| Abstract Scope |
During the fabrication of Navy ships, the shipbuilder in several manufacturing operations employ the use of labor-intensive manual Gas Tungsten Arc Welding (GTAW) processes for pipe, ship structures and tanks. Shipbuilders also have operations using challenging weld alloys for cladding (depositing a layer of secondary material of a sufficient thickness on the surface of specific components to minimize corrosion and ensure strength). Manual GTAW is relatively slow, particularly for large diameter circumferential welds. Also, manual GTAW requires a high level of skill due to the dexterity required to use both hands independently. A complementary process, Gas Metal Arc Welding (GMAW), is used for structural welding. GMAW has the advantage of high deposition rates, but it is susceptible to higher levels of porosity than GTAW, resulting in potential rework. Therefore, a one-to-one substitution of manual GTAW for GMAW is not a viable solution. It is proposed to replace manual GTAW with a semi- automatic variant of the GTAW process. This process features an automatic "hot-wire" filler metal feed that replaces the legacy manual hand fed wire into the weld pool. Development began by identifying candidate part applications. This entailed identifying base material combinations, weld joint geometry, and weld positions that would be encountered for ship fabrication. The base materials for weld development were HY-80, Inconel 625, duplex stainless steel, 304 stainless steel, CuNi alloy, and carbon steel. Weld development position was either flat or vertical. Once the candidate applications were identified, the project team, in collaboration with the Navy, determined the qualification requirements in accordance with Naval Sea Systems Command (NAVSEA) Technical Publication S9074 AQ GIB 010/248 (Tech Pub 248). Based on this collaboration effort, a weld quality test plan was created. Process development was performed in an iterative fashion, first establishing relationships between arc/cold wire amperage, wire/hot wire amperage, wire feed speed, and wire oscillation frequency. Preliminary parameters were first established on scrap plate material for each position. Once the initial weld parameters were developed, the procedures were applied to the different test assembly designs. A limited amount of nondestructive testing (NDT) and metallography was done to confirm weld quality based on the appropriate qualification testing in Tech Pub 248. Once the weld procedures were established for each test assembly, the detailed weld quality test plan was executed. The weld test matrix consisted of thirty-two assemblies which were grouped in three qualification lanes: Cladding, Structural, and Complex Geometry. After completing each test assembly, the welds were evaluated by NDT, consisting of visual examination (VT), liquid penetrant testing (PT), ultrasonic testing (UT), or Radiographic Testing (RT). All welds underwent limited destructive testing by examining weld cross section macrographs, hardness test mapping, tensile, and bend test. In the case of the duplex stainless steel a sigma phase analysis was done to investigate the effect of high and low cooling rates. After the weld quality execution plan was completed successfully, the process parameters/ procedures were transferred from the laboratory to a shipyard environment to validate the use and performance of the semi-auto GTAW process. As a final study, an investigation was explored to determine the productivity improvement from welding manually with conventional GTAW to that of the hot-wire semi-auto GTAW process. Weld process development for hot wire semi-auto GTAW yielded acceptable results during the execution of the weld quality test plan for all material types within the identified test assemblies. Performance testing met all the requirements for NAVSEA Technical Publication S9074 AQ GIB 010/248 (Tech Pub 248). NDT acceptance criteria was in accordance with Class 1 requirements of MIL STD-2035A. It was noted that the reciprocating wire frequency is to be considered an essential variable for Navy qualification. The laboratory results for parameter development transferred to the shipyard and yielded successful results as well. Shipyard welders embraced the process immediately. The development has demonstrated process improvements in welding, greater than 30%, for various welding applications in the shipyard. The technology is now being implemented to support the construction of upcoming Navy platforms. |