Experimental and numerical investigation of the fatigue crack growth behaviour of residual stress-modified additive manufactured aluminium samples

增材制造为近净形结构的制造提供了新的可能性。尽管增材制造工艺不断改进，并且越来越多的材料被引入，但关于微观结构和残余应力对疲劳行为的影响的知识相对有限。在这个项目中，残余应力改性的铝镁合金丝基激光金属沉积（LMD）制造的试样将被调查方面产生的疲劳裂纹扩展行为。基于线的LMD促进相对高的沉积速率。然而，为此目的需要相当高的能量输入，从而导致高的温度梯度，这导致残余应力以及粗晶粒结构。残余应力修改技术，例如激光冲击硬化（LSP），使得能够在增材制造结构中局部调整残余应力和修改微观结构的创新可能性成为可能。在这种情况下，LSP的目标是改善增材制造试样的机械性能。该项目的重点是研究初始和修改后的微观结构以及增材制造试样的残余应力对疲劳裂纹扩展的影响。为了回答这个问题，耦合的实验-数值整体的方法。为此，AA 5087结构通过基于线的LMD制造，后处理，随后在微观结构和残余应力方面进行表征。LMD工艺以这样一种方式进行调整，即产生两种不同的微观结构，用于实验研究它们对疲劳性能的影响。为了阐明残余应力的影响，LSP处理的试样进行了分析。除了研究LSP对残余应力的改性，预计这将导致疲劳裂纹扩展性能的改善，还将解决由于LSP在增材制造试样中的微观结构变化的影响。将进行LMD和LSP过程的数值过程模拟，并将其结合在多步模拟策略中，以预测疲劳裂纹扩展行为，并阐明微观结构和残余应力对疲劳裂纹扩展的重要性。数值方法将有助于量化增材制造结构中的微观结构和残余应力对疲劳的影响性能.

Additive manufacturing offers new possibilities for the manufacturing of near-net-shape structures. Although additive manufacturing processes are constantly enhanced and more and more materials are introduced, the knowledge about the effect of microstructure and residual stresses on the fatigue behaviour is relatively limited. In this project, residual stress-modified specimens manufactured by wire-based laser metal deposition (LMD) of an Al-Mg alloy will be investigated in regards to the resulting fatigue crack growth behaviour. Wire-based LMD facilitates comparatively high deposition rates. However, considerable higher energy inputs are required for this purpose, causing high temperature gradients that result in residual stresses as well as in coarse-grained structures. Residual stress-modifying techniques, such as laser shock peening (LSP), enable innovative possibilities for the local adjustment of residual stresses and modifications of the microstructure in additive manufactured structures. In this context, the goal of LSP is to improve the mechanical properties in additive manufactured specimens. The focus of this project is on the investigation of the influence of both the initial and modified microstructure as well as residual stresses of additive manufactured specimens on the resulting fatigue crack growth. To answer this question, a coupled experimental-numerical holistic approach is taken. For this purpose, AA5087 structures are manufactured via wire-based LMD, post-processed and subsequently characterised with regard to microstructure and residual stresses. The LMD process is adapted in such a way that two distinct microstructures, intended for the experimental investigation of their influence on the fatigue properties, are generated. To clarify the influence of the residual stresses, LSP treated specimens are analysed. Besides the investigation of the residual stress modification by LSP, which is expected to lead to an improvement of the fatigue crack growth properties, the influence of microstructural changes due to LSP in additive manufactured specimens will be also addressed. Numerical process simulations of the LMD and LSP process will be conducted and combined in a multistep simulation strategy to predict the fatigue crack growth behaviour and to clarify the importance of the microstructure and residual stresses on it. The gained knowledge by the coupled experimental-numerical approach will contribute to the quantification of the influence of the microstructure and the residual stresses in additive manufactured structures on the fatigue behaviour.