Laser Doppler in Green Compact Density Inspection of Powder Metallurgy

Abstract

The density of green compact in powder metallurgy (PM) is a critical characteristic to determine the strength and tenacity of the product. Due to the fragility and sensitivity to the humidity of green compact, detection method triggers by knocking or immersion cannot be applied. Current inspection method for green compact is to use a densitometer with Archimedes Principle, which has to immerse green compact into water. This action considers a destructive test due to the porosity of the PM product, and the oxidation reaction of metal particles inside products cannot be controlled. Since the current detection method is destructive, most of the inspection on forming process of PM only complies after setup of compaction machine. The next non-destructive test is executed after sintering, such as Acoustic emission testing (AE) and Magnetic Testing (MT), which require a solid object for knocking or liquid immersion. To reduce defective compaction flow into the manufacturing process, a Non-destructive test in forming process is a prerequisite. Laser Doppler Vibrometer (LDV) takes advantage of measuring without damaging the test object and obtaining characteristic spectrum signal from the sample for further analysis, which is preferable for green compact of PM. The LDV test setup included a Polytec PSV-400 scanning laser vibrometer with a crystal resonator attached to the test product to generate reference vibration. Integrate spectrum in the frequency range 0 - 5000 Hz was recorded from 30 points of an annular pattern. Comparison of the spectrum and statistical analysis from defective specimens demonstrate that velocity increases within a specific frequency, which is different from with normal samples. Increases in velocity refer to uneven density distribution with an absence of particles inside product decline. The study approves the possibility of density detection in green compact using an LDV. Further studies aim to construct a relational model of specimen and compaction machine and determine a fundamental database for Advance Process Control (APC) of forming process in PM.