Hot working and damage behaviour of additively manufactured Ti6Al4V

由于目前CFRP的使用趋势，铝结构部件在航空中越来越多地被钛部件所取代，因为超高强度铝合金与CFRP接触容易腐蚀。目前，钛合金在空客A350上的份额已经达到14%左右（相比之下，A320为3%）。对于锻造钛构件，材料利用率有时低于10%。由于钛的生产复杂，成本高，低的材料产量是无法忍受的。目前研究的增材制造（AM）工艺在单件和小批量生产中具有很大的优势，即近净形状生产。由于低堆积率和高成本，基于粉末床的增材制造工艺很难用于大容量钛部件。作为替代方案，使用激光、电弧和等离子焊接工艺（定向能沉积- DED）的增材制造工艺目前正在研究中。生产时间长、再现性差和残余应力高阻碍了工业生产。该提案的中心假设是，通过AM制造适应的预制体，可以节省锻造步骤和工具，并实现高材料利用率。如果能将增材制造预制体的优良组织转移到锻造件中，则可以实现机械性能的改善。迄今为止，关于增材制造预制体锻造的研究还很少。我们自己的初步工作表明，增材制造的样品具有显著降低屈服应力和较高的全球化率比传统轧制棒材。因此，由于在较低变形程度下已经实现了所需的球状体积分数，因此减小成形力和工具载荷以及具有小尺寸的锻件似乎是可能的。在初步工作的基础上，提出了增材制造过程中马氏体的形成及其在加热时的分解是造成这种异常成形行为的原因。本项目的目的是通过d - d生产的Ti6Al4V样品在锻造条件下的变形、球化、损伤和各向异性行为，作为微观结构和样品相对于焊接珠的位置的函数。此外，还应建立增材预制件的成形和转变行为以及损伤行为和各向异性的耦合材料模型。为了允许最大限度地改变β晶粒尺寸和马氏体组分，预制体的生产应通过粉末基激光金属沉积（小晶粒和完全马氏体结构可能）和线弧金属沉积（大β晶粒和Widmannstätten结构）进行。本研究旨在为在锻造工艺中使用增材制造预制体奠定基础。

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.