Numerical Design And Process Modeling Of High Depth-to-width Ratio Friction Stir Welding

Friction stir welding was developed and patented in 1991 by The Welding Institute,and is considered a solid state joining technique in which no melting of the base material occurs.Despite the efficiency and energy saving features,friction stir welding is regarded as a state-of-the-art joining process sparked with its high mechanical performance.However,for the specific application like friction stir butt welding of hollow aluminum profiles with a large thickness-to-width ratio,the conventional length-to-width ratio 0.33 of the welding tool is incapable of achieving the joining process.In this paper,high depthto-width ratio friction stir welding was proposed.The tool has a designed topology with a length-to-width ratio of 0.6,which can effectively apply for these specific demands.A sequential fluid-solid-interaction model was established based on FLUENT and ANSYS.A high-throughput screening was applied to preliminarily optimize the topology of the welding tools and the welding parameters via fracture analysis and welding defect prediction.Then the optimum design of the joint mechanical performance was proceeded based on the combination of the material flow model and the experimental approaches.Subsequently,the microstructure evolution was studied quantitatively in terms of precipitation dynamics and dynamic recrystallization,which achieved a theoretical design system with self consistency for high depth-to-width ratio friction stir welding.Firstly,the topology of the welding tools and the welding parameters were optimized via 3-dimensional fluid-solid-interaction model.The numerical evaluation model was proved to be accurate and practical for high-throughput screening.The thread structure could promote the material flow but increases the fracture risk.The welding defect prediction revealed that pins with milling facets were more beneficial to achieve sound joints;Optimized topology of tapered thread W6 steel pin with triple facets was determined at a welding speed of 30 mm/min and a rotational speed of 800 rpm.Sound joint without obvious thickness reduction was attained based on the optimal geometrical structure,which was consisted with the numerical results.Then,the mechanical property of the high depth-to-width ratio friction stir welding was further optimized via flow model combined with experimental methods.The preliminary high-throughput screening proved that the H13 steel tool had a better balance between strength and toughness,which can achieve larger parameter windows with nondefect joints.The heat input decreased effectively and the plasticized material flow was enhanced compared to the conventional friction stir welding.The tiny shoulder diameter of high depth-to-width ratio friction stir welding reduced the heat generation.The thread structure and the milling facets increased the strain rate greatly under the extremely low heat generation,avoiding the welding defects.The process-structure-property linkage was achieved via the comprehensive evaluation based on response surface method.The peak temperature and strain rate of 648 K and 151 s-1 led to the elimination of void defects and achieved the lowest coarsening degree of precipitate in high depth-to-width ratio friction stir welding;The optimum tensile strength and elongation of the joint obtained at the welding speed of 300 mm/min and rotational speed of 800 rpm were 265 MPa and 8.1%,equivalent to 86% and 52% of base material;No heat affected zone was obtained in the optimum joint,which inhibited the major weakness of the joint mechanical performance.Lastly,the microstructure evolution of high depth-to-width ratio friction stir welding was studied.The grain microstructure obtained by electron backscattered diffraction method affirmed the non-heat-affected-zone,non-thermo-mehanically-affected-zone phenomenon at the welding speed of 300 mm/min;Atypical grain coarsening in the welding nugget zone was observed at 300 mm/min,which has a microstructure of mainly unequiaxed coarse grains doped with a small amount of moderately fine grains.The average grain size was 266.3 μm,3.3 times of the base material and 74.0 times of the welding nugget zone at 30 mm/min,while the aspect ratio was 2.85;The atypical coarsening conditions were investigated based on the modified Wagner-Kampmann precipitation dynamics and Smith-Zener grain migration pinning effects for dynamic recrystallization:(a)extremely high plastic strain and strain rate;(b)peak temperature slightly higher than 0.5 melting temperature to inhibit dynamic recrystallization refinement and original shear induced refinement.