| Aluminum alloy thick plate has some excellent characteristics such as low density,high specific strength,high fatigue life and good corrosion resistance,which makes it present a wide application in aerospace,ship and vehicle industry fields.In general,the method of welding forming is very important for manufacturing aluminum alloy components.Friction stir welding(FSW)is a promising solid-state process,and it can avoid metallurgical defects including pores and weld cracks during fusion welding,which is beneficial to improve mechanical properties of the welds.The FSW technology is successfully used to welding aluminum alloy sheet components.However,there are many problems for FSW aluminum alloy thick plates.For example,the range of selected welding process parameters is narrow,the loose zone or hole-type defect is easily formed in the weld and the mechanical properties of the welds are poor,etc.The main reason is that plastic flow behavior of weld metal in extruding zone around pin tip is changed.Therefore,the material flow behavior in the weld for FSW 20 mm thick aluminum alloy was investigated.The detailed plastic flow pattern of local plastic material in the weld especially in the extruding zone was studied by experiments.Moreover,the forming mechanism of loose zone defect in the weld was revealed in detail.The effects of tool geometric parameters,base material mechanical properties and temperature field on the material flow behavior and the forming quality of the weld were investigated.The expected aim was to control the flow condition of plastic material in order to improve the quality of FSW joints.The results in this study can provide reference data and theoretical guidance for the weld forming mechanism and the weld quality control of FSW aluminum alloy thick plates.The main conclusions are as follows:(1)The center zone of FSW joint is comprised of shoulder affected zone above and nugget zone below.During FSW,the plastic material on the top of the weld continuously flows towards the tool pin tip,and then the extruding zone is formed.The plastic material in the extruding zone starts to horizontally flow and upwards expand,which can make it be metallurgic bonded with the metal in the shoulder affected zone,and forming a weld finally.When the shoulder affected zone and nugget zone is not close bonded,the loose zone or hole-type defect is formed.The flow pattern of plastic material in the extruding zone around the pin tip is investigated,and which mainly include axial extruding flow,horizontal friction flow and bypassing flow.Moreover,the changing of the plastic material flow is related to the temperature of the weld metal and the constrained condition of its surrounding metal.If the temperature of plastic material in the extruding zone is low and the flow resistance of its surrounding metal to plastic material horizontally transferring is big,which can make plastic material in the extruding zone flows upwards along the tool pin surface by way of bypassing flow pattern to the weld surface to form flash.There is no enough weld metal to fill in the cavity behind the pin,so the loose zone or hole-type defect is formed.(2)The size of transient cavity and the acting force of the weld metal around the pin are influenced by pin eccentricity,pin taper and pin surface profile,which can significantly change the temperature and the flow behavior of plastic material in the extruding zone.When the pin eccentricity is increased from 0mm to 0.2mm,the volume of the transient cavity around the pin increases,and it can make the quantity of the weld metal transferring into the extruding zone increase.The temperature of plastic material in the extruding zone from 1mm of the weld bottom increases by about 25?,and the force resistance of its surrounding metal to plastic material horizontally transferring reduces which cause the transfer distance of plastic material obviously increasing.The area of nugget zone increases by 52%,and the interior defect in the weld is eliminated.With increasing the pin taper from 10° to 20°,this pin can obviously enhance the downward acting force for plastic material.The total quantity of the weld metal transferring into the extruding zone increases and the temperature of plastic material in the extruding zone increases by about 20?,which can make the force resistance of its surrounding metal decrease.The area of nugget zone becomes larger while the area of loose zone decreases.However,when a pin with a bigger taper is used during FSW,the volume of the pin notably reduces which can decrease the frictional area with its surrounding metal.Also,the size of the cavity behind the pin also decreases.The flow capacity of the weld metal is significantly weakened.The pin with three flats on its surface can increase the volume of the cavity during FSW which can improve the flow capacity of the weld metal transferring into the extruding zone.This can make the volume of plastic material transferring in the extruding zone and the horizontal transfer distance obviously increase.Therefore,the area of nugget zone increases,and the loose zone defect in the weld disappears.(3)Tool shoulder and pin-tip profiles can change the driving force of the weld metal for flowing which influences its flow behavior.Compared with a shoulder with a concave profile,the shoulder profile with three raised grooves can enhance the driving force of plastic material on the weld above for flowing towards the pin end,which increases the quantity of plastic material transferring from the pin's surrounding into the extruding zone around the pin tip.Furthermore,the temperature on the bottom of the weld raises,the extruding zone is easier to expand outward.The area of nugget zone increases,and the loose defect disappears.The pin with a pin-tip profile of three-grooves can enhance its stirring force for plastic material in the extruding zone which results in the driving force for horizontally flowing increasing.Also,the frictional area between the pin tip and the weld metal is increased.The temperature of the metal on the weld bottom and the width of nugget zone are increased.(4)When the heat treatment conditions and their composites of aluminum alloys are different,the frictional condition of the contacted interface between the rotating tool and welding material is changing,and the number of heat generation is also varied during FSW,which can change the temperature field of welding joint,thus the plastic flow behavior of the weld metal is influenced.The quantity of the friction heating between the rotating tool and aluminum alloy metal is less because the viscosity of the weld metal is larger for FSW 7075 aluminum alloy with O state than T6 state,which makes the temperature of plastic material in the extruding zone and its surrounding metal be lower.Also,the amount of plastic material transferring into the extruding zone remarkably reduces.This results in the horizontal transfer distance of plastic material in the extruding zone and transfer height of deforming material in thermo-mechanically affected zones decreasing,and the width of nugget zone on the bottom for FSW 7075 aluminum alloy with O state become smaller.In comparison to 7075 aluminum alloy,the condition of the contacted interface between the weld metal of 5A06 aluminum alloy and the rotating tool is transformed into half-stick friction,which causes the temperature of plastic material in the extruding zone slightly decreasing.However,the deforming resistance of its surrounding metal due to lower yield strength of base material is smaller.Finally the transfer distance of plastic material in the extruding zone in the weld of 5A06 aluminum alloy is larger than that of 7075 aluminum alloy,and the size of nugget zone reduces a little.For FSW 2024 aluminum alloy,the contacted interface condition between the weld metal and the rotating tool is earlier to transfer from stick friction to slip friction during welding,which can obviously reduce the total heating production.The temperature of plastic material in the extruding zone is lower,and the total amount of the weld metal transferring into the extruding zone is less.Therefore,the horizontal transfer distance of plastic material in the extruding zone decreases,and the size of nugget zone is the smallest among them.(5)Raising appropriately the assisted-heating temperature can improve the flowability of plastic material in extruding zone.With increasing the assisted-heating temperature from room temperature to 150?,the temperature of plastic material in the extruding zone in the weld increases by 50?.Moreover,the temperature of its surrounding metal correspondingly increases and the horizontal flow capacity of plastic material significantly improves,which make the area of nugget zone and upward transfer height of deforming material in thermo-mechanically affected zones increase by 49% and 40%,respectively,and the loose zone in the weld disappears.However,when the assisted-heating temperature is increased to 250?,the temperature of plastic material in the extruding zone decreases by 25? than that in 150? condition.The flow resistance of plastic material increases,and the horizontal transfer distance reduces,which results in the area of nugget zone and upward transfer height of deforming material in thermo-mechanically affected zones decreasing by 38% and 27%,respectively,and the loose zone defect appears in the weld again.It is considered that this may be related to the change in the friction condition of the contacted interface between the rotating tool and weld metal during FSW. |
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