Study Of Pulsed Laser Welding Process And Corresponding Relationship Between The Microstructure And Properties Of DP500Steel

Weldability of0.65mm DP500is researched systematically by the SW500Nd: YAGlow-power pulsed laser. In this paper, the surface morphology and microstructure of joints areinvestigated by means of VHX-900digital microscope, optical microscope, scanning electronmicroscope (SEM) and transmission electron microscope (TEM). Moreover, The mechanicalproperties and forming properties of joints are also tested, compared and analyzed by means ofmicro-hardness tester, tensile testing machine and cupping test machine.The surface formability and the cross-sectional shape are analyzed comparatively underthe different pulse laser welding parameters, research the influence rules of pulse weldingvoltage, pulse width and pulse frequency on the surface morphology and section shape of weldseam. The results show that the obverse side of weld is superior to the reverse side under thedifferent pulse laser welding parameters, with the welding parameters increased, the HAZwidth increase gradually. In these three parameters, the pulse width has the biggest influence onthe HAZ width, nearly exponential growth. When the pulse voltage is increased to255V, thepulse width is increased to5ms, and the pulse frequency is increased to16Hz, the weldingpenetration mode change from "moderate penetration" mode to "overpenetration", weld sectionshape transform from "V" to"X", at the moment, the welding spot diameter of backface isinfluenced rapidly, welding spot overlap rate at this time also subsequently changes greatly andpulse frequency has a biggest influence on it; When the overlapping ratio of surface pulsepoints are reach to70%and the weld at “moderate penetration” mode,the0.65mm DP500steel plate will have well surface morphology of weld seam.The weld solidification characteristics under different pulse laser frequency and therelationship between microstructure grain and micro-hardness are analyzed. The resultshows that the welding spots consist of the weld seam can be divided into two parts:welding spot overlap region and unoverlap region. With the change of the pulse laserfrequency, not only the welding spot overlap factors and the morphology of weld isdifferent, but also the microstructure grain exist heterogeneity. Meanwhile, the fluctuant is appeared in micro-hardness distribution, the minimum micro-hardness value appears in thewelding spot overlap region, the maximum micro-hardness value appears in the unoverlapregion. With the increasing of the pulse laser frequency, the unhomogeneity ofmicrostructure grain is decreased and the fluctuation of the micro-hardness distributionalong the longitudinal section of the weld center is also gradually reduced. Although thedifference in grain structure, but the scan results show the distribution of the majoralloying elements Mn, Cr and Si have no segregation phenomenon.Macro-morphology of weld and microstructures of welded joint are analyzed underdifferent laser welding process. The results show that the welding pool has an obviouscolumnar crystal and crystal orientation, but the increase of the line energy has weakenedthe crystal orientation. The microstructure are mainly composed of martensite and upperbainite in weld. In the lower line energy, martensite volume fraction increased. Martensiteand bainite were observed through the TEM, and have the second phase particlesprecipitated. With the growth of the pulse energy, not only in the weld martensite volumefraction decreased, and intergranular ferrite appears in the weld.The test results of the mechanical properties of welded joints under different pulse laserprocess show that the joints maximum hardness appear in the fusion zone, and themicro-hardness of the weld is1.4-1.5times of the base metal. Softening of the heat affectedzone exist in higher energy parameter combinations, and the formation of soft zone is due to theeffect of welding thermal cycle, dual phase steel produces tempering. In the tensile test, thepercentage of elongtation and work hardening exponent have slightly reduced compared withthe base material. Fracture happened at the base metal in transverse tensile test, and longitudinaltensile test of welded joints are appear lightning-shaped fracture characteristic.During cupping test, the results show that there are three kinds of fracture locations,fractures happened vertically in the weld at the low laser power, In this case, the IE is higher, canreach more than83%of the base metal, with the growth of laser power, the fractures happenedparallel to the weld, in this case, erichsen value is greatly reduced.