| 6061 aluminum alloy has a series of characteristics and advantages,such as medium strength,low density,excellent thermal conductivity and corrosion resistance,which is one of the most important aluminum alloys for lightweight of structural parts.However,as a deformed aluminum alloy,the applicability of 6061 aluminum alloy to other forming processes is relatively poor,especially when used for liquid forming,the casting forming integrity is limited due to the poor fluidity of the alloy in the liquid state,which directly restricts the expansion of the application of the alloy.Research on 6061 aluminum alloy forming technology shows that the alloy has the possibility of using liquid forming technology,for instance,the use of liquid die-forging technology,which could compensate for the poor fluidity of the alloy itself,and has been successfully used in the production of aluminum alloy castings with simple shapes and small dimensions,and also in the automotive field.However,the high mechanical properties,good thermal conductivity and corrosion resistance of 6061 aluminum alloy are more suitable for the performance advantages of relatively complex shape and large size castings,which is also the development trend of this alloy in engineering applications.Therefore,around 6061 aluminum alloy liquid die forging forming technology,it is necessary to further study the performance characteristics of the alloy under liquid die forging conditions and the relationship with the forming process,so as to achieve further optimization of 6061 aluminum alloy liquid die forging technology for the production of castings with relatively complex shapes and large sizes,and to solve the problems of segregation and uneven properties of castings after forming,and finally to realize the production and application of large size and large aspect ratio castings represented by aluminum alloy castings for truck hubs.In view of the above situation,firstly,the thermophysical properties and solidification characteristics of 6061 aluminum alloy were studied by JMatPro simulation software in this paper.On this basis,the effects of cooling rate and alloying elements on the microstructures of the alloy were studied,and the liquid die forging characteristics and microstructures control of the alloy were analyzed and designed.Eventually,the 6061 aluminum alloy was used for the preparation of heavy-duty truck wheel products with large size and large aspect ratio using liquid die-forging technology,and the segregation existing in the product parts were studied in depth,and the corresponding solutions were proposed,achieved the preparation of 6061 aluminum alloy wheels by casting instead of forging.The physical properties of 6061 aluminum alloy were calculated and studied.It was found that the enthalpy of 6061 aluminum alloy could be divided into three stages with the change of temperature:the linear stage was below 600?,the parabola stage was between 600 and 654.42?,and the linear stage was above 654.42?.In the temperature range below 600?,the increase of enthalpy was about 32.569 J/mol/?;in the temperature range of 600?654.42?,the increase of enthalpy was about 465.016 J/mol/?;in the temperature range above 654.42?,the increase of enthalpy was about 32.614J/mol/?.The macro-solidification characteristics of 6061 aluminum alloy were calculated and studied.It was found that the mushy zone of 6061 aluminum alloy was 528?655?,the relationship between solid state and temperature in mushy zone of alloy was S=-9×10 T+0.1002T-27.141.The influence of alloy elements on the mushy zone was studied.It was found that with the increase of Mg and Si content,the number of solid phases in the mushy zone decreases,and the fluidity in the mushy zone increases.The effect of Cr on the mushy zone was related to the content of Cr.When the Cr content was more than 0.2%,with the increased of Cr content,the number of solid phases in the mushy zone increased,and the fluidity of the alloy decreased;Zn and Mn had little effect on the mushy zone of the alloy.The isothermal solidification characteristics of the alloy was studied,it was found that the types and order of precipitated phases were different when the alloy solidifies at different temperatures.Under the experimental conditions,the precipitation order of the second phase of 6061 aluminum alloy might be:L?a-Al+Al4Cu9+CrMn3+Cu16Mg6Si7+Mn5Si3?Cr3Si?Cu2Mg?FeCr+Cu16Mg6Si?Cu16Mg6Si+Fe-rich pahse?Al7Cu3Mg6+Mg2Si+a-AlFeSi.The harmful Fe rich phase in the alloy was formed when the temperature was below 700?,mainly concentrated in the grain boundary.The cooling rate had obvious effect on the second phase of 6061 aluminum alloy.It was found that when the cooling rate was higher than 0.615?/s,the main precipitated phases were ?-Al,Mg2Si,CrMn and Cu2Mg.When the cooling rate was lower than 0.615?/s,there were AlFeSi precipitates in the alloy.The smaller the cooling rate,the more AlFeSi precipitates.The influence of liquid die forging process on the structure and properties of 6061 aluminum alloy was studied.The suitable process of 6061 aluminum alloy liquid die forging hub was designed:the casting temperature of alloy was 710?,the preheating temperature of die was 300?,the pressure was 3000 t,and the reduction rate was 50 mm/s.The aluminum alloy wheel hub was successfully prepared by using the above design process.The microstructure and properties of different parts of 6061 aluminum alloy wheel hub were studied.It was found that the grain size of the outer rim was the smallest,with an average size of about 26 ?m;the grain size of the rim was different,and some of the grains were elongated;the grain size of the spoke was the thickest,with an average size of about 50 ?m;the comprehensive properties of the outer rim was the highest.The tensile strength could reach 371 MPa,the elongation was 16%;the tensile strength of the inner rim was 362 MPa,the elongation was 10%;the tensile strength of the spoke part was 346 MPa,the elongation was 9%;the tensile strength of the rim part was 358 MPa,the elongation was 10%.The segregation phenomenon and mechanism of 6061 aluminum alloy wheel hub by liquid die forging were studied.It was found that the segregation position was mainly at the R joint of the wheel spokes,and the segregation area was distributed in the form of floccules and strips,the largest size of the segregation area was about 10 mm;the segregation phase was mainly composed of Mg2Si phase and ?-AlFeSi phase.In order to avoid the segregation phenomenon in the liquid die forging of 6061 aluminum alloy wheel spokes,the cooling rate at this place should be at least greater than 1.0?/s. |
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