How to Future-proof and Qualify Legacy Parts Redesigned for AM Materials and Processes

How to Future-proof and Qualify Legacy Parts Redesigned for AM Materials and Processes

By Greg Vialle 

So now you’ve figured out that the best way out of your technical debt on a legacy aerospace product is to future proof the design for additive manufacturing (AM).  How do you recertify those parts, now that the manufacturing process has changed? If you’re still determining whether AM is the right way to go, please read this article Legacy Part Refresh: When tool life end, market demand, and technical debt mean you need to relook old parts in a new light, but assuming you’ve done that and need to work on the next steps, here they are: 

Step 1: Optimize the part design for AM 

Chances are good that your legacy part was designed for a forming technique (that worn blank/tooling is why you’re refreshing these parts, right?)   There are three levels of redesign[1] depending on how much you want to commit to future-proofing: 

Direct part replacement – Used when absolutely no change to the part is allowed, and the part must be reproduced as closely as possible to the original part. The main use for this approach is in low volume spare parts when lead-time alone is important enough to justify the use of AM. While less engineering intensive up front, direct part replacementit can lead to manufacturability issues down the road (cost and yield). 

Adapt for AM – Changes are made to the form of the part, often internally, to make the part easier to manufacture through AM. The external shape of the part might also be changed, but its use and function, and how it fits into the product does not.  

Design for AM – The entire part is redesigned to maximize the benefits of AM, and for how the part will be printed. Here we reconsider how the part fits in within its surrounding product and what and how it performs it function and one attempts to improve this. Part consolidation can be a major advantage of going this route. 

In an ideal world you’d redesign it for any manufacturing technology, and qualify it for the ones you actually use for manufacturing, as needed.  Practical design though means you have to commit early, and the basis for modern design for manufacturing (DfAM) is to know your method before the first mouse click of CAD.  If you need fractional engineering help with this, Nihilo has you covered. 

Step 2: Optimize the Design of Experiment (DoE) 

The good news is that part design variants in AM don’t carry the tooling commitment of the traditional forming technology you are used to, so you can afford to prototype and test multiple design alternatives. This is your opportunity to do a head-to-head comparison of design alternatives. Experimentally validating different design approaches opens up options, and helps you understand the design limits.  You’ll also want to do enough test articles to capture manufacturing variability.  Manufacturing variability is inevitable; the more you can account for in your qualification, the more options you have for manufacturing it.  For example, with DED extrusion, these are the top print parameters to deliberately vary at this stage: 

  • Infill specification, both type and percentage. 
  • Part orientation around z-axis   
  • Minimum outer wall thickness  

Be sure to get enough test articles to have statistical relevance in your qualification. With AM you can prototype and test the variants simultaneously rather than subsequently.  This not only allows you to streamline the schedule, it,  in many cases, reduces environmental/reliability testing costs through test consolidation. Don’t do in serial what you can do it in parallel; time is money. Another trick (especially useful down the road) is to also manufacture and test witness coupons to which you can correlate performance metrics (e.g., tensile strength, fatigue,etc) without needing to print full product parts in production for destructive testing. 

If you need help designing the reliability testing to collect qualification data, Nihilo can help. We have years of experience in MIL-STD testing, accelerated life testing, and six sigma design of experiment. Whomever you choose to proto/manufacture your parts, make sure they can document the process sufficiently to reproduce it.   

Step 3: Let the Qual Data Guide Process Specification 

If you did step 2 properly, you now know which design variants work best and how the range of each AM parameter affects quality, performance, and manufacturing cost. Ideally, you’ll even know how the different parameters interact, but if not, don’t feel bad- this often requires a second round of DoE/ testing.   

Once you can include process parameters in your part specification, your parts are nearly qualified.   

In the aviation (FAA) world, you are now ready to apply for an STC (Supplemental Type Certificate). In the space (NASA/ESA) world, it’s time for a MRR (Manufacturing Readiness Review) or MRB (Material Review Board). 

With that final approval, publish the process spec as either a supplemental document to the drawing and CAD file, or in the notes of the mechanical drawing.  

Step 4: Quality Process 

The last step ensures that your process controls are working. Those witness samples from Step 2 are a good way to make sure each batch is in control- you can either store them for later use or do destructive testing on them immediately.  Depending on the customer requirements (and step 3 qual data), you may need to control for porosity. Witness sample cross sectioning and x-ray are two ways of doing that. 

For AS9100 compliance, you’ll likely need a first article inspection (FAI) for each order, unless the process (i.e., GCODE file), raw material lot, and machine are identical from the last FAI. Best practice for 3DP manufacturing is to provide the GCODE filename/version on the CofC (Certificate of Conformance) along with the machine used.  Material should be covered on the CofA (Certificate of Analysis).  These are things to check for in your vendor approval process. 

Conclusion 

So there you have the 4 steps to qualification!  It would be disingenuous to claim they are all easy, but I hope it helps give you some idea of what’s involved. Nihilo is happy to help you on any, or all of the steps to get your legacy aerospace parts future-proofed, qualified, and delivered.

REFERENCES: 

[1] Diegel, Nordin, Motte. A Practical Guide to Design for Additive Manufacturing. Springer 2020. https://doi.org/10.1007/978-981-13-8281-9 

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