Better Defect Detection for Aerospace Castings?

Dec. 18, 2005
A commercialization effort for induced positron annihilation is in its second phase.

New nondestructive-testing technology promises the ability to find defects during the manufacture of aerospace castings, detecting much smaller defects and recognizing damage accumulation much earlier than with traditional NDT technologies.

To commercialize the technology, Positron Systems has been awarded a Phase Two Small Business Innovative Research grant by the National Science Foundation. NSF is funding development of an Induced Positron Annihilation (IPA) system for inspecting titanium- and nickel-alloy castings during manufacturing. Positron is conducting its Phase Two research together with an unnamed manufacturer of castings and forgings for aircraft engines and structural airframes.

“This effort will result in the development of a prototype field-inspection system applicable to a variety of manufacturers and industries,” says Steve Bolen, Positron Systems chairman.

Aerospace is the largest market for titanium products due to the performance characteristics of titanium in critical structural support and high-temperature applications. Those buyers, however, require titanium castings to be manufactured to exacting standards with a low tolerance for defects. Those facts drive the patented IPA technology, which uses a small linear accelerator or similar apparatus to generate a beam that penetrates materials. Positrons created in the process are attracted to nanometer-sized defects in the material. Eventually, the positrons collide with electrons in the material and are annihilated, releasing energy as gamma rays.

The gamma-ray energy spectrum creates a distinct and readable signature of the defects or damage present in the material. Because IPA examines materials at the atomic level, it can detect damage at its earliest stage, from initial manufacture through failure. The technology can detect damage in second-layer materials and may prove useful in determining the remaining useful life of a component.

“The initial commercialization will involve inspecting castings in the manufacturing environment,” says Scott Ritchie, operations and commercial accounts manager for Positron Systems. “Our goal in Phase Two, the validation phase (Phase One involved proving the concept) is to develop a prototype inspection capability for inspecting castings and then, in Phase Three, to commercialize that.”

IPA commercialization may extend beyond just manufacturers of turbine-engine and structural castings.

“If you can inspect in the manufacturing process, you also have the opportunity to take your capability downstream and inspect at engine manufacturers or in field service,” adds Ritchie.

The IPA system under development by Positron and its commercial partner consists primarily of off-the-shelf equipment, such as a linear accelerator and measurement equipment, according to Ritchie. This allows the partners to take advantage of industry-proven technologies.

“Positron annihilation spectroscopy has been around for 40 to 50 years, so the equipment, the science and the understanding of positron behavior in materials are pretty well understood,” says Ritchie. “But, our technology employs a unique way of creating positrons in material, allowing for deeper penetration in the material. Also, it uses equipment that is more portable and compact, allowing it to be taken into a factory or the field.

“The challenge,” he continues, “is not so much in the equipment but in building an informational database of the measurement results and validating those results via other known standards.”

That leads to another benefit: Correlating microstructural anomalies in various alloy materials to mechanical properties could reduce or eliminate inspection steps in the manufacturing of castings.

Positron is focusing on two technologies: surface testing (IPA-S) that is sensitive to a material depth of 3 mm, depending on density, and volumetric testing (IPA-V), sensitive to several inches deep.

Ritchie says the project’s goal is not necessarily to replace existing NDT, but to supplement it by detecting defects that fall below the threshold of those methods. As for when this API technology will reach the marketplace, a date has not been set, but SBIR Phase Two projects may last as long as two years. Phase Three, commercialization, is the next step, and a timetable for that has not been finalized.