Study On Microstructures And Properties Of Aluminum-steel Fusion-brazing Joint By Laser

The welding structures of aluminum-steel have comprehensive benefits, such aslight weight, high strength and corrosion resistance, etc. Their application has a hightechnical and economical potential in aerospace vehicle, automobile and nationaldefense industry. The article raised a novel molten brazing method for Aluminum-steellaser welding, trying to fulfill the demands of automotive industry. This methodsuccessfully realized molten solder connection between5A02aluminum-alloy plateand ST04Z galvanized sheet steel. A fusion-brazing welding process by Laser on5A02aluminum-alloy plate and ST04Z galvanized sheet steel has been systematicallyinvestigated as initial step. This paper proposed the use of laser welding with fillerwire to resolve the existed problem of aluminum-steel laser welding by adjustingdifferent processing parameters. The microstructure and phase of the joints underdifferent parameters was analyzed by means of metallographic microscope, SEM andXRD. The mechanical properties were estimated by tensile strength and microhardness experiments.Fist, the welding process of5A02aluminum-alloy plate and ST04Z galvanizedsheet steel by CO₂laser was studied. By investigating the influence of the weldparameters （laser power, spot diameter, welding speed） on the weld shaping, the bestregions of forming good weld shaping was found out: the range of laser power was1400W²000W; the range of welding speed was180²40mm/min; the range of spotdiameter was1mm³mm; the protection gas was argon, the flow of protection gas was3L/min. Furthermore, the cause of defects such as local incomplete fusion and weldingblowhole were analyzed and the solution was proposed.Finally, the microstructure and morphology of Fe-Al IMC （inter-metalliccompounds） layer of the welded joint for laser power of1600W and2000W wererespectively studied by the use of metallographic microscope MEF and scanningelectron microscopy6700F （SEM）. Line scan of the interface showed that Fe and Alelement diffusion occurred at the interface of aluminum-alloy and steel, and the greaterlaser heat input is, the more obviously spread. The component analysis of EDS aboutthe IMC and the weld seam with the region show: when laser power was1600W, themain phase of the IMC layer was FeAl, Fe₃Al and α-Fe solid solution; when laserpower was2000W, the main phase of the IMC layer was the brittle phase of Fe₃Al and α-Fe solid solution. It was found that when laser power was2000W, there was a lot ofFe₃Al IMC in the weld seam, and the greater laser heat input is, the more Fe₃Al IMCare inside, which seriously threaten the mechanical properties of the joints. Therefore,laser heat input must be strictly controlled in the welding process. Then the microhardness of the joints for power of1600W and2000W were test. Micro hardness testshowed that the hardness of IMC was much higher than other regions, and the hardnessincreased as the laser power increasing, and the maximum hardness was up to870HV.Tensile test showed that the joint of laser power1600W had a good toughness, and themaximum average tensile strength was204.38MPa,78.61percent that of the5A02aluminum alloy.