Surface Finish

AM surfaces tend to be rougher compared to conventional processes. Rougher surface finishes reduce fatigue strength compared to polished samples. Additive parts can be machined, ground, honed or polished to enhance the surface roughness, measured as Ra, or other surface finish attributes. Isotropic superfinishing processes might allow surface finish improvements without alterations to the geometry of the additive manufactured-parts. Extrusion honing or abrasive flow machining could be used to refine the surfaces of internal channels or hollows.

In the NASA technical report “Additive Manufacturing Overview: Propulsion Applications, Design for and Lessons Learned” by Kristin Morgan, engineering project manager from the NASA Marshall Space Flight Center, the fatigue performance of selective laser melted 718 nickel-based alloy (UNS N07718) was determined after various post-build surface finish enhancement treatments. Low-stress ground samples were the closest to approach the properties of the MMPDS design values for NO7718.

Heat Build-Up and Oxidation

As successive layers are deposited, heat can build-up within an additive part that could lead to grain or microstructure coarsening. The EBM process can take 5 to 80 hours to cool below 100° C after layer melting is completed, depending on part size and geometry, so an additive manufactured-part may experience a significant amount of annealing and recrystallization within the AM process chamber.

Heating certain metal powders or parts in an air atmosphere can result in oxidation or oxide scale formation, so the melting processes (LM, EBD, DED) use inert or vacuum atmospheres. If the atmosphere is not controlled within the metal deposition chamber, then oxidation and contamination of the deposited metal can occur, which can embrittle alloys like titanium. Oxidation can also result in brittle oxide inclusions, which introduces a surface where cracks can initiate. Aircraft grade alloys are often vacuumed arc remelted (VAR) to produce a cleaner, more uniform alloy product with the superior properties required for critical service applications. Current AM equipment from some suppliers makes the fabrication of parts from oxidation-prone materials difficult. In many systems, the metal powder is frequently loaded in open air. 

Residual Stress and Cracking

Residual stress and cracking are major problems in 3D-printed metal part manufacturing. Casting, welding, cold forming and machining processes induce residual stresses sometimes resulting in cracked components. Residual tensile stresses can cause warping or distortions and decrease fatigue strength. Stress relief heat treatments can be applied to parts to remove the residual stress, but part distortion and cracking can occur during this process. Mechanical peening, laser peening and ultrasonic peening can impart residual compressive surface stress in a part, which enhances fatigue properties.

High residual tensile stresses can cause cracks in components. Segregation, liquation and shrinkage can occur during AM with melting and solidification steps. Liquation occurs because the lower melting constituents in an alloy solidify first, separating out during solidification. Upon reheating, these liquated regions can cause liquation cracks, usually in the partially melted zone (PMZ) outside the weld pool.

Shrinkage from the liquid to solid volume change can cause solidification cracks, usually in the center of a weld or casting. Liquation and solidification cracking are more likely to occur in weld- or plasma arc-based additive processes where a hotter and larger melt pool heating forms.

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.