| In this work,in view of the problem that the 6061 aluminum alloy complex thin-walled castings by traditional casting process are prone to hot cracking defects,the method of semi-solid rheological high pressure die casting process(RheoHPDC)and alloy element content adjustment was attempted to significantly reduce the hot cracking tendency of 6061 aluminum alloy castings,while ensuring a high mechanical properties and good anodizing and coloring capabilities.The effects of alloy element content and pouring process on the hot cracking properties of 6061 aluminum alloy were studied,and the effects of serpentine channel pouring process(SCP),Rheo-HPDC process and alloy element on the microstructure and mechanical properties of semi-solid slurry and Rheo-HPDC samples were systematically studied.Besides,the evolution mechanism of non-dendritic structure of semi-solid slurry,the solidification behavior of semi-solid slurry in Rheo-HPDC process,the segregation phenomenon and fracture mechanism in Rheo-HPDC samples were described.In addition,the effects of T6 heat treatment on the microstructure and mechanical properties of Rheo-HPDC samples were further investigated.And the Rheo-HPDC 6061 aluminum alloy complex thin-walled blade was also studied.Moreover,the effects of alloy element content and T6 heat treatment on the anodizing and coloring properties of the modified 6061 aluminum alloy were discussed.The main research contents and results are as follows:Firstly,the hot cracking tendency of 6061 aluminum alloy was evaluated by a restraint rod die method.The results show that the Si content has the most significant effect on the hot cracking of 6061 aluminum alloy,and the hot cracking tendency of the alloy decreases with the increase of Si content.When the Si content increases from 0.6%to 2.6%,the hot cracking sensitivity(HCS)of the alloy decreases from 375 to 200.However,the Mg content,pouring temperature and mould temperature have little effect on the hot cracking of 6061 aluminum alloy.In this work,the optimal processing paraments to improve the hot cracking resistance of 6061 aluminum alloy samples are as follows:the Si content is 1.4%～2.6%,the Mg content is 0.8%～1.2%,the pouring temperature is 710～750℃,the mould temperature is 150～250℃.Secondly,the semi-solid 6061 aluminum alloy slurry was prepared by the SCP process.The results show that the quality of semi-solid 6061 aluminum alloy slurry can be effectively improved by reducing the pouring temperature,increasing the bends number or reducing the channel diameter.In addition,with the increase of Si content,the primary α-Al grains are refined,the roundness is improved,and the solidified shell ratio is reduced.When the Si content increases from 0.6%to 2.6%,the average diameter of primary α-Al grains decreases from 83 μm to 59 μm,the shape factor increases from 0.79 to 0.82,and the solidified shell ratio decreases from 14.3%to 11.4%.The optimal processing paraments for semi-solid 6061 aluminum alloy slurry preparation by a graphite serpentine channel are as follows:the pouring temperature is 675～690℃,the bends number is 5,the channel diameter is 25 mm,and the Si content is 1.0%～2.6%.Subsequently,the semi-solid 6061 aluminum alloy slurry was die-cast by Rheo-HPDC process.The results show that the average diameter of primary α-Al grains in the Rheo-HPDC samples increases with the increase of the pouring temperature and die temperature,and decreases with the increase of the injection pressure,injection punch speed and Si content.And the number of the secondary primary α2-Al grains increases with the increase of the pouring temperature and the Si content.The surface liquid segregation(SLS)of Rheo-HPDC samples can be reduced by decreasing the pouring temperature,increasing the injection pressure and the injection punch speed,and appropriately increasing the die temperature.The optimal processing paraments of Rheo-HPDC 6061 aluminum alloy samples are as follows:the pouring temperature is 690℃,the injection pressure is 90 MPa,the injection punch speed is 0.5 m/s,the die temperature is 200℃,and the Si content is 1.8%.Under this condition,the volume fraction of primary α-Al grains at edge of the Rheo-HPDC 6061 aluminum alloy samples is 41%,the average diameter is 36 μm,and the shape factor is 0.70.And the volume fraction of primary α-Al grains at center of the Rheo-HPDC samples is 76%,the average diameter is 38 μm,and the shape factor is 0.75.At the same time,the as-cast tensile strength of the RheoHPDC samples is 165±8 MPa,and the as-cast elongation is 3.90±0.42%.The fracture mechanism of the Rheo-HPDC samples is mainly a mixed-fracture mechanism of dimple ductile fracture and quasi-cleavage fracture,and the cracks develop in the form of transgranular fracture and intergranular fracture.Moreover,the Rheo-HPDC 6061 aluminum alloy complex thin-walled blade with smooth surface,no hot cracks and dense internal structures was successfully die casted by adopting the composite measures of high Si content and SCP with Rheo-HPDC process.Compared with the traditional high pressure die casting process(HPDC),the hot cracking tendency of the blade is significantly reduced.Especially,when the Si content is≥1.4%,the hot cracking defects of the blade can be completely eliminated.Under the same condition of slurry preparation and Rheo-HPDC,the thickness of the SLS at edge of the blade samples increases with the increase of Si content.When the Si content increases from 0.6%to 2.6%,the thickness of the SLS at edge of the blade samples(Location "C")increase from 55μm to 226 μm.Then,the Rheo-HPDC 6061 aluminum alloy samples were treated by T6 heat treatment.The optimal processing paraments of T6 heat treatment on Rheo-HPDC 6061 aluminum alloy samples are as follows:solution treatment at 530℃ for 1.5 h and aging treatment at 180℃ for 8 h.Under this condition,the tensile strength and elongation of Rheo-HPDC samples increase first and then decrease with the increase of Si content.When the Si content is 1.8%,the tensile strength and elongation reach the peak value,which are 324±8 MPa and 2.11±0.26%.And the hardness at edge of the Rheo-HPDC samples is 121±1.4 HV,the hardness at center of the Rheo-HPDC samples is 115±1.2 HV.After T6 heat treatment,the fracture mechanism of the Rheo-HPDC samples is mainly a mixed-fracture mechanism of dimple ductile fracture and brittle quasi-cleavage fracture,and the cracks develop in the form of transgranular fracture and intergranular fracture.Finally,the Rheo-HPDC 6061 aluminum alloy samples were treated by anodic oxidation and copper salt electrolytic coloring.The results show that with the increase of Si content,the color of the copper salt electrolytic coloring film on the Rheo-HPDC 6061 aluminum alloy as-cast samples gradually deepened,the density,continuity and thickness gradually decreased,and even the fault phenomenon appeared.After T6 heat treatment,the quality and luster of the colored film were improved significantly.When the Si content is 0.6%～1.8%and the aging time is 4～8 h,the Rheo-HPDC 6061 aluminum alloy samples have good anodic oxidation and electrolytic coloring properties,and the copper salt electrolytic coloring films have good color. |
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