【作者】 翟彦博；

【导师】 刘昌明；

【作者基本信息】 重庆大学 ， 材料加工工程， 2009， 博士

【摘要】 本文提出并试制了一种用于替代传统铁质水冷薄壁汽缸套材料的新型、轻质自生颗粒增强铝基骤变梯度功能复合材料。该骤变梯度复合材料筒状零件采用离心铸造工艺制备,从结构上分为内外两层,内层富含增强颗粒,具有高硬度、优良的耐磨性能、良好的热传导性能以及热稳定性,外层没有或存在少量增强颗粒,具有较高的拉伸强度以及良好的机械加工性能,从内壁到外壁,具有组织与性能的骤变功能梯度特征。为了达到这一目的,本文以自生颗粒选择、材料成分研究、离心成形工艺研究、试制气缸套毛坯、毛坯力学与热物理性能研究为主线,对这一设想进行了系统研究。离心铸造初晶Si颗粒单独增强和初晶Si、Mg2Si混合增强两种梯度复合材料组织与性能的对比发现,以Al-19Si合金为基体,采用热模离心铸造工艺,可以制备出单独初晶Si颗粒增强的渐变梯度复合材料;而以Al-19Si-5Mg合金为基体,采用相同的工艺参数,可以制备出初晶Si、Mg2Si颗粒混合增强的Al基骤变梯度复合材料。通过对离心场中颗粒的受力与运动关系的分析以及两种梯度复合材料的磨损性能研究发现,在初晶Si与Mg2Si混合颗粒增强材料中,初晶Si颗粒起提高材料耐磨性的作用,Mg2Si颗粒除起提高耐磨性的作用外,主要还起在离心力下推动初晶Si颗粒快速偏聚的作用。通过对材料成分的进一步研究发现,Mg2Si的形貌是形成初晶Si与Mg2Si颗粒增强骤变梯度复合材料的又一关键因素。只有块状或颗粒状的初晶Mg2Si相,才具有较大的向心运动速度,从而能够推动初晶Si颗粒一起向内壁偏移。而团簇状或网状的共晶Mg2Si组织不能够产生离心偏移或具有很小的离心偏移速度。因此,只有能够形成大量块状的初晶Mg2Si颗粒的Al-Si-Mg三元系合金才能通过离心铸造形成自生初晶Si与Mg2Si混合增强的骤变梯度功能复合材料。满足这一要求的合理合金成分是,Mg高于4wt%,Si高于19wt%。对离心成形工艺的研究发现,初晶Si与Mg2Si向内层的严重偏聚增大了工艺难度,容易在铸件增强层与非增强层之间形成缩孔和气孔。对缩孔形成的机制分析发现,铸型的冷却与颗粒向内层偏移结果导致在铸件厚度方向上形成固态——液态——半固态——固态这一双向凝固特征,内外两层之间的液体最后凝固形成缩孔。对气孔的形成过程分析发现,凝固过程中,铝熔体中过饱和气体的析出主要在660℃以下,而此时内层已经聚集了大量的增强颗粒,处于固相率很高的半固态或固态状态,从而导致气体从内壁溢出受阻,并最终在增强层以及界面处形成宏观气孔。为了控制缩孔和气孔,并形成骤变梯度功能复合材料,合理的离心成形工艺参数是:浇注温度—660～670℃;模具预热温度—200～300℃;喷水时间—60s;浇注方式—外端口、低速(100～150r/min)侧浇。得到的铸件,气孔基本消除;缩孔明显减小,并形成在铸件外端口。这样,可以通过车削加工将缩孔区域去除而不影响剩余铸件的使用。根据某型号水冷薄壁气缸套零件的尺寸,试制了气缸套毛坯,并对其进行T6热处理和车削加工。车削加工证明,铸件的内层具有高硬度、高耐磨性的特点,而外层具有良好的车削加工性能。不同热处理态下的组织演变特征显示,铸态下,增强区域颗粒细小,其中初晶Si的粒径在60μm以下,初晶Mg2Si颗粒粒径在25μm以下,均呈现不规则多边形状。非增强区域主要是α相、共晶Si与共晶Mg2Si,其中共晶Si呈现条状,共晶Mg2Si呈现网状和团簇状;经过T6热处理,初晶Si与Mg2Si颗粒发生圆润化,铸态下的多边形棱角消失,颗粒与基体的结合紧密。对于共晶组织,固溶以后,铸态下的条状共晶Si发生部分熔断,时效以后,形成短棒状;网状和团簇状的共晶Mg2Si固溶后,组织发生了明显的溶解,形成了大量的剩余点球状增强相,这些点球状的未溶解Mg2Si分布均匀,呈弥散状;时效以后,固溶处理中未溶解的点球状Mg2Si发生了明显的长大与粘连现象。毛坯的力学性能显示,固溶处理后,材料的硬度、拉伸强度与延伸率均处于最优状态,分别达到HRB70左右(非增强层)和HRB90左右(增强层)、210MPa和1.15%。而耐磨性却是时效后最好,该状态下,增强区域的磨损量仅有0.009g。造成这种结果的原因,主要是固溶过程中共晶Mg2Si未完全溶解,形成剩余增强相,而时效后,剩余增强相发生长大与粘连,形成了两种增强相、多种粒径增强颗粒共存的、类似于混凝土结构的“沙—石”增强机制,这种增强机制,有利于提高材料的耐磨性能。线膨胀系数测定结果显示,时效后铸件增强层的线膨胀系数为:18.9×10-6/℃(20～100℃)、19.61×10-6/℃(20～200℃)和20.48×10-6/℃(20～300℃),均优于相同温度区间的ZL109合金。对拉伸断口的分析发现,裂纹首先产生在颗粒增强区域,并以脆性的解理断裂为主。在拉应力与剪应力作用下,裂纹从增强区域向非增强区域延伸,导致非增强层粗大的α相发生滑移断裂。对磨损面的扫描电镜形貌分析发现,增强层以初晶Si与Mg2Si颗粒的疲劳磨损与磨粒磨损为主。对比这种骤变梯度功能复合材料与某型号铸铁气缸套零件发现,该材料的部分性能已经接近某型号铸铁气缸套零件的要求。因此具有替代传统铁质气缸套材料的潜质。更多还原

【Abstract】 A kind of original discontinuous function gradient composites (FGM) used in substituting the conventional thin wall cast iron cylinder liner was proposed and trial-produced by the centrifugal casting process. There are two layers in the radial wall of the tubes. The reinforcement layer in the inner layer with the large number of in-situ reinforcement particles, and that has the high hardness, the fine wear-resisting performance, the good heat conduction performance as well as the thermostability. The un-reinforcement layer in the outer layer without the particles or a little of particles, that has the high tensile-strengthen as well as the good machine-finishing performance. The structure and properties present the discontinuous gradient distribution and change from the inner to outer layer. In order to get the target, electing in-situ particles, designing material composition, establishing centrifugal casting parameter, fabricating rough shell, testing properties were systematically investigating, respectively.The structure and properties of only the primary Si and the primary Si, Mg2Si jointly reinforced Al based gradient function composites show that, the only primary Si particles reinforced continuous gradient function composites can be fabricated by hot mould centrifugal casting, but the primary Si and Mg2Si particles jointly reinforced discontinuous gradient function composites can be obtained by the same process. The stress and movement velocity of the particles in the centrifugal field shows that the primary Mg2Si particles have the higher centripetal moving velocity than the primary Si particles. During centripetal moving, the Mg2Si particles would impact and then drive the primary Si moving together towards to the inner layer, and form the discontinuous gradient function composites. Wearing properityies of two kinds of composites show that the primary Si is the main facter to increase the wear-resistance of composites, and the primary Mg2Si is the secondary reinforcement particles to exploit enough the wear-resistance of the primary Si particles.Studying on the material composition shows that the pattern of the Mg2Si is the alternative key factor forming the discontinuous gradient function composites by centrifugal casting. Only the massive primary Mg2Si has the higher centripetal velocity and may drive the primary Si to move together towards to the inner layer. The nodular and network eutectic Mg2Si does not shift or has very fine centripetal velocity. So only the Al-Si-Mg alloys that can form a lagge number of massive primary Mg2Si may fabricate the primary Si and Mg2Si jointly reinforced Al based discontinuous gradient function composites by centrifugal casting. And the reasonable alloying composition that satisfying the mention above is Mg≥4wt% and Si≥19wt%.The primary Si and Mg2Si segregating aggregation in the inner layer increases the processing difficulty and results the shrinkage cavity and blow hole forming in the face of the inner and outer layer in the process of centrifugal casting. Forming mechanics of shrinkage cavity shows that a freezing character of solid—liquid—semi-solid—solid was formed from the ectotheca to the inwall of tubes in centrifugal casting. So the liquid between the two layers freezes in the time of freezing terminal, and it results to the shrinkage cavity forming in the interface of the two layers. The blow hole forming process shows that, the supersaturation gas in the Al fused mass was separated out under the 660℃that a number of primary particles have already aggregated in the inner layer.So the gas with the smaller density is trapped in the interface and inner layer during outflowing from the inwall.The appropriate processing parameters are that: point of pouring is 660~670℃, the preheating temperature of the mold is 200~300℃, the spraying time is 60s, pouring procedure is controlled in the low velocity (100~150r/min), side pouring in the outer port. When these processing parameters are adopted the blow hole is reduced or vanish, and the shrinkage cavity is reduce and controlled in the outer port of the tubes that does not effect the residual tubes after the shrinkage cavity is removed by turning.The cylinder liner semifinished materials were produced according to some model water cooling thin wall cylinder liner components size, and then some of them was carried on the T6 heat treatment and turning. The turning processing proof that, the inner layer of the tubes is hard and owns the fine wear-resisting properties, and the outer layer has the good turning workability.The organization evolving characteristics of the different heat treatment condition demonstrate that, as casting, the primary Si and Mg2Si particles is tiny, and the size of the former is under 60μm,the latter is under 25μm .Both of them present the irregular multilateral shape, and has the union gap with the substrate between. The eutectic Si is tiny bangding, and the eutectic Mg2Si is clustering and network. After heat-treating, the primary Si and Mg2Si were dissolved in the edge angle and formed the rounded particles, and the gap between the particles and matrix vanishes. To the eutectic Si and Mg2Si during solution heat-treating, the former was dissolved and formed the short stick, and the latter was dissolved mainly and formed the tiny tubercles dispersing distribution among the Al matrix. And after aging heat-treating, the un-dissolved eutectic Si and Mg2Si grow up again that the size is over 5μm.The mechanical properties show that, the hardness, specific elongation and tensile-strength of the solution heat-treated condition are the best, and reach individual HRB70 (un-reinforcement layer) and HRB90 (reinforcement layer), 1.15% and 210MPa. But to wear-resisting property, the reinforcement layer of the artificial aging blank is the best and the wearing capacity is only 0.009g. The growed-up eutectic phase after artificial aging together with the two primary particles form the four grades reinforcement particles, and reinforcement model is similar to“sand-stone”of the concrete. The model is good at increasing the wear-resisting properties, but decreases the tensile-strength. The linear expansion factor of the artificial aging blank is 18.9×10-6/℃(20～100℃),19.61×10-6/℃(20～200℃) and 20.48×10-6/℃(20～300℃), and those are better than ZL109 alloys.The results of contrasting with the some cast iron cylinder liner show that the mechanical and thermophysics properties of the composites have already approached or reached application-required properties. So the composites own the possibility as the substituting materials of the conventional cast iron cylinder liner.更多还原