Influence of the Experimental Setup on the Damping Properties of SLM Lattice Structures

Background: Metal lattice structures obtained through Selective Laser Melting may increase the strength-to-weight ratio of advanced 3D printed parts, as well as their damping properties. Recent experimental results showed that AlSi10Mg and AISI 316L lattices are characterized by higher Rayleigh damping coefficients with respect to the fully dense material. However, some unclear or contradictory results were found, depending on the experimental setup adopted for modal analysis. Objective: In this work the influence of the experimental setup when performing modal analysis on different SLM AISI 316L lattice structures was deeply investigated. The study provides a critical comparison of various experimental modal analysis approaches, allowing to evaluate the influence of external damping sources and material internal damping phenomena. Methods: The dynamic behaviour of SLM AISI 316L specimens incorporating lattice structures was estimated by means of pulse testing and sinusoidal excitation through an electromagnetic shaker. The validity of the viscous damping model was assessed by means of sinusoidal excitation with different levels of vibration velocity. Moreover, the influence of experimental setup on modal analysis results was critically evaluated, by considering different actuators, contact and non-contact sensors and boundary/clamping conditions. Results: The classical viscous damping model describes with good approximation the damping properties of SLM lattice structures. When exciting single specimens in free-free conditions, those embedding lattice structure and unmelted metal powder filler were characterized by superior internal damping properties with respect to the specimens incorporating the lattice structure without any filler, which was however more effective than the full density equivalent material. Most of the other experimental setups introduced additional external damping sources, that could alter this important outcome. Conclusions: SLM lattice structures embedded into 3D printed components provide superior damping properties against mechanical and acoustic vibrations and the metal powder filler does significantly enhance such damping capacity. A correct estimation of material internal damping was achieved by applying non-contact sensors and free-free boundary conditions, whereas other experimental setups were partly inadequate

Upgraded Kalman filtering of cutting forces in milling

Advanced piezoelectric dynamometers with a wide frequency bandwidth are required for cutting force measurement in high-speed milling and micromilling applications. In many applications, the signal bandwidth is limited by the dynamic response of the mechanical system, thus compensation techniques are necessary. The most effective compensation techniques for a full 3D force correction require an accurate and complex identification phase. Extended Kalman filtering is a better alternative for input force estimation in the presence of unknown dynamic disturbances. The maximum bandwidth that can be currently achievable by Kalman filtering is approximately 2 kHz, due to crosstalk disturbances and complex dynamometer\u2019s dynamics. In this work, a novel upgraded Kalman filter based on a more general model of dynamometer dynamics is conceived, by also taking into account the influence of the force application point. By so doing, it was possible to extend the frequency bandwidth of the device up to more than 5 kHz along the main directions and up to more than 3 kHz along the transverse directions, outperforming state-of-the-art methods based on Kalman filtering

Effects of different cross-sections of Body Centered Cubic cells on pressure drop and heat transfer of additively manufactured heat sinks

In many industrial applications, heat loads management requires the design and production of compact heat exchangers which are expected to handle high thermal loads with acceptable pressure losses, while assuring good mechanical performances. These challenging targets can be achieved by filling the cavities where the cool/hot fluid circulates with lattice structures promoting the heat exchange between the fluid and the cavity boundaries. Such lattice structures can be only produced through Additive Manufacturing due to their high geometric complexity. Recent experimental investigations proved the effectiveness of some kinds of lattice structures having a circular cross section. Here the aerothermal behaviour of Body-Centred Cubic (BCC) lattice stagger arrays in a rectangular channel was experimentally investigated by considerably extending the previous studies to higher Reynolds numbers (up to 30′000) and to new types of lattice structures. Specifically, three new BCC structures having a cam-like, drop-like and elliptical cross section were explored in this work and compared against those having circular cross section. All the samples were manufactured by means of Laser Powder Bed Fusion and made from AlSi10Mg. At first, the heat exchangers were comprehensively characterized by means of optical non-destructive methods. Successively they were tested in a dedicated rig by imposing constant heat flux boundary conditions. The characteristics of the transitional or fully turbulent approaching flow to the test section are also reported thanks to dedicated flow field measurements performed by Particle Image Velocimetry. According to the obtained results, the BCC structure with the circular cross section of larger diameter is the most effective in terms of heat transfer, although it is largely penalized by the pressure losses. Similar heat transfer performances were achieved by the tapered cross section of elliptical shape with the advantage of a considerably lower friction factor. Pressure losses resulted almost identical for all the tapered cross sections but lower than those of the circular one having an equal frontal dimension. When considering the thermal performance factor the circular shape becomes unfavourable for Re>20′000, while the elliptical cross section is the best choice to efficiently promote heat transfer up to Re=30′000

Passive chatter suppression of thin-walled parts by means of high-damping lattice structures obtained from selective laser melting

Chatter vibrations arising during machining operations are detrimental for cutting process performance, since they may cause poor surface quality of the machined part and severe damages to machine tool elements. Passive approaches for chatter suppression are based on the integration of special mechanical components with high-damping properties within the machining system. They represent a good solution to this problem thanks to their intrinsic simplicity. Recently, the application of metallic lattice structures inside 3D printed parts obtained from the Selective Laser Melting technology have proven superior damping properties with respect to the same full density material. Here, this idea is further explored by considering the novel configuration where the unmelted powder grains are retained inside the lattice structure by an external shell, acting as a multiplicity of microscopic mechanical dampers. This concept is applied for passive chatter suppression of thin-walled parts that are of particular relevance for industry. Preliminary experimental investigation was first carried out on simple beam-like specimens, and then on thin-walled benchmarks that were identified through modal analysis and tested under real cutting conditions. The main conclusion is that the novel proposed configuration (lattice plus unmelted powder) has higher damping properties with respect to the full density and lattice alternatives. Accordingly, it may be successfully applied for passive chatter suppression in real machining operations

Digital upgrade of a bandsaw machine through an innovative guidance system based on the digital shadow concept

Nowadays, there is an increasing trend towards advanced CNC machine tools having a high level of automation. Nevertheless, manually operated equipment is still playing an important role in many industrial workshops. Operators’ experience is still essential in the perspective of increasing productivity, enhancing product quality, reducing manufacturing costs related to tool wear, waste and maintenance. Thus, even manual operations that are apparently less important in terms of product added value may deserve attention and need to be improved according to the principles of the digital transformation era. This paper introduces a structured approach for design, development and implementation of an operator guidance system for a manual bandsaw machine, based on the digital shadow concept and additional feedback sensors. This provides an actual example of how the digital transformation of a small-scale equipment may improve the manufacturing performance and ergonomics as well

A novel thermo-geometrical model for accurate keyhole porosity prediction in Laser Powder-Bed Fusion

When performing Laser Powder-Bed Fusion, undesired physical phenomena, such as balling, preballing and keyhole, must be avoided in order to achieve high-quality products. To date, keyhole-free process parameters can be identified either using demanding empirical methods or complex numerical simulations, while only a few analytical models can be found in literature for this purpose. In this work, state-of-the-art analytical models for predicting keyhole porosity were summarized and proved to be rather inaccurate because they are only based on thermodynamic principles, whereas they neglect the geometry and both the kinetics and kinematics of the keyhole cavity, which do also influence cavity collapse and porosity formation. Here an innovative physics-based semi-analytical model for predicting the formation of keyhole-related porosities was conceived, in which both thermodynamic and geometrical factors are taken into account. The proposed model was validated by performing single tracks experiments on Ti6Al4V according to a full factorial DoE on laser power and scanning speed. Produced samples were cross-sectioned and analyzed to evaluate keyhole porosity formation. The comparison between experimental data and model predictions confirmed the higher accuracy of the new model with respect to state of the art models. Besides improving the understanding of the keyhole phenomenon, the proposed model may provide a novel, effective and simple tool for fast process parameter optimization in industry

Effects of post-printing heat treatment on microstructure, corrosion and wet wear behavior of CoCrW alloy produced by L-PBF process

CoCr alloys are widely used as human implants because of both their superior corrosion resistance and superior mechanical properties (fatigue, wear resistance, etc.) respect to other metal alloys used in biomedical field. In particular, CoCrW alloys are used mainly to produce dental implants. In this study, the effects of thermal treatment on the corrosion resistance and wet wear resistance of CoCrW alloys produced via Laser-Powder Bed Fusion (L-PBF) were investigated, and the corrosion resistance and wet wear resistance of the L-PBF specimens were compared with those of the specimens obtained after forging. The heat treatment involved the solubilization of the alloy at 1150 °C in an Ar-saturated atmosphere, followed by furnace cooling. A detailed microstructural characterization of the L-PBF specimens was carried out using a light microscope and a scanning electron microscope in both the horizontal and vertical growth directions. Scanning Kelvin probe measurements were performed on the heat-treated specimens obtained by three-dimensional printing and forging. The void contents of the specimens were evaluated using the Archimedes’ method and image analysis. Vickers (HV2) hardness measurements were performed to evaluate the mechanical properties of the specimens. The corrosion properties of the specimens were evaluated by carrying out potentiodynamic tests in two different corrosive media (aqueous solution (9 g/L NaCl) at pH = 2 and 7). The corroded areas of the specimens were then examined using scanning electron microscopy (SEM). Finally, tribological tests were performed using the pin (Ti counter material)-on-flat configuration under dry and wet conditions, using the same corrosive environments as those used in the potentiodynamic tests and two different stroke lengths. The worn samples were characterized using SEM to investigate their wear mechanisms, and a stylus profilometer was used to determine the wear rates of the materials. The experimental results showed that the additively manufactured CoCrW L-PBF alloy had higher corrosion resistance than the wrought material. In addition, the additively manufactured material showed better dry and wet wear performances than the wrought material. Nevertheless, the heat treatment did not affect the properties evaluated in this study

Study of the Effect of L-PBF Technique Temporal Evolution on Microstructure, Surface Texture, and Fatigue Performance of Ti gr. 23 Alloy

Titanium alloys are widely used in various technological fields due to their excellent performance. Since the early stages of the 3D printing concept, these alloys have been intensively used as materials for these processes. In this work, the evolution of the performance of the 3D printing process has been studied by analysing the microstructure and the mechanical properties, fatigue and tensile, of the Ti gr. 23 alloy produced by two different models of Concept Laser M2 Cusing machines (an old model and a more recent one). The process parameters recommended by the manufacturer were adopted for each machine. Both microstructural and surface texture characterisations were carried out to better correlate the differences with the production process technique. For the same purpose, tensile tests and microhardness profiles were obtained, while the dynamic mechanical properties were evaluated by means of fatigue tests aimed at determining the fatigue limit of the material using a staircase approach. The mechanical tests were carried out on specimens with three different orientations with respect to the building platform, using two different SLM techniques. The fatigue behaviour was then analysed by evaluating the fracture surfaces and, in particular, the crack nucleation sites. By comparing the calculated fatigue values with the results of local fatigue calculations, an estimate of the residual stresses near the crack nucleation site was obtained. The results showed that the specimens produced on a newer machine had lower roughness (about 10%), slightly higher ductility, and a higher fatigue limit (10–20 MPa) compared to the specimens produced with the same material but on older equipment

Single tracks data obtained by selective laser melting of Ti6Al4V with a small laser spot diameter

Nowadays, advanced metal components with high geometrical complexity can be 3D printed by using the Selective Laser Melting (SLM) technology. Despite SLM resolution and accuracy are generally limited to some tenths of mm, it should be possible to produce finer and more precise details by applying lasers with a small spot diameter. However, to present date the data collected with small laser spot diameters are poor. In this work, experimental data describing the effects of laser power and scan speed on single track formation when applying a small laser spot diameter of 50 \ub5m on Ti6Al4V powder are reported. SEM images and the extracted geometrical data characterizing the obtained single tracks are provided here, as well as their microstructural analysis and microhardness measurements

Process parameters optimization in fused deposition modeling of polyether ether ketone

Fused Deposition Modeling is increasingly used for producing high-performing, creepresistant, biocompatible, fireproof, highly-stable parts from polyether ether ketone. However, the knowledge on this process is still poor and fragmented, and the lack of relevant data inhibits many applications. In this paper, the effects of the nozzle temperature, nozzle speed and layer thickness on the properties of PEEK processed by Fused Deposition Modeling were investigated by performing indentation, tensile, Scanning Electron Microscope, Computer Tomography and Energy Dispersive X-ray Spectroscopy tests on as-built samples. The outgassing behavior was also analyzed, while the synchrotron radiation was used to characterize the structure of selected samples on a hitherto unexplored scale. The samples morphology was finally used to identify the optimal process window. The results provided new insights on the process and novel data enabling new applications