3D printing company Renishaw has worked with Wassara, a Swedish based mining company to develop a new down-the-hole (DTH) hammer using additive manufacturing.
According to the companies, a DTH hammer is the most environmentally friendly percussion drilling method existing today since it is powered by water, and no oil is used for lubrication, so there is no contamination of air or water. The water is effectively suppressing dust as well. The incompressibility of water is a key factor in the function of the Wassara hammer system. It also gives a major energy cost saving compared to the more traditional air DTH technology, and reduces the energy demand to power the hammer.
However, the tool is made up of many complex parts including the sliding case that provides the bi-directional flow of water for the piston assembly. The sliding case requires several internal fluid channels to allow the flow of water, and due to its complexity, it cannot be fabricated from one single part. This complexity in the design makes these parts relatively expensive and they can faile due to wear or pitting corrosion. In an attempt to reduce cost and increase reliability of the component, Wassara approached Renishaw to investigate whether additive manufacturing (AM) could be a suitable alternative manufacturing method. One of the well-known benefits of metal AM is the ability to combine two or more complex machined parts into a single 3D geometry, or to simplify complex fabrication steps such as drilling cross-holes that require one end to be blind plugged or welded. Wassara’s sliding case geometry was redesigned to incorporate some of the benefits of AM design freedom.
Maximum hardness
The companies also had to choose correct metal alloy for the specific application. The standard steel alloy used for this part is 527M20, a structural alloy steel that would not normally be considered for metal AM due to the medium carbon content. A more suitable choice of steel alloy for AM is 316L stainless, but even though corrosion resistance is high for this alloy, it would not be expected to have sufficient wear resistance and withstand the erosion during usage, Renishaw says. An alternative suggested was using a maraging steel alloy to produce the test part. Maraging steel is a class of age hardenable tool steel that can be versatile, and is heat treatable to the extent of being able to tailor the properties required for certain applications. This was reportedly the first time that this steel had been tested in this kind of industrial mining application, and therefore the parts were heat treated post-build for maximum hardness.
To test the performance of the part, it was assembled into a full tool and used under standard mining conditions, creating typical long bore channels in a candidate rock face. The drill tool then underwent routine visual inspection and maintenance. The AM sliding case showed no signs of pitting and only minimal wear, compared to a standard part. The tool was re-assembled and further drilling tests were undertaken before re-inspection.
Following a second test the AM built sliding case did show some signs of wear but despite this there was no evidence of surface pitting which is the second most common failure mode. A further test followed, extending way beyond the expected drilling period to try and establish if the onset of pitting could be found but in actual fact there was no evidence of this. This has led to the initial conclusion that the AM maraging steel part has potentially superior pitting resistance to the conventionally chosen steel for the sliding case.
This story uses material from Renishaw, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier.
