A New Concept for Modeling Phase Transformations in Ti6Al4V Alloy Manufactured by Directed Energy Deposition.

Jérôme Tchoufang Tchuindjang,Anne-Marie Habraken,Hakan Paydas,Laurent Duchêne,Raoul Carrus,Anne Mertens,Hoang-Son Tran

doi:10.3390/ma14112985

Jérôme Tchoufang Tchuindjang, Anne-Marie Habraken + Show 5 more

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https://doi.org/10.3390/ma14112985

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Abstract

The microstructure directly influences the subsequent mechanical properties of materials. In the manufactured parts, the elaboration processes set the microstructure features such as phase types or the characteristics of defects and grains. In this light, this article aims to understand the evolution of the microstructure during the directed energy deposition (DED) manufacturing process of Ti6Al4V alloy. It sets out a new concept of time-phase transformation-block (TTB). This innovative segmentation of the temperature history in different blocks allows us to correlate the thermal histories computed by a 3D finite element (FE) thermal model and the final microstructure of a multilayered Ti6Al4V alloy obtained from the DED process. As a first step, a review of the state of the art on mechanisms that trigger solid-phase transformations of Ti6Al4V alloy is carried out. This shows the inadequacy of the current kinetic models to predict microstructure evolution during DED as multiple values are reported for transformation start temperatures. Secondly, a 3D finite element (FE) thermal simulation is developed and its results are validated against a Ti6Al4V part representative of repair technique using a DED process. The building strategy promotes the heat accumulation and the part exhibits heterogeneity of hardness and of the nature and the number of phases. Within the generated thermal field history, three points of interest (POI) representative of different microstructures are selected. An in-depth analysis of the thermal curves enables distinguishing solid-phase transformations according to their diffusive or displacive mechanisms. Coupled with the state of the art, this analysis highlights both the variable character of the critical points of transformations, and the different phase transformation mechanisms activated depending on the temperature value and on the heating or cooling rate. The validation of this approach is achieved by means of a thorough qualitative description of the evolution of the microstructure at each of the POI during DED process. The new TTB concept is thus shown to provide a flowchart basis to predict the final microstructure based on FE temperature fields.

Highlights

Understanding the mechanisms that trigger phase transformations during additive manufacturing (AM) remains a major issue, as the final microstructure strongly influences the mechanical properties
The present paper introduces a novel concept, namely the time-phase transformation-block, which will help numerical teams to select only parts of the thermal history and still keep the key thermal features governing the final microstructure
Each pair corresponds to the two adjacent tracks belonging to the same layer, located close to the related points of interest (POI)

Summary

Introduction

Understanding the mechanisms that trigger phase transformations during additive manufacturing (AM) remains a major issue, as the final microstructure strongly influences the mechanical properties. Each AM process produces different phases in type, morphology and volume fraction [13,18,22,24,25] Those microstructures lead to distinct mechanical properties that can be potentially improved by post heat treatments [10,16,18,24,26]. None of these approaches focus on the accurate prediction and control of microstructural evolution during AM

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