Johannes Preußner

Dislocation density is an important parameter for characterizing the microstructure of metals. It was measured by XRD on a quenched and tempered 42CrMo4 steel in various conditions, from the asquenched state to tempering at various temperatures between 200 °C and 650 °C. Dislocation density decreases continuously with increasing tempering temperature. Comparison with synchrotron measurements have shown the same trend. Secondary phases, such as cementite and Cr7C3 precipitates were also detected by XRD and the results were compared to equilibrium calculations done with JMatPro software and with SEM-EDXS analyses.

High strength and low density materials are needed to achieve lightweight design of components. Aluminum base metals alloyed with calcium are of potential interest because of the low density of calcium and its abundance. The additive manufacturing of dense and crack free samples out of an Al-10 wt.% Ca (Al-10Ca) alloy is presented. Both laser-based direct energy deposition (DED-LB) and laser powder bed fusion (LPBF) processes were applied to manufacture sample material. Preheating of the substrate plate is needed in LPBF to receive crack free samples. An analysis of the microstructure shows an Al-Al ₄ Ca lamellar eutectic.

Laser shock peening (LSP) is a mechanical surface treatment process to modify near-surface material properties. Compared to conventional shot peening (SP) the process parameters can be finely adjusted with greater precision and a higher penetration depth of compressive residual stresses could be reached. However, high process times of LSP leads to high production costs. In this study, ultrafast LSP (U-LSP) with an ultrafast laser source (pulse time in the picosecond range) was applied on specimens made of X5CrNiCu15-5 and AlZnMgCu1.5. The surface characteristics (surface roughness) and surface-near properties (microstructure, residual stresses, and phase composition) were compared to the as-delivered condition, to conventional laser shock peening (C-LSP), and to SP, whereas metallographic analyses and X-ray and synchrotron radiation techniques were used. The process time was significantly lower via U-LSP compared to C-LSP. For X5CrNiCu15-5, no significant compressive residual stresses were induced via U-LSP. However, for AlZnMgCu1.5, similar compressive residual stresses were reached via C-LSP and U-LSP; however, with a lower penetration depth. A change in the phase portions in the surface layer of X5CrNiCu15-5 after C-LSP compared to SP were determined.

The demand for advanced high-strength steel (AHSS) in the automotive industry has increased over the last few years. Nevertheless, it is known that AHSSs are susceptible to hydrogen embrittlement. Therefore, the influence of hydrogen on the localization and damage behavior of a CP1000 steel sheet was investigated in this work. The sheet metal was electrochemically charged to a hydrogen content of about 3 ppm (by weight). Tensile tests were performed at different nominal strain rates between 0.00004 s⁻¹ and 0.01 s⁻¹ to investigate the effects of strain rates on their susceptibility to hydrogen embrittlement. Nakajima tests were utilized to investigate the hydrogen effects on the steel’s formability under different stress states. Three different Nakajima specimen geometries were employed to represent a uniaxial stress state, a nearly plane strain stress state, and an equibiaxial stress state. Further, forming limits were evaluated with the standardized section line method. Hydrogen embrittlement, during tensile testing, occurred independent of the strain rate, unlike the Nakajima test results, which showed hydrogen effects that were strongly dependent on the stress state.

Most of the aluminum‐based components produced today by additive manufacturing (AM) are based on the Al–Si eutectic system due to its outstanding processability. The mechanical properties of these alloys, however, are well below comparable wrought alloys. Even though successful modifications of the cast and wrought alloys have been performed in the literature, the widespread application is still missing for various reasons. Herein, new alloys of the Al–Ni eutectic system with the composition Al–7.5wt%Ni are manufactured by laser metal deposition (LMD), and the microstructure and properties are described. The microstructure shows a different appearance over the track height: A coarse microstructure can be found at the interlayer boundary. The adjacent fibrous eutectic is followed by a microstructure of twinned dendrites, which only appear at certain process parameters. The appearance of the twinned dendrites is explained via simulations of the local cooling rate of the LMD process. The mechanical properties of samples manufactured by LMD are examined via tensile testing.

High-strength low-alloy steels (HSLA) are gaining popularity in structural applications in which weight reduction is of interest, such as heavy duty machinery, bridges, and offshore structures. Since the fatigue behavior of welds appears to be almost independent of the base material and displays little improvement when more resistant steel grades are employed, the use of bolted joints is an alternative joining technique which can lead to an increased fatigue performance of HSLA connections. Manufacturing a hole to allocate the fastener elements is an unavoidable step in bolted elements and it might induce defects and tensile residual stresses that could affect its fatigue behavior. This paper studies and compares several mechanical (punching, drilling, and waterjet-cut) and thermal (plasma and laser-cut) hole-making procedures in HSLA structural plates. A series of 63 uniaxial fatigue tests was completed, covering three HSLA grades produced by thermomechanically controlled process (TMCP) with yield strength ranging from 500 to 960 MPa. Samples were tested at single load level, which was considered representative in HSLA typical applications, according to the input received from end users. The manufactured holes were examined by means of optical and electron microscopy, 3D point measurement, micro hardness tests, X-ray diffraction, and electron backscatter diffraction (EBSD). The results give insight on cutting processes in HSLA and indicate how the fatigue failure is dominated by macro defects rather than by the steel grade. It was shown that the higher yield strength of the HSLA grades did not lead to a higher fatigue life. Best fatigue results were achieved with laser-cut specimens while punched samples withstood the lowest amount of cycles.

The Cu–Sn–Zn alloy system is technically very important. Brass (Cu–Zn) and bronze (Cu–Sn) alloys have been used and developed for tools and components for over thousands of years and are still used in a wide range of applications like marine components, bearings, music instruments and for components in contact with corrosive media. Even though it is a very old and well-studied alloy system, there are only few studies that analyze Cu–Sn–Zn coating systems. Furthermore very basic properties like the diffusion through different phases are not studied in detail. In this work experimental diffusion data in the literature is critically reviewed. On the basis of these studies a diffusion mobility database for the bcc phase of the ternary Cu–Sn–Zn is presented. The database was established using the DICTRA-type (DIffusion Controlled TRAnsformation) diffusion modeling. The resulting curves for the interdiffusion coefficient for various alloy concentrations are then compared to available literature data. Furthermore, experiments from diffusion couples are recalculated for validation. A good fit of these experimental data could be realized using only few parameters.

A new approach to structure metallic surfaces with laser radiation is structuring by remelting. In this process no material is removed but reallocated by melting. The laser power was adapted linearly to the increasing laser beam diameter for laser remelted (polished) samples. A carbon depleted area could be found close to the remelted zone accompanied with a local minimum in hardness. The surface residual stresses tend from tensile to compressive with increasing laser beam diameter/laser power and number of repetitions for laser structured and laser remelted samples. The residual stresses are a result of combined shrinkage (tensile) and transformation (compressive) stresses.