【作者】 王晓英；

【导师】 姚国凤；

【作者基本信息】 吉林大学 ， 固体力学， 2004， 硕士

【摘要】 随着对材料表面耐磨损、耐腐蚀、抗氧化、抗疲劳等性能要求日益提高，采用包括激光在内的高能束流进行表面改性强化处理的研究已成为表面工程领域的研究热点，激光熔凝加工工艺过程涉及到力学、热力学、凝固理论、传热传质学和材料学等学科的相互交叉的问题。为了达到优化参数、主动设计的目标，必须深入研究激光熔凝的机理，建立必要的数学模型，编制切合实际的软件，用数值模拟这个工具建立工艺参数和加工后性能特性之间的联系，认识激光熔凝加工过程的物理特性。激光熔凝处理的数学模型是以激光与材料相互作用的过程及其此后发生的物理规律为基础建立的。其目的在于用数学方法描述激光辐照后材料所发生的各种演化过程，包括热流过程、熔凝时的材料组织结构演化过程。可见，要以激光加工机理认识为基础，建立激光加工参数、材料参数和最终改性、改形的结果之间的联系是相当复杂的工作。但是，随着激光材料加工应用的扩展，人们对激光与材料相互作用的机理研究更加迫切，对其认识也日益深入。在以激光为基本手段的先进表面改性、改形加工技术中，对材料与激光相互作用的机理研究及实现主动可控的设计加工能力正在成为基础研究和工程技术人员努力追求的共同目标。激光熔凝硬化是在激光照射下使材料表面局部区域快速加热至熔化，随后借助于冷态的金属基体的热传导作用，使熔化区域快速凝固，形成组织结构极其细小的非平衡铸态组织，这种组织硬度高，耐磨，抗腐蚀性好。已有研究表明，激光对金属的加热，可以看作是一个一定形状和能量分布且极薄的表面热源，但在实际的数值模拟中，为了精度更高一点，也采用体热源模型，这完全由激光的波粒二相性及本身的特征决定的，自由电子模型，在表面层光能变成热能，其向金属深处的传播遵循一般的热传导规律。但实际上激光能量只有一部分能用于加热机体，其在激光总能量中所占比例，既材料对激光的吸收率与材料性质有关并且会随材料温度及表面粗糙度的变化而变化。 <WP=64>激光的作用强度达到或超过一定阀值后，金属表面将熔化，处于液体状态。随着激光作用时间的延长，由于热扩散作用，液固界面将向固体的深度和宽度方向扩展，使液体区域不断扩大。激光作用结束后，由于液态区的过热，液固界面继续扩展，随后向相反方向发展，直至液态区消失，材料完全凝固。显然，金属材料的激光熔凝过程本质上是固液及固固相变过程，固液界面两边的材料具有不同的热物理特性（如热导率、热扩散率、比热和密度等）。熔凝过程中，固液界面是按某一时间函数移动的，这个移动界面一般来说是未知的，热量在分界面上释放或吸收与温度场的变化的关系非常密切，因此必须考虑固液相变潜热问题。此外，应注意到金属加热到高温时，其热物性参数变化很大，主要与温度有关。目前国际上通用的有限元应用软件不适合本项目的应用研究，也没有其它应用程序可使用，从文献中看相关的研究者都使用自己编制的程序，并没有公开发表，根据以上情况，采用弹塑性有限元和相应的数值算法形成一套有效的分析软件程序是必要的。随着研究的步步深入，可以随时扩充完善该软件。温度场的分布，首先选取一个空间单元，对热传导方程进行有限元求解，得到结点温度关于时间的线性常微分方程组，根据精度要求采用常微分方程的精细积分算法，得到瞬时温度分布。利用三维有限元模型计算激光熔凝加工的温度场及残余应力非常复杂，因为整个过程涉及热弹性、热弹塑性、甚至热粘弹塑性及相变潜热，要想很好的模拟这一过程，还有很多困难工作要做，现在从国内外研究现状来看，数值模拟已成为相关领域的研究热点之一。本软件在保证精度要求的前提下，在原有研究的基础上，节省了计算时间，提高计算效率，计算效果很好的lapack方法用于本次研究的计算需要60小时左右，而本程序不足4个小时就可以完成。同时也可以节省大量的内存。本软件适用于大规模工件的瞬态温度场的计算，同时为进一步计算温度场的残余应力提供很好的计算方法。以激光熔凝表面强韧化处理为背景，应用空间弹塑性有限元和高精度数值算法自主编制应用软件，模拟工件的瞬时温度场，模拟温度场时考虑液固、<WP=65>固固相变潜热的影响，所有热物性参数和力学参数都随温度而变化，本软件能弥补大型商业软件在这些方面的不足，这为激光材料表面改性提供力学指导奠定了良好的基础。为开发高能束强化加工工艺技术提供理论依据，从而为进一步开发出更好的应用领域提供良好条件。最后用算例验证了模型的正确性，并给出了相应的数值模拟结果。更多还原

【Abstract】 With the increasing request of ability of resisting abrasivity, causticity, oxidation and fatigue in material surface, material surface hardening and remodel have been one of important subject in related domains, especially, laser melting employed. Laser melting is a crossed problem involving mechanics、thermodynamics、 solidification、 heat transfer as well as material. To optimize material parameter and design independently, it is necessary to research the theory of laser melting process further and set up a reasonable numerical model. Thus Application software is programmed using space elastic-plastic finite element method. With numerical simulation, the relationship of technical parameters and processed performance feature is set up, to show thermal physical specialty in laser melting process. The numerical model is set up on the basis of the action and mechanization generated by laser and material in laser melting process. Thus diversified courses are described due to laser radiation, including convection and material structure change. It is complicated to set up a relationship of laser parameters、material parameters and the processed result. We increase the cognition of the mechanization of the laser melting process with the expansion of the application of material laser process. The research of advanced surface character-change and shape-change by means of laser is becoming a corporate aim for those engaged in background research and engineering. The laser melting hardening is a process, during which parts of material surface are heated and melted, then rapidly solidified due to heat transfer of cold mental matrix. Cast nonequilibrium structure is formed in processed surface, which is possessed with high rigidity, and resisting <WP=67>abrasivity and causticity.Researchs indicate laser heat source can be supposed to be a surface heat source with a certain shape and thin energy distribution. To obtain higher precision, model is usually supposed to be one with a body heat source in practical numerical simulation, which is determined by laser characteristic. Light energy is changed into heat energy in free electron model surface. However the energy transmission in the depth of body is followed by heat transfer law. But in practical laser melting process, laser energy is partially used to heat matrix. The percentage of the energy is called material absorptivity; Material absorptivity to laser is related to material characteristic and changed according as material temperature and surface roughness. When laser intensity achieve a certain value or pass it, mental surface will melt and keep liquid. As the processed time going on, due to heat diffusion, the interface between solid and liquid will expand to the depth and width of solid matrix. After process, overheat of liquid leads to further expansion of the interface between solid and liquid, then to reverse direction, till liquid disappears and material solidifies completely. In nature, laser melting process of mental material is one with solid phase transformation and phase transformation between liquid and solid. The materials at each side of interface between solid and liquid are possessed with different thermal physical parameters (heat transfer rate, heat diffusion rate, specific heat and density etc). In laser melting process, the interface between solid and liquid is moving as time. The release and absorption of heat on interface is closely related with temperature variation, so it is necessary to consider latent heat due to phase transformation between solid and liquid. Moreover thermal physical parameter alters a lot in high temperature, chiefly related with temperature.At president, existing commercial application software is not fit for this <WP=68>project, and without other application program either; the researchers of related literatures did not issue their program. Based on these things, it is necessary to programme a set of effective analytical software using space elastic-plastic finite element method and corresponding numerical al更多还原