Applications

Laser additive manufacturing (LAM) of aluminium alloys is already used for various industrial applications. The dominating LAM technology in this field is the powder-based selective laser melting (SLM). However, the range of aluminium alloys available as qualified powder for SLM is very limited and powder handling is in general also laborious. For laser metal deposition (LMD) with wire conventional retrofitted laser welding facilities can be used and the handling of the feedstock material is facilitated. Moreover, a larger range of aluminium alloys is commercially available for this LAM technology in form of filler wires typically used for fusion welding. By using wire instead of powder as feedstock material considerable higher deposition and utilisation rates are possible. Thus, large-scale surface-coatings as well as structures can be realised. By coupling the knowledge about the process, the (new) materials and the microstructure it is possible to tailor the resulting properties of structures.

Laser additive manufacturing (LAM) of steel, titanium and aluminium is already widely used, whereas magnesium is practically not utilised for this fast-growing technology. The reason for this disregard of this lightweight material lies in the flammability of magnesium powder and the resulting extensive remedial measures for powder handling, since most of the LAM processes are powder-based. By using magnesium wire for laser metal deposition (LMD) the handling of the feedstock material is considerable facilitated and the deposition rates are increased. Enabling the LAM of magnesium, its lightweight potential can be fully exploited for example in structures for the automotive industry. By combining appropriate magnesium alloys and modifying the microstructure through the LMD process parameters the properties of these structures can also be tailored. In addition, newly developed magnesium alloys are tested with the LMD process for extending the material range for LAM.

Among the different laser additive manufacturing (LAM) techniques, the wire-based laser metal deposition (LMD) is used to manufacture structures consisting of aluminium alloys. The process is analysed through a process simulation and optimized accordingly. Advantages of wire-based LMD in comparison to powder-based LAM processes are the high process velocity, efficient use of material, applicability of existing laser welding facilities as well as the possibility of extensive scale-up regarding part size through high flexibility and mobility of the machinery. Currently, wire-based processes are mainly used for additive processing of titanium alloys. So far, wire-based LMD is hardly investigated for the usage of aluminium alloys as consumable, despite many aluminium alloys being available as wire. During the LMD process, a significant local heat input into the manufactured structure is occurring due to the large volume of material that needs to be molten for high deposition rates. This leads to a substantial non-uniform temperature distribution within the structure, which causes high residual stresses and distortions as well as damages within the structure, after cooling. Via process simulation, where the finite element method is applied, the LMD process can be optimized at more cost and time efficiency than through numerous experiments. Especially the temperature field during LMD processing as well as the resulting residual stresses and deformations can systematically be investigated and subsequently be predicted in dependence of the process parameters. For process optimization, the transient temperature field can be tailored in such way that residual stresses are minimized and distortions as well as defects are eliminated.

Titanium alloys exhibit promising mechanical properties at elevated temperatures. Titanium aluminides are applicable for even higher temperature ranges. For this reason, both materials are used for high-performance applications, as for example in the aerospace industry. However, the laser beam welding (LBW) of titanium aluminides is still challenging, in contrast to titanium alloys, and it generally requires special measures. For tailoring the properties of a high-performance aerospace structure, the combination of both materials becomes necessary. With the help of laser additive manufacturing (LAM), the joining of this challenging multi-material can be enabled. Understanding the peculiarities of both materials, the processes for LAM and LBW and thus also the microstructure and the resulting properties of the structure can be adjusted and optimised.

The presence of strongly inhomogeneous temperature fields in welding processes have a significant influence on the mechanical properties of weld joints and their microstructure. The simulation of temperature fields for both classical and also metal deposition processes represents the starting point here, which allows to investigate the microstructure, the residual stresses and distortion numerically as well as experimentally.

A focus of RSE's research activities represents the analysis of precipitation-hardenable aluminum alloys. This class of aluminum alloys shows complex microstructural changes during welding. The excellent mechanical properties get these alloys through precipitation of particles of a second phase. Their particle size lies in the range of a few manometers. A deep physical understanding oft the precipitation kinetics is one of our major research interests. To describe the complex physics of nucleation and growth of precipitates a combination of simulation and experiments is necessary. The aim is to develop a validated process model that can be used for process optimization.

Laser shock peening (LSP) can be used to improve the fatigue performance of lightweight structures and therefore provides a method to treat one of the main failure causes in the aircraft industry. The idea is to introduce compressive residual stresses in critical regions of fatigue to reduce crack driving tensile stresses and to retard the fatigue crack growth or even the fatigue crack initiation. Pressure pulses, induced by short high-energy laser pulses, cause local plastic deformations next to the surface of the material. The elastic relaxation of the plastically deformed area results in the residual stress field. By performing LSP with a Nd:YAG laser, which enables laser pulses with an energy of 5 J and a duration of 10 – 20 ns with a squared laser focus, a higher maximum and penetration depth of compressive residual stresses is achievable compared to traditional methods, e.g. shot peening. Residual stresses can be measured by the incremental hole drilling method based on the integral method.

Numerical simulations are used to gain a deeper insight of the high-speed process and its physical phenomena (e.g. shock wave propagation, plastically affected area, etc.). In a close interaction, numerical and experimental work drive the investigation of the relation between LSP-parameters and resulting residual stresses, answering the question how residual stresses after LSP can be predicted efficiently. Furthermore, it is necessary to study the effect of residual stresses on the fatigue behavior to find the residual stress field, which is needed to manipulate the fatigue crack growth in a certain way. In this way, for example, optimal shot patterns for the process can be identified. As final result a simulation strategy is develop including the prediction of LSP-generated residual stresses, the stress transfer to the fatigue propagation model and the prediction of the fatigue crack propagation rate as final result.

In the department of Joining and Assessment (WMF) a systematic property study of structures is carried out using the following methods: