脉冲激光离散重熔热物理及组织性能研究

IMECH-IR > 力学所知识产出(1956-2008)

	脉冲激光离散重熔热物理及组织性能研究
	罗耕星
导师	吴承康 ; 陈光南
	1999
学位授予单位	中国科学院研究生院
学位授予地点	北京
学位类别	博士
学位专业	工程力学流体力学
关键词	脉冲激光重熔加工热物理微观组织 Pulse Laser Remelting Processing Thermophysics Microstructure Wear
英文摘要	利用OM及FEM研究了铁基合金Nd:YAG脉冲激光熔凝区的几何形态及其变化规律、以及熔凝的热物理过程；利用OM、SEM、TEM、X-射线衍射仪及磨损实验机，研究了两种铁基热模具材料脉冲激光熔凝组织及其时效组织结构，以及熔凝区规则离散分布规律对材料抗磨损性能的影响。在10~5～10~7W/cm~2的脉冲激光平均功率密度范围内，可得到热传导型和深熔型两类强化区，当临界平均功率密度大于5 * 10~5W/cm~2，同时临界激光作用时间大于2ms时，热传导型强休区向深熔型强休区转变。熔化过程中，在熔池中形成上部以对流传热为主，底部以导热为主的传热模式，流场、温度场和压力场均随脉冲激光作用时间变化，最大流速、压力和温度梯度分别可达100m/s、数个大气压和10~(8-9) ℃C/m量级。凝固过程中，固液界面上的最大温度梯度、凝固速率和冷却速度时间和空间位置变化，分别可达10~(8-9) ℃/m量级、10~(-1)m/s量级和10~(7-8) ℃/s量级，其中冷却速度得到实验验证。亚共晶合金铸铁脉冲激光熔凝组织为δ-铁素体与M_3C的层片状共晶组织，还含有部分γ-奥氏体和少量的高碳孪晶马氏体组织，δ-铁素体和γ-奥氏体中均存在高密度位错亚结构。5CrMnMo钢脉冲激光熔凝组织由板条马氏体及少量的孪晶马氏体构成，马氏体中也存在高密度位错亚结构。上述两种组织经高温时效后，仍保持较细的晶粒，并有大量细小均匀弥散分布的碳化物析出，其中铸铁熔凝组织析出M_(23)C_6碳化物，M_(23)C_6可在M_3C/γ-奥氏体相界面或M_3C内部原位形核，亦可在δ-铁素体中弥散析出。两种材料的熔凝组织及其时效组织的显微硬度均明显高于相应的原始组织，也高于激光连续扫描熔凝的结果。脉冲激光规则离散熔凝加工在材料表面形成软硬相间的“原位”功能层，能显著降低裂纹形成的敏感性，提高材料表层的抗磨粒磨损性能，时效后仍具有较好的抗磨损性能。以熔凝强化区直径作为中心间距进行规则离散熔凝处理可使材料表面获得最佳抗磨损性能。; The thermophysics, microstructure, and mechanical property of pulsed Nd:YAG laser surface remelting were studied using finite element method, optical microscope, scanning and transmission electron microscope, X-ray diffractometer and wear tester. Two pool patterns having heat-conduction and deep-melting characteristics were obtained with the average power intensity of the pulsed laser being in a range of 10~5 to 10~7W/cm~2. The transition from heat-conduction to deep-melting occurred when the power intensity and pulse time surpassed critical values of 5 * 10~5W/cm~2 and 2 ms respectively. The main heat transformation modes were convection and conduction relevant to the upper and lower zone of the molten pool respectively. The interaction time of the pulsed laser on materials had an important influence on the flow field, temperature field and pressure field. Maximum values of the flow speed, pressure and temperature gradient were 100m/s, several atmospheric pressure and 10~(8-9) ℃C/m scales respectively. During solidification, the maximum temperature gradient, solidification rate and cooling rate of solid-liquid could reach 10~(8-9) ℃/m, 10~(-1)m/s and 10~(7-8) ℃/s scales respectively. The cooling rate of finite element analysis was well consistent with experimental results. Both the microstructural characterization and the tempered evolution of two kinds of metallic materials were analyzed. The remelted microstructure of the hypoeutectic cast iron was composed of lamellar eutectic i.e., δ-ferrite and M_3C carbide, γ-austenite and a few twin martensite of a high density of dislocations. The microstructure of a 5CrMnMo steel was martensite of a high density of dislocations. It was observed that a large quantity of carbides precipitated uniformly in tempered microstructures of both materials. M_(23)C_6 carbides nucleated uniformly either in an in situ transformation mode, such as, at the M_(23)C_6 carbides nucleated uniformly either in an in situ transformation mode, such as, at the M_3C carbide/γ-austenite interface and from the inner part of M_3C carbides, or from the δ-ferrite in the hypoeutectic cast iron. In addition, the as-solidified and as-aged microstructures of both materials possessed superior microhardness and temper resistance, as compared with those of corresponding counterparts. The pulsed laser surface processing synthesized an in situ function composite surface, with regular distribution of hard and soft regions. The formation of cracks was reduced remarkably. Both the as-solidified and as-tempered materials possessed abrasive resistance higher than corresponding original ones. The processed surface obtained an optimum abrasive resistance under the condition of the central distance of two adjacent molten pools being the diameter of the molten pool.
索取号	29867
语种	中文
文献类型	学位论文
条目标识符	http://dspace.imech.ac.cn/handle/311007/23542
专题	力学所知识产出(1956-2008)
推荐引用方式 GB/T 7714	罗耕星. 脉冲激光离散重熔热物理及组织性能研究[D]. 北京. 中国科学院研究生院,1999.

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