Meas. Sci. Technol. 11 (2000) 157–166. Printed in the UK PII: S0957-0233(00)08404-6 A new apparatus for non-destructive evaluation of green-state powder metal compacts using the electrical-resistivity method Gene Bogdanov, Reinhold Ludwig and William R Michalson Department of Electrical and Computer Engineering, Worcester Polytechnic Institute, Worcester, MA 01609, USA Received 5 October 1999, in final form and accepted for publication 16 November 1999 Abstract. This paper presents a new apparatus developed for non-destructive evaluation (NDE) of green-state powder metal compacts. A green-state compact is an intermediate step in the powder metallurgy (PM) manufacturing process, which is produced when a metal powder–lubricant mixture is compacted in a press. This compact is subsequently sintered in a furnace to produce the finished product. Non-destructive material testing is most cost effective in the green state because early flaw detection permits early intervention in the manufacturing cycle and thus avoids scrapping large numbers of parts. Unfortunately, traditional NDE methods have largely been unsuccessful when applied to green-state PM compacts. A new instrumentation approach has been developed, whereby direct currents are injected into the green-state compact and an array of spring-loaded needle contacts records the voltage distributions on the surface. The voltage distribution is processed to identify potentially dangerous surface and sub-surface flaws. This paper presents the custom-designed hardware and software developed for current injection, voltage acquisition, pre-amplification and flaw detection. In addition, the testing algorithm and measurement results are discussed. The success of flaw detection using the apparatus is established by using controlled samples, which are PM compacts with dielectric inclusions inserted. Keywords: nondestructive evaluation, green-state powder-metallurgy compacts, instrumentation, array sensor, resistivity testing (Some figures in this article appear in black and white in the printed version.) 1. Introduction 1.1. Powder metallurgy In a powder-metallurgy (PM) manufacturing process, the metal parts are formed by compressing metal powder at high pressure. The resulting ‘green-state’ compacts are then sintered in a furnace to produce the final products [1, 2]. This manufacturing process is fully automated, very fast and efficient. However, the PM manufacturing process is in need of quality assurance, because the occurrence of flaws in the compacts can significantly reduce the output efficiency, adversely affecting cost. The main quality hazard in PM compacts is cracking. Cracks occur mainly during compression and ejection of the green-state compact [3]. Unfortunately, quality assurance in powder metallurgy has been successfully applied only to the finished state [4]. The delay from compaction to quality-assurance inspection can range between hours and days. Therefore, a large number of flawed parts produced during that period may have to be scrapped before the process is corrected. In order to improve this situation, it is desirable that flaws in PM compacts be detected early in the process, preferably in the green state. Despite considerable efforts, traditional non-destructive evaluation (NDE) methods have been largely unsuccessful at detecting flaws in green-state PM parts [5–8]. Ultrasonic testing does not render repeatable results because the green- state PM materials strongly attenuate the elastic waves. Additionally, the individual powder particles randomly scatter the sound waves, further reducing accuracy. Eddy- current testing encounters limited field–medium interaction because the PM material has a very low conductivity compared with that of metals. The random particle distribution complicates the induced eddy current patterns and also degrades the reproducibility of the measurements. X-ray imaging, although it is usable for flaw detection in PM parts, cannot easily detect small near-surface and corner cracks. However, these locations are the preferred sites where in practice most flaws occur [3]. Thermal imaging is hindered by the relatively low thermal conductivity of the green-state compact. Because of these difficulties, classical 0957-0233/00/020157+10$30.00 © 2000 IOP Publishing Ltd 157