Microstructure And Properties Of Al/Fe Liquid Solid Bimetal Composites

Aluminum alloy,which is a kind of light alloy material,has become the preferred material to replace traditional steel in order to achieve the developing direction to lighten the weight of automotive parts,due to its high thermal conductivity,strong corrosion resistance and shock absorption as well as low specific weight.However,aluminum alloy is still inferior to steel in many aspects such as strength,hardness and wear resistance.For certain automotive parts working in high strength and wear resistance,steel can only be applied as the best choice at present.Therefore,the development of Al/Fe bimetallic material,which combines the very best of steel and aluminum alloy,can effectively improve the weight-strength ratio of automobile parts as well as ensure the automatic performance and safety,which plays significance role in the development of automobile parts with lightweight and high strength in the future.Compared to the traditional welding process of Al/Fe dissimilar metals,liquid-solid bimetal casting is a new technology with high preparation flexibility to realize the effective composite of Al and Fe dissimilar metals and the mass production of automobile parts with complex bonding surface.However,there are three difficult problems in the process of Al/Fe liquid-solid bimetal casting: Firstly,the oxidized product formed at the surface of aluminum and iron can result in the weaken wettability for the surface of metal substrate,leading to deterioration of the metallurgical bonding between aluminum and iron;Secondly,the easy formation of brittle Fe-Al intermetallic compound located at the interface layer of metallurgical bonding,can bring about the decrease of the interfacial strengths of metallurgical bonding;Thirdly,the significant difference in cooling shrinkage coefficient between the two metals can easily produce microcracks at the interface of aluminum and iron.So,focused on the key points in the process of Al/Fe liquid-solid bimetal casting,the main studying contents of this paper are as follows:Firstly,we investigated the influence of surface pretreatment and process parameters on the formation of metallurgy bonding interface via surface pretreatment of the matrix of QT500-7 ductile iron.The results indicate that for the matrix of QT500-7 ductile iron,it is fail to form the effective metallurgical bonding interface between the surface and the hot dip coating layer no matter we only apply the hot dip treatment nor combine the hot dip treatment with surface cleaning and a short period of low temperature.Subsequently,with the increase of the holding time and temperature,the sandwich structure consisting of Al layer,interdiffusion layer and Fe layer can gradually form the metallurgy bonding interface,which illustrates that for the matrix of QT500-7 ductile iron,the hot dip treatment with surface cleaning and optimized process parameters is a precondition for the formation of a continuous metallurgical bonding interface.Secondly,based on hot dip treatment,we focused on the influence of different process parameters on the metallurgical bonding interfacial strength of ZL101A/QT500-7bimetallic specimens.Through this analysis,transition layer consisting of Al8Fe2 Si phase on the side of aluminum substrate and Fe2Al5 and Fe Al3 phases on the side of iron substrate was formed on the sample liquid-solid interface,When the holding time is too short or the holding temperature is too low,the Al/Fe transition layer formed by the reaction is not flat and continuous enough,after which the morphology of the Al/Fe transition layer is improved gradually with the increasing of the holding time and increasing of the holding temperature,but its thickness will form a large number of intermetallic compounds,which cause irreversible damage to the interfacial bonding strength of the samples,moreover,the Fe can form a large number of plate strip like and agglomerate like Al-Fe-Si ternary phases in the aluminum matrix could equally affect the bonding strength of the interface,Therefore,the increase in shear strength with holding time,holding temperature shows a state in which first increases and then decreases,and it reaches a maximum value of 75.78 MPa when the holding temperature is 710℃ for 20 min.Finally,we investigated the effect of T6 heat treatment on the interfacial metallurgical bonding strength of ZL101A/QT500-7 bimetallic samples based on liquid-solid composite casting.The results indicates that the modified T6 heat treatment,in which the iron matrix was coated with thermal insulation material before quenching,can effectively change thermal field of bimetals,resulting in suppression of the cracks at the bonding interface originated from the significant difference in cooling shrinkage coefficient.In addition,the thicken Al/Fe transition layer due to heat preservation process can promote the formation of Kirkendall pores in the Al/Fe transition layer.Moreover,Al8Fe2 Si component with a plate strip and agglomerate like shape can remarkably decrease the bonding strength of the as-prepared samples with heat treatment.Consequently,the shear strength of samples decrease with the growing thickness of the Al/Fe transition layer.It was found that the as-cast composite with the heat treatment,which was holding for 10 min at 710℃,has delivered the best shear strength of 56.21 MPa.