喷丸处理对增材钛合金性能影响研究

EFFECT OF SHOT PEENING ON PROPERTIES OF ADDITIVELY MANUFACTURED TITANIUM ALLOY

江庆红

11749142

硕士

机械工程

张璧

2019-06-04

2019-06-28

哈尔滨工业大学

深圳

目前，金属增材制造技术直接成形试件的静态强度已经可以媲美传统锻造成形试件，但是较差的疲劳强度限制了其进一步工业化。本课题以激光选区熔化（Selective laser melting, SLM）增材制造Ti6Al4V钛合金为研究对象，在热处理的基础上，对试件进行机械喷丸（Mechanical shot peening, MSP）和激光喷丸（Laser shock peening, LSP）处理。从微观组织、显微硬度、残余应力、拉伸强度与超高周疲劳强度等多个角度对比分析两种喷丸处理技术对SLM增材钛合金性能的影响。主要研究工作为以下方面：SLM增材制造技术由于其“离散+堆积”的独特成形原理，使得其能够突破零件复杂程度的限制，实现高性能复杂零件的一体化制造。但同时也会引入气孔、熔合不良以及裂纹等典型缺陷，大大降低了材料的疲劳性能。本课题对SLM增材制造典型缺陷的形成机理进行分析，总结增材过程中减少缺陷的解决方案。然后在表面加工后对试件进行热处理，经过955°C保温1小时空冷处理后，大大降低了试件的残余应力。此外，SLM增材制造Ti6Al4V钛合金微观组织由完全的过饱和态针状α′马氏体相转化为稳定的板条状α+β相，虽然抗拉强度降低，但是塑性提升，材料力学性能各向异性效应降低，使用性能得到明显改善。使用激光共聚焦显微镜、金相显微镜、扫描电镜、超微小硬度计、增量钻孔残余应力测试系统以及拉伸试验机等设备，系统地对比分析了不同LSP与MSP喷丸参数对材料性能的影响。结果表明，与LSP相比，MSP处理后试件表面粗糙度明显增大，表层材料加工硬化效应更加明显。两种喷丸工艺都引入了残余压应力并提高了表层显微硬度，且随着喷丸强度的增大，残余应力与显微硬度的最大值与影响深度都增大。但是LSP处理试件显微硬度与残余压应力幅值更小，影响层深度更大。对激光喷丸前后的试件进行超高周疲劳试验，结果表明，SLM增材制造钛合金不存在传统的疲劳极限。LSP处理在试件表层引入残余压应力场，并产生高密度位错细化晶粒，对表层裂纹扩展具有延缓作用，但是对于缺陷过多的增材试件，不能提高疲劳寿命。

At present, although the static strength of a metallic part that is direct-formed by the additive manufacturing technique can be comparable to its as-forged counterparts, the poor fatigue strength limits its industrial applications. In this study, MSP and LSP are used to modify the mechanical properties of the selective laser melting （SLM）Ti6Al4V titanium alloy. The effect of two shot peening techniques on the property change of SLM-ed titanium alloy are analyzed in terms of its microstructure, microhardness, residual stress, tensile strength and ultra-high cycle fatigue strength. The main research work is as follows: SLM can be used to fabricate complex part with high precision, but typical material defects, such as pores, incomplete fusion holes, and cracks, are easy to introduce during the forming process, which greatly reduces the fatigue properties of a part. This study analyzes the formation mechanisms of the typical defects and find out solutions to reducing defects. Heat treatment is conducted at a temperature of 955°C after surface processing. The results show that the residual stress is greatly decreased after the heat treatment. Meanwhile, the initial acicular α′ martensitic phase of the SLM-ed titanium alloy is decomposed into α+β in the shape of colony containing parallel α lamellae. Although the SLM-ed titanium alloy has a low tensile strength, it demonstrates an improved plasticity and extensibility, and decreased anisotropy in the mechanical properties, which may improve its service performance. Confocal laser scanning microscope, metallographic microscope, SEM, micro-hardness tester, tensile testing machine, are used to analyze the effects of laser shot peening (LSP) and mechanical shop peening (MSP) with different processing parameters on property change of the SLM-ed titanium alloy. The results show that, compared with LSP, MSP has significantly increased surface roughness on the processed specimens, and more surface hardening. Higher micro-hardness and residual compressive stress are introduced into the specimens from both processes. With an increased strength due to shot peening, the specimen also has an increase in the maximum residual stress and its influence depth. The LSP processed specimen shows a lower micro-hardness and smaller amplitude of the residual compressive stress, but a larger influence depth. The results of the ultra-high cycle fatigue test show that there is no traditional fatigue limit for the SLMed titanium specimens. The LSP treatment introduces residual compressive stress, high-density dislocations, and grain refinement, which might have deferred the initiation and propagation of the crack, but longer fatigue life has not be achieved for the SLM-ed specimens with too many defects.

金属增材制造 选区激光熔化 机械喷丸 激光喷丸 超高周疲劳

metal additive manufacturing selective laser melting mechanical shot peening laser shock peening ultra-high cycle fatigue

中文

联合培养

南方科技大学

江庆红. 喷丸处理对增材钛合金性能影响研究[D]. 深圳. 哈尔滨工业大学,2019.