Jet on demand—A pneumatically driven molten metal jetting method for printing crack-free aluminum components

• A method to pneumatically deposit metal via an extended pressure pulse is described. • JOD deposits larger mass per deposition, causing lower cooling rates and good bonding. • High base plate temperatures can prevent hot cracking of printed Al-6061 parts. • T6 heat treatment produces similar microhardness in printed and wrought Al-6061 parts. Additive manufacturing (AM) of many traditional aluminum alloys is difficult due to hot cracking during cooling, which motivates investigating alternative AM methods that can mitigate this challenge. Here we demonstrate a new pneumatically driven molten metal jetting (MMJ) AM technique which uses a longer pressure pulse width to produce a jet of liquid metal that reaches the heated build plate. The “jet on demand” technique is utilized to build Al-6061 parts on heated build plates. Due to the large thermal mass contained in each jet, excellent adhesion is observed between droplets and layers while still maintaining dimensional control to produce parts with high relative densities (>98%). While as-printed parts exhibit different microstructure and hardness than traditional Al-6061, both microstructure and hardness are restored to traditionally processed values through a traditional T6 heat treatment. Microhardness values of 104 HV were obtained for printed Al-6061, which compares well to wrought properties. We observe that high build plate temperatures allow for lower solidification rates and eliminate hot cracking. These results point to a method for additively manufacturing traditional aluminum or other alloys that cannot currently be additively manufactured due to hot cracking.