Properties > Form & Finish
When we make a component it is not just what it is made of that is important, but how it is made. This is where metals generally excel over other materials as there are many ways to shape and join bits together. The thing to remember that the making of a component can change the properties of the material. This section describes some of the words engineers use to describe how easy it is to make a component from a material. These are also known as the manufacturing properties.
Synthesis is a technical word that means to make the material. Most advanced materials are not simple to make, and require very precise conditions to get the exact microstructure needed to give the material it’s amazing properties. In addition, to get a material to it’s final form before making it into something, may require a number of processes. As engineers, the more difficult a material is to make in the first place the more costly it is going to be. This means how easy a material is to synthesize is important when making jet engines.
When we make a part we call it fabrication. Fabrication can happen by a number of methods such as casting, sculpting (cutting a part out of a solid lump), forging (heating and beating it into shape), extrusion (squeezing it into shape), and lots and lots of other ways. Two terms used often that describe how easy something is to fabricate is machinability and the ability to join materials together. The design of the part also strongly influences how easy a component is to fabricate in a certain material.
When a component is formed it usually undergoes a series of processes where it is cut, ground, polished, drilled, etc. Now the materials we use in aerospace are pretty remarkable, but we still need to be able to make them into the shapes we need. How easy this is, is called the machinability of a material. Thinking about it, it is not only how easy it is to cut or drill a material, but also how that material responds to be cut and drilled.
Not all parts can be made out of one piece of material and so the ability to join bits together is important. The most well known method of joining things together is welding. This is where two bits of material are melted together, often with another material added to help out. This creates a weakness in the component. Other types of joining include brazing, soldering and even simply bolting something together. Each of these methods of joining bits together relies on different mechanical properties and so will work better for some materials than others. Also the design of the component and its environment are big influences on what type of joining is okay to be used. Thinking about a jet engine, drilling holes and bolting together bits is a bad idea simply because making a hole in a component makes it weaker, the aerodynamics are affected, it means more parts and therefore more weight, introduces new points for defects and concentrations of stress/temperature, etc.
Aerospace relies heavily on aerodynamics. A jet engine also experiences a lot of heat. These two things combined means that the finish of a component is very important as imperfections on the surface create points where stress and temperature can concentrate. The finish of a component depends on the material it is made of, how it was made and what other processes has it undergone. The finish of a component can be improved by things like polishing but what an engineer has to consider is the cost of this additional process and given some of the complex geometries of components, if it can be done at all.