Forging and extrusion
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In this video, Stella Rayanova, a research assistant based in the Department of Engineering at the University of Waikato, explains how titanium-based alloy powders can be forged and extruded into different shapes.
By carefully controlling the temperature and pressure, it is possible to produce, from powder alloys, highly consolidated materials similar to those traditionally formed from molten alloys after casting.
Extrusion and forging they have different purposes. Forging is to make the final shape of the product and extrusion is just… it’s a semi-product to produce. You can press only rods with a cylindrical diameter or it can be rectangular, and after that, this semi-product can be used to produce something else.
Traditionally, forging is used to make ingots from casting into the shapes. In powder metallurgy, we use forging in different terms because we are dealing with powders and we want to make our powders into a shape. It is basically, we are using a die, and the die has the shape that we really want to produce. We don't start directly from powders because powders are loose, and we need to heat the powders before we forge them.
So what we do is we make a cold compact of the powders. Normally we used a cold pressed technique to make the powder into the cylindrical shape, and then we heat that green sample – we call it ‘green sample’ because it’s just a powder sample, it’s very weak – and then we heat it up to very high temperatures and then we put it into the forging die, and then we use a very high pressure to apply on the sample, and then the powder becomes the solid part.
Extrusion is, again, it’s a metal to consolidate powders into the solid semi-products. Again, we have to make the cold compact, and then we have to heat it again, and then we put it into the extrusion die. And if the diameter of our cold compact is, for example, 20mm, after the extrusion, it may become 2mm. So normally we start with a cylindrical sample with a higher diameter and after extrusion we are producing a solid rod with a much smaller diameter. And it’s a great way for consolidation. If you imagine the density of the green sample, it’s around 65%, maximum 70%, and then after the extrusion, we have 100% density, and that’s the real product. After extrusion and forging, we are trying to achieve densities of 100%.