Laser-Powder Bed Fusion (LPBF) is a layer-based technique of 3-dimensional additive manufacturing. One of the major advantages of the LPBF technique is the short production time for individual parts. Applications are for example dental technology, biomedical application or aerospace engineering [1
]. The goal is to produce components for different industrial fields with high dimensional accuracy. The advantages of the LPBF process are the possibility of lightweight construction by lattice structures [3
], the flexibility of geometry and the integration of functions [6
]. In order to utilize these advantages, component designed for LPBF tend to have a higher level of complexity, with a significant amount of thin-walled parts. Unlike in conventional fabrication processes, the dimensional accuracy often poses a challenge in LPBF [8
In the melting process, the locally concentrated energy input of the laser leads to distortion and residual stresses in the part. There are two main theories for the generation of residual stresses, the Temperature Gradient Mechanism (TGM) and the Cool-Down Phase [9
]. The Temperature Gradient Mechanism describes the generation due to the layered structure of the process, first, a layer is melted and then the following layer. Because of the rapid heating by the laser and the low heat conduction, a high temperature gradient is created. The heat conduction is limited by the thin walls, which results in a small solidified cross-section available for heat transport in the solid, while the powder-bed acts like a thermal insulation. Therefore, stresses arise in the components due to the high temperature gradients [11
]. The Cool-Down Phase model describes the formation of the residual stresses by the melting material and the subsequent solidification. During solidification and cooling, the metal shrinks and contracts thereby stresses are generated [10
]. The already solidified layers inhibit the shrinkage and tensile stresses occur in the upper layer [9
]. Residual stresses are separated into three types: macro-residual stresses, which span several grains (type I), micro-residual stresses within one grain (type II), and sub-micro residual stress of several atomic distances within a grain (type III) [12
]. For the dimensional accuracy in the LPBF process residual stresses of type I are relevant [10
]. The residual stresses can lead to deformation and even cracks in the parts [9
]. Regarding component accuracy, residual stresses should be avoided. The main methods to reduce residual stresses are improving the scan strategy [13
], using powder bed preheating [15
] and via heat treatments after the fabrication process [18
With the help of the exposure strategy, the methodology of fusing the individual layers can be changed and thus the residual stresses can be reduced. With a change of the exposure strategy, the temperature profile within the layer is influenced, this can reduce the residual stresses, taking TGM into account. With the standard stripe exposure strategy, the test results vary. Cheng et al. found out in their experiments that a line exposure with a 45
angle for tension measurement gives the best results and exposed from inside to outside [19
]. In contrast to this, Kruth et al. reported that the successive island exposure with short vectors are preferable [14
]. Dunbar et al. and Ali et al. showed that the rotation of scan vectors lead to a decrease of the deformations [13
]. In the experiments of Mercelis and Kruth, residual stresses were lower in direction of exposure, compared to the perpendicular in-plane direction [10
]. Yu et al. studied the influence of re-melting on porosity and have shown that re-melting with same and opposite directions are the same in central areas of printed parts, the same direction re-melting have better results on porosity at edges [21
]. In summary, it can be stated that the experiments yielded different results. Common features in the investigations are that the exposure strategy influences the residual stresses.
In this work, the resulting residual stresses and the dimensional accuracy of specimens manufactured by the stripe and sectional scan strategies were analyzed. According to the models for the creation of residual stresses, which can lead to distortion, a suitable strategy for thin-walled components will be investigated. Lower residual stresses are expected from scan vectors along the component contour [9
]. The focus of this investigation is to improve the scan strategy with the scope of dimensional accuracy. A sectional scan strategy is developed and results are compared to the stripe scan strategy. With a scanning path orthogonal to the contour is deviations from the nominal dimension of the component width are observed. The best results are obtained with a scanning path along the contour path [22
]. The dimensional accuracy of thin-walled geometries before and after separating the part from the base platform will be investigated. To identify the influence of the scanning strategies on the residual stresses, the stresses after separation will be measured.
Two topics were discussed, the general result of the stripe strategy and the comparison with the sectional strategy. For thin-walled parts, the standard stripe scan strategy led to large deviations. X-ray diffraction results showed a formation of tetragonal martensite in these thin-walled specimens, which had no significant height dependence, which speaks against martensite shrinkage due to tempering processes. This led to a difficulty to quantify signal contributions to X-ray diffraction measurements of residual stresses. However, concerning dimensional accuracy, the investigation of the sectional strategy showed significantly better results with concern to the concavity of the outer surface and improvement of the diameter deviation of up to 81% compared to the stripe strategy. To improve the dimensional accuracy, the scaling should be adjusted to thin-walled components. For this purpose, the axis specific scaling has to be adapted to other aspects, such as geometry and machine parameters.
From an application point of view, the acceptance of additive manufacturing of metal components is growing with geometric accuracy. The required accuracy for the application is already achieved for several components. With the increasing functionality of the additively manufactured components, the requirements for dimensional accuracy increase. To date, this can only be achieved within limitations and often results in the necessity of mechanical post-processing.