Dieter De Baere

Hardware-in-the-loop control of additive manufacturing processes using temperature feedback

Laser-based additive manufacturing is a technology for the production of freeform metallic parts. In order to produce parts with high quality, it is important for the manufacturing processes to be controllable with a high degree of precision. Current additive manufacturing systems attempt to reach this goal by carefully tuning the operational parameters, often in combination with a feedback control system. These systems are based on low order, empirical models of the process, which may limit the performance that can be achieved. This paper introduces a control system based on a high order physical heat conduction model of the melt pool dynamics. The control system serves as a framework which can be applied to many laser material processes in which high precision is required such as laser cladding and selective laser melting. The controller is able to regulate the melt pool size by modulating the laser power using a number of surface temperature measurements as the feedback signal. A hardware-in-the-loop (...

Evaluation of SHM System Produced by Additive Manufacturing via Acoustic Emission and Other NDT Methods

During the last decades, structural health monitoring (SHM) systems are used in order to detect damage in structures. We have developed a novel structural health monitoring approach, the so-called “effective structural health monitoring” (eSHM) system. The current SHM system is incorporated into a metallic structure by means of additive manufacturing (AM) and has the possibility to advance life safety and reduce direct operative costs. It operates based on a network of capillaries that are integrated into an AM structure. The internal pressure of the capillaries is continuously monitored by a pressure sensor. When a crack nucleates and reaches the capillary, the internal pressure changes signifying the existence of the flaw. The main objective of this paper is to evaluate the crack detection capacity of the eSHM system and crack location accuracy by means of various non-destructive testing (NDT) techniques. During this study, detailed acoustic emission (AE) analysis was applied in AM materials for the first time in order to investigate if phenomena like the Kaiser effect and waveform parameters used in conventional metals can offer valuable insight into the damage accumulation of the AM structure as well. Liquid penetrant inspection, eddy current and radiography were also used in order to confirm the fatigue damage and indicate the damage location on un-notched four-point bending AM metallic specimens with an integrated eSHM system. It is shown that the eSHM system in combination with NDT can provide correct information on the damage condition of additive manufactured metals.

Feasibility study on integrated structural health monitoring system produced by metal three-dimensional printing

Numerous structural health monitoring systems have been investigated extensively in order to enhance safety level and reduce direct operational costs. This work demonstrates the feasibility study of a new concept, the effective structural health monitoring system. The effective structural health monitoring system detects cracks using a system of capillaries incorporated into a structure. The structure with the integrated capillaries is produced by additive manufacturing, a process of adding material layer by layer. The first objective of this study is to prove that the developed system has reached technological readiness level 3. In order to prove that, four-point bending specimens with the integrated effective structural health monitoring system were tested after being produced by additive manufacturing, more specifically by laser metal deposition. The second objective of the study is to indicate that during four-point bending fatigue tests, the integrated structural health monitoring system has no influence on the crack initiation behavior. To do so, the specimens were subjected to the so-called step method. We demonstrate that the effective structural health monitoring has reached technological readiness level 3 and that the presence of effective structural health monitoring did not negatively influence the fatigue initiation process. As higher technology readiness levels are required, further investigations are still in progress.

Fatigue of Ti6Al4V Structural Health Monitoring Systems Produced by Selective Laser Melting

Selective laser melting (SLM) is an additive manufacturing (AM) process which is used for producing metallic components. Currently, the integrity of components produced by SLM is in need of improvement due to residual stresses and unknown fracture behavior. Titanium alloys produced by AM are capable candidates for applications in aerospace and industrial fields due to their fracture resistance, fatigue behavior and corrosion resistance. On the other hand, structural health monitoring (SHM) system technologies are promising and requested from the industry. SHM systems can monitor the integrity of a structure and during the last decades the research has primarily been influenced by bionic engineering. In that aspect a new philosophy for SHM has been developed: the so-called effective structural health monitoring (eSHM) system. The current system uses the design freedom provided by AM. The working principle of the system is based on crack detection by means of a network of capillaries that are integrated in a structure. The main objective of this research is to evaluate the functionality of Ti6Al4V produced by the SLM process in the novel SHM system and to confirm that the eSHM system can successfully detect cracks in SLM components. In this study four-point bending fatigue tests on Ti6Al4V SLM specimens with an integrated SHM system were conducted. Fractographic analysis was performed after the final failure, while finite element simulations were used in order to determine the stress distribution in the capillary region and on the component. It was proven that the SHM system does not influence the crack initiation behavior during fatigue. The results highlight the effectiveness of the eSHM on SLM components, which can potentially be used by industrial and aerospace applications.

Modeling of laser beam and powder flow interaction in laser cladding using ray-tracing

Laser cladding, also known as direct metal deposition, is an additive manufacturing technique for the production of freeform metallic parts. In the laser cladding process, a high-power laser beam is directed onto the surface of a solid metallic workpiece while a jet of metallic powder is focused into the beam through a coaxial nozzle. The heating of the workpiece is governed by the laser light that is being absorbed, so that detailed simulations of the laser cladding process require an accurate knowledge of the light intensity pattern that reaches the workpiece after interaction with the powder jet. In the past, several statistical distributions have been proposed for modeling this intensity pattern. However, these require strong simplifications of the powder particle trajectories and do not take into account the complex powder flow profile that is present in practical systems. In this paper, the effect of the powder flow on the incident laser intensity is numerically studied under varying process conditions. A finite element simulation of the powder flow is performed and used to generate a set of powder particle trajectories using Monte Carlo simulation. A ray-tracing algorithm is developed to split the laser beam into multiple rays of light which get partly reflected and absorbed by the particles and the workpiece. Running the ray-tracing procedure over time allows the calculation of an averaged incident light intensity pattern as well as an averaged pattern of the energy absorbed by the particles that arrive at the workpiece. Several simulations are performed in order to study the effects of the used laser intensity pattern and the particle size distribution. The results are in good agreement with existing literature.

Proof of Concept of Integrated Load Measurement in 3D Printed Structures

Currently, research on structural health monitoring systems is focused on direct integration of the system into a component or structure. The latter results in a so-called smart structure. One example of a smart structure is a component with integrated strain sensing for continuous load monitoring. Additive manufacturing, or 3D printing, now also enables such integration of functions inside components. As a proof-of-concept, the Fused Deposition Modeling (FDM) technique was used to integrate a strain sensing element inside polymer (ABS) tensile test samples. The strain sensing element consisted of a closed capillary filled with a fluid and connected to an externally mounted pressure sensor. The volumetric deformation of the integrated capillary resulted in pressure changes in the fluid. The obtained pressure measurements during tensile testing are reported in this paper and compared to state-of-the-art extensometer measurements. The sensitivity of the 3D printed pressure-based strain sensor is primarily a function of the compressibility of the capillary fluid. Air- and watertightness are of critical importance for the proper functioning of the 3D printed pressure-based strain sensor. Therefore, the best after-treatment procedure was selected on basis of a comparative analysis. The obtained pressure measurements are linear with respect to the extensometer readings, and the uncertainty on the strain measurement of a capillary filled with water (incompressible fluid) is ±3.1 µstrain, which is approximately three times less sensitive than conventional strain gauges (±1 µstrain), but 32 times more sensitive than the same sensor based on air (compressible fluid) (±101 µstrain).

High Resolution Temperature Measurement of Liquid Stainless Steel Using Hyperspectral Imaging

A contactless temperature measurement system is presented based on a hyperspectral line camera that captures the spectra in the visible and near infrared (VNIR) region of a large set of closely spaced points. The measured spectra are used in a nonlinear least squares optimization routine to calculate a one-dimensional temperature profile with high spatial resolution. Measurements of a liquid melt pool of AISI 316L stainless steel show that the system is able to determine the absolute temperatures with an accuracy of 10%. The measurements are made with a spatial resolution of 12 µm/pixel, justifying its use in applications where high temperature measurements with high spatial detail are desired, such as in the laser material processing and additive manufacturing fields.

Spectroscopic monitoring and melt pool temperature estimation during the laser metal deposition process

Laser metal deposition is an additive manufacturing process that allows the production of near net shape structures. Moreover, the process can also be applied for the addition of material to an existing component for repair. In order to obtain structures with reproducible and excellent material properties, it is necessary to understand the thermal behavior of the process better and to monitor and control the process. One of the critical parameters in this process is the measurement of the melt pool temperature and its distribution. The varying emissivity in space and time for the melt pool forms a fundamental physical problem. This also prevents a correct temperature measurement of the melt pool temperature distribution with a thermal camera. The usage of the spectral information within the emitted light of the melt pool can form a key enabling element in the estimation of the emissivity and as such reveal the temperature information. In future, this information can be used in a controlling system in orde...

Evaluation of Different Topologies of Integrated Capillaries in Effective Structural Health Monitoring System Produced by 3D Printing

Over the last years the structural health monitoring (SHM) systems investigations have been focused on providing structures with similar functionality as the biological nervous system. There are numerous studies that have investigated this. In those studies a large number of sensors collects an extensive amount of data. In this study we demonstrate a novel effective SHM (eSHM) system which can monitor a structure with one single pressure sensor. The eSHM system can detect cracks by means of a system of capillaries integrated in a structure. This structure with the integrated capillaries can be produced by 3D printing, also known as additive manufacturing (AM). The principle of the eSHM system is monitoring the pressure variations in a network of capillaries. The effectiveness of this system is linked with the greatest strength of AM, which is the capability to create complex geometrical structures. Before the implementation in real structures, it is of crucial importance to be sure that the capillaries do not negatively influence the fatigue behaviour of the structures and the crack initiation. For this, the main objective of this study is to investigate different locations for a straight capillary incorporated into a four-point bending test specimen. The investigated titanium specimens with the integrated eSHM system are produced by AM. The capillary is located in the longitudinal dimension of the test specimen on the tension area of a four-point bending setup. We evaluate three different distances of the capillary to the outer surface of the test specimens. Furthermore, the results are also obtained by finite element simulations. We can conclude that –for the considered structure– the presence of the capillary does not influence the fatigue life negatively. On the other hand, cracks nucleate in the capillary region. Our future work will focus on the improvement of the capillary’s robustness. Other parameters like roughness effect and residual stresses should be also taken into account. doi: 10.12783/SHM2015/22

Fatigue Performance of Ti-6Al-4V Additively Manufactured Specimens with Integrated Capillaries of an Embedded Structural Health Monitoring System

Additive manufacturing (AM) of metals offers new possibilities for the production of complex structures. Up to now, investigations on the mechanical response of AM metallic parts show a significant spread and unexpected failures cannot be excluded. In this work, we focus on the detection of fatigue cracks through the integration of a Structural Health Monitoring (SHM) system in Ti-6Al-4V specimens. The working principle of the presented system is based on the integration of small capillaries that are capable of detecting fatigue cracks. Four-point bending fatigue tests have been performed on Ti-6Al-4V specimens with integrated capillaries and compared to the reference specimenswithout capillaries. Specimens were produced by conventional subtractive manufacturing of wrought material and AM, using the laser based Directed Energy Deposition (DED) process. In this study, we investigated the effect of the presence of the capillary on the fatigue strength and fatigue initiation location. Finite element (FEM) simulations were performed to validate the experimental test results. The presence of a drilled capillary in the specimens did not alter the fatigue initiation location. However, the laser based DED production process introduced roughness on the capillary surface that altered the fatigue initiation location to the capillary surface. The fatigue performance was greatly reduced when considering a printed capillary. It is concluded that the surface quality of the integrated capillary is of primary importance in order not to influence the structural integrity of the component to be monitored.