Electron beam melting (EBM) is a promising technology to manufacture various alloys with outstanding properties;however,the number of available alloys is limited.We propose in situ alloying to acceler-ate the development of advanced and novel alloys,based on thermophysical calculations and CALPHAD approach,during the EBM process.We demonstrate our concept through the design and fabrication of high entropy alloys (HEAs).Three CoCrFeNiMn-xTi (x=0.18,0.50,2.00,in molar ％) HEAs are manufac-tured.EBM-built HEAs achieve a homogeneous distribution of elements while forming multiphase alloys resulted from the hot powder bed.The topological structures formed by secondary phases contribute to an increase in the hardness of EBM-built HEAs up to 900 HV1.Considering alloy design,a systematic analysis on CoCrFeNiMn-038Ti HEA elucidates the microstructural evolution in detail.These findings pro-vide a deep understanding of in situ alloying and pave the way to develop new alloys specific to the EBM process.