摘要
本文采用反复压缩大塑性变形技术制备镁基复合材料，研究复合材料的组织和性能。将Si加入AZ31镁合金熔体制备了AZ31–Si原位镁基复合材料坯料，采用循环闭式模锻(CCDF)和反复镦压(RU)两种反复压缩大塑性变形技术细化和均匀组织结构，制备Mg2Si原位增强的均匀组织镁基复合材料，考察了Si含量对复合材料坯料组织、力学性能和耐磨性能的影响，研究了反复压缩工艺参数(变形道次、加工温度)对基体组织和增强相尺寸、形貌、分布，以及对力学性能的影响规律，分析了反复压缩对室温拉伸断裂行为的影响，探讨了AZ31–Si反复压缩过程中Mg2Si相的破碎机制，采用有限元法模拟了反复压缩过程中复合材料的温度场、流动场、应力场、应变场，优化了模具和工艺。通过高能超声技术将SiC纳米颗粒加入Mg熔体中制备了Mg–1wt.%SiC纳米复合材料坯料，采用往复挤压(CEC)大塑性变形技术细化和均匀组织，弥散SiC纳米颗粒分布，制备了SiC纳米颗粒均匀分布的镁基纳米复合材料，考察了往复挤压道次对Mg–1wt.%SiC纳米复合材料坯料组织和性能的影响。
研究了Si含量对AZ31–Si原位复合材料坯料组织和室温性能的影响，表明铸态AZ31–Si原位复合材料由α-Mg基体、Mg17Al12相、树枝状初生Mg2Si相和汉字状共晶Mg2Si相组成，随着Si含量从0增加到5%(wt.%)，Mg2Si相的尺寸和体积分数逐渐增大，复合材料的硬度、屈服强度和耐磨性能逐渐提高，由于基体中粗大的Mg2Si颗粒尖端附近容易产生应力集中，抗拉强度和延伸率逐渐降低，室温拉伸断裂形式由韧脆混合型穿晶断裂转变为解理脆性断裂。
考察了铸态AZ31–Si复合材料坯料的高温力学性能和耐磨性能，发现随着Si含量从0增加到5%(wt.%)，铸态AZ31–Si复合材料在150°C下的抗拉强度和延伸率逐渐减小；随着拉伸温度从100°C提高到200°C，AZ31–2wt.%Si复合材料的抗拉强度逐渐降低，延伸率逐渐提高；随着温度从30°C提高到190°C，由于复合材料的强度和硬度逐渐降低，高温下热应力与接触应力共同作用，磨损失重逐渐增大。
研究了反复压缩道次对AZ31–2wt.%Si坯料组织的影响，发现随着循环闭式模锻及反复镦压道次从0增加到5，由于动态再结晶，晶粒平均尺寸逐渐减小，尺寸分布均匀性逐渐提高；变形时基体产生的剪切应力将Mg2Si由粗大的汉字状和树枝状逐渐破碎为细小多角块状，破碎的Mg2Si颗粒在多道次加工过程中反复流动而重新分布，其分布均匀性逐渐提高，5道次后呈细小、弥散分布；坯料反复压缩过程的有限元模拟表明应力场中存在剪切应力，应变场中累积应变和应变的分布均匀性随道次增加逐渐提高，这些因素导致Mg2Si颗粒的分布均匀性随道次增加逐渐提高。
研究了反复压缩道次对AZ31–2wt.%Si坯料性能的影响，结果表明：随着循环闭式模锻及反复镦压道次从0增加到5，由于基体晶粒逐渐细化和Mg2Si相弥散化，复合材料的屈服强度、抗拉强度、延伸率和耐磨性能不断提高，拉伸断裂形式由解理脆性断裂转变为韧脆混合断裂。
AZ31和AZ31–2wt.%Si坯料在不同温度反复压缩后的组织和性能研究表明：随着循环闭式模锻加工温度从350°C提高到450°C：AZ31合金晶粒平均尺寸逐渐增大，形成的基面织构减弱，屈服强度和抗拉强度逐渐降低，延伸率逐渐提高；在400°C下加工时AZ31–2wt.%Si复合材料中Mg2Si颗粒破碎效果最好，且基体晶粒平均尺寸最小，强度和延伸率最高。
利用有限元法分析了反复镦压模具结构和工艺参数对材料流动的影响，发现减小型腔宽度可以提高每道次的等效应变，但降低应变分布均匀性和坯料的形状尺寸保持度；模具过渡角可提高坯料成型表面质量，随着过渡角半径的增大，坯料中应变分布均匀性有所提高；反复镦压过程中坯料的绝大部分在三个方向都处于压应力状态，变形过程中由于流速不均匀和流动方向不同，剪切变形总是存在的；随反复镦压道次的增加，累积应变和应变的分布均匀性都逐渐提高；采用路径B(每道次加工后坯料绕Z轴旋转90°)镦压获得的坯料中等效应变分布比路径A(每道次加工后坯料不旋转)更均匀；随着镦压温度的提高，最大镦压载荷不断减小，坯料中应变和应力分布更加均匀。
研究了采用路径A和B反复镦压后镁合金的组织和性能，表明在350°C、5道次，采用路径B加工对AZ31合金具有更强的晶粒细化和均匀化效果，对强度和塑性的提高更加显著。
往复挤压道次对Mg–1wt.%SiC纳米复合材料坯料组织和性能的影响研究表明：随着道次从0增加到8，复合材料的平均晶粒尺寸逐渐减小，纳米颗粒的分布均匀性、复合材料的硬度和耐磨性能逐渐提高；往复挤压过程中镁基体强烈的剪切变形使SiC纳米颗粒团簇解离并均匀分布；纳米复合材料性能的提高主要归结于弥散强化和细晶强化。
In this dissertation, magnesium matrix composites were fabricated by repeatedcompression severe plastic deformation (SPD) technologies. Microstructure and mechanicalproperties of the composites were investigated. AZ31–Si in-situ magnesium matrix compositebillets were fabricated by adding Si into AZ31magnesium alloy melt. Repeated compressionSPD technologies, cyclic closed-die forging (CCDF) and repeated upsetting (RU), wereadopted to refine and uniform the microstructure, and in situ Mg2Si reinforced magnesiummatrix composites with homogeneous microstructure were fabricated. Effects of Si content onmicrostructure, mechanical properties and wear resistance properties of as-cast compositebillets were examined. Effects of technological parameters (deformation pass, processingtemperature) on size, morphology, distribution of matrix structure and reinforcements,mechanical properties were studied. Influence of repeated compression on fracture behaviorduring tensile test at room temperature was analyzed. Breaking mechanism of Mg2Si phasesduring repeated compression of AZ31–Si was discussed. The temperature field, flow field,stress field, and strain field of composite were simulated with finite element method. The dieand technology were optimized. SiC nanoparticles were added into Mg melt by high intensityultrasonic method and Mg–1wt.%SiC nanocomposite billets were fabricated. Cyclicextrusion compression (CEC) severe plastic deformation technology was used to refine anduniform the microstructure, disperse the SiC nanoparticles, and magnesium matrixnanocomposite with homogeneous distribution of SiC nanoparticles was fabricated.Influences of CEC passes number on microstructure and properties of Mg–1wt.%SiCnanocomposite billets were studied.
Effects of Si content on microstructure and room temperature properties of AZ31–Si in-situ composite billets were investigated, and the results show that as-cast AZ31–Si in-situcomposites are composed of α-Mg matrix, Mg17Al12phase, dendritic primary Mg2Si phaseand Chinese script type eutectic Mg2Si phase. With Si contents increasing from0to5%(wt.%), size and volume fraction of Mg2Si phase gradually increase. Hardness, yield strength(YS) and wear resistance also gradually increase. Both elongation and ultimate tensilestrength (UTS) decrease due to stress concentrations occur easily in the matrix near the sharptips of Mg2Si particles. The tensile fracture mode at room temperature transforms from ductileand brittle mixed transgranular fracture to cleavage brittle fracture.
The mechanical properties and wear resistance properties of as-cast AZ31–Si compositebillets at elevated temperature were examined, it is found that with Si contents increasingfrom0to5%(wt.%), both UTS and elongation of as-cast AZ31–Si composites graduallydecrease. As tensile temperature increases from100°C to200°C, UTS of AZ31–2wt.%Sicomposite gradually decrease and the elongation increase. Wear loss of composite graduallyincrease with temperature rise from30°C to190°C, due to decrease of strength and hardness,interaction of thermal stress and contact stress at high temperature.
Influence of repeated compression pass on microstructure of AZ31–2wt.%Si billets wasstudied, and it is found that as CCDF and RU passes number increasing from0to5, averagegrain size of AZ31–Si composites decreases and homogeneity of size distribution improvesfor dynamic recrystallization. Both dendritic and Chinese script type Mg2Si are broken up intosmaller polygonal pieces due to the shear stress imposed by the matrix. The broken Mg2Siparticles repeatedly flow and redistribute during multi-pass processing, and their homogeneityis gradually enhanced. Fine and dispersive distribution of Mg2Si is obtained after5passes.Finite element simulation of the billet during repeated compression shows that shear stressexists in strain field. Accumulated strain and strain homogeneity in strain field graduallyimprove with increasing passes number. These factors result in homogeneity of Mg2Siparticles gradually improves with increasing passes number.
Effect of repeated compression pass on properties of AZ31–2wt.%Si billets wasinvestigated. It shows that with CCDF and RU passes number increasing from0to5, YS,UTS, elongation and wear resistance of AZ31–Si increase due to gradual refinement of matrixgrain and dispersion of Mg2Si phases. The tensile fracture mode transforms from cleavagebrittle fracture to ductile and brittle mixed fracture.
Research on microstructure and properties of AZ31and AZ31–2wt.%Si billets processed by repeated compression at different temperature show that as CCDF processingtemperature increases from350°C to450°C, basal texture weakens, the average grain sizeand elongation of AZ31alloy increase, while the YS and UTS decrease. For AZ31–2wt.%Sicomposite, the finest grain size and Mg2Si particles along with the highest strength andelongation are achieved at processing temperature of400°C.
Influences of RU die structure and technological parameters on material flow wereanalyzed by finite element method. It is found that reduction of cavity width could increasethe equivalent strain for each pass, however, it deceases homogeneity of strain distributionand retentivity of billet shape and size. Transitional angle can enhance the forming quality ofbillet. With increasing transitional radius, homogeneity of strain distribution is slightlyimproved. Most of the billet suffers compressive stress in three direction and sheardeformation always exists due to inhomogeneous flowing velocity and different flowingdirection during RU process. Both accumulated strain and strain homogeneity improve as RUpasses number increase. More homogeneous strain is obtained in the billet processed withroute B (rotate90°around Z axis after former processing pass) than route A (without rotationafter former pass). With increasing RU temperature, the maximum upsetting load decreaseswhile homogeneity of strain and stress distribution in the billet increase.
Microstructure and properties of Mg alloy processed by RU with route A and B wereinvestigated, and it shows that more intense grain refinement and homogenization of AZ31alloy is obtained when processed with route B at350°C for5passes. Thus, more significantimprovement of strength and ductility of AZ31alloy processed with route B is obtained.
Influences of CEC passes number on microstructure and properties of Mg–1wt.%SiCnanocomposite billets were studied, and it is found that with passes number increasing from0to8, a finer average grain size and more uniform nanoparticle distribution are obtained alongwith significant improvement in hardness and wear resistance. Nanoparticle declusteringoccurs due to intense shear deformation of Mg matrix during CEC and the SiC nanoparticlesdisperse homogeneously. The property improvement is mainly attributed to dispersionstrengthening and fine grain strengthening.
引文
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