# The Current State of Research of Wire Arc Additive Manufacturing (WAAM): A Review
## 2 Materials, Processes and Methods
### Path Planning and Building Strategies  
Path planning and building strategies have a significant impact on the geometrical correctness and thermal management in wire arc additive manufacturing, as well as reducing residual stresses and distortion [146]. The thermal conditions have a direct impact on the microstructure and mechanical properties of the manufactured part. This has been shown by various working groups. For example, [PERSON] et al. showed for high-strength steel that the distance between welding paths has a direct influence on the building heights [62]. Therefore, [PERSON] et al. identified the path planning and building strategies as the main challenges in wire arc additive manufacturing [147]. To overcome this issue, different approaches have been created to figure out how process parameters like the wire feed speed, travel speed and others influence the weld bead width and height and make them controllable [51, 62, 117]. [PERSON] et al. developed a prediction model for the weld seam width and height on the basis of the dynamic response of a WAAM process [148]. These approaches to make the necessary building height for each layer and the path distance predictable are necessary for further path planning. The general adaption of layer heights and layer widths is shown in Figure 13. A common way that the deposition parameter for the different processes could be identified was described by [PERSON] et al. [149]. Based on these process-related parameters, different path planning strategies for various applications are currently being investigated [50, 59, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171]. [PERSON] et al. proposed path planning based on medial axis transformations, with good results for a fully filled part without a significant amount of defects [150]. [PERSON] et al. showed that division of the part into different zones can be beneficial for path planning [151]. [PERSON] et al. presented a holistic and data-centric approach through a knowledge base to achieve a welding path plan [152]. [PERSON] et al. pointed out that lateral extensions are a critical point and have a significant possibility to generate failures, and they developed a strategy to realize lateral extensions [172]. Aside from the normal geometric aspects of path planning, the thermal history is the second key to reach the proposed material properties, and this in conjunction with adequate process surveillance can lead to material property-oriented path planning, which is supposed in Figure 8. The thermal history, on one hand, has an influence on the welding-related residual stresses and the distortion of the part, and on the other hand, the thermal history has an influence on the microstructure of the material. Both aspects need to be taken into account for a material property-oriented path planning. The influence of the path planning strategy on the residual stresses has been studied as well [173]. [PERSON] et al. showed that the path planning strategy has a direct influence on the residual stress distribution and identified the distance between single welding passes as the key point for accumulation of residual stresses [173]. The occurrence of residual stresses and distortion of welded parts is closely related to the obstruction of thermal expansion and constrictions. If the occurring stresses exceed the yield strength, the material will react and result in geometrical distortion. The distortion of WAAM-generated parts due to welding-related residual stresses has been investigated by [PERSON] et al. [174]. They studied the angular distortion of various materials with bead on-plate welds and transferred their results to a finite element model (FEM). Furthermore, they focused on the relationship between characteristic material properties like the yield strength, Young's modulus and thermal expansion coefficient [174].