【作者】 曹丽琴；

【导师】 王正东； 轩福贞；

【作者基本信息】 华东理工大学 ， 化工过程机械， 2010， 博士

【摘要】 Ti6A14V合金由于优异的综合性能被广泛应用于航空航天、生物医学等高新技术领域,然而硬度低耐磨性差等自身不足致使其进一步推广受到局限,促使人们开展了钛合金表面强化技术的研究与应用探索。高硬度耐磨损的氮化钛防护涂层可有效改善钛合金材料表面性能,而激光气体氮化有传统技术无可代替的氮化钛涂层制备优势。激光氮化技术的关键是控制工艺参数,进而获得满足工艺需要的渗氮层,达到提高表面性能的目的。然而,由于影响参数众多,这一技术仍存在诸多急需解决的问题,距离真正广泛的实际生产应用尚有很大的距离。本文针对激光氮化工艺存在的不足,采用试验和理论分析并重的研究思路,开展了相关技术和理论研究。主要内容和创新如下：(1)对于钛合金激光辅助氮化过程的热质传递,存在界面浓度梯度驱动和高阶温度梯度驱动的耦合效应影响。采用数值模拟结合微观尺度试验研究的方法,首次分析了渗氮层内氮元素最大溶解极限对于扩散传质的影响,获得了激光渗氮层内组分的不均匀性分布规律。研究发现,在高阶温度梯度和浓度梯度的共同驱动下,氮原子不断向内扩散的同时也产生沿温度梯度方向的扩散,最高浓度峰值向内偏移至距表面一定深度处；分析了各加工参数对激光氮化表面强化层性能的影响,为激光氮化工艺优化提供指导。(2)根据工艺参数优化分析的结果,利用连续CO2激光和脉冲Nd:YAG激光在钛合金表面成功制备了高硬度高结合强度的枝晶状分布的氮化钛保护层。同时,试验分析发现,由于基体约束作用和氮化层相变等因素的影响,激光渗氮层表面存在残余应力和萌生表面裂纹等问题。(3)针对激光渗氮层开裂这一技术瓶颈问题,首次提出了施加压缩外载荷的激光氮化工艺。研究发现当施加一定的压缩载荷时,可以有效地减少甚至避免在激光渗氮工艺过程中形成裂纹;随着外载荷的加大,残余应力降低,氮化层内氮原子浓度峰值降低,表面硬度降低,摩擦系数增加。(4)基于纳米压痕和纳米划痕测试技术,测定了不均匀激光氮化层的弹塑性力学性能,并与采用不均匀组分相关的力学模型计算获得的弹塑性力学性能进行比较,揭示出激光渗氮层力学性能沿深度方向的分布规律与激光渗氮层内氮浓度分布规律的一致性。本研究为钛合金激光表面氮化技术的推广和工业应用提供了有力的支撑。更多还原

【Abstract】 In view of excellent strength and lightweight properties, Ti6A14V alloy is an attractive candidate material to guarantee the comprehensive performances for metal components in many innovative and high technological fields, ranging from aeronautic industry to biomedicine. However, hardness and tribological properties are too low to satisfy the extensive application. For this reason, surface strengthening of titanium alloys has attracted much attention. Titanium nitride with high hardness and wear resistance is an excellent material known suitable for surface strengthening. Laser gas nitriding of titanium alloy in nitrogen environment is developed as a promising technology recently to form titanium nitride layer with metallurgical bonding to the titanium substrate for modification of the traditional technology in mechanics, tribological properties. The quality of nitriding layer for industrial demands is the key to performance by adjusting the process parameters. Nevertheless, due to the influence of many process parameters, there are still some confronting problems for the actual production for extensive application. These problems are further studied and discussed in this work by experimental and theoretical analysis on the basis of previous studies. The main contents are as follows:(1) A self-consistent diffusion model is employed to simulate the atomic nitrogen transport in titanium alloy with the coupling effect of both initial activated nitrogen concentration and high temperature gradient. It is the first attempt at investigation of the effect of the maximum concentration limit of nitrogen in titanium nitride layer on nitrogen transport. It is worth noting that high temperature and concentration gradients lead to inhomogeneous distribution of nitrogen with the local maximum close to the surface which is confirmed by both the experimental and simulated results. Meanwhile, transient theoretical models are employed to investigate the effect of process parameters on the distribution characteristics of temperature and concentration field. It is used as the applicable reference for process optimization by trial-and-error.(2) The densely packed dendrites layers with high hardness and strength are prepared successfully under trial optimized condition by continuous CO2 laser and pulsed Nd:YAG laser. Meanwhile, considering the influence of substrate constraint and phase transformation during laser gas nitriding process, the distribution of residual stresses and the crack defects formed in nitriding layers are analyzed and summarized by experiments.(3) For the technical bottleneck problem of laser nitriding layer cracking, we report a laser surface nitriding process coupled with an applied stress field for the first attempt. A surprising finding is that the continuous and defect-free nitrided layer on the surface of Ti-6A1-4V alloy is achieved using a millisecond Nd:YAG laser coupled with an optimum applied stress field. The distribution rules of nitrides and stress states are not influenced by the applied stress. Meanwhile, the decreased residual stress, nitrogen concentration near the surface, and surface micro-hardness of the nirided layer is associated with increased friction coefficient and the increasing applied stress levels.(4) The elastic-plastic mechanical properties induced by the inhomogeneous composition in laser nitriding layer are quantitatively evaluated by nanoindentaion and nanoscratch experiments. A composition-dependent predicted model is developed to characterize the interplay between the inhomogeneous composition and mechanical performance which resembles the concentration profile of nitride generated from diffusion by laser irradiation. It is a new feasible method to confirm the mechanical properties by extracting the depth loading curvature profile involving the composition dependence.This study is the foundation of titanium alloy for extensive applications. Moreover, this gives us confidence in the industrial application of laser surface nitriding technology for rapidly developing and new green industrial revolution.更多还原