Characterizing aluminum sintering using in-situ NDE

Abstract

INTRODUCTIONThe versatility of powder-metallurgy (P/M) technology in manufacturing complex shapes has enabled an increasing amount of wrought-and-cast production capacity to be converted to P/M technology; however, ultrasonic inspection of green components produced using these techniques remains a challenge and is not generally practiced. 1 Inspecting P/M components in the green state is highly desirable, because the green part can be reclaimed, with only accepted components being subjected to further costly processing. Additional potential benefits are the characterization and greater understanding of the sintering process itself. An increased scientific understanding of the sintering process enables intelligent process control, greater reliability of the production process, and a lower rate of rejection of the finished product. In order to realize these potential benefits, it is necessary, therefore, that accept/reject criteria be based on real-time nondestructive evaluation (NDE) of the green-state components and the sintering process.Most current nondestructive inspection techniques suffer from their inability to perform inspections below the near-surface region, with the exception of x-ray techniques and, to some extent, pulsed eddy current. 2 X-ray inspection requires extensive shielding of production personnel and expensive film and film processing. While the development of pulsed eddy-current techniques has lessened the impact of the skin effect, the penetration depth of the induced eddy currents still remains somewhat of an issue. 2 It is generally known that ultrasonic inspection is capable of making density measurements by recording the change in velocity of the sound waves with changes in porosity. 3,4 However, the use of ultrasonic techniques has been severely limited by the need for a medium to conduct the acoustic wave into the compact, rendering this inspection technique previously unusable for most cases of P/M compact production. 5-7 Although air-coupled transducers eliminate the need for a liquid medium to conduct the acoustic wave, these methods require a density change for characterizing sintering; previously reported work has only examined samples at room temperature and not during sintering. 8 Recent advances in electromagnetic acoustic transducer (EMAT) technology have overcome this limitation by using the natural electrical conductivity of the metal compact to induce and detect eddy currents. A coil carrying an alternating-current signal of the desired frequency is placed near the component to be inspected. The field generated from this coil induces eddy currents at the surface of the component, causing atom movement and generation of an ultrasonic wave. This wave propagates through the component and is detected due to the atom motion and subsequent field generation at a surface near a receiving coil. IN-SITU MEASUREMENT PROCEDURESTwo types of aluminum powder were examined in this work-99.99% pure aluminum powder made at the Ames Laboratory and 99.7% pure-air atomized aluminum. The Ames L...

Determining the role of surfaces and interfaces in the powder metallurgy processing of aluminum alloy powders

“…Typical waveforms taken with an in situ non-destructive evaluation method 13,14 during different stages of sintering of the GARS Al sample after die pressing are shown in Fig. 10.…”