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In a recent article published in the journal Additive Manufacturing Letters, researchers discuss the laser melting process for copper composites based on 316L stainless steel.
Research: Synthesis of 316L stainless steel-copper composites by laser melting. Image credit: Pedal in stock / Shutterstock.com
Although heat transfer within a homogeneous solid is diffuse, heat can travel through a solid mass along the path of least resistance. In metal foam radiators, it is recommended to use anisotropy of thermal conductivity and permeability to increase the heat transfer rate.
In addition, anisotropic thermal conduction is expected to help reduce parasitic losses caused by axial conduction in compact heat exchangers. Various methods have been used to change the thermal conductivity of alloys and metals. Neither of these approaches is suitable for scaling up directional control strategies for heat flow in metal components.
Metal Matrix Composites (MMC) are produced from ball milled powders using laser melting in powder bed (LPBF) technology. A new hybrid LPBF method has recently been proposed to fabricate ODS 304 SS alloys by doping yttrium oxide precursors into a layer of 304 SS powder prior to laser densification using piezoelectric inkjet technology. The advantage of this approach is the ability to selectively adjust the material properties in different areas of the powder layer, which allows you to control the material properties within the working volume of the tool.
Schematic representation of the heated bed method for (a) post-heating and (b) ink conversion. Image credit: Murray, J. W. et al. Letters on Additive Manufacturing.
In this study, the authors used Cu inkjet ink to demonstrate a laser melting method for producing metal matrix composites with better thermal conductivity than 316L stainless steel. To simulate a hybrid inkjet-powder bed fusion method, a stainless steel powder layer was doped with copper precursor inks and a new reservoir was used to control oxygen levels during laser processing.
The team created composites of 316L stainless steel with copper using inkjet copper ink in an environment simulating laser alloy in a powder bed. Preparation of chemical reactors using a new hybrid inkjet and LPBF technique that takes advantage of directional thermal conduction to reduce the overall size and weight of the reactor. The possibility of creating composite materials using inkjet ink is demonstrated.
The researchers focused on the selection of Cu ink precursors and the manufacturing procedure for composite test products to determine material density, microhardness, composition, and thermal diffusivity. Two candidate inks were selected based on oxidation stability, low or no additives, compatibility with inkjet printheads, and minimal residue after conversion.
The first CufAMP inks use copper formate (Cuf) as the copper salt. Vinyltrimethylcopper(II) hexafluoroacetylacetonate (Cu(hfac)VTMS) is another ink precursor. A pilot experiment was conducted to see if drying and thermal decomposition of the ink results in more copper contamination due to carryover of chemical by-products compared to conventional drying and thermal decomposition.
Using both methods, two microcoupons were made and their microstructure compared to determine the effect of the switching method. At a load of 500 gf and a holding time of 15 s, the Vickers microhardness (HV) was measured at the cross section of the fusion zone of two samples.
Schematic of the experimental setup and process steps repeated for fabricating 316L SS–Cu composite samples fabricated using the heated bed method. Image credit: Murray, J. W. et al. Letters on Additive Manufacturing.
It was found that the thermal conductivity of the composite is 187% higher than that of 316L stainless steel, and the microhardness is 39% lower. Microstructural studies have shown that reducing interfacial cracking can improve the thermal conductivity and mechanical properties of composites. For directional heat flow inside the heat exchanger, it is necessary to selectively increase the thermal conductivity of 316L stainless steel. The composite has an effective thermal conductivity of 41.0 W/mK, 2.9 times that of 316L stainless steel, and a 39% reduction in hardness.
Compared to forged and annealed 316L stainless steel, the microhardness of the sample in the heated layer was 123 ± 59 HV, which is 39% lower. The porosity of the final composite was 12%, which is associated with the presence of cavities and cracks at the interface between the SS and Cu phases.
For the samples after heating and the heating layer, the microhardness of the cross sections of the fusion zone was determined as 110 ± 61 HV and 123 ± 59 HV, respectively, which is 45% and 39% lower than 200 HV for forged-annealed 316L stainless steel. Due to the large difference in the melting temperature of Cu and 316L stainless steel, about 315°C, cracks in the fabricated composites were formed as a result of fluidization cracking caused by the fluidization of Cu.
BSE image (upper left) and map of elements (Fe, Cu, O) after sample heating, obtained by WDS analysis. Image credit: Murray, J. W. et al. Letters on Additive Manufacturing.
In conclusion, this study demonstrates a new approach to create 316L SS-Cu composites with better thermal conductivity than 316L SS using sprayed copper ink. The composite is made by putting ink in a glove box and converting it to copper, then adding stainless steel powder on top of it, then mixing and curing in a laser welder.
Preliminary results show that the methanol-based Cuf-AMP ink can degrade to pure copper without forming copper oxide in an environment similar to the LPBF process. The heated bed method for applying and converting ink creates microstructures with fewer voids and impurities than conventional post-heating procedures.
The authors note that future studies will explore ways to reduce the grain size and improve the melting and mixing of the SS and Cu phases, as well as the mechanical properties of the composites.
Murray J.W., Speidel A., Spierings A. et al. Synthesis of 316L stainless steel-copper composites by laser melting. Additive Manufacturing Fact Sheet 100058 (2022). https://www.sciencedirect.com/science/article/pii/S2772369022000329
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Surbhi Jain is a freelance technology writer based in Delhi, India. She has a Ph.D. He holds a PhD in Physics from the University of Delhi and has participated in several scientific, cultural and sports activities. Her academic background is in materials science research with a specialization in the development of optical devices and sensors. She has extensive experience in content writing, editing, experimental data analysis and project management, and has published 7 research articles in Scopus indexed journals and filed 2 Indian patents based on her research work. She is passionate about reading, writing, research and technology and enjoys cooking, playing, gardening and sports.
Jainism, Surbhi. (May 25, 2022). Laser melting allows the production of reinforced stainless steel and copper composites. AZ. Retrieved December 25, 2022 from https://www.azom.com/news.aspx?newsID=59155.
Jainism, Surbhi. “Laser melting enables the production of reinforced stainless steel and copper composites.” AZ. December 25, 2022 . December 25, 2022 .
Jainism, Surbhi. “Laser melting enables the production of reinforced stainless steel and copper composites.” AZ. https://www.azom.com/news.aspx?newsID=59155. (As of December 25, 2022).
Jainism, Surbhi. 2022. Production of reinforced stainless steel/copper composites by laser melting. AZoM, accessed 25 December 2022, https://www.azom.com/news.aspx?newsID=59155.
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