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Additive Manufacturing Essentials

2.6 Solid Material-Addition Processes

Additive Manufacturing Essentials2.6 Solid Material-Addition Processes

2.6 Solid Material-Addition Processes

Learning Objectives

By the end of this section, students will be able to:

  • Understand sheet lamination processes.
  • Comprehend cold spray technology.

Sheet Lamination Process Architecture

Sheet lamination process architecture is defined as an additive manufacturing process in which an ultrasonic transducer attached to a 3-axis gantry fuses feedstock of thin metallic foil that is coiled approximately .5” wide with a thickness similar to aluminum foil used in everyday household needs. After a few layers of ultrasonic bonding of the foil feedstock, a mill cutter is introduced into the process and material is machined away to provide the feature definition needed to fabricate the part. After the part is completed, the part exhibits a smooth machined surface. This process is known as ultrasonic consolidation.

Due to the relatively low temperature solid state bonding process, ultrasonic consolidation has unique benefits compared to other AM processes. These include:

  • The ability to embed temperature sensitive objects into the part during fabrication. Some examples include strain gage wires, sensors, fiber optics, printed circuits, etc.
  • Being able to bond very dissimilar metals together that would typically be difficult to bond using a high heat fusion process.

It is worth noting that there exists an alternative sheet lamination process that involves adhesive backed paper rolls that are layered. At each layer, the paper is cut to the desired profile with a laser cutter on a gantry system. The resultant wooden part is mainly used for basic prototyping. This type of AM technology is known as Laminated Object Manufacturing (LOM) or Selective Laminated Deposition (SLD) and is very niche with limited application space. For the sake of brevity, it will be excluded from the sheet lamination discussion with a sole focus on ultrasonic consolidation.

Commercially available ultrasonic consolidation machines are capable of a build volume of up to 6ft x 6ft x 3ft. The fundamental systems that make up an ultrasonic consolidation system includes:

  • Foil Feed Mechanism
  • Transducer
  • Horn
  • Booster
  • 3 Axis Gantry System
  • Baseplate Substrate
  • Mill Cutting System
  • Computer / HMI

Delivery Systems

The transducer and horn fusion head are directly mounted to a X-Y gantry system. The gantry system moves the horn to the desired location for fusion. The foil feedstock is fed into the work area. A specified amount of force is applied from the horn onto the foil to hold it in place while the ultrasonic vibration motion takes place. After a few layers are deposited, a CNC programmed mill cuts away undesired material from the consolidated metal part profile. Water soluble material may be used as a support structure if needed for overhanging features.

Energy Sources

Ultrasonic consolidation occurs primarily through the energy transfer of the transducer which generates ultrasonic vibrations at 20 kHz. The frequency is a static value whereas the amplitude value may be varied depending on the type of metal being fused. The sonotrode, which is a disc shaped horn, transmits the ultrasonic vibrations to the foil feedstock that are being fused together.

Materials

There are a wide variety of metals available for the ultrasonic consolidation process, some of which may be bonded together to create functionally gradient metal. Some examples have been demonstrated include, steel-nickel, tantalum-steel, aluminum-titanium, aluminum-copper. For single metal bonding, the typical materials used in the ultrasonic consolidation process include:

  • Aluminum alloys
  • Copper alloys
  • Low alloy steel
  • Titanium

Cold Spray Process Architecture

Cold spray technology is commonly used as a coating process; however, it has been demonstrated to be effective for freeform AM. Despite being named ‘cold’ spray, the process is heated just below the melting temperature of the metal being deposited and is anything but ‘cold’.Cold spray is an impaction process where metal powder is discharged from a gas nozzle jet at extremely high velocities. These metal powder particles impact onto a surface and buildup material based on force. Care must be taken to tune process parameters to ensure part integrity and avoid delamination and porosity in the resulting built up structure. Once the bulk geometry is deposited, the structure is transported to a 5-axis mill for final machining.

Figure 2.8 Functional diagram of cold spray system

Delivery Systems

There are a number of variations to the delivery systems for cold spray. The first includes a gas nozzle jet moving in a linear motion spraying material on feedstock on a lathe producing a gradual buildup. Second, the nozzle jet may remain stationary below a tilt table shooting material vertically while a 5-axis tilt table receives the material from the nozzle creating a 3D freeform structure (shown in F02_19). A third approach includes a substrate plate mounted vertically in a stationary format while a nozzle delivers material horizontally against the build up baseplate.

Energy Sources

The originating source of energy for cold spray technology is highly compressed heated gas. The heated gas acts as a supersonic velocity carrier to transfer the metallic powder to a substrate to be impacted on the surface up to 4000 feet/sec. The kinetic energy of the particles is transferred to plastic deformation energy in the bonding process. The gas composition is typically either helium, nitrogen, or compressed air above 15 bar resulting in a flow rate of more than 71 ft3/min. The power required for the heating the gas. can range from 3-5kW.

Material

Similar to other AM powder based systems, cold spray is sensitive to powder particle size variation. Ceramics and metal powders can all be used for cold spray. Some of these include:

  • Aluminum
  • Copper
  • Nickel
  • Titanium
  • Zinc
  • Tantalum
  • Niobium
  • Tungsten
  • Zirconium
  • Steel
  • Copper-Tungsten
  • Al-SiC
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