Why it's not always easy to get the right surface finish when printing metal

Dr Jody Muelaner

The full advantages of additive manufacturing are realised when components can be topology optimised with complex lattice features
The full advantages of additive manufacturing are realised when components can be topology optimised with complex lattice features

As additive manufacturing becomes increasingly capable of producing high-quality parts in metal alloys, its use in aerospace applications is growing.

However, surface finish remains relatively poor, with partially fused powder and visible deposition layers. These surface imperfections reduce fracture toughness and fatigue life, while also making inspection difficult.

External surfaces on many parts can be improved by shot peening – a cold finishing process used to produce a compressive residual stress layer. 

However, the full advantages of additive manufacturing are realised when components can be topology optimised with complex lattice features, and such features are, however, often not compatible with shot peening. 

This is restricting the uptake of topology-optimised components, since if they cannot be inspected they cannot be certified. They may also not be able to achieve the required fracture toughness and fatigue life. Improved methods of achieving the required surface finish are therefore required.

Particle size crucial

Although the as-deposited surface finish of powder-bed fusion processes can be acceptable for many applications, issues with inspection and crack initiation remain. The process uses a laser or electron beam to fuse metallic powder in a powder bed, and the surface finish is therefore related to the powder particle size. 

The outer layer may not fully fuse all of the powder, resulting in some partially fused powder on the surface. Surface texture depends on powder particle size, melt parameters, layer thickness and the orientation of surfaces relative to the build plate.

Inspection for surface cracks is often carried out using a liquid penetrant die. When applied to the part, the die is drawn into cracks by capillary action and excess die can then be removed, giving the cracks a high contrast for visual inspection. When the surface is very rough this doesn’t work as the die cannot be removed from the surface and the cracks do not stand out. 

Ultrasonic testing is normally used to identify internal voids or pores. The probe used to send pulses of ultrasound into the part must be in good contact – a smooth surface is, therefore, also required.

Although many materials exhibit either brittle or ductile behaviour, the high-strength aluminium alloys used in aerospace structures often show both types of behaviour. Although these materials have significant ductility, they are also susceptible to fracture, especially owing to low-cycle fatigue. Brittle and fatigue failure is caused by crack growth and anything that can act as a crack initiation site will make this type of failure more likely, reducing fracture toughness and fatigue life.

The surface finish of additive-manufactured parts may need to be improved, both to enable inspection and to improve fracture toughness and fatigue life. The most common way of doing this is shot peening but not all parts are suitable for this. Thin-walled and slender features may be too delicate, while internal features are simply not accessible. 

Abrasive flow machining

Alternative processes are available to improve surface finish, which are compatible with delicate or internal features. The most established is abrasive flow machining (AFM). A viscous, putty-like fluid, containing abrasive particles, is pumped through the part. Hydraulic rams at either end are used to push this thick abrasive fluid back and forward, acting like a file. 

This can be effective for smoothing internal features but the rate of material removal is dependent on the flow rate. Features around flow restrictions are, therefore, abraded more than those in dead spots. It is also possible to use AFM on external features, with the part placed in an enclosure and fluid pumped around it. 

Electropolishing is a newer process related to electrochemical machining; it uses a cathode tool and the part becomes an anode. While electrochemical machining normally requires a liquid electrolyte, the electropolishing process uses solid particles. It can be used to produce a surface finish equivalent to grinding, deburring or polishing.

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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.


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