Cutting mechanism and surface integrity in milling of Ti-5553 processed by selective laser melting†

Titanium alloys are of significant importance in several high performance applications such as aerospace components or medical implants. Advances in additive technologies lead to an increase of additively built workpieces, offering new possibilities regarding functional integration and lightweight structures. Several authors have shown that microstructural, mechanical and thermal material properties differ significantly from those of cast alloys. Although additively produced parts are near net-shape, most of them are machined after the building process, to achieve the requirements regarding surface finish and dimensional accuracy. Titanium is generally known as a hard-to-cut material due to its thermo-mechanical properties. Although there is a profound knowledge about the machinability of conventionally cast and wrought titanium alloys, there is a lack of understanding regarding the machining of additively built titanium. In this paper, the machinability of an additively built Ti-5Al-5V-5Mo-3Cr alloy (Ti-5553) is analyzed regarding chip formation, cutting forces and tool wear. Three different generation methods, conventional wrought, selective laser melting and selective laser melting with in-process (insitu) heat treatment are investigated. It is shown that the machinability differs significantly compared to a conventional wrought alloy, which is linked to decreased contact length on the tool rake face and higher mechanical tool load (up to + 23 % force increase). Highest tool load is found for titanium alloy, which is built using selective laser melting and in-process heat treatment. Furthermore, the Surface integrity after machining is analyzed regarding hardness, roughness and residual stresses. Hereby, high compressive residual stresses for additively built titanium with in-process heat treatment are obtained due to the higher mechanical tool load. Therefore, it could be shown that the generation method (e.g. selective laser melting) needs to be considered for the later process design in finish machining.