Engineering development of the additve manufacturing process for a structural component produced by robotic GMAW CMT additive manufacturing

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Insinööritieteiden korkeakoulu | Master's thesis
Mechanical Engineering
Degree programme
Master's Programme in Mechanical Engineering (MEC)
The production of metal parts with Additive Manufacturing (AM) comprises a series of promising technologies with great challenges yet to overcome. There are not many existent processes able of producing metal parts with high quality, namely good surface finishing, no defects and good mechanical and corrosion performance in a costly-effective manner. Generally, metal-AM processes are only the preferable method for parts made of expensive materials, with high geometric complexity and low batch sizes that would require a really extensive machining process. The application of wire and arc (WAAM) process has proved to increase productivity and spread out the limits of AM but mechanical properties still demand further investigation and development. This thesis details the development of a wire and arc AM (WAAM) process using Gas Metal Arc Welding (GMAW) with Cold Metal Transfer (CMT) to manufacture an industrial mixer impeller made of 480 MPa steel, which included several computational and experimental studies. The work done in this thesis is a first approach to the proper development of WAAM manufacturing at the Aalto University, with ongoing work to ensure reliability and quality of the process. The CMT enables high controllable short circuit transfer mode with low heat input compared to conventional GMAW. The CMT was selected to explore the capacity to produce narrow thickness blades of about 4 mm. Numerical fluid-dynamic and structural models of the mixer impeller were created with ANSYS to provide a procedure to measure the performance of different geometries and to compare them. Two models were created, one with straight blades and another one with tapered blades. The models were also used to provide an estimation of the real performance of the component. The fluid-dynamic simulations exhibit a similar performance between both models. The power consumption due to fluid flow was in accordance with literature values. Static-structural simulations showed maximum stress values endurable by the material with safety factors within the typical range of design for mixing systems. A new cooling and clamping system were design and manufactured to implement this WAAM action. Three GMAW processing conditions were implemented and tested to evaluate the influence of the different manufacturing conditions in the component produced. This conditions were two different sets of parameters with CMT and one with conventional short circuit transfer. The processing parameters were tuned to meet the geometrical requirements with stability of the arc and metal transfer. E.g. an increased hold time of 0.5 s at the end of the weld seams to ensure layer homogeneity. After achieving layer homogeneity, several blade samples were produced to extract specimens to be tested varying the transfer mode and the wire feed speed. Prototypes of the impeller were successfully produced using the processing conditions delivering better superficial finishing. Finally, a sustainability assessment of the WAAM process was carried out in comparison with one machining process for a functional unit of one impeller. The assessment using LCA methods showed that the manufacturing of the impeller was more sustainable for WAAM process. An analysis of the processes as a function of the geometry showed results aligned with the literature, justifying the WAAM process over machining for higher geometrical complexity levels.
Vilaça, Pedro
Thesis advisor
Kretzschmar, Niklas
additive manufacturing, wire and arc additive manufacturing, gas metal arc welding, cold metal transfer, CFD, structural FEM
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