Degradation von Werkstoffen
Filtern
Dokumenttyp
- Vortrag (6)
- Zeitschriftenartikel (2)
- Beitrag zu einem Tagungsband (1)
Sprache
- Englisch (9)
Schlagworte
- Porosity (9) (entfernen)
Organisationseinheit der BAM
Eingeladener Vortrag
- nein (6)
The contactless magnetohydrodynamic technology has been considered as a potential and promising method to improve the weld qualities of deep penetration laser beam welding. In this paper, numerical investigations are conducted to study the influence of the auxiliary oscillating magnetic field on the porosity suppression in laser beam welding of 5754 aluminum alloy. To obtain a deeper insight into the suppression mechanism, a three-dimensional transient multi-physical model is developed to calculate the heat transfer, fluid flow, keyhole dynamic, and magnetohydrodynamics. A ray tracing algorithm is employed to calculate the laser energy distribution on the keyhole wall. A time-averaged downward Lorentz force is produced by an oscillating magnetic field. This force acts in the molten pool, leading to a dominant downward flow motion in the longitudinal section, which blocks the bubble migration from the keyhole tip to the rear part of the molten pool. Therefore, the possibility for the bubbles to be captured by the solidification front is reduced. The electromagnetic expulsive force provides an additional upward escaping speed for the bubbles of 1 m/s ~ 5 m/s in the lower and middle region of the molten pool. The simulation results are in a good agreement with experimental measurements. Based on the results obtained in this study, a better understanding of the underlying physics in laser beam welding enhanced by an auxiliary oscillating magnetic field can be provided and thus the welding process can be further optimized reducing the porosity formation.
Magnetohydrodynamic technology is increasingly recognized as a promising approach for enhancing the quality of deep penetration laser beam welding. This study employs experimental and numerical methods to investigate the effects of an auxiliary oscillating magnetic field on reducing porosity in the laser beam welding of 5754 aluminum alloy. The experimental results clearly demonstrate a significant reduction in the porosity ratio, thereby validating the efficacy of applying MHD technology in mitigating porosity during the laser beam welding process. In addition, a transient 3D multi-physical model has been developed, integrating the magnetohydrodynamic and metal vapor plume, to gain a more comprehensive understanding of the porosity suppression mechanism. The introduction of an oscillating magnetic field generates a time-averaged downward Lorentz force. This Lorentz force, in turn, induces an electromagnetic expulsive force, which effectively increases the upward escape velocity of bubbles in the molten pool. Furthermore, the molten pool shape is significantly enlarged, which further facilitate the escape of bubbles. The simulation results agree well with the experimental results.
This study presents mechanisms of the evolution of a narrowed region in the weld pool center during deep penetration laser beam welding. In numerous numerical studies presented in this study, it was also found that the local reduction of the weld pool size can cause detrimental effects on the melt flow behavior and the resulting properties of the welds. A particularly large influence of this effect was identified in three aspects. First, the local variation of the solidification sequence of the weld pool causes an increase in the hot-cracking susceptibility due to a locally delayed solidification. Second, it was proven that a change in the local length and width of the weld pool is associated with an adverse impact on the potential flow routes of the molten material that induces stronger local variations of its solidification. Thus, the element mixing, e.g., during the welding with filler materials, is blocked. This leads to a non-homogeneous chemical composition of the final weld and can cause undesired effects on the final material properties. Finally, another observed effect is related to the reduced ability of process pores to reach the top surface. As this type of porosity is usually produced around the keyhole tip, the change of the fluid flow regime above this area plays a significant role in determining the final path of the pores until the premature solidification in the middle of the weld pool captures them. This study summarizes mainly numerical results that were supported by selected experimental validation results.
In this seminar I present the microstructure and micromechanical properties of diesel particulate filter materials, and then particularize them to porous microcracked aluminum titanate.
I show that neutron diffraction is particularly suited for bulk studies, especially under applied load or at high temperatures. The combination of macroscopic and microscopic tests with modeling and simulation yields great added value to understand the mechanics of microcracking.
In this Seminar cycle I will present first diffraction methods to determine residual stress and investigate micromechanical properties of complex materials, and then particularize the treatment with applications to porous microcracked ceramics for diesel particulate filter applications.
I will show that neutron diffraction is particularly suited for bulk studies, where 3D stresses are needed. The advantages to use Time-of-Flight or steady state sources will be discussed, together with the problematic of the determination of absolute RS values.
Finally, the behavior of DPF materials under applied load or at high temperatures will be discussed under the combination of macroscopic and microscopic tests.
In this Seminar cycle I will present first diffraction methods to determine residual stress and investigate micromechanical properties of complex materials, and then particularize the treatment with applications to porous microcracked ceramics for diesel particulate filter applications.
I will show that neutron diffraction is particularly suited for bulk studies, where 3D stresses are needed. The advantages to use Time-of-Flight or steady state sources will be discussed, together with the problematic of the determination of absolute RS values.
Finally, the behavior of DPF materials under applied load or at high temperatures will be discussed under the combination of macroscopic and microscopic tests.
We investigated lattice structure manufactured by laser beam melting with computed tomography on difference scales, such as powder scale, strut scale and lattice scale.
The raw powder has been evaluated by means of synchrotron computed tomography (CT) at the BAM-Line (HZB Bessy II, Berlin). Therefore, the particle size distribution and even the pore size distribution was investigated and compared with results received by the producer by means of sieving. Studies with laboratory X-ray CT of porosity and roughness of manufactured struts in dependence of the build angle exhibited the tendency that elongated pores appear solely in a certain range near the edge. The integrity and load-bearing capacity of a lattice structure was investigated by means of in-situ CT during compression. The lattice structure was compressed by 10 % in height with an applied maximum force of 5 kN. We applied digital volume correlation algorithm on volumes of different load steps to quantifies the displacement within the structure.
The combination of microstructural data with other experimental techniques and with modeling is paramount, if we want to extract the maximum amount of information on porous material properties. In particular, quantitative image analysis, statistical approaches, direct discretization of tomographic reconstructions represent concrete possibilities to extend the power of the tomographic 3D representation to insights into the material and component performance. I will show a few examples of possible use of X-ray tomographic data for quantitative assessment of porosity in ceramics.
Moreover, I will show how not-so-novel 2D characterization techniques, based X-ray refraction, can allow a great deal of insights in the damage evolution in microcracked (and porous) ceramics. I will show how X-ray refraction can detect objects (e.g. microcracks) below its own spatial resolution.
Finally, I will discuss the link between the microstructural findings and the mechanical properties of porous microcracked ceramics.
An improved diffusion model is proposed for pore annihilation during HIP of single-crystal nickel-base superalloys. The model assumes the pore dissolution by emission of vacancies and their sink to the low angle boundaries. Calculation, considering distribution of the pore sizes, predicts the kinetics of pore annihilation similar to the experimental one.