Research On Welding Characteristics Of 6082 Aluminum Alloy Laser Filler Wire Welding

To meet the requirements for the lightweight of high-speed trains,more and more high-speed trains use aluminum alloy as the main structural material of the vehicle body.For the high-speed and low-deformation welding demand of 6082-T6 alloy on high-speed rail trains,this paper studies the welding of 6082-T6 alloy flat plate by laser filler wire welding method.By optimizing and adjusting the process parameters,the “keyhole penetration type” forming mode of the weld is converted into the “keyhole non-penetrating type” welding mode,which solves the problem of weld seam collapse and obtains a well-formed weld seam.For the typical molten pool morphology and welding wire droplet transfer using high-speed video observation,found that in the case of negative defocus,the molten pool transition form is an unstable large-drop transition mode,this time the tendency of the pore is very large,the porosity rate reaches 4.8 %.A stable liquid bridge transition mode can be obtained with zero defocus.In the liquid bridge mode,the heat input of the welding has a great influence on the stability of the transition.Microhardness analysis revealed that there is a “resoftening zone” in the softened HAZ that is more severely softened.The softening at this point will cause the joint to break from the heat affected zone during the tensile test and at the same time make the strength of the joint from 249 MPa dropped to 228 MPa.It was found that when the heat input of welding is less than P=3250W,vs=0.025m/s(Q=130J/mm),the "resoftening" zone does not affect the tensile test fracture position of the joint,and the tensile fracture breaks at the weld.At this time,the joint strength reached 78% of the base material.X-ray inspection found that in the liquid bridge transition mode,the porosity rate shows a "V" distribution with the increase of welding heat input,and the heat input is P=4250W,vs=0.04m/s(Q=106.25J/ When mm),the lowest porosity was 0.12%.Differential scanning calorimetry(DSC),X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),and finite element modeling were used to analyze welds during welding.With the phase change in the heat affected zone,the re-softening phenomenon of the heat affected zone of welded joints and the mechanism and conditions for its occurrence are studied.By comparing the "resoftening zone" of the heat-affected zone with the parent metal.High-resolution TEM analysis found that the acicular ?” phase is abundant in the base metal but cannot be observed in the “re-softening zone” of the heat-affected zone.The diffraction pattern in the [001]Al direction in the base metal characterizes the "cross pattern," and the elongated diffraction line characteristics resulting from lattice distortion in the [011]Al direction all disappear in the "resoftening zone.".Therefore,it can be confirmed that the disappearance of the ?? phase,which plays a major reinforcing role in the 6082 aluminum alloy,in the “re-softening zone” is the main cause of its softening.The DSC analysis combined with the finite element simulation of the temperature field of the welded joints resulted in the “resoftening” condition that caused the fracture of the joint to break from the heat affected zone: the residence temperature of the zone thermal cycle above 400°C was greater than 0.55 s.