Analysis and optimization of hybrid WAAM-milling process

The hybrid additive-subtractive manufacturing processes combine the advantages provided by the metal additive manufacturing with the high accuracy of the machining processes. Among these technologies the combination of WAAM (Wire-Arc-Additive-Manufacturing) and milling is an attractive option. The WAAM process is a metal additive manufacturing technology that uses arc welding to create metal components. It provides a high deposition rate and enables to manufacture large components. It requires a reduced investment compared laser based technologies both in terms of installation and operation. Moreover, WAAM operations can be performed on existing machine tools by a simple retrofitting to provide them arc welding capability. Despite its advantages, the hybrid WAAM-milling process has several drawbacks which limit its diffusion among the manufacturing companies. The goal of the Ph.D. work presented in this thesis is to analyze such criticalities, proposing solutions to overcome or mitigate the process issues. Since the considered technology is a hybrid process, the overall performance depends on both the involved technologies. Hence, part of this thesis is strictly related to the WAAM process, while a further one is related to the milling of WAAM manufactured parts. For what concerns the WAAM process, this thesis is focused on the thermal issues induced by the arc welding. The heat input of the process can cause large distortions, residual stresses and even lead to the structural collapse of the workpiece. This thesis pinpoints the process simulation as an efficient and effective approach to overcome the WAAM thermal issues. The current simulation techniques are analyzed, proposing improvements that aim at increasing the simulation time efficiency without losing accuracy. The proposed modelling techniques are validated comparing simulations with the actual process, both in terms of temperature field and workpiece distortions, confirming their accuracy. The proposed simulation techniques are applied to tackle the heat accumulation issues, responsible for the structural collapse of WAAM workpieces. To overcome this issue, two different approaches are proposed: i) an innovative cooling system, developed by using the proposed simulation technique ii) a simulation-based algorithm to schedule inter-layer idle times for workpiece cooling. These techniques are validated by simulation and experiments, showing their effectiveness in preventing the detrimental effect of the heat accumulation phenomenon. For what concerns the milling of WAAM components, this thesis pinpoints two main criticalities: the poor machinability of WAAM material and the issues related to machining the thin walled features of the WAAM workpieces. The machinability aspect is tackled by a comparative cutting force analysis on a reference material. This analysis highlights an increase of specific milling cutting forces on an AM processed material with respect to the traditional material. To overcome the issues related to thin walled features, the thesis proposes a spindle speed optimization algorithm based on FE modelling of the workpiece. The algorithm is experimentally validated, highlighting its accuracy in predicting the workpiece dynamics and comparing the results achieved by different optimization strategies. In summary, the aim of the thesis is to contribute to the development of the hybrid WAAM-milling, providing tools to support the process planning of such operations.