A new apparatus for non-destructive evaluation of green-state powder metalcompacts using the electrical-resistivity method

This paper presents a new apparatus developed for non-destructive evaluatio n (NDE) of green-state powder metal compacts. A green-state compact is an i ntermediate 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 gr een state because early flaw detection permits early intervention in the ma nufacturing cycle and thus avoids scrapping large numbers of parts. Unfortu nately, traditional NDE methods have largely been unsuccessful when applied to green-state PM compacts. A new instrumentation approach has been develo ped, 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 potentia lly dangerous surface and sub-surface flaws. This paper presents the custom -designed hardware and software developed for current injection, voltage ac quisition, pre-amplification and flaw detection. In addition, the testing a lgorithm and measurement results are discussed. The success of flaw detecti on using the apparatus is established by using controlled samples, which ar e PM compacts with dielectric inclusions inserted.