Liquid Metal Jetting for Printing Metal Parts

Liquid Metal Jetting (LMJ) is solid freeform fabrication process for producing metal mechanical parts and electronic interconnects. It is a technology similar to ink jet printing where individual molten droplets are accurately printed. LMJ will produce metal parts on demand from a CAD database with functional performance parameters similar to metal parts produced by machining or casting. By controlling solidification rates and metal alloy composition, LMJ is able to produce parts with unique properties such as metal matrices and functionally graded materials. This paper will review the current status of LMJ and future applications for this technology. One emerging manufacturing technology that addresses many challenges in solid freeform fabrication (SFF) is liquid metal jet printing (LMJP). The process is based on technology analogous to ink-jet printing. This agile additive method dispenses individually controlled microballs of molten metals to precise locations. Unlike spray forming and spray deposition process which spray materials in an uncontrolled manner, LMJP dispenses and controls every "single molten droplet" of material to a specific location using digitally stored computer-aided design (CAD) data in a highly reproducible manner. The direct-write, additive nature ofan LMJP system offers an agile approach. Potential applications for LMJP include the ability to rapidly fabricate 3-d mechanical parts and electronic circuitry. This paper discusses research issues in the development ofliquid metal jet printing systems. The technical issues that affect jet operation and the quality ofjetted materials are also discussed. Background of Jetting Research The Frenchman Nollet wrote in 1754 ofobservations made on a low-speed stream issuing from a small diameter nozzle [1]. He commented on the formation of drops, and the ability of a charged rod to deflect them. Lord Raleigh undertook the first thorough and accurate mathematical analysis of liquid jets in the 1870s [2,3]. Rayleigh's theoretical work explained the droplet disintegration mechanism as driven by surface tension induced instabilities. Basset [4] published a theory confirming the role of surface tension induced instabilities and the stabilizing effect of viscosity. Experimentalist A. Haenlein built a system to produce very long (up to 5 meter) water-, glycerinand gasoline-air jets under positive pressures and no external oscillation in 1931 [5]. Weber [6,7], used the data and observations ofHaenlein to generate the first cogent and useful analysis of a viscous cylindrical jet with both symmetric and transverse aerodynamic wave actions although the experimental results did not completely agree with the theory. Electro-mechanical forced stimulation, was studied by Hansell [8] in the 1950's. This greatly broadened the application for jetting. Jet applications changed from fuel injection to rocket propulsion to ink-jet printing. Lee and Spencer [9] used fuel injection mechanisms with fairly broad nozzle length to diameter ratios in the generation of high speed photographic studies ofliquid jets. McCormack et. al. [10] in 1965 described the essential elements ofa modem forced oscillation experimental water jet system employing a vibrating PZT ceramic crystal Jetting for building mechanical structures and parts, which is often called solid freeform fabrication (SFF), started with the use of wax and wax like materials. For example, a patent by Mitchell [11] discloses the generation of an object with liquid wax or similar type material using a jet printer. A later patent of Sanders Prototype shows a desktop jetting machine for generating wax parts. These systems used piezoelectric crystals which limited the systems to low melting point temperature waxes. Considerable research on solidification issues in jetting wax was performed by Gao and Sonin[12]. The jetting of molten metals became the natural next step for mechanical and electronic structures. The field of liquid metal jet printing started in electronics with low temperature solder applications on a suggestion by ffiM in 1972 [13]. Work during the 1980s by Heiber in solder jetting resulted in the first LMJP patent for Philips North American in 1989 [14]. The described drop on demand method utilized a lead zirconium titanate piezo-electric (PZT) crystal to generate