Defect and Diffusion Forum Vol. 422

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: As part of the project “Thermostat for universal use in (electro) mobility”, an analysis of three currently used thermostats failed during operation was performed. In all three cases, cracks occurred in the brass outer casings. The cracks were open and fractographical observation was performed. The microstructure of the material was evaluated using both the light and scanning electron microscopy. The local composition of the material was determined by EDS microanalysis. Furthermore, hardness profiles were measured. The cracks were predominantly intergranular with a smaller portion of transgranular cleavage. The microstructure was formed by a mixture of α- and β-phase grains and lead particles. In addition to the stress caused by the overpressure of the molten wax, a higher level of residual stresses caused by deformations can be expected. The failure was caused by the mechanism of stress-corrosion cracking. Metal induced embrittlement or/and corrosion fatigue could interact too.

Abstract: A significant number of different metals are present in aluminum alloy scrap and waste. Secondary aluminum cast alloys, made by recycling from scrap and waste, have as the main impurity Fe. Fe reduction is a very economically and technologically expensive process and therefore there is a growing interest in researching such materials. Moreover, the higher content of Fe leads to the formation of brittle Fe-rich phases, leading to faster propagation of fracture in castings. Therefore, this study reflected on secondary aluminum cast alloy with a higher concentration of Fe and research their effect on brittle Fe-rich phase formation (in the needle; plate-like form) and propagation of fracture in the castings. This study confirms the increasing amount of needle Fe-rich phases in the melt with higher content of Fe. The increasing amount of such phases leads to the formation of a large number of cleavage fractures on fracture surfaces. Although the cleavage fracture increased, the experimental results show low changes in the properties of all experimental melts.

Abstract: The article is focused on the analysis of fracture mechanisms of specimens made of austenitic steel, which have been subjected to dynamic tests. Austenitic stainless steels are characterized as high corrosion resistant materials with high bio-tolerance and relatively high strength. They are made by cold working, where plastic deformation occurs and they are deformed especially by slipping and twinning. Deformed regions are characterized by deformation twins and slip deformation. Specimens were used in two states, in the initial state and after chemical-thermal treatment. Dynamic tests to which specimens were subjected were the impact test and the three-point bending test. Fracture areas were evaluated by scanning electron microscope.

Abstract: Polycrystalline cast nickel-based superalloy IN738LC is employed for critical parts of gas turbine components in the power industry, aircraft engines, and the marine sector. These components undergo severe degradation by low cycle fatigue caused by thermal gradients, particularly during start-up and shut-down periods. The present work reports the cyclic deformation behaviour and fatigue damage of IN738LC during high-temperature isothermal fatigue. Cylindrical specimens were cyclically deformed under strain control with constant total strain amplitude in symmetrical cycling at 800 °C and 950 °C in air. The microstructure is typical of coarse dendritic grains with carbides, eutectic, and shrinkage pores. SEM imaging revealed a γ matrix with coherent L1₂ γ′ precipitates with bimodal morphology. Cyclic hardening/softening curves, cyclic stress-strain response, and fatigue life diagrams were determined. An increase in testing temperature is associated with a significant decrease in stress amplitude and an increase in plastic strain amplitude. The fatigue life gradually decreases with increasing temperature. The fracture surfaces and polished sections parallel to the specimen axis were examined to study damage mechanisms in cyclic loading at high temperatures.

Abstract: The metallography was used for evaluation of fractured elements from different aluminium alloys´ elements applied in Czech transmission system. Although the fractured elements were produced from different aluminium alloys (hypereutectic Al-18%Si, AlSi10Mg, Al0,7MgSi) the main reason of fractures was very similar for all of them – the low quality of casting, inhomogeneous structure and composition of matrix. These analyses were performed on fracture surfaces and on reference matrix. Typical defect evident on fractured areas was dark stains. In some cases, the quartz concentration was ca twice into comparison with standards composition and reference matrix.

Abstract: The main goal of this work was to conduct metallographic and fractographic studies, the purpose of which was to determine the cause of electric locomotive axle failure. The padding weld with a depth about 3 mm in the surface of the axle during fractographic observations was revealed. In addition, several sharp cracks inside of this weld were noticed. According to the fractographic analysis these cracks were the direct cause of the gradual spread of fatigue failure.

Abstract: The absorbed impact energy KV and the dynamic fracture toughness K_Id of the low-alloyed steel OCHN3MFA were measured in the respective temperature ranges 〈–40, 90〉 and 〈–60, –20〉 °C. The values of impact energy in the range of 〈14.1, 21.3〉 J were obtained using Charpy V-notch samples after subtracting parasitic energies (friction in bearings, etc.). All these values corresponded to an extended transition region since the temperature –40 °C was still higher than the lower shelf impact-energy (shear lips present) and the temperature 90 °C was lower than the upper shelf (only 20 % of ductile morphology). Dynamic fracture toughness was determined using Charpy V‑notch samples with fatigue pre-cracks. The related K_Id-values were obtained in the range 〈55.8, 77.5〉 MPa.m^1/2. They were found to be valid linear-elastic fracture toughness and were somewhat higher than the previously reported static K_Ic-values. The study revealed that OCHN3MFA steel has sufficient resistance to dynamic fracture, particularly at low temperatures.

Abstract: The paper deals with results of long-term stress rupture tests on „cross-weld“ specimens made of HR3C–P92 heterogeneous welds. Stress rupture tests were carried out in air at temperatures of 625 and 650 °C up to ca 20 000 h to rupture. Creep rupture strength values of HR3C–P92 welds for 10⁴ h at both 625 and 650 °C were calculated. The preferential location of failure was the intercritical part of the heat affected zone in P92 steel. Local changes of hardness during creep exposure were evaluated by hardness profiles across the welds. Metallographic studies were performed in individual parts of heterogeneous welds. A special attention was paid to precipitation reactions in both base materials and heat affected zones.