【作者】 李东南；

【导师】 罗吉荣；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2004， 博士

【摘要】 半固态金属加工技术被认为是 21 世纪最具有发展前途的近净成形技术之一,其关键环节是半固态浆料的制备。针对目前半固态镁合金研究较少和制浆机剪切速率较低的特点,本文首次采用高剪切速率双螺杆机械搅拌制浆机和半固态流变压铸成形机组,研究了 AZ91D 镁合金半固态浆料制备工艺及材料组织与力学性能之间的关系。该技术为镁合金产品的推广和应用开辟了新的途径。 率先比较研究了单螺杆和双螺杆制浆机的制浆效果。高剪切速率的双螺杆制浆机可获得固相率为 15%～60%、晶粒平均直径为 35μm～55μm左右的超细球团状组织。单螺杆制浆机由于其剪切速率低,对物料的混合作用不强烈,制备的非枝晶组织呈条状或不规则块状分布,晶粒平均直径为 80μm～100μm左右。 系统地研究了镁液浇注温度、筒体温度和剪切速率对双螺杆制浆机制备的半固态浆料质量的影响。得出镁合金半固态浆料制备的最佳工艺参数范围为:镁液浇注温度 605℃～625℃,筒体温度 575℃～590℃,剪切速率 4100 s-1～6830s-1。结果表明,随着镁液浇注温度或筒体温度的降低,半固态浆料的固相率提高,晶粒变得更加细小、圆整。螺杆剪切速率越大,半固态浆料的固相率越低,晶粒的圆整度变化不大。一方面,随着剪切速率的提高,螺杆对熔体的剪切作用加强,另一方面,剪切时间相应缩短。剪切速率低于 1365s-1时,组织形态明显恶化。半固态镁合金浆料具有很好的充填性能,采用半固态流变压铸成形技术,成功地生产出壁厚只有1.1mm 的薄壁圆盘类零件,该铸件表面光洁,字迹清晰。 分别采用半固态流变压铸和半固态流变挤压成形工艺,成功地制备出直径为φ50mm、高度约为120mm的优质半固态坯料,晶粒平均尺寸分别为40~60μm、50~70μm,半固态流变压铸坯料的晶粒形貌更圆整。该坯料可以用于半固态触变成形。 探明了镁合金半固态流变压铸成形组织中浆料固相率和初生α-Mg相晶粒平均尺寸与材料力学性能之间的关系。当半固态浆料固相率保持在 30%～50%时,既有较好的充填能力,又具有较细小、园整的非枝晶组织,此时试样的力学性能最佳。 I<WP=5>随着初生α-Mg 相晶粒尺寸的增大,试样的抗拉强度逐渐降低;晶粒大小在 35~45μm 范围时,试样的延伸率变化不大;当晶粒平均尺寸增大到 50μm 以后,延伸率急剧下降。通过非线性回归分析,建立了半固态流变压铸成形试样力学性能的数学模型。研究了热处理工艺对镁合金成形试样力学性能的影响。结果表明,半固态流变压铸成形试样克服了液态压铸成形试样在固溶处理或时效处理过程中出现析出气孔或显微疏松等缺陷而不能热处理的缺点,可通过热处理工艺进一步提高其力学性能。在铸态时,镁合金半固态流变压铸成形试样与液态压铸成形试样的力学性能相当,抗拉强度为 192MPa,延伸率为 2.3%,硬度值为 74HB。固溶处理之后,抗拉强度提高了 15%,延伸率提高了 210%,硬度值略有降低。时效处理之后,抗拉强度提高了 13%,延伸率略有升高,而硬度值提高了 16%。提出了高剪切速率条件下非枝晶组织的形成机理,既有破碎枝晶的机械球化现象,又有熔体内部均匀形核-平面生长的过程,并建立了半固态组织演化过程的物理模型。螺杆搅拌引起的熔体流动及机械剪切力使得初始生成的树枝晶在长大的过程中发生碰撞、破碎、熔断而机械球化。对半固态流变压铸成形组织进行面扫描分析的结果表明,未见明显的溶质扩散层,细小的网状共晶相中析出了大量的晶粒尺寸为 5μm～10μm 的二次α-Mg 相。由此可见,在高剪切速率的双螺杆作用下,晶粒结晶界面前沿的温度梯度和溶质浓度梯度极小,一方面,导致液相中各处的形核几率相等,晶粒在各处形核更加趋向于同时性,另一方面,抑制了树枝晶的生长,有利于晶粒按照平面方式生长。筒体内壁和双螺杆螺棱处首先结晶的晶核随着螺杆的运动被很快地带到高温熔体中,极大地增大了熔体中的晶核数,从而减小了晶粒尺寸。高速旋转的晶核无择优生长的条件,最终使得晶核逐步演变成球状晶粒。更多还原

【Abstract】 Semisolid metal processing technology is regarded as one of the near-net shapeforming technologies with wide application in 21 century, and the preparation ofsemi-solid slurry is the key technique. However, research on semi-solid magnesiumalloys is seldom to date, and generally the shear rate of stirring mixer is small. In thispaper, the semi-solid slurry preparation of AZ91D magnesium alloy with twin-screw inhigh shear rate was studied, and the correlation of microstructure and mechanicalproperties of magnesium alloy in semi-solid state was also systematically investigated byrheo-diecasting process. Therefore, this advanced technique opens a new route forgeneralization and application of magnesium alloy products. Effects of semi-solid slurry preparation by single or twin-screw stirring werecomparatively studied for the first time. The results showed that fine round granularcrystals with average diameter of 40μm to 60μm of the prepared slurry could beobtained by twin-screw stirring, and the range of solid fraction could be controlled from15% to 60%. In contrast, because of the weak effect of mixture with low shear rate, thesolidified microstructure of the slurry by single screw was in strip shape or anomalousblock, the range of the primary grain size varied from 80μmto 100μm. The influences of the pouring temperature、the barrel temperature and the shear rateon the quality of the slurry by twin-screw stirring were systematically investigated. Theresults showed that the pertinent processing parameters were as following: a pouringtemperature of the Mg melt of 605℃～625℃; a barrel preheating temperature of 575℃～590℃; and shear rate of 4100s-1～6830s-1. The solid fraction in the slurry increasedand that the grains became finer and rounder with reducing pouring temperature of theMg melt or the barrel temperature. The solid fraction of the semi-solid slurry decreasedwith the increase of the shear rate, and the roundness of the crystals changed slightly. Thereason is that the higher shear rate enhances the shear action on one hand, whilst it III<WP=7>shortenes the shear time on the other hand. But the microstructure was distinctlydeteriorated when the shear rate was under 1365s-1. A thin-walled circular plate of 1.1mm thickness was fabricated successfully with a clear contour and good soundness withthis technique, this process has advantages of improvement in the fluidity and feedingcapacity of the semi-solid slurry. An excellent semi-solid feedstock of magnesium alloy was successfully producedby rheo-diecasting or rheo-squeezing process, which was a round stick of 50 mmdiameter and 120 mm height. The average size of grains were 40μm ~60μm and 50μm~70μm, respectively, and the grains made by rheo-diecasting were rounder. These slugfeedstock with non-dendritic primary phase are applicable for thixoforming. The relation of the mechanical properties of semi-solid magnesium alloy with thesolid fraction in slurry and the average size of primary α-phase grains was studied. Theresults showed that the tensile strength and elongation of rheo-diecasting AZ91D alloywere best when the solid fraction varied in 30% to 50%, because of high feedingcapacity of the semi-solid slurry and finer and rounder non-dentritic microstructure. Thetensile strength decreased gradually with the increase the average size of primary α-phase grains. The elongation was almost unchanged when the average size of primaryα-phase grains was varied in 35μm~45μm, but the elongation decreased rapidly whenthe average size of primary α-phase grains was over 50μm. Mathematical model ofmechanical property of semi-solid magnesium alloy sample fabricated by rheo-diecastingprocess was constituted with non-linear regression analysis. The effect of heat treatment on the mechanical property of magnesium alloy wasstudied. The results indicated that the mechanical property of semi-solid magnesiumalloy coul更多还原