November 14, 2023
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Steel and aluminum are key players in supporting economic growth, yet materials joining them remain unexplored due to their fusion zones’ brittleness. A new 3D printing method’s fix may be a step toward a steel-aluminum hybrid renaissance.
A new 3D printing method shrunk brittle zones plaguing steel and aluminum’s juncture to a size of less than two microns, overcoming a fundamental barrier to fusions of these titans of the automotive, aerospace, and critical infrastructure sectors.
These two metals have been rivals for market share, especially in the auto industry. Steel is stronger and cheaper. But aluminum has a better strength-to-weight ratio. Combining them can deliver weight savings without sacrificing structural integrity—valued by automakers as it is a step toward slashing carbon emissions. Yet fusions of steel and aluminum remain largely unexplored due to the brittle intermetallic compound (IMC) formed where their contrasting metallurgical properties meet.
“The challenge in combining aluminum alloys with ferrous materials, like the stainless steel used in our study, is the formation of the extremely brittle intermetallic compound. To improve joint strength, a joining method must suppress IMC formation to an ultra-thin layer,” said research co-lead Motomichi Yamamoto, professor at Hiroshima University’s Graduate School of Advanced Science and Engineering.
He and his co-researchers developed a 3D printing method that combined the hot wire technique, diode laser, and fluxes—which aids the proper spread and fusion of metals by preventing harmful oxidation—to control IMC thickness in the joint zones of stainless steel and aluminum (aluminum-magnesium) alloy.
They presented their findings at the 76th Annual Assembly of the International Institute of Welding and the International Conference on Welding and Joining held in July at the Marina Bay Sands Convention Center in Singapore.
Through a technique known as the hot wire method, scientists heated an aluminum alloy close to its melting point before introducing it to the molten pool, an area where dissimilar metals amalgamate under the influence of laser irradiation.
Two methods of flux application were employed for the experiment, making use of two distinct types of aluminum alloy wires: a solid wire and a flux-cored wire (FCW). The chloride flux was coated on a 15-millimeter stainless steel base plate in the first method, wherein solid wire without flux was used. In the second scenario, the researchers switched to FCW as a source of flux and kept the base plate uncoated.
An evaluation was conducted to find the optimal combination of laser spot sizes and process speeds for flux activation, reducing the formation of intermetallic compounds (IMC), and ensuring accurate and uniform prints. The results showed that the bead formation was most stable when a laser defocus distance of +15 mm was used. However, if this was exceeded, it led to undesirable outcomes such as pre-melting of the flux and formation of aluminum blobs at the filler wire’s tip, both of which interfered with a proper bead formation.
Moreover, the findings revealed that low-speed modelling yielded the best results, reducing IMCs to as low as 1–2 microns when the print speed was set at one meter per minute.
Next, the research team assessed the impact of laser power on the appearance of the bead and the breadth of Intermetallic Compound (IMC). During these experiments, a constant processing speed of 1.5 m/min was used. The investigators found that although laser power does not significantly influence IMC thickness, it does affect the shape of the bead.
When laser power was set to 4.7 kilowatt (kW), it was found to be insufficient, leading to defects in the center of the bead. Raising the power to 6 kW resulted in unstable bead shapes due to excessive fuming. The sweet spot of power, which resolved bead defects, was determined to be between 5 kW and 5.5 kW.
In addition to this, the role of laser spot size in activating flux coated on the stainless steel base was highlighted. Simultaneously, it was found that laser power dictates the size of the molten pool in flux-cored wire (FCW) method.
Utilizing their findings, the team determined the optimal settings for laser power and spot size and created one specimen per flux delivery method to test tensile strength. Each of the specimens consisted of nine layers of aluminum, and each layer had a height of 12mm. For the subsequent layers in each sample, solid wire was used.
The optimized calibrations achieved stainless steel and aluminum bonds that withstood separation stress of up to 17,404.5 pounds per square inch on average. Their IMC layers were also suppressed to less than two microns.
Observing the fractures sustained by the specimens under a scanning electron microscope, the researchers found what differentiated the strong bonds from weak ones. The samples that took the most force to break apart showed the presence of dimples, suggesting a ductile instability fracture. This happens in highly ductile materials which are more likely to deform than break when subjected to excessive strain. Once it cannot sustain further deformation, it fractures abruptly.
Meanwhile, those with the lowest strength exhibited particles. Analysis using energy-dispersive X-ray spectroscopy revealed the presence of oxides and flux elements such as potassium, fluorine, and carbon in the particles. This suggests that the weak bond is due to the entrapment of flux and other defects in the interface.
The researchers hope their method could help usher in a renaissance for designs combining aluminum and steel.
“We hope that this new process will help to create innovative product designs and revolutionary improvements in product performance by enabling high-strength direct joining of stainless steels and aluminum alloys,” Yamamoto said.
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“Why did the 3D printer go to therapy? Because it had too many layers of unresolved issues!”
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