De-Lubrication During Sintering of P/M Parts

Mr. Deepak Saha, Prof. Diran Apelian

The primary motive of this work was to direct research towards the development of sensors and controls and thus, mitigate the various problems due to improper de-lubrication. Currently, there exists a myriad of lubricants being used during the process of compaction. They include metallic based lubricants, polymers and non-metallic lubricants. In this work, research was limited in understanding the de-lubrication of EBS (Ethylene Bisstearimide), as, it the most commonly used lubricant in the industry. It has replaced commonly used lubricant due to cleaner burnouts, absence of metallic residue and, cost effectiveness. The entire work was divided into three phases:

Ascertained the most important parameters that affect the kinetics of de-lubrication
Investigated the type of gases released during the decomposition of EBS
Recommended a control strategy

TGA (Thermo-gravimetric analysis) was used in phase I, the results clearly show that the rate of heating is the most important parameter during de-lubrication (Figure 1). Identification of gases was performed using the FTIR (Fourier transform infrared spectroscopy) and DUV (Deep ultraviolet spectroscopy). This constituted the second phase of our experiments. The primary gases identified in Phase II were carbon dioxide and a hydrocarbon (possibly hepta-decane). A reaction mechanism has been proposed for the decomposition of EBS in the presence of moisture. Finally, an empirical model for de-lubrication has been proposed in Phase III. The model is as follows:

a=(Weight fraction at any temperature ‘T’)
W₀=Initial weight of the lubricant
W_t=Weight of the lubricant at any temperature ‘T’
T=Temperature (K)
T_max=Temperature (K) at the point of inflection (Point of Maximum weight loss rate in the theoretical model)
b=Constant depending on the conditions

The model was verified in an industrial furnace. It has been observed that there exists a very good correlation between the proposed empirical model and the experiments performed in Phase II of this study (Figure 2).

This study lays down the following guidelines for the development of future sensors and controls:

The development of future sensors should focus in the detection of CO2 and hepta-decane
Rate of heating determines how fast or slow the lubricant decomposes and finally escapes from the compacted part

The empirical model may be used, as a means to determine the time a part should reside in a furnace for complete lubricant burnout at a given heating rate. The empirical model has been extended to Polywax, Kenolube and Zinc Stearate.

Figure 1: Percentage Contribution of various parameters and their effect on the de-lubrication of Acrawax in PM compacts.

Figure 2: Predicted curve (for Acrawax heated at 10 0C/min) versus FTIR / DUV analysis for same experimental conditions.

Publications from this work:

Deepak Saha and Diran Apelian, "Control strategy for De-lubrication of P/M compacts", International Journal of Powder Metallurgy, May 2002, Vol. 38, No. 3, Pages 71 – 79
Deepak Saha, Marc Baum and Diran Apelian, "Mechanism of de-lubrication during sintering: Reaction kinetics and decomposition stages", Proceedings of the 2001 International Conference on Powder Metallurgy and Particulate Materials, New Orleans, Vol. 5, Page 82 - 94.
Deepak Saha and Diran Apelian, "Control of de-lubrication utilizing a logistic function based empirical model", Proceedings of the 2001 International Conference on Powder Metallurgy and Particulate Materials, New Orleans, Vol. 5, Page 103 – 112.
Deepak Saha and Diran Apelian, "Optimization of De-Lubrication during Sintering", Proceedings of the 2000 International Conference on Powder Metallurgy and Particulate Materials, New York City, Vol. 5, Page 183 – 190

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