Under a crushing amount of vertical force and forced translation, the friction generated on a metal surface can be used to accomplish useful things from joining dissimilar metals to chemically and physically modifying the metal surfaces and even 3D printing metals. These are some unconventional applications of useful friction.
5 millimetres is just the right size to do something big, claims Apple. Their unusually sleek iMac which is just 5 mm thick, is as awe-inspiring to a material scientist as to a gadget geek, because joining the ultra-thin monitor panels is a challenging material engineering problem. Which is why, Apple overruled conventional metal welding processes and used a relatively new approach called friction-stir welding, creating a product enclosure that was too thin and seamless to take apart.
Friction-stir welding is a process where pressure and friction-generated heat are used to join materials. Since ancient times, humans have known that thermal and mechanical processes can be used to morph materials to our needs – liquid water can be solidified on cooling, carbon can turn into diamond at high pressures and temperatures, and sheets of metals can be joined when their edges are melted by hot flame. Friction-based processes are similar, albeit they use friction to generate heat under intense pressure (imagine a crushing 800 kilos weight supported on an area equivalent to two finger tips). The study of these processes is one of the main focuses of the Materials Joining group at IITM’s Metallurgical and Materials Engineering department. The group uses friction to achieve a variety of things – from improving the strength of metals, to coating their surfaces and welding or compositing very different materials.
Prof. Ranjit Bauri, one of the faculty members in the group, describes his work as ‘surface engineering’. He uses a method called friction-stir processing to enhance hardness and wear resistance of metal surfaces. The apparatus is as basic as shown in the image below:
The vertical tool, which is under intense downward pressure, rotates and translates along the metal substrate. This produces frictional heat and local mixing at the interface, causing what’s called a plastic deformation of the material. Plastic deformation is almost like flow in solid state. Flow in solid state may seem counter-intuitive but this phenomenon of plasticity is universal in our daily life. Whenever we pound a bar of iron, the bar gets permanently deformed, literally ‘flowing’ into its new shape without melting. Or even when we iron our clothes which are typically made of polymers, the creases flow out.
Metals, which are the consideration of the study here, get such plasticity from their grainy internal microstructure. One way to understand microstructure is to look at metals as not one big solid slab, but as made of many microscopic interlocking polygons. Each of these polygons is a ‘grain’ and shares boundary with other neighbouring grains in three dimensions. Sizes and boundaries of these grains strongly affect almost all industrially useful properties like strength, ductility, hardness, corrosion resistance and wear-resistance of the material. Hence, understanding and improving microstructure of metals has been the holy grail of the metallurgical sciences.
Friction-stir processing (FSP) is one such process that helps refine the grain size, says Prof. Ranjit. When this method was discovered in late nineties, the group here was the first to apply the process to make surface composites. Composites are an immensely useful class of materials as they give us new properties – say, a mix of strength of one metal and low weight of the other. Aluminium (Al) and nickel (Ni) fit this bill, as nickel gives higher hardness to a widely used, low density metal like aluminium.
Now, the easiest and crudest way would be to melt the metals and mix them. But melting them to make an alloy results in formation of brittle intermetallic compounds like Al3Ni. These unwanted compounds arise because energy supplied to melt the metals also makes chemical reactions between them thermodynamically feasible. One way to overcome this trouble, as this group discovered, is to make the composite using a solid-state process like friction stirring. The second metal can be introduced into the ‘stir zone’ in a variety of ways and embedded into the other metal’s surface by the movement of the vertical rod tool.
The end product in case of aluminium and nickel is a metal-metal composite that’s three times harder than aluminium on the surface. This means it can resist wear more effectively. “The beauty of the process though”, as Prof. Ranjit puts it, “is that it doesn’t decrease the ductility, which is the ability of aluminium to be drawn into wires, too much; we are able to retain 80 percent of aluminium’s ductility.” This is a big deal because there is often a trade-of between strength and ductility in conventional strengthening processes. Prof. Janaki Ram, another faculty member in the group, has achieved similar results with metal-ceramic composites despite ceramics being a completely different class of materials from metals.
Making composites though is just one of the multitude uses of friction-related processes. Prof. Janaki Ram is also keen on applications of friction-related processes in additive manufacturing. Additive manufacturing or 3D printing is a computer operated layer-by-layer manufacturing of an object. This is unlike in a normal setting, where different parts of an object are casted first and then welded together. In a journal paper in 2011, this group was the first to propose that friction surfacing, a process very similar to friction-stir processing, could be used for layer-by-layer manufacturing of metal objects. The only difference, in fact, between friction stir processing and friction surfacing is that the rod tool used in the latter is consumable. While traversing the substrate, the rod tool itself significantly softens at the interface because of high temperature and pressure. This leads to establishment of metallurgical bonds between metal atoms of the rod and the substrate, causing some material to come of the rod and deposit on the surface of the substrate as one layer in a step-by-step layer addition process.
Friction surfacing’s biggest advantage is in the fact that it is a solid state process. This means that it is suitable for use with dissimilar materials, which would say, be incompatible with each other in melt state. There are many ideas-in-waiting for products using dissimilar materials – like a turbine with the input end made of a material optimised for heat resistance, and the output end optimised for strength. Or a bottle with a magnetic bottom to be held in place by a magnetic holder. The possibilities are unlimited.
One of the more established uses of friction is the friction-stir welding process used in building products like Apple’s iMac, NASA’s rovers or more traditionally, aerospace components produced by the likes of Boeing and Airbus. Invented in 1990s by The Welding Institute in UK, friction-stir welding was competitively patented until recently, closing of avenues for external research, says Prof. Gandham Phanikumar, another faculty member in this group. Once the patent expired, research opened up and techniques like friction processing and surfacing were proposed as a modifications of the original welding process. While these techniques are yet to be undertaken on a large commercial scale in India, organisations like Naval Research Board are looking at possibilities of using friction welding and surfacing for in situ repairs or application of coatings for marine vehicles.
Reading through the Ph.D. thesis of H Khalid Rafi, who worked in this group and graduated in 2011, one gets the idea of immense potential of friction-based processes to serve as an alternative to the conventional techniques. In fact, one of his papers on friction welding aluminium alloys has already been cited over fifty-five times in five years. While there is a still long way to go, the group hopes to continue drawing more insights into friction and its applications in material processing.
Materials Joining Laboratory at IIT Madras comprises of Prof. Gandham Phanikumar, Dr. GD Janaki Ram, Dr. Ranjit Bauri, and Dr. Srinavasa Rao Bakshi from the Department of Metallurgical and Material Science Engineering. Their research interests span surface engineering, microstructure analysis, additive manufacturing, welding and welding simulation, study of composites and alloys. Housed in a large Central Workshop bay, they use a wide array of testing and analytical tools to investigate and improve material behaviour.
Raghavi Kodati is a senior undergraduate student in the Chemical Engineering department, whose research interests are in microfluidics and materials. While working on this article, she got fascinated by the history of material joining processes – from their use in iron pillars in ancient India to today’s aluminium-lithium SpaceX rockets. Excited about science writing, she has written for three issues of Immerse.
Cover image : Image of Friction Welding, copyright of The Welding Institute, UK. Available via Flickr.