Feasibility Study of Infrared Detection of Defects in Green-State and Sintered PM Compacts

Souheil Benzerrouk and Reinhold Ludwig

Introduction

Active thermography is an established NDE technique that has become the method of choice in many industrial applications that require non-contact access to the parts under test. However, when conducting on-line infrared inspection of powder metallic compacts, complications arise due the generally low emissivity of the metal powders and the thermally noisy environment in manufacturing plants.

In our research at MPI we have developed a patent pending active IR imaging approach for on-line and off-line P/M compact inspection. At present, we explore the suitability of this approach to detect surface and sub-surface defects in pre-sintered, or green-state, P/M compacts. This method can be applied to on-line inspection in a manufacturing environment in a go/no-go fashion to ensure hundred percent quality assurances. Additional off-line tests can be carried out for a statistical quality analysis.

Our IR imaging of sub-surface defects is based on a transient instrumentation approach that relies on an electric control system which synchronizes and monitors the thermal response due to an electrically generated heat source.

Objective

The objective of this research is the development of an IR-based NDE technique that will enable full quality control early in the P/M manufacturing process. To achieve this goal we have designed and tested an apparatus that is capable of detecting surface and sub-surface flaws in green-state compacts. The theoretical approach is based on the concepts of heat flow, i.e. thermal energy that is generated through electric excitation, followed by temperature recording over the compact's exterior surface. Subsequent temperature data acquisition into a computer and processing enables us to gain valuable on-line information regarding part integrity as well as process stability.

The research involved detailed theoretical studies of heat deposition and transport mechanisms in an effort to set general baseline parameters for our testing system. The resulting computational modeling permits an assessment of the limitations inherent in the methodology in terms of resolution and inspection speed.

Currently we are testing complex parts with subsurface defects exposed to induction heating in an effort to achieve high-resolution imaging and real-time processing. Two systems are being completed:

An off-line inspection arrangement that is based on the principles of pulsed thermography whereby the PM specimen is subjected to a heat pulse (or step input) that is synchronized with a detection system, and
An on-line inspection arrangement that relies on convection to exploit the natural cooling of the parts in real time as they leave the compaction press.

Methodology

To meet our research objectives we have devised an approach that includes the theoretical foundation of all physical aspects of the IR inspection methodology and a set of experiments that evaluate the stability and repeatability of the technique in a manufacturing environment. The major tasks identified include:

A theoretical investigation that involves a comprehensive solution for realistic temperature predictions over the part geometry and in the presence of defects. Figure 1 shows an image of an idealized P/M part, and Figure 2 is the corresponding plot of the analytical temperature solution.
A complete finite elements model (FEM) to test the validity of our analytical representation and to perform a sensitivity analysis.
A general, qualitative on-line testing method that can be carried out at various manufacturing sites and that includes complex, multilevel gears. The results shown in Figure 3 were collected with defective two level gears.
The design of a user-friendly software package for the data analysis that will enable the operator a reliable pass/fail feedback. In addition, the software will enable a more comprehensive failure analysis by providing defect features.

Figure 1: Idealized P/M compact with sub-surface defect representation as an embedded point source.

Figure 2: Dynamic temperature rise in K over the compact surface due to an embedded heat source of strength 8 Watts located at x=5cm and y=5cm.

A statistical analysis of monitoring the P/M compacts over a 45-second inspection duration, or 1,350 frames, is shown in Figure 3. Here, a fixed focal observation point in space is chosen for the IR camera; all compacts that move past this observation point will generate a characteristic thermal signature that can be recorded as a time-amplitude response.

Defects can be introduced, for instance, by changing press settings during operation. As Figure 3 clearly demonstrates, the onset and duration of the process modification are detectable.

Figure 3: Temperature (in K) recorded at a fixed spatial location (one spot) over time.

An investigation of the thermal data sequence reported in Figure 3 allows us to conduct detailed analysis, as depicted in Figure 4. Specifically, the separation of the top level of the gear part and the misalignment of the lower, or gear level, can be delineated.

Figure 4: Zoomed-in temperature (in K) recorded at a fixed spot location.

Publications

S. Benzerrouk, R. Ludwig, and D. Apelian, "Active Thermography for the Detection of Defects in Powder Metallurgy Compacts, " in press in 2006 Proceedings of Review of Progress in Quantitative Nondestructive Evaluation Proceedings.
S. Benzerrouk, R. Ludwig, and D. Apelian, "Electrothermal Defect Detection in Powder Metallurgy Compacts", in 2005 Proceedings of Review of Progress in Quantitative Nondestructive Evaluation, Vol. 25, pp. 1201-1208, American Institute of Physics.
S. Benzerrouk, R. Ludwig, and D. Apelian: "Contact-less Active Infrared Imaging System for the Detection of Defects in Green-State P/M Compacts", Proceedings of the 2006 International Conference on Powder Metallurgy & Particulate Materials (PowderMet 2006 -San Diego), published by MPIF, Princeton, NJ, Part 11, pp. 25-42, 2006.
S. Benzerrouk, R. Ludwig, and D. Apelian: "On-Line and Off-Line Infrared Flaw Detection", Proceedings of the 2005 International Conference on Powder Metallurgy & Particulate Materials (PM2TEC 2005 -Montreal), published by MPIF, Princeton, NJ, Part 11, pp. 25-42, 2005.
S. Benzerrouk and R. Ludwig: "Infrared Testing of Green-State P/M Parts", Proceedings of the 2004 International Conference on Powder Metallurgy & Particulate Materials (PM2TEC 2004 -Chicago, Il), published by MPIF, Princeton, NJ, Part 11, pp. 25-42, 2004.

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