| Under the environment of energy shortage,automobile lightweight is an inevitable trend of energy saving and emission reduction.In order to meet the requirements for automobile lightweight,researchers and steel companies have developed high-strength grade automotive steels;At the same time,in order to meet the requirements of corrosion resistance,the surface of the steel plate is usually coated,for example,the steel plate is coated with Al-Si layer.The continuous development of automobile steel also puts forward higher requirements for welding process.Laser welding is an efficient and accurate welding technology to realize self fusion welding between steel plates.Laser welding has many advantages:stable laser beam quality,small generator size,and low maintenance cost,which promotes the application of laser welding in automobile manufacturing process.In this paper,laser welding was used to conduct welding experiments on hot-dip Al-Si coated 22MnB5 steel plate.In this paper,the hot-dip Al-Si coating of 22MnB5 steel plate welded at different welding speeds?70-110mm/s?after 950?heat treatment was studied firstly,the microstructure transformation,mechanical properties and corrosion resistance of the welded joints were studied.The results showed that the welded joint could be divided into weld fusion zone?FZ?,heat affected zone?HAZ?and base material?BM?.The heat affected zone?HAZ?of the welded joint was divided into three parts:internal quenched zone,central incomplete quenched zone and external tempered zone.Its microstructure was mainly martensite;Since the cooling speed of laser welding was very fast,the microstructure of the weld fusion zone was mainly consisted of lathy martensite,bainite and Fe3?Al,Si?phase formed while the Al-Si coating melting into the fusion zone.The brittle Fe3?Al,Si?phase in the fusion zone would reduce the mechanical properties of the welded joint.With the increase of welding speed?70-100mm/s?,the heat input of welding decreased,the cooling speed increased,the martensite was refined,the content of Al in fusion zone decreased,Fe3?Al,Si?phase decreased,and the hardness and tensile strength of joint increased;When the welding speed was110mm/s,the welded joint had micro crack defects and its mechanical properties were reduced due to incomplete penetration.The results of electrochemical test and full immersion test showed that with the increase of welding speed,the Al content in the melting zone decreased and the corrosion rate decreased.When the welding speed was110mm/s,due to lack of penetration,there were micro cracks in the welded joint,and the corrosion rate increased.The hot-dip Al-Si coated 22MnB5 steel plate after 950?heat treatment at different welding speeds?70-110mm/s?was also studied,the microstructure transformation,mechanical properties and corrosion resistance of the welded joints were studied.The results showed that the heat affected zone and the base metal of the welded joint were transformed into a martensite-based microstructure after heat treatment.The high temperature thermal diffusion of Al in the fusion zone was reduced,a small amount of Fe3?Al,Si?phase was transformed into ferrite,so the microstructure of the fusion zone was martensite,bainite,Fe3?Al,Si?phase and a small amount of ferrite.The brittle Fe3?Al,Si?phase in the fusion zone would also reduce the mechanical properties of the welded joint.With the increase of the welding speed?70-100mm/s?,the heat input decreased,and the Al content in the fusion zone decreased,the brittleness Fe3?Al,Si?phase was also reduced,as a result,the hardness and tensile strength of the welded joint were improved;Similarly,when the welding speed was 110mm/s,the welded joint had micro-crack defects due to incomplete penetration,which reduced its mechanical properties.Compared with the samples welded after heat treatment at the same welding speed,the brittle Fe3?Al,Si?phase was reduced,and the mechanical properties were better.The results of electrochemical test and full immersion test showed that with the increase of welding speed,the Al content in the fusion zone decreased,the corrosion rate decreased,and the corrosion resistance increased;When the welding speed was 110mm/s,although the weld was not fully penetrated,after heat treatment,the surface cracks and other defects disappeared,leading to the increase of corrosion resistance. |
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