Copper is a highly conductive material which makes it challenging to fabricate in high-power-energy-beam AM systems. This project explores Binder Jetting as an Additive Manufacturing method to create complex geometries out of copper. By studying process parameters and material properties in the context of copper, the project aims to establish a methodology for new material development and optimization in Binder Jetting of metals or ceramics. Novel material systems (binder and powder) are proposed for this project and will be studied while printing copper.
- Successfully printed copper with complex truss structures via Binder Jetting
- Improved sintered density and reduced shrinkage is achieved through a new powder composition
- 93% fractional of theoretical density
- 45% volumetric shrinkage
- Thermal conductivity: 87.3% of theoretical conductivity
- Electrical resistivity: 127% of theoretical resistivity
- Hot Isostatic Pressing (HIP) of sintered copper parts has achieved near full density
Cellular materials, metallic bodies in which gaseous voids are interspersed, are valued for having high strength accompanied by a relatively low mass. Cellular metals have ranges of properties that suggest their implementation in ultralight structures, as well as for impact/blast absorption, heat dissipation media and acoustic isolation. Among them, thermally conductive cellular structure (e.g. Copper) allows the creation of complex structured heat exchangers that can potentially enable a new generation of power generator for portable electronic devices that use high energy density fuels.
The traditional manufacturing technologies for cellular materials are only capable of fabricating a predetermined mesostructure and a planar macrostructure, and are thus unable to realize recent structure design advances. Additive Manufacturing (AM), a manufacturing tool that offers the utmost geometrical freedom, has demonstrated its success in fabricating metal cellular structures therefore is proposed for fabricating copper cellular materials.
Highly conductive and reflective materials (e.g. Copper) are usually challenging to make in many direct metal Additive Manufacturing systems that involve high energy sources. To circumvent the difficultly in controlling heating/cooling process and residual stress, a Binder Jetting AM process is proposed to create complex structured copper components. Binder Jetting (also known as Three-dimensional Printing) relies on inkjet printing to selectively deposit liquid bonding agents into a powder bed and join particulate materials. The deposited binder droplets interacts with the powder particles to form granules, which are stitch together to form a cross-sectional layer. A three-dimensional solid green part can be printed following this pattern, layer by layer from bottom up. For metals and ceramics printed in this manner, a thermal processing is required to decompose binder and strengthen/densify materials via sintering.
The feasibility of Binder Jetting of copper has been demonstrated on an ExOne R2 3D printer with gas atomized high purity copper powder and a standard polymeric binder. Solid state sintering of printed copper is conducted in a furnace with reducing atmosphere (hydrogen and Argon). In order to improve material strength and conductivity by reducing porosity content in the sintered parts, powders with different particle size distribution was explored. It has demonstrated a revised particle size distribution realized by bimodal powder mixture can improve sintered density from 85% to 93%. Furthermore, dimensional shrinkage associated with sintering has been reduced by using powder mixtures to improve green density.
Current research aims at optimizing printing/sintering parameters, the use of suitable post-processing methods to achieve near-full density of the parts. Research is also being done on developing and incorporating copper nanomaterials in the printing process.
Bai, Y. and Williams, C.B. (2015), “An exploration of binder jetting of copper”, Rapid Prototyping Journal, Vol. 21 No. 2, pp. 177–185
Bai, Y., Wagner, G., Williams, C. B. 2015. “Effect of Bimodal Powder Mixture on Powder Packing Density and Sintered Density in Binder Jetting of Metals” Proceedings of the 26th Annual International Solid Freeform Fabrication Symposium, Austin, TX.
Bai, Y., Williams, C. B. 2014. “An Exploration of Binder Jetting of Copper.” Proceedings of the 25th Annual International Solid Freeform Fabrication Symposium, Austin, TX. (Best Paper Award)
Yun Bai, Ashwath Yegyan Kumar
NSF CAREER Award (CMMI 254289)