| Dual-phase and martensitic steels are the largest amount AHS steels thatapplicate in white car body structure, and the main materials that used in the processof lightweight. Gas metal-arc welding (GMAW) welding is one of the inevitablewelding processes, in the forming of the car. Thus, investigative the weldability ofDual-phase and martensitic steels during GMAW has great significance for theapplication of AHSS, promote energy conservation, upgrade the level of automobilemanufacturing of our country and promote the safety of the car.This article researched the GMAW technology of cold rolling2mm Dual-phasesteel (DP780) and martensitic steel (MS1180) in detail. Firstly, chose tensile strengthof the joint as the experimental indicators, studied various process parameters’influence to the joint performance and optimize the process parameters through themethod of orthogonal experiment. Analysis the orthogonal experiment found that,joint tensile strength showing first increases and then decreases with the increases ofwire feed speed and welding speed, respectively, in the wire feed speed and weldingspeed of4.4m/min and0.85m/min reached the maximum joint strength. With thewelding voltage increases joint strength and inclination of the exhibits have changedthe law of diminishing. While joint strength demonstrated a law of diminishing withthe increase of voltage and welding angle. Orthogonal results shows that wire feedspeed4.4~5.2m/min, welding speed0.85~0.95m/min, welding voltage18.5V,welding angle0~15°, while in the range of welding parameters in this process,lap joints shows good mechanical properties.Secondly, established a mathematical model that used the process parameters asindependent variables, and the joint strength of the joint as dependent variable, basedon multivariate regression analysis. Modeling process found that, it’s necessary toestablish a second-order model, and a prerequisite second amendment to achieve themodeling requirements. Finally, designed the comparison experiments, and used the methods ofmicro-hardness testing, optical microscopy and scanning electron microscopy,studied the distribution of structure and hardness in the joint cross-sectional,simultaneously fracture failure mechanism in different areas of the joint was analyzed.Peak hardness of dual-pase steel side appears in coarse grain area, and the peakhardness of martensitic steel side is the hardness of the base material itself. The valleyhardness of the two plates is both appear in the incomplete recrystallization zone, thisdue to the decomposition of martensite. The valley hardness of dual-phase steel side islower than martensitic steel side, the greater the heat input is, the serious the softeningwill be. SEM analysis showed that fracture of the boundary line of fusion zone ofmartensitic steel side mainly in pore coalescence fracture mode, small area appearedin cleavage fracture, while fracture of heat affected zone of dual-phase steel sideshowing mainly in stratification fracture form, small area appeared in quasi-cleavagefracture. Low heat input cause segregation and inclusions on the board the fusion zoneleads to a small effective shear, crack forming in low stress. After the heat inputincreases, result in decomposition of martensite in incomplete recrystallization region,and that leads to serious softening effect so creates a new mechanics weak area. Anumber of elongated M-A components were found in both fracture zones, which issignificantly affected the static load ability of each joint. |
PDF Full Text Request