Purchase Order NNJ07HF31P by National Aeronautics And Space Administration with Georgia Institute Of Technology

THE ATTRACTIVE COMBINATION OF LIGHT WEIGHT AND HIGH STIFFNESS OF AL-LI ALLOYS HAS MADE THEM IMPORTANT CANDIDATE MATERIALS FOR AEROSPACE APPLICATIONS. THESE ALUMINUM ALLOYS HOWEVER, ARE CONSIDERED DIFFICULT TO WELD BY CONVENTIONAL FUSION WELDING DUE TO THE DENTRITIC STRUCTURE TYPICAL OF A FUSION WELD. THE DENTRETIC STRUCTURE CAN SERIOUSLY WEAKEN AND DEGRADE THE WELDED JOINT MECHANICAL PROPERTIES DEEMING THE WELD INEFFECTIVE FOR STRUCTURAL APPLICATIONS. FRICTION STIR WELDING (FSW) WAS DEVELOPED RECENTLY AS A PROMISING SOLID STATE PROCESS WITH ENCOURAGING RESULTS. THIS WELDING TECHNIQUE HAS POTENTIAL SIGNIFICANT APPLICATION IN DIFFERENT INDUSTRIAL APPLICATIONS, AND HAS RESULTED IN WELDED JOINTS BEING USED IN CRITICAL LOAD BEARING STRUCTURES, AND STRUCTURALLY DEMANDING APPLICATIONS. THIS WELDING PROCESS TRANSFORMS THE METAL INTO A PLASTIC STATE AT A TEMPERATURE BELOW THE MELTING TEMPERATURE OF THE MATERIAL, AND THEN MECHANICALLY STIRS THE MATERIAL TOGETHER UNDER PRESSURE TO FORM A WELDED JOINT. THE FSW CONSISTS OF A WED NUGGET, A THERMO-MECHANICAL AFFECTED ZONE (TMAZ), AND A HEAT AFFECTED ZONE (HAZ). FSW TAKES PLACE AT TEMPERATURE BELOW THE MELTING TEMPERATURE; THEREFORE, RESIDUAL STRESSES MAY BE CONSIDERABLY LESS THAN THOSE IN FUSION WELDS. NEVERTHELESS, RESIDUAL STRESSES THAT DEVELOP DURING THE FSW PROCESS CAN REACH VALUES UP TO 75% OF THE YIELD STRESS. THESE RESIDUAL STRESSES CAN HAVE A SIGNIFICANT EFFECT ON THE SERVICE PERFORMANCE OF THE WELDED MATERIAL. RESIDUAL TENSION STRESSES IN THE WELD CAN ALSO RESULT IN STRESS CORROSION CRACKING (SCC). ALUMINUM ALLOYS MAY FAIL BY CRACKING ALONG GRAIN BOUNDARIES WHEN SIMULTANEOUSLY EXPOSED TO CERTAIN ENVIRONMENTS AND STRESSES OF SUFFICIENT MAGNITUDE. THE STRESSES CAPABLE OF CRACK INITIATION AND CRACK GROWTH CAN BE FAR BELOW THE STRESSES REQUIRED FOR YIELDING. INSPECTION AND MAINTENANCE OF THE WELDMENTS CONTINUES TO BE EXTREMELY IMPORTANT FOR PROPER DETECTION AND REPAIR OF STRESS CORROSION CRACKS. HOWEVER, SOMETIMES IT MAY BE DIFFICULT TO INSPECT THE WELDS DUE TO ACCESS RESTRAINTS. THEREFORE, LASER SHOCK PEENING WAS INVESTIGATED AS A MEAN OF MODERATING THE TENSILE RESIDUAL WELDING STRESSES. LASER SHOCK PEENING IS A PROCESS IN WHICH A LASER BEAM IS PULSED UPON A METALLIC SURFACE, PRODUCING A PLANAR SHOCKWAVE THAT TRAVELS THROUGH THE WORKPIECE AND INTRODUCES COMPRESSIVE RESIDUAL STRESSES IN THE MATERIAL. LASER PEENING HAS BEEN PROVEN TO BE CAPABLE OF ENHANCING THE FATIGUE AND MECHANICAL PROPERTIES OF A NUMBER OF METALLIC MATERIALS. THE LASER PEENING TECHNIQUE GENERATES A COMPRESSIVE RESIDUAL STRESS AT THE SURFACE THAT CAN BE SIGNIFICANTLY HIGHER AND DEEPER THAN FOR CONVENTIONAL SHOT PEENING. IN THIS STUDY, SHOT PEENING, AND A VARIETY OF LASER PEENING INTENSITIES WERE USED TO INTRODUCE COMPRESSIVE RESIDUAL STRESSES INTO FSW AA 2195. THE INFLUENCE OF THE DIFFERENT PEENING TECHNIQUES ON STRESS CORROSION CRACKING FOR FSW SPECIMENS WERE CHARACTERIZED AND ASSESSED BY SLOW STRAIN LOAD TESTING. THE BENEFITS FROM THE VARIOUS PEENING TECHNIQUES WERE ASSESSED AND DOCUMENTED.