Wire and Arc Additive Manufacturing of Thin Structures Using Metal Cored Wire Consumables: Microstructure, mechanical properties, and experiment based thermal model
In recent years, Wire and Arc Additive Manufacturing (WAAM) is gaining attractions both from academia and industry, because it has the potential to substitute or complement the traditional manufacturing methods. Traditional materials and methods are reaching their limitation when demanding and special applications are considered. The current growth in WAAM applications will boost the WAAM consumable material development in the coming future. WAAM applications mostly refer to small batch production or prototypes, which often requires special wire compositions or can benefit from tailoring the consumable composition for the desired components. On one hand, solid wire production is only economically viable when large volumes are involved. On the other hand, metal cored wires are particularly suitable to produce tailored or small batch consumable compositions, which is very attractive for WAAM. However, only limited research has been carried out in the use of metal cored wires in additive manufacturing. In this research, the focus is to explore the use of metal cored wire in WAAM applications. Two chemical compositions, one ferrous, medium carbon low steel alloy (AM XC 45) and one non ferrous, Stellite 6 (cobalt based superalloy) metal cored wires were investigated based on industrial interests at RAMLAB. The microstructure of the WAAM AM XC 45 thin wall was characterized using optical microscopy and scanning electron microscopy. Pearlite, ferrite, bainite, and martensite are present in the deposited wall. Columnar grains are found near the fusion line. The repeated thermal cycles cause the grains to become finer from the top to bottom layers. The mechanical properties including microhardness and tensile strength were tested and compared with the traditional processing methods like casting, milling, and forging. It showed a comparable or superior microhardness and tensile strength to the traditional process whereas the relative lower elongation of the deposited AM XC 45 thin wall indicates that further post heat treatment is needed to improve the ductility of the part. Stellite 6 is a cobalt based superalloy, which has good wear and corrosion resistance and retains these properties at high temperatures. In this study, Stellite 6 metal cored wire (WEARTECH(r) WT 6 GMAW C, AWS A5.21 ERCCoCr A) is selected and optimized to obtain results comparable to the commonly employed and more costly laser deposition, in terms of microstructure and mechanical properties. The optimal deposition parameters were developed using the S355 steel substrate, and then adopted on depositing on the AISI 420 stainless steel substrate. Characterization showed that Stellite 6 layer mainly contains Co Cr Fe solid solution with FCC crystal structure as the matrix and the Cr7C3 and Cr3C2 were identified in the microstructure through SEM, EDS, and XRD. These carbides can contribute to the strength. In addition, the identified Co4W2C carbides can contribute to the wear properties of the Stellite 6 WAAM deposits. The dilution and hardness of the WAAM deposit can reach the same level as laser deposition. Additionally, a 3D temperature distribution model is developed by adopting the moving mesh technique to numerically study the WAAM process. The model helps to gain a better understanding of the physical phenomena (heat and mass transfer) during the WAAM deposition. The AM XC 45 was used to validate the developed model. The simulated and experimental results were compared. The dilution and HAZ of WAAM deposited single bead was used as validation criteria. The A1 line (1100K) simulation is in a good agreement with the experimental result. The fusion line (1800K) shows some deviation. Compared with laser deposition, the main differences between the two processes lie on the heat source, the boundary conditions, and the material responses to the process.