Improving the Surface Finish of Additive Manufactured Parts

A new chemical immersion treatment could revolutionise the aerospace industry

South West Metal Finishing has been working on an additive manufacturing surface treatment process for the last three years and believes it could be the future choice of aircraft manufacturers around the world, such as the likes of Safran, UTC Aerospace and Airbus.

Figure 1. Left top: AM part resolution at 500 microns. Left bottom: Treated part resolution at 500 microns. Middle: AM part without treatment. Right: AM part treated with new immersion process.

Almbrite™ is a chemical immersion process designed to modify and enhance the surface of additive manufactured (AM) parts by removing foreign object debris whilst smoothing and brightening the surface of a part, as illustrated in Figure 1.

Aerospace and defense manufacturers have been searching for a surface treatment solution since additive manufacturing started to be used. One of the challenges regularly encountered is the poor finish of AM components. They are often rough or porous, with semi-melted powder particles. That can obviously affect the performance of the component, which is detrimental when you’re making an aircraft.

AM uses various techniques to construct a three-dimensional object including direct energy deposition and powder bed fusion processes. AM is a process in which a component is built up in discreet layers by using a high-energy heat source to fuse powders. The processes are driven by data from computer aided designs (CAD) which are then sliced into individual layers. In some cases, fine metal powders are deposited on top of a build platform and the energy beam is used to melt the shape of the design. The build then proceeds with a new layer of metal powder which is then melted, such that the component is built up in a layer by layer fashion.

Figure 2. Improved performance.

This layer manufacturing approach means that more complex parts can be produced compared with traditional processes. One of the benefits of AM for manufacturers is that increased complexity generally doesn’t have a detrimental impact on the cost of the process. Parts treated with the new technology are more cost-effective than machined parts as they can increase in geometric complexity without increasing the cost of build (Figure 2).

AM allows component designers to have greater design freedom, knowing that the end result will be more representative of the final design than is possible with traditional processes.

The use of AM is on the rise in every sector, including medical and automotive, because of the versatility of creating bespoke designs, one-off prototypes, or complex components that cannot be machined. But without the correct finish, these components may fail at the early assessment stage in an industry that tests and re-tests to the breaking point.

The aerospace and defense industry has adopted AM, though it needed time to collate data and carry out stringent tests before it was confident the components could withstand the operating conditions they would be subjected to. Everything had to be tried and tested and then tested again. Now the processes are considered safe enough, they must make sure the finish of these components fulfills the necessary requirements.

The highly skilled team developing this technology knows the testing, time and effort it takes to achieve approval certificates in aerospace and are fully accredited with NADCAP, ISO 9100, ISO9001 and ISO14001, holding approvals for all the major UK tier one suppliers. AM surface treatment is being taken to the next level and many of the issues currently facing those using additive manufacturing in the aerospace industry are being addressed. This innovative surface treatment process greatly improves the finish of components made using additive manufacturing, by chemically removing material from each surface to achieve the final condition required.

Research and development began on the AM treatment project in 2014. It has taken a long time to fully develop, but there was significant demand for this type of post processing. Large aerospace manufacturers using additive manufacturing presented the need for a more refined and enhanced surface finish on their AM parts. Both commercial and technical challenges were overcome before launching Almbrite™ as a production capable finishing solution. This is a fantastic opportunity for the aerospace industry to really push the quality and finish of AM parts being used to build aircraft in the market today and going forward.

Figure 3. No geometric constraints. Sheffield University’s Formula 1 team optimised rocker arms for their vehicle’s suspension system, treated with the new technology for optimum performance.

So, how exactly does it work? The surface treatment process essentially refines the surface of the component by chemically removing material from each surface to achieve a surface roughness of below 3.2 microns, whilst enhancing edge and feature definition.

For reference, metal AM parts tend to have an average roughness between 10 to 30 microns depending upon the AM process used. This means that the technology can reduce the roughness of an AM part by up to 88%. It could also be argued that the innovative surface treatment could almost increase the quality of AM aerospace components ten-fold.

Almbrite™ can enhance surface quality regardless of the complexity of a component’s geometry (Figure 3). This complements AM designed components, which use either traditional or topology optimized approaches, where conventional treatments are unable or too costly to be used.

An AM part’s topology describes the way in which its geometrical properties and measurements are interrelated and arranged. As AM is increasingly adopted by big players in the aerospace and defense industry, the complimentary and innovative treatment similarly has the potential to generate substantial interest among top manufacturers.

Almbrite™ is a chemical immersion process; the immersion bath used during the treatment is changed, or refreshed, depending on throughput. If many AM parts need their surfaces treated in a certain period to a high level of material removal, the bath will need to be replenished regularly. The level of material removal during the ALM surface treatment process is controlled using a combination of process parameters. The immersion times required during the treatment process are the same regardless of component size; however they do vary depending upon materials.

This technology is currently being used to finish components made of titanium alloys whilst applications on polyether ether keytone (PEEK), a thermoplastic polymer used widely in engineering, as well as aluminium alloys are in development. The surface treatment process is also being looked at for application on nickel-based alloys in the future.

Titanium and aluminium alloys are the primary metallics used for manufacturing in the aerospace and defense industry currently. Aircrafts are also made up of a huge range of polymers; high performance polymer PEEK is highly valued in aerospace manufacturing. Nickel-based alloys are primarily used in the engines and mechanical systems of aircraft, and this is where the technology is branching out to in the future.

The material on which Almbrite™ is being used does impact the treatment’s chemical compositions, however, the process requires a chemical reaction to occur when treating either Titanium or PEEK thermoplastic polymers, which removes the unwanted material from the component surface.

Figure 4. Support structure removal

In metal additive manufacturing, support structures are used to help transfer heat away from the part as new fused powder layers are added whilst helping to hold the part’s shape as it forms. Until now, metal AM has lacked an efficient way to remove supports after the build is complete. In fact, supports have often been removed with hand tools – e.g. hammer and chisel – which is a bit primitive considering the advanced technology involved in the aerospace industry.

However, the new surface treatment process dissolves supports used in the AM process, removing the need to machine away or manually remove support structures, as illustrated in Figure 4. This is hugely beneficial as manually removing supports constrains the geometric freedom of the part, restricting the design possibilities of aerospace components.

Figure 5. Max stress tested against cycles to failure comparing machined parts and treated parts’ performance.

Further advantages include improving surface related material properties such as fatigue strength (Figure 5) and fracture toughness, whilst offering a controlled, cost efficient and repeatable treatment. The process can be used for any type of part, but a significant advantage is that it is also suitable for internal surfaces where a high-quality finish can be achieved. The surface treatment technology is currently being used on hydraulic aerospace components such as pumps, gears, pipes and filters whilst development of application on more complex mechanical parts is underway. Additionally, the shiny, bright, aesthetically pleasing finish that is produced means that it is being used on interiors such as gear sticks and dashboards. There is a broad scope of diverse applications in the future of AM surface treatment.

The application possibilities of this innovative surface treatment technology are endless. With high-level skill and precision engineering experience, AM is now being used to produce a vast range of components. These are usually small scale, mechanical parts due to the stage at which AM is at in its technological development. Aerospace manufacturers are looking at building larger structural aircraft parts with AM such as stringers or wing sections as the capabilities expand. It’s key that AM can replace manufacturing processes for components that are already in use, to speed up production time and optimize performance without having to redesign the parts. Whilst AM took its time to be adopted by the aerospace industry due to the strenuous testing involved, Almbrite™ is already being rapidly accepted by aerospace and defense manufacturers.

This article was written by James Bradbury, Lead Researcher, South West Metal Finishing (Exeter, UK). For more information, Click Here .