Study On Material Plastic Flow Behavior During Friction Stir Welding Of Die Casting AZ91D Magnesium Alloy

Die-casting magnesium alloy is the main composition in the practical engineeringapplication of magnesium alloy. While, serious tendency of weld porosity has restrictedthe further expansion of die casting magnesium alloy. This time, friction stir welding(FSW) technology is considered to be an effective way to solve the problem of diecasting magnesium alloy weld porosity. During FSW, due to the invisibility of metalplastic, theoretical research is extremely difficult to carry out, so no unifiedunderstanding of the mechanism of plastic flow has been formed currently, and thedesign of the pin tool and the optimization of welding process can only be solvedthrough a lot of trial and error experiments. Therefore, carrying out effective analysis ofexperimental methods of plastic flow behavior during FSW in die-casting magnesiumalloy，and mastering regularity of weld shaping has important theoretical and practicalsignificance in the promotion of FSW in the application of die-casting magnesiumalloys.In this paper,4mm thick die-casting AZ91D magnesium alloys were welded byfriction stir welding, and the plastic flow behavior of weld material was investigated byusing tracking of a tracer technique. Based on this, the formation mechanism of internaltunnel defects was analyzed, the results are as follows:①Weld microstructure has differences in horizontal and thickness direction,Therefor, the plastic flow of weld material is " three-dimensional asymmetry". Weldmaterials flow in horizontal direction as well as thickness direction: In the thicknessdirection, top-down, and then down-top flow happened, but the flow trend is notobvious, is a secondary flow. In the horizontal direction, on the same thickness area,material flow in advancing and retreating side experience different patterns, the flow inthe horizontal direction is asymmetry. The paper established a two-dimensional physicalmodel of horizontal flow to explain the experimental phenomena; on the differentthickness area, the migration distance in the horizontal direction decrease with theincrease of the thickness, this is due to the temperature gradient and the different drivemode in the direction of the thickness. Material flow in the horizontal direction directlyaffects the quality of the weld, which plays a dominant role in the whole material flow.②the welding speed and the rotation speed ratio (v/n) has a great influence on thecenter weld material flow in the horizontal direction: with the increase of v/n, the amount of material flowing along the direction of welding increase, and the intensity offlowing forward also increase; On the contrary, with the increase of v/n, the amount ofmaterial flowing backward the direction of welding decrease, and the intensity offlowing backward also decrease.③The improper selection of welding technology will lead to producing tunneldefects in the weld. When v/n is less than0.1,the internal weld is perfect without defect.When v/n is larger than0.1: At the macro level, welding heat is Insufficient, weldingheat input is too low, the volume of plastic metal in the weld is too small; At the microlevel, the material at center weld on the advancing side experience "layered"phenomenon by “pin driving”, and a large amount of material flow forward, which leadto the rear hole can not be filled in time, so the tunnel defects occur in this area. Withthe increase of v/n, the tunnel defect become larger and have a tendency to spread to thebottom of the weld.