating material) are important in designing the cutting tools.
Following are the considerations to take into account in terms of cutting tools design for the specific case of titanium machining.
Positive tool geometry with high rake angle: The tendency of titanium chips to stick to the cutting tools (sticky behaviour) often results in the formation of built-up edge (BUE) during machining. This BUE often takes away the cutting tool material with it, leading to unexpected and uncontrolled chipping off at the cutting edge. It is, thus, imperative to use cutting tools with sharp cutting edges. A high positive rake angle is frequently used in the tools’ design for titanium machining.
Thicker carbide inserts with higher carbide mass: Low heat dissipation into titanium chips leads to a higher amount of heat entering the cutting tools and higher temperatures at the cutting edges. To counter this, a higher carbide mass resulting from thicker inserts in general can cope up with the heat better, leading to a better tool life.
Combination of a deformation resistant and a tough carbide substrate: Low thermal conductivity of titanium alloys results in higher temperatures at the cutting edge. The ability to withstand the higher temperatures without plastically deforming under the cutting loads is important. What makes the choice of carbide substrate difficult is the fact that the chips of titanium alloys are highly segmental; in other words, the chip thickness varies a lot during chip formation. Given the fact that the cutting forces are controlled by chip thickness, we have a situation that even during continuous cutting, the cutting forces fluctuate a lot, and this needs to be supported by adequate toughness from the substrate of the cemented carbide tool. Thus, a carefully chosen fine grained carbide substrate needs to be implemented in the cemented carbide tools design.
Coating suitability: In general, PVD coated tools are better for titanium alloys machining, as the edge sharpness of PVD coated tools is better. High stickiness of the workpiece material poses a risk that during machining, the chips sticking to the tool could peel the coating off. Thinner coatings deposited in a PVD process offer a more suitable option for the machining of titanium alloys. The possibility of the chemical interaction between the typical coating material, such as TiN or TiAlN and the workpiece material – titanium itself poses a big risk of adhesive wear mechanism of the cutting tools. To overcome this problem, modern carbide grades, such as Seco Tools grade MS2050, designed for milling titanium, feature a Niobium Nitride (NbN) coating as the outermost coating material. This silver coloured NbN coating eliminates any possible chemical interaction with the workpiece and offers a long life of the carbide insert.
Drills with a higher back taper: Low thermal conductivity of titanium alloys results in higher cutting-edge temperatures. In drill design, this aspect becomes critical as the drill can expand and can get jammed in the workpiece material. Consequently, drills for titanium are often designed with a higher value of back taper, compared to the drills used for steel & cast iron drilling.
What leads to successful machining of titanium alloys
In the end, the optimisation of cutting parameters (speed, feed & depth of cut) with the coolant conditions is equally important for the successful machining of titanium alloys.