微合金化及不同冷速作用下ZA48合金的微观组织、性能研究

【作者】 闫淑卿；

【导师】 谢敬佩；

【作者基本信息】 郑州大学 ， 凝聚态物理， 2009， 博士

【摘要】 锌铝合金具有优良的减磨耐磨性能、机械性能和显著的经济性,可取代铜合金作耐磨材料,代替青铜作低、中速中温重载轴承。所以,尽管锌铝合金问世时间不长,其相关科研技术的发展非常迅猛,应用范围也越来越广泛。随着经济发展,大件和异型件的应用增多,对合金产品性能提出了更高的要求。世界发达国家目前已经开发了一系列的高铝锌基减磨材料,供不同工况下的轴瓦、轴套、滑块等选用。而国内由于长期受传统观念的影响,绝大多数冶金企业仍使用常规牌号的ZA27合金。大量研究表明,高铝含量的偏析锌合金比共晶或共析锌合金具有更加优良的性能。因此,本文以高铝锌合金为研究对象,在不改变现有生产工艺的前提下,通过添加微合金元素、改善浇注工艺,采用不同的凝固、修复等手段,提高合金的力学性能、高温承载能力以及改善产品成型中的铸造缺陷,对提高高铝锌合金的性能,扩大其应用范围具有重要的理论及应用价值。本文首先研究了高铝含量的锌合金组织及性能,结果表明：随着铝含量的增高合金的抗拉强度和耐磨性均增强。分析认为,锌铝合金的微观组织主要由初生α枝晶和(α+η)共析体组成。随着铝含量的增加,组织中的初生α枝晶增多,(α+η)共析体减少；树枝状的α-Al相是铝基固溶体,面心立方晶格,属于强化相,其强度和硬度都要高于η相。根据测试结果,选择性能较好的ZA48合金作为本课题的主要研究对象。首先采用电解加钛、Si对合金进行强化。通过研究电解加钛、Si对合金力学性能的影响,结果表明：电解加钛有效提高了合金的力学性能,当钛含量达到0.04%时,合金的力学性能达到最大值。其细化机理是Ti在锌铝合金中形成Al66Ti25Zn9颗粒,可作为合金的异质形核核心,从而有效细化α(Al)相,强化基体。合金的力学性能随硅含量的增加而减弱,特别是当合金中出现初生硅相时,力学性能明显下降。分析认为,合金中的共晶硅相和初生硅相的尖角或棱边处易产生应力集中,削弱硅相与基体的结合力,在外加应力的作用下易形成微裂纹,降低合金的力学性能。系统研究了常规铸造条件下冷却速度对ZA48合金组织的影响。确立了合金的二次枝晶臂间距λ与冷却速率T之间的关系λ=47(T)-0.325。研究发现,在较快的冷却速度下,合金中的α相内析出大量细小弥散的质点。这些析出物均匀弥散地分布在α相上,有利于合金强度的提高。同时能谱分析表明,冷却速度的提高,有效避免了比重偏析,各元素的偏析程度也随着冷却速度的提高而减小。分析认为,合金在凝固过程中的溶质再分配是产生偏析的根本原因,在非平衡凝固条件下,固液界面实际溶质分配系数k*随着凝固速率的增大更加趋近于1,固液界面固相成分更加趋近于液相成分C0,因而使枝晶偏析减小。采用单辊甩带法制备快速凝固ZA48合金条带,根据其传热特点,结合传导理论和凝固理论,应用数学解析法推导计算了单辊甩带法制备ZA48合金薄带的冷却速度。得到单辊甩带快速凝固制备50μm厚合金薄带的冷却速度约为105K/s。同时用与时间有关的非均质形核理论说明了ZA48合金快速凝固过程中的形核特点。随着冷却速度的提高,快速凝固ZA48合金的相选择顺序是：α-Al→η-Zn相,同时铝元素的含量对合金中α-Al相的形核孕育期有较大的影响。系统研究了冷却速度对电解加钛ZA48合金、含硅ZA48合金组织的影响。在常规铸造条件下,随着冷却速度的加快,电解加钛ZA48合金的晶粒得到细化,并且随着Ti含量的增加,合金晶粒尺寸减小；但当冷却速度达到一定值时,晶粒尺寸不再受Ti含量的影响,冷却速度成为晶粒细化的主要原因。合金中的共晶硅相随着冷却速度的加快也得到细化,但是初生硅相的形态改变不大。当冷却速度高达105K/s,即快速凝固后,合金中的共晶硅和初生硅完全固溶于α-Al固溶体中,大的过冷度抑制了硅相的形核与生长,使硅不能析出,大量的硅和其它合金元素固溶于基体中。模拟使用工况,系统分析了ZA48合金的耐磨性。根据磨损表面及磨损亚表面的形貌,分析了其磨损机理。结果表明,ZA48合金的高耐磨性是α相和η相共同作用的结果,η相首先被磨去并储存在对磨表面,起到自然润滑的作用,减轻试样和对磨轮之间的摩擦；同时摩擦副上的Fe从对磨轮转到试样表面,填充被抹去的部分及磨损表面,在磨损表面形成一层耐磨层,阻止磨损的进一步发生。研究了电解加钛对ZA48合金耐磨性的影响。无论是润滑条件还是无润滑条件下,电解加钛后合金的耐磨性均提高,原因在于晶粒尺寸的减小、晶界面积增加,使试样的剪切力随着晶粒尺寸的减小而增加；因而,细的晶粒尺寸使合金具有较高的耐磨性。系统研究了硅对ZA48合金耐磨性的影响。无论是润滑条件还是无润滑条件下,加硅合金的耐磨性均提高,且其提高的幅度远大于电解加钛后的合金。硅颗粒的硬度高达757HV,远高于ZA48合金基体的硬度,硅颗粒的加入在合金中起到硬质点的作用,有效地提高了合金的耐磨性。同时,合金的耐磨性受硅颗粒的大小、形态的影响。组织中粗大、不均匀硅粒子处易形成裂纹,使裂纹易于沿着脆性硅相扩展,对材料的耐磨性起负面影响。弥散分布的短棒状、或颗粒状共晶硅可以提高基体的硬度,增强合金抗犁沟磨损能力,同时也可以减小基体与初生硅相之间的硬度差,从而提高两相之间的协调性,增强合金的耐磨性。更多还原

【Abstract】 Zinc-aluminum (ZA) alloy with excellent wear resistance, mechanical properties and a significant economy, could replace copper alloy for wear-resistant materials, and instead of bronze for low-, medium-speed medium temperature overloading bearings. Despite the advent of ZA alloy is not long, their relevant scientific research and technological development is very fast and it also has a very wide range of applications. Along with economic development, large and special-shaped pieces of products are more and more widespread; a higher demand has been set to the performance alloy products. The developed countries have developed a series of high aluminum zinc-based anti-friction materials for different conditions to select. However, the majority metallurgical enterprises in domestic are still using conventional brands of ZA27 alloy due to the long-standing traditional value. A large number of studies have shown that high aluminum zinc-based alloy has a more excellent performance than of eutectic or eutectoid ZA alloy. Therefore, the high aluminum zinc alloy was chosen in this paper and the alloying, metamorphic, improving casting technique, different solidification, repair, and means to improve the mechanical properties of the alloy. It would have important theoretical and application value for improving the performance of ZA Alloy. It also expands the scope of application of ZA alloy.The influence of different high aluminum content on the microstructure, tensile and wear properties of zinc-based alloy was studied firstly in this paper. The test results show that the tensile strength and abrasion resistance are increased with increasing aluminum content. Zn-Al alloys basically comprise a mixture of primary phase a and eutectoid (α+η).The increase of aluminum content in the alloys conducted the increase of quantity of primary phase a and the decrease of eutectoid (α+η).Theα-Al rich solid solution has a face-centred cubic structure and acts as a strengthening and toughening phase. Its strength and hardness were higher than of theηphase.According to test results, ZA48 alloy with a better performance was chosen as the main object of study. Si and Ti elements were used to strengthen. The effect of different Ti, Si contents on the mechanical properties of ZA48 alloy was studied and the following results were gotten. Titanium improved the mechanical properties effectively, and when the titanium content reaches 0.04%, the mechanical properties achieve maximum value. The refinement mechanism of Ti is the form of Al66Ti25Zn9 particles in ZA alloy and it can be used as heterogeneous nuclei. Then the primary phase a gets refined effectively and the matrix is strengthened. Mechanical properties of ZA48 alloy decreases with the increase of silicon content, especially as the primary silicon phase appears in alloy. The sharp points or cutting edge of eutectic Si and primary Si phase are prone to produce stress concentration and weaken the combination between silica and matrix. The micro-cracks are formed under the effect of applied stress, reducing the alloy mechanical properties.Effect of cooling rate on the microstructure of conventional casting ZA48 Alloy has been systematic studied. The relationship between the dendrite arm spacing and cooling rate was established. The study found that a large number of small particle dispersion precipitated from a phase at higher cooling rate. The precipitate uniform mass distribution in the a phase, then the strength is improved effectively. The spectrum analysis showed that the proportion of segregation was avoided effectively with the increase of the cooling rate. The microsegregation of various elements was also reduced as the cooling rate increased to a certain degree. It believes that the solute redistribution is the root to generate segregation during solidification. Under the non-equilibrium solidification conditions, the actual solute partition coefficient k* of the solid-liquid interface more tends to 1 with the increase of solidification rate. The solid-phase composition of the solid-liquid interface is more close to the liquid phase composition Co, and thus reduces the dendrite segregation.The rapid solidification ZA48 ribbon was prepared by single-roller spinning. According to the heat transfer characteristics, combined with conduction theory and solidification theory, the mathematical analysis method was applied to calculate the cooling rate of ZA48 ribbon. The cooling rate of a 50μm thick ZA alloy ribbon prepared by single-roller spinning is about 105K/s. The time-related non-homogeneous nucleation theory describes the nucleation characteristics of rapidly solidified ZA48 ribbon. With the increasing cooling rate, the phase selection sequence of rapidly solidified ZA48 ribbon isα-Al→η-Zn phase, aluminum element content also has great effect on the nucleation incubation period of the a-Al phase.Effect of cooling rate on the microstructure of ZA48-Ti alloy and ZA48-Si alloy has been systematic studied. Under the conventional casting conditions, the grains of ZA48-Ti alloy get refined with increasing cooling rate and Ti content. When the cooling rate increased to a certain value, Ti elements had no effect on the grain size and the cooling rate as the main reason for grain refinement. The eutectic silicon phase gets refined and the primary silicon phase morphology has no much changed with the increasing cooling rate. After rapid solidification, Ti and 5% Si completely dissolved intoα-Al solution. The large undercooling suppressed the nucleation and growth of silicon phase, the silicon phase does not precipitate and a large number of silicon and other alloy elements solid solution in the matrix.Simulated working conditions, the wear resistance of ZA48 alloy was analyzed systematically. According to the worn surface and wear sub-surface morphology, its wear mechanism was analyzed. The results show the improving wear resistance is the combined action of a andηphase. The softηphase prior to theαphase was removed and the hard a phases were protruded from matrix and acted as a loading phase. Extensive zinc transfer occurred and helped to act as a natural lubricant in sliding wear situations wherein the smearing behavior is facilitated and a lubricating film on mating surfaces is formed. Meanwhile, the iron transfer from the steel ring to block and forced to recess continuously during sliding wear, which forms a thin film at the contact surface between the composite and the counter face. It is equivalent to a number of reinforced particulates added to ZA48 alloy, and its load bearing capability would be improved. The influence of Ti elements on the wear resistance of ZA48 alloy was also studied. The results showed that whether the lubrication conditions or non-lubricated conditions, the adding Ti by electrolysis improved the wear resistance of ZA48 alloy. The reason can be attributed to the decrease of grain size and the increasing of grain boundary area, which induced the shearing force increased with the decrease of grain sizes.Whether lubrication or no lubrication conditions, Si can improve the wear resistance of ZA48 alloys significantly and the extent of the increase is far greater than that of Ti. The hardness of silicon particles of up to 757HV, far higher than that of ZA48 alloy, silicon particles are added to play a role of hard particles, then the wear resistance is improved effectively. The size and shape of silicon particle also have great effect on the wear resistance of alloy. Thick and uniform silica particles are prone to form cracks, and cracks easily along the brittle silicon phase expansion. It has a negative impact on the wear resistance of the material. Diffuse distribution of short rod-like or granular eutectic silicon can increase the hardness of the matrix and enhance furrow abrasion resistance. It also reduced the hardness difference between substrate and primary phase silicon. Thereby improving the coordination of the two phases, enhance the wear resistance of alloys.更多还原