Study On Fracture Mechanism Of Al/Steel Resistance Spot Welding Joints And Improvement Stategy Of Resistance Spot Weld Bonding Performance

In recent years,aluminum/steel hybrid structure design has received extensive attention and is increasingly being applied to vehicle body manufacturing,which can achieve high structural strength,low manufacturing cost and light weighting.As the main connection method in the automotive manufacturing,resistance spot welding?RSW?,featured by low cost,high efficiency,high reliability and easy automation,has great application potential and practical significance in aluminum/steel joining.However,due to the special dual-nugget structure of the aluminum/steel resistance spot welded joint and the brittle Al-Fe intermetallic compound?IMC?present at the interface,the mechanical properties,fracture mode and fracture mechanism are very different from the simmilar material joint.At present,there is a lack of systematic research in this area,which is not conducive to parameter optimization and structural design.In addition,the introduction of adhesive layer in the spot welding joint is expected to greatly improve the mechanical properties of the aluminum/steel joint and solve the problem of electrochemical corrosion between dissimilar metals,but the adhesive layer will seriously deteriorate the quality of the weld by causing inclusions,porosity and thickening of IMC,resulting in joint strength doesn't meet the requirements of production and utilization.This paper aims to study the influence of macroscopic and microscopic characteristics of aluminum/steel resistance spot welding on the mechanical properties and fracture modes of joints,reveal the main factors affecting the mechanical properties and clarify the fracture mechanism.On this basis,innovative welding schedules and electrode were developed for reliable connection of aluminum/steel resistance spot weld bonding?RSWB?joints.Based on the quantified analysis of macro features,the micro-observation of aluminum nugget and interface,as well as the calculation of Al-Fe IMC growth for the Al/steel RSW joints,the effects of welding current and welding time on the nugget size,microstructure and growth of Fe₂Al₅ were revealed.The growth model of Al-Fe intermetallic compounds was proposed.The increase in welding current led to an increase in the diameter of the aluminum nugget,an increase in the steel side ridges,and helps to eliminate dendrites and inclusion defects at the central area of weld,while avoiding an increase in the IMC thickness,but excessive current resulted in expulsion at aluminum/steel interface.The extension of the welding time helped to obtain a larger aluminum nugget while avoiding the occurrence of expulsion,which transformed the microstructure of aluminum nugget into from cellular to dendrite and eliminated inclusions and the shrinkage within the dendrites,but extending the welding time excessively causes a significant thickening of the IMC and changes the IMC thickness profile from a Gaussian to a bimodal.The aluminum/steel interface was mainly composed of Fe₂Al₅.With the thickening of Fe₂Al₅,it evolved from randomly oriented fine equiaxed grains to coarse columnar grains with preferred orientation along[001]direction,while the fraction of iron phase between the Fe₂Al₅ grains decreased.The growth of Fe₂Al₅ mainly occured in the high temperature region above 900°C,and the root cause of Fe₂Al₅ thickening caused by prolonged welding time was the increase of the high temperature duration.The mechanical properties,fracture mode and fracture mechanism of aluminum/steel spot welded joints were analyzed and compared under lap shear?LS?,cross tension?CT?and coach peel?CP?loading conditions,respectively.The fracture modes of Al/steel RSW joints under three loading conditions were revealed.The correspondence between fracture modes and behaviors and the IMC thickness was explained.The influence of Fe₂Al₅ growth on interfacial mechanical properties was clarified.The study found that the fracture modes included the thickness fracture,the button pullout fracture and the interfacial fracture,and the button pullout fracture had the best mechanical properties.The main influencing factors of the LS were IMC thickness,aluminum nugget size and inclusion defects.In the CT tests,the fracture mode was divided into partial thickness fracture,partial button pullout fracture and interfacial fracture.CT mechanical properties mainly depended on the thickness of the intermetallic compound,the size of the nugget,the inclusion defects and the thickness of the aluminum sheet in the weld area.For the CP testing,the fracture mode was consistent to be a partial button fracture.Compared with the LS and CT tests,the CP peak load was the lowest but the energy absorption was the highest.Its mechanical properties mainly depended on the size of the nugget,the thickness of the aluminum sheet in the weld area,and the inclusions.As the thickness of Fe₂Al₅ increased from 3?m to 6.5?m,the interfacial fracture toughness was reduced from 1.07 MPam^1/2 to 0.35 MPam^1/2,and it droped to 0.56MPam^1/2 when it reached 4?m.The enhancement of the texture of Fe₂Al₅,the coarsening of the grains,and the reduction of the iron phase between the grains decreased the interfacial fracture toughness.Based on the basic theory obtained in the research of Al/steel RSW and the experimental analysis of the issues in Al/steel RSWB,solutions to expel the adhesive before welding by introducing the pre-cleaning schedule and modifying the electrode geometry were proposed.The 2-step schedule with multi-pulse pre-cleaning schedule and multistep electrode were designed.Based on this,the control objectives were achieved and reliable Al/steel RSWB joints were obtained.The results showed that when the tranditional1-step welding schedule without pre-cleaning and the dome electrode were used for aluminum/steel RSWB,there were a large amount of inclusions in the aluminum nugget near the interface,and the IMC excessively grew,leading to deterioration of the weld strength.The multi-pulse pre-cleaning schedule was employed to expel the adhesive in the aluminum/steel interface by inducing an appropriate expulsion.In the subsequent welding stage,the appropriate welding time?600 ms?avoided excessive growth of intermetallic compounds,obtained a large aluminum nugget and dissolved the inclusions.The multistep electrode arranged on the aluminum side promoted expulsion generation in the pre-cleaning stage by enlarging the contact angle,while the steps on the surface cut off the oxide film on the aluminum surface,reducing the interfacial reaction between the electrode and aluminum.Compared with the Al/steel RSW joint,the LS peak load of the cured Al/steel RSWB joint increased by 130%?from 4.9 kN to 11.1 kN?,and the energy absorption increased by about 44 times?from 3.4 J to 153.3 J?.In addition,the CP peak load and energy absorption increased by 18%?from 0.71 kN to 0.84 kN?and 83%?from12.6 J to 25.6 J?,respectively.