Johanna Müller

The surface characteristics of direct energy deposition (DED)-Arc specimens have been previously explored, revealing the dual effect of the surface topography on fatigue strength. This effect includes a reduction in the nominal load-bearing cross-section due to the waviness of the surface and the initiation of fatigue cracks at combined geometric and metallurgical notches from the surface. To complement these findings, this study focuses on the fatigue life and crack initiation of milled DED-Arc samples, e.g., after removal of the geometric notches. Fatigue tests on DED-Arc specimens validate the impact of surface topography on fatigue strength, emphasizing the significance of surface characteristics in determining structural integrity. The additional tests conducted on milled surfaces provide insights into the failure mechanisms specific to these samples. Fracture surface analysis, microstructure characterization, and hardness measurements are performed. For both surface conditions, cracks originate in the interlayer zone, where local hardness is reduced. In unmilled specimens, this is due to the stress concentration effect, and in milled samples, this is due to softened interlayer zones. The fatigue life of milled specimens is comparatively high, demonstrating the importance of optimizing surface characteristics for improved fatigue resistance. Highlights: 1. Both, unmilled and milled specimens, show crack origins in the interlayer areas. For unmilled specimens, this is because of the local stress concentration in the notch; for milled specimens, it was shown that soft zones between the layers act as the weakest link. 2. The fatigue strength of unmilled test coupons is significantly lower than those of milled specimens. 3. Unmilled samples exhibit multiple crack origins.

Thermography is a technique that uses, e.g., an infrared camera to visualize and measure the temperature of an object. It is often used in industrial and scientific applications to identify areas of heat loss, overheating, and other thermal anomalies. In the context of the DED-Arc (Direct Energy Deposition) process, thermography can be used to monitor the process and evaluate the temperature profile of the produced part. This can help to ensure the quality and reliability of the product, as well as to predict the resulting mechanical properties of the produced part. However, for other AM processes like LPBF (laser powder bed fusion), thermography is already used in industrial applications while for DED-Arc it is still a challenge to reliably determine the dynamically changing emission coefficient, as the emissivity strongly depends on the surface conditions. This means the emission coefficient differs for changes in surface conditions like impurities from soot and annealing colors. This work focuses on the potential of thermography for monitoring the DED-Arc process. A workflow for generating a calibration function for the emission coefficient ε is presented. In the context to the focus of this work, the resulting ε(T) function differentiates between the first three deposited layers and shows the change of emissivity for higher temperatures. This function is then used to correct the measured temperature profile with regard to different surface conditions and thus emission coefficients of a DED-Arc part. Highlights: 1. Temperature-dependent function for correcting the emissivity for DED-Arc with steel was defined. 2. Emissivity is also influenced by the purity of the surface, leading to higher emissivity coefficients in layers that are previously deposited. 3. t_8/5 times are also affected by a wrongly set emission coefficient; deviations from the corrected t_8/5 time depend on the cooling speed.

Steel structures, such as bridges, must be continuously maintained. Due to increasing trac loads, design and execution shortcomings, many steel bridges are subject to various types of damage, such as cracks. The bridges must be maintained by crack removal and re-welding the cracks. During this work, the bridge is closed for trac in order to suppress the crack ank movement during welding. As a result, trac loads on other routes are higher, which increases further damage on detour routes and subsequently additional welding work and costs on theses structures. Therefore, the aim is to provide a method that allows welding work on steel structures under running trac, respectively under cyclic loads. First, measurements at a representative steel bridge were conducted to derive gap opening parameters of amplitude and frequency for welding tests under cyclic loads. In a test setup for reproducible and comparable welding tests, specimen were welded under cyclic loads. Deep grinding of the root layer allowed defect-free welding within the range of 0.1 mm amplitude at a frequency of 2 Hz loadings. For such welds, the weld quality and the fatigue strength was comparable to the standard requirements.

Additive manufacturing with steel opens up new manufacturing possibilities for individual parts or complex supply chains. The direct energy deposition process DED-arc, also known as wire arc additive manufacturing (WAAM), is particularly suited for manufacturing large structures with a high degree of geometrical freedom. The design freedom makes WAAM ideal for creating force-flow-optimized steel nodes and spaceframes for use in the construction industry. Thick-walled parts can be manufactured with manifold strategies and a variety of infill patterns, like hachure, spiral, or meander as well as with and without a contour path. For parts with varying thicknesses, not all of them apply. If the wall thickness changes in increments smaller than the width of a weld bead, the desired shape cannot be made by selecting one of the available deposition strategies but the distance (overlap) between the individual weld beads needs to be varied. A variation of the weld bead overlap can often lead to compromised build quality due to the presence of voids. Thus, utilizing variable bead overlap requires the development of a deposition parameter set that allows deviation from the defined overlap values without running the risk of introducing voids between beads or between layers. In this study, four parameter sets were analyzed regarding their weld bead geometry, and tolerance ranges for the weld bead overlap were identified for each parameter set. They were found to vary between 15 and 50% overlap, depending on the parameter set and its corresponding weld bead geometry. The geometry of the surface of the last deposited layer was evaluated regarding the height difference. It can be stated that, with decreasing wire feed, the surface of the last welded layer becomes smoother. The findings obtained are transferred to the manufacturing of complex parts. As a result, parts with variable wall thickness in building direction were manufactured by variation of weld bead distances.

Additive manufacturing with the DED-arc process offers limited freedom in terms of the geometric shape of work pieces. The process and fabrication systems restrict the part geometry producible, which must be taken into account during design already. For this reason, a design process was investigated in which geometry generation is based on a self-organizing system. The aim of using a self-organizing system is the possibility to directly control the geometry-defining points. Next to load cases, the design method considers geometric boundary conditions from the production process when generating the geometry. In order to identify these geometrical constraints from production experimentally, a concept of Case Study Demonstrators was applied. This was used to investigate how path planning and production can be carried out for specific geometrical features and to identify restraints of the process and the manufacturing system, e.g., smallest producible wall thickness and overhangs. Subsequently, the obtained restraints were considered as boundary conditions for the design process and were included in the modification of an example geometry. By applying the presented design method, it was possible to maintain a minimum wall thickness throughout the structure while generating a topologically optimized geometry. In contrast to compliance with the minimum wall thickness, no satisfactory behavioral rule could be found for limiting the overhang.

Additive manufacturing with steel opens up new possibilities for the construction sector. Especially direct energy deposition processes like DED-arc, also known as wire arc additive manufacturing (WAAM), is capable of manufacturing large structures with a high degree of geometric freedom, which makes the process suitable for the manufacturing of force flow-optimized steel nodes and spaceframes. By the use of high strength steel, the manufacturing times can be reduced since less material needs to be deposited. To keep the advantages of the high strength steel, the effect of thermal cycling during WAAM needs to be understood, since it influences the phase transformation, the resulting microstructure, and hence the mechanical properties of the material. In this study, the influences of energy input, interpass temperature, and cooling rate were investigated by welding thin walled samples. From each sample, microsections were analyzed, and tensile test and Charpy-V specimens were extracted and tested. The specimens with an interpass temperature of 200 °C, low energy input and applied active cooling showed a tensile strength of ~ 860–900 MPa, a yield strength of 700–780 MPa, and an elongation at fracture between 17 and 22%. The results showed the formation of martensite for specimens with high interpass temperatures which led to low yield and high tensile strengths (R_p0.2 = 520–590 MPa, R_m = 780–940 MPa) for the specimens without active cooling. At low interpass temperatures, the increase of the energy input led to a decrease of the tensile and the yield strength while the elongation at fracture as well as the Charpy impact energy increased. The formation of upper bainite due to the higher energy input can be avoided by accelerated cooling while martensite caused by high interpass temperatures need to be counteracted by heat treatment.