| Aluminum/Steel composite tapes which combine the low density,corrosion resistance and high thermal conductivity from aluminum and good mechanical properties from steel,have been widely utilized in various fields of people's life.Among many applications of aluminum/steel composite tapes,applying to braze could solve the problem that aluminum and steel is difficult to weld.However,during the welding,the Intermetallic Compound(IMC)at interface of Al/Steel composite tapes must be inhibited to prevent interfacial cracking,and to ensure excellent interface property of Al/Steel composite tapes.Previous studies indicated that,by adding Si element of 0.41~0.83 wt.% in the pure aluminum,the IMCs at interface of Al/Steel composite tapes were effectively inhibited during annealing and brazing,and by multiply adding Er of 0.075 wt.% in the pure aluminum,the effective composition range of Si could be widened.But,in practical production of aluminum alloy,general aluminum which inevitably contains impurity Fe element is commonly adopted as raw materials.Impurity Fe element is one of the main elements for forming Fe-Al IMC,and it may influence the formation and growth of IMC at the interface of Al/Steel composite tapes.In the present study,firstly,pure Al/08 Al steel composite tape was annealed at 645? for 1h to study its interface morphology by SEM and FIB,and to study the cause of interface crack.The results show that,after annealing at 645? for 1h,the IMC grow completely continuously along the interface with the thickness of 2-6 ?m,while,at some regions grow abnormally with the thickness up to 10-30 ?m.Besides,the SEM observation of mechanical polished sample showed that,some micro cracks exist along Al/IMC interface.After the digging craters by FIB,the SEM observation showed that,the micro cracks are intrinsic defects of annealed Al/Steel composite tapes.In order to get the reason for formation of the micro cracks,the thermal expansion rates of Al and IMC were compared,and the results indicated that the micro cracks formed at Al/IMC interface could be attributed to larger difference of thermal expansion rate between Al and Al-Fe IMC.The produced micro cracks would grow,and result in separation of Al and steel.So the IMC must be strictly inhibited.Secondly,the factory existing products(general Al(0.28Fe)-0.8Si/steel composite tapes)were isochronally annealed for 1h at different temperature and isothermally annealed and then simulatively brazed,to study the interface properties of general Al(0.28Fe)-0.8Si/steel composite tapes.The results showed that,the critical temperature of forming IMC of general Al(0.28Fe)-0.8Si/steel composite tape is lower than the critical temperature of forming IMC of pure Al-0.83Si/steel composite tapes.After comparison of composition of two composite tapes,the results indicated that,the impurity Fe element that existed in the aluminum alloy may reduce the brazing property of Al/Steel composite tapes.On the other hand,the general Al(0.28Fe)-0.8Si/steel composite tapes were isothermally annealed at 520? and 550? for 12~36h and 3~24h and then simulately brazed,respectively.The results showed that no IMC generated and the bonding condition of interface is fine.Thirdly,Al-0.32Fe/steel and Al-0.8Si-(0~0.51)Fe/steel composite tapes were isochronally annealed for 1h at different temperature and isothermally annealed and then simulatively brazed,to study the effect of impurity Fe element on the interface properties of pure Al/Steel and Al-Si/Steel composite tape.It indicated that,when the contents of Fe are of 0.32 wt.%,it would decrease the thickness of interface IMC of Al/Steel composite tapes after isochronally annealing at 635?~645? for 1h,and increase the critical temperature of forming IMC of Al/Steel composite tape after isochronally annealing for 1h.Besides,when the contents of Si are constant,if the impurity elements of Fe increase,the critical temperature of forming IMC of Al-Si/Steel composite tape after isochronally annealing for 1h would decrease.Then,Al-0.8Si-(0~0.51)Fe/steel composite tapes were isothermally annealed at 520? and 550? for 12~36h and 3~24h and then simulately brazed,respectively.The results showed that no IMC generated.It follows that,the impurity elements of Fe should be controlled below 0.51 wt.% when the content of Si is about 0.8 wt.%,so that Al-Si/Steel composite tapes could ensure great property for brazing after isothermally annealing at 520? and 550? for 12~36h and 3~24h and then simulately brazing,respectively.Then,Al-(0~1.08)Si-0.3Fe/Steel composite tapes were isochronally annealed for 1h at different temperature and isothermally annealed for different time and then simulatively brazed,to study the effect of Si elements on the interface of general Al/Steel composite tapes.It indicated that,after annealing at 475?~645? for 1h,the critical temperature of forming IMC of Al-Si/Steel composite tapes reached maximized number which is of 617.5?±2.5?.After comparing the experimental results with Al-(0~1.13)Si/Steel composite tapes,it showed that,impurity Fe elements existed in aluminum alloy would decrease the critical temperature of forming IMC of Al-Si/Steel composite tapes from 622.5?±2.5? to 617.5?±2.5?.Then,the Al-(0~1.08)Si-0.3Fe/Steel composite tapes were isothermally annealed at 520? and 550? for 12~36h and 3~24h and then simulately brazed,respectively.The results showed that,all of their Al/Steel interface grown no IMC except the interface of Al-(0~0.23)Si-0.3Fe/Steel composite tapes which isothermally annealed at 550? for 3h and then simulately brazed.Therefore,for general Al-Si/Steel composite tapes that contains impurity Fe elements of 0.3wt.%,in order to ensure great property for brazing after isothermally annealing at 520? and 550? for 12~36h and 3~24h,the content of micro alloying element Si added to the Al alloy is about 0.47~1.08 wt.%.Finally,Al-xSi-0.3Fe-0.075Er/Steel composite tapes were isochronally annealed for 1h at different temperature and isothermally annealed for different time and then simulatively brazed,to study the effect of Er element on the interface of general AlSi/Steel composite tapes.It indicated that,multiply adding Er to Al alloy could increase the critical temperature of forming IMC of general Al-Si/Steel composite tapes from 617.5?±2.5? to 622.5?±2.5?,and corresponding Si content is about 0.50~0.75 wt.%.This method could also broaden the range of effective Si content adding into Al side of general Al-Si/Steel composite tapes. |
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