Taking a cue from the structural complexity of trees and bones, Washington State University engineers have created a way to 3D-print two types of steel in the same circular layer using two welding machines. The resulting bimetallic material proved 33% to 42% stronger than either metal alone, thanks in part to pressure caused between the metals as they cool together.

The new method uses commonplace, relatively inexpensive tools, so manufacturers and repair shops could use it in the near term. With further development, it could potentially be used to make high-performance medical implants or even parts for space travel, said Amit Bandyopadhyay, senior author of the study published in the journal Nature Communications.1

The research team borrowed the idea from nature, noting that trees and bones get their strength from the way layered rings of different materials interact with each other. To mimic this with metals, the WSU researchers used welding equipment commonly found in automotive and machine shops, integrated inside a computer numerical control or CNC machine. The new hybrid setup creates parts using precise computer programming and two welding heads.

In a demonstration, the two welding heads worked one right after the other on a circular layer to print two metals, each with specific advantages. A corrosion-resistant, stainless-steel core was created inside an outer casing of cheaper "mild" steel like that used in bridges or railroads. Since the metals shrink at different rates as they cool, internal pressure was created -- essentially clamping the metals together. Tests on the result showed greater strength than either stainless steel or mild steel has on their own.

Currently, 3D printing with multiple metals in a welding setup requires stopping and changing metal wires. The new method eliminates that pause and puts two or more metals in the same layer while the metals are still hot.

  • 1. "It has very broad applications because any place that is doing any kind of welding can now expand their design concepts or find applications where they can combine a very hard material and a soft material almost simultaneously," said Bandyopadhyay, a professor in WSU's School of Mechanical and Materials Engineering.
Fig. 1: Natural radial structures.
Fig. 1: Natural radial structures. - a Variation in compositional architecture is exhibited in natural multi-material structures such as bone. Variation is not observed along a single plane, but is found in complex structural and material arrangements that largely govern mechanical performance and unique functional properties. Adapted from Zimmermann et al.; b Annular architectural variations in wood anatomy is easily recognized in the concentric rings commonly exposed with cross-grain cuts. Each region fills a function vital to the overall survival of a tree, with the interaction between regions responsible in part for a flexible yet strong structure.

Lile Squires, Ethan Roberts, Amit Bandyopadhyay. Radial bimetallic structures via wire arc directed energy deposition-based additive manufacturing. Nature Communications, 2023; 14 (1) DOI: 10.1038/s41467-023-39230-w

Bimetallic wire arc additive manufacturing (AM) has traditionally been limited to depositions characterized by single planar interfaces. This study demonstrates a more complex radial interface concept, with in situ mechanical interlocking and as-built properties suggesting a prestressed compressive effect. A 308 L stainless core is surrounded by a mild steel casing, incrementally maintaining the interface throughout the Z-direction. A small difference in the thermal expansion coefficient between these steels creates residual stresses at their interface. X-ray diffraction analysis confirms phase purity and microstructural characterization reveals columnar grain growth independent of layer transitions. Hardness values are consistent with thermal dissipation characteristics, and the compressive strength of the bimetallic structures shows a 33% to 42% improvement over monolithic controls. Our results demonstrate that biomimetic radial bimetallic variation is feasible with improved mechanical response over monolithic compositions, providing a basis for advanced structural design and implementation using arc-based metal AM.

Fig. 7: Impact and role of residual pressure.
Fig. 7: Impact and role of residual pressure. - a Comparison of average compressive test results for monolithic and bimetallic specimens, with bimetallic compressive strengths averaging 37.8% above monolithic controls. Error bars indicate statistical uncertainty from repeated experiments; b 3D model of the multi-dimensional annular bimetallic cylinder, sectioned to reveal the interlocking interface between the 308 L stainless core and the mild steel casing; c Sectioned bimetallic specimen showing the as-deposited interface, as well as an illustration of the location for a drilled relief passage; d Analytical model indicating mechanical interactions between the two materials with pressure exerted at wedge boundaries; e Illustration exploring the effect of slight CTE differences, depicting each additional disc of inner core deposition shrinking at a higher rate than the surrounding casing ring upon which it partially overhangs, creating a clamping relationship and net residual stress in the growing specimen. The outer casing adds further compressive hoop stress as it also cools, constrained by the rapidly shrinking core; f Post-compression bimetallic specimen with a passage drilled through the core, showing severe deformation; g Compression test results for the unconstrained drilled bimetallic specimen compared to the as-deposited bimetallic specimen; h Heating of a bimetallic specimen to 550 °F, for subsequent cooling to room temperature to validate the accuracy of the analytical model; i Analytical model predicting the percent change in overall physical displacement during cool-down from 550 °F to room temperature.