The Physics Problem Behind Copper LPBF

Copper conducts electricity at 100% IACS and heat at 398 W/m·K — second only to silver, and about 60% higher than aluminum. Yet point a standard infrared laser at it and almost nothing happens: solid copper absorbs only about 5% of the 1064 nm beam. The physics problem behind copper LPBF is not whether copper can melt. It is whether the light goes in at all.

The Laser Reflectivity Problem

Pure copper reflects 90–98% of infrared laser energy. The reason is electronic, not chemical: copper's conduction electrons act as a near-perfect mirror for infrared photons, re-radiating the incoming field instead of absorbing it. What does penetrate — roughly 5% at 1064 nm — is all the laser delivers to the part. Laser reflectivity this high is a property of the free-electron metal, not a process defect.

Two Losses Compound

The absorbed fraction then meets a second obstacle. Copper's 398 W/m·K thermal conductivity drains heat away from the melt spot faster than it can accumulate, so powder bed fusion copper on infrared machines often stalls at 80–90% relative density and 50–70% IACS. The loss is mechanical, not chemical — pores, not impurities, break the conduction path.

The Green Laser

Green laser AM changes the physics. At 515 nm, solid copper absorbs roughly 40% of the beam — about eight times the infrared figure. The mechanism is an interband transition: green photons carry enough energy to lift electrons from the d-band to the s-band, so the free-electron mirror breaks down. The inversion is instructive — in the green, solid copper absorbs more than liquid copper (40–60% versus 25–50%), the opposite of infrared behavior. With coupling fixed, green-laser LPBF reaches ~99.8% relative density and 98–100% IACS, essentially annealed-copper conductivity.

The Boundary Condition

The rule that infrared cannot print copper has a limit. High-power infrared systems, running in keyhole mode where the beam reflects internally inside the vapor cavity instead of escaping, also densify copper — once density passes ~98.5%, infrared prints approach 90–96% IACS. The deeper rule, confirmed by de Terris et al. (2023) and a 2025 Metals review, is that conductivity tracks density: fix the porosity and the conductivity returns, regardless of wavelength.

Why It Matters

The payoff concentrates in two places. Thermally, conformal cooling and LPBF heat exchangers gain up to 30% efficiency. Electrically, parts at 98–100% IACS behave like machined copper. Understanding why the light bounces off is the first step to making it stay — by Jiangsu Vilory.