| Embedded aluminum-steel composite sheet works as a key material to manufacture aluminum-steel composite finned tube which is used in large air-cooling system of the thermal power plant.The aluminum layer is covered outside the base tube for being soldered with aluminum fins into finned tube.Embedded aluminum-steel composite sheet has characteristics of very thin aluminum layer(<100 ?m),great thickness difference between aluminum layer and steel layer.In order to conveniently weld the composite sheet into the composite tube,symmetric no aluminum layer(about 10 mm)on each side of the composite sheet is also needed.Cold roll bonding(CRB)process is an effective method for large-scale production of the embedded aluminum-steel composite sheet.However,the CRBed composite sheet has a large thickness fluctuation of the thin aluminium layer and poor interfacial bonding strength,which can easily lead to the aluminium layer being perforated or pierced and aluminum-steel separation.The above issues are adverse to the yield and quality of the composite sheet.In order to solve the problems mentioned above,a series of researches were conducted.The deformation behavior of aluminum/steel layer and the thickness ratio variation of aluminum layer to steel layer in the deformation zone were investigated,and the deformation mechanism and the cause of aluminum/steel layer thickness ratio variation were explained.Then the formation mechanism of the aluminum layer thickness fluctuation was analyzed and the measures to improve the thickness uniformity of the aluminum layer were also proposed.The influence of sheet surface hardening state on the interfacial bonding strength of the composite sheet and the related mechanism were studied,and the aluminum/steel interface formation were analyzed and the bonding mechanism was revealed.Finally,methods for improving cold roll bonding strength of the embedded aluminum/steel composite sheets were also proposed.The reseach achievements of the thesis include:The thickness ratio of aluminum/steel(TRAS)in the deformation zone from rolling entrance to exit initially decreased and then remained unchanged under a certain reduction rcon.While the reduction was higher or lower than rcon,TRAS initially decreased and then increased.With the increase of reduction,the interfacial shear stress increased,resulting in that relative sliding(reduction<rcon),non-relative sliding(reduction = rcon)and adhesive shearing(reduction>rcon)occurred successively at the interface of the aluminum and steel layer.The composite sheet had a good interfacial bonding quality and there were no delamination of aluminum layer and steel layer on the tensile fracture when the reduction was higher than rco.For the composite sheet which was cold roll bonded with 3.75 mm steel sheet by steel-brush surface treatment and 0.25 mm aluminum sheet,the thickness fluctuation percentage of the aluminum layer increased from ±2.6%to ±17%when the reduction increased from 20%to 60%,while the thickness fluctuation percentage of the aluminum layer decreased to ±10%under the condition of surface treated by flap disc at the reduction of 60%,which was ascribe to that the surface hardness of the steel sheet treated by flap disc was much less than that of the steel sheet treated by steel brush.The inhomogeneous deformation of the steel surface and nonuniform bond strength of the interface during cold roll bonding caused by the work-hardened steel surface layer after scratch brushing were mainly responsibe for the thickness fluctuation of aluminum layer.Proper surface mechanical preparation method(such as flap disc replacing steel brush),which decreases the surface hardening extent of the steel sheet and reduces the number of cracks between hardened surface layer and steel matrix during the CRB,can work as an effective method to improve the interfacial bonding strength of the embedded aluminum-steel composite sheet.Work-hardened surface layer of the steel sheet was an important factor affecting the bonding strength during cold roll bonding.The nano-hardness of the steel surface treated by flap disc was 4.5 GPa which was close to that(4.4 GPa)of the steel matrix,while the nano-hardnesses of the steel surfaces treated by the steel brushes made of ? 0.3 mm wires and ? 0.1 mm wires were 8.6 GPa and 5.7 GPa,respectively.For the thickness reduction of 25%,the peel strengths of the composite sheets whose original steel sheet surface were treated by the steel brush made of ? 0.3 mm wires and ? 0.1 mm wires were 0.9 N/mm and 2.9 N/mm,respectively,while the peel strength of the composite sheet whose steel sheet surface was treated by flap disc significantly rose to 14.9 N/mm.The work-hardened surface layer was liable to crack with steel matrix during cold roll bonding,which was the main reason for the reduction of the bonding strength.It is an effective method for reducing the work-hardened surface layer hardness to improve the bonding strength of the composite sheet.Study of the interface microstructure in the CRB deformation zone of the composite sheet under a reduction of 65%with 0.5 mm aluminum sheet showed that the aluminum layer and the steel layer were mainly combined by mechanical lock at the deformation amount of 35%in the deformation zone.When the deformation amount increased to 50%,over ten nm thick diffusion layer containing about 2 nm amorphous layer formed at the interface between the aluminum layer and the steel layer,indicating that preliminary metallurgical bonding was established.Instead of deformation-induced heat,severe shear deformation at the interface was the main reason for achieving preliminary metallurgical bonding of aluminum/steel interface with 2 nm amorphous layer.When the initial thickness of the aluminum sheet reduced from 1.0 mm to 0.10 mm,the reduction for achieving preliminary metallurgical bonding of the composite sheet decreased from 60%to 40%.To meet the required mechanical properties of the composite sheet to manufacture aluminum-steel composite finned tube used in large air-cooling system,the appropriate annealing temperature was 525?550?.After annealing,the cracks formed between the broken work-hardened surface layer and its steel matrix during cold roll bonding still remained,and the diffusion layer in the interface arear containing broken work-hardened surface layer was thinner than that in the interface arear containing fresh aluminum and steel metals.The composite sheet CRBed with the steel surface treated by flap had higher diffusion rate of Al and Fe atoms at the interface and larger metallurgical bonding extent under the same conditions of heat treatments,compared with the composite sheet CRBed with the steel surface treated by steel brush.Based on the above research results in this thesis,a measure is proposed that flap disc treatment or on-line belt grinding substituted for the commonly used steel brushing can decrease the thickness fluctuation of the aluminum layer and improve the bonding strength of the embedded aluminum-steel composite sheet.This measure was successfully applied to the "863 project" cooperating partner of Yin Bang Clad Material Company,which not only improved the quality of the embedded aluminum/steel composite sheet but also improved the yield and production efficiency. |
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