Laser-foil-printing (LFP) additive manufacturing is a laminated object manufacturing process developed at the Missouri University of Science and Technology. It has been used to build 3D-structural parts of Zirconium-based amorphous metals and crystalline metals such as carbon steel and stainless steel layer by layer. LFP uses a dual-laser system to weld each layer of metal foil onto the substrate or a previously fabricated layer and then cut the cross-sectional contour for the fabrication of each layer.

The LFP system consists of a continuous-wave (CW) fiber laser (IPG YLP-1000) for welding and an ultraviolet (UV) pulsed laser for cutting, as schematically shown in Fig. 1. The CW fiber laser subsystem includes a galvo-mirror scanner (SCANLAB) and an F-θ lens. The UV pulsed laser (Coherent AVIA-355X) subsystem includes optical reflection mirrors, a focal lens, and high-precision Aerotech motor-driven stages.

Fig. 2 Schematic illustrations of the six steps in LFP for the processing of each layer: (a) foil supply; (b) IR laser spot welding; (c) IR laser pattern welding; (d) UV laser contour cutting; (e) excess foil removal; (f) surface flattening.

• Higher Cooling Rate: The cooling rate of melt pool using the metal foil as the feedstock is high enough to generate fine crystalline grain structures or even amorphous structures if desired because the thermal heat of the melt pool can be conducted away efficiently through the foil, instead of powder whose thermal conductivity of powder is significantly lower than the foil.

• Lower Porosity: The formation of shrinkage pores can be minimized because the usage of foil does not involve high volumetric reduction during the melting and solidification process.

• Thicker Layer Thickness: In SLM, each layer of the powder-bed thickness is usually limited to 20-100 μm due to concerns on balling behavior and formation of pores. In a commercially available SLM machine, the laser power is normally limited to <400W in order to avoid possible powder blown away by the recoil pressure in case of a high-power laser. However, in LFP, the layer thickness of foil can be tens of micrometers to hundreds of micrometers to potentially enhance product productivity.

• Lower Material Cost: Compared with the cost of powder, the foil cost is only one fifth (e.g., 304L foil: $11-16/Kg vs. 304L powder: $70-80/kg).

• Higher Strength and Ductility: The tensile test results indicate that the LFP fabricated parts achieve ~15% and ~10% higher in yield strength and ultimate tensile strength, respectively, compared to the SLM fabricated parts. This is mainly because the use of foil feedstock in LFP leads to a higher cooling rate during the solidification of molten metal than the use of powder bed in SLM, due to higher thermal conductivity in foils than powders. The grain structure in SLM is coarser than the grain structure in LFP and shown in Fig. 6.

• Smaller Surface Area: The surface area of powder is ~10 times of surface area of foil per unit volume.

• Lower Oxygen Content Absorption: Because of smaller surface area, the oxygen content absorption could be reduced during melting and solidification processes in LFP.

• Safer Environment: Using foil as the feedstock can prevent explosion and inhalation hazards compare using the powder as the feedstock.

• More Material Manufacturability: Because of smaller oxygen content absorption, the LFP process can fabricate metal parts with a variety of materials, especially those materials are sensitive to the oxygen. For example, a dense Al-1100 part is hard to fabricate using SLM because aluminum is highly sensitive to the oxygen. However, a dense Al-1100 part can be easily fabricated using LFP.