【摘要】：機床導軌表面疲勞缺陷的出現(xiàn)嚴重影響加工產品精度而造成器件的報廢,更為嚴重的是由內部不可見的疲勞裂紋擴展而引起的巨大安全隱患。為了解決這一難題,工業(yè)上一般采用在灰鑄鐵床身鑲嵌硬質的鋼片作為工作接觸面,同時具備灰鑄鐵床身具有良好的減震性這一優(yōu)異特點,從而最大程度上地降低疲勞缺陷出現(xiàn)的可能性。盡管鑲鋼導軌能顯著地改善滾動導軌的服役壽命,但對于具有較長行徑的導軌,需將多片鋼片通過特殊方式拼接,以滿足行徑要求,導致在連接處疲勞失效更容易產生。同時,鋼片底面與灰鑄鐵表面不能完全的契合,制約機床對震動吸收程度的進一步提高,因此,相應的缺點嚴重制約滾動導軌抗疲勞磨損性能更有效地改善。鑒于此,探索一種特殊的處理方式直接對灰鑄鐵材料進行強化處理,使其作為一個整體能替代甚至超越傳統(tǒng)的鑲鋼導軌。對于灰鑄鐵而言,材料中彌散大量片狀石墨,其尖端應力集中現(xiàn)象更易萌生疲勞裂紋,使灰鑄鐵材料疲勞失效機理更為復雜。因此,改善在滾動疲勞接觸條件下服役產品抗疲勞磨損性能以及了解疲勞缺陷形成機理,不僅可提高經濟效益而且對實際生產和加工安全至關重要。基于耦合仿生學原理,利用激光局部處理灰鑄鐵表面,形成與耐磨生物體相似的體表結構,提高其抗疲勞磨損性能,揭示不同的表面仿生形態(tài)及其特征量、碳化物含量及合金元素改善灰鑄鐵材料抗疲勞磨損機理。結果表明:1.仿生單元體形態(tài)及其特征量嚴重影響仿生灰鑄鐵材料抗疲勞磨損性能改善程度;(a)表明不同的仿生形態(tài)(點狀,條狀及網(wǎng)狀)對材料的抗疲勞磨損性能的影響程度。當單元體呈網(wǎng)狀形態(tài)時,仿生單元體對輥子起到連續(xù)支撐,并在滾動方向形成明顯軟硬相間的仿生結構,仿生試樣有最優(yōu)的抗疲勞磨損性能,與基體相比其抗疲勞磨損性能改善程度達53%。(b)揭示了仿生試樣抗疲勞磨損性能與單元體取向和滾動方向的關系。當單元體與滾動方向呈60°時,能將所產生的接觸切應力分散到無限個切應力平面,最為有效地降低應力集中現(xiàn)象,從而具有最佳的抗疲勞磨損性能。(c)建立回歸方程,揭示單元體間距對材料抗疲勞磨損性能的影響規(guī)律:當單元體間距大于2mm時,強化區(qū)域面積起主導作用,材料的抗疲勞磨損性能隨間距的減小而增大;當單元體間距小于2mm時,不一致變形對疲勞壽命的影響更為顯著,材料的抗疲勞磨損性能隨間距的減小而減小。2.對碳強化仿生單元體對灰鑄鐵抗疲勞磨損性能的影響進行實驗性研究。相對于熔凝單元體碳化物含量40-45%,滲碳單元體碳化物含量明顯提高,高達60-70%。隨著激光加工能量密度的降低,所制備的仿生單元體截面積尺寸減少,滲碳單元體中碳化物含量也相應地減少,仿生試樣抗疲勞磨損性能也隨之降低。不同預涂層厚度對滲碳單元體截面積尺寸影響較小,但過渡區(qū)組織隨著預涂層厚度增加而變化,當厚度為0.3mm時,其組織完全為索氏體;晶粒尺寸隨預涂層厚度減小而減少,并且整體材料的軸向和切向抗變形能力與晶粒尺寸密切相關,因此,仿生試樣抗疲勞磨損性能的改善程度與預涂碳層厚度成反比。3.闡明了合金元素強化對仿生處理灰鑄鐵抗疲勞磨損性能的強化機制。通過激光合金化處理,使混合合金元素Cr+W滲入到熔池內部,形成較為復雜的共熔碳化物組織,例如(Fe,Cr,W)xCy,CrxCy,WxCy等。探明了Cr和W元素在單元體中的分布規(guī)律:Cr元素同時分布在晶界和晶粒上,而W元素則主要分布在晶界上。相對于單一合金元素的添加,混合合金元素Cr和W的添加使單元體內部晶粒和晶界很大程度上的增強。隨著單元體的強化,單元體對疲勞微裂紋的擴展的阻礙作用更為明顯,使整體材料抗疲勞磨損性能有效提高。4.揭示了仿生處理材料疲勞磨損過程的一般規(guī)律。接觸疲勞缺陷產生的原因有:疲勞裂紋的擴展形成金屬顆粒的移除,高應力區(qū)顆粒的壓碎,粘著磨損形成的點蝕以及依附裂紋或石墨片而形成的金屬移除。對于未出現(xiàn)疲勞缺陷的區(qū)域,在循環(huán)應力的作用下,表面發(fā)生明顯的塑性變形,出現(xiàn)加工硬化現(xiàn)象,而其表面粗糙度明顯降低,直到一恒定范圍(285nm-325nm)。因此,在長期的疲勞磨損過程中,接觸表面的磨損失重率趨于平穩(wěn)并明顯低于前期磨損失重率。5.對于激光處理局域,其內部組織晶粒致密度明顯提高,晶粒尺寸明顯降低,碳化物含量明顯增多,并且出現(xiàn)大量的增強相,整體材料抗變形能力得到很明顯的改善,疲勞缺陷出現(xiàn)的循環(huán)周期也隨之而延長。此外,單元體的存在不僅降低基體區(qū)域所產生的接觸應力,延緩疲勞微裂紋的萌生和擴展,而且增加高應力區(qū)強度,降低疲勞缺陷出現(xiàn)的可能性。
[Abstract]:The appearance of fatigue defect on the surface of machine tool guideway seriously affects the precision of machining products and results in the scrap of the device. What is more serious is the huge hidden danger caused by the fatigue crack propagation inside the machine tool. In order to solve this problem, the hard steel sheet embedded in the grey cast iron bed is generally used as the working contact surface in industry, and the tool is also used. Grey cast iron lathe bed has excellent shock absorption, thus reducing the possibility of fatigue defects to the greatest extent. Although steel-encased guideways can significantly improve the service life of rolling guideways, for guideways with longer trajectories, it is necessary to splice multiple pieces of steel in a special way to meet the trajectory requirements, resulting in fatigue defects. At the same time, the bottom of the steel sheet and the surface of gray cast iron can not fully fit, which restricts the further improvement of the vibration absorption of the machine tool. Therefore, the corresponding shortcomings seriously restrict the fatigue wear resistance of the rolling guide to improve more effectively. For gray cast iron, a large amount of flake graphite is dispersed in the material, and the stress concentration at the tip of the material is easier to initiate fatigue cracks, which makes the fatigue failure mechanism of gray cast iron more complex. Therefore, the service production under rolling fatigue contact condition is improved. The fatigue wear resistance of gray cast iron and the formation mechanism of fatigue defect can not only improve the economic benefit but also is very important to the safety of production and processing. The results show that: (1) the morphology of bionic unit and its characteristic amount seriously affect the improvement of fatigue wear resistance of bionic gray iron materials; (a) the fatigue wear resistance of gray iron materials is improved by different bionic forms (dot, strip and mesh). When the element is in a network shape, the bionic element acts as a continuous support to the roll and forms a bionic structure with obvious soft and hard phases in the rolling direction. The bionic specimen has the best anti-fatigue wear performance, and its anti-fatigue wear performance improves by 53% compared with the matrix. The relationship between fatigue wear resistance and element orientation and rolling direction is studied. When the element and rolling direction are 60 degrees, the contact shear stress can be dispersed to infinite shear stress planes, and the stress concentration phenomenon can be reduced most effectively. Thus the fatigue wear resistance of the element can be optimized. (c) The regression equation is established to reveal the relationship between element spacing and material. The influence law of fatigue wear resistance is as follows: when the space between elements is larger than 2 mm, the area of strengthening zone plays a leading role, and the fatigue wear resistance of materials increases with the decrease of space; when the space between elements is less than 2 mm, the effect of inconsistent deformation on fatigue life is more significant, and the fatigue wear resistance of materials decreases with the decrease of space. The effect of carbon-strengthened bionic unit on the fatigue wear resistance of gray cast iron was studied experimentally. Compared with the carbide content of melting unit body 40-45%, the carbide content of carburizing unit body increased significantly, up to 60-70%. With the decrease of laser processing energy density, the cross-sectional area of bionic unit body decreased and the carburizing unit body decreased. The content of carbide in the cementite decreases correspondingly, and the fatigue wear resistance of the bionic specimen decreases accordingly. Different pre-coated thickness has little effect on the sectional area size of the cementite element, but the microstructure in the transition zone changes with the increase of the pre-coated thickness. When the thickness is 0.3 mm, the microstructure is completely sorbite. Therefore, the improvement of fatigue wear resistance of bionic specimens is inversely proportional to the thickness of pre-coated carbon layer. 3. The strengthening mechanism of alloying elements on the fatigue wear resistance of bionic gray cast iron is clarified. The laser alloying treatment can mix the bionic gray cast iron. The alloying elements Cr+W penetrate into the molten pool and form more complex eutectic carbide structures, such as (Fe, Cr, W) xCy, Cr xCy, W xCy, etc. The distribution of Cr and W elements in the element body has been studied. The distribution of Cr and W elements in the element body has been found out: Cr elements distribute simultaneously on the grain boundaries and grains, while W elements distribute mainly on the grain boundaries. The addition of Cr and W greatly enhances the internal grain and grain boundary of the element. With the strengthening of the element, the inhibiting effect of the element on the fatigue crack propagation becomes more obvious, and the fatigue wear resistance of the whole material is improved. 4. The general rule of the fatigue wear process of bionic materials is revealed. The reasons are as follows: the propagation of fatigue crack leads to the removal of metal particles, the crushing of particles in high stress zone, the pitting corrosion caused by adhesive wear and the removal of metal formed by attaching cracks or graphite flakes. As a result, the wear weight loss rate of the contact surface tends to be stable in the long-term fatigue wear process and is significantly lower than that of the previous wear. 5. For the laser-treated locality, the grain density of the internal microstructure increases, the grain size decreases and the carbide decreases obviously. In addition, the existence of the element not only reduces the contact stress in the matrix region, but also delays the initiation and propagation of the fatigue micro-cracks, and increases the strength of the high stress region. Reduce the possibility of fatigue defects.
【學位授予單位】：吉林大學
【學位級別】：博士
【學位授予年份】：2016
【分類號】：TG502.4;TG143.2

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