Study on morphological characteristics of the molten pool and defects in powder fed laser cladding

送粉式激光熔覆技术以其加工过程柔性化程度高、适用粉末材料体系广泛及制造过程绿色无污染的优势，广泛应用于航空航天、大型工程机械和船舶等领域构件的直接成形制造，同时可以实现具有特定力学、材料学和化学性能的功能涂层与基体构件的冶金结合。因此，送粉式激光熔覆技术得到了国内外研究者和工程技术人员的高度关注和广泛的研究。在送粉式激光熔覆工艺过程中，粉末流-激光、粉末流-气流的耦合作用决定粉末流空间传输行为、空间气流场和衰减的激光能量分布。粉末的空间传输特性、衰减后的激光能量分布和气流的动力学行为直接作用于熔池的气液界面，进而决定熔池特征及其演化规律。同时，熔覆过程高温度梯度和高冷却速率的特点使得熔池内存在复杂的热-力耦合作用，导致精确控制熔池形貌特征，实现无缺陷熔覆工艺存在很大的困难和挑战。因此，深入分析送粉式激光熔覆的工艺过程并建立合理的分析方法，总结熔池形貌特征、缺陷特征的形成规律并探究其形成机理是深入研究送粉式激光熔覆技术、提高其工程价值的根本途径。针对这一情况，本文采用数值模拟和工艺实验相结合的方式，基于激光和粉末流的相互作用研究熔池特征，分析送粉式激光熔覆粉末的两相传输行为、激光-粉末相互作用、熔池形貌特征成形规律及工艺过程导致的缺陷形貌和演化行为。本文主要内容及研究成果如下： 1. 采用数值模拟结合在线光学诊断的方法研究了同轴送粉过程中粉末的传输机理，开发了一种试错匹配数值模拟和实验结果来量化描述颗粒与喷嘴壁之间非弹性碰撞恢复系数的方法。实验统计和数值计算的粒子出口速度和空间浓度分布的一致性表明，恢复系数取值0.9可用于描述颗粒与喷嘴壁之间的非弹性碰撞行为。基于确定的恢复系数，提出了一种基于光学在线观测半定量和数值模拟定量分析粉末空间传输行为的方法，研究粉末从单层到多层射流的传输特性。研究发现：外保护气流对粉末在多层射流流场的传输行为有很大影响，当外环保护气流量为20L/min时最有利于粉末聚焦。由同轴送粉头内部结构确定的载粉气流的速度分布和颗粒与壁之间的非弹性碰撞是决定颗粒出口速度的两个相互耦合的因素。频繁的非弹性碰撞和载粉气流速度的降低导致粒子速度的发散和传输轨迹的波动。当颗粒的入口速度为1.33m/s时，出口速度范围为0.4m/s至0.9m/s。本部分研究提供一种确定粒子与同轴送粉头壁面非弹性碰撞恢复系数的通用方法，全面探究了粉末颗粒在同轴送粉头内部和自然射流区的输送机理。 2. 从理论和实验两个角度研究了激光熔覆在大工艺范围的激光能量输入和粉末质量添加条件下单道熔覆工艺及熔池几何特征的成形规律。建立了9个熔池形貌的几何特征与激光功率和送粉速率组合工艺参数(记为P&alpha;F&beta;)之间的线性回归模型，以定量分析熔池几何特征与工艺参数之间的关系，大的回归系数和残差的均匀分布证明了所建立模型的正确性。可以得出结论：随着能量输入的增加，熔池内具有向外流动特征的Marangoni对流增强。因此，熔池经历了从无稀释、浅稀释、平坦稀释到波动稀释的演变。熔池宽度W与激光功率P之间几乎是线性关系，表明熔池表面激光能量积累在熔池宽度W的演变中起主要作用。随着送粉速率的增加，熔池高度hc的增加率在640%至360%之间，说明熔池高度hc的演变由送粉速率主导。熔池横截面的总面积A、熔池在基体之上的面积Ac、基体上熔池的面积Am、熔池总高度H、熔池在基体之上的高度hc、熔池在基体内的深度hm、熔池宽度W、稀释率D、润湿角&theta;由能量输入和质量累积的复杂耦合关系确定，分别与组合工艺参数P0.5F0.2、P0.2F0.5、P0.5/F0.2、P0.3F、P0.5/F0.2、P0.2/F0.2和P0.2/F0-2构成线性回归模型。这项工作旨在探究激光能量输入和粉末质量添加对熔池几何特征的影响及其影响机制，为促进送粉式激光熔覆技术的实际工程应用提供重要的参考价值。 3. 气孔、裂纹和欠熔合是送粉式熔覆缺陷的主要形式。系统分析了不同激光能量输入和粉末质量输入导致的缺陷大小、形状及其分布规律。建立了熔覆过程的热-力耦合模型，解释了裂纹的动态演化机制。分析了涂层的微观形貌特征、裂纹萌生行为、扩展规律及其扩展机理。结果表明：熔池内气孔形貌是激光能量输入和粉末质量输入相互耦合作用的结果，其分布规律反映熔池内部的物理过程机制。单道裂纹主要来源于熔池表面，并沿熔池深度方向扩展至基体。激光功率和扫描速度是影响裂纹萌生和扩展行为的主要因素。当扫描速度相对较低时，可以诱发基体产生裂纹，并且裂纹在平行于激光扫描速度的方向上扩展，表现为基体被撕裂。在多道搭接熔覆的工艺过程，裂纹主要表现为网状裂纹的形成和横向裂纹的持续扩展。合理工艺参数可以实现无裂纹熔覆工艺。未熔合缺陷是熔池能量输入不足的表现，并且影响裂纹缺陷的动态演化行为。本工作旨在深入理解熔池内的缺陷形貌及其动态演化行为，并为无缺陷熔覆工艺提供理论指导。</p

The level of self-awareness effect the cognition of self-information

探讨意识水平对自我信息认知的影响.采用内隐社会认知研究范式,测量被试对自我信息的内隐态度.自我意识水平对自我信息的认知具有影响.高自我意识水平的被试,对自我美化信息的内隐态度显著好于丑化信息;低意识水平被试对他人的丑化和美化信息的内隐态度差异不显著,而高意识水平被试对他人的丑化信息更敏感;高水平自我意识被试对自我美化信息的内隐态度显著好于对他人的美化信息,而低水平自我意识对此两类信息的态度差异不显著.提示高自我意识水平被试的自我信息加工能力强于低自我意识水平的被试

Morphological Characteristics and Printing Mechanisms of Grid Lines by Laser-Induced Forward Transfer

Laser-induced forward transfer (LIFT) is an innovative metallization technique used in the processing of grid lines of solar cells for the photovoltaics industry. A study on the morphology and transfer mechanisms of formed lines with high-viscosity silver paste and small gap was performed in this paper. There were three different transfer states under different laser fluences: non-transferred lines or transferred but no continuous lines, continuous transferred lines, and explosive transferred lines. There was a critical transfer threshold for the continuous line transfer under different processing speeds. Higher processing speed required a larger critical transfer threshold. The line width increased as the laser fluence increased. For all continuous formed lines, the cross-sectional morphologies with single and double peaks were shown at critical and above transfer threshold, respectively. Two symmetrical protrusions with steep edges were observed for the formed line with double peaks. By comparing the silver paste remaining on the donor and transferred to the acceptor under different laser fluences, it can be found the transferred silver paste exhibited a retracting characteristic under the critical and above transfer threshold. While a stretching characteristic was obvious when the laser fluence was much higher than the transfer threshold. Morphological characteristics with single or double peaks were determined by the distance between the rupture position of the bridge and the bottom of the bubble, under the action of the axial combined forces. This work can provide insights for improving fine-line metallization and understanding transfer mechanisms in the photovoltaic application and flexible electronics devices

Exit morphology and mechanical property of FDM printed PLA: influence of hot melt extrusion process

In order to study the hot melt extrusion process in fused deposition modeling (FDM), this study mainly explores the effects of printing temperature, heated block length, feeding speed on the exit morphology and mechanical properties of FDM printed Polylactic acid (PLA) samples. High-speed camera is used to capture the exit morphology of molten PLA just extruded to the nozzle. According to exit morphology, the outlet states of extruded molten material can be divided into four categories, namely, bubbled state, coherent state, expanding state, and unstable state. Tensile test results show that printing temperature, heated block length and printing speed have significant influence on tensile properties and fracture mode of FDM printed samples. When the heated block length is 15 mm and 30 mm, there is a ductile-brittle transition in fracture mode with the increase of printing speed. The printing process window under different heated block lengths and printing temperatures has been figured out and the distribution of printing process window under different printing speeds has been discussed. There is a maximum printing process window under the heated block length of 30 mm. This finding provides a frame work for performance prediction of FDM printed parts and theoretical guidance for expanding the scope of printing process window

Effect of Surface-Active Element Oxygen on Heat and Mass Transfer in Laser Welding of Dissimilar Metals: Numerical and Experimental Study

The effects of the surface-active element oxygen on the laser welding of 304 stainless steel (304SS) and nickel were numerically and experimentally studied in pure argon and argon-oxygen mixed gas atmospheres containing 21% oxygen (AMO). In this study, the molten pool morphology, thermal behavior, solidification phenomenon, correlation between dilution and convection flow, and microhardness of welding joints were analyzed. As a result of oxygen effects, the molten pool was deeper, the maximum temperature was higher, and the maximum flow velocity was lower in the AMO. The cooling rate (GR) and combination parameter (G/R) were studied by the direct simulation of temperature gradient (G) and solidification growth rate (R). Combined with the solidification microstructure, it was found that oxygen had little effect on grain size. The major elements Fe, Cr, and Ni within the solidified molten pool in the AMO were uniformly diluted, while the distribution of the above elements was non-homogenous in pure argon. Stronger flow and multiple directions of convection inside the molten pool contributed to uniform dilution in the AMO. The distribution of microhardness was similar to the content of Cr, and the microhardness at the substrate interface of the joint was higher in the AMO than in pure argon. The preliminary conclusions of this study provide in-depth insights into the effects of surface-active element oxygen on heat and mass transfer in laser dissimilar welding

Effect of Laser Beam Profile on Thermal Transfer, Fluid Flow and Solidification Parameters during Laser-Based Directed Energy Deposition of Inconel 718

The profile of the laser beam plays a significant role in determining the heat input on the deposition surface, further affecting the molten pool dynamics during laser-based directed energy deposition. The evolution of molten pool under two types of laser beam, super-Gaussian beam (SGB) and Gaussian beam (GB), was simulated using a three-dimensional numerical model. Two basic physical processes, the laser-powder interaction and the molten pool dynamics, were considered in the model. The deposition surface of the molten pool was calculated using the Arbitrary Lagrangian Eulerian moving mesh approach. Several dimensionless numbers were used to explain the underlying physical phenomena under different laser beams. Moreover, the solidification parameters were calculated using the thermal history at the solidification front. It is found that the peak temperature and liquid velocity in the molten pool under the SGB case were lower compared with those for the GB case. Dimensionless numbers analysis indicated that the fluid flow played a more pronounced role in heat transfer compared to conduction, especially in the GB case. The cooling rate was higher for the SGB case, indicating that the grain size could be finer compared with that for the GB case. Finally, the reliability of the numerical simulation was verified by comparing the computed and experimental clad geometry. The work provides a theoretical basis for understanding the thermal behavior and solidification characteristics under different laser input profile during directed energy deposition

中国脑血管病临床管理指南（第2版）（节选）——第5章 脑出血临床管理 Chinese Stroke Association Guidelines for Clinical Management of Cerebrovascular Diseases (Second Edition) (Excerpt) ——Chapter Five Clinical Management of Intracerebral Hemorrhage

自发性脑出血是卒中的一种严重亚型，在中国卒中患者中占23.4%，以急性发病、病情迅速变化以及高致死和致残率为显著特点。鉴于管理脑出血所面临的紧迫和复杂挑战，本指南旨在为其临床处理提供系统性和全面性的推荐意见。本章节覆盖了从脑出血院前评估、医疗干预到二级预防和康复的各个关键环节。在具体推荐方面，本指南根据不同的证据等级，为早期诊断、影像学评估、急性期干预、内科和重症监护、外科干预以及二级预防策略提供了指导，旨在全面提升自发性脑出血诊疗的科学性和系统管理质量。 Abstract: Spontaneous intracerebral hemorrhage represents a severe subtype of stroke, accounting for approximately 23.4% of stroke cases in China. It is characterized by its rapid onset, swift disease progression, and high rates of mortality and disability. Given the urgency and complexity in managing intracerebral hemorrhage, this guideline aims to provide systematic and comprehensive recommendations for its clinical management. This chapter covers all key aspects ranging from pre-hospital evaluation and medical intervention to secondary prevention and rehabilitation. With respect to specific recommendations, this guideline offers exhaustive guidance based on varying levels of evidence for early diagnosis, imaging assessments, acute-phase interventions, medical and ICU management, surgical interventions, and secondary prevention strategies, with the ultimate aim of enhancing the scientific rigor and systematic quality of spontaneous intracerebral hemorrhage management