The influence of thermally sprayed aluminum coatings (Al99%; arc spraying) on the fatigue strength of gas metal arc welded (GMAW) non-alloyed structural steel specimens with respect to foundations for offshore wind turbines was investigated. Additionally, the corrosion protection effect of such coatings for water conditions similar to the Baltic Sea was determined. Wöhler tests were carried out on test specimens with different weld details in the as-welded condition as well as in the thermal spray coat under the consideration of different kinds of surface preparation (blast cleaning with corundum and grit). Substrate and coating were characterized by scanning electron microscopy and the influence on the residual stress states was determined. Corrosion rate monitoring via LPR measurements was carried out as well as the monitoring of the galvanic current between coated and uncoated steel to characterize the coatings’ sacrificial capability for minor defects. Fatigue strength was significantly increased through thermal spraying, especially for test specimens with welded transverse stiffeners (Δσ_c,var = 127 MPa after coating compared to Δσ_c,var = 89 MPa as welded). With a characteristic value of the stress range of Δσ_c,var = 153 MPa, the welded butt joint specimens already exhibited a high fatigue strength in the as-welded condition. The corrosion studies demonstrated that thermally sprayed Al99% coatings have a high resistance to corrosion in seawater environments and are suitable as planar sacrificial anodes sufficiently polarizing bare steel below 0.8 V. The combination of fatigue strength improvement and corrosion protection makes the thermally sprayed Al coatings promising for design and operation of e.g., offshore structures.

Arc brazing with low-melting copper-based filler materials, which has long been established and standardized in the thin sheet sector, offers numerous advantages in the processing of predominantly electrolytically galvanized steel structures. In steel and shipbuilding, on the other hand, equipment parts made of thick steel sheets are hot-dip galvanized at low cost and with good corrosion-inhibiting properties. Quality welding of such constructions is not possible without special precautions such as removing the zinc layer and subsequent recoating. With regard to greater plate thicknesses, arc brazing was analyzed in these investigations as an alternative joining method with regard to its suitability for practical use. Within the scope of the investigations, CuSi3Mn, CuMn12Ni2, and four different aluminum bronzes were examined on different sheet surface conditions with regard to the geometrical and production parameters. This was carried out by build-up and connection brazing, executed as butt and cross joints. Quasi-static tensile tests and fatigue tests were used to assess the strength behavior. In addition, metallographic analyses are carried out as well as hardness tests. The suitability for multi-layer brazing and the tendency to distortion were also investigated, as well as the behavior of arc brazed joints under corrosive conditions.

Never before has an optical method been used to determine the arc temperature inside a submerged arc welding cavern with frequencies up to 5 kHz. To be able to do that, a combination of high-speed imaging and spatially resolved high-speed spectroscopy with up to 5000 fps has been performed. A DCEP (direct current electrode positive) process with 600, 800, 900 and 1000 A, and an AC (alternating current) process at 800 A were included in this research. The Bartels method has been used to calculate these temperatures for the first time. It generated temperatures from approximately 7000–9000 K. Also, it was found that a decrease of arc temperature for rising currents appeared until 900 A. After that, it is reversed for currents higher than 900 A.

The effect of High-Frequency Mechanical Impact (HFMI) treatment on the fatigue resistance of structural steel S355 butt welds with macro geometric imperfections outside of the usual fatigue design limits was investigated. Thin-walled specimens (t = 8 mm) were manufactured with different degrees of angular and linear misalignment and fatigue tested in the as welded and HFMI treated condition. The fatigue test results show that HFMI treatment can compensate the fatigue strength-reducing effects of macro geometric imperfections (Δσ_C,HFMI = 127 N/mm², m = 5), but also that linear and angular misalignment does not reduce the fatigue strength of butt welds as severely as rules and guidelines suggest (Δσ_C,AW = 107 N/mm², m = 3). Underestimating the fatigue strength determined by testing by more than 40%, numerical analysis and strain gauge measurements show that the application of the effective notch stress approach and the structural hot-spot stress approach delivers conservative assessment results, which might not always be representative for the actual fatigue strength benefit that can be gained by HFMI treatment.