Investigation of effects of heat treatment and hot isostatic pressing on micro-drilling of additively manufactured Ti6Al4V alloy

Additive manufacturing (AM) is an innovative and promising method through its limitless product design possibilities in critical sectors such as medicine, aviation, and automotive. However, the material properties and finish part quality of the parts fabricated by AM must meet the requirements of these sectors. Therefore, investigation of the effects of post-processing applications (heat treatment (HT) and hot isostatic pressing (HIP)) on the material properties, and machinability characteristics of AMed parts is one of the critical issues for both academia and industry. This study investigates the micro-drillability of HT and HIPped Laser Powder Bed Fusion (LPBF) and wrought Ti6Al4V alloys under dry and minimum quantity lubrication (MQL) conditions using conventional and peck drilling strategies. The micro-drilling tests were performed at two spindle speeds (5000 and 10000 rev/min) and a feed per tooth (4 μm/tooth) using uncoated WC-Co micro drills with a diameter of 0.7 mm. Some critical performance criteria such as thrust force, surface roughness, burr formation, tool wear, and the changes in subsurface microstructure and microhardness of the hole walls were analyzed in the scope of the micro-drillability tests. The test results showed that due to their different microstructural, physical, and mechanical properties, HT LPBF and HIP LPBF Ti6Al4V alloys exhibited different machinability characteristics than wrought ones. In this context, it was found that the HIP LPBF specimen contains higher β phase fraction in its microstructure and thus, has maximal hardness value while the wrought one has lower β phase fraction. The HIP LPBF specimen produced higher thrust force values due to its higher hardness value. Minimal roughness value was measured after micro-drilling of the wrought specimen with a lower force value. The wrought and the HIP LPBF specimens produced minimal and maximal burr heights in harmony with the thrust force results, respectively. In the HIP LPBF specimen, the thicker plastically deformed layer was measured owing to its higher thrust forces.