Abstract

Highly-stressed rotating components of jet engine compressors are often made from titanium alloys. For welding these components, the electron beam (EB) welding technique is favoured for its advantageous welding properties.

When these components are welded, undesirable gas pores of sizes up to 0.5 mm dia. may be produced, especially when large sections are involved. Generally, these pores can be detected by ultrasonic inspection. In the assessment of ultrasonic inspection data, a major parameter apart from the incidence and spatial distribution of flaws is the detectability of pores, or POD (probability of detection), as a function of pore size.

This investigation was carried out to study the detectability of pores in titanium alloy welds by the ultrasonic immersion technique. High-definition radiography (HDR) was used as a reference method.

To determine the POD within confidence limits, the a-versus-a approach was selected. Principal problems embarrassing this approach proved to be:

(i)	For real pores, the amplitude scatter is remarkably wider than for round bottom reference holes of known size, and
(ii)	the a-versus-a relation for the round bottom holes is not linear on a logarithmic scale, as would be expected, and the POD results vary widely with the decision threshold selected.

Further work on these problems is planned.