Colloquia

Summary

This project investigates the fundamental science and engineering of microstructure in Ti-6Al-4V additively manufactured by laser powder bed fusion (LPBF). Enabling a way to tailor microstructure necessitates a breakthrough to enhance the understanding of the process and make it more robust. However, there is limited knowledge on how to allocate microstructures for obtaining site-specific properties. In this project, the following knowledge gap is addressed; Can alterations be made to the LPBF process to be able to control the outcome of microstructures of Ti-6Al-4V?

LPBF is a highly anticipated technique with unique opportunities. It is an additive manufacturing technique that finds its basis in powder bed manipulation by laser illumination. With this technique, it is possible to create objects that are impossible to manufacture using conventional techniques. During the process powder (metallic, ceramic, plastic) is deposited onto a bed. A cut-through of a designed product is projected onto the bed whereafter a laser illuminates the designated areas and fuses the powder. Hereafter a new powder layer is deposited on top of the previous and the cycle repeats until the build is finished. During this process, high heating and cooling rates are very common. This is due to the small spot size of the laser and the high energy input needed to melt the powder. When a material is cooled down rapidly this has implications on the microstructure that forms in the material. Microstructures have a big impact on the mechanical properties of a product. The driving factors that determine the formation of a certain microstructure are cooling rate and cooling time. If unwanted microstructures occur in a product it is needed to give the product a heat treatment which is costly and very time-consuming.  

To explore the main mechanism that influences microstructural evolution and formation, modeling tools built upon the Rosenthal equations are addressed and two different approaches were developed. The first approach depended on the temperature difference on the surface, and the second on the difference at melt pool depth. Accordingly, a set of parameters that have an influence on microstructural evolution was investigated. 

A range of re-heating power and cycles was chosen. To evaluate the impact that the Jonker heating method had made on the samples X-ray diffraction and scanning electron microscopy analyses were used. Assess the data showed that using the Jonker heating method the microstructural composition can be changed using the Jonker heating method, leading to a method that can be used to influence the microstructure.