Key Engineering Materials Vol. 813

Abstract: The use of additive manufacturing (AM) technique is recently increased due to its ability in producing complex-shaped components. AlSi10Mg alloy is largely employed for AM and, in particular, for selective laser melting (SLM). Hence, the interest on this alloy is growing, together with the studies to control its mechanical performances, which can be increased by microstructure modification. With this focus, low temperature heat treatments to enhance AlSi10Mg mechanical behavior have been proposed in the recent literature. The present work focuses on two post-additive thermal treatments, with temperatures specifically selected for SLMed AlSi10Mg alloy. The study investigates their effect on mechanical performances, with a particular attention to residual stresses. Experimental measurements of residual stresses obtained by an X-ray diffractometer (XRD) are presented, considering samples produced with their main dimension along the in-plane scanning directions (XY configuration) and the Z direction. Different conditions are accounted: 1) as-built; 2) after heat treatment at 244°C for 180 minutes; 3) after heat treatment at 290°C for 45 minutes. Tensile properties are correlated with the measurements of the residual stresses, allowing for a critical discussion and for a deeper insight on the correlation of the mechanical performance with the process parameters and the following thermal treatments.

Abstract: The present investigation aims at studying the effect of carbon nanotubes dispersion on the surface hardness of carbon fiber-reinforced polymer composites, manufactured by means of compression resin transfer molding. The influence of the weight fraction of nanofiller, distributed in the epoxy matrix, on the Vickers hardness values was investigated. Furthermore, the evaluation of carbon nanotubes filtering effect was also taken into account by comparing the hardness values measured at the top and bottom surfaces of the laminate composites. It was observed that the different weight loads affect the surface properties of the nano-composites, both in terms of hardness and filtering effect.

Abstract: Recently, the growing attraction to the development of new eco-sustainable composite materials is driving the research interest toward the replacement of synthetic reinforcing fibres with natural ones and exploiting the inherent recyclability of thermoplastic resins even for uses in which thermosetting matrices are well consolidated (e.g. naval and aeronautical fields). Among the natural fibres, a growing interest of the research is addressed to basalt fibres. Focusing the attention on thermoplastic composites, many experimental findings already available in literature highlight the outstanding mechanical properties of composite materials including basalt fibres and their potentiality concerning glass ones. On the other hand, some issues are related to the surface properties of the bio-laminates: in particular, the wear ones, the flame resistance and the aesthetic appearance have to be improved to extend the application fields of these materials. Aiming to these goals, this paper deals with the study of the deposition ofaluminium coating through cold spray process on polypropylene/basalt fabric composite laminates. The specimens were obtained by film stacking, and compression moulding technology and their performances were studied in terms of low-velocity impact behaviour, considering the influence of the surface modification with the aluminium coating. The results obtained from the reference laminates and the coated ones are compared in terms of impact parameters: the aluminium deposition seems to affect the damage mechanism propagation even if the impact response seems to be similar in both conditions.

Abstract: It was shown that under dry contact conditions, under normal load of to 2 MPa, all coatings demonstrated a significant increase in wear resistance compared to that of the substrate. However, among them, the Mo coating showed the highest wear resistance: ~20 times higher than that of the uncoated steel. That was caused not only by the Mo high microhardness and the lowest initial roughness, but also by the structure of this coating. Meanwhile, the Ti + SiC samples displayed the highest microhardness among investigated coatings. A correlation was established between the microhardness of the coating and the friction coefficient: the larger the microhardness of the coating, the higher is the coefficient of friction. An X-ray analysis of the coatings obtained by ESA on steel with compositions (Ti + Al + C), (Ti + AlN) and (Ti + SiC) revealed phases of titanium carbide, titanium nitride, intermetallic compound AlFe₃, and small amounts of aluminum nitride, silicon dioxide and titanium dioxide. This could explain the high microhardness (from 6.8 up to 13.8 GPa) of the obtained coatings.

Abstract: Landing gear is an aircraft component often subjected to wear, fracture, mechanical failure and erosion, principally caused by impact with sand and other small particles. Erosion wear can cause deformation and material removal with consequent efficiency reduction. Coatings can protect stressed structural part and impede the erosion of the metallic components. This work focus on the investigation of the erosion resistance of two ceramic multilayer coatings, AlSiTiN and AlSiCrN, deposited by Physical Vapour Deposition (PVD) on a high speed steel (H11) usually used for landing gear application. Erosion test were carried out with an erosion machine using alumina particles. Powder was directed to the specimens (coatings and substrate) at nominal impingement angles of 90° and 20° with different impact speed (50, 75, 100 and 125 m/s at 90° and 100, 125, 150 and 175 m/s at 20°), at a nozzle-specimen distance of 10 mm. All the tests were performed for two minutes. Hardness and Young's modulus were obtained by nanoindentation, and adhesion between coating and substrate was evaluated by scratch test. Volume lost was measured with Taylor Hobson profiler while cracking behaviour and microstructure modifications were examined with a scanning electron microscope (SEM). AlSiCrN coating significantly enhanced the erosion resistance of H11 substrate, showing higher resistance also with respect to AlSiTiN coating. Indeed, the coating was not completely removed from the surface neither at 90° nor at 20°. The erosion wear rapidly increased by increasing the impact speed in the case of substrate and AlSiTiN, while such parameter was not significantly influent in the case of AlSiCrN. The results suggest that adhesion should play an important role to explain the highest erosion resistance of AlSiCrN coating. Erosion mechanism was principally driven by the intrinsic brittleness of both ceramic coatings.

Abstract: In the present study, the influence of stationary shoulder friction stir processing (SSFSP) tool wear on the processed surface was investigated. SSFSP is a solid-state process that uses a rotating tool plunged in the material to promote microstructure modification, mainly grain refinement, on the workpiece without melting the material itself. It differs from conventional Friction Stir Processing (FSP) since in the SSFSP the tool shoulder does not rotate and simply slides on the surface. The workpiece processed with SSFSP shows a smoother surface without shoulder marks, and with smaller flash than conventional FSP. The tool wear could slightly change the profile of the shoulder and affects process soundness. SSFSP testing made up of 2880 experiments, each one with a path length of 50 mm, was realized on AA 6082 T6 sheets with 3.0 mm thickness using 1500 rpm tool rotation speed and 300 mm/min welding speed. Optical and confocal microscopy analysis and Vickers micro hardness measurements allowed putting in evidence the characteristics of the processed surfaces and their modifications as a function of tool wear.

Abstract: Heat treatments are widely used in industry to improve the surface behavior of components exposed to external mechanical actions and, therefore, undergoing wear phenomena. If compared to the conventional thermal treatments, induction hardening is an interesting candidate solution when the effect should be limited to the surface without affecting the microstructure and properties at the material core. Furthermore, this solution is appealing considering energy saving and cost reduction. In this process, an intense electric alternating current flows through a conductive coil enveloping the component to be treated. Such current generates a magnetic field and, as a consequence, eddy currents arise in the conductive work-piece providing a heat generation by the Joule effect. Due to this mechanism, the induction heating is characterized by faster heating rates with respect to the heating in a hot furnace. On the other hand, the induction hardening process requires a more challenging control of the operative parameters, namely the current density and frequency and the coil advancing speed. Numerical modeling and simulation are recognized as a very useful tool to predicting the effects induced by a treatment, avoiding undesired insufficient- or over-heating. The present work deals with a finite element numerical approach to the simulation of an induction hardening treatment of a steel component. The model is based on the subsequent solution of two numerical submodels. Firstly, the electro-magnetic field generated by the current flowing through a coil surrounding the processing part is inferred solving the Maxwell governing equations. Then, the magnetic field is used as input load for the subsequent heat transfer transient finite element model. The influence of the current density and frequency as well as other processing parameters on the magnetic and thermal fields is discussed.

Abstract: In the present investigation, friction stir processing (FSP) is carried out with multi pass processing having 100 % overlap zone on the workpiece material of aluminum alloy 6061 with constant FSP parameters and varying multi pass processing conditions. Novel processing concept of multi pass FSP was performed with different rotation directions (such as clock wise and anti-clock wise directions) and processing directions (such as forward, reverse and revert directions). Surface inspection, macrographs and microstructures of the processed regions are evaluated and compared with each other. Multi-pass FSP with 100 % overlapping of two passes caused intense dynamic recrystallization and resulted in reduced grain size. Hardness of processed zone was found increased in case of two pass FSP. Minimum tensile strength was reported with double sided FSP compare to single pass and two pass FSPs. No major variations in tensile strength were reported in case of single pass and two pass FSPs.

Abstract: In this paper, we report the effect of multi-layer cold spray deposition on the residual stress formation in the coating and substrate. A method is proposed to separately measure the thermal and mechanical residual stresses induced in cold spray coating. Fiber Bragg Grating (FBG) sensors were employed for in situ monitoring of the strain evolution during the cold spray of multi-layer coating Al7075-Zn on AZ31B Magnesium substrates. Utilizing the capability of the FBG sensors in recording both thermal and mechanical strain gradients, first the effect of temperature on the substrate was investigated when the sample was only treated under carrier gas temperature. Then, the sensors were employed to evaluate the mechanical strain behavior of substrate during the coating process and cooling. Therefore, the effect of thermal mismatch on inducing mechanical strains was observable during the process. Finally, the interaction between the peening process of cold spray and thermal mismatch after cooling was studied. It is shown that the thermal expansion coefficient (CTE) plays a critical role in residual stress development in the substrate and consequently affects the mechanical properties of the coated sample. Hence, careful selection of layers in multilayer deposition can provide desired residual stress in the coating and substrate.

Abstract: In the scope of this study, quenched and tempered H13 steel samples were subjected to conventional (CN) and low temperature (LTN) gas nitriding in a fluidized bed reactor. Structural examinations revealed that surfaces of the CN sample were covered with about 1-2 µm thick compound layer (CL) with an underlying ~30 µm thick nitrogen diffusion zone (NDZ), while outer surface of the LTN sample consisted of ~25 µm thick NDZ. The surface hardness values were measured as 1320 HV_0.1 for LTN sample and 1220 HV_0.1 for CN sample. Under impact sliding conditions, wear mechanisms of the CN and LTN samples were determined as “oxidation + fatigue” at RT and “plastic deformation” at 600 °C. As a general trend CN sample exhibited better impact sliding wear resistance compared to LTN sample both at RT and 600 °C.