The Royal Australian Navy has a fleet of customized Collins Class submarines, which were developed from five generations of submarines designed and built by the Swedish Navy. These state-of-the-art computer-designed submarines, however, have to surface and dock at the nearest port if they develop a major fault that requires lengthy repairs.

Australia’s dedicated submarine sustainment organization, ASC, is working on a solution that would avoid such situations and save time and costs. It is partnering with Australia’s national science agency, the Commonwealth Scientific Industrial Research Organisation (CSIRO), and DMTC Limited, to enhance the cold spray additive manufacturing (CSAM) technology to repair damaged metal surfaces in situ, or on site.

If successful, the Australian submarines will be able to use CSAM to perform lengthy repairs while at sea without the need to dock.

(Image courtesy of ASC PTY Ltd.)

Cold spray is an additive manufacturing and repair method that uses a stream of gas at supersonic speeds (MACH 3) to accelerate metal powder particles at a surface. When the metal powder particles collide with the part’s surface, the high kinetic energy causes plastic deformation, creating mechanical interlocking and metallurgical bonding. This allows for bonding of the exposed metal surfaces under high local pressure, and thick layers of deposited material to be built up rapidly.

High-pressure systems, as opposed to low-pressure ones, achieve higher deposition rates in production.

(Image courtesy of Digitalalloys.com.)

The heat generated during this interaction is not sufficient to melt the material since the process occurs below the melting temperatures of the metals involved—hence, the term “cold spray.” This process prevents the structural integrity of the components and surrounding area from becoming damaged.

“The use of additive manufacturing for the repair of critical submarine components, including the pressure hull, will mean faster, less disruptive repairs for our front line Collins Class submarine fleet,” said ASC Chief Executive Officer Stuart Whiley in a June 2 press statement.

CSIRO and ASC have been jointly exploring ways to use the cold spraying of nickel to repair corrosion for the last 4 to 5 years. CSIRO has spearheaded the adoption of cold spray by the Australian industry since first introducing the technology 18 years ago.

The organization has developed cold spray-based solutions for the printing industry, aerospace, and rail, as well as for combating marine biofouling, according to research team leader at CSIRO, Peter King. The project will also leverage DMTC’s long history in developing cold spray as a repair technology for defense applications.

CSAM is gaining a lot of traction in the manufacturing sector. Conventional metal additive manufacturing (MAM) technology has its benefits, but its low deposition rate presents an often-insurmountable hurdle when it comes to manufacturing large parts and prototypes, according to the National Research Council (NRC) of Canada.

In comparison, cold spray technology combines high feed rates (currently up to 14 kg/h) with excellent deposition efficiencies on all types of materials and without any heat affected zone—the heat input being very limited.

Even the U.S. Army is sold on the potential of the cold spray AM technique.

In September 2019, Worcester Polytechnic Institute (WPI) in Massachusetts said that it received a three-year, $25 million award from the U.S. Army Combat Capabilities Development Command Army Research Laboratory (CCDC-ARL) to develop cold spray additive manufacturing for use in the repair and production of metal parts.

WPI’s primary focus in this research is on developing, characterizing, and testing new alloys optimized for use in cold spray. Unlike metals used in other metal manufacturing processing, including casting and forging, the alloys used in cold spray do not have to be capable of being heat treated. This gives the WPI researchers access to a wide range of potential materials. The properties of cold spray powders can be fine-tuned with the careful application of heat.

Since cold spray doesn’t require heat to bind the materials, maintenance units can perform repairs on thermally sensitive components that would normally melt or would otherwise be compromised with a traditional approach like thermal spray or welding.

The technology also grants the user a tremendous amount of control and precision over the location of the coating and its properties. The Army expects to save an estimated $23.6 million per year and almost $300 million in total due to cold spray.

CSAM has typically been limited to malleable materials. This makes it well suited to structural elements and alloys but not for functional materials that are typically brittle.

Researchers at Lawrence Livermore National Laboratory (LLNL) said in May that they have partnered with TTEC Thermoelectric Technologies to extend the range of materials that can be cold sprayed as part of the U.S. Department of Energy’s Technology Commercialization Fund (TCF) program. CSAM, hence, remains a technique to watch out for.