Adrian Maguire
Stainless steel complex test shape, before and after.
Before and after: stainless steel complex test shape.
Surface finishing costs can be up to 60% of an additive manufacturing (AM) component. Currently, poor quality of surface finish can make them unsuitable for some industrial uses. Post-processing, such as CNC machining or linishing, is time consuming, often inconsistent and usually costly. This is suppressing AM uptake, despite the potential benefits. To mitigate the scarcity of information, Innovate UK sponsored detailed research, led by Croft Additive Manufacturing with Liverpool John Moores University (LJMU), Manufacturing Technology Centre (MTC) and Fintek.
Integration of AM and finishing
The first aim was to reduce the variability and overall surface roughness of an AM part by optimising the build parameters and make mass finishing more effective and quicker. The second aim was to improve mass surface finishing techniques to suit increased part complexity. Capturing process informatics from build and finishing stages, along with mechanical properties measured at key points, were vital to providing data for developing a new process optimisation system (POSY), designed and developed by MTC to help manufacturers predict the best build parameters to achieve near net shape while maintaining tensile strength and reducing initial surface roughness.
Meaningful benchmarks
To start, 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 development. This process was repeated to create a sizeable database. A set of test bars were also produced for mechanical testing by Liverpool John Moores University, who carried out further surface finishing in a centrifugal disc finishing machine and a drag finishing machine. An identical set of samples went to Fintek, who processed them in a centrifugal machine and a new generation high-energy stream finishing system from OTEC Präzisionsfinish. Measurements of surface roughness before and after processing, tensile strength and mechanical properties were then supplied to MTC.
Using disc finishing, LJMU found that roughness differed depending on if the AM bars were built layer-by-layer horizontally, vertically or at 45 degrees. During the finishing cycle, they responded differently with plastic process media over time – vertical built bars saw the greatest reduction in surface roughness, followed by the horizontal build and then the 45-degree build as the cycle time increased. Drag finishing proved to be more aggressive over the same time.
Fintek found highly variable cycle times were necessary to achieve smoothing. 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 had better results using plastic media. Both LJMU and Fintek found that the greatest roughness decrease occurred in the first 20 minutes with further incremental improvements up to 80 minutes at between 190rpm and 250rpm.
The studies also showed that the rate of material removal had implications for the initial part build; suggesting that for more complex shapes it would be beneficial to design in material to be strategically added to compensate. OTEC's high energy stream finishing performed best in achieving a commercially viable smoothness.
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Complex test part
The next tests represented a real world, complex AM component. With experimental design from MTC to help validate the POSY software, Croft created an AM part comprising flat, curved inner and outer surfaces. Identical test pieces were supplied to LJMU and Fintek. The results from mechanical testing and low and high energy stream finishing were added to POSY.
To refine the stream finishing process, Fintek called on OTEC. With an adjustment of the plastic media, the SF machine was able to surface finish external facets to Ra 0.05µm in 12 minutes – a more commercially acceptable time comparable to subtractive engineering. However, the smaller internal spaces were still 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 build parameters and orientation necessary to achieve the target. The resulting part was tested, and the surface roughness was found to be within 6% of the POSY prediction. This showed the software enabled the creation of a part nearer to net shape from first build, reducing lengthy and costly trial and error work. Significantly, the surface finish of the part was already improved.
As more data is added, POSY is expected to become even better at predicting build parameters based on a target surface roughness and known post-processing method. Importantly, stream finishing is one of the newest forms of mass finishing, highly adaptable to inline production. 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 this purpose.
