合金改性

激光表面改性工艺及性能研究

摘要：钛及钛合金密度小，比强度高，具有良好的耐蚀性、疲劳抗力，广泛应用于航天、航空、国防、汽车、医疗等领域。然而，钛合金摩擦系数高、对粘着磨损和微动磨损非常敏感、耐磨性差及高温抗氧化性差等缺点，制约了它的应用。在保持钛合金固有性能优点的条件下，激光表面改性是从根本上解决钛合金表面性能较差这一缺陷的有效、经济、灵活且具有较强可设计性的方法之一。

本文运用CO2激光，在Ti6A14V钛合金表面进行激光表面改性，直接在Ti6A14V钛合金表面制得了具有优异耐磨性能和抗高温氧化性能，并与Ti6A14V钛合金基体之间呈现牢固冶金结合的复合材料涂层。

首先，为确保激光表面改性的顺利进行，研制了激光表面改性专用的同轴保护送粉装置，并探索了适合该同轴保护送粉装置的工艺参数，获得较高的粉末利用率和较好的保护效果。

其次，设计了激光表面改性材料体系。根据Ti6A14 V钛合金和激光表面改性的特点，设计出激光熔覆Ni-Ni基合金梯度涂层、激光熔覆Mo-Ni基合金梯度涂层、激光表面原位生成TiC涂层和激光表面原位生成TiC+TiB:涂层四类材料体系。

再次，研制了激光表面改性工艺。分别研制出各种材料体系对应的激光工艺参数，制得激光表面改性层，着重研究了不同工艺参数对激光熔覆Ni-Ni基合金梯度涂层的影响，探索最佳工艺参数。对激光表面改性涂层进行了微观分析，从表面至基体测试了显微硬度，发现涂层组织呈快速凝固特征，硬度梯度明显，有效地改善了涂层的应力分布状况。

最后，研究了激光表面改性涂层的摩擦学性能和抗高温氧化性能。研究发现，激光表面改性能够大大地提高了Ti6A14V钛合金的耐磨性能和抗高温氧化性能。

关键词:Ti6A14V钛合金，激光表面改性，微观组织，硬度梯度，耐磨性，抗高温氧化性

Abstract

Titanium alloys are used widely in aviation, national defence, automobile, medicine and other fields because of their advantages in lower density, excellent corrosion resistance, and good fatigue resistance etc. However, their high friction coefficient, high sensitivity to adhesive wear and fretting wear, as well as their weak resistance to wear and bad resistance to high temperature oxidation restricts the application of titanium alloys. Laser surface modification technique can change these surface defects of titanium alloys under the condition of keeping the virtues of titanium alloy substrate. For the reason, laser surface modification is one of effective, economic, appropriate and designable methods.

In this paper, surfaces of Ti6A14V alloy were modified by CO2 laser. Special composite metallurgy coatings, which have excellent wear resistance and high temperature oxidation resistance, were obtained directly on the surfaces of Ti6A14V alloy. Furthermore, the coatings showed good metallic combination with the substrate of titanium alloy.

In order to carry out the laser surface modification process successfully, a coaxial powder feed nozzle with gas protection was designed at first. Through investigations of suitable technical parameters of the coaxial powder feed nozzle with gas protection, a higher duty factor of powder and better protection effect was obtained.

Moreover, based on properties of the Ti6A14V alloy and characteristics of the laser surface modification, four types of coating were designed: laser cladding Ni-Ni base alloy gradient coating, laser cladding Mo-Ni base alloy gradient coating, laser surface in-suit formed TiC coating and laser surface in-suit formed TiC+TiB2 coating.

Furthermore, the suitable technical parameters of laser surface modification for each type of coating above-mentioned were established. Influences of different technical parameters to the laser cladding Ni-Ni base alloy gradient coating had been investigated emphatically and the most suitable technical parameters had been obtained. By means of SEM and micro-hardness tester, microstructure of the coatings was investigated, and micro-hardness was measured. The results showed that the coatings appeared representative characteristics of quick solidification, the micro-hardness appeared gradient character clearly from deep substrate to surface and the stress distribution was improved effectively.

At last, tribological characteristics and characteristics of high temperature oxidation resistance were tested. The results showed that laser surface modification could increase wear resistance and high temperature oxidation resistance of Ti6A14V alloy.

Key words: Ti6A14V alloy; laser surface modification;crostructure; hardness gradient; wear resistance; high temperature oxidation resistance

1钛合金表面改性技术

钛合金具有许多优异的性能，应用面越来越广，对其存在的表面性能较差的状况，也已引起越来越多工程技术人员的高度重视，并开发了多种工艺技术以解决这一问题。但这些方法不同程度地存在一些不足，影响了它们的应用范围。目前，钛合金常用的表面强化方法主要有以下几种：

1.1离子注入

离子注入是将注入元素离子化后，在电场中获得高能量，强行注入材料表面，从而改变其性能。钛合金表面主要的注入离子有N十、C十、Pb+等。离子注入最大的特点是原子与基材间呈原子级混合，不会出现剥落等现象;同时，离子注入可在常温下进行，工件不会发生变形。但是，因离子注入能量的制约，强化层很浅，一般不超过1µm，这对该技术的应用带来许多限制。

1.2离子渗氮

离子渗氮是钛合金表面改性的一种较好工艺。通过4h左右的离子渗氮处理，可在钛材表面获得一层0.2一0.3的渗氮层，表面硬度可达1000HV以上。钛合金离子渗氮层硬度高、抗疲劳性能好，但离子渗氮处理温度较高，一般都超过了800℃，因而工件的变形在所难免，对薄壁件、长杆件和大型件较难实施。

1.3双层辉光离子渗金属

双层辉光离子渗金属是利用离子轰击的原理，将一种或多种金属元素渗入工件表层的工艺技术，通过2一4h的处理，可获得.005nnn以上的合金化层，有效地提高钛合金表面的耐磨、耐蚀及疲劳性能。但是，由于金属元素向内扩散的需要，双层辉光离子渗金属处理必须在830一1000℃的高温下进行，必然会引起工件变形，而较薄的合金化层，不可能留出后续机加工的余量;其次，受渗入元素在钛基体中溶解度的影响，合金成分的设计受到限制，从而影响性能的进一步提高;另外，双层辉光离子渗金属处理的金属元素来源于金属靶(源极)，加工过程中存在明显的视线效应，对形状较复杂的工件难以做到所有面均匀强化，对内孔还会产生空心阴极效应，使工件过热。

1.4热喷涂