Effect of Laser Power on the Microstructure and Mechanical Properties of TiN/Ti_3Al Composite Coatings on Ti6Al4V

Laser nitriding is one of the effective techniques to improve the surface properties of titanium alloys and has potential application in the life extension of last-stage steam turbine blades. However, cracking of surface coating is a common problem due to heat concentration in laser nitriding process. Conventionally, the cracks can be avoided through heat treatment, which may have an important influence on the mechanical properties of coating. Crack-free TiN/Ti3Al IMC coatings on Ti6Al4V are prepared by plasma spraying and laser nitriding. The microstructures, phase constitutes and compositions of the coating are observed and analyzed with scanning electron microscopy(SEM), X-ray diffraction(XRD) and X-ray energy-dispersive spectroscopy(EDS). Microhardness, elastic modulus, fracture toughness of the coating are measured. The results show that the crackand pore-free IMC coatings can be made through the proposed method; with increasing laser power, the amount and density of TiN phase in the coating first increased and then decreased, leading to the similar trend of microhardness and elastic modulus and the reverse trend of fracture toughness of the coating. Both the average microhardness and elastic modulus of the coating increase three times higher than those of the substrate. The volume fraction of the TiN reinforced phase in composite can be controlled by varying the laser power and the cracking problem in laser nitriding process is successfully solved.

蒸汽发生器手孔镍基合金堆焊层开裂分析

蒸汽发生器是核电厂主要的承压边界,某核电厂蒸汽发生器手孔152镍基合金堆焊层发生开裂,对核电安全、经济地运行造成一定影响。预制手孔模拟件复现缺陷并分析开裂处的微观特征,采用横向可调拘束试验分析152熔敷金属的开裂倾向,并利用有限元分析手孔堆焊层的塑性应变分布,复现出了与实际缺陷特征和位置一致的裂纹,发现152堆焊层开裂位置处存在长条状、颗粒状碳化物沿晶界析出,明确了蒸汽发生器手孔152堆焊层开裂机理为碳化物诱导型高温失塑裂纹(DDC),碳化物在晶界析出与焊接应变共同作用导致手孔堆焊层发生开裂。

低压氢气-空气混合物爆炸试验研究及数值模拟

为了预防或控制密闭容器内氢气爆炸事故,运用20 L密闭球形容器试验研究不同初始低压（0.025-0.1 MPa）下氢气-空气混合物的最大爆炸压力、最大压力上升速率;并采用Fluent数值模拟软件,通过标准k-ε湍流模型和概率密度函数（PDF）燃烧模型,模拟不同初始压力下氢气-空气混合物燃烧过程,直观再现不同初始压力下火焰传播过程及流场扰动状况。研究表明：氢气体积分数一定时,氢气-空气混合物的最大爆炸压力和最大压力上升速率与初始低压均成线性关系;初始压力从0.1MPa降低至0.025 MPa,最大爆炸压力降低75.1%-75.9%,最大压力上升速率降低77.1%-83.7%。另外,初始压力降低,火焰前沿到达器壁的时间变长。

Magnification Ratio of Mechanical Displacement Amplifier in Sensing Device

In order to monitor deformation of high temperature components for a long time,a sensing device integrating a bridge-shaped mechanical displacement amplifier has been designed.This sensing device has higher resolution and accuracy than conventional extensometers.However,the relation between the magnification ratio and the structure size of the displacement amplifier is a bottleneck of sensing device design.Addressing this,the magnification ratio of a mechanical displacement amplifier is analytically derived based on its geometry structure.Six prototypes of the displacement amplifier made in propathene are manufactured,and an experimental system is set up to validate the accuracy of the established magnification ratio equation.Theoretical magnification ratios and experimental magnification ratios are compared and agree well,which verifies that the proposed equation is reliable.This analytical equation provides an effective design method for bridge-shaped mechanical displacement amplifiers with an expected magnification ratio.

Quality Evaluation of Diffusion Bonded Joints by Electrical Resistance Measuring and Microscopic Fatigue Testing

Micro-structure related behavior of diffusion bonding joints is a crucial issue in device and reactor fabrication of Micro Chemo Mechanical Systems.However,the previous studies have been focused on the macro mechanical performance of diffusion bonded joint,especially diffusion bonding conditions effects on tensile strength,shearing strength and fatigue strength.The research of interfacial micro-voids and microstructures evolution for failure mechanism has not been carried out for diffusion-bonded joints.An interfacial electrical resistance measuring method is proposed to evaluate the quality of bonded joints and verified by using two-dimensional finite-element simulation.The influences of micro void geometry on increments of resistance are analyzed and the relationship between bonded area fraction and resistance increment is established by theoretical analysis combined with simulated results.Metallographic inspections and micro-hardness testing are conducted near the interface of diffusion bonded joints.For the purpose of identifying the failure mechanisms of the joints,both microscopic tensile and fatigue tests are conducted on the self-developed in-situ microscopic fatigue testing system.Based on the microscopic observations,the mechanism of interfacial failure is addressed.The observation result shows that for 316LSS diffusion-bonded joints,microstructure evolution and effect of micro-voids play a key role in interfacial failure mechanism.Finally,a new life prediction model in terms of the increment of electrical resistance is developed and confirmed by the experimental results.The proposed study is initiated that constituted a primary interfacial failure mechanism on micron scale and provide the life prediction for reliability of components sealed by diffusion bonding.

创新创业任务导向的“工程导论”课程建设与教学实践

近年来,世界各国均十分关注卓越的创新型工程科技人才的培养,因为这是关乎人类可持续发展又与一个国家的社会经济发展紧密相关的重大议题。国内外多所高校针对大一新生缺乏对工程和工程问题的了解开设了“工程导论”课程,但普遍缺少创新创业开放性任务的导向。本文介绍了基于逆向教学设计理论的教学实践,从学习目标、教学组织与评价方式等方面细化“工程导论”的教学设计,组织校内外教学资源,改革教学效果评价方式,最后以新生创新创业研讨会形式结束课程,较好地达成了培养学生工程意识与创新创业能力的目标。

基于学生视域的工程教育风险识别与防控研究

工程本质上是一种带有一定风险的赋能活动。旨在培养工程师的工程教育,在帮助学生实现这种能动性和可行能力的同时,不仅需要识别到工程教育以及工程中的风险,而且应通过有效防控确保参与者乃至公众的健康与安全,进而增进社会的福祉。当前,我国的工程教育正通过新工科的推进、工程专业认证等举措,系统修正着工程教育的某些痼疾,但不容否认的是工程实践的回归以及工程教育的再创新,也让技术的风险、组织的风险等随之而来,作为工程教育最直接受体的学生,将会直接面对或承担这些风险故必须以学生健康安全底线思维为首位透视工程教育中的风险。本文提出以风险/安全来建构底线意识、强化科学技术的禁忌、加强知识与信息的传递、严守质量关卡、推进社会保险介入、加强组织沟通和标准的制定,从而系统构建起工程教育的风险防控体系。