Research On Thixoforming And Defect Controlling Of High Performance Wrought Aluminum Alloys

Wrought aluminum alloys are widely used for aerospace applications. Nowthese alloys are mainly prepared by forging, which involves high cost and lowefficiency, and it will cause much waste after maching. Based on the requirement bymodern industry to near net shaping and high performance products, there is astrong drive to develop a novel processing technology which could not only exertthe high performance of alloys, but also has high processing efficiency. Semi-solidmetal processing is one potential near net shaping technology, which provides newroute for the application and development of high performance wrought aluminumalloys. Thixoforming is one of SSP technology, which bases on the thixotropicproperty of semi-solid alloys, and it’s very suitable for processing wroughtaluminum alloys. However, there are also some difficulties. For example, largecrack sensitivity, small processing windows (for temperature) and so on. Besides,some high performance aluminum alloys are difficult to be recrystallized, such as7075alloy, so it is difficult to prepare semi-solid feedstock with sphericalmicrostructures by regular solid phase route. In this research, based on thedifficulties of thixoforming of wrought aluminum alloys, some specific researchideas were proposed: billet preparingâ†'research on semi-solid tensile meachnicalbehaviour and fracture behaviourâ†'defect analysis in thixoformingâ†'formingcontrolling.7075,2A50and2024aluminum alloys were prepared by thixoforming,and the defect mechanism and controlling methods were studied innovatively andsystematically.Multistep reheating treatment was introduced in Recrystallization and partialremelting (RAP) route, and the forming mechanism of spherical structures andmicrostructure evolution of semi-solid7075aluminum alloy prepared byconventional and multistep reheating RAP routes were studied. The results showthat,7075aluminum alloy was fully recrystallized during hyperthermally reheating;the semi-solid grain sizes increase when hyperthermal holding time or isothermalholding time increase, and the effect of the former one is larger. By reheating to665℃for5min holding, followed by isothermally holding at620℃with total heatingtime of10min, the microstructure consist of homogeneous and fine spherical grainswith size of about45μm. The semi-solid grain size is about120μm underconventional RAP route.The tensile properties of7075aluminum alloy in high temperature solid andsemi-solid states (400℃～580℃) were studied. The results show that the tensileand fracture behaviors could be divided into three stages: solid dominating stage, combined influences of solid and liquid stage and liquid dominating stage, while thefracture mechanism changes from ductile fracture to brittle fracture, and liquidresulting in fracture. The crack is intergranular in the semi-solid state. A brittletemperature region exists in the semi-solid state, and at these temperatures, the alloycould still sustain some tensile stress but with amost no ductility, which is prone tohot cracks.Tpical parts of2A50and2024aluminum alloys prepared by direct and indirectthixoforming were studied, respectively. The effect of mechanical conditions on themicrostructures and mechanical properties were discussed, and the generationmechanism and controlling method of micro-cracks and micro-shrinkages were alsoanalysed. The results show that the distribution of mechanical conditions has greateffect on thixoformed microstructures, and the thixoformed parts will behomogeneous and with no defect under three-dimensional compression stress; theinfluencing factors of microcracks include tensile deformation during thixoforming,tensile stress caused by solidification shrinkage and mechanical constraint; the bondstrength between solid grains near the microshrinkage is very low, so microcracksare likely to generate here. The compound loading-local feeding method wasproposed to be applied in thixoforming of aluminum alloys. In this ways, thesemi-solid billet could be in three-dimensional compression stress state duringthixoforming process, therefore, the microcracks could be controlled, and themicrostructures and mechanical properties could be homogenized.The effect of compound loading-local feeding method on the mechanicalconditions during thixoforming of7075aluminum alloys was studied, and the effectof feeding amount on the microstructures and mechanical properties was alsoresearched. A novel heat treatment suitable for7075aluminum alloy was proposed.The results show that, the microstructures and mechanical properties of thixoformedparts prepared by multistep reheating RAP route are better, compared withconventional RAP route. When the feeding amount increases from1mm to5mm,the defects at flange region disappeared gradually, the ultimate tensile strengthincreases a little, and the yield syrength even decreases little, however, theelongation increases evidently; the yield strength and the ultimate tensile strength ofcentral region increases a little, but the elongation decreases evidently. Byemploying compound loading-local feeding method, the thixoforming defects werecontrolled effectively, and the homogeneities on microstructures and mechanicalproperties of the thixoformed parts were improved evidently. The ultimate tensilestrength and the yield strength both increase evidently after reinforcing T6heattreatment (solution at465℃for2tâ†'solution at480℃for8tâ†'quenchingâ†'aging at125℃for24t), however the elongation decreases a little.In this paper, the semi-solid billet preparing of high performance wrought aluminum alloys, tensile and feacture behaviors in the semi-solid state, thixoformingdefects and controlling of thixoforming were studied systematically, and some novelmethods were proposed, such as multistep reheating RAP route and compoundloading-local feeding method. The results could build theoretical and experimentalfoundation for thixoforming and defects controlling of high performance wroughtaluminum alloys, and provide useful instruction to the research and application ofsemi-solid processing in wrought aluminum alloys. Thixoforming of wroughtaluminum alloys is proved to be a feasible near net shaping technology withpromising industry applications.