Highly conductive and strong CuSn0.3 alloy processed via laser powder bed fusion starting from a tin-coated copper powder

Abstract

Despite the high demand, the successful fabrication of fully dense, highly conductive, and strong copper components via laser powder bed fusion (LPBF) is not readily evident. This is mainly due to the low optical absorption of copper, which inhibits the complete melting of copper powders when using commercially available fiber-laser-based LPBF machines. Accordingly, this article proposes a novel approach of using optically absorptive metal-coated copper powders for the fabrication of fully dense, highly conductive, and strong copper components via LPBF. To validate this approach, the surface of the copper powder is modified by applying a very thin (62 ± 14 nm) layer of metallic tin via an immersion plating technique. The application of only a 0.28 wt% of metallic tin coating significantly improved the room temperature powder optical absorption by ~170% at the fiber laser wavelength. Consequently, crack-free and fully dense copper parts combining high thermal conductivity of 334 ± 4 W/(m•K) and electrical conductivity of 80 ± 1% international annealed copper standard (%IACS) with a good tensile strength of 256 ± 14 MPa, yield strength of 203 ± 4 MPa, and ductility of 21 ± 2 % have been fabricated using a fiber laser with an output laser power of 500 W.Furthermore, the article also describes the negative influence of the presence of a high amount of (0.091 wt%) sulfur, which originates from the organic additives during sub-optimal coating conditions, on the LPBF processing behavior of CuSn alloys. As such, high sulfur-containing tin-coated copper powders do not allow the fabrication of fully dense parts due to the occurrence of solidification cracks and the formation of pores. Subsequently, the optimum tin coating methodology, which limits the amount of sulfur below 0.0025 wt%, is described. The use of sulfur-free powder is recommended for the successful LPBF processing of fully dense parts made of copper and copper alloys.

A Micro-Computed Tomography Comparison of the Porosity in Additively Fabricated CuCr1 Alloy Parts Using Virgin and Surface-Modified Powders

“…Hence, while a high E v is needed to start the melting process, the same high E v causes vortices, metal vaporization, and high fluid speeds entrapping metal vapors or shielding gases at the bottom of the laser line track. This behavior was discussed already by Jadhav et al for different copper alloys [17,42] and is one of the causes for their small LPBF processing windows.…”

“…Thanks to new advancements, the processability problem of pure copper or high-copper-containing alloys has been almost completely solved. It concerns both LPBF machine hardware improvements, with lasers having higher power or different emitted wavelengths [14,15], and advancements on the proper selection and modification of the starting raw powder material, with surface-modified powders [16][17][18] or tuned copper alloys [19,20]. Nevertheless, still a higher porosity and higher number of defects are expected from parts manufactured in pure or nearly pure copper, with densities often below the industrial average requirement of ≥99.7% for LPBF.…”

“…For example, densities up to 99.1% were reported by Yan et al for pure copper; similar densities were achieved by Jadhav et al [21] and others [22] with the employment of a high-power (≥1 kW) fiber laser. In the realm of high-copper-content alloys, densities above >99% are reached for, e.g., CuCr1Zr (99.84% in [19]), CuSn0.3 (99.6% in [17]), or CuCr1 (99.1% in [16]).…”

Laser-based powder bed fusion additive manufacturing of pure copper

“…Therefore, the use of green or blue laser may broaden the L-PBF processing window, and copper parts exhibiting full density could be fabricated in both conduction and keyhole modes, similar to the conventional infrared fiber laser (λ = 1080 nm) based L-PBF processing windows of Ti-6Al-4V and SS 316L alloys. Besides, a broader L-PBF processing window against infrared lasers for highly conductive copper and copper alloys can be achieved by utilizing surface-modified copper powders which exhibit high optical absorption, as validated by Jadhav et al [27,[50][51][52] and Lindström et al [53]. However, the latter approach is valid when, at least, a small amount of alloying is permitted.…”

“…As such, the keyhole mode melt pool exhibits much higher effective laser absorption compared to the conduction mode melt pool [46]. Since the production of bulk solid copper parts with full density requires a high effective laser absorption, the L-PBF parameters which have a greater influence on the keyhole mode formation will logically have a greater influence on the part densification, especially in the case of highly conductive and optically reflective metals [50,55]. Consequently, the laser power has the highest influence on the part densification behavior, followed by the laser scan speed inside the designated array of L-PBF settings (However, within a broader L-PBF parameter set, the melt pool depth is supposed to increase linearly with increasing applied laser power, while it is supposed to decrease at ≈ (1 / (1 + v)) rate with increasing laser scan speed (v), refer to Equations (1 & 2)).…”