Characterization of the application behavior of self-lubricating powder pressing tools

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The most frequently used process for shaping of metal powders is the double-sided pressing in dies. The properties of the green compact resulting from this pressing process, such as the green density, are essential for the final density of the sintered component and thus for the mechanical properties, like tensile strength and fatigue strength. In general, green properties are dependent on material- and process-specific factors. For instance, the reduction of friction during pressing has a positive effect on compressibility. Usually, in order to reduce friction the used powders are “lubricated” by adding a lubricant (wax) to the powder in small amounts and subsequently mixing it. An alternative method is the die lubrication, where a thin lubricant is applied to the die wall by means of a nozzle prior to each pressing operation. This leads to various advantages, such as higher green and sintered densities, lower density gradients and the elimination of the dewaxing process. However, the application of this lubrication method is not widely used. The reasons for this are mainly the extra efforts, such as the additional process step, but also problems with the production of complex geometries.A highly promising approach to counteract these disadvantages while at the same time benefiting from the advantages of a die lubrication is the application of self-lubricating coatings. Consequently, the aim of the project is to validate the suitability of self-lubricating coatings in die pressing of metal powders to directly improve the green or indirectly improve the sintered properties.The investigations are carried out with two coatings and a steel powder (with and without lubricant/wax). For this purpose the characterization of the powder and the tool design are carried out. Besides the influence of the compression pressure on the resulting relative density, the dependencies between the different lubrication strategies and the green strengths are determined via pressing investigations. In order to interpret the results, the force profiles are recorded during pressing and correlated with the material characteristics. After that, the green parts are sintered in order to show the influences of the investigated lubrication strategies on the final material properties. For this, micrographs are created and density and hardness distributions are determined. By analyzing the determined characteristic values (density, hardness, pressing and ejection forces) as a function of the lubrication strategy, the potentials of self-lubricating die coatings should be emphasized. Finally, the acquired knowledge should be transferred to complex component geometry, including a comparative study of the material properties.