摘 要

TC4钛合金（Ti-6Al-4V）具有高比强度和比刚度、良好的耐腐蚀性性能、高温力学性能、抗疲劳性能和生物相容性，综合性能优异，是一种极具发展潜力的金属，在航空航天、汽车船舶、生物医学等诸多领域中应用广泛，然而钛合金较低的硬度和较差的耐磨性限制了其进一步的应用。

激光熔覆是一种新型表面改性技术，其在提高钛合金耐磨性和表面硬度、修复受损钛合金工件方面，具有良好的表现。激光熔覆加热速度快，对基体热影响小，可与基体实现良好的冶金结合，研究价值很高。然而，因为加工过程中材料的快速融化和凝固，工件内必然产生残余应力，影响工件的机械性能和使用寿命。因此对工件温度场和应力场的数值模拟对提高熔覆层性能与质量具有重要的意义。

本文采用有限元数值模拟的方法，使用Visual Environment软件平台，对钛合金TC4的激光单道熔覆和双道熔覆过程进行数值模拟，分析其温度场和应力场的特点与规律，研究不同工艺参数对单道激光熔覆过程的影响。本文考虑了热传导、相变潜热、热弹塑性理论等问题；使用了Visual Mesh建立工件三维模型并划分网格；使用Visual Weld设置模型工艺参数，并调用Sysweld内核进行计算；最后使用Visual Viewer查看计算结果，总结温度场与应力场规律。

根据温度场动态演示云图与特征路径上的热循环曲线，分析了激光熔覆过程中温度场的特点；分析了激光熔覆过程中典型熔池的形貌与特点；分析了端部效应产生的效果与原因；分析了单道激光熔覆与双道激光熔覆温度场的异同；分析了激光功率、扫描速度对单道激光熔覆温度场的影响。

根据应力场动态演示云图与特征路径上的应力应变分布曲线图，分析了激光熔覆过程中应力场的特点，分析了易产生应力集中的危险区；分析了应力与应变产生的机理；分析了工件残余应力与应变的大小与分布状况；分析了单道激光熔覆与双道激光熔覆应力场的异同；分析了激光功率、扫描速度对单道激光熔覆应力场的影响。

最后，通过钛合金TC4的激光单道熔覆和双道熔覆试验，观察并分析了熔覆层的宏观形貌与尺寸，验证了本文数值模拟结果的正确性。

关键词：钛合金TC4，激光熔覆，数值模拟，visual environment，温度场，应力场，残余应力

Abstract

TC4 titanium alloy (ti-6al-4v) has high specific strength and specific stiffness, good corrosion resistance, high temperature mechanical properties, fatigue resistance and biocompatibility, and excellent comprehensive performance. It is a metal with great development potential and has been widely used in many fields such as aerospace, automobile and ship, biomedicine and so on.However, the low hardness and poor wear resistance of titanium alloys limit their further applications.

Laser cladding is a new type of surface modification technology, which has good performance in improving wear resistance and surface hardness of titanium alloy and repairing damaged titanium alloy workpiece. Laser cladding has the advantages of fast heating speed, little effect on the heat of the substrate, good metallurgical combination with the substrate, and high research value. However, because of the rapid melting and solidification of materials in the processing process, residual stress will inevitably occur in the workpiece, affecting the mechanical properties and service life of the workpiece. Therefore, the numerical simulation of the temperature field and stress field of the workpiece is of great significance for improving the performance and quality of the cladding layer.

In this paper, finite element numerical simulation method and Visual Environment software platform are used to conduct numerical simulation of single laser cladding and double laser cladding process of titanium alloy TC4, analyze the characteristics and laws of temperature field and stress field, and study the influence of different process parameters on single laser cladding process. The problems of heat conduction, latent heat of phase transition and thermo - elastoplastic theory are considered. Use Visual Mesh to build 3d model of workpiece and divide Mesh. Set the model process parameters using Visual Weld, and create the Sysweld kernel to create the model. Finally, Visual Viewer is used to view the calculation results and summarize the laws of temperature field and stress field.

The characteristics of the temperature field in the laser cladding process are analyzed according to the dynamic cloud diagram of the temperature field and the thermal cycle curve on the characteristic path. The morphology and characteristics of typical pool in laser cladding process are analyzed. The effect and reason of end effect are analyzed. The difference of temperature field between single laser cladding and double laser cladding is analyzed. The influence of laser power and scanning speed on the temperature field of single-channel laser cladding is analyzed.

According to the dynamic cloud diagram of the stress field and the stress-strain distribution curve on the characteristic path, the characteristics of the stress field in the laser cladding process are analyzed, and the hazard areas which are prone to stress concentration are analyzed. The mechanism of stress and strain is analyzed. The size and distribution of residual stress and strain of workpiece are analyzed. The difference of stress field between single laser cladding and double laser cladding is analyzed. The effects of laser power and scanning speed on the stress field of single-channel laser cladding are analyzed.

Finally, the laser single-channel cladding and double-channel cladding experiments of titanium alloy TC4 were carried out to observe and analyze the macroscopic morphology and size of the cladding layer, which verified the correctness of the numerical simulation results in this paper.

Key word：titanium alloy TC4, laser cladding, numerical simulation, visual environment, temperature field, stress field, residual stress

目 录

摘 要 I

Abstract II

第1章 绪论 1

1.1 选题的背景及意义 1

1.2 钛合金的特性与应用 1

1.2.1钛合金的基本特性与应用 1

1.2.2钛合金的限制因素 3

1.3 激光熔覆技术 3

1.3.1激光熔覆简介 3

1.3.2熔覆材料 4

1.3.3钛合金激光熔覆的国内外研究现状 5

1.4 激光熔覆的数值模拟技术 6

1.4.1激光熔覆温度场数值模拟的国内外研究现状 7

1.4.2激光熔覆应力场数值模拟的国内外研究现状 7

1.5 主要研究内容与研究方法 8

1.6本章小结 9

第2章 激光熔覆有限元分析的理论基础 10

2.1有限元法概述 10

2.2激光熔覆温度场分析理论 10

2.2.1激光熔覆传热模型 10

2.2.2激光与基体、粉末的相互作用 11

2.2.3相变潜热及其处理 13

2.3激光熔覆应力场分析理论 13

2.3.1塑性理论 13

2.3.2热弹塑性有限元方法 15

2.4生死单元法 16

2.5本章小结 17

第3章 有限元模型的建立 18

3.1软件介绍与基本假设 18

3.1.1软件介绍 18

3.1.2基本假设 19

3.2三维模型的建立 19

3.3有限元模型的建立 20

3.3.1网格划分 21

3.3.2材料属性 22

3.3.3热源模型选取与加载 23

3.3.4设定边界条件 26

3.4模拟实验方案设计 27

3.5本章小结 27

第4章 激光熔覆温度场模拟结果及分析 28

4.1激光熔覆温度场的动态演示 28