选区激光熔化TC4合金的凝固组织演化规律研究

发布时间：2018-10-26 11:19

【摘要】：选区激光熔化技术成形过程中,金属粉末在高能量密度的激光热源作用下,迅速熔化形成微熔池,激光热源移走后熔池内熔体快速冷却凝固,温度梯度和冷却速度非常高。温度分布对成形件组织影响很大,材料的组织决定力学性能,但SLM实际成形过程温度的测量非常困难。本文从微熔池的熔化和凝固入手,研究SLM成形TC4合金的温度场分布和组织演变及工艺参数的影响规律。利用ANSYS有限元软件对SLM成形TC4合金过程进行温度场模拟,考虑TC4合金的热物性参数随温度非线性变化,以及粉末颗粒尺寸和形貌对粉末密度和导热系数的影响。采用APDL语言实现生死单元技术和热源的动态加载,得到SLM成形TC4合金过程的温度场分布。随激光热源加载,热量不断积累,温度升高,光斑作用点中心的瞬时温度最高,光斑作用中心前端的等温线比后端等温线更密集,主要是由两端的导热不同引起。温度场模拟得到熔池的温度梯度最高达到106 K/m,冷却速度最高可达105 K/s。在激光作用区域内不同节点和工艺参数下经历热循环规律基本相似,但会影响热循环曲线的温度峰值,激光功率越大或扫描速度越小,温度峰值越高。温度场模拟得到TC4合金粉末受激光热源作用形成的微熔池宽度和深度仅几十微米,发现了温度分布影响微熔池的形态。工艺参数直接影响熔池的尺寸,随激光功率的增大或扫描速度降低,熔池的宽度和深度均增加,相比于激光功率和扫描速度,扫描间距对熔池尺寸的影响不显著。SLM成形所用TC4合金粉末颗粒近似为球形,微观组织为细小的针状马氏体,马氏体长度小于10μm。SLM成形单层单道TC4合金,得到单道熔池内的组织由大量针状马氏体组成,针状马氏体的平均长度约为35μm,比粉末的组织更粗大。熔池的形态对工艺参数比较敏感,主要考虑激光功率和扫描速度,随激光功率的增大或扫描速度减小,热输入增加,熔池的熔深和熔宽增大,单道熔池与基体上表面的夹角也逐渐增大。SLM成形TC4合金块体试样,分析宏观组织和微观组织,发现块体试样上表面为相互垂直的棋盘状组织,纵截面组织为沿堆积成形方向外延生长的β柱状晶,柱状晶内是大量针状马氏体,室温下主要相组成是α′相。同时,改变工艺参数,组织发生变化,热输入增大,柱状晶组织变粗大。
[Abstract]:In the forming process of selective laser melting technology, the metal powder rapidly melts into a micro-melting pool under the action of high energy density laser heat source. After the laser heat source is removed, the melt in the molten pool is rapidly cooled and solidified, and the temperature gradient and cooling rate are very high. The temperature distribution has a great influence on the microstructure of the forming parts, and the microstructure of the material determines the mechanical properties, but it is very difficult to measure the temperature in the actual forming process of SLM. In this paper, the temperature field distribution, microstructure evolution and the influence of process parameters of SLM forming TC4 alloy are studied by means of melting and solidification of micro-melting pool. The temperature field of SLM forming TC4 alloy was simulated by ANSYS finite element software. The nonlinear variation of thermal properties of TC4 alloy with temperature and the influence of particle size and morphology on the density and thermal conductivity of TC4 alloy were considered. The temperature field distribution of SLM forming TC4 alloy was obtained by using APDL language to realize the technology of birth and death element and the dynamic loading of heat source. With the loading of laser heat source, the heat accumulates and the temperature increases. The instantaneous temperature of the center of the spot action point is the highest. The isotherm of the front end of the spot action center is more dense than the back end isotherm line, which is mainly caused by the difference of heat conduction between the two ends. Temperature field simulation shows that the maximum temperature gradient of molten pool is 106K / m and the highest cooling rate is 105K / s. The temperature peak value of the thermal cycle curve is affected by the different nodes and process parameters, but the higher the laser power is or the smaller the scanning speed is, the higher the temperature peak value is. The temperature field simulation shows that the width and depth of the micropool formed by the laser heat source of the TC4 alloy powder are only several tens of microns, and the shape of the micro-melting pool is affected by the temperature distribution. Process parameters directly affect the size of the weld pool. With the increase of laser power or the decrease of scanning speed, the width and depth of the pool increase, compared with the laser power and scanning speed. The effect of scanning distance on the size of molten pool is not obvious. The powder particles of TC4 alloy used in SLM forming are approximately spherical, the microstructure is fine acicular martensite, the length of martensite is less than 10 渭 m.SLM to form monolayer single-channel TC4 alloy. It is found that the microstructure in the single pool is composed of a large number of acicular martensite, the average length of the acicular martensite is about 35 渭 m, which is thicker than that of the powder. The shape of the molten pool is sensitive to the process parameters. The laser power and scanning speed are mainly considered. With the increase of laser power or scanning speed, the heat input increases, and the penetration depth and width of the weld pool increase. The angle between the single pool and the surface of the substrate gradually increased. SLM formed the TC4 alloy bulk sample, analyzed the macro and microstructure, found that the upper surface of the block sample is a mutual vertical chessboard structure. The microstructure of the longitudinal section is 尾 columnar crystal grown along the stacking forming direction, and there is a large amount of acicular martensite in the columnar crystal. At room temperature, the main phase composition is 伪 'phase. At the same time, the microstructure changes, the heat input increases, and the columnar crystal structure is thickened by changing the process parameters.
【学位授予单位】：哈尔滨工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG146.23;TG665

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