Wire arc additive manufacturing is a method that uses an electric arc together with a metal wire to produce 3D objects. Molten metal from the wire is deposited layer-by-layer along a defined path to get the desired shape upon solidification. This method has a greater deposition rate and could be used to build large-scale products compared to other additive manufacturing methods. The gas metal arc welding-based cold metal transfer process has been widely used for wire arc additive manufacturing due to its reliability and low cost. Materials such as aluminum alloys, titanium alloys, magnesium alloys, and steel had been tried out.

High strength low alloy steel has a high demand for wire arc additive manufacturing applications in a vast area of applications in construction, marine, automotive, power, and machining industries because it has good material qualities such as high strength, toughness, low cost, and good formability. However, one of the key problems in wire arc additive manufacturing of the high strength low alloy steel is surface oxide accumulation. These oxides are floating in the melt pool and accumulate at the top of the build structure. This creates issues when large structures are built and may start to trap inside the component as inclusions. Frequent cleaning is required between layers to minimize the effect of the oxides. Therefore, minimizing the formation of these oxides is crucial for such materials.

This study was carried out to investigate how to minimize the formation of surface oxide during the cold metal transfer wire arc additive manufacturing of Aristorod 89, a high-strength low alloy steel. A two-level, three-variable design of experiment was used. Heat input, CO2 percentage of shielding gas, and the use of a gas trailing shield (yes or no) attached to the arc to protect the developed material layer from immediate exposure to the atmosphere were used as variables. In the study, i) accumulation of surface oxides, ii) current, voltage, and power variations, iii) geometric variations, iv) microstructure, and v) hardness variation were investigated. Real-time images of the melt pool during the deposition were captured to observe the formation of oxides. 

A larger amount of surface oxides accumulated on the deposited bead. Generally, oxides were not trapped inside the built material. The results of the study revealed that reducing the CO2 in the shielding gas (from 20% to 5%) reduced surface oxide formation significantly. The use of the gas trailing shield also reduced surface oxides by more than 30%. Changing the heat input did not affect the oxide formation significantly. However, deposition efficiency (deposition volume per unit of heat input) was increased when the heat input was increased. Therefore, a combination of high heat input, low CO2, and a gas trailing shield is a suitable condition for the WAAM of Aristorod 89. Surface geometry measurements did not reflect any relationship with the variables used. Martensite-ferrite, tempered martensite, martensite-bainite, and granular bainite were observed in different zones in the microstructure. The hardness of the material ranged from 320HV to 400HV showing different value ranges in different zones. The highest hardness values were observed in critically heat-affected zones. 

Further analysis of the microstructure using the SEM to identify any oxide inclusions in the microstructure, examine more cross-sections from samples to correlate defects with input variables, and map the hardness of different samples are recommended for future work. Further, a comparison of the tensile strength for different scenarios could provide a better overview to decide on using the gas trailing shield and less CO2 in the shielding gas.