Study On Aluminum/Steel Bimetal Casting Process And Interface Microstructure

Steel has the good mechanical properties,and the aluminum alloy has the low density high thermal conductivity and corrosion resistance performaces,and Al/steel bimetallic composite materials can effectively combine these excellent properties.Therefore Al/steel bimetallic composite materials have a very broad application prospect in marine gas turbine block materials.Due to its simple process and low cost,the liquid-solid composite casting method is widely used in the preparation process of Al/steel bimetal composite material.However,with the large difference in physical and chemical properties between aluminum and steel and a short contact time during the casting process,it is difficult to form a good combination,which has been s the bottleneck in the production and application of Al/steel bimetallic composites.Aiming at the problems in the liquid-solid composite process of aluminum/steel bimetal,the pretreatment process of hot dip aluminizing,electroplating nickel and electroplating nickel combined with hot dip aluminizing is used to improve the wetting property between aluminum melt and steel before casting.After the compounding,the samples are subjected to post-treatment of different processes to promote metallurgical bonding between aluminum and steel.The microstructure and composition of the Al/steel composite interface were analyzed by optical microscopy,SEM,EDS,and XRD.And the microhardness and shear strength of the samples were tested to investigate the effect of different post-treatments on the bonding properties of Al/steel.In addition,the growth kinetics of the interface diffusion layer of Al/aluminized steel under different holding temperatures and different holding time were also studied.The bonding interface of the Al/steel bimetal composite specimens pretreated with hot dip aluminizing before pouring forms a continuous diffusion layer.This diffusion layer consisted mainly of Al₅Fe₂ phase adjoining the steel substrate and Al₅FeSi phase and Al₈Fe₂Si phase adjoining the aluminum substrate.In addition,Fe atoms diffuse into the aluminum matrix and forms complex Al-Fe-Si intermetallic compounds.It is found that the microhardness of the diffusion layer is more than 700HV.After heat preservation at 760?30min and then furnace cooling,the maximum shear strength of the sample is 15.9 MPa.The study shows that the Al/steel composite process is mainly controlled by diffusion.Different post treatments have a great influence on the interface structure.When the holding time is30min,the thickness of the interface diffusion layer increases with the holding temperature increasing.The thickness of the diffusion layer and the holding temperature meet the growth equation of x=7.59×10^-3exp?-28587/RT?.When heat treatment at 760?for different times,The thickness of the diffusion layer increases as the holding time increases.When the heat treatment is performed for 60 minutes,the thickness of the diffusion layer reaches about 350?m.The growth equation of diffusion layer with holding time is x=2.55×10^-5t^0.31676.Electroplating nickel pretreatment before casting can prevent the growth of the diffusion layer.However,with the subsequent heat treatment,the thickness of the diffusion layer gradually increases and there is obvious stratification.The Al/nickel-plated steel diffusion layer is also mainly consisted of Al₅Fe₂ phase?Al₅FeSi phase andAl₈Fe₂Si.It has been tested that the hardness of the diffusion layer is up to about 700 HV,which is much higher than the matrixstructure.Afterheatpreservation15min,theshearstrengthofthe aluminum/nickel-plated steel sample reaches the maximum value of 13.4 MPa.The fracture is mainly initiated in the Al₅Fe₂ phase layer and belongs to brittle fracture.In order to improving Al/steel bimetal composite casting process,this paper attempts to electroplating nickel combined with hot dip aluminizing before pouring.After the casting,the sample is furnace cooling at 500?.The analysis results show that this process can control the thickness of the diffusion layer and reduce the production of Al₅Fe₂ phase.And the diffusion layer is mainly composed of Al₅FeSi phase and Al₈Fe₂Si phase.However,as the hot dip plating time increasing,the thickness of the diffusion layer increases significantly.Tested and found,the microhardness of the Al/steel interface diffusion layer decrease to 400⁶00HV.When electroplating nickel and hot dip aluminizing for 10min before pouring,the combine of Al and steel is the best,and the sample shear strength reaches 14.6MPa.