Research On Aluminum/Steel Dissimilar Metal Resistance Element Welding

When resistance spot welding is used to weld aluminum alloys and high-strength steels,the intermetallic compounds formed at the aluminum-steel interface are the main load-bearing parts,which causes the joint mechanical properties to be greatly reduced.The resistance element welding(REW)process,by converting the main bearing part of the joint into the connection of the same material,cleverly avoids the problem of hard and brittle phases easily caused by direct resistance spot welding of dissimilar metals,so that the bearing area It has become a nugget formed by the steel rivet and the steel plate,thereby improving the mechanical properties of the joint.This makes the REW process one of the important methods for dissimilar metal connection under the background of light weight.At present,most of the research on REW process starts with welding materials.This article starts with steel rivets and is committed to the optimal combination of steel rivets and steel plates,so as to achieve the high efficiency,safety,reliability,and the low-cost resistance unit welding connection is suitable for applications in the automotive industry.The main research contents are as follows:(1)Three different shapes of rivets are designed: countersunk head rivets,round table rivets and flat head rivets.The diameter of the head of the rivet is 10 mm,the height of the head of the rivet is 1mm,and the diameter of the end of the rivet is 6mm.Three different rivets are matched with 5052 aluminum plate and connected to the lower layer DP780 steel plate by REW.Experimental results show that the combination of countersunk head rivets,round table rivets,flat head rivets and DP780 high-strength steel plates can achieve reliable connection of dissimilar metals of aluminum / steel.The countersunk head rivet REW joint has the locking effect of the rivet cover compared with the round table rivet joint,and can obtain a larger nugget diameter than the flat head rivet joint under the same welding current,thereby ensuring the tensile and shear mechanical properties of the joint.Compared with the other two types of rivets,the process window is wider.(2)Three types of Q235 steel countersunk head rivet shaft diameters(4mm,6mm,8mm)were designed,and then matched with 2mm thick 5052 aluminum plate,and connected with 2mm thick DP780 cold rolled steel plate by REW.The experimental results show that when the diameter of the rivet shaft is 6mm,compared with the 4mmm rivet,it can withstand the electrode pressure intact,and the rivet will not collapse.Compared with 8mm rivets,although 6mm rivets are upset due to welding heat input and electrode pressure,the diameter of the rivet shaft is smaller than the diameter of the rivet cover,which can better lock the aluminum plate between the steel rivet and the steel plate..(3)The macrostructures and microstructures of the 6mm countersunk head rivet REW joint was studied,and it was found that the intermetallic compound formed between the DP780 / 5052 aluminum alloy interface is Fe2Al5 and FeAl3,and the intermetallic formed between the Q235 rivet / 5052 aluminum alloy interface The compound is FeAl3.The tensile-shear experiments show that there are three fracture models for the countersunk head rivet resistance element welding head,including interface failure model,nugget pullout failure model,and aluminum heat affected zone failure model.The welding parameters significantly affect the tensile-shear mechanical properties of the resistance element welding joint.As the welding current increases from 12 kA to 22 kA or the welding time increases from 9C to 19 C,it will increase the welding heat input,increase the nugget diameter,and thus increase the mechanical properties of the joint.On the other hand,the increase in heat input simultaneously reduces the average hardness of the heat affected zone and the bearing area of the aluminum plate,which reduces the mechanical properties of the joint.As the electrode pressure increases from 0.15 MPa to 0.4 MPa,the indentation rate on the Q235 side is reduced,which improves the mechanical properties of the joint,on the other hand,it reduces the nugget diameter and reduces the mechanical properties of the joint.