Effect Of Shot-Peening On The Microstructure And Mechanical Properties Of Al/Steel Dissimilar Welding Joints By Laser

The joining of steel and aluminum dissimilar alloys becomes the key questions of weight reduction of automobiles.But aluminum and ferrite are easy to react and produce brittle intermetallic phases,which deteriorate the joint properties.It is difficult to weld dissimilar materials.Due to the low energy input,high energy density,high heating and cooling rate,laser welding has the ability to limit the formation of brittle intermetallic compounds.However,the brittleness of dissimilar joints has not been solved.Researches showed that the high energy shot peening is an effective surface strengthening technology to change the metallurgical structure and mechanical properties of material in the surface layer of the components.The weld joint is impacted with high speed and high energy metal projectiles in the HESP treatment and better surface strengthening effects could be obtained.In this study,the aluminum-steel weld joints specimens are treated using HESP,the effect of HESP on the joint is studied.In this paper,the 2mm thickness of 6061 aluminum alloy and 1mm thickness of carbon steel were weld by laser in overlap joint.At first,in order to decrease formation of intermetallic components during laser welding,the microstructure of the specimen under various laser power,welding speed and defocus amount were observed using optical microscopy,scanning electron microscopy,energy disperse spectroscopy and X-ray diffraction.Tensile test and microhardness was performed to identify the effect of each parameter on the weld.Results obtained show that good joints can be acquired at various800-950W in laser power.The best joints can be acquired at 850W in laser power,30mm/s in welding speed and 5mm in defocus amount.The biggest tensile strength reaches 2407N.The brittle phase always occurred at the interface in the joints of different power.The main component is FeAl phase and FeAl₃ phase.The weld joints are treated using HESP method on a shot peening machine.The grain size and metallographic microstructure of the specimen surface layer are analyzed.Results show that plastic deformation happened in the surface layer of the joints,the hardness and the strength all increase.The biggest tensile strength reaches 2690N.The fracture surface morphology was characterized by a quasi-cleavage fracture pattern with some dimples,indicating the mixed fractures of plastic and brittle deformation during tensile test.All these factors contributed to the significant improvement in mechanical properties of the joints.