Additive Manufacturing of Ti-6Al-4V: Relationship between Microstructure, Defects and Mechanical Properties

Sammanfattning: Additive manufacturing (AM) is a relatively new technology that is labelled to be innovative, disruptive, near-net shaping, enabling manufacturing of complex and customised products, for limitless number of applications, directly from the CAD model into real physical parts. For titanium alloys in aerospace applications, AM moreover stands for a reduced material cost, but also for large challenges when considering consistency and qualification of material properties and components in serial production. In the AM process the feedstock material is melted by a heat source that moves according to a building sequence defined by the CAD model. Layer-by-layer the material solidifies into the wanted shape and accordingly the microstructure forms, which determines the average mechanical properties of the manufactured component. However, even if the AM process seems to be very straight forward, the prediction of mechanical and metallurgical properties is complex, partly because of its building in layer nature which generates a complex thermal history dictating the mechanical properties, and partly because of the number of parameters involved during the AM process itself. The objective of the present work was to increase the fundamental understanding of the relationship between microstructure, defects and mechanical properties of AM:ed Ti-6Al-4V. Three AM techniques were investigated, namely laser metal-wire deposition (LMwD), electron beam melting (EBM), and Shaped Metal Deposition (SMD), with the main focus on LMwD. The different techniques were evaluated with regard to microstructure and tensile and fatigue properties. In addition, the EBM Ti-6Al-4V was tested in a hydrogen atmosphere to simulate the working environment for a certain engine application. One of the core findings in the present work was that AM:ed Ti-6Al-4V exhibited a columnar microstructure with elongated prior beta grains growing through several layers following the temperature gradient direction in the built material. To cover the different characteristics of the columnar microstructure, the mechanical properties were evaluated in both these orientations of the built Ti-6Al-4V. The mechanical properties, both static and dynamic, were found to be anisotropic, which was further evaluated in detail with respect to the microstructure evolution and defects generated by the AM process. Among the results, when different process conditions were tested, it was concluded that the thickness of the grain boundary alpha along the prior beta grain boundary did not influence the level of anisotropy. However, the prior beta grain boundary was observed to be the weakest microconstituent when the load was applied perpendicular to its prevalence in both tensile and LCF testing. In order to get a better understanding of how the columnar microstructure influences the fatigue properties, the fatigue crack propagation characteristics were investigated with respect to the columnar prior beta grains and crystal orientation. An extensive fractographic study was carried out on all tested specimens. Lack of fusion (LoF) defects were concluded to be the individually most detrimental type of defect to the material properties. The influence of the LoF defects was further concluded to be very dependent on its prevalence in relation to the loading direction; the largest impact on the fatigue life was observed when the LoF defect was perpendicular to the loading direction. Finally, a part of the aim of the present work was to support the development of a microstructure model that will be implemented in a thermo-mechanical model when simulating AM of Ti-6Al-4V. In order to validate the material model developed, the alpha lath thickness and the fraction of grain boundary alpha were quantified at specific locations in single and multiple bead walls of SMD:ed Ti-6Al-4V and compared with the results of the simulated AM process of Ti-6Al-4V.