| In the current development of automobile manufacturing field,improving fuel efficiency,reducing carbon emission and improving safety have been the three major concepts in the development of automobile design.Among them,weight reduction of automobiles is considered to be the key issue in reducing fuel consumption,but the ensuing issue of light weight of automobiles contradicts with the high standard of crashworthiness.Therefore,although the light weight of automobiles can ensure the fuel economy,its safety will be seriously affected.In recent decades,the lightweight alloys for automobiles have been widely researched,such as aluminum alloys,magnesium alloys and high-strength steels,etc.Although aluminum and magnesium alloys can meet the demand for lightweight automobiles,their production and manufacturing costs are still higher.However,the development of advanced high-strength steel（AHSS）is considered to significantly reduce their manufacturing costs while ensuring the overall quality of automobiles,which in turn can contribute to China’s Manufacturing 2025,carbon peaking and carbon neutrality goals.The current welding of high-strength steel for automobiles varies depending on the material properties and plate thickness,as does the choice of welding method and its weld formation.Among them,tungsten arc welding（TIG）uses high-efficiency non-melting electrodes and is easy to maintain a constant arc length,welding current and stable welding process,which in turn can obtain smooth,uniform welds and high-quality joints.However,when the pulsed current was used for welding,it can effectively adjust and control the welding heat input,and realize single-sided welding double-sided forming.So this method is suitable for welding and manufacturing of thin plate structures for automobiles.This study is applied to the design of front and rear crossmember components of automobiles.By combining two steel plates with different strengths and thicknesses,an optimal combination of weight and performance of the final component can be achieved.In the event of a collision,the front half of the beam will be deformed to absorb energy due to its low strength and thin thickness,while the rear half of the beam will maintain a stable high strength to ensure passenger safety.In this thesis,the unequal-thickness high-strength steel CR22MnB5/DH1050 used in automobiles was study,and the DC pulse tungsten arc welding（P-TIG）process was adopted.By adjusting the welding process parameters,the welded joint with excellent weld morphology was obtained,and the process parameters are as follows:the welding current is80A,welding speed is 3.42mm/s,and argon flow is 12L/min.The formation of joints were good without obvious welding defects,and different tempering treatment processes were designed and conducted.Some testing methods,metallography,scanning electron microscopy（SEM）and energy dispersive spectroscopy（EDS）,were used to analyze the microstructure of welded structure.And some mechanical properties testing,tensile and flexural properties,hardness and electrochemical properties was also used to analyze the properties of the joint before and after tempering treatment.As a result,the influence mechanism of the microstructure transformation on the mechanical properties and corrosion properties of welded joints before and after heat treatment were discussed.The research results are as follows.The microstructural analysis indicated that the P-TIG weld of CR22MnB5/DH1050heterogeneous steel joint is mainly composed of the base material（BM）,weld zone（WZ）and heat-affected zone（HAZ）.The BM of CR22MnB5 steel shows a lath martensite structure,and the BM of DH1050 steel shows a martensite island structure evenly distributed on the ferrite matrix.Without tempering treatment,the WZ is composed of coarse lath martensite,and there are typical completed quenching and uncompleted quenching zones in the both side of HAZ.The completed quenching zone consists of the coarse and fine crystal zones（martensite structure）,while the uncompleted quenching zone consists of a mixture of martensite and ferrite two-phase structure.After tempering at 250°C and 550°C,with the increase of tempering temperature,the structure division boundaries of each region of the joint became less and less obvious,in which the coarse martensite structure decomposed continuously.In this time,a large number of granular carbides precipitated along the martensite grain boundaries,and the ferrite content increased,and the structure distribution also became more uniform.The mechanical and electrochemical corrosion analysis indicated that the WZ of the welded joints before and after the heat treatment obtained a hardness maximum.There is the obvious softening zone in the both sides of HAZ.The hardness in HAZ of CR22MnB5 side is277.6HV,and the hardness is significantly lower than that of the BM（376.6HV）.After high temperature tempering at 550℃,the content of martensite structure is reduced.However,the content of ferrite in softened structure is increased,and the overall hardness value distribution of the joint tends to be flat,with an average value of 300.4HV.The tensile strength results indicated at room temperature the joints of the samples were all fractured on the side of the DH1050 steel,and showing a significant ductile fracture mode.The tensile strength（890MPa）and elongation（4.85%）of joint at the untempered and 250°C low-tempered are relatively similar.After high temperature tempering at 550°C,the martensite island in the BM gradually dissolved,and some carbides precipitated.However,the tensile strength decreased significantly（724MPa）,but the plastic deformation process was longer and the elongation increases significantly（9.65%）.This promoted the improvement of joint plasticity and toughness.The bending strength tests at room temperature showed that the maximum bending load（516N）of the joint after the 250°C tempering is not significantly decreased compared with the untempered joint（558N）.However,the bending resistance of the specimen（438N）is significantly decreased after the 550°C tempering.The maximum mismatch at fracture damage（6.0mm）is significantly decreased compared with the untempered treatment（6.8mm）.In addition,compared with the untempered welded joint,the bending angles of 250℃tempering and 550℃tempering is 7.43%and 13.22%,respectively.The electrochemical corrosion performance analysis shows that the electrochemical corrosion potential of the joints was positively shifted after tempering treatment,which can effectively improve the corrosion resistant of the joints.The corrosion potential of the weld zone after the 250℃tempering is-0.648V,and the corrosion resistant is improved compared with weld zone of untempered（-0.728V）.At the same time,after 250℃tempering,the minimum corrosion current density in the WZ area of the joint is 6.470x10-7 A/cm~2.The WZ area experienced the low temperature tempering,the martensite begins to decompose.The carbides begin to precipitate with ferrite content increase,the tissue distribution is more uniformly,and its corrosion resistance is better. |
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