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Machining Exotica

Machining Exotica

Machining Exotica

Here’s a look at machining strategies when dealing with exotic metals. Article by Andrei Petrilin, ISCAR.

What are exotic metals, why are they so rare, and how are they machined? To understand this subject, let’s start by defining it.

Mainstream engineering materials are iron-based alloys such as steel, stainless steel, and cast iron. Another group of regularly used materials includes alloys based on nonferrous metals, such as aluminium alloys, brass, and bronze.

In addition, there are exotic types of material that were developed to answer specific demands. These exotic materials feature a dedicated application; they are rare and not commonly used and are generally more expensive to fabricate.

A strict agreed definition of an exotic material does not exist. Many experts refer to them as metals like beryllium, zirconium, and their alloys, ceramics, composites, and superalloys. When considering the use of structural materials, superalloys and composites should be distinguished first. The metalworking industry mainly deals with these types of materials due to several reasons, one of which is machining exotic materials is problematic. Superalloys, or more specifically, high temperature superalloys (HTSA), are intended for operating under a heavy mechanical load in combination with high temperatures. They are largely used in gas turbines and in various valves and petrochemical equipment. The “exoticism” of superalloys is their metallurgical design, which provides high creep resistance to keep strength at high temperatures. According to the main component, HTSA can be divided into three groups: Nickel (Ni)-, Cobalt (Co)-, and Iron (Fe)-based superalloys. A superalloy chemistry, especially in case of Ni- and Co-based HTSA, results in poor machinability. 

Composites are multicomponent materials. When compared with a traditional engineering material, such as steel or Aluminum, composites workpieces are nearer-to-net shape and do not require significant material removal. Nevertheless, the components of a composite have different properties, and when combined, they produce a heterogeneous structure that makes machining problematic. The process of machining composites differs from machining metals and it often looks more like shattering than cutting. High composite abrasiveness can lead to intensive tool dulling, and various performance problems such as a degradation of accuracy or non-repairable machining defects.

The metalworking industry has made significant progress in machining exotic materials. Advanced machining tools and effective machining strategies have already lifted the performance of machining operations to a totally new plane. An impressive leap forward in 3D printing, which may significantly diminish machining operations, looks very promising. But there is one “exception”, which still limits taking full advantage of the considerable increase of machine tool capabilities. This “exception” is the cutting tool. Despite the distinct progress, cutting tools remain the bottleneck for machining efficiency. Hence, the plans of a breakthrough in the productive machining of exotic materials have much to do with the cutting tool.

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