A new manufacturing method combines 3D printing with traditional casting to produce damage-tolerant components

A new manufacturing method created by Oak Ridge National Laboratory and Rice University combines 3D printing with traditional casting to produce damage-tolerant components composed of multiple materials. Composite components made by pouring an aluminium alloy over a printed steel lattice showed an order of magnitude greater damage tolerance than aluminium alone.

The process, published in Materials and Design, was developed for potential automotive and other applications where thermal and mechanical properties must be optimised simultaneously.

“This scalable processing strategy can be used to fulfill specific component functions, giving materials designers unprecedented control over both microstructure and material properties,” said ORNL’s Amit Shyam.

Researchers modified the shape and density of 3D-printed lattice structures to achieve desired material properties in composite metal castings

To date, product designers and manufacturers have viewed additive manufacturing as an alternative (or even a competitive threat) to more established forming processes, like moulding or machining. And those assumptions are correct: AM or 3D printing can be used in place of casting parts, with advantages to be determined by the scale of production or the needs of a specific design. But these technologies can also be used to optimise or improve aspects of the casting process, as in printing sand moulds and cores, or defining and repairing casting moulds.

But now it’s possible to imagine additive manufacturing as a pathway to products and materials that were never conceivable, except as some designer’s dream. Now, such dreams can be realised in the research lab, at least.

Researchers at Oak Ridge National Laboratory (ORNL) and Rice University have reported their results in developing a new approach to consolidating multiple materials into a single cast part, creating what they term “damage tolerant” components. They contrast their method to established metal 3D printing techniques (referred to as “fusion-based metal additive manufacturing”), which often achieve geometric design goals but fall short of mechanical or material requirements in a functional sense.

In details reported by the researchers, the two materials they combined were stainless steel and aluminium. Their two-stage process “infiltrates” a 316L stainless steel lattice structure with a molten aluminium alloy, A356. According to their report, the process helps to overcome “issues with intermetallic formation, cracking, and poor resolution” that are common in many metallic parts formed by standard additive manufacturing.

The first step developed by the Oak Ridge team involves producing a lattice preform structure in the stainless material using selective laser melting, an established metal additive manufacturing method. The lattice structure then is the framework of the finished part.

In the second step, the frame is set within a permanent mould and then surrounded with a liquid metal at a melt temperature lower than that of the inset framework. The molten aluminium fills the space around the frame within the mould, and the resulting part is described as “interpenetrating phase composite” (IPC) that has a higher tolerance to damage than a comparable aluminium casting.

Dual material components were created by pouring an aluminium alloy over a printed stainless steel lattice, and the results showed significantly improved damage tolerance than a simple aluminium part

According to the process developers, these parts could have commercial application in automotive design and production, where lightweight parts with more optimised thermal and mechanical characteristics are needed. In designing such a part, the new two-step process could be used to tailor the desired thermal and mechanical properties needed for a specific final application.

Compression tests conducted on the stainless steel/aluminium part showed that adjusting the volume fraction and topology of the stainless steel lattice can control its stress-strain response. Also, according to the researchers, tension tests on composites with a 39-vol% of steel demonstrated “an order of magnitude improvement over the strain to failure” compared with the aluminium alloy on its own.

“Inspection of the as-tested tensile specimens suggested that this exceptional damage tolerance is a result of the interpenetrating structure of the constituents,” the researchers wrote.

“These results together demonstrate that this infiltration processing route avoids problems with intermetallic formation, cracking, and poor resolution that limit current fusion-based additive manufacturing techniques for printing metallic composites.”

“This scalable processing strategy can be used to fulfill specific component functions, giving materials designers unprecedented control over both microstructure and material properties,” stated Amit Shyam, one of the ORNL researchers and an author of the report.

“The key advantage of this processing strategy over other fusion-based metal additive manufacturing techniques is that in this two-step process, liquid-phase mixing of the constituents is excluded. As a result, we are able to overcome problems with cracking and poor resolution that limit most of the other fusion-based additive manufacturing techniques for printing composites,” according to their report.

The research report, “Damage-tolerant metallic composites via melt infiltration of additively manufactured preforms” is available for download at http://bit.ly/2vNVdJr