Aerospace Materials Light Heavy Many aerospace components used in high and low-pres-
sure sections of a turbine are made from high-nickel super- alloys which may be cast, forged or sintered using powder metallurgy techniques. Tese alloys are notorious for being difficult to machine due to their high strength, corrosion and fatigue resistance, and low thermal conductivity. Te same attributes which improve their engine performance also result in making the alloys more difficult to machine. Many of these components have as much as two-thirds of their original weight removed to produce the finished component, while turning, milling and broaching processes are traditionally utilized to remove most or all of the material.
Tool Failure When machining high-nickel alloys, tool failure primar-
ily occurs due to tool edge breakdown through chipping or plastic deformation, or notching at the depth-of-cut line. Tese materials also have a chemical affinity at high temper- ature to the tool material, resulting in welding of workpiece material to the cutting edge. Te welded material may break free randomly during cutting, taking a small portion of the cutting edge with it, resulting in reduced tool life. Te higher temperatures encountered during ma- chining these materials also lead to the formation of a work-hardened layer on the part surface. On subsequent passes, this hardened layer causes increased tool wear, known as depth-of-cut notching. If this notch gets too large, the tool can fail catastrophically. Tese alloys produce long chips and when the chips are not cleared from the cutting zone, they can lodge between the work and tool thereby breaking the tool. If a component has an interrupted surface, repeated entry and exit of the tool can also cause force and temperature shocking which further reduces tool life, especially in the case of ceramic cutting tools. In combination, these failure modes lead to an unpre- dictable tool life. If a tool should fail on, or close to, final dimension, then
builders typically use conservative values for tool life and operating parameters. Tey also employ lower cutting speeds than conventional high-speed machining, which is detrimen- tal to productivity. Another reason to employ lower cutting speeds is
to preserve the surface integrity of the components. If machining parameters are too aggressive or the tool is left in the cut too long, the cutting temperature will rise and cause the formation of white layer. White layer occurs due to phase transformation resulting from rapid heating and cooling of the work surface, grain refinement due to severe plastic deformation of the surface, and chemical reaction of the work surface with the environment. White layer affects the fatigue life of the parts significantly, thereby reducing the service life of the part.
Wheel Capacity Testing Testing was done at the Norton Higgins Grinding Technol-
Open structure of the TG2 wheel (left) in contrast with the 38A wheel.
ogy Center on IN718 material to demonstrate the capacity of the latest wheel technology combining Norton Vitrium³ bond with the high aspect ratio TG2 grain. One-half inch (12.7-mm) wide slots ¾" (19.1-mm) deep were ground into two stacked 1" (25.4-mm) thick parts. Te removal rate was increased incremen- tally until either visual burn or excessive wheel corner break- down occurred. In the
past, grinding
the part is removed from production and sent for review to determine if final part performance has been compromised. In addition, operators are asked to not index the insert while tak- ing the final pass to avoid leaving marks on the part surface. Consequently, to minimize production interruptions and to minimize risk of damaging these high-value parts, engine
156 Aerospace & Defense Manufacturing 2014
processes have been unable to reach high stock removal rates due to a variety of factors, including insufficient space in the wheel face to accommodate large volumes of material, weak grains that dull and fracture prematurely, or weak bonds that take up a large portion of the wheel volume and release the grains prematurely. In wheels without sufficient space, chips can pack into the face of the wheel and these chips rub on the part surface damaging it due to increased frictional heat. Wheels made with the new Norton Vitrium³ bond coupled with their TG2 grain mark a major step in overcoming limi- tations in wheel construction. Te Norton Vitrium³ bond is able to hold onto the TG2 grain under very high forces while at the same time due to the bond’s chemistry taking up less volume in the wheel. Tis lower bond content, along with the natural porosity of the high aspect ratio TG2 grain, means there is more space to accommodate coolant and a larger volume of chips.
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