The Effect Of Coated Zn Layer On The Microstructure And Mechanical Properties Of Friction Stir (Spot) Welding Of Galvanized DP590 High Strength Steel Plates

Friction stir welding(FSLW)is a kind of solid-state welding technology,which shows unique advantages in the welding of light metals such as aluminum alloys and magnesium alloys.The emergence of high wear resistance stirring tools has promoted the rapid development of friction stir welding of high strength steel.Friction stir spot welding(FSSW)and friction stir lap welding(FSSW)are important variants of FSW.In this study,hot-dip galvanized DP590 steel plates were subjected to the FSSW and FSLW processes.In order to study the flowing behavior of the Zn layer in the welding process,FSSW was carried out with a gradually increased penetration depth and the distribution of the Zn layer in each different welding conditions were investigated.During FSLW,the interface formation,microstructure,mechanical properties and immersion corrosion performance of the joint under different welding parameters were studied by changing the test parameters.The main conclusions are as follows:In the FSSW test of DP590 galvanized high-strength steel,a 1.4 mm thick steel plate was welded,and a spot welded joint with no internal defects and good welding quality was obtained.The FSSW was performed at a constant rotation speed of 800rpm,while the PD of the tool was varied from 0.6 mm to 0.9 mm,the interface changes from a hook-like interface to a straight interface,and the bonded length of the interface increases from 0.44 mm to 2.01 mm accordingly.With the increase of penetration depth,the size of martensite in the stir zone increased from 8 mm to 14.2?m.In order to study the flow behavior of the galvanized layer in the welded joint,welding without residence time was increased.When the penetration depth was increased from 0 mm to 0.6 mm,the results showed that the Zn layer has a tendency to enter the stir zone at the beginning of welding.After the welding was completed,the accumulation of Zn in different states was found at the interface,but no accumulation of Zn was detected in the stir zone far away from the interface.The strength of the FSSW joint is mainly determined by the bonding method of the interface,the bonding length of the interface and the thinning of the upper plate.The maximum shear tensile load was about 21.35 kN through the plug failure mode at a penetration depth of 0.8mm.In addition,the presence of zinc at the HOOK interface may reduce the mechanical properties of the joint.However,for straight interface joints,no significant effect of zinc on mechanical properties was found.In the FSLW test of DP590 galvanized high-strength steel,a polycrystalline cubic boron nitride(PCBN)ceramic stirring tool was used to perform a lap test on a 1.4 mm thick galvanized steel sheet.All the FSLW was performed at a constant rotation speed of 500 rpm,while the welding speeds was varied from 25 mm/min to 150 mm/min.With the increase of welding speed,the heat input gradually decreases.The stir zone was mainly composed of martensite,bainite and ferrite.The formation of a typical HOOK interface appears on the advance side of the joint under all parameters.An interlayer consisting of Feand Fe₃Zn₁₀ was observed at the interface.With the increase of welding speeds,the thickness of the interface was increased with a large number of pores appeared.At a welding speed of 50 mm/min,the Zn layer was destroyed before welding,with an intermittent interfacial transition layer with a smaller thickness was formed,which helped to reduce the harm of the zinc layer.Fracture occurred in the base material area.The immersion corrosion experiment of welded joints showed that with the increase of travel speed,lower heat input resulted in more residual zinc,and the corrosion rate of joints decreased.The corrosion products on the joint surface are mainly composed of FeOOH and Fe₃O₄.