Research On The Three-dimensional Characterization Of The Flow Field Of FSW With The Tag Embedded Technology

In this study, AZ31magnesium alloy plates were butt-welded by friction stirwelding to investigate the three-dimensional characterization of the flow field based onthe “marker insert technology”. And finite element simulation was made with“FLUENT” software to reveal the flow pattern of friction-stir welding.The flow way of material is three dimensional asymmetric.In terms of the horizontal direction, the plastic materials in different positions havedifferent flow patterns. Perpendicular to the welding direction, the flow way of theadvancing side is different from the retreating side. Material from the advancing sidecrosses the pin and is divided into two directions: part is dragged forward by the shoulderwhile part is extruded backward. However, material from the retreating side movesbackward only. At different depths, the flow range of plastic material is different. Thecloser to the bottom of the weld, the flow range of plastic material is smaller. And withthe increase of the depth, the forward moving material from the advancing side isdecreased. Along the welding direction, the flow range of the plastic material changeslarger from the initial stage to the end stage.Considering the flow pattern of the plastic metals along the thickness direction, atthe top of the weld, part metals migrate to the center of the joint affected by the shoulderwhile other plastic metals move downward. In the middle part of the weld, plasticmaterials move downward only and form the onion–like structure. At the bottom of theweld, the downward currents are stopped by solid material and backing board andsqueezed up to the high plastic zone.The microstructure of joint is also asymmetric due to the complex material flow.Along the transverse direction, the joint organization is divided into four zones: the weldnugget zone, the thermal mechanical affected zone, the heat affected zone and the basemetal. Along the welding direction, the grain size of the weld nugget zone increasesgradually, and the bending deformation of the grain in the thermal mechanical affectedzone is more and more serious, and the grain size of the heat affected zone increasesslightly. Along the thickness direction, from the top to the bottom, the weld has threeparts. In the most upper zone, very tiny and uniform grains are observed in the coronaryzone. And next to the coronary zone is a center zone, the weld nugget zone (WNZ), with finer and very uniform equiaxed dynamic recrystallization microstructure. At the bottomof the weld, there exists a pin extrusion zone. The organization of this zone is chaotic anddisorderly.Finite element analysis method is used to study the flow field of the friction stirwelding. A three-dimensional model of the friction stir welding in steady state isestablished based on the fluid mechanics theory. The fluid is incompressible,non-Newtonian fluid. And the simulation results agree well with the experimental results.