Investigation of Friction Stir Weldability of V-95 CHT1 and D-19CHT Russian Aluminum alloys and evaluating metallurgical feasibility of replacement of riveted joint of stringer-panel of fuel tank of Sukhoi-30MKI aircraft with FSW

Abstract

Friction stir welding (FSW) is a solid-state joining technique that can weld alloys not easily weldable using conventional weld techniques. FSW is a solid state weld process and is undertaken at significantly lower temperatures compared to the conventional fusion weld process. Hence, problems such as plate distortion and solidification and liquation cracking/inclusion common in fusion welds are avoided. . V-95CHT1 and D-19CHT Russian are heat treatable, high strength precipitation hardenable alloys used extensively in airframe structure of Russian aircrafts and are not weldable by conventional fusion weld techniques. These plates are presently being joined on the airframe of the aircraft by riveting which causes significant increase in the all-up weight of the aircraft as well as increased manufacturing time and cost. No study has been reported that has studied the replacement of the riveting with FSW for these alloys. This study aims to investigate the friction stir weldability of V-95CHT and D-19 CHAT aluminum alloys with the specific objective of exploring the metallurgical feasibility of replacing the riveted joint of the D-19 stringer to V-95 panel of Outer Wing Tank 3 of Sukhoi 30 aircraft with lap FSW. The static and dynamic strength of FSW and riveted panels have been compared during the study. Weld parameters were optimized based on hooking defects and joint strength. The study included fabrication of welded specimen, static and dynamic strength testing of welded and riveted specimen, optical, SEM and TEM study of welded microstructure and electrochemical evaluation through immersion and cyclic polarization tests and effect of post weld heat treatment on the mechanical and electrochemical characteristics of the welded alloys. The static joint and panel strength of FSW specimen welded at different process parameters was compared to the strength of similar riveted joint and panel riveted in the same configuration as found on the aircraft panel. Fatigue strength of the welded and riveted specimen was also compared. The friction stir welded joint and panel was observed to have higher joint and panel strength compared to the riveted specimen. The double pass FSW joint and panel was observed to have substantially higher static and dynamic strength compared to single pass weld and riveted panels. The electrochemical characterization of the friction stir welded joints and plates were undertaken as per ASTM G34 EXCO immersion test as well as through cyclic polarization test. Both the tests have revealed that the weld operation has made the V-95 plate more susceptible to pitting and intergranular corrosion while the D-19 plate is hardly affected. The nugget of the weld is least affected and the hottest regions of HAZ were identified most susceptible to corrosion. Joints with hotter parameters employed during the weld were shown to have lower corrosion susceptibility. The post weld Retrogression and Re-ageing treatment (RRA) was shown to have improved the corrosion resistance of the weld nugget as well as the HAZ of the joint. RRA treated specimen was shown to increase strength of joints fabricated at hot weld process parameters while it slightly degraded joint strength for welds joined using low heat input process parameters. The RRA treated welded samples were also demonstrated to provide higher joint and panel strength compared to riveted joints/panels. Hardness testing of the SP and DP welded specimen indicated that the second pass of the DP weld significantly reduced the hardness of the weld nugget of the first pass while the nugget hardness of the second pass was comparable to that of the nugget of the SP weld. ‘S’ bend Hammer test on both the welded as well as the base metal plate specimen were satisfactory with no cracks revealed on the weld area within the maximum bend angle providing crack free performance in the base metal plate. Micro-structural analysis through optical methods and TEM analysis indicated varied grain size and structure in the three weld zones with the nugget having fine recrystallized grains compared to the HAZ and base metal. The grain size in the HAZ was slightly larger than that of the base metal although the size was by and large comparable. The intermetallics in the nugget was also found significantly broken down and fine as compared to that in the TMAZ and HAZ. The strengthening precipitates of the nugget were not visible in the optical microstructure, however, these nano- scale particles were found densely populated in the nugget in the TEM image of the nugget. The precipitates in the Retrogression and Re-aged sample were larger and dense compared to the as-received V-95CHT1 base metal. The grain boundary precipitates of the RRA treated alloy were also more spaced apart and larger in size compared to the non heat treated alloy. The electrochemical characterization of the friction stir welded joints and plates were undertaken as per ASTM G34 EXCO immersion test as well as through cyclic polarization test. Both the tests have revealed that the weld operation has made the V-95 plate more susceptible to pitting and intergranular corrosion while the D-19 plate is hardly affected. The nugget of the weld is least affected and the hottest regions of HAZ were identified most susceptible to corrosion. Joints with hotter parameters employed during the weld were shown to have lower corrosion susceptibility. The post weld Retrogression and Re-ageing treatment (RRA) was shown to have improved the corrosion resistance of the weld nugget as well as the HAZ of the joint. RRA treated specimen was shown to increase strength of joints fabricated at hot weld process parameters while it slightly degraded joint strength for welds joined using low heat input process parameters. The RRA treated welded samples were also demonstrated to provide higher joint and panel strength compared to riveted joints/panels. Hardness testing of the SP and DP welded specimen indicated that the second pass of the DP weld significantly reduced the hardness of the weld nugget of the first pass while the nugget hardness of the second pass was comparable to that of the nugget of the SP weld. ‘S’ bend Hammer test on both the welded as well as the base metal plate specimen were satisfactory with no cracks revealed on the weld area within the maximum bend angle providing crack free performance in the base metal plate. Micro-structural analysis through optical methods and TEM analysis indicated varied grain size and structure in the three weld zones with the nugget having fine recrystallized grains compared to the HAZ and base metal. The grain size in the HAZ was slightly larger than that of the base metal although the size was by and large comparable. The intermetallics in the nugget was also found significantly broken down and fine as compared to that in the TMAZ and HAZ. The strengthening precipitates of the nugget were not visible in the optical microstructure, however, these nano- scale particles were found densely populated in the nugget in the TEM image of the nugget. The precipitates in the Retrogression and Re-aged sample were larger and dense compared to the as-received V-95CHT1 base metal. The grain boundary precipitates of the RRA treated alloy were also more spaced apart and larger in size compared to the non heat treated alloy.