Increasing the time allowed between cleaning and welding of aluminum joints may result in fewer reworked weld joints and lower fabrication costs on several Navy ship classes. NSWCCD photo
This project’s objectives were to better quantify the effects of the delay time between cleaning and welding on aluminum weldment mechanical properties.
Aluminum welded joints are sensitive to defects if the faying surfaces (i.e., surfaces to be joined) are not cleaned properly prior to welding. These defects include porosity, inclusions, entrapped oxides, and other discontinuities that can degrade the mechanical properties of the joint. Usually, a weld joint is prepped, cleaned, and tack welded and then left in this condition for some time prior to final welding. There is concern that moisture and shipyard debris may contaminate the joint surfaces during this delay, so Navy procedures call for re-cleaning of the joint prior to final welding. However, there is no standard procedure for re-cleaning the tack-welded joints, and the guidance on a delay time- limit varies from platform to platform. For example, the current requirement for the Littoral Combat Ship (LCS) 2 Independence Class is to re-clean the joint and adjacent surfaces if it sits longer than 16 hours, but, for other platforms (e.g., CG 47), this requirement is as little as six hours. Furthermore, commercial shipyards contend that cleaning only the exposed surfaces is adequate, and that breaking the tack welds to fully access the faying surfaces is too costly and unnecessary for a sound weld joint. This issue also impacts the LCS 1 Freedom Class, JHSV, LHA, and future Ship- to- Shore Connector (SSC) new construction, as well as Landing Craft, Air Cushion (LCAC), FFG 7, and CG 47 Class repairs.
The mechanical property test results may provide guidance on aluminum welding procedures across the Fleet. Alternatively, if degraded mechanical properties are found, it may be possible to address those weaknesses in the joint design.
Project results may show that longer delays or simpler cleaning procedures (e.g., not breaking tack welds) may result in acceptable weldment mechanical properties and, thus, lead to significant cost savings. A second cost benefit may result from relaxing the requirement for detailed inspection of joints that are not classified as high stress but are required to be inspected due to noncompliance with current Naval specifications. Another potential benefit is optimization and standardization of aluminum welding requirements across multiple platforms, thereby reducing the administrative cost of maintaining multiple welding guidance documents.
Project test results (along with other factors) have contributed to the conclusion that tack-welded joints do not need to be broken, cleaned, and re-welded in cases where the exposure time has exceeded the requirement for LCS 2 Class; these results were implemented in early FY12. However, project results concerning the potential for extending the delay time for re-cleaning weld joints were inconclusive due to significant data scatter in tensile strength test results. Implementation of these results on various ship classes will depend on completion of an additional test program and positive test results. The first anticipated implementation will be on development of the fracture control plan for LCS 2 Class primary hull structures being fabricated at Austal USA in Mobile, AL; as of FY12 Q4, this plan is in development at NAVSEA. Another ship type that may benefit from these results is the Joint High Speed Vessel (JHSV), which also is being fabricated at Austal USA.
Naval Sea Systems Command (NAVSEA)
Naval Surface Warfare Center, Carderock Division