Effect Of Tool Pin Eccentricity On Microstructure And Mechanical Properties Of Friction Stir Welded Thin-Plate Aluminum Alloy

Due to the advantages of small deformation,green non-pollution,high quality,friction stir welding has been widely used in aerospace,rail transit,ship building and other fields.The stirring pin is one of the core components of the friction stir welding equipment,which directly determines the plastic metal flow during the welding process,and affects the joint performance and the weld formability.The reasonable stirring pin design is essential for obtaining the ideal welded joint.At present,researches on the design of the stirring pin mostly focus on changing the shape of the stirring pin,try to strengthen the flow of the plastic metal material and improve the joint quality with complicated shape design(such as a tapered stirring pin,a spiral pin).However,complex geometric pins require a longer processing time,which increases the welding cost.Therefore,in order to reduce costs,the design of the mixing pin should be simplified as much as possible while ensuring the quality of the joint.Based on the above discussion,eccentric stirring pins were designed.It is expected that the eccentric motion of the stirring pin during the welding process will increase the movement dimension of the weld metal,promote the metal plastic flow,and improve the joint quality.In this paper,three eccentric pins are designed,and eccentricities are 0mm,0.4mm and 0.8mm respectively.The paper studies the effect of eccentric pins on the welding process through similar metal(5052 alloy)and dissimilar metal(5052-6061 aluminum alloy).Firstly,the microstructure of the welded joint was observed by metallographic microscope(OM).The stress of the weld material using eccentric and non-eccentric stirring pin was analyzed and compared.In addition,the plastic metal flow model during the eccentric friction stir welding process was established.Then,field emission scanning electron microscopy(SEM),electron backdiffraction(EBSD),transmission electron microscopy(TEM)and other means were used to further reveal the micro structure differences of the joints prepared with eccentric and non-eccentric stirring pins.It is found that the dynamic volume of the eccentric stirring pin is larger than that of the non-eccentric stirring pin,which makes the surrounding plastic metal transfer path longer,increases the welding heat loss,and reduces the peak temperature of the weld nugget zone,and promotes the refinement of the grains.The area of the onion ring in the weld nugget zone reflects the mechanical stirring behavior of the stirrer pin,this is to say,the larger the area of the onion ring,the stronger the mechanical stirring behavior of the pin.The area of the onion of similar welded joints produced by 0mm,0.4mm,0.8mm eccentric tools is 0.5mm2,5.5mm2,and 6.5mm2 respectively,which proves that the pin-eccentric use helps to strengthen the stirring behavior of the pin.According to the force analysis of the weld metal and the established physical model of the flow,the eccentricity of the stirring pin increases the friction force with the weld metal.The force causes the weld metal to be transferred throughout the circumference of the stirring pin.Compared with the non-eccentric amount,the metal transfer path is increased,so that the advancing side metal and the receding side metal are alternately combined in the weld nugget region to form an onion ring.Organization,and the greater the amount of eccentricity of the stirring pin,the more obvious this tissue.The tensile test shows that for the eccentric friction stir welding of the same metal,The yield strength of the welded joints with the eccentric 0mm pin is 125MPa,while the welded joint produced by the 0.4mm eccentric has the highest yield strength of 147MPa,which is about 20%higher than the yield strength of the joint produced by the non-eccentric pin.For the eccentric friction stir welding of dissimilar metals,the yield strength of the eccentric joint is also better than that of the joint with the non-eccentric pin.When the 5052 aluminum alloy is placed on the advancing side,the yield strength of the joints corresponding to the 0 mm,0.4 mm,and 0.8 mm eccentric pins is 122MPa,141MPa,and 146MPa,respectively.In addition,the strength of the joint is also affected by the placement position of the metal.When the 6061 is placed on the advancing side,the yield strength of the joints with the 0mm,0.4 mm,and 0.8 mm eccentric pins is 118MPa,124MPa,and 128MPa,respectively.On the basis of confirming that the eccentric stirring has a beneficial effect on the welding,an eccentric friction stir welding experiment in which an intermediate metal(copper foil,zinc foil)is added to the 5052 aluminum alloy weld is carried out.Research shows that during the welding process,the added metal is broken into fine particles and distributed in the matrix,and the plastic metal is mixed with the rotation of the stirring pin.Under the action of welding thermal cycle,the added metal and the weld matrix mutually diffuse to form a solid solution,which increases the hardness of the weld nugget zone.The use of the eccentric stirring pin further promotes the mixing of the added metal and the matrix compared with the use of non-eccentric pin.The joint produced by the eccentric stirring pin has a more uniform hardness distribution in the weld nugget region produced by the non-eccentric stirring pin.