Additive-subtractive manufacturing of parts produced by WAAM

Wire and Arc Additive Manufacturing (WAAM) is an additive manufacturing technique that uses an electric arc as a heat source to melt the metal wire and deposit a component layer-by-layer. WAAM allows to produce medium to large metal parts with moderate geometrical complexity in a material- and cost-efficient way. However, the WAAM parts are characterised by low surface quality and dimensional accuracy leading to the necessity of further post-processing using conventional subtractive techniques (i.e., milling, turning, grinding). To produce a part that meets the requirements, the entire manufacturing process plan, starting from the design of the workpiece for WAAM to the final part, should be considered. This thesis focuses on the investigation of the complete WAAM based Additive-Subtractive Manufacturing (ASM) process chain.

First, a standard welding equipment for Gas Metal Arc Welding (GMAW) and a standard industrial welding robot arm were adapted for production of metal components using WAAM. For this purpose, the robot cell was integrated into CAM software and empirical models for weld bead geometry prediction depending on welding parameters were developed for conventional GMAW and CMT processes. Conventional GMAW and CMT processes were compared showing that multiple factors determine the applicability and productivity of the GMAW process.

Second, the impact of welding heat input on microstructure and mechanical properties of solid WAAM parts, such as hardness, tensile strength and Charpy impact toughness, was determined. The properties of the WAAM material were compared to the properties of filler wire and hot-rolled structural steels showing that the 3Si1 wire is suitable to produce functional components taking static and impact loads using WAAM.

Third, the required processes and their sequence within the ASM process chain were experimentally investigated. The optimal ASM process chain for hybrid WAAM parts was determined and includes the preparation of the reference surfaces and 3D scanning as intermediate processes. The necessity of post-weld heat treatment depends on the precision of the part and its application.

Fourth, the amount of stock material to be removed was determined. The effective wall width was experimentally determined depending on the WAAM process parameters. By using empirical models for weld bead width and effective wall width, a minimum allowance to remove initial wavy WAAM surface was estimated. Additional allowance was investigated experimentally for thin-walled and solid WAAM parts considering the impact of thermal induced deformations and substrate removal step on dimensional and geometrical accuracy of the components. Recommendations for assignment of the allowances for subtractive manufacturing were provided.

Fifth, the machinability of thin-walled WAAM components was investigated. The impact of WAAM process parameters on as-deposited part’s characteristics (i.e., hardness, flatness deviation and total wall width) was analysed. Further, the impact of the as-deposited part characteristics along with the milling parameters on the final surface quality obtained after post-processing was determined.

The dynamic behaviour of the WAAM part during milling depending on WAAM and milling parameters was investigated. The types of vibrations that might excite for specific milling parameters, dimensions and geometry of the tool and natural frequency were predicted and compared to the time-domain displacement signals and surface roughness profiles. An example of cutting parameters optimization to avoid the chatter vibrations during milling of WAAM parts and improve the final surface quality was provided.

Finally, the obtained research results were applied to produce industrial case study parts using the WAAM-based ASM process chain. The valorisation activities performed during this dissertation were described. Close cooperation with the industrial companies revealed the main barriers to the industrial application of the WAAM based ASM process chain. The future valorisation strategy is focused on the bilateral contract research as the most promising strategy for the industrial implementation of the gained knowledge and future technology development.