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Productivity-oriented process parameters effect of the fatigue strength of SLMed Inconel 718
No full textMetal additive manufacturing, in particular Selective Laser Melting (SLM), emerged as a
technology suitable for the industrial production of structural components featuring
complex geometries, i.e. internal cooling cavities, like the ones typical of the aerospace and
power generation industries. In this field, nickel-based superalloys are used in a wide range
of critical applications, characterized by the occurrence of severe environmental conditions
and elevated stresses.
The SLM process parameters play a fundamental role both in determining the mechanical
performances of the component, in particular the fatigue strength, but are also the key
parameter for increasing the productivity of the process and thus the industrial spread of
the technology.
In the present work, it is presented an experimental investigation, through HCF tests on
plain and notched specimens, fractographic examinations, and metallographic analyses, of
the effects of productivity-oriented process parameters on fatigue strength, surface quality,
and microstructural defects, namely hot tearing cracks and porosities, of the Inconel 718
alloy. The experimental approach, along with a theoretical investigation of the thermal field
produced by the SLM process, will be used to define a feasible region in terms of scan
velocity, laser power, layer thickness, and scan strategy
SLM process parameters effects on the fatigue strength of AMed Inconel 718
No full textSelective Laser Melting (SLM) emerged as a technology suitable for the industrial production of structural components featuring complex geometries. In the field of elevated temperature applications, the possibilities offered by the SLM can be successfully used to produced complex geometries as internal cooling channels or lattice structures, as long as the process doesn’t jeopardize the mechanical properties, in particular the fatigue strength.
The SLM process parameters play a fundamental role in determining the mechanical performances of the component but are also the key parameter for increasing the productivity of the process and thus the industrial spread of the technology.
In the present work, it is presented an experimental assessment of the effects produced by different sets of productivity-oriented SLM process parameters on the Wöehler curves of cylindrical plain specimens. The adopted process parameters were defined on the basis of a previously developed thermal analytical model aimed to predict the melt pool dimensions and shape.
HCF tests were carried out at room temperature in an axial load configuration with a stress ratio of 0.05 and a loading frequency of about 150 Hz, by using a resonant testing machine. Test frequency was monitored to detect the occurrence of crack nucleation and monitor the propagation phase.
In order to understand the causes of the fatigue behavior, metallographic analyses were carried out to investigate the microstructural properties or the presence of internal defects, i.e. porosity and hot tearing cracks, produced by each set of process parameters. The surface quality was also investigated in detail through optical microscope analyses. Fractographic analyses were used to identify the nucleation and crack propagation region, as well as the presence of the defects in proximity to the fracture onset.
The experimental data, along with an analytical model of the thermal field produced by a single scan line, allowed to define a preliminary feasible region for the SLM process on Inconel 718, in terms of scan velocity, laser power, layer thickness, and scan strategy
SLM process parameters effects on the fatigue strength of AMed Inconel 718
No full textSelective Laser Melting (SLM) emerged as a technology suitable for the industrial production of structural components featuring
complex geometries. In the field of elevated temperature applications, the possibilities offered by the SLM can be successfully
used to produced complex geometries as internal cooling channels or lattice structures, as long as the process doesn’t jeopardize
the mechanical properties, in particular the fatigue strength.
The SLM process parameters play a fundamental role in determining the mechanical performances of the component but are also
the key parameter for increasing the productivity of the process and thus the industrial spread of the technology.
In the present work, it is presented an experimental assessment of the effects produced by different sets of productivity-oriented
SLM process parameters on the Wöhler curves of cylindrical plain specimens. The adopted process parameters were defined on
the basis of a previously developed thermal analytical model aimed to predict the melt pool dimensions and shape.
HCF tests were carried out at room temperature in an axial load configuration with a stress ratio of 0.05 and a loading frequency
of about 150 Hz, by using a resonant testing machine. Test frequency was monitored to detect the occurrence of crack nucleation
and monitor the propagation phase.
In order to understand the causes of the fatigue behavior, metallographic analyses were carried out to investigate the microstructural
properties or the presence of internal defects, i.e. porosity and hot tearing cracks, produced by each set of process parameters. The
surface quality was also investigated in detail through optical microscope analyses. Fractographic analyses were used to identify
the nucleation and crack propagation region, as well as the presence of the defects in proximity to the fracture onset.
The experimental data, along with an analytical model of the thermal field produced by a single scan line, allowed to define a
preliminary feasible region for the SLM process on Inconel 718, in terms of scan velocity, laser power, layer thickness, and scan
strategy
HCF assessment of additively manufactured notched specimens in Inconel 718
No full textExploiting the potential of metal Additive Manufacturing (AM) mainly means the ability to produce complex geometries in high-performance materials. It implies that AMed components present several notches (holes, grooves, and shoulders), both on external and internal surfaces. A fatigue assessment of AMed components of industrial interest must deal with notch effects: multiaxial stresses and stress concentration.
Inconel 718 is a widespread material for AMed components used in high-temperature applications, which typically feature complex geometries and notches. Criteria developed for dealing with notches in traditionally manufactured components, such as the Average Strain Energy Density (ASED), can be extended to AMed specimens including the effects introduced by the manufacturing process.
In the present work, it is analyzed the room temperature HCF behavior of as-built notched and smooth cylindrical specimens in the framework of the criteria present in the literature. Three geometries of V-notches, resembling the ones typically present in industrial components and featuring a k_t in the range between 1.5 and 3.0, were considered.
High cycle fatigue tests were carried out at room temperature in an axial load configuration with a stress ratio of 0.05 and a loading frequency of about 150 Hz, by using a resonant machine. It was analyzed the life region between 10^4 and 10^6 cycles.
Fractographic analyses, carried out with a Scanning Electron Microscope (SEM), were used to identify the nucleation and crack propagation region, as well as the presence of the defects in proximity to the fracture onset. Microstructural investigations were also carried out to investigate grain dimension and growth direction, as well as the presence of internal porosity and hot tearing cracks. Optical microscope analyses were finally used to investigate the surface quality in proximity to the fracture nucleation regions
Productivity-oriented SLM process parameters effect on the fatigue strength of Inconel 718
Full text linkThe low productivity of the SLM process is known to be a limiting factor, but speeding up the process can lead to material defects. Two sets of SLM process parameters enhancing its productivity by 50 % were devised and tested in comparison with baseline sets, in terms of material microstructure, porosity, surface roughness, static mechanical properties, and HCF behavior, in the as-built and aged conditions. The as-built surface was investigated. Despite a significant increase in the porosity and surface roughness, the fatigue strength was reduced by 5 %. The Murakami √(area_R) parameter effectively correlates the fatigue strength and surface roughness variations
Productivity-oriented SLM process parameters effect on the fatigue strength of Inconel 718
No full textThe low productivity of the SLM process is known to be a limiting factor, but speeding up the process can lead to material defects. Two sets of SLM process parameters enhancing its productivity by 50 % were devised and tested in comparison with baseline sets, in terms of material microstructure, porosity, surface roughness, static mechanical properties, and HCF behavior, in the as-built and aged conditions. The as-built surface was investigated. Despite a significant increase in the porosity and surface roughness, the fatigue strength was reduced by 5 %. The Murakami areaR parameter effectively correlates the fatigue strength and surface roughness variations
Boundary conditions modeling in the FE hydrogen migration models in high-strength steels
No full textDespite the improved strength and toughness, the susceptibility to the hydrogen-assisted delayed fracture limits the use of the high strength steels in the automotive industry. The component strength strongly depends on the hydrogen distribution, and it can significantly drop due to stress-assisted hydrogen migration and accumulation phenomena. Several hydrogen migration models are published in the literature, and some of them are already implemented in finite element software. Nevertheless, the definition of the proper boundary conditions that can model the hydrogen evaporation from specimen surface are still an open issue.
In the present study it is presented an experimental procedure to assess the hydrogen desorption form a planar specimen extracted from a sheet metal by measuring the hydrogen content via Hot Extraction Method (HEM). Hydrogen content was measured at increasing time intervals on coupons extracted from the same sheet metal specimen, where hydrogen had been introduced by electrochemical charging. The experimental data were analyzed in the framework of the Fick’s migration model, deriving the value of the hydrogen desorption coefficient
Validation of a multi-scale simulation strategy based on the Pointwise Strain Superposition Method
Full text linkThis paper details the experimental validation of a multi-scale simulation strategy that we developed for predicting the stresses and distortions induced by Powder Bed Fusion processes. The strategy comprises a meso-scale model, a macro-scale model, and a scaling method named Pointwise Strain Superposition. The first model evaluates the temperature, stress, and strain fields produced by a single scan line. The scaling method transfers the meso-scale results to the macro-scale model, which is then able to simulate the entire manufacturing process with a reasonable computational cost. The simulation strategy was validated by comparing its results with the stresses and distortions measured on several specimens made of selective laser melted Inconel 718. Stresses were measured through the blind hole drilling method on a cylindrical specimen printed with two different scanning strategies, while distortions were measured on a hollow cylinder and on a cantilever-shaped specimen after removing its supports. In both cases, the simulation showed first- or higher-order accuracy despite the significant uncertainties regarding the input parameters and material properties. This robustness, coupled with its computational efficiency, leads us to believe that our simulation strategy could enhance the process optimization and provide a better understanding of the underlying physical phenomena along with their effects on the manufactured parts
Improved model for the prediction of the residual stress field in autofrettaged cylinders
No full textThe stress-strain curve in a loading-unloading process of high-strength steels can be significantly affected by the previous stress-strain history. In particular, if the material has accumulated plastic strain, the elastic modulus can decrease and the Bauschinger effect can impact the plastic response.
The combined effect of the reduction of the elastic modulus and the Bauschinger effect can affect the residual stress field in autofrettaged cylindrical pressure vessels. In particular, the value of the compressive residual stress at the bore is, at the same time, the one most impacted by the loss of either strength or stiffness and the one that bottle-necks the pressure vessel performances.
An experimental campaign has been performed to accurately measure the constitutive behavior for the high-strength steel of an autofrettaged component. Uniaxial tests were performed on specimens extracted from the above mentioned autofrettaged component. A complete 3-D incremental elastic-plastic constitutive model has been tuned to reproduce the observed behavior. This model, implemented in the ANSYS® commercial Code via User Programmable Features, has then been used to simulate the autofrettage process.
The residual stress field, obtained via the proposed FE simulation, was compared with the distribution measured on the component by means of the slitting method. The comparison was used to assess the validity of the proposed constitutive model and its numerical implementatio
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