Effect Of Micro-alloying And Heat Treatment On Microstructures And High Temperature Properties Of Al-Mn-Based Alloy

The process of production and working environment of composite brazing heat exchangers call for materials with good elevated temperature properties.Therefore,this thesis focuses on the core material of exchanger materials.To be specific,the effect of micro-alloying and heat treatment process on the recrystallization and microstructure of the 3003 aluminum alloy were studied in detail.Based on the micro structure control and creep mechanisms,the effects of alloying elements on the high temperature properties,especially creep properties of Al-Mn-based alloys were also studied.Firstly,the effect of alloying elements on the recrystallization and microstructure of the Al-Mn-based alloys were studied.The results show that the addition of Mg?Cu?Ce or La mainly causes fine-grain strengthening;with Ni or Zr micro-alloying,the grain of Al-Mn-0.3Mg alloy elongated.Besides,Mg speeds up the precipitation of AlMnSi phase during the process of heat treatment;the addition of Ni increases the type and amount of crystalline phase and dispersion phase in alloy(AlMnFeSiNi,AlMnSiNi);the addition of Ce/La causes a lot of AlRESi phases formed in the matrix.The critical temperature Tc of the interaction between precipitation and recrystallization of the four alloys Al-Mn,Al-Mn-0.3Mg,Al-Mn-0.3Mg-0.2Ni and Al-Mn-0.3Mg-0.2Zr are 396?,405?,430? and 433?,respectively.When annealing at 600?(above Tc),recrystallization starts before precipitation,fine-grain structure is obtained.While annealing at 350 ?(below Tc),precipitation occurs preferentially at high-angle grain boundaries before the onset of recrystallization.Due to the Zener pinning effect,coarse and rolling direction elongated grains are observed.Subsequently,in the brazing process at 600?,these dispersoid particles could also retard the movement of grain boundaries,and keep the long grains of alloy,which are good for creep property.The high temperature tensile properties and creep properties of Al-Mn-based alloys were tested.The results show that the addition of 0.3%Mg alone is better than the addition of 0.1%Ni alone in improving the high temperature performance of Al-Mn-based alloys.Al-Mn-0.3Mg-0.2Ni alloy has the best elevated temperature properties,the ultimate tensile strength at 200?,250? are 131MPa,101MPa,which are improved 90%and 98%,respectively.The steady-state creep rate under 200?/40MPa is reduced to 5.71×10-9s-1,obviously lower than Al-Mn alloy.The steady-state creep rate of Al-Mn-0.3Mg-0.2Ni and Al-Mn-0.3Mg-0.2Zr alloys under 250?/40MPa are strongly decreased to 3.93×10-8s-1 and 5.21×10-8s-1,two orders of magnitude smaller than Al-Mn alloy.With Ce or La micro-alloying,the ultimate tensile strength and creep resistance of Al-Mn-0.3Mg alloy are further improved,but the effects doesn't change significantly due to the grain refinement.The creep behaviors of Al-Mn-based alloys were studied at the temperature range of 175?250? and under the stress of 25?50 MPa.The results show that the stress exponent n=3.4 at the temperature of 175? of Al-Mn alloy,which indicates that the creep is controlled by the dislocation viscous glide mechanism.However,the n values lie in a range of 5.9?8.3 at the temperature of 200?250?,which is corresponding to dislocation climb mechanism.The addition of Mg,Mg and Nior Mg and Zr makes the creep is still controlled by the dislocation viscous glide mechanism at the temperature of 200?225? for n values lie in a range of 2.9-3.9.The dislocation viscous glide rate is lower than climb rate,so that creep property is improved a lot with micro-alloying.From TEM images of dispersoids before and after creep,we can conclude that the high temperature performance of Al-Mn-based alloy is mainly related with the amount and type of dispersoids.With dispersoids complicated or increased,the tensile strength and creep resistance of Al-Mn-based alloy inproved significantly.