Predicting Thermal History, Microstructure and Hardness of Wire Arc Additive Manufactured Parts

Abstract

Wire Arc Additive Manufacturing (WAAM) is a manufacturing technique with the ability to produce large metal parts with relatively complex geometrical shapes. One obstacle limiting full exploitation of WAAM in the industry is uncertainty on the mechanical properties of manufactured parts. Mechanical properties are strongly influenced by the microstructure of the material which is a product of the thermal history of the material. Thermal history of parts manufactured with WAAM is complex which leads to uncertainty on mechanical properties and microstructure. Therefore a methodology to predict thermal history, microstructure and hardness of metal-based additive manufactured parts is presented, validated and applied in this study. The methodology is applied to high strength low alloy s690 steel parts produced with WAAM. First a part level thermal process model is utilised to predict the thermal history. Subsequently, a model is developed to predict the microstructure by modelling microstructure phase transformations. Based on thermal history and microstructure phase fractions, Vickers hardness is predicted. Predictions are validated with experimental measurements and observations obtained from a collaborating master thesis. Part temperatures of the thermal model agreed well with measurements. Although the predicted microstructure phase fractions show a lower ferrite content than experimentally observed, the trend of the microstructure phase fractions along the part height is the same in the prediction and the experiment. The predicted hardness is 49 HV higher than the measured hardness. However, hardness predictions and measurements show the same trend along the part height. The methodology to predict the thermal history, microstructure and hardness was applied on a study in which the height of a wall was varied and on a study on a geometry in which two walls cross each other at the center. For both studies the martensite content decreases over the part height, whilst the bainite content increases over the part height. This is caused by the decrease of cooling rate over the part height. Hardness also decreases over the part height. For both the change in microstructure phases and the hardness over the part height, the largest change was observed at the bottom of the part. For the study with the crossing geometry, no significant difference was found for the microstructure and hardness between the center and side of the crossing.