Strategies for machining ISO-S materials

For example, established HRSA such as nickel-base Inconel 738 and cobalt-based SFX414 were engineered to operate in temperatures in the range of 850 degrees Celsius to 1200 degrees Celsius. Some of the latest HRSA compositions such as GTD 262 and Rene 108 are intended to perform at temperatures of 1200 degrees Celsius to 1600 degrees Celsius. The new alloys present proportionately greater machining challenges.

Seco recently assisted with the machining of a new, higher-performance alloy used in static components of a power generation turbine. With the material’s higher heat resistance came increased machining difficulty; only 18 m/min cutting speed could be achieved, compared to a speed of 25-35 m/min typical with Inconel 718 reference material.

Existing tooling wore out after only one turbine segment (320mm cutting length), and the turbine manufacturer sought greater tool life. Seco developed a special cutter based on the Jabro 780 tool geometry, which features a dual-core design that provides increased stability in tough cutting conditions. The tool was applied at the parameters used originally: cutting speed of 18 m/min, feed per tooth of 0.015mm and table feed of 43 mm/min. The new tool machined two turbine segments (640 mm), an increase of 100 percent in tool life. Then, by reducing cutting speed to 16 m/min. and increasing feed per tooth 0.017mm, application engineers were able to further extend tool life to 800 mm (+ 150 percent tool life).

Aerospace components
Because HRSA maintain strength at high temperatures and offer superior creep and corrosion resistance, the alloys make up as much as 50 percent of the weight of a modern aerospace engine.

Applications of ISO-S materials in aerospace turbines are similar to those in turbines used in energy production. In many cases, however, aerospace tolerances are tighter. For example, Seco develops special tools to machine the fir-tree-shaped root profile of turbine blades. Root profile tolerances for some energy applications are in the range of 10 microns, while tolerances for some aerospace profiles are as tight as 0-5 microns (0. – 0.005).

Structural titanium
In addition to application in the low-temperature sections of turbines, titanium’s light weight, and strength is exploited in structural aerospace parts such as landing gears. By nature the landing gear components are massive and strong, but also very heavy when manufactured from standard materials.

Newer, lighter, and stronger titanium alloys used to produce lighter landing gears are more difficult to machine than the titanium alloys previously applied. One such recently developed alloy is titanium 5553, so called because it includes 5 percent aluminium, 5 percent molybdenum, 5 percent vanadium, and 3 percent chromium content. The benefit of titanium 5553 is its high tensile strength: 1160 MPa compared to 910 MPa for Ti6Al4V reference material. The higher tensile strength limits cutting speeds to levels 50 percent lower than those applied with Ti6Al4V.

Stacked alloys
If a single ISO-S material poses machining difficulty, processing two different materials together offers an even greater challenge. Some aerospace applications involve machining components composed of stacks of differing materials. The challenge is to machine the “sandwich” or “hybrid” with adequate chip control and no vibration or burrs.

Seco’s solution for machining an engine mount featuring a titanium 6Al4V/austentic stainless steel stack was application of a carbide Jabro JHP 770 tool specially designed for machining titanium. The tool incorporates differential flute spacing, radial relief, and a specially formed chip space. A through-coolant channel minimises workpiece adhesion and clears chips. In machining the stacked materials, the tool passed first through the stainless steel then through the titanium. The parameters for more difficult-to-machine material (titanium) were applied throughout. In recognition of the alloy’s low thermal conductivity, a moderate cutting speed of 50 m/min was used, with a feed of 0.036mm/rev feed, and a 3mm depth of cut, descending in circular interpolation.