Papers by Author: Radomila Konečná

Abstract: Aluminum-based alloys are widely used in high-performance structural applications. Therefore, the opportunity to fabricate aluminum components using Laser Powder Bed Fusion (L-PBF) is a matter of great interest. In particular, the Al2024 alloy is extensively used for conventional part production but its processability by L-PBF remains a challenge because of its hot cracking sensitivity upon solidification. The new Reactive Additive Manufacturing (RAM) technology by Elementum 3D enables the production of innovative powders characterized by metal matrix and nanoceramic particles that can be processed using L-PBF. The ceramic nanoparticles of 2 % by weight improves properties and prevents Al2024-RAM2 alloy cracking during solidification. The present study investigates the fatigue performance of Al2024-RAM2 alloy manufactured by L-PBF using an SLM 280 HL equipment with a nominal layer thickness of 60 µm. A set of miniature vertical fatigue specimens were manufactured then underwent to a heat treatment T6. The specimens were tested in the as-built state (i.e., without any surface post-processing) under cyclic plane bending at a load ratio R = 0 at a frequency of 25 Hz. The fatigue performance was determined and compared to that of another Al-alloy produced by L-PBF. Specimens were examined by using optical microscopy and SEM analysis to determine the microstructure. The fracture surfaces of vertical specimens were investigated in the SEM to determine the mechanisms of crack initiation.

Abstract: Challenging structural applications such as customized jet engine parts are increasingly fabricated by Selective Laser Melting (SLM) of Inconel 718 powder. The as-built surface quality of SLM parts is however inferior of the machined version and the fatigue behavior is negatively affected. The as-built fatigue response of SLM Inconel 718 was quantified here using three sets of directional specimens. Since the surface quality is influenced by powder characteristics, process parameters and layer-wise fabrication, fatigue results showed a directional contribution that was interpreted using metallography and fractography.

Abstract: Growth of long fatigue cracks in Ti6Al4V alloy manufactured by direct metal laser sintering (DMLS) was investigated. Two DMLS systems, EOSINT M270 and EOSINT M290, with different process parameters were used for production of CT specimens having three different orientations of crack propagation with respect to the DMLS build direction. The as-built specimens were stress relieved at 740 °C. The fatigue crack growth curve and the threshold values of the stress intensity factor for crack propagation were experimentally determined. It has been found that the chosen DMLS processing parameters and the used stress relieving procedure results in material exhibiting isotropic crack growth behavior, i.e. the crack growth was found to be independent of the DMLS build direction. The fatigue crack growth rates and the threshold values for the crack growth were compared with published results characterizing the as-built material and material after different post processing heat treatments.

Abstract: Direct Metal Laser Sintering (DMLS) is additive manufacturing (AM) process that can produce near net shape parts from metal powders such as titanium alloys. DMLS is a layer by layer additive manufacturing technique based on high power fiber laser that creates solid layers from loose powder material and joins them in an additive manner. The specific DMLS process conditions, lead to a specific and complex microstructure and to mechanical properties that show a degree of directionality. It was found that microstructural characteristics are related to the building process parameters. The aim of this work is to evaluate the fatigue performance of the Ti6Al4V alloy depending on the process parameters, building orientations and post-process heat treatment.

Abstract: Direct Metal Laser Sintering (DMLS) is a complex process where a part is build-up by localized melting of gas atomized powder layers by a concentrated laser beam followed rapid solidification. The microstructure of DMLS produced material is substantially different from that of conventionally manufactured materials, although the ultimate strength is similar. However, yield strength and elongation and especially fatigue behavior may vary considerably according to the process parameters and post fabrication heat treatment because they affect structural heterogeneity, porosity content, residual stresses, and surface conditions. Fatigue tests of DMLS Ti6Al4V alloy are interpreted in the light of a thorough metallographic and fractographic investigation. The fatigue crack initiation for three different cyclic stress directions with respect to build direction is determined by fractography.

Abstract: High cycle fatigue life of Ti6Al4V alloy specimens manufactured by Direct Metal Laser Sintering (DMLS) was experimentally determined. The DMLS fabrication process was characterized by a 400 W laser power and 50 μm layer melted thickness. Post-fabrication heat treatment consisted in stress relieving for 3 h at 720 °C in vacuum with subsequent cooling in argon atmosphere. Fatigue testing of specimens oriented in three different directions with respect to the material build direction was performed with the aim to examine the influence of the layered microstructure on the fatigue behavior. Results of measurement of surface roughness, metallographic examinations of the layered material and fractographic investigation of the fatigue fracture surfaces were employed in the discussion of fatigue crack initiation in DMLS fabricated Ti6Al4V alloy.

Abstract: In the paper fatigue specimens are extracted from different regions of cast aluminum cylinder heads produced by two foundries. A high strength region and a low strength region were identified within the cylinder head and the A356-T6 material locally characterized in terms of microstructure and defect population. High cycle fatigue testing according to a reduced staircase method was performed to determine the local fatigue strength at 10⁷ cycles in the cylinder heads of the two foundries. The implications of the experimental observations are discussed.

Abstract: Eutectic Al-Si alloys are typically used for the production of internal combustion engine pistons. A high-cycle, high-temperature fatigue characterization of AlSi12 alloy performed using specimens extracted from actual pistons is presented and discussed. Fatigue strength at 10⁷ cycles were obtained at test temperatures of 250 °C, 300 °C and 350 °C. The fatigue strength reduction was quantified. The micro structural features were quantified by quantitative metallography and fatigue fracture surfaces inspected to identify the initiation causes.

Abstract: Fatigue life of a cast Ni-base superalloy IN 713LC under combined cycle loading consisting of a superposition of low- and high-cycle fatigue at 800 °C was experimentally determined. No measurable effect of combined cycle was found for studied loading conditions. High scatter of fatigue life related to the initiation of cracks on casting defects was observed. Size of the largest defect in a specimen was predicted by the largest extreme value distribution method. The predicted size was compared with fractographic observation of defects resulting in final fatigue failure.

Abstract: The fatigue strength of cast Al-Si alloys is strongly sensitive to the presence of casting defects. Extensive rotating bending fatigue testing of cast AlSi7Mg alloy at room temperature and 50 Hz is reported showing that shrinkage pores are the critical casting defect. The porosity of selected fatigue specimens extracted from castings is characterized with metallography using different pore sizing criteria. Data are fitted to EVS distributions and used for critical size prediction. Fatigue fracture surfaces are inspected in the SEM and the critical pores originating the fatigue cracks identified and measured according to criteria used in the metallographic inspection.