【作者】 张谦；

【导师】 周杰；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2022， 硕士

【摘要】 TC21钛合金是我国自主研发的一种新型的损伤容限型两相(α+β)钛合金,被广泛应用于飞行器上的各个重要结构上,随着我国的航空航天领域的发展,锻件向着精密化、高质量方向发展。TC21钛合金在锻造完成后,其表面常出现粗大组织,综合性能较差的贴模层,必须通过机械加工去除掉,如何减少这种性能不合格的表层组织的厚度,实现钛合金精密化成形,是本文的研究问题之一。其二,为了获得强韧性的网篮组织,TC21钛合金一般使用准β锻工艺成形,而准β锻最关键的是变形均匀,且变形量要在合理区间,期望变形量区间及如何实现变形均匀性是本文要研究的问题之二。本文的主要研究内容及结果如下:(1)通过对不同工艺参数和不同锻件结构参数下的TC21钛合金锻件表层组织变化规律进行研究,将锻件表层组织定义为贴模层组织和近表层组织,贴模层组织为一层粗大颗粒组织,其组织厚度尺寸从大到小排列依次为:型腔区域、筋条区域、圆角区域;粗大颗粒组织形成的主要原因是锻件贴模层温降速度过大,其综合性能相较于细密组织更差。进一步分析了保压时间、润滑条件、包套工艺、模具温度对近表层组织的影响,最后,发现贴模层和近表层的大角度晶界含量都大于各自的小角度晶界的含量,贴模层大角度晶界的含量比近表层更多。(2)通过对TC21钛合金阶梯腹板试验件进行准β锻造试验,建立了TC21钛合金准β锻不同变形量与力学性能之间的关系:随着变形量的增大,TC21钛合金横、纵向抗拉、屈服强度都呈现先增大再减小趋势,在应变量区间15%～21%时最小;随着变形量的增加,纵向伸长率无明显变化规律,横向伸长率呈现逐渐增加的趋势,横向和纵向的伸长率都在10.5%以上;除了在变形量区间15%～21%之外,其它变形量区间纵向收缩率都要大于对应区间的横向收缩率,纵向和横向收缩率全部都大于13%;纵向冲击功随着变形量增加,先增加后减小,在变形量区间为21%～32%之间时,纵向冲击功达到最大值56.9 J/cm~2;断裂韧性随变形量的增加而降低。(3)以某TC21钛合金航空锻件为例,根据前述研究成果设定期望变形量区间为21%～32%,通过数值模拟,分析了工字形截面结构参数与工艺参数对其变形均匀性的影响规律,并以此设计了预锻件结构并优化了成形工艺参数,最后,采用所设计的预锻再终锻的工艺方案,期望应变区间的占比达到了55%,相较于直接终锻成形提升了20.5%,有效实现了锻件的变形均匀性和大范围期望变形量区间。(4)对该TC21钛合金航空锻件进行生产试制,使用预锻后终锻成形方案,锻件理化检测结果满足性能指标要求,组织为网篮组织;使用包套工艺能够减小表层粗大颗粒组织厚度;在该锻件腹板剖面上,表层粗大颗粒组织的厚度尺寸分布与结构的关系符合第二章得出的规律(型腔区域>筋条顶部区域>凹圆角区域)。更多还原

【Abstract】 TC21 titanium alloy is a new damage tolerant two-phase alloy developed independently in China(α+β)Titanium alloy is widely used in various important structures of aircraft.With the development of China’s aerospace field,forgings are developing towards precision and high quality.After the forging of TC21 titanium alloy,its surface often appears coarse microstructure,and the die layer with poor comprehensive performance must be removed by machining.How to reduce the thickness of this unqualified surface structure and realize the precision forming of titanium alloy is one of the research problems in this paper.Second,in order to obtain a strong and tough basket structure,TC21 titanium alloy is generally used quasi-βforging process.The key of the quasi-β forging is uniform deformation,and the deformation should be in a reasonable range.The expected deformation range and how to achieve the deformation uniformity are the second problems to be studied in this paper.The main contents and results of this paper are as follows:(1)Based on the study of the change law of the surface structure of TC21 titanium alloy forgings under different process parameters and different forging structural parameters.The surface layer microstructure of the forging is defined as the die layer microstructure and near surface microstructure.The die layer microstructure is a layer of coarse particle microstructure,and its thickness and size are arranged in order from large to small: cavity area,rib area and fillet area.The main reason for the formation of coarse particle microstructure is the excessive temperature drop rate of the die layer,and its comprehensive performance is worse than that of fine microstructure.Besides,the effects of packing time,lubrication conditions,cladding process,and die temperature on near-surface microstructure were then analyzed.Finally,it is found that the large-angle grain boundary content of the die layer and the near surface layer are greater than the content of their respective small-angle grain boundaries,and the content of the large-angle grain boundary of the die layer is more than that of the near surface layer.(2)Through the quasi-β forging test of TC21 titanium alloy stepped web test part,The relationship between different deformation and mechanical properties of TC21 titanium alloy quasi-β was established: With the increase of deformation,the transverse and longitudinal tensile strength and yield strength of TC21 titanium alloy first increase and then decrease,and the minimum value is in the range of 15%～21%.With the increase of deformation,there is no obvious change law in longitudinal elongation,and the transverse elongation shows a gradually increasing trend.The elongation rate of both lateral and longitudinal directions is above 10.5%.In addition to the deformation range of 15%～21%,the longitudinal shrinkage rate of the other four deformation areas is greater than the lateral shrinkage rate of the corresponding area,and the longitudinal and transverse shrinkage rates are all greater than 13%;the longitudinal impact work increases with the increase of deformation,first increases and then decreases,and when the deformation area is between 21% and 32%,the longitudinal impact work reaches a maximum of 56.9 J/cm2;the fracture toughness decreases with the increase of deformation.(3)Taking a TC21 titanium alloy aviation forging as an example,according to the above research results,the expected deformation range is 21%～32%.Through numerical simulation,the influence of the structure parameters and process parameters of the I-shaped section on its deformation uniformity was analyzed.Based on this,the pre-forging structure is designed and the forming process parameters are optimized.Finally,the designed pre forging and final forging process scheme is adopted,and the proportion of the expected strain range reaches 55%,Compared with the direct final forging,the forming rate is increased by 20.5%,which effectively realizes the deformation uniformity of forgings and a wide range of expected deformation.(4)The TC21 titanium alloy aerospace forgings were produced on a trial basis,using the pre-forging and then final forging forming scheme.The physical and chemical test results of the forging met the performance index requirements,the microstructure is a net basket microstructure,the temperature drop rate of the die layer is slowed down by using the cladding process.In the web section of this forging,the relationship between the thickness size distribution of the surface coarse particle microstructure and the structure is in accordance with the rule derived in Chapter 2(cavity area> rib area> concave fillet area).更多还原