A Study On Microstructure And Properties Of Steel/aluminum Laser Deep Penetration Welded Joint

The steel-aluminum hybrid body is the most important development directions in the field of automobile manufacturing.Thus,it has became a research hotspot to be solved to study the joining technology of dissimilar materials of aluminum alloy and steel,recently.However,due to the obvious differences in the physical and chemical properties of aluminum alloy and steel,and the formation of brittle intermetallic compounds(IMC),it is difficult to achieve good quality connection by conventional welding methods.Laser welding has the advantages of small welding heat input,narrow heat-affected zone and high welding efficiency,which provides a new exploration direction for obtaining excellent joints between those.In this study,the DP980 placed on A6061 aluminum alloy and the high-power fiber laser was usded to obtain joints between those.The microstructure and mechanical properties of welded joint were studied.Then,nickel and copper foil with different thickness(0.10mm,0.20mm and 0.30mm)were preset at lap joint,which used to study the effect of nickel and copper content on the microstructure and properties of DP980/A6061 laser welded joints.And it will provide theoretical basis and basic data for the toughening of steel/aluminum dissimilar metal laser lap weld joints.The main research contents and conclusions are as follows:(1)During the laser welding process,aluminum entered the molten pool,expanded the ?phase region,and caused some of the ? phase to freeze to room temperature.And it caused uneven longitudinal hardness distribution(255?300HV).In addition,the IMC layer was composed of Fe2Al5 and FeAl3 in the interface,the peak thickness was 25?m,the microhardness was 540HV,and the tensile fracture location broken at the interface that the tensile-shear strength was just only 42MPa;(2)The effect of Ni on the microstructure and mechanical properties of DP980/A6061 dissimilar laser welded joints was summarized as two aspects:On the one hand,Ni inhibited the precipitation of ? ferrite in WS,significantly.Finally,the microstructure was LM(0.10 and 0.30mm),which was improved the average and uniformity of microhardness,significantly.On the other hand,Ni had changed the metallurgical reaction in interface,a certain amount of Ni-Al IMC(NiAl3 or Ni2Al3)was precipitated to reduce the precipitation of Fe-Al IMC.Thus,under the condition that the nickel foil thickness was 0.10mm,the peak thickness of IMC layer was reduced to 15 p?m,and the tensile-shear strength was up to 61MPa.(3)It was difficult to inhabit the formation of ? ferrite in the weld by Cu.Consequently,the microstructure was still ? ferrite and LM.However,the Cu inhibited the interdifusion of Fe and Al,and the IMC layer was composed of(Fe,Cu)2Al5 and(Fe,Cu)Al3,which reduced the peak thickness to However,it caused thermal cracking,resulting in a maximum tensile-shear strength of only 45MPa.(4)In contrast,nickel inhibited the formation of 8 ferrite and reduce the precipitation of Fe-Al intermetallic compounds,significantly.Thus,the line load of steel-aluminum welded joint increased by 48%.Howerer,the Cu just only modified the Fe-Al IMC.And compared with previous research work,nickel alloying has obvious cost and safety advantages.