Non-destructive lock-in thermography of green powder metallurgy component inhomogeneities: A predictive imaging method for manufactured component flaw prevention

Pre-sintered ("green") compressed metal powder components were studied using non-destructive imaging by means of Lock-In Thermography (LIT) for assessing the capability of this methodology to produce quantitative information on crucial material integrity properties through preventive non-destructive evaluation (P-NDE). A major goal of the work was to develop the LIT potential to reduce economic and environmental waste in automotive manufacturing that impacts mechanical strength and integrity, such as density non-uniformity. Using pre-optimized experimental parameters including infrared-camera phase stabilization by means of high sampling rates, highly reproducible quantitative phase image results from LIT were shown to reveal distinct phase shift bimodalities and widespread non-uniformity in an industrial sample which exhibited surface cracks in the green (pre-sintered) state. In juxtaposition, LIT phase images from another green sample without cracks were much more uniform across the entire sample. A regression-fitting program was used to determine the local thermal diffusivity at critical phase-varying locations of the cracked sample in the green state and a positive correlation was found between the thermal diffusivity and the measured phase shift with a lone exception occurring at the exact location of cracks. Local sample porosity measurements in selected regions of interest (ROI) of the sample using concomitant high-resolution optical imaging were introduced and revealed a positive correlation between the experimental LIT phase shift results and porosity which determines local density. The combination of these procedures led to the development of a potential industrial imaging modality for P-NDE as an important new inspection approach. This approach may lead to identifying catastrophic flaws/cracks before the emergence of such flaws and before proceeding to irreversible sintering of manufactured products, thereby allowing the operator to minimize material waste and effectively optimize production costs with the added advantage of developing environmentally friendly manufacturing processes.