Ductility

What is the meaning of ductility?

Ductility is the physical property of the metal which means if we pull the metal it’s going to stretch rather than break. In other words, a material’s ability to undergo significant plastic deformation under tensile stress before rapture is called the ductile properties of the material. The ductile materials are nickel, copper, steel etc.  

The property of metal which explains the breaking of metal without forming stretch under tensile stress is called brittle. It is also called sudden failure. The brittle materials are glass, cast iron etc.

Ductility is an important consideration in engineering and manufacturing, defining a material suitable for certain manufacture and capacity to absorb mechanical overload.

Ductility and malleability are the two physical properties associated with the metal. This shows that there is not much resistance to deformation of the structure but that a large cohesive force holds the structure together. The difference between ductility and malleability is that ductility is the result of the application of tensile stress on metal and malleability is the compressive stress applied to the metal.

Applications of Ductility

  • Metals with high ductility such as gold, copper and steel can be drawn into long, thin wires without breaking. 
  • Gold is the most ductile metal; a wire of about 2 km in length can be drawn from one gram of gold.
  • Steel cables are possible because of the ductility of the alloys being used in them.
  • Ductility may be expressed as percent elongation or percent area reduction from a tensile test.

Ductility is an important factor in allowing a structure to survive extreme loads, such as those due to large pressure changes, earthquakes, and hurricanes, without experiencing a sudden failure or collapse.

The property of a metal which breaks without forming stretch under tensile stress is called brittle. It is also called sudden failure. The brittle materials are glass, cast iron, concrete etc.

Why is it Important in Machining?

You will often come across terms like:

  • Malleable cast iron.
  • Mild steel.
  • Ductile material.
  • Gummy material.
  • Soft stainless steel.

All the above mean that the material we are about to machine is soft. An inexperienced machinist might think that machining soft raw materials is more straightforward; however, it presents several unique challenges.

Chip Control

It isn’t easy to break the chip when machining ductile materials. Chip breaking is achieved when the chip breaker forces the bending of the evolving chip until it breaks. When the material is ductile, the chip bends without breaking, leading to very long chips.

Build-up edge

Built-Up Edge (Nicknamed BUE) is a wear mechanism that can occur on both milling and turning operations. It is caused by the welding of chips to the insert body. 

Best Practices When Machining Ductile Materials

  • Utilize an insert/tool with a sharp and positive cutting edge, preferably ground.
  • Work with polished or post-treated inserts.
  • Use an aggressive/tight chip breaker with a deflector that is very close to the cutting edge.
  • Use high coolant pressure.
  • Raise the cutting speed to generate enough heat. 
  • Adopting some or all the above tips will increase your chances to break the chips and decrease or postpone the forming of the build-up edge.