IIT Bombay's New Research On Friction Welding Yields Significant Results
In a new study, researchers from the Indian Institute of Technology (IIT) Bombay, the ISRO Propulsion Complex, the Indian Space Research Organisation (IPRC-ISRO), and the Defence Metallurgical Research Laboratory (DMRL) have developed a simple and innovative method to improve the bonding in a rotary friction weld. Researchers are constantly working on addressing the shortcomings of rotary welding and improving the bonds.
Rotary friction welding, for example, is a ‘solid-state’ modern process in which, instead of heating the metals to a melting point, one metal rod is rotated at high speed while pressing it against the other stationary rod. The friction between them generates enough heat to soften but not melt the metal surfaces. When enough force is applied, the softened materials bond together, creating what is known as a solid-state weld.
In particular, they worked on improving the bond between stainless steel (SS321) and titanium alloy (Ti6Al4V), commonly used metals for aerospace, defence and other industrial applications.
From its earliest days when medieval blacksmiths hammered glowing metal in forges, humanity has sought ways to weld different metals together, building tools, structures, and eventually complex machines. Modern welding encompasses a vast array of techniques, moving far beyond simple fire and force.
Joining distinct metals, like steel and titanium, often leads to metallurgical incompatibility at the weld interface. Under the heat and pressure of welding processes, atoms from both metals diffuse across the weld boundary and react to form intermetallic compounds (IMCs), making the final product incompatible for industrial use. For example, in the case of steel and titanium, the iron-titanium (Fe-Ti) IMCs that form are brittle and readily form microscopic cracks, severely weakening the joint. To mitigate this issue, a thin nickel interlayer is introduced between the steel and titanium.
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“The nickel (Ni) prevents the formation of brittle Fe-Ti IMCs. Instead, Ni encourages the formation of Ni-Ti IMCs, which are more ductile and improve joint strength,” explains Dr. Neeraj Kumar Mishra, who led this study as a PhD student under the guidance of Prof Amber Shrivastava, an Associate Professor at the Mechanical Engineering Department, IIT Bombay.
However, using nickel as an interlayer has one major challenge. The friction welding process inherently generates high amounts of flash, which is the plasticised material, including the nickel interlayer, squeezed out from the joint. Flash is typically considered waste and machined away, representing lost material and an extra processing step. The interlayer, thus, adds complexity and cost to the process. Its effectiveness relies heavily on maintaining its integrity and sufficient thickness during the intense deformation and heating of the welding cycle.
In the new study, the researchers devised a simple solution to reduce the loss of material through flash formation and retain a thicker layer of the nickel interlayer. They changed the interface geometry, or the shape of the surface where the two rods meet, so that the flash is not simply pushed out. Specifically, they created a tapered end on one of the surfaces, titanium in this case, creating a cavity near the outer edge when the rods are pressed together.
“In this study, tapering was applied to the Ti6Al4V side because Ti is softer than SS and deforms more readily, ensuring a continuous and uniform interface bond”, says Prof Shrivastava for choosing to taper the titanium side.
According to Prof Shrivastava, “The interface geometry plays a crucial role in heat generation, material flow, and flash retention. A flat-taper interface improves bonding by creating a cavity that traps highly deformed flash material, which then undergoes further plastic deformation and remains within the joint, contributing to more refined grains at the interface and enhanced mechanical properties.”
To test the performance of their method, they ran experiments by rotary-welding rods of stainless steel (SS321) and a titanium alloy (Ti6Al4V) using a rotary friction welding machine. They compared two scenarios: the first, a flat-flat interface, used standard flat-ended rods, and the second, a flat-taper interface, where the titanium rod had a tapered end. In both cases, a thin nickel layer is sandwiched between the SS321 and Ti64 rods.
After welding, the samples were sliced near the joint periphery, the area experiencing the highest temperatures and deformation, and examined using Electron Back Scattered Diffraction (EBSD). EBSD allows the visualisation of the different material phases, the size and orientation of the crystalline grains, and the degree of internal strain. They performed tensile tests by putting increasing amounts of tension on the welded rods until they broke to measure the strength of the joints. They also examined the fracture surfaces under a Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS), which shows the surface structures and any tiny chemical phases that form when metals bond.
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The team also refined various parameters, like the tapering length and angle, to ensure the best outcome. In particular, they had to ensure uniform flash trapping without defects, optimise interlayer thickness retention, prevent excessive heat buildup at the interface, and determine the taper length and minimum mating diameter. They worked meticulously to optimise these parameters for the strongest weld between the two metals.
The results were striking. The flat-taper geometry proved highly effective not only at trapping the flash but also at retaining up to five times thicker nickel interlayer compared to the conventional flat-flat joint. The EBSD and EDS analysis of the flat-taper joint showed that the retained, thicker nickel interlayer successfully prevented the intermixing of iron and titanium. This inhibits the formation of the brittle Fe-Ti IMCs that plagued the interface in other scenarios.
The tensile strength of the joints with the flat-taper interface also showed significant improvement. It was, on average, 334.7 MegaPascals (MPa), a remarkable 105% improvement over the 163.3 MPa achieved with the traditional flat-flat interface. Fractography, the study of fractured surfaces, confirmed the reason: the flat-flat joints underwent brittle failure with evidence of Fe-Ti IMC due to intermixing. In contrast, the flat-taper joints failed primarily at the Ni-Ti interface without significant iron presence, indicating the interlayer held firm.
Microstructural analysis of the structure also revealed that the trapped flash within the cavity underwent a phenomenon called dynamic recrystallisation. Under extreme temperatures and pressures, metals can experience dynamic recrystallisation, where their grain structure reorganises or recrystallises into a finer, more random pattern while still hot and deformed. Compared to the standard joint, the new method resulted in significantly finer grains, particularly on the titanium side. The finer grain also contributes to the increased tensile strength of the weld. However, “additional research on deformation strain, strain rates, and process parameter optimisation could further refine the control over recrystallisation and enhance joint performance”, remarks Dr. Mishra.
The study suggests that sometimes, the solution to a stronger connection lies not just in the materials themselves but in the smarter shaping of the space between them. While this study focused on a specific steel-titanium-nickel combination and peripheral properties, the principle of using interface geometry or changing the shape to control flash and interlayer behaviour offers a promising avenue for optimising friction welds in various material systems.
“The technique needs to be extended to other dissimilar material combinations. Further, there is a need to develop models that capture the combined influence of taper angle, interlayer thickness, and process parameters on joint performance,” concludes Prof Shrivastava about the following steps to improve the research.
