Wim Devesse

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 (...

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...

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.

Negative Pressure Waves Analysis for Crack Localization and Crack Size Estimation for 3D Printed SHM System

A new Structural Health Monitoring system was developed to allow a faster introduction of 3D printed components into safety critical applications. Additive manufacturing techniques are used to embed capillaries in a 3D printed structure that are then pressurized. Continuous monitoring of the capillary pressure allows the system to indicate the existence of a crack when the pressure deviates from the initial pressure level. A specifically developed experimental set-up enables the study of the impact of different parameters on the leak flow behavior and the occurring Negative Pressure Waves. Negative Pressure Waves are analyzed to demonstrate the crack localization and crack size estimation feasibility. It will first be theoretically proven that the size and location of the crack can be derived from the Negative Pressure Waves. Secondly, measurements will validate the crack localization and size estimation feasibility of the new Structural Health Monitoring system.Copyright