| Currently, metal matrix particle reinforced composites have great porosity, segregation and interface reaction tendency which seriously affect the quality of weld joints. In this paper, the Friction Stir Welding(FSW) technology of in-situ TiB2/A356 composites which prepared by mixed salts reaction are studied. The influence of process parameters and the shoulder forms on weld appearance, microstructure and mechanical properties(include tensile property and hardness) has been researched. At the same time, this paper also analyzes the flow way of weld materials through marker-insert technology and finite element simulation made with FLUENT software.It was founded that the quality of weld appearance improves with the increase of rotational speed or decrease of welding speed after FSW. And the quality of weld appearance in concave surface shoulder is better than plane shoulder. The microstructure and mechanics property of weld are investigated. The conclusion is that the grain of weld is finer and the distribution of TiB2 and Al-Si eutectic more uniform than base metal. It also showed that the mechanical properties of the joints are significantly affects by the welding parameters. The mechanical properties(tensile property and hardness) improve with the increase of rotational speed or decrease of welding speed.The transfer behavior of the plastic metal in the weld thickness can be divided three areas which are Shoulder Affect Zone(SAZ), Turbulence Zone(TZ) and Weld nugget Zone(WZ). In SAZ, the plastic metal circles the stir head in horizontal direction. The plastic metal converges in TZ, where come from multiple directions. The metal of WZ makes helical movement under the action of thread and exhibit the onion ring structure. In addition,the transfer behavior of the plastic metal has relationship with the welding parameters. With the increase of the rotational speed, the area of SAZ enlarges. And the area of WZ decreases with the increase of welding speed. It also showed that different forms of shoulder have impact on metal flow in SAZ. In terms of the horizontal direction, material from the advancing side crosses the pin and is divided into two directions: part is dragged forward by the shoulder while part is extruded backward. However, material from the retreating side moves backward only. At different depths, the flow range of plastic material is different.And with the increase of the depth, the forward moving material from the advancing side is decreased.Finally, a three-dimensional flow model is established based on the fluid mechanics theory. Comparing simulation results with marker-insert test results, they can better match.With the increase of the depth, the temperature decreases and the flow velocity of material also reduce. In terms of the horizontal direction, velocity gradient of material flow in advancing side is more than retreating side, which indicating the materials of advancing side flow more intensely. The 2-D numerical model of the longitudinal section along the weld is established, which takes the thread into account. The simulation results can agree well with the material flow of keyhole. Finite element simulation of FSW has a certain theoretical and practical significance of the weld and weld quality evaluation. |
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