Research On The Al Alloy Clad Sheets With Excellent Collapse Resistance

The cold-rolled 4343/3003/4343 Al alloy clad sheets,with a clad/core layered structure,are the most widely used aluminum brazing materials for automotive heat exchangers.The 4343 alloy clad layer made from an Al-Si hypo-eutectic alloy acts as the brazing filler to assembling fin stock and multiport Al tube;the 3003 alloy core layer made from an Al-Mn alloy provides the structural rigidity during the high temperature brazing.The weight reduction of vehicular components calls for thinner aluminum clad sheets,however,the clad sheets thinner than 0.08 mm might lead the heat exchanger to collapse during brazing.The current research is aimed at researching the collapse mechanism in the brazing process and develop new aluminum clad sheets with lower brazing temperature and better collapse resistance,to provide a solid foundation for the development of thinner aluminum foils.Firstly,4343/3003/4343 clad sheet was electropolished to get separated 3003 core sheet.The effects of annealing process on the hardness,microstructure and Si diffusion of clad sheets and core sheets were researched.The effects of microstructure evolution and Si diffusion on the collapse resistance were especially analyzed to get the main factor.The experiment results show that with the increase of annealing temperature,the hardness of both clad sheet and core sheet decreased,while the collapse resistance of both clad and core sheets achieved maximum values after annealing at 380°C for 1h.After annealing at 380°C,the clad layer contained fine and equiaxed recrystallized grains;the core layer was composed of fully recrystallized coarse grains elongated along the rolling direction.The Si diffusion from the clad layer into the core zone was limited.After annealing at a higher temperature of 440°C or 550°C,the collapse resistance of clad sheets worsened precipitously.The grains in the clad layer grew up,obscuring the interface between the clad and core layer;the recrystallized grains in the core layer became finer.Significant amount of Si penetrated into the core layer through the accelerated diffusion.Although the microstructure evolution of the clad sheet was similar to that of the core sheet under the same annealing condition,the latter exhibited distinctly better collapse resistance,indicating the Si diffusion is the main factor influencing the collapse resistance.Secondly,0-1.0%(wt.%)Cu or Sn element was added to 4343 alloy in order to develop a new brazing filler metal with lower melting point.The results show that with the increase of Cu or Sn element,the solidus and liquidus of the new alloys decrease gradually,and when the Cu or Sn element is increased to 1.0%,the solidus and liquidus will be reduced by 10-15 ? respectively.With the increase of additions,the Cu bearing intermetallic phases or the Sn particles increase in the cast alloy,while during hot extrusion the Cu bearing intermetallic phases dissolve and Sn particles attribute to dissolution of the Fe-Si bearing intermetallic phases;The tensile strength increases gradually while the elongations decrease a little.The brazing experiment results show when the Cu or Sn element is increased to 1.0%,the brazing temperature is reduced by more than 15?.Finally,we changed the contents of the main alloying elements and added Zn,Re to the current 3003 core alloy by orthogonal analysis method.Then the effects of different elements on the microstructure,mechanical properties,corrosion resistance and collapse resistance were researched to get excellent core alloy with better comprehensive properties.The results show that the addition of Cu and Si has the most distinct influence on the mechanical property.The Si and Mn has the biggest impact on the collapse resistance;The collapse resistance is worsened with the increase of Si or Cu content,while improved with the increase of Mn content.The optimum alloy(Cu0.5%,Zn2.5%,Si0.8%,Mn1.2%,RE0%)increase the tensile strength from 138.52 MPa to 183.64 MPa,and improved the elongation from 17.3% to 19.2%,and the collapse resistance is improved by 130% compared with 3003 alloy.