| Summary: | The main attraction of PM steels related to automotive industry is to produce large number of parts of complex geometry with a net shape manufacturing approach achieving very tight dimensional tolerances without the need for secondary operations and using most of the raw material. The associated reduction of process costs makes PM steels very competitive in comparison with alternative processing and shaping methods. For the manufacturing of parts with this technology, it is necessary to know not only the microstructural events that induce the dimensional changes during sintering but also their relationship with the mechanical properties required for the material application. PM is consequently an economical way for obtaining a large number of components with high geometrical complexity in a single pressing operation. In order to avoid the costly set of precision tools needed, the powder blend is commonly pressed using relatively low pressures, which consequently results in attaining green densities between 6.1 - 6.8 g/cm3.
Linked to the dimensional changes this research work aims at gaining a deep understanding of the behavior and the redistribution kinetics of alloying elements during sintering. Special attention has been devoted to the study of the Fe-Cu-C system typically used by the PM industry for the manufacturing of shock absorber parts. For this matter, this study considers not only the influence of the chemical composition of the powder blend (Cu and C, wt%) but also different graphite grades (natural and synthetic) whose dissolution in Fe takes place at different temperatures. For comparison purposes, the Fe powder grade (sponge or atomized) has been another aspect taken into account in this study.
As Cu and Ni are commonly found in alloy systems for high demanding applications, the influence of Ni was also introduced preparing selected powder blends. The results show that the presence of Ni causes important alterations in the behavior of the material, particularly influencing events like Cu swelling and the redistribution of Cu and C during sintering. With these data, the mechanisms involving the interaction between Ni, Cu and C can therefore be determined. Hence, with this thesis it is expected to understand the micromechanisms involved in the microstructural development of steels within the Fe-Cu- Ni-C system to determine the role of these elements in the dimensional changes of the compact and their diffusion paths. To this aim, several factors must be considered both during sintering (temperature and time) and those related to compaction (density, particle size, Cu, Ni and C concentration, Fe base grade, etc.).
Firstly, dilatometry tests were carried out for different Fe-Cu-Ni-C combinations. Green compacts containing Cu, C and Ni within ranges 0.5-3.5%Cu, 0.3-0.9%C and 0, 1, 4, 10 and 28%Ni where die pressed for the sintering experiments. This technique is not used to quantify the exact dimensional change in the compact but to identify the most important features in the alloy system and relate them with the micromechanisms engaged. Moreover, as dilatometry is a helpful method to reveal the key temperatures where micromechanisms take place, then, interrupted sintering experiments for selected specimens have been done at those temperatures showing the microstructural development of the material. Mechanical properties and total dimensional change at the end of the cycle in a tridimensional measuring machine were also evaluated.
Complementarily, another study collaborating with PMG Polmetasa was carried out. PMG Polmetasa assumes the competence relative to marketing, development and manufacturing of the components for shock absorber applications being de biggest European producer of these components. Dimensional variation and quality of the final parts during process is mainly, if not entirely, a consequence of the events that occur at the sintering temperature. In this regard, a study comparing laboratory sintering conditions (extremely controlled conditions, same number of specimens in each cycle and controlled atmospheres with vacuum purges before de beginning of the sintering process, etc.) with industrial conditions (high productivity, changes in furnace mass depending on industrial objectives, etc.) is also performed. Based on the results obtaining in the laboratory, the study has been directly applied in an industrial part, particularly in a shock absorber component, to check them in the industrial scope. The outcome of this analysis is essential to guarantee the robustness of the sintering cycle. Along those lines, sintering window between 1100-1120 °C and within ranges 0 - 15 minutes of isothermal holding has been developed to optimize the process preserving a good relationship between microstructural development, control over dimensional changes and mechanical properties required in the material application.
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