Aerospace materials drive cutting tool innovations.

Aerospace machining is changing rapidly, primarily because the materials used in aerospace engines and structural components are changing. These changes present some big challenges in terms of machining. Before looking at those challenges, though, let's take a brief look at the materials themselves.

Through the years, the need for stronger, tougher, and more corrosion-or oxidation-resistant materials has been the driving force behind many technological advancements. Particularly since widespread use of the jet engine, there has been a demand for materials with outstanding high temperature, mechanical, and chemical properties. Originally, steels and stainless steel alloys were used in aerospace applications. But the drive for hotter, more powerful, and fuel-efficient engines led to the development of "superstainless" alloys or superalloys.

Similarly, the need for a class of strong, lightweight materials led to the development of titanium alloys. Titanium alloys are ideal for aircraft applications because of their outstanding strength-to-density ratios. They are also in high demand in many other industries due to their good corrosion resistance and the ability to incorporate a wide range of mechanical and physical properties.

The term superalloy describes a broad range of nickel, iron, and cobalt-based alloys developed for applications that demand exceptional mechanical and chemical properties at elevated temperatures. Roughly two-thirds of the superalloys produced are consumed by the aerospace industry for the manufacture of power turbines (jet engines) and related components. The remaining third is used in the chemical, medical, and construction industries for applications that require optimal high temperature properties and/or exceptional corrosion or oxidation resistance.

Unfortunately, almost every metallurgical change made to enhance these alloys makes them more difficult to machine. For instance, while these strong and heat-resistant alloys can increase engine life, allowing smaller engines to do the work of larger ones, improving fuel efficiency, and decreasing operating costs, they also take their toll on cutting tools. Their heat resistance leads to higher temperatures at the tool tip, increasing the chance of tool breakage. Similarly, the carbide particles in these alloys significantly increase abrasion, shortening tool life.

The trade-offs

Overall, tools are subjected to tremendous heat, pressure, and abrasion during aerospace machining. As high-temperature strength increases, the alloy can become harder and stiffer at high cutting temperatures, resulting in increased force on the cutting edge. This can cause edge breakdown by chipping or deformation. High cutting temperatures also result because stronger materials generate more heat during chip formation, and have relatively low thermal conductivity. As high-temperature strength, toughness, and ductility increase, it becomes more difficult to break chips, adding to the challenge of chip control.

To make matters worse, as the alloys are heat treated to modify the as-cast or solution-treated properties, abrasive carbide precipitates or other second phase particles often form. In the case of oxide dispersion strengthened (ODS) alloys, relatively large and hard particles are part of the alloying recipe, which can cause rapid wear of the cutting edge.

For the majority of these materials, work hardening takes place rapidly, which means that a hardened surface is created during machining. As a result, depth of cut line notching is often a problem. Work hardening also makes it difficult to maintain precision tolerances, and the metallurgical integrity of the part surface. A damaged part surface can in turn compromise fatigue strength, which is one of the reasons why maintaining the edge condition of the insert is so important when machining superalloys.

Titanium alloys today, intermetallics in future

Titanium alloys began to be widely used during the 1950s, when it became apparent that the requirements of airframe and engine components were exceeding the capabilities of existing lightweight alloys. Time has since justified this interest. With exceptional strength-to-weight ratios, low density, and a high melting point, titanium continues to gain in popularity today. Not only are these alloys used in aircraft applications, but also in many chemical and structural components such as chemical processing equipment, marine equipment, pressure vessels, cryogenic components, and even bicycles.

What should we expect in the future? Machinability will continue to decline as service demands increase. And this is only the beginning. Several difficult-to-machine alloys are now being introduced, including single crystal alloys and directionally solidified eutectics reinforced with long carbide fibers, as well as the new "intermetallics."

New titanium alloys are also being developed, including titanium aluminides. Titanium aluminides have such impressive high temperature properties that they are likely to compete with nickel-based alloys for use in aircraft engines and automotive engine valves.

In addition, new processing techniques such as superplastic forming and advanced powder metallurgy methods are being used on conventional alloys and their derivatives to improve toughness, high temperature properties and fatigue properties. Carboloy Inc., www.rsleads.com/ 503tp-262

Don Graham

Turning products manager

Seco-Carboloy