Metallurgical Integrity in Metal Additive Manufacturing

Every materials processing or fabrication method has an impact on the material structure and therefore the properties of the processed material.

Additive manufacturing (AM) metallurgy has its own unique set of processing-structure-property relationships, although many aspects involve powder and welding metallurgy. Processing can impact material microstructure (size, shape and orientation of grains or crystals), which will alter the mechanical properties of the metal alloy. The material properties and structure also alter how a material can be processed.

For example, certain alloying additions can make an alloy too brittle for rolling, forging or other wrought processing. Casting, powder metal and additive processes might be the only way to produce certain highly alloyed materials. The chemistry or composition of the alloy can also change. For instance, titanium alloys will pick up oxygen, which will strengthen titanium up to a point. But if oxygen levels become too high, the titanium alloy will be brittle and crack. Powdered metals are also susceptible to contamination by oxygen and nitrogen, depending on the metal alloy. NASA researchers found increased nitrogen levels in nickel superalloys resulted in increased grain sizes in AM parts.

Powder bed electron beam melting (EBM) processes tend to generate lower residual stress levels and less cracking compared to processes using laser melting (LM) powder bed and direct energy deposition (DED) powder or wire feeding, most likely due to slow cooling and in situ aging. DED powder or wire fed AM processes can be used to deposit multi-materials, which could enable parts with tougher cores and wear-resistant surface layers. Wear, mold, die or tooling surfaces can be rebuilt or repaired with DED processes. Most of the processes that employ melting rapidly solidify the metal deposits, which reduces elemental segregation and can aid in developing refined or unique microstructures. However, rapid cooling can cause gas entrapment, delamination, retain undesirable metastable phases and increase residual stress levels. Binder jet deposits are not prone to delamination, but the as-built “green” parts can be delicate and have high porosities until sintered or fired.