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 (HIL) system was built to enable safe and cost-effective testing of the controller hardware in different simulation environments. The HIL setup includes a real-time image processing module for extracting the required temperature information from hyperspectral data generated by detailed numerical simulations of the melt pool dynamics. Hyperspectral measurements are performed on a laser cladding system to validate the simulation results.

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.

Laser cladding is a technique that is frequently used for the coating and repair of metallic components. More recently, the technology is used in the additive manufacturing domain for building freeform three-dimensional parts. A lot of attention is dedicated to the optimization of process parameters and to real-time feedback control strategies. This paper presents a feedback control scheme in which a hyperspectral camera is used to provide absolute temperature measurements of the melt pool surface with a high spatial resolution of 12 ${\mu }$m/pixel. A combined linear state feedback and proportional-integral controller actuates the laser based on the measured temperature profile in order to maintain a constant melt pool width. The controller includes a model-based state observer for suppressing the noise introduced by powder particles that are present in the laser cladding process with coaxial powder feeding. The performance of the controller is evaluated by creating beads with varying thicknesses on a base plate of AISI 316L stainless steel. The experimental results demonstrate that the controller is able to successfully regulate the melt pool size with a standard deviation that is smaller than 10 ${\mu }$ m during laser melting (without powder) and 50 ${\mu }$m during laser cladding (with powder).

Directed energy deposition (DED) is a widespread laser additive manufacturing process characterized by the simultaneous laser and powder delivery. During the layerwise bounding of the material, many complex interconnected physical phenomena take place in a very short time. Presently, one of the main challenges faced by DED is to enhance the powder deposition efficiency. It is defined as the ratio of powder that has been effectively solidified in the part over the total amount of powder that flowed through the nozzle while the laser was on during the deposition process. Increasing the powder efficiency would allow us to minimize the powder waste, minimize the overall costs of the DED process, and therefore reduce the printed part cost. The present work, therefore, proposes to study the influence of laser beam diameter, stand-off distance, and gas/powder settings on the DED powder efficiency. The considered gas/powder settings are carrier and shielding gas volumetric flow rate, powder mass flow rate, and particle diameter. The efficiency is computed by means of the geometry of 316L stainless steel single-track deposits. The track geometries are evaluated based on the deposition width, deposition height, and area of deposition, all extracted from the deposition profiles measured by means of laser triangulation. Optical micrographs of the single-track transversal cross sections are linked to the obtained powder efficiencies. The final aim is to get insight into the effect of process parameters on the powder efficiency and print quality and to identify the optimal process parameter combination in order to maximize the powder efficiency.

Laser cladding is a technique that is used for the coating, repair and production of metallic parts. Material is added to the surface of the part by injecting a flow of powder into a melt pool that is created with a high power laser beam. When the beam scans the surface of the substrate, strong local heating and cooling results. A good knowledge of the temperature distribution history during the laser cladding process is vital to predict and optimize the material properties of the final part. This paper presents a contactless temperature measurement system with high temporal and spatial resolution based on a hyperspectral line camera. High temperature measurements were made during laser cladding of AISI 316L stainless steel. A good correlation is shown between the temperature measurements and microscope images taken after creation of the clad

This article presents a novel approach for assessing the effects of residual stresses in laser-directed energy deposition (L-DED). The approach focuses on exploiting the potential of rapidly growing tools such as machine learning and polynomial chaos expansion for handling full-field data for measurements and predictions. In particular, the thermal expansion coefficient of thin-wall L-DED steel specimens is measured and then used to predict the displacement fields around the drilling hole in incremental hole-drilling tests. The incremental hole-drilling test is performed on cubic L-DED steel specimens and the displacement fields are visualized using a 3D micro-digital image correlation setup. A good agreement is achieved between predictions and experimental measurements.