Hot working and damage behaviour of additively manufactured Ti6Al4V

由于目前碳纤维复合材料的使用趋势，航空中的铝结构部件越来越多地被钛部件取代，因为超高强度铝合金与碳纤维复合材料接触时容易腐蚀。目前，钛合金在空客A350中的份额已经在14%左右(相比之下，A320-gt；的份额为3%)。对于锻造的钛部件，材料利用率有时不到10%。由于钛的生产复杂，成本高，低的材料产率是无法容忍的。目前研究的添加制造(AM)工艺在单件小批量生产，即近净成形生产中具有很大的优势。由于积累率低，成本高，基于粉末床的AM工艺很难适用于大容量的钛部件。作为替代方案，目前正在研究使用激光、弧焊和等离子焊接工艺(定向能沉积-DED)的AM工艺。在这里，由于生产时间长、可重复性和高残余应力，阻碍了工业应用。这一建议的中心假设是，通过利用AM制造适应的预制件，可以省去锻造步骤和工具，并且可以获得高的材料利用率。如果能够将AM预制件的细小组织转移到锻件中，则可以改善力学性能。到目前为止，关于添加制造的预制件的锻造工作还很少发表。我们自己的初步工作表明，与传统的轧制棒材相比，添加制造的样品具有显著更低的屈服应力和更高的球化率。因此，由于所需的球化体积分数已经在较低的变形度下实现，因此降低成形力和刀具载荷以及锻造较小的超大尺寸似乎是可能的。在前期工作的基础上，推测AM过程中马氏体相的形成及其加热时的分解是导致异常成形行为的原因。本项目的目的是通过DED生产的Ti6Al4V样品在锻造条件下表征变形、球化、损伤和各向异性行为，作为组织和样品相对于焊道位置的函数。此外，还应开发一个耦合的材料模型，用于描述添加生产的预制件的形成和转变行为以及损伤行为和各向异性。为了尽可能大范围地改变β晶粒度和马氏体组分，应采用粉末激光金属沉积(细晶和完全马氏体组织)和线弧金属沉积(大β晶和魏德曼斯特组织)的方法进行预制坯生产。这项研究旨在为在锻造过程中使用添加制造的预制件奠定基础。

Due to current trends in the use of CFRP, aluminum structural components are increasingly being replaced by titanium components in aviation, since the ultrahigh-strength Al-alloys tend to corrode in contact with CFRP. Currently, the share of titanium alloys in the Airbus A350 is already around 14% (compared to A320 -> 3%). For forged titanium components, the material utilization is sometimes less than 10%. Due to the complex production and high costs of titanium, the low material yield is intolerable. The currently investigated additive manufacturing (AM) processes offer great advantages in single-part and small-batch production, i.e., near-net shape production. Due to the low build-up rates and high costs, powder bed based AM processes are hardly usable for large-volume titanium components. As alternatives, AM processes using laser, arc and plasma welding processes (Directed Energy Deposition - DED) are currently being investigated. Here the industrial take-up is hindered by the long production times, the reproducibility and the high residual stresses. The central hypothesis of this proposal is that by manufacturing adapted preforms by means of AM, forging steps and tools can be saved and a high material utilization can be achieved. If the fine microstructure of the AM preforms can be transferred into the forged component, improved mechanical properties may be realized. Very little work on forging of additively manufactured preforms has been published up to now. Our own preliminary work shows that additively manufactured samples have significantly lower yield stress and higher globularization rates than conventional rolled bar stock. Thus, a reduction of the forming forces and tool load and a forging with a small oversize appears possible, since the required globularized volume fractions are already achieved at lower degrees of deformation. On the basis of the preliminary work it was hypothesized that the formation of martensite in the AM process and its decomposition upon heating are responsible for the unusual forming behavior. The aim of this project is to characterize the deformation, globularization, damage and anisotropy behavior by means of DED-produced Ti6Al4V samples under forging conditions as a function of the microstructure and the sample position relative to the weld beads. Also, a coupled material model for the forming and transformation behavior as well as the damage behavior and anisotropy of the additively produced preforms shall be developed. In order to allow for the widest possible variation of beta grain sizes and martensite fractions, preform production shall be carried out by means of powder-based laser metal deposition (small grains and fully martensitic structure possible) and wire-arc metal deposition (large beta grains and Widmannstätten structure). This research is intended to lay the foundations for the use of additively manufactured preforms for forging processes.