If the surface finishing alone amounted to between 40 to 60% of a subtractively engineered component’s final cost, most would never be made. This is the challenge facing additive manufacturers. While the many advantages of additive manufacturing (AM) are well documented, it is the poor quality of surface finish of completed builds that still makes them unsuitable for some industrial applications. Post-processing, such as CNC machining or linishing of individual surfaces, takes time and imposes significant overheads. This is suppressing AM uptake, despite the real benefits to be had.

The ability to create more intricate and complex shapes, as demanded in aerospace and other high value applications, compounds the problem as modern mass finishing methods are currently optimised to subtractively engineered parts. A dearth of research and information leaves AM manufacturers and mass surface finishing companies with little more than trial and error as a means of addressing the issue. To fill this void, Innovate UK has sponsored detailed research, bringing together a team of experts under the project leadership of Croft Additive Manufacturing (CAM) and that includes Liverpool John Moores University (LJMU), the Manufacturing Technology Centre (MTC) and Fintek.

Integrating AM and finishing

The research had two aims. First, reduce the variability and overall surface roughness of an AM part by optimising the build parameters and so make mass surface finishing more effective and quicker. Second, improve mass surface finishing techniques to suit the increased part complexity. Capturing process informatics from build and finishing, along with mechanical properties measured at key stages have been vital to providing data for developing a process optimisation system (POSY).

As a software tool, POSY is a tangible outcome from the project that is being designed and developed by MTC. This will help AM manufactures to predict and set the best build parameters to achieve near net shape while maintaining tensile strength and also reducing initial surface roughness. In addition, it will help AM part design teams to allow for the tolerances required by further post processing using mass finishing. Meeting these two objectives will enable AM parts to achieve a comparable surface smoothness to a subtractively engineered component.

Meaningful benchmarks

The MTC team was also pivotal in helping to design the experimental project. At the outset, Croft additively manufactured simple test bars in stainless steel 316L, having defined a series of different laser parameters and build orientations. Surface roughness measures for each set of parameters formed the basic data matrix to begin the POSY software development. This process was repeated to create a sizable database.

A set of test bars were also produced and subjected to mechanical testing at LJMU, who also processed them in centrifugal and drag finishing machines. Identical samples were provided to mass finishing specialist Fintek who processed them in a centrifugal machine and also a high energy stream finishing system. Measurements of surface roughness before and after processing, tensile strength and mechanical properties were then supplied to MTC to use in developing the process optimisation system.

In the centrifuge machine, LJMU found that roughness on the AM bars, built layer-by-layer horizontally, vertically and at 45° differed. During finishing, they responded differently with plastic media over time – vertical built bars saw the greatest decrease in surface roughness, followed by the horizontal build and then 45° build as time increased. Drag finishing was more aggressive over the same time scale.

Fintek found highly variable cycle times were necessary to achieve smoothing. Surprisingly, they also discovered that the usual silicon carbide media used in stream finishing was unsuccessful, sometimes resulting in pitting on the part surface due to its grain structure. Like LJMU, they achieved better results using plastic media. Both LJMU and Fintek found that the greatest roughness decrease occurred in the first 20 minutes with increments showing further improvement up to 80 minutes of processing time at between 190 and 250rpm.

These initial studies showed that the rate of material removal in post processing had implications for the initial AM build. The high energy stream finishing performed best in achieving a commercially viable smoothness but it could also rapidly remove material. This would need to be mitigated by designing-in material to be strategically added during the AM build.

With experimental design from MTC to help validate the POSY software, for the next series of tests representing a real-world component of greater complexity, Croft created an AM part comprising a series of flat, curved, inner and outer surfaces. Initial surface roughness measurements and part build parameters were supplied to MTC for inclusion in POSY. Again, CAM manufactured identical sets of test pieces for LJMU and Fintek to process. The results from mechanical testing and low and high energy surface finishing were added to the developing software tool.

To refine the stream finishing, Fintek called on the laboratory facilities of the machine makers, OTEC Präzisionsfinish. With an adjustment of the plastic media, the SF machine proved capable of surface finishing external facets to Ra 0.05µm in just 12 minutes – a time comparable to processing a subtractively engineered component. The smaller internal spaces were challenging to penetrate for current process media types.

Validation of POSY

To validate the effectiveness of the data and POSY, a desired surface finish of an AM part was entered into the software tool which then predicted the required build parameters and orientation necessary to achieve the target. Built using this data, the AM part was tested and the actual surface roughness of the build was within six percent of the software prediction. This shows using POSY significantly reduces the trial and error work often required and produced a part nearer the net shape from the first build. In addition, as the surface finish was already much improved, it would subsequently require less post processing time.

As more data is added to POSY it is expected to become even better at predicting the AM build parameters based on a target surface roughness and the known post processing method. Importantly, stream finishing represents one of the newest forms of mass finishing that is highly adaptable to inline production needs. While internal spaces and channels are still problematic, there is hope in the form of an abrasive flow polishing system being developed by Croft for just this purpose.

The project is already going ‘real world’ with Croft and Fintek taking on complex parts for two end users and are working with them to achieve their required standards.

Louise Geekie, director at Croft Additive Manufacturing, concludes: “This is just the beginning. With the help of Innovate UK in sponsoring this project, we have made a leap forward in understanding the correlation between AM build parameters and surface finishing. And the benefits are more than cost saving - they also come from opening up the areas of AM part applications that are beyond current technology. Further testing of different AM materials and refinements of mass finishing media will provide a richer data set for the development of POSY by MTC - which creates a virtuous circle.”

www.fintek.co.uk