The principal challenge in producing aerospace components using Ti-6Al-4V alloy is to employ the optimum process window of deformation rate and temperature to achieve desired material properties. Qualitatively understanding the microstructure-property relationship is not enough to accomplish this goal. Developing advanced material models to be used in manufacturing process simulation is the key to compute and optimize the process iteratively. The focus in this work is on physically based flow stress models coupled with microstructure evolution models. Such a model can be used to simulate processes involving complex and cyclic thermo-mechanical loading.

2.

Charles, Corinne

Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.

The microstructure and the mechanical properties of titanium alloys are highly dependent on the temperature history experienced by the material. The developed microstructure model accounts for thermal driving forces and is applicable for general temperature histories. It has been applied to study wire feed additive manufacturing processes that induce repetitive heating and cooling cycles. The microstructure model adopts internal state variables to represent the microstructure through microstructure constituents’ fractions. This makes it possible to apply the model efficiently for large computational models of general thermo-mechanical processes. The model is calibrated and validated versus literature data. It is applied to Gas Tungsten Arc Welding -also known as Tungsten Inert Gas welding- wire feed additive manufacturing processes.Four quantities are calculated in the model: the volume fraction of α phase, assumed interpreted to be Widmanstätten α, grain boundary α, and martensite α. The phase transformations are modelled based on diffusional theory described by a Johnson-Mehl-Avrami-Kolmogorov formulation, except for diffusionless α martensite formation where the Koistinen-Marburger equation is used. A parabolic growth rate equation is used also for the α to β transformation upon heating. An added variable, grain size indicator of assumed Widmanstätten α, has also been tested through the implementation of a simple Arrhenius law after parameter calibration.The coupling with physically based constitutive model gives first steps towards a more comprehensive and predictive model of the properties that evolve during processing.

The microstructure and consequently the mechanical properties of titanium alloys are highly dependent on the temperature history endured by the material. The manufacturing process of metal deposition induces repetitive cooling and heating in the material determining a specific microstructure. The presented study is devoted to developing and implementing a microstructure model for Ti-6Al-4V intended to be coupled to a thermo- mechanical model of the metal deposition process. Microstructural analysis of the metal deposited samples was first performed to understand the formed microstructure. A set of representative parameters for microstructure modelling were then selected as representative for the known impact of Ti-6Al-4V microstructure on mechanical properties. Evolution equations for these parameters were implemented for thermal finite element analysis of the process. Six representative state variables are modelled: the phase volume fraction of total alpha, beta, Widmanstätten alpha, grain boundary alpha, martensite alpha, and the alpha lath thickness. Heating, cooling and repeated re-heating involved in the process of metal deposition are taken into account in the model. The phase transformations were modelled based on a diffusionnal theory described by a Johnson-Mehl-Avrami formulation, as well as diffusionless transformations for the martensite alpha formation and the beta reformation during reheating. The Arrhenius equation is applied as a simplification to model temperature dependent alpha lath size calculation. Grain growth is not included in the present formulation, but would have to be added for capturing alpha lath coarsening during long term heat treatment. The temperature history during robotised tungsten inert gas deposition welding is simulated together with the microstructure. The implementation of the model handles well the complex cyclic thermal loading from the metal deposition process. A particular banded structure observed in the metal deposited microstructure is partially explained using the proposed microstructure model. It is concluded that although qualitatively interesting results have been achieved, further calibration testing over a wider range of temperature histories must be performed to improve the transformation kinetic parameters for reliable quantitative predictions of the microstructure.

An integrated design of material and process is necessary when designing a component where the effect of the manufacturing route on its performance must be accounted for. This is particularly the case for welded components even when post weld heat treatment is performed. The paper describes developments done at Luleå University of Technology in cooperation with Volvo Aero in the Swedish National Programme for Aeronautical Research (NFFP) and in different European projects. The paper focuses on two particular issues of importance. The first is of more administrative character, the transfer of data between different finite element models used in each of the manufacturing steps. The other aspect is the extremely important issue of material modeling.Material models for simulation of a chain of manufacturing processes include additional complications besides large variations in strain rates and temperatures. These complications are caused by the changing microstructure that may occur. The authors expect that physically based models can have a larger range of applicability than engineering type of models. Physical based models are formulated by considering the underlying physics of the deformation whereas engineering type of models are more of a curve-fitting nature. The physical based models may also have a natural coupling to models of the microstructure evolution. However, the models must still be tractable for large-scale computations. Thus, they should be of the internal state variable type with relatively few additional parameters and equations to solve at the integration point level of finite elements. The paper describes a basic dislocation density model used in modelling different manufacturing processes and how it can be coupled to microstructure models. It is based on dislocation glide as the dominating mechanism for the plastic deformation. This may be models for phase changes, like in Ti6-4, or precipitate growth/dissolution as in Alloy 718. The coupled models will not only make it possible to describe the material behavior more correct over the process cycles but also predict the obtained microstructure. It is expected that future research may couple this information with defect predictions in order to contribute to life assessment. The paper includes some example of manufacturing simulations and also an example of simulation of a chain of manufacturing processes.