New Method Of Friction Stir Welding To Stabilize Grain Structure In Weld Nugget

Friction stir welding(FSW) is a relatively new solid-state joining technique. It has commendably solved the problem of joining high-strength Al alloys, which are difficult or impossible to weld conventionally. FSW can produce a weld with better microstructure and mechanical properties as compare with conventional welding. However, the problem of abnormal grain growth(AGG) occurring in weld nugget after heat treatment may cause a loss in formability and mechanical properties in some process of rocket, railway and automobile industries. It has become one of the key points in FSW territory to improve the stability of the fine grain structure in FSW nugget.In this dissertation, new FSW method by adding solder into FSW nugget is studied. By improving welding process, well-distributed second phase particles were brought into weld nugget to exert a pinning effect on grain boundaries. Using optical microscope、scanning electron microscope and transmission electron microscope, the reaction mechanism between solder and base material was studied, as well as the distribution of second phase particles and their effect on grain structure. Moreover, the new method to improve the stability of the fine grain structure in FSW nugget was proved to be true.Because of the better macro- and poorer micro-material flow in FSW process, micron-sized metal and nano-sized intermetallic particles made different behavior in distribution. Micron-sized metal particles were well distributed throughout the nugget after 4 passes, while most of the intermetallic particles still showed an accumulation. The content of second phase particles in weld nugget was obviously higher than that in base material. The pinning effect on grain growth after recrystalization was stronger with higher content of second phase particles thus that the grain structure could better stay fine.The heat treatment results showed no AGG occurring in weld nugget. Regions with higher content of second phase particles presented less grain growth than regions with lower content. Regions where AGG was supposed to occur theoretically remained a fine grain structure due to the existence of second phase particles. Regions with larger grain size avoided AGG due to lack of requirement for AGG.Experiment results proved that the existence of second phase particles restricted grain growth both after recrystalization and heat treatment.