Coaxial laser metal deposition with wire (LMD-w) is an innovative additive manufacturing technology in which a wire is coaxially fed through the center of a hollow laser beam into a laser-induced melt pool. This special configuration results in a direction-independent process, which facilitates the manufacturing of thin-walled metal components at high deposition rates. However, laborious experimental test series must be conducted to adjust the process parameters so that the substrate and the part do not overheat. Therefore, models are needed to predict the resulting temperature field and melt pool dimensions efficiently. This paper proposes a finite element simulation model using an innovative heat source, which considers the unique intensity distribution of the annular laser spot. The heat source parameters were calibrated experimentally based on fusion lines obtained from metallographic cross sections of aluminum alloy samples (AA5078 wire and AA6082 substrate). Subsequently, the temperature distribution in the substrate plate was measured by means of thermocouples to validate the developed model. It was shown that the proposed heat source replicates the heat input accurately. With the presented model, essential features for process development, such as the temperature field and the melt pool dimensions, can be reliably predicted. The model contributes to a better understanding of the LMD-w process and facilitates an efficient process development in future research work as well as for industrial applications. Key words: thermal simulation, annular laser spot, heat source, laser metal deposition, coaxial wire feeding, directed energy deposition
Coaxial Laser Metal Deposition with wire (LMD-w) is a valuable complement to the already established Additive Manufacturing processes in production because it allows a direction-independent process with high deposition rates and high deposition accuracy. However, there is a lack of knowledge regarding the adjustment of the process parameters during process development to build defect-free parts. Therefore, in this work, a process development for coaxial LMD-w was conducted using an aluminum wire AlMg4,5MnZr and a stainless steel wire AISI 316L. At first, the boundaries for parameter combinations that led to a defect-free process were identified. The proportion between the process parameters energy per unit length and speed ratio proved crucial for a defect-free process. Then, the influence of the process parameters on the height and width of single beads for both materials was analyzed using a regression analysis. It was shown that linear models are suitable for describing the correlation between the process parameters and the dimensions of the beads. Lastly, a material-independent formula is presented to calculate the height increment per layer needed for an additive process. For future studies, the results of this work will be an aid for process development with different materials.
Due to their outstanding characteristics, additive manufacturing processes are attracting increasing industrial interest. Among these processes, laser metal deposition (LMD) is an innovative technology for the production of metal components. In order to create three-dimensional parts, wire or powder is deposited layer-wise onto a substrate. When wire is used as feedstock, major drawbacks of the powder-based process, such as the low material usage, contamination of the process cell with metal powder, and health or safety issues, can be overcome or even avoided. In addition, recent developments in laser optics allow for a coaxial wire feeding in the center of an annular laser beam. This eliminates the strong directional dependence of the process when feeding the wire laterally. However, wire-based LMD is highly sensitive to process disturbances, which impedes its broader industrial application. Since it is necessary to completely melt the fed wire to achieve a stable process, self-regulating effects such as overspray in powder-based LMD are not present. In contrast to the widely investigated thin walls, the build-up of multi-track solid structures poses a particular challenge. Therefore, process strategies for producing such solid structures are presented in this paper. The experiments were carried out using a laser processing head that enables coaxial wire feeding (CoaxPrinter, Precitec). By systematically varying the lateral overlap between adjacent weld beads, it was shown that an optimum exists at which minimum surface waviness is achieved. Based on this, defect-free multi-layer solid components could be generated in a reproducible manner. During the process, the melt pool temperature was evaluated using a pyrometer. Furthermore, a microscopic examination of the resulting parts was conducted. The results obtained show the need for process monitoring and control, for which a novel and holistic approach has been developed.
Laser metal deposition (LMD) is an additive manufacturing process in which a metal powder or wire is added to a laser-induced molten pool. This localized deposition of material is used for the manufacturing, modification, and repair of a wide range of metal components. The use of wire as feedstock offers various advantages over the use of powder in terms of the contamination of the process environment, the material utilization rate, the ease of handling, and the material price. However, to achieve a stable process as well as defined geometrical and microstructural properties over many layers, precise knowledge on the effects of the input variables of the process on the resulting deposition characteristics is required. In this work, the melt pool temperature was used as an input parameter in LMD with coaxial wire feeding of stainless steel, which was made possible through the use of a dedicated closed-loop control system based on pyrometry. Initially, a temperature range was determined for different process conditions in which a stable deposition was obtained. Within this range, the cause-effect relationships between the melt pool temperature and the resulting geometry as well as the material properties were investigated for individual weld beads. It was found that the melt pool temperature is positively correlated with the width of the weld bead as well as the dilution. In addition, a dependence of the microhardness distribution over the cross section of a weld bead on the melt pool temperature was demonstrated, with an increased temperature negatively affecting the hardness.
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