Although magnesium is known for being lightweight, riveting pieces of it together can be an involved, energy-intensive process. A new technique is claimed to make doing so much simpler, possibly opening up new uses for the metal.
Rivets are basically mushroom-shaped metal pins, the cylindrical "stems" (or shanks) of which are pushed through aligned holes in the two sheets of metal that need to be joined. The "cap" (or head) of the rivet ends up resting on the surface of one piece, while the shank protrudes out from the other. That shank is then pounded with a hammer, causing it spread out and flatten. The sheets are thus held together by the head on one side, and the flattened shank on the other.
Unfortunately, because magnesium has a tendency to be brittle, a rivet made of it would likely shatter if pounded at room temperature. This problem can be addressed by preheating such rivets in an oven – in order to make them softer – but that takes time and consumes energy. As a result, magnesium rivets aren't used very often.
And while aluminum rivets can be used to join sheets of magnesium, they're approximately 30 percent heavier than their magnesium counterparts. So, utilizing them partially counteracts the weight savings of using magnesium sheet metal.
Scientists at Washington state's Pacific Northwest National Laboratory have now developed what they claim is a better approach to riveting magnesium, known as Rotating Hammer Riveting (RHR). It builds upon an existing technique called friction stir welding.
The process begins with the shank of a magnesium rivet being inserted through the holes in two sheets of magnesium, at room temperature. A tool known as a hammer is then pushed up against the tail end of the shank. Despite its name, this hammer isn't like the one in your toolbox – instead, it's a solid metal cylindrical device with a concave end, and it rotates rapidly like a drill bit.
As the hammer spins against the shank, it creates friction that in turn produces heat. That heat softens the magnesium, enough so that the pressure exerted by the hammer is able to quickly flatten it out.
"It took many attempts to find the right balance between the rotational speed of the hammer and the speed at which we plunged it into the rivet," says the lead scientist, Dr. Tianhao Wang. "But we ultimately found a sweet spot in the processing range where the rivet head did not adhere to the tool, nor crack during the RHR process."
Importantly, the technique can also be applied to rivets made of commonly utilized aluminum alloy 2024. Because this metal is actually too hard to use for rivets if stored at room temperature, it has to first be heated in order to make it softer, then stored in a freezer to keep it in that state. It subsequently has to be used soon after being removed from the freezer, before it warms up and re-hardens.
If RHR is used on untreated aluminum alloy 2024 rivets, however, they will readily flatten at room temperature. What's more, the application of each rivet is claimed to take just a quarter of a second, as opposed to the 1 to 3 seconds that would ordinarily be required.
"That would mean one full week less spent riveting on just a fraction of one commercial airliner if RHR were adopted," says Dr. Scott Whalen, corresponding author of a paper on the research. That paper was recently published in the Journal of Magnesium and Alloys.
Source: PNNL
