Research On Resistance Spot Welding Of Unequal Thickness DP590 Steel/6061 Aluminum Alloy In Lightweight Body

With the call for energy conservation and emission reduction,the lightweight of automobile has been paid more and more attentions.Steel and aluminum alloy are the main materials in automobile manufacturing.The increase in the use of lightweight materials,such as aluminum alloy,in the automobile design and manufacturing process is the most efficient and economical way to achieve the lightweight of automobile.However,the physical and chemical properties of steel and aluminum alloy are different,and the Fe-Al mutual solubility is extremely low,and brittle intermetallic compounds are easily formed.Therefore,the mechanical properties of steel and aluminum alloy welded joints are highly poor.This has become one of the key technical issue for the development of automobile lightweight,which needs to be overcome urgently.In this study,the spot welding process of 1.2mm thick DP590 GA dual phase steel plate and 2mm thick 6061-T6 aluminum alloy plate was studied in depth in the way of the combination of process research and ABAQUS finite element analysis.The results of the study were as follows:Under the welding current of 14kA-22 kA,with the increase of welding current,the indentation rate,the diameter of the nugget and the tensile shear force increased accordingly.The concrete manifestation was that the indentation rate of aluminum alloy increased from 13% to 38%.The nugget diameter increased from 6.25 mm to10.05 mm,and the tensile shear force increased from 2.013 kN to 6.215 kN.In the welding time of 200ms-400 ms,the indentation rate,nugget diameter and tensile shear force increased with the prolonged welding time.The specific manifestations were as follows: the indentation rate of the aluminum alloy increased from 22% to 43%,and the nugget diameter increased from 8.21 mm to 10.28 mm,tensile shear force increased from 5.335 kN to 6.615 kN.In addition,the electrode force rose from700 LBS to 1500 LBS,and the nugget diameter and tensile shear force showed the law of increasing first and then decreasing.The peak values at 1100 LBS are 9.67 mm and6.056 kN,respectively,and the indentation rate fluctuated within a certain range.In order to obtain the optimal welding process parameters,an orthogonal process test was designed on the basis of the above tests.The results showed that the joints with the best mechanical properties of the welding can be obtained at I=18 kA,t=300ms,F=1300 LBS.The nugget diameter and shear force were 8.64 mm and 4.759 kN,respectively.The different welding currents affected the tensile shear force most,the welding time did less,and the electrode force did the least.The failure modes of the above test joints were button-type fracture and fracture at the joint surface,and the nugget diameter was a key factor affecting the value of the tensile shear force.The macroscopic cross-sectional morphology of the joint aluminum alloy presented a "spoon" shape.The microstructure of the nugget bottom,center region,and edge region were columnar crystal,equiaxed crystal and cell crystal,respectively.The nugget region of dual-phase steel was shown as non-uniform lath martensite microstructure.The intermetallic compound layer with non-uniform thickness was developed at the interface,the thick of middle area is the thickest,and the edge is the thinnest.The results of ABAQUS finite element analysis under the best parameters showed that the stress concentration occurred near the electrode edge of the interface between electrode and workpiece during the pre-compression phase,and the value of stress reached the maximum.During the welding heating phase,under the combined action of the temperature and the electrode force,the stress distribution was quite different from that in the pre-compression phase.The analysis of temperature field showed that when the electrode and workpiece were electrified for 0.066 s,the heat generation is dominated by the contact resistance,and the temperature of the interface rose rapidly.Electrified for0.15 s,the aluminum alloy began to melt at the interface,and a nugget occurred at 0.23 s.At the end of the welding,the temperature distribution of the aluminum-steel interface showed a decreasing trend along the radial direction,and the temperature of interface center was the highest,reaching 1243?,The difference between the simulated nuclear diameter and the actual nuclear diameter is about 9.2%.