Abstract
In the following master thesis, an attempt was made to use the high strength Al-Zn alloy EN A W 7075, which is considered to be non-weldable, for the laser deposition welding process. For this purpose, deposition welding tests with various parameters were carried out on a NC machine with a glass fibre laser with appropriate optics. The aim ofthe work was to identify the influence of certain parameters on defect formation, such as pores and cracks, and thus to develop a parameter set that produces a structure with as few defects as possible. To this end, multilayer wall-shaped build-up welds were produced and investigated, using various destructive and non-destructive test methods.
As a result of this work, the generation of low defect structures can be regarded as successful. By varying parameters such as welding speed, wire feed and correspondingly adjusted laser power, it was possible to produce crack-free and largely pore-free specimens. The susceptibility to crack formation could be clearly correlated with the line energy. At low line energies, strong crack formation occurs in the structures, some of which propagate through previously crack-free layers. Here a high temperature gradient in the cooling phase is assumed to be the cause. A further finding is the balance, between low microporosity and high hardness on the one hand and dimensional accuracy and stable process of the additive manufactured structures on the other hand. In order to achieve the lowest possible microporosity, it is necessary to achieve low intensities and low heat input. lt is assumed that this results in a smaller melt pool with a lower temperature, which leads to a lower evaporation of alloying elements. The other side behaves in the opposite direction. Here a stable process is achieved by a large heat input with high intensity. As a result, a large melt pool with higher temperatures is assumed, in which evaporation of the volatile alloying elements, especially Zn and Mg, is favoured, but which results in the build-up of more uniform layers.
In order to be able to achieve the positive properties from both extremes and thus compete in mechanical properties with components from conventional production, the process conditions must be better understood and controllable.
