| The rapid development of the economy has made energy-saving and emission reduction a pressing global issue at the moment.Using a lightweight aluminum alloy instead of some steel to achieve energy-saving and emission reduction through the car is the next effective measure.Therefore,it is necessary to study the welding process of aluminum/steel dissimilar metals and to explore the methods to improve the mechanical properties of welded joints.In this paper,a new method of improving aluminum/steel dissimilar metal welded joints is proposed based on the modification of the surface microstructure and the pre-placement of Al Si12 powder,using 1 mm thick A6061 aluminum alloy and 1.2 mm thick DP600 duplex steel as research objects.The core idea is to prepare a certain microstructure on the steel surface by fiber optic laser and pre-place Al Si12 powder,to improve the wettability and spreadability of the liquid aluminum alloy by capillary force and Si elements and to form a "pinned" structure to further enhance the mechanical engagement,and to regulate the microstructure and thickness of the IMC layer by Si elements.This paper investigates aluminum/steel heterogeneous metal connections using a laser fusion brazing method under aluminum on steel.The main content includes three aspects: 1,the COMSOL finite element analysis software was used to establish and analyze the threedimensional transient temperature field model of A6061 aluminum alloy and DP600 duplex steel.The temperature field distribution and thermal cycling curve characteristics of the aluminum/steel welded joints under different process parameters were compared and analyzed to provide theoretical support for the next experiments.2.Explored the effect of process parameters on joint weld formation,optimal process parameters were obtained 3.A systematic study of the weld formation,interfacial microstructure,and mechanical properties were carried out by preparing the steel surface microstructure and the modulation method of the pre-set powder.Firstly,the best process parameters are laser power of 405 W,welding speed of 6mm/s,the amount of defocused +0.9mm,the shielding gas is argon,and the flow rate of 10L/min.Under the best process parameters,it is to obtain the maximum mechanical properties of the welded joint 116.7N/mm.The microstructure observation shows that the IMC with a thickness of about 10μm is formed at the interface between aluminum and steel,which is mainly Fe2Al5 and the Fe Al3 compound layer,which also shows that the intermetallic compound is from the steel side to the aluminum side of the growth.Based on obtaining the optimum process parameters,the joint weld formation,metallographic organization,mechanical properties,microstructure morphology,composition,main phases,and fracture mechanism of the interfacial IMC layer were investigated using the universal tensile testing machine,SEM,EDS,and XRD respectively,based on the modulation method of changing the surface microstructure and pre-positioning Al Si12 powder.The results show that element Si can refine the weld structure,improve the fluidity of the melt pool,aggregate at the interface,and preferentially react with Fe-Si and Al to form Fe-Si binary compounds and Fe-Al-Si ternary compounds,thus inhibiting the generation of brittle Fe-Al compounds.With the addition of Al Si12 powder,the surface of the welded part is well-formed,the effective joint width of the weld cross-section becomes wider,the wetting angle becomes smaller,and the fluidity of the aluminum solution is improved.The thickness of the IMC layer is reduced,and the metallurgical replacement reaction between Si and Fe and Al occurs,which suppresses the generation of Fe-Al metal compounds while generating new phases such as Fe3 Si,Al0.5Fe3Si0.5,and Al8Fe2 Si,increasing the toughness of the joint.17.1%.The addition of structure to the added powder resulted in a further increase in mechanical properties,reaching a maximum value of 145.1N/mm at a groove line spacing of 200 μm.Analysis suggests that this is due to the addition of structure,the intermetallic compound growth at the interface changing from a flat state to a curved state along with the groove growth.The presence of the grooves prevents further crack expansion.After the liquid aluminum alloy flowing into the groove has cooled and solidified,a "nailing effect" is formed,which not only increases the connection area of the aluminum-steel interface but also enhances the mechanical engagement between aluminum and steel,further improving the mechanical properties. |
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