| Advanced high strength steel(AHSS)QP980 is the representative of the third generation of AHSS,and because of its characteristics of high strength,high plasticity and excellent energy absorption,it has a good application prospect in lightweight of automobile.The problems of component segregation,weld hardening and hydrogen induced cracking(HIC)are easy to occur when using conventional fusion welding.The micro structure of QP980 steel is mainly composed of martensite with high density dislocations,ferrite and residual austenite in metastable state.The structural characteristics of QP980 steel cause it to be easily transformed under welding heat cycle.At present,there are few studies on the welding of QP980 steel,and there is a lack of in-depth study on the evolution law ofthe joint micro structure.Also,the inhomogeneity of joint micro structure will lead to more complex hydrogen embrittlement problem.Therefore,in order to solve the welding problems,the welding tests were carried out by friction stir welding(FSW)technology and laser welding technology.The influence of welding parameters on the microstructure evolution and properties of joints was studied,and the relationship between properties and micro structure evolution was also established.The micro structure and properties of the HAZ were studied by using a thermal dilatometer.In addition,the hydrogen embrittlement sensitivity of FSW joints and laser welded joints was studied by electrochemical hydrogen charge and slow strain rate tensile(SSRT)test.The influence of micro structure in different regions on the formation of HIC was studied,and the fracture mechanism of the joints was also discussed.Research on FSW of QP980 steel shows that:By optimizing welding process parameters and controlling field test operation,the welds with good surface morphology were obtained.The cross section of the FSW joints presents a bowl-shaped morphology,and the joints can be divided into the stirring zone(SZ),the heat affected zone(HZA)and the base metal(BM).The peak temperature and cooling rate are different under different welding process parameters,which leads to different micro structure in the SZ.The peak temperature is largely dependent on the rotation speed,and the increase of welding speed will significantly improve the cooling rate after FSW.The hardness distribution trend of the joints under different welding process parameters is almost the same,and the hardness of the SZ is obviously higher than the BM,there is a softening zone on both sides of the weld.Due to the large amount of ferrite in the SZ when the rotation speed is 200 r/min and the welding speed is 50 mm/min,the hardness of the SZ is significantly lower than the other conditions.The joints with equal strength to the BM can be obtained by optimizing the welding process parameters,and the elongation is lower than the BM.When the rotation speed is 200 r/min,the joints break in the SZ,while under the other welding process parameters,the joints break in the softening zone or the BM,it indicated that the joints with good welding performance can be obtained by FSW.Research on laser welding of QP980 steel shows that:The joints with full penetration and well-formed surface can be obtained under the selected welding process parameters.The cross section of the laser welding joints shows hourglass shape,and the joints can be divided into the fusion zone(FZ),the coarse-grained heat affected zone(CGHAZ),the fine-grained heat affected zone(FGHAZ),the intercritical heat affected zone(ICHAZ),the subcritical heat affected zone(SCHAZ)and the BM.The microstructure of the weld is coarse lath martensite,and the growth direction tends to be perpendicular to the FZ boundary.The hardness of the FZ and some HAZ of the joints under different welding process parameters are higher than the BM.The highest hardness is in the FGHAZ and there is a softening zone at both sides of the weld.With the increase of heat input,the width of the FZ and the HAZ becomes larger,and the softening zone is further away from the weld center.Under different welding process parameters,the tensile strength of the joints can reach the strength of the BM,the yield strength is higher than the BM,and the elongation of the joints is lower than the BM,it indicated that the joints with good performance have been obtained under the selected welding process parameters.Research on welding thermal simulation of QP980 steel shows that:With the increase of heating rate,the Ac1 and Ac3 of QP980 steel also increase gradually.The temperature of the simulated SCHAZ is 300?700?,the martensite is decomposed and carbides are precipitated at peak temperatures of 600? and 700?.The temperature of the simulated ICHAZ is 800? and 900?,and the micro structure is ferrite and martensite.The temperature of the simulated FGHAZ is 1000? and 1100?,and the micro structure is fine martensite.The temperature of the simulated CGHAZ is 1200? and 1350?,and the martensite is obviously coarsening at 1350?.The results of microhardness test shows that the hardness is the lowest at the peak temperature of 700? due to martensite decomposition and carbide precipitation,and the hardness is the highest at the peak temperature of 1000? due to the small martensite in the FGHAZ.The simulated-HAZ continous cooling transformation(SHCCT)diagram of QP980 steel is established by thermal expansion method.When the cooling rate is 0.1?1?/s,the microstructure is ferrite and bainite,when the cooling rate is 2?/s,the micro structure is bainite,a little ferrite and a little martensite,when the cooling rate is 3?/s,the microstructure is bainite and martensite,when the cooling rate is higher than 5?/s,the micro structure are all converted to martensite.The mechanism of HIC in the joints under electrochemical hydrogen charge was studied.For the FSW joints,HIC first forms in the ICHAZ,then in the BM,and finally in the SZ.The HIC in the ICHAZ mainly propagates along the grain boundary of ferrite and martensite and the interior of martensite,while HIC in the SZ mainly propagates through the martensite.The physical model of HIC formation and propagation under the condition of electrochemical hydrogen charging is established.For laser welding joints,HIC is more likely to occur in the CGHAZ,and then appears in the FZ and the BM with the increase of hydrogen charge time.The SSRT test of the precharged hydrogen joints shows that the mechanical properties of the two welded joints decrease with the increase of hydrogen charging time,and the fracture mode of the joints gradually changes from ductile fracture to brittle fracture.The TRIP effect in the tensile process of QP980 steel joints will increase the hydrogen embrittlement sensitivity.In addition,the presence of hydrogen will reduce the resistance of HIC formation.It can cause the joints to deform under low stress,which leads to the appearance of microcracks and cleavage characteristics in the fracture.With the increase of hydrogen content,large cracks appeare in the middle of the samples,which cause the transition of the joints from ductile fracture to brittle fracture. |
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