【作者】 陈刚；

【导师】 杜之明；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2013， 博士

【摘要】 变形铝合金在航空航天等领域应用十分广泛。目前变形铝合金主要采用锻造技术进行加工，加工成本高、效率低，且由于机械加工量过大资源浪费严重。基于现代工业对近净成形和高力学性能产品的双重需求，迫切需要一种能同时发挥出合金的性能优势和成形效率优势的新型加工技术。半固态金属加工技术(Semi-Solid Metal Processing, SSP)是一种新型的近净成形技术，为高强变形铝合金的应用和发展开辟了新的途径。触变成形是半固态加工技术的一种，该技术利用了半固态金属的触变特性，比较适合于变形铝合金等高性能合金。但变形铝合金的触变成形存在很多技术困难，如成形裂纹倾向大，半固态加工温度区间小等。此外7系高强铝合金发生再结晶较为困难，采用常规的固相法难以制备球晶化良好的半固态坯料。本文基于变形铝合金触变成形的可控难点，提出了针对性的研究思路：半固态坯料制备→半固态拉伸力学与断裂行为研究→触变成形缺陷分析→成形缺陷控制，对7075、2024和2A50等高强变形铝合金触变成形及缺陷产生机理和控制方法开展了创新性的系统研究。本文将分级热处理工艺引入到再结晶重熔法(Recrystallization and partialremelting，RAP)中，研究了常规RAP法和分级热处理RAP法制备7075铝合金半固态坯料的球晶组织形成机制和组织演变规律。研究表明：7075铝合金在过高温加热过程中发生了充分的再结晶；半固态晶粒尺寸随着过高温保温时间和半固态重熔时间的延长而增加，但前者的影响更为明显。在665℃保温5min后，再于620℃保温5min的条件下，挤压态7075铝合金的微观组织由均匀细小的球状晶粒组成，晶粒尺寸约为45μm，远优于采用常规RAP法(620℃保温40min)所制备的半固态坯料(晶粒尺寸约为120μm)。利用Gleeble1500D热力模拟机进行了7075铝合金从高温固态到半固态温度(400℃～580℃)的拉伸试验。研究发现7075铝合金的拉伸力学及断裂行为可分为三个阶段：固相决定阶段、固液共同影响阶段和液相决定阶段；断裂方式从延性断裂→脆性断裂→液相导致断裂转变；半固态温度下为延晶断裂。7075铝合金在半固态温度存在一定的脆性温度区间，在此温度区间内，合金仍能承受一定的拉应力，但几乎不能承受拉应变，因此表现出较高的裂纹倾向。分别通过对2A50和2024铝合金典型零件的直接和间接触变成形研究，分析了成形力学条件对制件微观组织和力学性能的影响，以及触变成形裂纹和缩孔等缺陷的产生机理和控制措施。结果表明：裂纹和缩孔等缺陷的影响因素包括成形过程中半固态坯料的拉伸变形、凝固收缩导致的拉应力以及模具结构限制等；微观缩孔附近的晶粒之间结合强度较低，因此容易成为微裂纹的扩展源；使半固态坯料在触变成形末期处于三向压应力状态，可以获得组织性能均匀且无缺陷的触变成形件。本研究提出将复合加载-局部补缩技术应用于铝合金半固态触变成形，保证半固态坯料在触变成形过程中处于三向压应力状态，抑制成形缺陷的产生，实现成形件组织和性能均匀化控制。在前期研究的基础上，分别采用常规和分级热处理的RAP法制备7075铝合金半固态坯料，系统研究了复合加载-局部补缩技术对7075铝合金触变成形过程中力学条件的影响，以及不同补缩量对制件微观组织和力学性能的影响；研究了适用于7075铝合金触变成形件的强化热处理工艺。结果表明：采用分级热处理RAP法制备的7075铝合金半固态坯料，其触变模锻件的微观组织相对于常规RAP法更为均匀，力学性能也相对更高。复合加载补缩量从1mm增加到5mm时，制件法兰区域的成形缺陷逐渐消失，抗拉强度增加幅度较小，屈服强度甚至稍有下降，但延伸率增幅为97.4%；中心区域的屈服强度和抗拉强度有小幅增加，延伸率却小幅下降。通过采用复合加载-局部补缩形式，触变模锻件的成形缺陷得到抑制，且整体组织和力学性能的均匀化程度大幅提高。7075铝合金触变模锻制件采用强化T6热处理(465℃固溶2t→480℃固溶8t→淬火→125℃时效24t)后，制件的延伸率稍许降低，但屈服强度和抗拉强度明显提升。本论文通过高强变形铝合金半固态坯料制备、半固态拉伸力学及断裂行为、触变成形缺陷和成形过程控制等一系列的系统研究，提出了分级热处理RAP法、复合加载-局部补缩技术等创新思想，为高强变形铝合金的半固态触变成形和缺陷控制打下了坚实的理论和试验基础，为半固态加工技术在变形铝合金领域的研究和应用提供了技术支持。变形铝合金触变成形是一种可行的近净成形技术，具有广阔的工业化应用前景。更多还原

【Abstract】 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 preparing→research on semi-solid tensile meachnicalbehaviour and fracture behaviour→defect analysis in thixoforming→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℃for2t→solution at480℃for8t→quenching→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.更多还原