| Cast aluminum alloy has become an excellent lightweight structural material due to its low density,high specific strength and low cost,and is widely used in the automotive field.In recent years,as the requirements for the strength,toughness and elastic modulus of cast aluminum alloys in automotive structural parts continue to improve,it is difficult for existing cast aluminum alloys to meet these higher requirements.Therefore,it becomes an important research direction to develop new high-performance cast aluminum alloys,which can be used in structural parts with high requirements for strength,toughness and rigidity.This article takes Al-Mg-Zn-Cu-（Mn/Cr）alloy as the research object,focusing on the alloy composition-casting process-microstructure-property relationship as well as their application.Firstly,based on the existing research results in our group,the Al-5Mg-3Zn-1Cu（wt.%）alloy was designed.Its casting castability was studied and the heat treatment process including solution treatment and aging treatment was optimized.The effect of cooling rate on the microstructure and mechanical properties of Al-5Mg-3Zn-1Cu cast alloy was studied and the relationship of cooling rate-microstructure-properties was established.Meanwhile,the aging precipitation behavior and its influencing factors were also discussed deeply.Secondly,based on the Al-5Mg-3Zn-1Cu（wt.%）alloy,Mn/Cr alloying is used to improve the elastic modulus;and the ductility was improved by reducing the Mg content increase.Thus,a high modulus Al-2.5Mg-3Zn-1Cu-2Cr（wt.%）cast alloy is developed.Finally,the Al-5Mg-3Zn-1Cu and Al-2.5Mg-3Zn-1Cu-2Cr caliper shells were trial-produced by gravity casting and tilting casting.The main results are as below:The castability of Al-5Mg-3Zn-1Cu alloy was studied firstly.The results showed that the Al-5Mg-3Zn-1Cu alloy has good casting properties,including low hot cracking tendency and excellent flow properties.Refining treatment further improved the flow properties.The flow length of the unrefined and refined alloy reached 697 mm,755 mm,respectively,when the pouring temperature was 700°C and the mold temperature was350°C.However,due to the large solid-liquid phase interval of the Al-5Mg-3Zn-1Cu alloy,the volume shrinkage reached 4.3%as it solidified from 700°C to room temperature.The effect of solution treatment[（460,470,485,500）°C×（0～48）h]on the microstructure and mechanical properties of Al-5Mg-3Zn-1Cu cast alloy was systemly studied.The results showed that the as-cast microstructure of this alloy consists ofα-Al dendrites and network-like T-Mg32（Al Cu Zn）49 phase.As the solution temperature exceeds 470°C,the microstructure tended to overburn,and the T phases were eliminated after solution-treated at 470°C for 24 h.As the solution treatement progresses,the dissolution rate of T phase first increased and then decreased.Meanwhile the Cu concentration in the residual phase increased significantly,but no transiformation from T phase to S phase was found.As the solution treatment continues,the strength,elongation and impact toughness first increased and then decreased.There is a good linear relationship between the impact toughness（IT）and the elongation（EL）:IT=8.43EL-3.46.The optimized combination of mechanical properties is acheieved at470°C×24 h,with the yield strength（YS）,ultimate tensile strength（UTS）,elongation（EL）and impact toughness（IT）being 241 MPa,388 MPa,19.5%and 154.7 k J/m2,respectively.Based on the above optimized solution-treated Al-5Mg-3Zn-1Cu alloys,the effects of aging treatment[（120,150,175）°C×（0～150）h]on the aging precipitation behavior and mechanical properties were explored.The results showed that the aging precipitation sequence of the alloy is SSSS→GP zone→GP II→T’→T precipitates.As the aging temperature increases,the peak-aged time is significantly shortened,the average size of the precipitates increases,and the proportion of T’precipitates aslo rises.The optimized solution-treated alloys can obtain a wide range of mechanical properties after different aging treatments.When the ageing temperature increases from 120°C to 175°C,the average size of the precipitates increases from 12.3 nm to 16.8 nm,and the proportion of T’precipitates increases from 30%to 47%.As the aging temperature increases from120°C to 175°C,the YS of the alloy increases from 377 MPa to 451 MPa,the UTS increases from 465 MPa to 527 MPa,while the EL decreases from 13.8%to 5.2%,the IT drops from 104.0 k J/m2 to 84.0 k J/m2.The optimized aging treatment is 150°C×16 h.Under this condition,this alloy possessed good mechanical properties with UTS,YS,EL and IT being 496 MPa,441 MPa,9.6%and 93.0 k J/m2,respectively.The effect of cooling rate（2.2～24.1 K/s）on the microstructure and mechanical properties of Al-5Mg-3Zn-1Cu cast alloy was studied by casting step-like samples with thickness ranging from 5 mm to 50 mm.The results showed that as the cooling rate increased,the secondary dendrite arm spacing（SDAS）gradually decreased,and the T-Mg32（Al Zn Cu）49 phase was gradually refined.The aging precipitation kinetics was accelerated as the cooling rate rises and the average size of the eutectic T phases increases.As the cooling rate increased,both the strength and ductility of the as-cast,solution-treated and peak-aged alloys increased.In comparison,the cooling rate has the least impact on the mechanical properties of the as-cast alloy,and the greatest impact on the mechanical properties of the peak-aged alloy.As the cooling rate increased from 2.3 K/s to 24.1 K/s,the EL of the peak-aged alloy increased from 6.1%to 10.6%with an increase of 73.8%;the YS increased from 402.5 MPa to 441.9 MPa with an increase of 9.8%.The dominant strengthening mechanism for the peak-aged alloy is precipitation strengthening,and the cooling rate mainly adjusts the YS by affecting the precipitation strengthening process.The effect of Mn/Cr alloying on the microstructure and mechanical properties of Al-5Mg-3Zn-1Cu alloy was comparatively studied.The results showed that with the increase of Mn/Cr content,the elastic modulus of Al-5Mg-3Zn-1Cu-x Mn/Cr alloy rised rapidly.Adding an equal amount of Mn or Cr produces similar elastic modulus increments.After adding 4 wt.%Mn/Cr,the elastic modulus of the alloy increased from67.6 GPa to 76.3 GPa/76.1 GPa.However,with the increase of Mn/Cr content,the mechanical properties of Al-5Mg-3Zn-1Cu-x Mn/Cr alloy decreased significantly.In comparison,the solution-treated Al-5Mg-3Zn-1Cu-2Cr alloy had better ductility with the EL being 2.4%.As the Mg content decreased,the strength of the Al-x Mg-3Zn-1Cu-2Cr alloy decreased,while both the EL and elastic modulus increased.Al-2.5Mg-3Zn-1Cu-2Cr alloy has better comprehensive mechanical properties,with YS at 220 MPa,UTS at310 MPa,and EL at 4.3%under solution-treated condition.After aged treatment,the YS,UTS and EL became 300 MPa,350 MPa and 2.3%,respectively.Compared with the Al-5Mg-3Zn-1Cu alloy,the elastic modulus of Al-2.5Mg-3Zn-1Cu-2Cr alloy increased from 67.4 GPa to 73.6 GPa with an increment of nearly 9.2%.Based on the developed Al-Mg-Zn-Cu-（Cr）alloy,caliper shells were trial-produced by gravity casting and tilting casting.The Al-5Mg-3Zn-1Cu caliper castings were prepared by gravity casting process,and there are large shrinkage cavities and some shrinkage porosity in the castings,which were related to the wide solid-liquid interval of this alloy,the larger volume shrinkage and the sand core used in the mold.The results of numerical simulation for tilting casting process showed that some isolated liquid phase regions were formed during solidification,resulting in shrinkage defects.After expanding the flow channels and feeding channels of the mold,the filling process became stable and sequential solidification was achieved,thus these isolated liquid phase regions were removed.Based on the numerical simulation results,Al-5Mg-3Zn-1Cu caliper castings were prepared by the tilting casting.The outer surface of the castings was of good quality.There were no shrinkage cavities in the castings except for some loose defects.The solution-treated alloy has better mechanical properties with UTS at 356 MPa,YS at 277 MPa and EL at 4.9%.Moreover,Al-2.5Mg-3Zn-1Cu-2Cr caliper castings were also prepared by tilting casting.The outer surface of these castings was of good quality.There were no shrinkage cavities and less loose defects in the castings.The solution-treated alloy has higher mechanical properties with UTS at 306 MPa,YS at 221 MPa and EL at 2.3%. |
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