Additive manufacturing (AM) can now produce high-quality parts in metal alloys. It is increasingly being used for structural aerospace applications. This is possible because, unlike with early metal AM processes, parts are now fully dense with few pores and gain structures often equivalent to a forged part. Powder bed fusion (PBF) processes are also able to achieve tight tolerances, while directed energy deposition (DED) wire-fed processes give rapid material deposition rates. However, AM results in relatively poor surface finish. For PBF, the surfaces include partially fused powder and deposition layers may be visible.
Rough surface finish makes inspection difficult and reduces performance in terms of fatigue and fracture toughness. Although surface finish is continuing to improve, surface machining or shot peening are often needed to improve surface finish. Internal surfaces and some small features may be difficult to finish in this way. This often means that topology optimized components cannot be inspected for crack initiation defects, and therefore cannot be certified for aerospace applications.
New methods of improving surface finish, such as abrasive flow machining and electro-chemical milling, may enable internal and delicate features to obtain good surface finish. This would allow intricate topology optimized components to achieve the fracture toughness and inspectability required for aerospace structures, fully realizing the potential of additive manufacturing.
Why Additive Manufacturing Produces Rough Surfaces
Wire-fed DED processes, based on established welding methods, can rapidly and cost-effectively produce large, complex, near net shape components. They key advantage of DED processes is the rapid deposition rate, although this also creates some issues. The speed of deposition means that heat dissipation becomes an issue, meaning post-process heat treatments are often required for stress relief and distortion may occur. The large melt pool also results in weld-bead shaped deposits, meaning the entire surface of DED parts is normally machined. Less invasive surface finish improvement is therefore not relevant to these processes.
PBF processes give an as-deposited surface finish that can be acceptable for many applications. They can also produce fine features with intricate detail. This all means that fully machining the surfaces would not be appropriate. These processes use a laser or electron beam to fuse metallic powder in a powder bed. The surface finish obtained is related to the powder particle size, which typically ranges from 5 to 60 µm. The as-deposited surfaces of PBF components feature partially fused powder. Deposition layers may be visible, although surface roughness measurements are no higher when profiles are taken across the layers than when they are taken along a single layer. The surface texture depends on powder particle size, melt parameters, layer thickness and the orientation of surfaces relative to the build plate.
Issues for Inspection
Rough surface finish makes inspection difficult and reduces performance in terms of fatigue and fracture toughness. Although surface finish is continuing to improve, surface machining or shot peening are often needed to improve surface finish. However, internal surfaces and some small features may be difficult to finish in this way. Liquid penetrant dye is typically used to inspect parts for surface cracks. The dye is applied to the part and is drawn into any cracks by capillary action, excess penetrant is then removed from the surface before visual inspection is carried out. However, if the surface is too rough, the excess dye cannot be removed and the cracks will not become visible.
Inspection for internal voids or pores is typically carried out using active ultrasound testing. This process uses a contact probe to send short pulses of ultrasonic vibration into the part. The time it takes the reflected wave to be returned to the probe is recorded, giving the distance to the nearest free edge of the material. Defects inside the material can be detected since the distance to the free edge will be less than the material thickness. The issue with this type of testing is that the contact probe, which sends the pulses of ultrasound into the part, requires a good contact with the surface. If the surface is too rough, the test will be unreliable.
Reduced Fatigue Life and Fracture Toughness
Materials can fail under static stress or fatigue, and failure under static stress may be due to brittle failure (fracture) or by ductile failure (yielding). Although under normal operating conditions many materials can simply be considered as either brittle or ductile, certain conditions can change this behavior. For example, glass is generally brittle but can be ductile at high temperatures, and steel is generally ductile but can be brittle at very low temperatures or when subjected to hydrogen embrittlement. Significantly for aerospace components, aluminium alloys more commonly exhibit both brittle and ductile behavior. A single application of a large load will typically cause a high-strength aluminium alloy to yield, but such parts are more likely to fracture due to relatively low-cycle fatigue. The actual mechanism for brittle and fatigue failures is crack growth. Small cracks, sharp internal corners, pores, holes and rough surfaces can all act as crack initiation sites. They will, therefore, reduce a material’s fracture toughness and fatigue life.
Post-Processing to Improve Surface Finish
The most established way of improving surface finish is shot peening. This involves bombarding the surface with round metal, glass or ceramic particles. The resulting localized plastic deformation on the surface of the part closes small cracks, smooths the surface and creates a compressive residual stress layer. Although this can be a highly effective way to produce a smooth and hard surface, it is not suited to many of the features that give additive manufacturing its unique advantages. Thin walled and slender features may be too delicate for shot peening, while internal features are completely inaccessible to it.
For internal features, abrasive flow machining (AFM) may instead be used. This involves pumping a fluid containing abrasive particles through the part. The optimum flow characteristics for AFM are typically obtained using a highly viscous putty-like fluid. An AFM machine typically uses hydraulic rams to drive the fluid backward and forward through the part, acting much like a file. Although AFM can be effective at removing burrs and polishing surfaces, the rate of material removal cannot be easily controlled. The most material is removed from areas where flow is the greatest, such as restrictions. AFM may also be used to smooth delicate external features by first placing the part in an enclosure through which the fluid is then pumped.
Electropolishing is a novel process developed by DLyte, which uses solid particles flowing over a part to conduct electricity that removes material from the surface, much like electroplating in reverse. This can be accurately controlled to produce a surface finish equivalent to grinding, deburring or polishing.
Major drivers for the use of additive manufacturing in aerospace components are weight reduction and the ability to create intricate internal details such as cooling channels. AM components have more refined grain structures than cast components, often comparable with wrought components. However, surface imperfections can act as crack initiation sites, meaning the grain structure doesn’t always translate into the expected structural performance. Rough surfaces can also prevent inspection for cracks, a requirement for many aerospace applications. The processes currently used to improve the surface finish, such as shot peening, have significant limitations. Internal features cannot be reached and intricate details may be damaged. This means that, currently, the advantages of topology optimization and internal features often cannot be realized. Processes such as abrasive flow machining and electropolishing have potential.