Efeito da vibração mecanica na estrutura de peças fundidas

Orientador : Trevor William ClyneDissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia de CampinasResumo: Procurou-se analizar a eficiência de um tipo específico de agitação mecânica (vibração senoidal aplicada no sentido vertical do sistema metal-molde), como agente controlador da estrutura bruta de fusão de ligas de alumínio solidificado em es cala de laboratório. Pela variação das condições dinâmicas impostas ao sistema, determinou-se a existência de um campo de utilização efetiva do processo, atuando a agitação mecânica como efetivo agente de refino para o alumínio comercialmente puro. A pesquisa mostra também a influência conjunta da adição de soluto (cobre) e inoculação (Nb) com a agitação mecânica sobre a estrutura de solidificação do alumínio. A vibração mecânica demonstrou em todas as ocasiões ser um excelente refinador da estrutura, provando sua viabilidade técnica agente refinador da estrutura bruta de fusão de alumínio comercialmente puroAbstract: This study concerns the effect of a specific form of mechanical vibration (sinusoidal in a vertical plane) on the solidification structure of aluminium alloys, with a view to refining the grain size and inproving the mechanical properties of the casting. It was found that, in terms of the variables of the process, an optimal field of operation could be defined, corresponding to conditions of effective grain refinement combined with good physical stability and reasonable energy expenditure. The investigation encompassed commercially pure aluminium and Al-Cu alloys and also included the effect of simultaneous inoculation (with niobium). Observed results were correlated with mechanisms of refinement, and the viability of the process was considered in practical termsMestradoMestre em Engenharia Mecânic

Effect of Refrigerant Gases (HFC134a and R600a) on the Tribological Behaviour of a Multifunctional DLC Coating

Multifunctional DLC coatings have been extensively studied and recognized as a promising solution to avoid wear and friction problems. However, the effect of the environment on the tribological behaviour, in particular in the refrigeration industry, is little studied yet. The present work aims to study the influence of refrigerant gases (HFC134a and R600a) on the tribological behaviour of multifunctional CrN-Si rich DLC coatings, in particular on the tribo-chemical reactions. Tribological behaviour was evaluated using constant and incremental load reciprocating sliding tests. The constant load tests were conducted to access the friction coefficient and wear rates of specimen and counter-bodies whereas the incremental load tests (increments of 2 N at 15 min. intervals) allowed the assessment of the scuffing resistance of the coatings. The scuffing resistance was defined as the work (N.m) at which the value of the friction coefficient first rose above 0.20 (lubricity effect). The characterization of wear scars (tribolayers) was performed by optical interferometry, energy dispersive X-ray spectrometry associated with scanning electron microscopy, and micro Raman spectroscopy. It was verified a strong influence of the atmosphere via the formation of a tribolayer at the interface between body and counter body. This influence resulted in some disturbance of friction coefficient and contact resistance values. Tests performed in R600a atmosphere presented lower (36%) friction coefficient and lower (40%) scuffing resistance (surface durability) than those performed in HFC134a refrigerant gas. The observed differences were justified in terms of the tribo-chemical reaction between the multifunctional coating, the counter body and the refrigerant gases. Both results presented traces of Oxygen, but in the tests with HFC134a gas it was observed the presence of Fluorinated compounds in the tribo-layer. In the tests performed with R600a gas atmosphere it was found only Carbon and Silicon, indicating different interactions of the environment. It is worth noting that these results can serve as a guide to the application: the low friction induced by the presence of R600a recommend it for applications where sustainability and energy saving are required while the use of HFC134a is recommended for applications requiring increased reliability (longer life)

Tribological behaviour of sintered iron based self-lubricating composites

Abstract This work is a review of previous works, presenting and discussing the most important results obtained by an ongoing research program towards the development of innovative, low-cost, self-lubricating composites with a low friction coefficient and high mechanical strength and wear resistance. Special emphasis is given to uniaxial die pressing of solid lubricant particles mixed with matrix powders and to metal injection moulding associated with in situ generation of solid lubricant particles. Initially, a microstructural model/processing route (powder injection moulding followed by plasma-assisted debinding and sintering) produced a homogeneous dispersion of in situ generated solid lubricant particles. Micrometric nodules of graphite with diameter smaller than 20 μm were formed, constituting a nanostructured stacking of graphite foils with nanometric thickness. Micro Raman analysis indicated that the graphite nodules were composed of turbostratic 2D graphite having highly misaligned graphene planes separated by large interlamellae distance. Large interplanar distance between the graphene foils and misalignment of these foils were confirmed by transmission electron microscopy and were, probably, the origin of the outstandingly low dry friction coefficient (0.04). The effect of sintering temperature, precursor content, metallic matrix composition and surface finish is also reported. Furthermore, the influence of a double-pressing/double-sintering (DPDS) technique on the tribological performance of self-lubricating uniaxially die-pressed hBN + graphite-Fe-Si-C-Mo composite is also investigated. Moreover, the tribological behaviour of die-pressed Fe-Si-C matrix composites containing 5, 7.5 and 10 wt% solid lubricants (hBN and graphite) added during the mixing step is analysed in terms of mechanical properties and wear mechanisms. Finally, the synergy between solid lubricant particles dispersed in a metallic matrix and fluid lubricants in a cooperative mixed lubrication regime is presented