Nanoparticles Make Mega Difference For “Unweldable” Aluminum

Though much of it is hidden from view, welding is a vital part of society. It’s the glue that holds together the framework of the cars we drive, the buildings we occupy, the appliances we use, and the heavy machinery that keeps us moving forward. Every year, the tireless search continues for stronger and lighter materials to streamline our journey into the future of transportation and space exploration.

Some of these futuristic materials have been around for decades, but the technology needed to weld them lagged behind. A group of researchers at UCLA’s Samueli School of Engineering recently found the key to unlocking the weldability of aluminium alloy 7075, which was developed in the 1940s. By adding titanium carbide nanoparticles to the mix, they were able to create a bond that proved to be stronger than the pieces themselves.

In the simplest terms, welding is defined as ‘the joining of metals and plastics without the use of fasteners’. The most common type is known as fusion welding, where the parent metals are melted together with a flame or an electrode. Non-fusion welding includes soldering and brazing. In these methods, a third metal is used as a filler to help join the pieces.

Welding dates back to the Middle Ages, and the first weldors were blacksmiths. These brave, soot-covered men both cut and joined pieces of iron together using nothing but fire, hammers, and a deep well of patience. The Industrial Revolution increased the demand for welding by several orders of magnitude, because much of the machinery from that era was made by casting molten metal. This brought about an entire sub-industry built on a new cast welding process, which involved heating the broken bits, bolting a mold around them, and pouring in molten metal.

When electricity arrived in the 20th century, the carbon-arc rods used in lighting fixtures sparked the idea of arc welding. Arc welding works by creating a circuit between a power supply (the arc welder) and the metals to be welded. The ground lead is clamped to the work piece, and the positive lead runs to a spring-loaded clamp that holds a 12-14″ electrode. This rod consists of a parent-matched filler metal coated with a flux material that turns into a gas when heated. This gas shields the work piece and the filler metal from impurities in the air while the bead is formed. The downside is that it also creates a solidified slag of filth that must be chipped away.

Oxy-acetylene gas welding came soon after arc welding, and WWI advanced both of these methods. As the aircraft industry began to take off, the demand for lightweight, durable metals and the people to weld them together skyrocketed. A newer style of welding known as GTAW (gas tungsten arc welding), Heli-Arc, or TIG (tungsten inert gas) began to gain popularity. Though difficult to master, TIG welding offers finer control and gives excellent results.

Many commonly welded metals are alloys of several different metals. This is because pure metals are too soft (and valuable) for the frames of cars and buildings. The only problem is that some alloys’ constituent metals don’t melt together well. When heated, the different metals flow unevenly, and cracks develop along the welded joint. This Achilles heel renders a number of otherwise strong and reputable alloys useless for welded applications.

AA7075 is one of these alloys. This decades-old concoction of aluminium, zinc, magnesium, and copper is extremely strong yet lightweight. It’s ideal for a number of applications, especially where fuel efficiency and battery conservation are valued. The only problem is that AA7075 is highly susceptible to cracking when welded. Though it is widely used in riveted-together airplane fuselages, AA7075 generally considered to be unweldable by any means.

A UCLA research team led by graduate student Maximilian Sokoluk and Professor Xiaochun Li have given the alloy a new lease on life. They’ve found a way to TIG weld two pieces of AA7075 together without any cracking whatsoever.

TIG (tungsten inert gas) welding uses a non-consumable tungsten electrode situated inside a torch. During welding, the torch releases helium or argon, which shields the weld from impurities. A separate filler wire made from a compatible alloy can be fed in to complete the joint, though it’s not required for thicker base metals.

The paper describes how adding titanium carbide nanoparticles to the mix allowed them to create a bond that proved to be stronger than the pieces themselves. The wire’s filler metal is infused with titanium carbide nanoparticles, which strengthen the mechanical properties of the metal in the melt zone. Using electron microscopes, the researchers studied cross-sections of the joints and found that the nanoparticles changed the alloy’s solidification mechanisms. In fact, they provide so much reinforcement that the metal in the melt zone actually becomes harder than the parent metals.

The resulting joint is quite strong, with a tensile strength up to 392 megapascals. To put that in slightly more accessible terms, that means it can withstand more than three times the pressure at the bottom of the Mariana, the deepest oceanic trench in the world. Not only is this great news for AA7075, it could create new opportunities for other high-grade, previously unweldable alloys.