Browsing by Author "Costa, J.M."
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- Fatigue behaviour of selective laser melting steel componentsPublication . Santos, L.M.S.; Ferreira, J.A.M.; Jesus, J.S.; Costa, J.M.; C. CapelaSelective laser melting (SLM) is a laser based rapid manufacturing technology that builds metal parts layer-by-layer using metal powders and a computer controlled laser. Various important aspects strongly affect the mechanical properties of sintered metal components, such as: porosity, surface roughness, scan speed, layer thickness, and residual stresses. Therefore, properties of SLM manufactured parts must be carefully analysed, particularly under fatigue conditions. The purpose of this work was to study the effect of scan speed, porosity and microstructure on the mechanical properties and fatigue strength of sintered laser samples. Sintered laser parts were manufactured in maraging steel AISI 18Ni300. Fatigue behaviour is related to process parameters, such as: surface residual stresses, microstructure and porosity. The results showed that a very high scan speed (400 or 600 mm/s) causes the appearance of high percentages of porosity and a consequent drastic reduction of tensile strength and stiffness. Fatigue behaviour was assessed in terms of the traditional S-N curves and of the da/dN–DK crack propagation curves. Fatigue life predictions based on Hartman and Schijve’s equation underestimated significantly fatigue lives, particularly for low stress levels. The results of the tests performed at variable amplitude loading were well fitted by Miner’s law.
- Fracture toughness of the heat affected zone on Nd-YAG laser welded jointsPublication . Costa, J.M.; Ferreira, J.M.; Capela, C.; Capela, CarlosLaser deposit welding based on modern Nd-YAG lasers is a new mould repair process with advantages relatively to the traditional methods (Micro-plasma and TIG methods). For moulds steels there are no research studies about the laser deposit welding process effects in respect to fracture toughness, hardness and residual stresses variations in the laser-deposited layer and in the heat affected zone. These variations will have an important influence in the thermal-mechanical fatigue strength of the moulds parts. This paper is concerned on the study of the fracture toughness of transient microstructure regions in Nd-YAG laser welded joints performed in mould steels. Fracture toughness tests were performed in two hot-working tool steels: AISI H13 and AISI P20. Small welded specimens were prepared with U notches and filled with laser welding deposits. The hardness profiles were obtained at the middle cross-section of specimens against distance to the surface in order to identify the microstructures present at the crack tip region. Fracture toughness was evaluated and plotted against the Vickers hardness measured at the fatigue crack tip. Depending of the crack tip depth two very different toughness levels were observed in the H13 steel, while a lower variation of toughness was observed for the P20 steel. The values of fracture toughness observed in each specimen are consistent with the correspondent crack tip microstructure hardness.