Objective With its excellent thermal conductivity, copper is an ideal material for the critical complex cooling parts in the combustion chambers of aerospace engines. Increasingly, complex copper parts are being fabricated by selective laser melting, an additive manufacturing method that forms complex 3D parts and structures. However, bulk copper surfaces and spherical copper particles absorb less than 10% and 45% of 1064 nm laser energy, respectively. Melting the copper powder completely and fabricating sound copper parts without defects such as unmelted particles and holes are difficult tasks. Recently, we improved the laser absorptivity of copper powder by nano-TiC modification, and fabricated a sample with a density of 99.8% under a 1064 nm laser (340 W, 500 mm/s). Our modification technique provides a novel solution to the above problem.
Methods The reflectivities of nano-TiC, unmodified pure copper powder, and copper powder modified with 0.2%(mass fraction) nano-TiC were measured under lasers of different wavelengths. Measurements were performed in a UV-3600 plus ultraviolet spectrophotometer. The morphologies of the powders and nano-TiC particles were observed by scanning electron microscopy with energy-dispersive X-ray spectroscopy. The unmodified and modified copper powders were subjected to selective laser melting in a metal 3D printer (EOS M290) operated with a 400 W and 1064 nm laser under the same scanning conditions (340 W, 500 mm/s). The relative densities of the samples were determined by the Archimedes method. Their morphologies and hardness values were analyzed by a Nikon optical microscope and a Brinell hardness tester, respectively. The thermal and electrical conductivities of the samples were measured by a laser thermal conductivity meter and a Hall effect tester, respectively.
Results and Discussions The observation and analyses confirmed that: a) modifying the copper powder with 0.2% nano-TiC significantly reduced the reflectivity of copper to 1064 nm laser, and consequently increased the laser absorptivity from 22% to 53.7% (Fig. 4); b) the sample modified with the powder had a much higher relative density (99.8%) and significantly fewer defects than the sample fabricated from unmodified pure copper powder under the same conditions (340 W, 500 mm/s) (Fig. 5); c) modification with nano-TiC improved the Brinell hardness of the samples from 64.5 to 87.7 HB (Fig. 6); d) the modification improved the thermal and electrical conductivities of the samples from 120.8 W/(m·K) and 29.1% IACS, respectively, to 189.2 W/(m·K) and 42.1% IACS, respectively. The improved relative density fundamentally improved the performance of the samples.
Conclusions The proposed modification increased the relative density of copper powder from 90.7% to 99.8% and improved its laser absorptivity from 22% to 53.7% under a 1064 nm laser (340 W, 500 mm/s). The high relative density correspondingly improved the Brinell hardness, thermal conductivity, and electrical conductivity of the modified sample. Modification by nano-TiC is a novel and effective technique for increasing the laser absorptivity of copper and fabricating high-density copper parts by selective laser melting under low-power laser scanning.
