Investigation of wire and arc additive manufacturing methods for high integrity, high productivity fabrication of large steel structures.

This thesis describes advances in aspects of deposition process control, mechanical performance optimisation and improved material property development, undertaken for high productivity and integrity wire and arc additive manufacture of large heavy-walled components. One key objective of this study was to undertake a fundamental study to understand the important factors at play during tandem-GMAW deposition of single bead and multi-layer wall structures, with a focus on achieving substantial improvement in deposition rate and surface quality of WAAM components. The study of the influence of key deposition parameters on the instantaneous arc characteristics and deposited bead geometry was conducted. The study found that while wire feed speed and travel speed significantly affected the arc and bead characteristic, the influence of contact tip to work distance was minimal. However, the later strongly influences the arc stability, due to its effect on the mode of metal transfer. The study also showed that consistent high quality single bead deposit is achievable when the lead wire is set about 1-2m/mins lower than the trail wire and increase in total wire feed speed of tandem process above 24m/mins, produced insignificant effect on bead width. Furthermore, tandem parallel wire configuration, produced the optimum surface quality and metal deposition efficiency; however, it was more susceptible to defect formation at higher travel speed, compared to tandem series wire set-up. The fundamental experience and knowledge gained through the aforementioned process study was instrumental in building a rectilinear wall structure, with excellent surface and geometrical quality and, subsequently a large skin and core part, with significant improvements in deposition rate and surface quality of the component. Thermal cycle generation during deposition of a thick-walled WAAM structure, built using 2.25Cr 1Mo type steel composition and response of the structure to heat treatment were studied. The main focus was to improve understanding on the effect of thermal cycle on the mechanical performance of the component and develop a suitable heat treatment regime to restore an acceptable material property, without requiring the more expensive and complex austenitisation heat treatment process. The study found that the generated thermal cycle produced microstructural heterogeneity, high hardness level, with large gradient in hardness of the multi-layered structure, in as-deposited condition. This resulted in poor impact toughness. However, the applied post deposition heat treatment parameters, was beneficial in homogenising the material, reducing the high hardness and variation in hardness, and restored the impact toughness. Tandem GMAW modification of alloy 2.25Cr 1Mo type wire composition, (using ER90S-G wire), with ER120S-G wire, having higher nickel content, was studied to exploit the potential for compositional modification, through this manufacturing route. The mixing of the two wires was carried out to determine whether higher nickel content can improve the impact toughness and eliminate the need for PDHT. The focus was to increase the as deposited charpy impact toughness, while maintaining excellent all-round static mechanical properties. The result showed that significant increase in charpy impact toughness, in both testing direction, at -30deg C, was achieved with the modified composition, containing equal proportions of ER90S-G and ER120S-G wire materials. Finally, laboratory sour service corrosion test was conducted to compare the corrosion performance of WAAM alloys, obtained under different processing conditions, to a similar but conventional wrought steel variant. The results showed that both the exposure temperature and time, accelerated the corrosion degradation of the materials. Also, while 2.25Cr 1Mo WAAM alloy and the wrought variant showed comparable corrosion performance, the modified WAAM alloy containing equal blend of ER90S-G and ER120S-G wire, exhibited superior corrosion performance compared to the wrought alloy, which was attributed to the higher nickel content of the former.