Wire arc additive manufacturing (WAAM) is the process by which large, metallic structures are built, layer-by-layer, using a welding arc to melt wire feedstock. In this process, the proper selection of the shielding gas plays a vital role in the achievement of structurally acceptable part geometries and quality surface finishes. In this study, the authors used either a ternary mix (He, Ar and CO2) or a binary mix (Ar and CO2) of shielding gases to deposit wall geometries using an open loop-controlled WAAM system developed at Oak Ridge National Laboratory’s Manufacturing Demonstration Facility. The binary blend produced a wider and shorter geometry, while the ternary blend resulted in a narrower build that was more equivalent to the CAD geometry. The data indicated that the binary blend provided a higher oxygen concentration in the weld as compared to that of the ternary blend. The results imply that the arc characteristics and heat input had a significantly higher impact on the weld penetration than the surface tension effect of surface active elements. This was further verified by developing and applying a high-fidelity computational fluid dynamics (CFD) model of the thermophysical properties of gas mixtures. The results from the model showed that, while the influence of increased oxygen concentration on the surface tension for the binary blend led to a deeper penetration, the ternary blend gave rise to heat flux to the workpiece.
The applications for metal additive manufacturing (AM) are expanding. Powder-bed, powder-fed, and wire-fed AM are the different kinds of AM technologies based on the feeding material. Wire-Arc AM (WAAM) is a wire-fed technique that has the potential to fabricate large-scale three-dimensional objects. In WAAM, a metallic wire is continuously fed to the deposition location and is melted by an arc-welding power source. As the applications for WAAM expands, the quality assurance of the parts becomes a major concern. Nondestructive testing (NDT) of AM parts is necessary for quality assurance and inspection of these materials. The conventional method of inspection is to perform testing on the finished parts. There are several limitations encountered when using conventional methods of NDT for as-built AM parts due to surface conditions and complex structure. In-situ process monitoring based on the ultrasound technology is proposed for WAAM material inspection during the manufacturing process. Ultrasonic inline monitoring techniques have the advantages of providing valuable information about the process and parts quality. Ultrasonic technique was used to detect the process condition deviations from the normal. A fixture developed by the authors holds an ultrasonic sensor under the build platform and aligned with the center of the base plate. Ultrasonic signals were measured for different process conditions by varying the current and gas flow rate. Features (indicators) from the radio frequency (RF) signal were used to evaluate the difference in signal clusters to identify and classify different build conditions. Results show that the indicator values of the ultrasonic signals in the region of interest (ROI) changes with different process conditions and can be used to classify them.
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