NDT / NDT Exclusives

Have Technology Advances Made Nondestructive Testing More Complex?

The manufacturing industry is using additive manufacturing to create large metal parts for aircraft and other complicated shapes.

August 11, 2014

The reality is that it would be pretty hard to say “no” to this question.

To illustrate this point, I have taken a single new process and analyzed it from a nondestructive test examiner’s point of view. When I’m finished, I hope you will understand the additive manufacturing process, be able define what it is, and understand the complexity of inspections.

What is Additive Manufacturing?

The consumerization of 3-D printing has made it possible for anyone to create a three-dimensional solid object of any shape from a digital model. Visit your local office supply or computer store, and you will find 3-D printers on sale.

The more technical term for 3-D printing is Fused Disposition Modeling (FDM), which is an additive manufacturing technology commonly used for modeling, prototyping and production applications. This technology is being tested all over the world, and one example of this is fused extrusion of small thermoplastic parts. During this process, a plastic filament is unwound from a coil and laid down in layers to produce a part.

The manufacturing industry is also using additive manufacturing to create large metal parts for aircraft and other complicated shapes. This helps to reduce the amount of time needed for machining large sections of metal.

Electro Beam Manufacturing

Another additive manufacturing process, used by all the major aerospace industries that have been involved in prototype development, is the electro beam manufacturing process, which uses a computer-aided design/computer-aided manufacturing (CAD/CAM) file to build up a part through the electro beam welding process. During this process, weld metal is applied to the degree necessary to achieve the near-net-shape, then final machining steps finish the part.

With electro beam manufacturing, the current method of machining large parts is to take a sheet of metal plate with all the chemical, mechanical, and grain direction and remove excess metal (a.k.a. hog out) the part to the final dimension.

Some parts are extremely wasteful in time and material to machine. This is mostly due in part to their shape and complexity. The old method would take a solid round stock of metal and machine many hours, or even days to get it to a near-net-shape for final machining. It would take a lot less time to complete the final machining process for this type of part using the electro beam process.

Nondestructive Testing of New Process Parts

Both the FDM and electro beam manufacturing processes create new challenges for NDT.

This is because engineers and designers must take the welding process and visualize it as a flowing foundry, or one bead of liquid metal at a time, to produce large parts. This leads to some tough questions for engineering departments:

  • Is the inspection criteria based on the raw material that is extruded or forged or the welding process?

  • Assuming this replacement part must meet the same chemical, mechanical and environmental conditions, what types of in-service inspections are required to identify the detrimental discontinuities and defects?

  • Will there be new nondestructive test methods to achieve the degree of mitigated risk during manufacturing, in process, final and in-service surveillance inspections?

At the present time, all raw material is subject to nondestructive and mechanical testing, and inspected prior to being processed. This would only be a verification and certification of the filler metal, as well as verification of the weld parameter in-process inspection.

I believe the only way to verify the material properties using these new additive manufacturing processes, including electro beam manufacturing, would be to insert an inspection coupon at the start and finish of the uninterrupted process. I would also recommend a tensile test coupon and fracture toughness coupon.

Now you have seen a demonstration of how an ASNT examiner views these new processes. How do you view them? I would love to hear your thoughts. Contact me at joesorrent@aol.com.

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Charles J. Hellier has been active in the technology of nondestructive testing and related quality and inspection fields since 1957. Here he talks with Quality's managing editor, Michelle Bangert, about the importance of training.
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